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Iron-Modified Mesoporous Silica as Efficient Heterogeneous Lewis Acid Catalysts
Mémoire
Wan Xu
Maîtrise en chimie Maître ès sciences (M. Sc.)
Québec, Canada
© Wan Xu, 2018
Iron-Modified Mesoporous Silica as Efficient Heterogeneous Lewis Acid Catalysts
Mémoire
Wan Xu
Sous la direction de :
Freddy Kleitz, directeur de recherche
Thierry Ollevier, codirecteur de recherche
iii
Résumé
Les catalyseurs hétérogènes acides de Lewis ont principalement attiré beaucoup d'attention
à cause de leurs applications dans de nombreux processus chimiques tels que le raffinage de
pétrole. De plus, la facilité de séparer avec la phase liquide, et peu de déchets dangereux
générés dans les processus répondent aux exigences de la chimie verte. Par conséquent, il
est nécessaire de concevoir les catalyseurs acides de Lewis de façon simple, efficace, et peu
couteux. Pour ce faire, la silice mésoporeuse peut agir comme support potentiel pour ce type
de catalyseurs hétérogènes. La silice mésoporeuse possède les propriétés physiques et
chimiques adaptées tel que la surface spécifique élevés, le volume de pore grand, la taille des
pores accordable et ajustable, et la facilité de fonctionnalisation de la surface.
Par conséquent, l'objectif de cette thèse est d'explorer un procédé de synthèse facile pour
préparer 'un catalyseur d'acide de Lewis hétérogène efficace en utilisant la silice mésoporeuse
ordonnée fonctionnalisé par des métaux peu couteux. Pour cela, les silices mésoporeuses du
type MCM-41, et SBA-15 ont été choisis comme supports de catalyseur. Par la suite, Fe-
MCM-41 et Fe-SBA-15 ont été correctement synthétisé en utilisant un procédé polyvalent.
Le traitement de l'ammoniac au cours de la synthèse a été trouvé être un moyen efficace
d'augmenter la teneur en fer tout en préservant la dispersion convenable des cations
métalliques. Les paramètres physicochimiques de la silice mésoporeuse finale contenant du
fer ont été obtenus par l’analyse d'adsorption-désorption d'azote à la base température, et
l'environnement de la coordination des éléments en fer a été validé par la spectroscopie de
UV-Vis réflectance diffusée et la spectroscopie photoélectronique à rayons X. L'acidité de
surface a été sondé à l'aide des indicateurs de Hammett. Pour distinguer en outre les sites
acides de Lewis sur la surface, l’adsorption de pyridine suivie par FTIR a été mise en œuvre.
Ces catalyseurs préparés ont été criblés dans la réaction d'aldolisation de Mukaiyama, qui est
une réaction modèle catalysée par l’acide de Lewis. L'activité catalytique acide de Lewis des
matériaux était peaufinées et les produits aldol ont été obtenus avec un bon rendement et la
sélectivité. De plus, les catalyseurs hétérogènes sont très stables et peuvent être réutilisés au
moins neuf fois en conservant l’activité catalytique.
iv
Abstract
Heterogeneous Lewis acid catalysts have primarily attracted much attention for their
applications in many organic processes such as petroleum refinery. Moreover, their ease of
separation and no hazardous waste during the processes meet the requirements of cleaner and
environmentally friendly technologies requested by public in modern society. However, in
contrast to extensive studies of homogeneous Lewis acid catalysts, fewer efforts have been
dedicated to the study of heterogeneous Lewis acid catalysis. It is necessary to design
efficient Lewis acid catalysts through a straightforward and cost-effective method for the
generation of different chemicals. Scientific interest has focused on ordered mesoporous
silica because of their potential application, particularly in catalysis. Indeed, mesoporous
silicas can act as potential catalysts or catalyst supports owing to their physical and chemical
properties such as high surface area, larger pore size than zeolites for better support for active
sites, high pore volume, tunable pore size and ease of surface functionalization.
Therefore, the objective of this thesis is to explore a facile synthetic method for preparing an
efficient heterogeneous Lewis acid catalyst using ordered mesoporous silica functionalized
by cheap metals. For this, MCM-41, and SBA-15 materials were chosen as catalyst supports.
Subsequently, Fe-MCM-41 and Fe-SBA-15 were synthesized by using a versatile method.
pH adjustment during the synthesis route was found to be an efficient way to increase the
iron content while preserving suitable dispersion of the metal cations. Physicochemical
parameters of the final iron-containing mesoporous silica were obtained by low temperature
nitrogen adsorption-desorption equilibrium isotherms, and the bonding environment of iron
elements was validated by UV–vis diffuse reflectance spectroscopy and X-ray photoelectron
spectroscopy. The surface acidity was probed by using Hammett indicators. To further
distinguish the Lewis acid sites on the surface, pyridine sorption probed by the FTIR method
was implemented. These prepared catalysts were screened in the Mukaiyama aldol reaction,
which is a model reaction catalyzed by Lewis acid. The Lewis acid catalytic activity of the
materials was fine-tuned, and the corresponding aldol products were obtained in good yield
and selectivity. More importantly, the solid catalysts were very stable and could be reused at
least nine times maintaining the same catalytic activity.
v
Table of Contents
Résumé ............................................................................................................................................... iii
Abstract .............................................................................................................................................. iv
Table of Contents ................................................................................................................................ v
List of Schemes ................................................................................................................................. vii
List of Figures .................................................................................................................................. viii
List of Tables ...................................................................................................................................... xi
List of Abbreviations ......................................................................................................................... xii
Acknowledgement ............................................................................................................................. xv
Chapter 1 ............................................................................................................................................. 1
Introduction ......................................................................................................................................... 1
1.1 Need for green Lewis acid catalysts .......................................................................................... 1
1.2 Mesoporous solid acid catalysts ................................................................................................ 2
1.3 Aims and organization of the thesis .......................................................................................... 3
Chapter 2 ............................................................................................................................................. 5
State-of-the-art .................................................................................................................................... 5
2.1 Heterogeneous catalysis ............................................................................................................ 5
2.1.1 Fundamentals of heterogeneous catalysis .......................................................................... 5
2.1.2 Supported catalysts ............................................................................................................. 7
2.2 Synthesis of mesoporous silica supports ................................................................................... 8
2.2.1 Mechanisms for formation of mesoporous silica ............................................................. 10
2.2.2 Mesostructure tailoring .................................................................................................... 12
2.3 Functionalization of mesoporous silica ................................................................................... 15
2.3.1 Surface properties ............................................................................................................. 16
2.3.2 Metal-modified mesoporous silica ................................................................................... 16
2.3.3 The post-synthesis method using acetylacetonate-metal precursors (acac) ..................... 17
2.4 Mukaiyama aldol reaction ....................................................................................................... 18
Chapter 3 ........................................................................................................................................... 22
Characterization techniques for synthesized materials ..................................................................... 22
3.1 Nitrogen adsorption ................................................................................................................. 22
3.2 Powder X-Ray Diffraction (XRD) .......................................................................................... 26
3.3 Electron microscopy ................................................................................................................ 28
3.3.1 Scanning electron microscopy (SEM) .............................................................................. 28
vi
3.3.2 Transmission electron microscopy (TEM) ....................................................................... 29
3.3.3 Energy dispersive X-ray spectroscopy (EDX) ................................................................. 30
3.4 Diffuse reflectance ultra-violet visible spectroscopy (DR-UV-vis) ........................................ 31
3.5 X-ray photoelectron spectroscopy (XPS) ................................................................................ 31
3.5 Pyridine adsorption probed by FT-IR spectroscopy ................................................................ 32
3.6 Inductively coupled plasma mass spectrometry (ICP-MS) ..................................................... 33
Chapter 4 ........................................................................................................................................... 35
Iron-Modified Mesoporous Silica as an Efficient Solid Lewis Acid Catalyst for the Mukaiyama
Aldol Reaction .................................................................................................................................. 35
Résumé .......................................................................................................................................... 36
Abstract ......................................................................................................................................... 37
4.1 Introduction ............................................................................................................................. 38
4.2 Experimental section ............................................................................................................... 40
4.3 Catalyst Characterization and Testing ..................................................................................... 41
4.3.1Titration of the Lewis Acid Solids with Hammett Indicators ........................................... 43
4.3.2 Pyridine Adsorption FT-IR Experiments. ........................................................................ 44
4.4 Results and Discussions .......................................................................................................... 44
4.4.1 Synthesis and Characterization of the Materials .............................................................. 44
4.4.2 Surface Acidity ................................................................................................................. 53
4.4.3 Catalytic Tests .................................................................................................................. 57
4.5 Conclusion ............................................................................................................................... 67
4.6 Supporting information ........................................................................................................... 69
4.6.1 Materials and general procedure of catalytic reactions .................................................... 69
4.6.2 Catalytic reactions ............................................................................................................ 69
4.6.3 Schemes of Mukaiyama aldol reactions ........................................................................... 71
4.6.4 Characterization of the catalysts....................................................................................... 72
4.6.5 Lewis acidity tests ............................................................................................................ 76
4.6.6 Catalytic tests ................................................................................................................... 77
Chapter 5 ........................................................................................................................................... 93
Conclusions ....................................................................................................................................... 93
5.1 General conclusions ................................................................................................................ 93
5.2 Future prospect ........................................................................................................................ 94
References ......................................................................................................................................... 96
vii
List of Schemes
Scheme 2.1 Mukaiyama aldol reaction catalyzed by a stoichiometric amount of TiCl4
Scheme S4.1 Mukaiyama aldol reaction of 1-(trimethylsilyloxy)-cyclohexene and
benzaldehyde
Scheme S4.2 Mukaiyama aldol reaction of 1-phenyl-1-(trimethylsiloxy)-propene and
different
viii
List of Figures
Figure 2.1 General classification of catalysts.
Figure 2.2 Representation of hurdles in a heterogeneous catalyzed reaction (center); potential
energy diagram (left); volcano plot of catalyst activity and adsorption forces (right).
Figure 2.3 Two pathways for the synthesis of ordered mesoporous silica: A, cooperative self-
assembly; B, true liquid-crystal templating.
Figure 2.4 Schematic representation of different types of cooperative interactions for the
inorganic-organic hybrid mesophase.
Figure 2.5 Representations of the pore topology with symmetries of (A) p6mm, (B) Ia3d, (C)
Pm3n, (D) Im3m, (E) Fd3m, and (F) Fm3m.
Figure 2.6 Micropore formation in mesoporous SBA-15 silica where steps (i) and (ii)
correspond to treatment with sulfuric acid and calcination, respectively.
Figure 2.7 Schematic routes for the functionalization of mesoporous silica via various
methods.
Figure 2.8 Various types of surface silanols/siloxanes on mesoporous silicas.
Figure 2.9 Hydrolysis constants and water-exchange rate constants for determining Lewis
acidity.
Figure 3.1 Different types of the physisorption isotherms as classified by IUPAC.
Figure 3.2 Classification of adsorption-desorption hysteresis loops.
Figure 3.3 Typical low-angle powder XRD patterns obtained for MCM-41(Left) and SBA-
15(right), showing reflections of the hexagonal plane group.
Figure 3.4 The main types of the signal generated by the electron beam-specimen interaction.
Figure 3.5 TEM images of (a) Fe-MCM-41 (b) Fe-SBA-15.
Figure 3.6 Schematic representation of an ICP-MS instrument.
Figure 4.1 N2 adsorption-desorption isotherms measured at 77.4 K (-196 °C) for (A) Fe-
MCM-41 and (C) Fe-SBA-15 with various iron contents and the corresponding NLDFT pore
size distributions for (B) Fe-MCM-41 calculated from the adsorption branch of the isotherm
and (D) Fe-SBA-15 calculated from the desorption branch of the isotherm.
Figure 4.2 High-resolution transmission electron microscopy images of (A) Fe-MCM-41
(10%); (B) Fe-MCM-41 (10%, pH 10) and (C) Fe-SBA-15 (10%); (D) Fe-SBA-15 (10%, pH
10) and their corresponding energy dispersive X-ray spectroscopy data.
ix
Figure 4.3 XPS spectra of (A) Si 2p, (B) O 1s and (C) Fe 2p for the Fe-SBA-15 (20%) sample
(taken as representative).
Figure 4.4 UV-vis absorption spectra of different indicators and the catalysts with the
indicators adsorbed.
Figure 4.5 FT-IR spectra for Fe-MCM-41 (20%) and Fe-SBA-15 (20%) materials with and
without pyridine materials.
Figure S4.1 Low-angle XRD patterns for Fe-MCM-41 (A) and Fe-SBA-15 (B) with various
iron contents, as indicated.
Figure S4.2 Wide-angle XRD patterns for Fe-MCM-41 (A) and Fe-SBA-15 (B), with various
iron contents, as indicated, and the reference pattern of Fe2O3 is also shown.
Figure S4.3 SEM images of (A) MCM-41, (B) Fe-MCM-41 (10%), (C) Fe-MCM-41 (10%,
pH=10), and (D) SBA-15, (E) Fe-SBA-15 (10%), (F) Fe-SBA-15 (10%, pH=10), and their
corresponding EDX spectra, with surface Fe/Si molar ratio of (G) 2%, (H) 13%, (I) 1.7% and
(J) 11%, for samples in (B), (C), (E) and (F), respectively.
Figure S4.4 UV–vis diffuse reflectance (DR-UV–vis) spectra of calcined Fe-MCM-41 and
Fe-SBA-15 samples.
Figure S4.5 1H NMR spectrum of 1-(trimethylsilyloxy)-cyclohexene
Figure S4.6 1H NMR spectrum of 1-phenyl-1-(trimethylsiloxy)-propene
Figure S4.7 1H NMR spectrum of 2-(hydroxyphenylmethyl)-cyclohexanone
Figure S4.8 13C NMR spectrum of 2-(hydroxyphenylmethyl)-cyclohexanone
Figure S4.9 1H NMR spectrum of 3-hydroxy-2-methyl-1,3-diphenylpropan-1-one
Figure S4.10 13C NMR spectrum of 3-hydroxy-2-methyl-1,3-diphenylpropan-1-one
Figure S4.11 1H NMR spectrum of 3-hydroxy-3-(4-methoxyphenyl)-2-methyl-1-phenyl-1-
propanone
Figure S4.12 13C NMR spectrum of 3-hydroxy-3-(4-methoxyphenyl)-2-methyl-1-phenyl-1-
propanone
Figure S4.13 1H NMR spectrum of 3-hydroxy-3-(4-chlorophenyl)-2-methyl-1-phenyl-1-
propanone
Figure S4.14 13C NMR spectrum of 3-hydroxy-3-(4-chlorophenyl)-2-methyl-1-phenyl-1-
propanone
x
Figure S4.15 1H NMR spectrum of 3-hydroxy-3-(4-cyanophenyl)-2-methyl-1-phenyl-1-
propanone
Figure S4.16 13C NMR spectrum of 3-hydroxy-3-(4-cyanophenyl)-2-methyl-1-phenyl-1-
propanone
Figure S4.17 1H NMR spectrum of 3-hydroxy-3-(4-nitrophenyl)-2-methyl-1-phenyl-1-
propanone.
Figure S4.18 13C NMR spectrum of 3-hydroxy-3-(4-nitrophenyl)-2-methyl-1-phenyl-1-
propanone
Figure S4.19 1H NMR spectrum of 3-hydroxy-3-(2-nitrophenyl)-2-methyl-1-phenyl-1-
propanone
Figure S4.20 13C NMR spectrum of 3-hydroxy-3-(2-nitrophenyl)-2-methyl-1-phenyl-1-
propanone
Figure S4.21 1H NMR spectrum of 3-hydroxy-2-methyl-1-phenyl-1-hexanone
Figure S4.22 13C NMR spectrum of 3-hydroxy-2-methyl-1-phenyl-1-hexanone
xi
List of Tables
Table 2.1 Parameter to be considered for the selection of catalyst supports.
Table 4.1 Indicators Used for Acid Strength Measurements.
Table 4.2 Chemical Composition and the Structural Properties of All Prepared Materials.
Table 4.3 Effect of Catalyst Amount for Mukaiyama Aldol Reaction over Fe-MCM-41 (20%,
pH 10) Catalyst.
Table 4.4 Effect of Catalyst Amount for Mukaiyama Aldol Reaction over Fe-MCM-41 (20%,
pH 10) Catalyst.
Table 4.5 Effect of the Solvent for Mukaiyama Aldol Reaction over Fe-MCM-41(20%, pH
10)
Table 4.6 Effect of the Iron Content in the Mukaiyama Aldol Reaction of 1-phenyl-1-
(trimethylsiloxy)propene with Benzaldehyde.
Table 4.7 Effect of Iron Content for Mukaiyama Aldol reaction of 1-
(trimethylsilyloxy)cyclohexene with Benzaldehyde.
Table 4.8 Catalytic performances of Fe-SBA-15 (5%)(A) and Fe-SBA-15 (20%)(B) in
Mukaiyama aldol reactions (substrate scope)
Table 4.9 Reusability of catalysts for Mukaiyama aldol reaction of 1-(trimethylsilyloxy)-
cyclohexene with benzaldehyde over Fe-SBA-15 (20%) catalyst
Table 4.10 Catalytic test of Fe-MCM-41-HMDS (20%) and Fe-SBA-15-HMDS (20%)
catalyst.
Table S4.1 Results of titration of Fe-modified mesoporous materials with Hammet indicators.
Table S4.2 Effect of the addition of the surfactant (SDS) in the Mukaiyama aldol reaction.
Table S4.3 Mukaiyama aldol reaction using FeCl3 as catalyst
Table S4.4 Reusability of catalysts for Mukaiyama aldol reaction of 1-(trimethylsilyloxy)-
cyclohexene with benzaldehyde over Fe-SBA-15 (5%) catalyst
xii
List of Abbreviations
2D 2-dimensional
3D 3-dimensional
BE Binding energy
BET Brunauer–Emmett–Teller
BJH Barrett–Joyner–Halenda
CMC Critical micellar concentration
CTAB Cetyltrimethylammonium bromide
DMC Dimethyl carbonate
EDX Energy-dispersive X-ray spectroscopy
FT-IR Fourier transform infrared spectroscopy
GNP Gross national product
HRTEM High-resolution transmission electron microscopy
HMS Hexagonal mesoporous silica
HMDS Hexamethyldisilazane
IUPAC International Union of Pure and Applied Chemistry
ICP-MS Inductively coupled plasma mass spectrometry
ICDD International Center for Diffraction Data
KE Kinetic energy
KIT-6 Mesoporous silica Korean Institute of Technology number 6
MCM-41 Mobil Composition of Matter Number 41
MCM-48 Mobil Composition of Mater number 48
MCM-50 Mobil Composition of Mater number 50
MOF Metal-organic framework
NLDFT Nonlocal density functional theory
NMR Nuclear Magnetic Resonance
Pluronic P123 Triblock copolymer, PEO20PPO70PEO20
PEO Poly(ethylene oxide), -(CH2CH2O)-
PPO Poly(p-phenylene oxide)
SBA-15 Santa Barbara Amorphous Number 15
xiii
SBA-15 Santa Barbara Amorphous Number 16
SEM Scanning electron microscope
SDS Sodium dodecyl sulfate
TON Turnover number
TLCT True liquid-crystal templating
TEOS Tetraethyl orthosilicate
TEM Transmission electron microscopy
TS-1 Titanium silicalite-1
UV-vis DRS Ultraviolet-visible diffuse reflectance spectra
WERCs Water exchange rate constants
XRD X-ray diffraction
XPS X-ray Photoelectron Spectroscopy
xiv
To my parents, grandmother and my sister,
xv
Acknowledgement
I would first like to give my sincere gratitude to my supervisor Prof. Freddy Kleitz for giving
me the opportunity to work in his laboratory. His guidance and support throughout my entire
MSc. study and research also helped me a lot. I am truly grateful to him for his patience,
insightful comments, and encouragements on the thesis as well as on all the time of the
research. I also would like to thank to my co-supervisor Prof. Thierry Ollevier. He is always
willing to spend a lot of time to give me advices and share his immense knowledge and skills
with me. I would like to express my deepest thanks to both of my supervisors, my research
and related works would not have been possible without their guidance and help.
My sincere thanks also go to professors, administrative staff and technicians in Laval
University for their assistance during my MSc life. I would like to thank Prof. Peter Mcbreen
for his help in my course and seminar. Great thanks to André Ferland, for his kind help for
SEM analysis and interesting talks. I would also like to thank Jean Frenette for XRD, Alain
Adnot for XPS analysis and Serge Groleau for ICP-MS analysis. I would like to acknowledge
Jongho Han and Prof. Ryong Ryoo from KAIST, Korea for providing the data of XRD
measurement images of high-resolution TEM and element mapping.
I am also very appreciating and grateful to all the members in Kleitz group and Ollevier
group, for their working experience, stimulating discussions and friendship. It is my honor
and pleasure to work with them all. I would like to thank to Dr. Justyna Florek, Dr. Angela
Jalba, Dr. Maria Zakharova and Estelle Juère for their training, supports, and friendship. I
would like to thank Dr. Nima Masoumifard, for his great help and suggestions at the
beginning of my research. He is a good friend, teacher and researcher because of his earnest,
patience, and diligence. Thanks to Yimu Hu, her intelligence, and hardworking always
inspires me a lot. In addition, the abstract in French of this thesis was completed with her
great help. Great appreciation also goes to Dazhi Li, Di Meng, Dandan Miao and Mao Li for
all their help both in my research and my life.
Finally, I need to thank my dearest sister, parents, grandma, and my boyfriend, who give me
endless love, inspiration, and power to help me go through all the difficulties.
1
Chapter 1
Introduction
1.1 Need for green Lewis acid catalysts
Catalysis played an important role in the chemical industry over the past century. It now
continues to be essential to economic growth, contributing about 20% of the world GNP in
total.3,4 Since the beginning of the 21st century, catalytic technologies have been widely
studied to meet the requirements of greener production of chemicals and sustainable fuels.
For a greener process to be carried out in a reactor, the catalysts must be designed for ease of
separation and the ability to reuse. Also, the design of a catalyst requires
the high productivity as well as the reduction of waste.5,6
Lewis acid catalyzed reactions are of high importance in catalytic technologies for green
chemistry as many fine and pharmaceutical chemicals manufacturing processes depend on
the application of homogeneous Lewis acid catalysts. Since many of these industrial
processes were invented almost more than one hundred years ago, their only goal was to
improve the productivity of the catalysts, neglecting the influence of the hazardous and toxic
waste on the environment. Most of the waste is produced during the isolation step of the
reaction mixture and therefore it is necessary to add a step to break the bond existing between
the products and the catalysts to destroy the acid-base adduct. Lewis acid catalysts usually
decompose completely during this step, not only producing undesired by-products but also
making it hard to regenerate catalytic activity.7-9 In addition, it is common for a Lewis acid
catalyzed reaction to employ a greater amount of catalyst than the stoichiometric equivalents
of Lewis acid because the product usually has stronger bases than the reactants. Therefore,
the percentage of the waste and by-products are mostly derived from the conventional Lewis
acid catalysts. In summary, there is no surprise that the reactions catalyzed by Lewis acids
are considered as processes that are less environmentally-safe in the chemical industry.6
Therefore, it is necessary and urgent to design a heterogeneous solid Lewis acid catalyst that
could be employed in this kind of reactions. The efficient use of solid acid catalysts facilitates
the separation of the catalysts, simplifies the isolation of the products and improves the
2
selectivity. Meanwhile, only conventional organic solvents or water are required to
manipulate the reagents and products in the processes using the solid acid catalysts.3,10,11
1.2 Mesoporous solid acid catalysts
One way to synthesize environmentally friendly Lewis acid solid catalysts is supporting
corrosive Lewis acids or introducing metal species on solids such as zeolites, silica, graphites
or alumina, which exhibit high specific surface areas. The catalytic performance of the
obtained supported materials is strongly affected by the type of material used as support.
When a material is employed as a support, there are two critical factors that it should satisfy.
First, as a catalyst, it should be stable when it is used in a reaction process, both thermally
and chemically. Second, good accessibility and homogeneous dispersion for active sites are
necessary for a support material. Regarding this, nanoporous materials have been considered
as very suitable supports in recent years.
Porous materials, solids with channels or cavities that are deeper than they are wide,12 have
been widely studied regarding applications as heterogeneous catalysts or catalyst supports.
On the basis of different pore size, the classification of porous materials defined by IUPAC
has three types: microporous materials with a pore size less than 2 nm, macroporous materials
with a pore size larger than 50 nm, and mesoporous materials with a pore size between 2 nm
and 50 nm.13 The most famous members among the class of microporous materials are
zeolites, which exhibit small molecular size pores, have been extensively studied in the
literature.14-16
For instance, zeolites as heterogeneous acid catalysts, are nowadays considered as a
substitute for conventional homogeneous acid catalysts. However, zeolite-type catalysts
could prevent the accessibility of reactants with sizes larger than the dimensions of the
micropore openings (most often <0.8 nm), especially in the case of liquid-phase reactions.
Taking the Mukaiyama-aldol reaction as a representative example, Corma17 and co-workers
reported that Ti-containing mesoporous silica catalysts showed an excellent yield up to 98%
in a solvent-free system. They demonstrated that well-prepared Ti-MCM-41 could be a
better catalyst for the Mukaiyama aldol reaction compared to Ti-zeolites, owing to the
possibility of introducing isolated Ti (IV) on the walls whereas bigger pore size allowed
3
less diffusional restriction for reactants. In this respect, for carrying out catalytic reactions
involving larger molecules, larger pore sizes are necessary. Therefore, attempts were focused
on broadening the pore diameter of zeolites into the mesoporous range, which can also
maintain the porous structure and allow the entrance of large molecules into the pore system.
Meanwhile, the diffusion of the reactants to the active sites could be facilitated because of
the presence of mesoporous system.
It has been found that several factors have impacts on the catalytic activity of mesoporous
solid acid catalyst, such as their Brønsted/Lewis acidity, their strength and number of
introduced active sites and the surface area/porosity of the support.3 By fine-tuning these
properties, a range of novel heterogeneous mesoporous acid catalysts, which possess high
product selectivity could be obtained and employed in various reactions that require acidity.
For example, a novel solid acid catalyst, zinc triflate supported hexagonal mesoporous silica
(HMS) was synthesized by Wilson and co-workers for the rearrangement of α-pinene oxide.18
This solid acid catalyst showed excellent catalytic activity and selectivity and also exhibited
high stability for their high selectivity in the in the recycling experiments (no ie).18
Meanwhile, the mesoporous solid acids have been proven to be effective for a typical acid
catalyzed reaction, Friedel-Crafts reaction. Armengol found that mesoporous aluminosilicate
MCM-41 could be employed as a catalyst for Friedel-Crafts alkylation of cinnamyl alcohol
and a bulky aromatic compound. This discovery exemplified the application of MCM-41 as
a catalyst.19 It was found that different metal-incorporated mesoporous silica-containing acid
sites, especially Fe-MCM-41, showed excellent catalytic activity in the Friedel-Crafts
benzylation of benzene.20 It was also reported that several metal modified mesoporous
materials could be used as catalysts for the Mukaiyama aldol reaction, which is usually
catalyzed by homogeneous Lewis acid catalysts, such as TiCl4 and Sc(OTf)3.21-23 However,
studies of mesoporous Lewis acid catalysts are still limited. Thus, it is of particular
importance to develop a straightforward and cheap method for the synthesis of this type of
catalyst, which enables the diffusion of bulkier substrates.
1.3 Aims and organization of the thesis
Because of the need for solid Lewis acid catalysts, we have sought to explore the synthesis
metal-modified mesoporous materials as potential catalysts for Lewis acid catalyzed
4
reactions. To reach this goal, two typical mesoporous silicas, MCM-41 and SBA-15, were
firstly synthesized. The powder mesoporous silicas were then used as supports for the
introduction of metal sites through the post-grafting method. The resulting metal-modified
mesoporous silicas were applied as heterogeneous catalysts in a model Lewis acid catalyzed
carbon-carbon bonding reaction, i.e., the Mukaiyama aldol reaction. The catalytic activity,
selectivity as well as the stability of the catalysts were examined. Furthermore, the influence
of mesopore size, as well as that of the surface hydrophobicity of the mesoporous support on
the catalytic activity, were also be investigated.
The organization of this thesis consists in five parts, introduction, state-of-the-art,
experimental section, result and discussion and conclusions, as follows:
Chapter 2 introduces the state-of-the-art including the fundamental principles of
heterogeneous catalysis. This chapter also explains the basic concepts of the catalyst supports
as well as the principles behind the synthesis of the mesoporous materials and the final
catalysts. Chapter 3 presents the experimental techniques applied in the thesis for the
characterization of the synthesized materials. In Chapter 4, a simple approach based on the
post-grafting method to synthesize iron modified mesoporous silica is introduced in detail.
The resulting solids were used as catalysts for Mukaiyama aldol reaction to investigate their
catalytic activity. Finally, the last chapter presents conclusions of the work and indicates
some perspectives.
5
Chapter 2
State-of-the-art
2.1 Heterogeneous catalysis
2.1.1 Fundamentals of heterogeneous catalysis
In general, a substance that can participate in a chemical reaction to convert the reactants into
products and increase the reaction rate is the catalyst. The catalyst may be transformed into
some other different entities during the reaction, but it will regenerate to its original form
after each reaction complete catalytic cycle. There are three types of catalysts: heterogeneous,
homogeneous and biological catalysts, respectively. Figure 2.1 shows the range of the
classification.
Figure 2.1 General classification of catalysts.24
Heterogeneous catalysts are estimated to be used in 90% of all the chemical processes. Raw
materials can be converted into beneficial compounds in an efficient and environmentally
friendly way by using heterogeneous catalysts, which is of great importance for numerous
traditional industrial applications such as chemical production, food, pharmaceutics, and oil
6
industries.25,26 Also, heterogeneous catalysts can be applied in novel areas, e.g., green
chemistry27, biotechnology, fuel cells28, and nanotechnology.29
There are several advantages of using a heterogeneous catalyst for a reaction. One of the
most important ones is that the separation step for a solid catalyst removal from either gas or
liquid reactants and products is very straightforward. Active sites on the solid surface are the
most important part of heterogeneous catalysts because they are the active ingredients for the
reactions.
In general, there are some activation barriers and hurdles which need to be overcome for the
reactants and products of a heterogeneous catalyzed reaction (Figure 2.2). First, the reactants
are diffusing to the surface of the catalyst, and then adsorption at the active site occurs. Once
the active site is reached, the surface reaction takes place, the products are formed. The
products are finally transported away from the catalyst particle through desorption and
diffusion.30 Thus, in a heterogeneous catalytic reaction cycle, the rate of the reaction is not
only affected by the actual chemical reaction but may also be influenced by the diffusion rate
of the reactants and products, the adsorption and desorption processes.30 If the adsorption
force is too weak, the catalyst will have little capability to break the bond, resulting in a
reaction of slow rate. On the contrary, if the interaction is too strong, the desorption of the
products will be very difficult, and the active sites will be “poisoned”, a “volcano” effect on
activity may be expected 30 (Figure 2.2 (inset)).
7
Figure 2.2 Representation of hurdles in a heterogeneous catalyzed reaction (center); potential energy
diagram(inset, left); volcano plot of catalyst activity and adsorption forces (inset, right).31
The performance of a catalyst is determined by a lot of parameters. First, it is obvious that a
good catalyst should exhibit high activity and high selectivity. This means that desired
products need to be obtained at a required conversion on a period of time. The elimination of
by-products should be easy and minimal purification cost required. Good selectivity is the
primary objective compared with high activity/conversion in catalyst development. It
represents the ability of a catalyst converting the starting substance to the desired products.
Furthermore, a catalyst should exhibit sufficient stability under the reaction conditions over
an extended period of time (lifetime), or it should be possible to regenerate high activity and
selectivity by appropriate treatment of the deactivated catalyst.32
2.1.2 Supported catalysts
One of the largest series of heterogeneous catalysts in the chemical industry are supported
catalysts. Supported catalysts can be obtained by introducing small amounts of active sites,
such as metal ions, onto the surface of a porous material, i.e., supports. The reasons for the
predominant role of supported catalysts in heterogeneous catalysis are the low costs, high
activity, good selectivity and ability to maintain activity and selectivity after several cycles
8
of reaction. The main factors that would affect these properties are the type of support
materials and the arrangement of the active sites in the pore structure of the supports.33
In general, the catalytic activity of a catalyst improves as the specific surface area increases.
Higher surface area of a support can increase the accommodation of the active sites on the
support surface. However, the relationship between the catalytic behavior (reaction rate) and
the surface area is not always linear since the catalytic activity may also be influenced by the
structure of the support. The diffusion of the reactants can be highly affected by various pore
structure of the catalyst, leading to a different reaction rate. Moreover, the catalytic activity
may also be affected by the pore size of the catalyst because different pore sizes may have
different impact on the dispersion of the active sites on the surface of the catalyst. In summary,
the catalytic activity of a supported catalyst is highly depended on its surface area, pore
structure, and pore sizes. Therefore, ordered mesoporous materials which have various pore
structures, high specific surface area, and different pore sizes, are of great interest in catalyst
field and there are a lot of challenges and possibilities left to explore since it was discovered
only two decades ago. In this thesis, ordered mesoporous materials will be used as the
catalyst supports.
2.2 Synthesis of mesoporous silica supports
In the early 90s, scientists at Mobile Oil Corporation prepared a novel class of ordered
mesoporous aluminosilicate materials named as M41S.2,34 Among these materials, MCM-41
(Mobil Composition of Matter No. 41) shows one-dimensional (1D) cylindrical pores
accompanied with a relatively narrow pore size distribution. The pores are highly ordered
and hexagonally arranged. However, the walls of this new type of material are amorphous
silica. Other related materials with well-defined structures: MCM-48 with cubic
mesostructure and MCM-50 with lamellar mesostructure, were also reported in these early
publications.2,34 Shortly after the discovery of these mesoporous siliceous materials, the
synthesis of various non-siliceous mesoporous oxides was studied under different
conditions.35 Since then, numerous non-siliceous mesostructured materials, such as oxides,
metals, and phosphates were discovered.36-38 However, compared to non-siliceous materials,
silica-based systems have attracted considerably much more attention, due to a great variety
of possible mesostructures, the controllable hydrolysis-condensation reactions, the enhanced
9
thermal stability of the amorphous wall and a great variety of available methods for
functionalization.39 Over the recent years, great progress has been obtained in the synthesis
of mesoporous silica.40 A large variety of synthetic approaches have been discovered for the
formation of mesoporous silica, e.g., SBA-15,41 SBA-16,42 KIT-6,43 etc., with different
morphology, structure, and texture.
Most of the synthetic routes for mesoporous silica are based on the use of organic amphiphilic
molecules acting as a template or structure-directing agent, around which the silica precursors
can condense and thus result in the formation of an open framework. After the organic
template is removed by calcination or solvent extraction, a cavity or a pore, which retains the
structure and the morphology of the template, forms.
A large variety of synthetic approaches have been developed for the formation of different
mesoporous silicas by using various surfactants and synthesis methods; both can be
controlled and altered. Different reagent ratios, organic additives as well as the synthesis
conditions (time, temperature, pH) and the nature of surfactants are being used to control the
structural properties and morphology of the obtained materials. Basically, for mesoporous
materials, there are three main features involved in their synthesis: the specific synthesis
mechanism, the combination of surfactant and solvent type, the interactions between the
inorganic compounds and the template molecules.40,44 In general, several steps are involved
for the preparation of the mesoporous silicas: first, the molecules used as a template are
dissolved in the solvent (pH, temperature, and co-solvents, additives could be altered or
added in this step), and the silica source is then added. The mixture is stirred for a period of
time at a certain temperature to allow hydrolysis and condensation of the silica source. The
temperature is increased for aging (sometimes microwave synthesis or pH adjustment or
hydrothermal treatment could be used as assistant method). The products will be
subsequently recovered by filtration, washing, and drying. In the final step, the porous
structure of the final materials could be opened after removal of the template by calcination
or solvent extraction. Calcination is the most common way used in practice to eliminate the
template of the as-synthesized materials. The materials are heated to a given temperature
often under a flow of nitrogen, oxygen or air, in the calcination step. The solvent extraction
method may be used as an alternative method to calcination. For instance, the block
10
copolymer template of SBA-15 silica can easily be extracted using a mixture of acid and
ethanol solutions at low temperature.45
2.2.1 Mechanisms for formation of mesoporous silica
Two different pathways have been proposed for the synthesis of mesoporous silica, that is,
true liquid-crystal templating (TLCT) route and cooperative self-assembly route. The TLCT
mechanism was originally suggested by Beck and co-workers.2,34 It suggested that the liquid
crystal mesophase can be formed without the presence of the inorganic species if the
concentration of the surfactant was high enough (above CMC). The LC could serve as the
template for the construction of the mesoporous structure (Figure 2.3, pathway B). On the
other hand, another mechanism pathway, i.e., the cooperative self-assembly route, was
plausible to explain the formations of the mesophase at lower concentration of the surfactant
(Figure 2.3, pathway A). The cooperatively self-assembly of the silica species and the
surfactant micelles results in the inorganic-organic hybrid micelles, which further aggregate
into silica-surfactant rods through inorganic-organic interactions. These rods subsequently
Figure 2.3 Two pathways for the synthesis of ordered mesoporous silica: A, cooperative self-
assembly; B, true liquid-crystal templating.2
11
assemble into hexagonal mesostructure with further condensation and polymerization of
inorganic networks around them through hydrothermal or ammonia treatment.46,47
Following the cooperative self-assembly route, the formation of the mesostructure is strongly
dependent on the inorganic-organic interactions between the inorganic precursor (I) and the
head group of the surfactant (S). As shown in Figure 2.4, there are six plausible cooperative
interactions for the inorganic-organic hybrid mesophase.44,48,49 When the synthesis takes
place under alkaline conditions, the silicate present as anions I−, interact with the oppositely
charged surfactant cations S+ (S+I− mechanism). These electrostatic interactions are
applicable for the synthesis of MCM-41 and MCM-48, which are both carried out in basic
conditions. On the other hand, if the synthesis is carried out under acidic medium, S−I+
mechanism should be proposed since the silicate species are positively charged cations. In
the case of the inorganic species and the surfactant both positively charged or negatively
charged, two other indirect routes are proposed. It is necessary to introduce a counter-ion to
make the interaction possible. Under the acidic media, the use of halide anions (X− = Cl–,
Br–) make the S+ X− I + route possible. Conversely, for a base catalyzed synthesis, S–M+I–
route is plausible with the assistance of alkali metal ion (M+ = Na+, K+). For synthesis using
a neutral surfactants (S0) or non-ionic surfactant (N0) in neutral media, new assembly routes
denoted as S0I0 and N0I0 were proposed.48 In these cases, the driving force of the interaction
is supposed to be hydrogen bonding. In 1998, the synthesis of a new ordered mesoporous
silica named SBA-15 was reported by Zhao et al.41 The synthesis was performed in acidic
conditions, and non-ionic triblock copolymers were used as the surfactant. Since the silica
species are present as cations under acidic condition (I+) and the triblock copolymer could
also be positively charged, a more realistic pathway (N0H+)(X−I+) derived from N0I0 was
proposed.
12
Figure 2.4 Schematic representation of different types of cooperative interactions for the inorganic-
organic hybrid mesophase.44
2.2.2 Mesostructure tailoring
Concerning the prospects of applications of ordered mesoporous silica in catalysis, sensing,
sorption and so on, diversity in mesostructure is essential. Figure 2.5 displays the most
common mesostructures synthesized. Among these, the 2D mesostructures of mesoporous
silica consist of cylindrical pore channels that are hexagonally close-packed, with the p6mm
symmetry. The two most important representatives among the hexagonal phases of
mesoporous silica are MCM-41 and SBA–15.
13
Figure 2.5 Representations of the pore topology with symmetries of (A) p6mm, (B) Ia3d, (C) Pm3n, (D)
Im3m, (E) Fd3m, and (F) Fm3m. Adapted from reference 50
MCM-41. MCM-41 was first synthesized by using cationic surfactants as templates and
tetraethyl orthosilicate (TEOS) as the silica source under alkaline conditions. MCM-41
materials consist of an amorphous-silicate framework with a highly ordered hexagonal array
of cylindrical mesopores. The thickness of the pore wall usually varies from 0.7 to 1.1 nm.
MCM-41 has high BET surface area up to 1000 m2/g and pore volumes exceeding 1 cm3/g.45
The high surface area and large pore volume of MCM-41 make it a possible material for
catalysis and sorption applications.51 In addition, it presents a quite narrow pore size
distribution due to the relatively uniform mesopores.52 In general, the pore sizes of MCM-41
can be finely tuned by changing the chain length of the surfactant, and a larger pore size
could be obtained by using surfactants with a longer hydrophobic chain. The conventional
material with the highly ordered arrangement and pore size of 2-10 nm can be easily obtained.
SBA-15 (Santa Barbara acids No 15). SBA-15 is the most important and extensively studied
hexagonal mesoporous silica after MCM-41. The synthesis of SBA-15 involves the non-ionic
triblock copolymer P123 (EO20PO70EO20) as the surfactant, which consists of large
polypropylene oxide (PO)m and polyethylene oxide (EO)n blocks, combined with silica
molecules such as TEOS or tetraethyl orthosilicate (TMOS) under strongly acidic aqueous
condition. The cationic silica species can interact with EO units of P123, resulting in the
formation of the mesostructure. SBA-15 exhibits 2D hexagonally tunable uniform mesopores
as does MCM-41 but consists of both micro- and mesopores. According to the study of
14
Impéror-Clerc and co-workers,53 the presence of micropores within the walls results from
penetration of the hydrophilic PEO part of the surfactant into the wall (Figure 2.6). With
different synthesis conditions, the size of the micropores differs from 0.5 to 3 nm. The use of
triblock copolymers as a surfactant improves the available pore size range of the mesopores.
In contrast to MCM-41, SBA-15 exhibits larger pore sizes adjustable from 6 to15 nm and
thicker pore walls of 3−7 nm. The large mesopores of SBA-15 make it a favorite in the
catalysis field because of the increase in the diffusion and accessibility of any active sites in
the pores. The thermal stability and hydrothermal stability could be significantly improved
because of the thicker pore wall of SBA-15, compared to other mesoporous silica exhibiting
thinner pore walls (about 1 nm). The characteristic of the large specific surface area and pore
volume also make it a promising support in catalysis field. Because of these desirable features,
in addition to the simple synthesis route, SBA-15 attracted massive attention and many
publications about its applications in heterogeneous catalysis have appeared.54,55,56
Figure 2.6 Micropore formation in mesoporous SBA-15 where steps (i) and (ii) correspond to
treatment with sulfuric acid and calcination, respectively. Adapted from 1
15
2.3 Functionalization of mesoporous silica
The use of pristine mesoporous silicas in the chemical industry, especially in heterogeneous
catalysis, was restricted by their poor chemical activity due to the lack of active sites to the
amorphous silica wall. In the past years, numerous efforts have been dedicated to the
functionalization of mesoporous silicas, of which the chemical properties could be
significantly improved, and therefore the field of application could be broadened. There are
two primary methods to incorporate functionalities into mesoporous silicas (Figure 2.7). The
first one is “direct synthesis” which is also known as co-condensation. Namely, a given
functional organosilane is added into the silica precursor (tetraalkoxysilanes)/surfactant sols,
and the final functionalized mesoporous material can be obtained by the co-condensation of
organosilane and silica precursors. In this way, the functional group can be partly
incorporated into the framework of the silica walls while most of them were dangling on the
surface. The other method is based on the post-grafting or impregnation or adsorption,
involving a grafting reaction after the first formation of the mesoporous silicas.
Figure 2.7 Schematic routes for the functionalization of mesoporous silica via various methods. Adapted
from reference 57
16
2.3.1 Surface properties
For the mesoporous silica, there are abundant silanol groups on the surface that are accessible
as anchoring sites silane coupling or metal species. (Figure 2.8) Even though the silanol
density of pristine mesoporous silica (MCM-41 or SBA-15) is relatively low (1-3
SiOH/nm)58,59 compared to the regular hydroxylated silica (4-6 SiOH/nm),60 some silanol
groups are still available for surface modification after calcination. Surface modification of
the mesoporous silica is always performed by mixing the silica powder and the organic
solutions containing silylating agents, such as hexamethyldisilazane or
trimethylchlorosilane,61,62 under reflux for a period of time. Therefore, the silanol groups are
passivated by the introduction of the alkylorganosilanes on the surface of the mesoporous
silica, which can be used as a strategy to provide the walls more hydrophobicity, and thus to
improve the structural stability of the materials.
Figure 2.8 Various types of surface silanols/siloxanes on silica.
2.3.2 Metal-modified mesoporous silica
Much effort has been focused on the activation of mesoporous silica, and one facile pathway
is inclusion of heteroatoms such as boron, aluminum, and different transition metals, into the
silica frameworks for the modification of the composition of the siliceous walls. These metal-
modified mesoporous silicas are of particular importance in regards to applications in
catalysis,63 Because in this process, a vast number of acidic sites could be formed because
of the substitution of the silicon in the siliceous frameworks with ion-exchange capacity, and
thus resulting in a high catalytic activity, in a similar pathway as for the amorphous silicates
and zeolites. An element of the third main group, for example, boron, aluminum, gallium,
etc. and some of the transition metals, such as titanium, iron, and copper, are the most
favorable substitution elements for mesoporous silica.63
17
2.3.3 The post-synthesis method using acetylacetonate-metal precursors (acac)
Essentially, one can obtain metal-modified mesoporous silica through two different pathways.
Namely, the position of the inorganic components are preferentially located on the surface
of the mesopores, mostly as metal oxides, or silicon atoms in the frameworks are replaced by
metal ions. In general, the modification of the siliceous frameworks can be obtained by
mixing the heteroelements, e.g., Al, Ti, Cr, and Fe, with the synthesis sols containing silica
source and surfactants. In this way, the silicon atoms in the framework can be substituted by
tetrahedrally coordinated trivalent or tetravalent element, leading to homogeneously
incorporated heteroatoms in the materials. In another way, pore surface functionalization can
be carried out through the post-synthesis grafting of metal elements on the silanol groups,
resulting in a higher heteroatom concentration on the surface without altering the structure
of the mesophase. For instance, iron oxide could be deposited on M41S materials using
iron(III) nitrate as a metal precursor through the incipient wetness method. The presence of
iron oxide nanoparticles was reported by different authors.64,65 However, even though the
pathway based on the post-synthesis can benefit from the high concentration of metal species
on the silica surface, it also suffers from aggregation and formation of metal oxide, leading
to a low dispersion of heteroatoms on the silica surface.66 Since the dispersion of the metal
sites on the silica surface can affect the performance of the catalyst, it is important to develop
a grafting method in order to obtain a catalyst with high dispersion of the metal atoms. To
reach this goal, it is crucial to select a proper metal precursor, which could have adequate
interactions with the silanol groups on the silica surface, and therefore generate a catalyst
with highly dispersed metal atoms.
Recent studies in our laboratory have shown that chelated metal complexes are among the
most promising metal precursors for the post-grafting of metal atoms on the silica surface.
Conventional metal precursors, such as metal chlorides or metal alkoxides, can easily
aggregate during the synthesis process, resulting in large clusters. On the contrary, due to the
relatively stability of the metal chelated complexes, and therefore limited hydrolysis and
condensation, various metals such as V, Cu, Co, Fe, etc.67 have been successfully grafted on
the silica surface via a generalized post-grafting method using corresponding metal acetates
or acetylacetonates as precursors, and the thus-synthesized materials showed potential
18
catalytic activity.68 For example, our group demonstrated that the titanium-modified SBA-15
exhibited extensively higher catalytic activity for the epoxidation of cyclohexene compared
to materials synthesized by co-condensation method.69 Besides, the resulting Ti-SBA-15
material showed a high stability during the recycling tests, indicating high stability of the
active sites.70 The main advantages of this generalized post-grafting method using metal
acetates or acetylacetonates as precursors compared to traditional post-grafting methods
using conventional metal precursors are: (1) It facilitates the synthesis procedure, and (2) it
is highly tunable by modifying the related grafting parameters, e.g., pH, temperature, and
acac/metal ratio.67-70
Therefore, in this thesis, the metal-modified mesoporous materials were prepared according
to the generalized post-grafting method, whereby the MCM-41 and SBA-15 were used as
silica supports and Fe(acac)3 as the metal precursor.
2.4 Mukaiyama aldol reaction
The Mukaiyama aldol reaction was discovered by Mukaiyama and co-workers more than
four decades ago.71,72 It is a directed cross-aldol reaction in the presence of a Lewis acid. The
formation of the bond between an aldehyde and a preformed silicon enolate (a silyl enol ether
derived from a ketone or a ketene silyl acetal derived from an ester), gives rise to the final
aldol adduct.73,74 The Mukaiyama aldol reaction has greatly inspired the development of
various related carbon–carbon bond-forming reactions in organic chemistry, for instance, the
Sakurai–Hosomi allylation reaction75 and hetero-Diels–Alder reactions of Danishefskys
dienes.71,76 For electrophiles such as acetals, ketimines, thioacetals and imines and for
nucleophiles such as allylsilanes, silyl cyanides and silicon dienolates have both been studied
as substrates under acidic conditions.73 In a word, the Mukaiyama aldol reaction has extended
the organic synthesis into a larger scope. Moreover, classical aldol reactions performed under
alkaline media generally suffer from side reactions such as polymerization, dehydration and
self-condensation, leading to a low yield, and selectivity of the reactions.77 On the contrary,
Mukaiyama reactions catalyzed by Lewis acids can prevent these side processes.73
19
Scheme 2.1 Mukaiyama aldol reaction catalyzed by a stoichiometric amount of TiCl4
The catalyst applied for the Mukaiyama aldol reaction in the first report published in 1973
was a stoichiometric amount of TiCl4 (scheme 2.1).78 Since then, a series of metal halides
used as conventional homogeneous Lewis acid catalysts such as BF3, AlCl3 and FeCl3 were
also developed as the catalysts by the same authors.23 However, strictly anhydrous conditions
and quite low temperatures were always necessary, otherwise moderate or low yields were
obtained. On the other hand, organic reactions in water are of current interests for
environmental and economic concerns. However, during a long period of time, it was
considered that the Lewis acid catalysts were incompatible with water because of their
moisture sensitivity, thus being easily decomposed or deactivated during the processes.
Therefore, much effort has been made to produce water-tolerant catalysts for the Mukaiyama
aldol reactions. In the 1990s, rare earth triflates were revealed to exhibit excellent
performance in the Mukaiyama aldol reactions in the water/organic solvents systems.79-82 For
instance, it was discovered that Sc(OTf)3 was efficient in the Mukaiyama aldol reactions of
different silicon enolates with aldehyde in water-THF mixture.81 The discovery of rare earth
metal triflate as catalysts for the Mukaiyama aldol reaction has inspired extensive research
to expand the availability of Lewis acid catalysts. Heterogeneous catalysts were then
employed as alternatives for going further toward a greener process due to their easy recovery
and recycling. The most prominent and early developed examples of heterogeneous catalysts
comprising Lewis acidic sites are based on microporous zeolites.83-85 Kumar found that TS-
1 and Ti-Beta zeolites could be used as active solid catalysts in the Mukaiyama aldol
reaction.86,87 However, the low yields of reaction led to the speculation that the too small
micropore environment in zeolites prevents the contact of the reactants with the Lewis acid
sites, decreasing the ability for the chemical transformation. Therefore, the pores of supports
for the acid sites were expanded to the meso region. For example, Ti-containing mesoporous
materials showed satisfactory yields as high as 98% in solvent-free systems.17 According to
the study of novel mesoporous Lewis acid catalysts obtained by immobilizing Sc(OTf)3 on
20
mesoporous silica functionalized with sodium-benzenesulfonate for the Mukaiyama aldol
reaction, the thus-prepared materials displayed a much higher reactivity than that of
homogeneous catalysts, such as Sc(OTf)3.88-89 Surprisingly however, there is still only very
few supported or heterogeneous catalysts reported for the Mukaiyama aldol reactions. More
scientific work needs to be done to expand the use of the heterogeneous catalysts in the
Mukaiyama aldol reactions, and further in other organic synthesis.
It should be highlighted that Kobayashi and co-workers have established criteria to
understand the Lewis acidity of different metals, which are efficient for Mukaiyama aldol
reactions in the aqueous medium. The catalytic activities of metal cations in water could be
somehow determined by their hydrolysis constants (Kh) and water-exchange rate constants
(WERCs). WERCs correspond to the exchange rate constants for the substitution of water
ligands. The pKh and WERC values of different ions are shown in Figure 2.9. The range of
pKh of these active metal cations varied from 4.3 (Sc (III)) to 10.08 (Cd(II)) while the WERC
values are all larger than 3.2 × 106 M–1 s–1. In general, cations with small pKh values tend to
be efficiently hydrolyzed. For pKh values less than 4.3, the metal cations undergo easy
hydrolysis and the protons produced from hydrolysis result in the decomposition of the silyl
enol ethers. On the contrary, if the pKh values are larger than 10, the metal cations will be not
able to catalyze the aldol reaction because of its weak Lewis acidity. The concept of the
hydrolysis constant and WERC can help us understand the catalytic activity and Lewis
acidity of different metals in an aqueous medium, thus choosing an appropriate metal to be
introduced into the support for Mukaiyama reaction.90
21
Figure 2.9 Hydrolysis constants (Kh) and water-exchange rate constants for determining Lewis acidity.
Adapted from 90
22
Chapter 3
Characterization techniques for synthesized materials
The materials synthesized in this thesis were characterized by various techniques to
understand their physical and chemical properties thoroughly.
3.1 Nitrogen adsorption
Adsorption can be categorized into two types: physisorption and chemisorption. When a gas
adsorptive contacts the surface of an adsorbate, physisorption occurs. The interaction
involved in physisorption is mainly the van der Waals forces, while chemical bonding occurs
in them in the chemisorption process. Gas (commonly nitrogen, argon or krypton gas)
adsorption is one of the most commonly used and efficient experimental methods for
measuring the pore information of the porous materials including specific surface area, pore
size distribution, pore volume, and porosity. The processes of physisorption are usually
presented as graphs known as adsorption isotherms, which illustrate the relationship between
the gas amount adsorbed on the surface of the adsorbate and the relative pressure P/P0 at a
given temperature, where P0 is the saturation pressure of the gas used in the measurement.
The shape of the adsorption isotherms is strongly affected by the porous structure of the
measured materials as well as the strengths of fluid-wall and fluid-fluid interactions.91
Therefore, one can deduce the porous structure and some other pore-related information on
the basis of the adsorption isotherms. According to the notation of the IUPAC, the adsorption
isotherms (Figure 3.1) can be classified into six shapes or types, and the pores are proposed
to be categorized by their pore width.
23
Figure 3.1 Different types of the physisorption isotherms classified by IUPAC.92
Type I is the characteristic isotherm of microporous materials such as zeolites whereas, type
II and III usually refer to the physisorption isotherm of macroporous or non-porous materials.
Type IV and V isotherms corresponds to mesoporous materials. Type VI is an unusual type
of isotherm, representing a layer by layer adsorption, which is rarely observed. The presence
of a hysteresis loop in type IV and V isotherms can be seen only for mesoporous materials.
For microporous materials, the adsorption behavior is mainly determined by the fluid-wall
interactions, resulting in a continuous filling of gases in the pore without phase transition.
However, the fluid-fluid interactions also exist during the adsorption process of gases into
the mesopores, leading to multilayer adsorption, in other words, pore condensation, which is
often accompanied by the hysteresis loop.93-95 Therefore, the pore size range can be
determined based on the presence or the absence of the hysteresis loop. In a word, when using
nitrogen as the adsorptive at 77 K, no hysteresis loop will be observed if the pore size was
below 4 nm.96
The pore structure can also be identified based on the shape of the hysteresis loop in the
isotherms. It was classified into four different types by IUPAC. (Figure 3.2) Type H1 loop
represents highly uniform cylindrical pores with a narrow pore size distribution. Type H2
corresponds to the presence of ink bottle mesopores. H3 type hysteresis is associated with
plate-like particles with slit-like mesopores. Type H4 hysteresis loop corresponds to the
24
adsorption-desorption process in narrow slit-like mesopores, can also be caused by the
structural defects in mesoporous materials.63
Figure 3.2 Classification of adsorption-desorption hysteresis loops.97
3.1.2. The BET specific surface area
The specific surface area is increased if a particle exhibits pores. Surface area is one of the
most significant parameters when a porous material or functionalized porous material is
designed as a catalyst. Hence, to evaluate the catalytic activity of a material, its surface area
should be measured. The most commonly used method is the BET method proposed by
Brunauer, Emmett, and Teller.98 The BET equation (3.1) is frequently applied for the
evaluation of specific surface area:
1/(𝑛(𝑝0/𝑝) − 1)) = 1/𝑛𝑚𝐶 + (𝐶 − 1)/𝑛𝑚𝐶 × 𝑝/𝑝0 (3.1)
in which n is the total gas amount adsorbed at p/p0; nm is the monolayer (saturated) capacity
of the adsorbate, C is the BET constant which gives some indication of the heat of adsorption
and condensation in the first adsorption layer.99
25
Usually, there are two stages involved in the calculation of the BET specific surface area.
First, the monolayer capacity of the adsorbate, namely nm in the BET equation, should be
determined. It can be obtained by transforming a physisorption isotherm into a plot. In
general, the BET equation could be expressed as a linear equation f(p/p0) = a(p/p0) + b. A
linear line, which is called BET plot, could be observed for most mesoporous materials in
the relative pressure region of 0.05-0.3.93,94 The monolayer capacity nm and BET constant C
can both be calculated from the slope of the plot and the Y-intercept. The specific surface
area is obtained from the following equation (3.2):
𝑆 = 𝑛𝑚 × 𝑁𝑎 × 𝜎 (3.2)
in which σ is the cross-sectional area of the adsorbate (16.2 Å2 if nitrogen was used as an
adsorbate), Na is the Avogadro constant (6.02 x 1023 mol-1). It should be noted that the BET
equation is applicable in most cases such as mesoporous and nonporous materials, however,
is not suitable for the microporous adsorbents because of the difficulty to distinguish the
multilayer adsorption from the micropore filling process.100
3.1.3. Pore size analysis
Different theoretical methods are applied to determine the pore size of mesoporous materials.
The Barret-Joyner-Halenda (BJH) method101 which originates from Kelvin equation is used
for the mesopore size analysis.
ln(𝑝/𝑝0) = −2γcosθ/RTρ(𝑟𝑝 − 𝑡𝑐) (3.3)
p, equilibrium vapor pressure, P0, the saturated pressure, γ, the surface tension of the liquid
condensate generated from nitrogen gas, R, the gas constant, and T, the temperature. θ is
contact angle of the liquid meniscus against the pore wall, r is the radius of the pore, and tc
the thickness of an adsorbed multilayer film formed before the pore condensation.
However, the BJH method fails to assess the thermodynamic and thermophysical properties
of the confined pre-adsorbed fluid, leading to an underestimation of the pore size, especially
for those materials with pores under 10 nm.93,94,96 Therefore, nonlocal density functional
theory (NLDFT) and Monte-Carlo methods are considered as an alternative and more
26
suitable method for the pore size analysis. The details of the local fluid structure near the
solid wall of the surface can be provided by NLDFT, as well as the arrangement of the gas
molecules adsorbed in pores. The advantage of the DFT method is that it considers the
characteristics of the hysteresis, which is determined by the pore shape of the porous
materials. Consequently, one can choose corresponding theoretical isotherms or kernels for
pores of respective pore geometry with different pore sizes to obtain the pore size
distributions. The NLDFT method could be employed for analyzing the pore size of materials
in all the range when an appropriate kernel model is chosen.45,102
3.2 Powder X-Ray Diffraction (XRD)
Powder XRD analysis is one of the most widely used and powerful techniques for the
characterization of solid materials. When a monochromatic X-ray beam with a wavelength
of λ strikes a solid crystalline sample, the scattered X-ray will interfere constructively, which
gives rise to a diffraction peak. The geometry of the diffractogram can be described by using
the Bragg law. Since atoms in a single crystal present excellent periodicity, the distance d
between two lattice planes (dhkl) can also be calculated based on the Bragg law.
nλ = 2dsinθ (3.4)
where n is an integer number of wavelengths also called, λ is the wavelength of the X-ray
radiation, and θ is the diffraction angle.
The powdered XRD diffraction patterns can be classified into low-angle XRD and wide-
angle XRD according to the diffraction angles. For the crystalline materials, due to the small
distance between lattice planes, the diffraction peaks are usually collected in the 2 theta
region between 10-80º. On the other hand, in the case of ordered mesoporous silica, which is
amorphous at the atomic scale, the reflected intensities in the diffraction patterns are not from
the ordered arrangement of atoms, but from the periodicity of mesopores arrays. Therefore,
the diffraction peaks of are usually observed at low diffraction angles, typically in the 2-theta
range of 0.8 and 5 º. From these reflection peaks determined by the unit cell parameters and
symmetry of the investigated solids, on can reduce the size and symmetry of the lattice. Like
liquid phases, mesoporous silica exhibits hexagonal, lamellar, or cubic phases. Figure 3.3
display the typical low-angle X-ray diffraction patterns of MCM-41 and SBA-15, both with
27
hexagonal symmetry. The typical XRD diffraction patterns of MCM-41 and SBA-15 usually
exhibit four or five reflections indexed to (100), (110), (200), (210) and (300), corresponding
to the hexagonal space group. Furthermore, the unit cell parameter in a hexagonal lattice can
be calculated as follow:
1
𝑑2= (
4
3𝑎2) (ℎ2 + 𝑘2 + ℎ𝑘) + (𝑙2/𝑐2) (3.5)
Hence, the unit cell parameter can be obtained as k = 0 and l = 0,
𝑎 = 2𝑑100/√3(𝑤𝑖𝑡ℎ𝑙 = 0) (3.6)
In summary, powder XRD is a powerful method for the investigation of mesoporous
materials. It provides the necessary information of the mesostructures, as well as the domain
size of the crystals.
Figure 3.3 Typical low-angle powder XRD patterns obtained for MCM-41(A)34 and SBA-15(B)41,
showing reflections of the hexagonal plane group.
28
3.3 Electron microscopy
Electron microscopy is one of the most useful methods to determine the mesostructure. There
are two typical electron microscopy techniques for imaging the siliceous materials, namely,
scanning electron microscopy (SEM) and transmission electron microscopy (TEM). In
principle, an electron microscope uses a focused beam of electrons to examine the materials
on a very fine scale. As illustrated in Figure 3.4, when the electron beam hits a specimen,
different energy signals can be produced depending on various interactions between an
electron beam and a sample.
Figure 3.4 The main types of the signal generated by the electron beam-specimen interaction.103
3.3.1 Scanning electron microscopy (SEM)
Scanning electron microscopy (SEM) characterization is a powerful technique primarily for
the study of the morphology and the topography of the materials in the form of a solid on a
scale down to 10 nm. The information of topographical features, particle aggregation, could
be imaged by SEM. When it is coupled with energy-dispersive X-ray spectroscopy (EDX),
the compositional difference within the material could also be obtained by SEM.
29
SEM technique is based on the principle that an electron beam passing through an evacuated
column, is then focused by electromagnetic lenses onto the material. The beam is scanned
over the surface of the specimen in synchronism with the beam of a cathode ray tube display
screen. The emission of inelastically scattered secondary electrons from the sample surface
occurs, and they are collected to form the image. The brightness of the cathode ray tube is
modulated by the obtained signal. The picture on the screen including the information about
the sample surface can be achieved by the secondary electron emission from the specimen.
Usually, changes in the topographical features of the sample surface result in different
secondary electron emission.40
In most cases, materials can be adequately studied by SEM only if they were electrically
conductive. Therefore, non-conductive samples need to be coated with a thin layer of
conductive material before the imaging process. Gold or carbon is commonly used for the
coating. However, in some other cases, new high-resolution set-ups function at low voltage
and no coating is needed.
3.3.2 Transmission electron microscopy (TEM)
TEM is the ultimate technique to obtain structural information, periodic lattice spacings and
symmetry at a nanometer scale for mesoporous materials. Unlike a SEM instrument, the
detector is placed on the same side of the sample with the electron beam to detect the scattered
secondary electrons, in which the detector of the TEM is mounted on the other side of the
sample to detect the electrons transmitted through the ultrathin sample (less than 100 nm).
The sample is oriented so that some of the electrons are transmitted while some of them are
scattered or diffracted. An image is formed from the interaction of the electrons with the
sample when the beam is transmitted through the specimen. The image is then magnified and
focused onto a fluorescent screen. Since structural variation could cause a different fraction
of the electron beam to be scattered, images with variations, correspondingly, can be
obtained.40 Nowadays, the improvement of TEM microscopy makes the atomic resolution
possible since its discovery in the 1930s. They are known as high-resolution TEM (HRTEM).
30
Figure 3.5 TEM images of Fe-MCM-41 quoted from 104b) Fe-SBA-15 quoted from reference 105
3.3.3 Energy dispersive X-ray spectroscopy (EDX)
Energy dispersive X-ray spectroscopy (EDX), also refers to as EDXS or EDS, is a commonly
used technique in conjunction with SEM or TEM for quantitative and qualitative
microanalysis, providing the chemical information of the elements present in the samples.
When an electron beam bombarded on a solid sample, the characteristic X-rays is emitted
from the sample because of the interactions between electrons and the sample. These X-rays
are detected by the EDX, and the signal is shown as a spectrum, in which the X plot represents
the energies of the X-rays, while Y plot represents the intensity of the X-rays (X-rays counts
or X-rays number). From the spectrum, the elemental composition of different elements can
be identified by the characteristic X-rays energies, as the concentration of each element can
be quantified by the intensity of the X-rays. The output of the EDX can also be presented as
an image or “map”. As the electron beam is rastered over a selected area of the sample, the
X-rays element distribution images or “maps” can be obtained by counting the number of the
X-ray photons generated at a selected energy. With the X-ray mapping, one can
straightforwardly identify how the concentration of a specific element varies in the sample
without quantitative elemental calculation.106 In this thesis, EDX techniques implemented in
both SEM and HRTEM were applied to detect the mesostructure, the particle distribution as
well as the chemical composition of the resulting mesoporous catalysts.106
31
3.4 Diffuse reflectance ultra-violet visible spectroscopy (DR-UV-vis)
When a molecule is irradiated with light, the molecule will adsorb the light in the visible or
ultra-violet region, the electrons of the molecule will be excited and transferred in atomic
or molecular orbitals from low energy to high energy. Thus, the spectroscopy in the
ultraviolet-visible region can provide the information of such electron transfers. For
example, in the case of the transition metal ions, the electron transitions include metal-to-
ligand or ligand-to-metal charge transfer and d-d transitions. Such electron transitions can
also affect the color of the chemicals. Usually, transmitted light should be collected in
conventional spectroscopy investigations of crystal or solutions. However, for the
spectroscopy investigation of heterogeneous catalysts (powders or solids), from which is
not easy to obtain a transparent film to let the light penetrate through, diffuse reflected light
should be collected to obtain the spectra. There are two different forms of reflection possible
when incident light irradiates a powdered sample, i.e. specular reflection and diffuse
reflection. In specular reflection from either smooth or non-adsorbing surface, the angle of
the reflected light beam is the same as that of the incident light beam. On the other hand,
the light scattered from a non-adsorbing surface in all directions is called “diffuse
reflection”. By using the DRS technique, information about the coordination environment
and the oxidation state of the transition metals can be all obtained, by the probing the outer
shell electrons of the ions. Furthermore, DRS can be routinely applied for the quantitative
analysis of the transition metal ions, providing information about the species amount.107
3.5 X-ray photoelectron spectroscopy (XPS)
XPS is among the most extensively used methods for the characterization of solid catalysts.
Information about the chemical composition on the external surface of a sample can be
obtained, as well as the electronic and oxidation state of the surface elements. The analysis
of photoelectron spectroscopy is based on the photoelectric effect: when a photon of
sufficient energy hits upon an atom, the emission of electrons will occur because of the
interaction between an atomic orbital electron and the photon, leading to a total transfer of
photon energy to the electron.108 The different kinetic energies of these emitted
photoelectrons can be measured by the spectrometer. This photo emission process can be
explained by the equation (3.7) stated as follow:
32
BE = hν – KE (3.7)
Where BE represents the binding energy of the electron in the atom, which is a function
determined by the atom’s own type and its environment, hν is a known value for the energy
of the X-ray source, and the KE is the value measured by the XPS for the kinetic energy of
the emitted photoelectrons. Therefore, one can obtain BE values, which contain valuable
information about the photo-emitting atoms, only based on the equation.
It should be noted that the binding energy of an electron in the atom can only be affected by
the type of the atom and the other atoms bound to it. A negatively charged electron can be
bound to an atom because of the positively charged nucleus. The shorter the distance between
the electron and the nucleus, the higher the binding energy. It should be fully understood that
the binding energy of an electron will change as the nuclear charge of an atom changes. Also,
when another atom is added to be bound to that atom, the electron distribution around the
atom of interest will be modified, leading to the change of the binding energy. The binding
energy cannot be altered by different isotopes due to their same nuclear charge nor by weak
interactions such as hydrogen bonding between atoms or by crystallization. Therefore, one
can observe the variations in the binding energy as the chemical environment of an atom
associated with ionic or covalent bonds between atoms changes. These changes are known
as binding energy shifts or chemical shifts, which is the characteristic of the atom.109
The energy of photoelectrons can also be transferred to other electrons in the atom resulting
in a loss of kinetic energy and thus produce satellite peaks at higher binding energies. These
satellite peaks preferentially appear in the spectra of the metals, such as copper, nickel, and
iron. They are highly useful characteristics while their properties are associated with the
oxidation state of the photo-emitting atom.
In this thesis, XPS was adopted to investigate the chemical state of the metal species inserted
into the mesoporous silicas and to calculate the molar ratio of Metal/Silicon.
3.5 Pyridine adsorption probed by FT-IR spectroscopy
Fourier transform infrared spectroscopy (FT-IR) is a sensitive technique for identifying the
structural details of organic chemicals and some inorganic materials. The application of FT-
33
IR relies on the existence of the internal molecular vibrations with specific frequencies. When
the molecule or a sample is exposed to a beam of infrared radiation, the sample adsorb the
radiation in the infra-red region to reach the excited vibrational state. The transmitted or
scattered light can then be recorded by the detector of the equipment and displays as a single-
beam adsorption band at a specific frequency, which is the characteristic of the molecule or
the sample.110 FT-IR with an appropriately chosen probe molecule is a commonly applied
method to explore the surface acidity of the solid catalysts. Pyridine, ammonia, substituted
pyridine, benzene are frequently chosen as probe molecules because the IR spectrum of the
adsorption of probe molecules with Lewis acidic sites and Brønsted acidic sites can be easily
distinguished.111 For example, the characteristic of pyridine-Lewis acid adduct appears at
1450 and 1600 cm–1 while that of the pyridine-Brønsted acid adduct at 1540 cm–1. Therefore,
in this thesis, pyridine adsorption probed by FT-IR spectroscopy was used as an additional
method to investigate the difference between Lewis acid sites and Brønsted acid sites.
3.6 Inductively coupled plasma mass spectrometry (ICP-MS)
A mass spectrometric technique ICP-MS was applied to identify and quantify the trace
elements (Fe in the present study) in the samples. The fundamental principle of ICP-MS is
based on the generation of positively charged ions, which are transported through different
parts of the instrument and finally analyzed by the MS. Figure 3.6 shows a schematic
representation of an ICP-MS instrument. In ICP-MS, the liquid sample is first converted into
droplets aerosol in the nebulizer with argon flow and these aerosol droplets pass through the
spray chamber, in which only the finest aerosol droplets (<10 μm) are transported to the
plasma. A quick process of drying, dissociation, vaporization, atomization and finally
ionization of the fine-droplet aerosol occurs under high temperature. Supersonic expansion
then occurs when the ion beams pass over the interface region (Figure 3.6(3)). Hereafter, the
ion beams are focused and introduced into the mass separation device/mass analyzer, which
works as a filter to separate the ions with a selected function of mass-to-charge (m/z) ratio,
Finally, the amount of these filtered ions separated by the mass analyzer is counted by the
detector by transforming them into electrical pulses. These pulses are counted by an
integrated system, and the ion signal of the samples can be compared with the ion signals of
the calibration standards. The concentration of the sample can be obtained in this way.112,113
34
Figure 3.6 Schematic representation of an ICP-MS instrument.114
35
Chapter 4
Iron-Modified Mesoporous Silica as an Efficient Solid
Lewis Acid Catalyst for the Mukaiyama Aldol Reaction
Wan Xu,a Thierry Ollevier,*a Freddy Kleitz*a, b
a Département de Chimie, Université Laval, 1045 Avenue de la Médecine, Québec, QC G1V
0A6, Canada
b Department of Inorganic Chemistry-Functional Materials, Faculty of Chemistry, University of
Vienna, Währinger Straße 42, 1090 Vienna, Austria
Published as Xu, W.; Ollevier, T.; Kleitz, F. "Iron-Modified Mesoporous Silica as Efficient
Solid Lewis Acid Catalyst for the Mukaiyama Aldol Reaction", ACS Catalysis 2018, 8,
1932–1944
36
Résumé
Fe-MCM-41 et Fe-SBA-15, deux silices mésoporeuses différentes contenant du fer ont été
synthétisées par une méthode simple et polyvalente utilisant l'acétylacétonate de fer comme
précurseur de métal. Le traitement de l'ammoniac au cours de la synthèse a été trouvé être un
moyen efficace d'augmenter la teneur en fer tout en préservant la dispersion convenable des
cations métalliques. Les paramètres physicochimiques de la silice mésoporeuse finale
contenant du fer ont été obtenus par l’analyse d'adsorption-désorption d'azote à la base
température, et l'environnement de la coordination des éléments en fer a été validé par la
spectroscopie de UV-Vis réflectance diffusée et la spectroscopie photoélectronique à rayons
X. L'acidité de surface a été sondé à l'aide des indicateurs de Hammett. Pour distinguer en
outre les sites acides de Lewis sur la surface, l’adsorption de pyridine suivie par FTIR a été
mise en œuvre. Ces catalyseurs préparés ont été criblés dans la réaction d'aldolisation de
Mukaiyama, qui est une réaction modèle catalysée par l’acide de Lewis. L'activité catalytique
acide de Lewis des matériaux a été optimisée et les produits aldol ont été obtenus avec un
bon rendement et une bonne sélectivité. De plus, les catalyseurs hétérogènes sont très stables
et ont pu être réutilisés au moins neuf fois en conservant leur activité catalytique.
37
Abstract
Fe-MCM-41 and Fe-SBA-15, two different iron-containing mesoporous silicas were
successfully synthesized by a straightforward and versatile method using iron acetylacetonate
as a metal precursor. pH adjustment with ammonia during the synthesis was found to be an
efficient way to improve the iron content. Physicochemical parameters of the iron-containing
mesoporous silicas were obtained by nitrogen physisorption measurements, and the
coordination environment of iron elements was validated by UV–vis diffuse reflectance
spectroscopy and X-ray photoelectron spectroscopy. The surface acidity was tested by using
a series of Hammett indicators. To further distinguish the Lewis acid sites on the surface,
pyridine adsorption probed by FT-IR method was implemented. These prepared nanoporous
catalysts were screened in the Mukaiyama aldol reaction as a model reaction catalyzed by a
Lewis acid. The Lewis acid catalytic activity of the materials was fine-tuned, and the
corresponding aldol products were obtained in good yield and selectivity. More importantly,
the solid catalysts were very stable and could be reused for at least nine times while
maintaining the same catalytic activity.
KEYWORDS. Mesoporous silica, grafting, iron acetylacetonate, water-tolerant Lewis acid,
Mukaiyama aldol reaction
38
4.1 Introduction
Homogeneous Lewis acid catalysts, such as AlCl3 and TiCl4, are applied in the production of
petroleum-derived chemicals.6,115 However, the use of these Lewis acids has some
disadvantages. For example, strictly anhydrous conditions are necessary for homogeneous
Lewis acids because of their easy decomposition or complete deactivation in water.
Furthermore, higher amounts of catalysts are usually required for reactions catalyzed by
conventional Lewis acid catalysts, which can generate large aqueous effluents during the
postsynthesis workup process5 and difficulty in the recycling of the catalysts for repeated use.
Therefore, it has become a target of great interest to design efficient heterogeneous catalysts
with Lewis acid sites to solve these issues.
The most prominent and early developed examples of heterogeneous catalysts comprising
Lewis acid sites are microporous zeolites.83-85 The most prominent and early developed
examples of heterogeneous catalysts comprising Lewis acid sites are based on microporous
zeolites.83-85 In organic chemistry, in particular, Kumar found that TS-1 and Ti-β zeolites
could be used as active solid catalysts in the Mukaiyama aldol reaction in the absence of
water.86,87 However, low yields led to the speculation that the too small microporous
environment in zeolites prevent the contact of the reactants with the Lewis acid sites,
decreasing the ability for the chemical transformation. Thus, silica-based materials with
larger pore sizes, such as ordered mesoporous MCM-41 and SBA-15 silicas,34,41 have been
highlighted as promising alternatives. In addition, their high surface area (800-1000 m2/g),
uniform and tunable pore diameter (usually between 2 and 15 nm), ease of surface-
functionalization, and well-defined particle morphology make them potential heterogeneous
catalysts for a larger variety of organic reactions.68,116-123
Since the discovery of the Mukaiyama aldol reaction 40 years ago, different types of catalysts
including both homogeneous and heterogeneous catalysts have been studied.22,124-129 In
organic synthesis, the Mukaiyama aldol reaction is an essential way to chemoselectively
synthesize β-hydroxycarbonyl compounds.22,130,131 A few Mukaiyama aldol-type reactions in
the presence of iron compounds have been described since the reaction is generally catalyzed
by a Lewis acid.124,125,132,133 Also, using some metal triflates, such as Sc(OTf)3, as catalysts
for Mukaiyama aldol reaction of aldehydes and silyl enol ethers, Kobayashi and co-workers
39
showed their good catalytic reactivity and selectivity in comparison to metal halides.21 On
the other hand, heterogeneous catalysts, such as Ti-containing mesoporous materials, also
showed satisfactory yields as high as 98% in solvent-free systems, which was much higher
than that of Ti-microporous zeolites.17 Furthermore, according to a study on mesoporous
Lewis acids obtained by immobilizing Sc(OTf)3 on mesoporous silica functionalized with
sodium-benzenesulfonate,88,89 for the Mukaiyama aldol reaction, the thus-prepared materials
displayed a much higher reactivity than that of homogeneous catalysts, e.g., Sc(OTf)3.
In comparison to other metals, iron-based catalysts are cheap, low in toxicity, stable, and
easily accessible. Furthermore, the Lewis acidity of Fe(III) is another significant benefit for
the use of this metal in catalysis. As an illustration of the applicability of iron in catalysis, a
series of Fe-containing porous materials used as catalysts in different organic reactions have
been disclosed. Dhakshinamoorthy and co-workers synthesized Fe-containing porous metal-
organic-frameworks (MOFs), which were used as solid Lewis acid catalysts for the
isomerization of α-pinene oxide into camphonelal and isopinocamphone in the absence of
solvent.134 According to other studies, Fe-containing mesoporous silica shows promise as an
efficient catalyst for several other organic reactions. For example, studies have demonstrated
that Fe-containing mesoporous materials are very active catalysts for Friedel-Crafts
alkylations,31 which are usually catalyzed by Lewis acids. In addition to being less sensitive
to water (water-tolerant Lewis acids), they demonstrate better recycling performance in
contrast to usual Lewis acids.135-137 However, surprisingly, Fe-containing mesoporous silica
has not yet been used as a Lewis acid catalyst for Mukaiyama aldol reactions.
In the present study, in order to develop an efficient Lewis acid Fe-containing mesoporous
material for the Mukaiyama aldol reaction, a series of Fe-MCM-41 (pore size 3-4 nm) and
Fe-SBA-15 (pore size 7-8 nm) with various iron contents were prepared by a specific
postgrafting methodology, inspired by our previous work on Ti-SBA-15 oxidation
catalysts.67,68 MCM-41 and SBA-15 were chosen as two different silica support to probe pore
size effects, and iron(III) acetylacetonate was used as the metal precursor in this procedure.67
The surface iron content and bulk iron content were analyzed using X-ray photoelectron
spectroscopy (XPS) and inductively coupled plasma mass spectrometry (ICP-MS),
respectively. Attenuated total reflectance infrared spectroscopy (ATR-IR), powder X-ray
40
diffraction (XRD), transmission electron microscopy (TEM), diffuse reflectance UV–visible
spectroscopy (DR-UV-vis), and N2 physisorption measurements were also applied for the
characterization of the final materials. The Lewis acidity of the prepared materials was
studied by the Hammett indicator method to substantiate the nature of the active sites in the
materials. Pyridine sorption probed by FT-IR analysis was performed to assess the Lewis
acid sites. The Lewis acid heterogeneous catalysts prepared above were then tested in the
Mukaiyama aldol reaction of various aldehydes with 1-(trimethylsiloxy)cyclohexene or 1-
phenyl-1-(trimethylsiloxy)propene at ambient temperature in an aqueous environment,
giving high yield and selectivity.
4.2 Experimental section
MCM-41 Silica Support. Mesoporous silica MCM-41 was synthesized using tetraethyl
orthosilicate (TEOS, 98%, Aldrich) as the silica source and cetyltrimethylammonium
bromide (CH3(CH2)15N(Br)(CH3)3, CTAB, Sigma-Aldrich) as a structure-directing agent
under basic aqueous conditions. A typical synthesis is as follows: 9.65 g of CTAB was first
dissolved in 480 g of distilled water with vigorous stirring at 35 °C to form a homogeneous
solution. A 36.5 mL portion of NH4OH (28%, Anachemia) was subsequently added to the
solution, and the temperature of the mixture was then reduced to 25 °C. After 15 min, 40 g
of TEOS was added, and the mixture was stirredbou for 2 h. The mixture was subsequently
aged at 90 °C under static conditions and the aging time was 3 days. The obtained white
precipitate was filtered off and washed with distilled water. After being dried at 100 °C for
24 h to remove the surfactant, mesoporous silica MCM-41 was obtained after calcination at
550 °C for 5 h.138, 139
SBA-15 Silica Support. The synthesis procedure of SBA-15 in this study was reported by
Choi et al.140 In a typical synthesis, the amphiphilic block copolymer Pluronic P123 was
adopted as the structure-directing agent and TEOS as the silica source. First, a homogeneous
solution was obtained after dissolving 13.9 g of Pluronic P123 (Aldrich, Mw = 5800 g/mol)
in a mixed solution of HCl (7.7 g, 37%, Anachemia) and distilled water (252 g) at 35 °C for
2 h. After addition of TEOS to the solution, the mixture was stirred at the same temperature
for 24 h, followed by aging of the mixture at 100 °C for 24 h. The precipitated white solid
41
was held in a mixture containing concentrated HCl and ethanol (100%, Fisher Scientific) for
30 min and then recovered by filtration. The resulting mesoporous silica SBA-15 was
obtained by heating the product at 550 °C for 5 h.
Preparation of Fe-MCM-41 and Fe-SBA-15 Catalysts. Impregnation of iron species into
MCM-41 and SBA-15 with different molar ratios was performed by a simple postgrafting
method reported earlier by our group67 with some modifications. The method used Fe(acac)3
as a metal precursor and calcined mesoporous silica MCM-41 or SBA-15 as a support. The
procedure was as follows: the given amount of Fe(acac)3 and 100 mL of 1-propanol (certified
ACS grade, Fisher Scientific) were mixed at 45 °C with vigorous stirring until a
homogeneous solution was formed. A 1 g amount of calcined mesoporous silica MCM-41 or
SBA-15 was then added to the grafting solution and finely dispersed by stirring. The mixture
was continuously stirred at the same temperature for 2 h. In some cases, after the addition of
the silica, concentrated ammonia (28%) was used to modify the pH of the mixture to 10. The
yellow product was then filtrated, washed with 1-propanol, and dried at 100 °C in the air for
24 h. The resulting material was calcined at 550 °C for 3 h. The resulting Fe3+-deposited
mesoporous silica samples with different iron contents are designated as Fe-MCM-41 (X%),
Fe-SBA-15 (X%), Fe-MCM-41 (X%, pH 10), and Fe-SBA-15 (X%, pH 10) (X = initial iron
molar percent of the synthesis solution).
Passivation of Iron-modified Mesoporous Silica. The conventional silylating agent
hexamethyldisilazane (HMDS, 99.9%, Aldrich) was used for the passivation of the surface
silanol groups. Iron-modified mesoporous silica was first degassed at 150 °C in a vacuum
oven overnight, and 0.5 g of the material was dispersed in 25 mL of dry hexanes, and then 1
mL of HMDS was subsequently added to the mixture with vigorous stirring. The mixture
was stirred for 24 h at room temperature. The excess HMDS was removed by Soxhlet
extraction refluxing for at least six runs at 60 °C using dichloromethane (100 mL, certified
ACS grade, Fisher Scientific) as the solvent. The obtained passivated materials were then
dried in a 60 °C oven for 12 h. The obtained materials were denoted Fe-MCM-41-HDMS
and Fe-SBA-15-HDMS.141
4.3 Catalyst Characterization and Testing
Low-angle powder X-ray diffraction (XRD) patterns were obtained using a Rigaku Multiplex
42
instrument with Cu Kα radiation (30 kV, 40 mA). Wide-angle X-ray diffraction (XRD)
patterns of all samples were collected on a Siemens Model D5000 diffractometer with Cu Kα
radiation (λ = 0.15496 nm). The wide-angle XRD pattern of Fe2O3 as reference was acquired
from the Powder Diffraction File 2 (PDF-2) database licensed by the International Center for
Diffraction Data (ICDD). The nitrogen adsorption-desorption isotherms were measured at -
196 °C with a Quantachrome Autosorb-1 sorption analyser (Quantachrome Instruments,
USA). Prior to the measurement, nonpassivated samples were outgassed under vacuum at
200 °C and passivated materials at 80 °C for at least 12 h. The specific surface area was
calculated through the Brunauer-Emmett-Teller (BET) equation using the obtained
adsorption data at P/P0 values between 0.05 and 0.2. The amount of nitrogen adsorbed at
P/P0 = 0.95 was used to estimate the total volume of the mesopores. The nonlocal density
functional theory (NLDFT) method was applied to determine the pore size distributions. The
selected NLDFT kernel considers sorption of N2 on silica at -196 °C, assuming the model of
equilibrium isotherm on the basis of the desorption branch and cylindrical pore geometry.142
All the data were extracted by using the Autosorb-1 1.55 software. Attenuated total
reflectance infrared (ATR-IR) spectra were measured with a Nicolet Magna 850 Fourier
transform spectrometer with a liquid-nitrogen-cooled narrow band mercury cadmium
telluride (MCT) detector. Each spectrum was gathered from the acquisition of 128 scans at 4
cm–1 resolution varied from 4000 to 700 cm-1 using a Happ-Genzel apodization. All samples
were dried in vacuum oven at 80 °C overnight before the ATR-IR measurements. Diffuse-
reflectance UV-vis spectra were recorded in the range of 200 nm-800 nm using a Varian
Cary 500 spectrophotometer equipped with a Praying Mantis accessory. X-ray photoelectron
spectroscopy (XPS) spectra were collected on a Kratos Axis-Ultra electron spectrometer
(UK), using a monochromatic Al Kα X-ray source (Al K = 1486.6 eV) at a power of 300 W
and operated at a base pressure of 5×10-10 Torr. Charge compensation was performed using
a low-energy electron beam perpendicular to the surface of the samples. Survey spectra used
for determining the elemental composition were collected at a pass energy of 160 eV. The
bulk iron contents of Fe-SBA-15 and Fe-MCM-41 were determined by ICP measurement
using a Varian 800 MS spectrophotometer. Scanning electron microscopy (SEM) images and
energy-dispersive X-ray spectroscopy (EDX) spectra were obtained using a JEOL JSM-840A
43
instrument. Before the analysis, a small amount of the sample was dispersed on the aluminum
sample holder, both sides of which were coated with gold and palladium. High-resolution
transmission electron microscopy (HR-TEM) and EDX analysis were recorded on a Cs-
corrected Titan G2 ETEM, which was operated at an accelerating voltage of 300 kV. Energy
dispersive X-ray mapping images (EDX) were obtained from a square area 15 × 15 nm of
TEM images and the acquisition time was 1 s. For the analysis, the samples were crushed in
the analytical reagent grade acetone solvent, and a few drops of the resulting suspension were
placed and dried on a carbon film supported on 300 mesh grids. For the catalytic tests, the
products were analyzed using 1H and 13C nuclear resonance spectroscopy (NMR; Varian
Inova NMR AS400 spectrometer 300, 400, 500 MHz). Chloroform-d was used as an internal
standard for 1H NMR (7.26 ppm) and 13C NMR (77.23 ppm). The syn/anti aldol ratio was
determined by the 1H NMR analysis of the crude product. High-resolution mass spectra
(HRMS) were recorded on an Agilent 6210 ESI TOF (time-of-flight) mass spectrometer.
4.3.1Titration of the Lewis Acid Solids with Hammett Indicators
Titration with different Hammett indicators was performed to determine the surface acidity
of the prepared catalysts, according to classical protocols.143 In this method, about 0.1 g of
dried solid was transferred to a glass vial after the addition of benzene, three or five drops of
a 0.1% freshly made solution of the indicators in benzene was added. According to the final
color of the mixture, it could be readily determined if a catalyst was in a basic or acidic form
to all indicators or had an H0 value lying between two adjacent indicators. Before the color
tests, all of the samples were dried and stored in a glass vial in a desiccator to avoid water
adsorption, which would have caused a shift to a lower acid strength and decreased the color
intensity of the adsorbed indicators. In addition, the titration solutions were suspended in a
quartz cuvette and were analyzed using a Varian Cary 500 spectrophotometer equipped with
a Praying Mantis accessory. The benzene solvent was used as a blank reference. All of the
catalysts here were tested with three different indicators. These Hammett indicators were
chosen on the basis of their pKa value, which is presented in Table 1 as well as the colors of
the acidic and basic forms of the indicators.
44
Table 4.1 Indicators Used for Acid Strength Measurements.
4.3.2 Pyridine Adsorption FT-IR Experiments.
Prior to the tests, the iron-containing mesoporous silica was freshly degassed at 150 °C for
24 h to remove the physisorbed water. A 50 mg of portion degassed Fe-MCM-41 and Fe-
SBA-15 was suspended in 3 ml of 5% pyridine solution in dry hexanes, respectively. After
the mixture was stirred for one hour, the solvent of the mixture was evaporated under reduced
pressure, and the obtained solids were dried at 100 °C for one hour afterward and finally
exposed at room temperature for 16 h. The final gathered powders were analyzed using ATR-
IR spectroscopy.
4.4 Results and Discussions
4.4.1 Synthesis and Characterization of the Materials
Both Fe-MCM-41 and Fe-SBA-15 materials were synthesized by a postgrafting method
using Fe(acac)3 as the iron source. There are several advantages of using Fe(acac)3 for the
grafting procedure. In contrast to metal nitride and chlorides or other metal precursors, which
can easily generate an aggregation of the metal oxide forming large clusters, the relatively
higher stability of acetylacetonate precursors can limit their hydrolysis and condensation.67,69
Furthermore, acetylacetonate complexes are less moisture-sensitive.144,145 There are two
documented mechanisms of the interaction of acac compounds with the silica support surface
according to a previous report.67 First, the Fe(acac)3 compound could be connected to the
silica surface through hydrogen bonding. Second, the ligand exchange between the acac and
silanol groups forming a covalent Fe-O-Si bond could also exist. Therefore, a high iron
dispersion on the silica surface can be obtained using such Fe(acac)3 precursors. These
previous results showed that this method was highly reproducible and could be fine-tuned by
indicators basic color acidic color pKa
methyl yellow yellow red +3.3
phenylazodiphenylphosphine yellow purple +1.5
dicinnamalacetone yellow red -3.0
45
just changing the molar ratio of metal-to-silicon in the grafting gel or adding ammonia
during the synthesis procedure. In the present study, the initial Fe/Si molar ratios of the
grafting gel were 1%, 3%, 5%, 10% and 20%. Another batch of samples was synthesized
using Fe(acac)3 as the precursor with the same iron concentrations, except for the addition of
ammonia during the grafting process.
The N2 adsorption-desorption isotherms and the respective pore size distributions of the
prepared Fe-MCM-41 and Fe-SBA-15 samples are shown in Figure 4.1, and structural
properties of all the prepared materials are compiled in Table 4.2. All the samples exhibited
similar type IV isotherms which are the typical feature of a well-defined mesoporous
Figure 4.1 N2 adsorption-desorption isotherms measured at 77.4 K (-196 °C) for (A) Fe-MCM-41 and
(C) Fe-SBA-15 with various iron contents and the corresponding NLDFT pore size distributions for (B)
Fe-MCM-41 calculated from the adsorption branch of the isotherm and (D) Fe-SBA-15 calculated from
the desorption branch of the isotherm.
46
structure.34 In comparison to Fe-MCM-41, isotherms of Fe-SBA-15 showed a well-resolved
H1 hysteresis loop with a steep increase in the adsorption volume within a relative pressure
(P/P0) between 0.6 to 0.8, indicating the highly ordered mesostructure of the catalysts with
large uniform cylindrical channels.146 Both pure mesoporous silica supports, MCM-41 and
SBA-15, show high BET surface area (1168 and 1067 m2/g) and pore volume (1.0 and 1.4
cm3/g), respectively, which slightly decrease in the cases of Fe-MCM-41 (5%, 10% and 20%)
and Fe-SBA-15 (5%, 10% and 20%), after the insertion of iron into the mesopores of the
supports. Meanwhile, when the catalysts were synthesized through the grafting procedure
performed at pH = 10, using the same iron concentration in the initial grafting gel, a
significant reduction of the surface area, pore volume, and pore size was observed. In addition,
the onset of the capillary condensation of the isotherms for Fe-MCM-41 and Fe-SBA-15 with
ammonia treatment shifted toward lower relative pressures (0.2-0.3 and 0.6-0.7, respectively)
demonstrating smaller average pore size of these samples in comparison to native
mesoporous siliceous materials. A drastic decrease in pore volume was also observed in these
samples. Such a phenomenon could be attributed to higher loading of iron species both on
the external surface and inside the channels of mesoporous silica.
In order to investigate the location of the iron species grafted in the silica, the surface Fe/Si
molar ratio and bulk Fe/Si molar ratio were measured by XPS and ICP-MS, respectively, and
are summarized in Table 4.2. One can observe that all the Fe/Si molar ratio in the obtained
materials are lower than the initial gel iron concentration, suggesting that only a fraction of
the iron precursors could be inserted into the silica matrix. It was expected that the Fe/Si
molar ratio measured by XPS should be close to the bulk Fe/Si molar ratios measured by
ICP-MS, showing a uniform grafting of the iron precursor on the surface of silica. As shown
in Table 4.2, only materials with lower Fe concentration in the initial gel showed a well-
dispersed iron loading on the silica. Samples obtained with pH adjustment met a strong
increase in Fe/Si molar ratio values. This increase in iron content on the silica might be
attributed to small size iron oxide particles on the external surface of silica when the iron
loading was too high.67 It was postulated before that the grafting of the iron acetylacetonate
could depend on its coordination stability.67 For Fe(acac)3, the interaction between the
precursor complex and the silanol groups is believe to take place through single-layer ligand
exchange that will be limited by the number of the silanol groups available on the silica
47
surface. Therefore, although 20 mol% of the precursor solution was used, both the surface
and bulk Fe/Si molar ratios of the resulting Fe-MCM-41 and Fe-SBA-15 seemed to reach a
plateau. This plateau might be attributed to the decreasing amount of available silanol groups
on the silica surface, most of which have already reacted with the iron precursor. As was
previously observed in the case of acac-substituted titanium alkoxide precursors69, the
interaction between the titanium precursors and the negatively charged silica surface Si-O-
under basic condition could be stronger than that with Si-OH and Si-OH2+ groups on the
silica surface at lower pH.69 pH adjustment was confirmed to be an efficient way to improve
metal loading. This conclusion was further proven here. Indeed, when the pH of the initial
precursor solution gel was adjusted to 10 by adding concentrated ammonia at 20% gel ratio,
iron loading substantially increased to 15%, in comparison to only 2.4% for the sample with
no pH adjustment. Under basic conditions, decomplexation of Fe(acac)3 could occur, and the
surface of silica becomes negatively charged. There are three possible iron cationic species
produced from the hydrolysis of Fe(acac)3, [Fe(acac)2(H2O)2]+, [Fe(acac)(H2O)4]
2+ and
[Fe(H2O)6] 3+ ,147 which are then available for the ligand exchange with the SiO
- on the silica
surface. In addition, the products of hydrolyzed Fe(acac)3 showed a low coordination stability,
which probably leads to a multilayer grafting, forming small iron oxide particles.67 Therefore,
it could be expected that the pH adjustment method can increase the iron loading.
Table 4.2 Chemical Composition and the Structural Properties of All Prepared Materials.
entry catalyst S BET
a
(m2/g)
dpb,c
(nm)
Vpd
(cm3/g) Fe/Sie (mol%) Fe/Sif (mol%)
1 MCM-41 1168 3.8 1.0 0 0
2 Fe-MCM-41 (1%) 970 3.9 0.8 1 0.4
3 Fe-MCM-41 (3%) 991 4.0 0.8 1 0.7
4 Fe-MCM-41 (5%) 1088 3.8 0.9 1 0.6
5 Fe-MCM-41 (5%, pH 10) 683 3.5 0.5 13 4
6 Fe-MCM-41 (10%) 1071 3.8 0.9 2 0.7
48
a S BET is the specific surface area calculated from the values of the relative pressure ranging from 0.05 to
0.20;
b Pore diameter calculated using the NLDFT method for the materials with MCM-41 as a silica support
(NLDFT adsorption conditions, i.e., adsorption branch);
c Pore diameter calculated using the NLDFT method for the materials with SBA-15 as a silica support
(NLDFT equilibrium conditions, i.e. desorption branch);
d Vp is the adsorbed volume obtained at P/P0 = 0.95;
e Measured by X-ray photoelectron spectroscopy (XPS);
f Measured by ICP-MS.
Low-angle XRD measurements of Fe-MCM-41 and Fe-SBA-15 are shown in Figure S4.1 (in
the Supporting Information). The diffractograms of Fe-MCM-41 and Fe-SBA-15 are
consistent with those of their corresponding bare mesoporous silicas.2, 41 Both diffractograms
showed three well-resolved peaks associated with (100), (110) and (200) reflections, which
originated from the highly ordered hexagonal arranged (p6mm) mesopores. This observation
indicated that the mesostructure of MCM-41 and SBA-15 was preserved after the insertion
7 Fe-MCM-41 (10%, pH 10) 834 3.5 0.6 13 6
8 Fe-MCM-41 (20%) 1088 3.8 0.9 2.4 0.7
9 Fe-MCM-41 (20%, pH 10) 758 3.5 0.5 15 10
10 SBA-15 1067 8.5 1.4 0 0
11 Fe-SBA-15 (1%) 879 8.2 1.2 1 0.4
12 Fe-SBA-15 (3%) 836 8.2 1.1 1 0.8
13 Fe-SBA-15 (5%) 960 8.5 1.3 0.9 0.5
14 Fe-SBA-15 (5%, pH 10) 466 8.2 0.9 7 3
15 Fe-SBA-15 (10%) 868 8.5 1.2 1.7 0.7
16 Fe-SBA-15 (10%, pH 10) 521 8.2 0.9 11 7
17 Fe-SBA-15 (20%) 742 8.5 1.0 1.7 0.9
18 Fe-SBA-15 (20%, pH 10) 563 8.2 0.9 15 10
49
of the Fe species. However, the diffraction peaks shifted to a higher angle region, suggesting
that the d(100) spacing and the hexagonal unit cell parameters a0, calculated as 2d(100)/√3,
decreased with the increased iron loading. The explanation suggested by Bouazizi148 was that
a slight framework compaction could occur because of the electrostatic attraction between
Fe and lattice O atoms. In addition, the diffraction peaks of Fe-MCM-41 (10%, pH 10) in the
range of 3.5-5 broadened, indicating a slight decrease in the structural regularity of the
sample. This structural deformation can be ascribed to the fact that small iron oxide clusters
or nanoparticles started to form in the mesopores or on the external surface of those materials
upon pH adjustment.149 The mesoporous materials were also characterized by wide-angle
XRD measurements, and the results are shown in Figure S4.2 (in the Supporting Information).
No clear diffraction peaks were observed in the wide-angle XRD patterns for the samples
synthesized either with or without pH adjustment. This phenomenon indicates the absence of
iron oxide species with large crystal domain size. The presence of smaller crystals of size
below the XRD detection limit (< 4-6 nm) however cannot be excluded.149 The absence of
diffraction peaks may also be attributed to the amorphous nature of metal layers coated on
the walls of the pores, even after calcination at high temperature. The high dispersion of iron
species on the mesoporous surface could also be demonstrated by other methods. SEM
images of Fe-MCM-41 and Fe-SBA-15 (Figure S4.3, in the Supporting Information) showed
no large crystal (FeO)n particles on the external surfaces of the materials, which is in
agreement with the XRD results. Fe-MCM-41 (B and C) shown in Figure S4.3 exhibited
spherical-like particles around 1 μm in size, as for pure MCM-41 material (A) while Fe-SBA-
15 (E and F) showed typical thread-like morphology as for SBA-15 (D). The elemental
compositions of Fe-MCM-41 and Fe-SBA-15 were obtained by EDX analysis. No Fe signal
could be seen in the spectra of samples with low iron content (i.e., 2% and 1.7%), which
seemed beyond the sensitivity of our instrument. In contrast, significant signals for Fe
appeared in EDX spectra of Fe-MCM-41 (10%, pH 10) and Fe-SBA-15 (10%, pH 10),
indicating the higher iron content of these samples. In this case, the Fe/Si molar ratio obtained
from EDX analysis of a selected area of the Fe-MCM-41 (10%, pH 10) and Fe-SBA-15 (10%,
pH 10) samples were 17% and 62%, respectively (in the Supporting Information). The much
higher molar ratios of the Fe/Si in comparison to values obtained from XPS analyses may be
attributed to the higher iron content within the mesopores of the silica since in the selected
50
area XPS analyses can only detect the elements of the outer surface of the materials. Further
in-depth analysis of the Fe distribution is nevertheless needed.
Thus, high-resolution transmission electron microscopy (HRTEM) imaging of representative
samples was carried out to study the structural characteristics of the catalysts. The TEM
images of both Fe-MCM-41 and Fe-SBA-15 (Figure 4.2) show uniform and parallel channels,
which further confirmed that the mesoporous structure was preserved after the modification
with iron species. More importantly, no visual iron oxide particles were detected for any of
the catalysts from the TEM images. Energy Dispersive X-Ray mapping (EDX) mapping of
the individual elements (Si, O, and Fe) present on the surface of mesoporous silica is also
depicted in Figure 4.2. The TEM-EDX maps revealed that the iron species are uniformly
distributed, while the iron elements content of Fe-MCM-41 (10%) (Figure 4.2 A) and Fe-
SBA-15 (10%) (Figure 4.2 C) was relatively lower in these catalysts. The Fe/Si atomic ratios
obtained from EDX analyses were as follows: Fe-MCM-41 (10%), 2.4%; Fe-MCM-41 (10%,
pH 10), 13.3%; and Fe-SBA-15 (10%), 2%; Fe-SBA-15 (10%, pH 10), 7.9%. These are in
agreement with the trends obtained from XPS analyses.
51
Diffuse reflectance UV-vis (DR-UV-vis) spectroscopy was performed to investigate the
dispersion and the coordination environment of Fe species in the mesoporous materials
because the absorption of visible light in the 200-800 nm region can be affected by the
coordination number of the metal sites (Supporting Information). The results are shown in
Figure S4.4 in the supporting information. For all of the Fe-MCM-41 and Fe-SBA-15
samples, one can observe a distinct absorbance in the wavelength range of 200-340 nm with
two maxima at about 220 and 250 nm, which can be assigned to the electron transition from
anion O2– and to the t2g and eg orbitals (ligand-to-metal charge transfer, LMCT) of four-
coordinated Fe3+ in tetrahedral coordination.145,150-152 In addition, the weak absorption band
Figure 4.2 High-resolution transmission electron microscopy images of (A) Fe-MCM-41 (10%); (B) Fe-
MCM-41 (10%, pH 10) and (C) Fe-SBA-15 (10%); (D) Fe-SBA-15 (10%, pH 10) and their corresponding
energy dispersive X-ray spectroscopy data.
52
appearing in the range of 280-300 nm could be assigned to small oligomeric (FeO)n
species.145,153 Nevertheless, the absence of visible absorption bands above 350 nm in all of
the spectra can exclude the presence of large clusters of iron oxide, which is usually featured
by a broad absorbance in the 440-480 nm region,154 this is in good accordance with the
SEM/TEM and XRD observations. This phenomenon could be explained by the covalent
bonding between the Fe(acac)3 and the surface of silica. Such a bonding could anchor the
iron complexes firmly and keep the iron species from becoming aggregated.155 Therefore, it
can be assumed that the deposition of the iron species on the surface of the silica supports
may be predominantly presented in the form of mononuclear Fe3+ cations as well as small
fractions of oligomeric (FeO)n species.
XPS is a powerful technique for obtaining surface chemical composition information and
exploring the chemical state of different metals. All catalysts show very similar results for Si
2p, O 1s, and Fe 2p values, and the spectra of Fe-SBA-15 (20%) shown in Figure 4.3 were
chosen as representative examples. The Si 2p spectrum of Fe-SBA-15 samples is shown in
Figure 3A. The single peak of this spectrum centered at binding energy (BE) of 103.4 eV,
which is in agreement with the values reported in the literature,156,157 is characteristic of
silicates. The spectrum of O 1s (Figure 3B) can be deconvoluted into two peaks. The largely
predominant peak centered at BE = 532.9 eV corresponds to SiO2 oxygen, and the other peak
centered at about 530 eV can be attributed to the presence of oxygen within iron oxides.158
The characteristic peak for adsorbed -OH groups at BE = 531.5 eV was not observed because
the weak signal was probably masked by the intense signal attributed to oxygen within
SiO2.157 The Fe 2p spectrum of Fe-SBA-15 (Figure 4.3 C) can be fitted with four peaks. The
specific peaks centered at BE = 711.6 eV corresponding to Fe 2p3/2 and BE = 725.2 eV
corresponding to Fe 2p1/2, accompanied by a satellite peak for Fe 2p1/2 at 733 eV, can be
attributed to the presence of iron(III).149 In addition, the binding energies for pure iron oxide
appear at around 710.6 and 711.2 eV in most cases, therefore, the higher binding energy
value for Fe 2p3/2 here was indicative of strong interactions between the iron species and the
silica framework.159 Meanwhile, the appearance of the peak at around 717.0 eV could be
53
attributed to the presence of isolated Fe3+ cations bound to surface O atoms, which
corroborates the UV-vis results.149
4.4.2 Surface Acidity
The surface acidity of a solid is defined by its ability to convert an adsorbed neutral base,
known as the Hammett indicator, into its conjugated acid. 116,160 The concept of the Hammett
acidity function H0 was introduced to determine the extrapolated pH range of the solid and
therefore to provide information on the acidity strength of the solid by using a series of
specific Hammett indicators. The mechanism of this titration is known as follows: when a
basic Hammett indicator (B) molecule reacts with the surface Brønsted acid sites (HA) or the
Lewis acid sites (LA), the conjugated base (HB+ + A−) or Lewis pair adduct (LA-B), which
are specified as the acidic form of the Hammett indicators, will be formed respectively.
During each titration, a dramatic color change is observed which is due to the intense color
of the conjugated acid of the indicator.12, 160 Thus, the acidity of a solid material could be
Figure 4.3 XPS spectra of (A) Si 2p, (B) O 1s and (C) Fe 2p for
the Fe-SBA-15 (20%) sample (taken as representative).
54
directly identified by the naked eye using titration method. If the color of the solid changed
to that of the acidic form of the Hammett indicator when a solid was titrated, this indicated
that the H0 value of the solid is equal to or lower than the pK a value of the conjugated acid
of the indicator.161
Since the color intensities of the Hammett indicators could be affected by the water absorbed
in the solid, all of the catalysts were dried at 150 ℃ and were then immediately used for color
tests or were stored in a sealed glass vial in a desiccator before carrying out the titration
tests.162 The acidic strengths of Fe-MCM-41 and Fe-SBA-15 with various iron contents are
given in Table S4.1. The colors of the prepared catalysts immersed in the indicator solution
changed dramatically. As shown in Table S4.1, all the catalysts were able to change the color
of methyl yellow indicator, as did typical Lewis acids such as AlCl3 and FeCl3, showing a
Hammett constant value estimated to be under +3.3. Since the native mesoporous silica
MCM-41 and SBA-15 were not able to modify the color of solutions of the indicators, this
proves the insertion of the iron sites on the silica surface increased the acidity of the materials.
It could also be shown that the acidity of Fe-MCM-41 (1% and 3%) and Fe-SBA-15 (1% and
3%) was weaker than that of materials with higher iron content. Since Fe-MCM-41 (1% and
3%) and Fe-SBA-15 (1% and 3%) could only change the color of methyl yellow, the
Hammett constant value may be estimated to be lying between [+1.5, +3.3]. Meanwhile, most
of the catalysts were able to change the color of dicinnamalacetone, which means that these
samples exhibited a stronger acidity with an H 0 value lower than -3. However, it is hard to
compare the acidity of the catalysts with higher iron content simply by using the Hammett
titration method. Although the indicator solution dispersed with different catalysts showed a
different color shade, it was not apparent whether the shade of the color was affected by the
iron content or by the original color of the solids, since the catalysts with higher iron content
had an orange or brown color.
Since the catalysts with higher iron content were not colorless, and thus the color changes
cannot be simply determined by the naked eye in this case, the color change was further
monitored with the aid of UV-vis. The UV spectra of three different indicators adsorbed on
the catalysts are shown in Figure 4.4. The neutral indicator of methyl yellow in benzene
solution is yellow and showed an absorption band around 450 nm. (Figure 4.4(A)).
Takahashi163 reported that the absorption maximum of methyl yellow absorbed on zeolites
55
appeared at 520 nm. Spectra of methyl yellow adsorbed on both Fe-MCM-41 (10%) and Fe-
MCM-41 (10%, pH 10), which were taken as representative samples, showed the appearance
of the band near 520 nm. The spectra of phenylazodiphenylphosphine and the catalysts with
this adsorbed indicator are shown in Figure 4.4. The color of the LA-B species was purple in
benzene as the absorption band appeared near 540 nm according to the literature.164 The
spectra of this indicator adsorbed on the catalysts proved the formation of the LA-B because
of the presence of a band near 540 nm. As in the case of dicinnamalacetone, Figure 4.4, the
interaction between the molecule and the Lewis acid sites might lead to the appearance of an
absorption band between 400 and 500 nm, which is responsible for the orange color.164
56
Although the surface acidity of a solid can be determined by simple titration with Hammett
indicators, a problem remained since both Lewis acid sites and Brønsted acid sites had the
capability to modify the color of the Hammett indicator solution without any distinction.
Therefore, pyridine adsorption probed by FT-IR spectroscopy was used as an additional
method to investigate the difference between Lewis acid sites and Brønsted acid sites.165 The
IR spectra of iron-modified mesoporous silica with and without the pyridine adsorption
process are shown in Figure 4.5 in the wavenumber region of 1400-1650 cm-1. The spectra
of the pyridine-adsorbed materials contain two distinct absorption bands at 1445 and 1600
cm-1, which were both related to pyridine-Lewis acid adduct. Meanwhile, the observation of
Figure 4.4 UV-vis absorption spectra of different indicators and
the catalysts with the indictors adsorbed.
57
absorption bands at 1495 cm-1 could be attributed to the chemisorbed pyridine on both Lewis
and Brønsted acid sites. The band of pyridine adsorbed on a Brønsted acid site was supposed
to appear at 1540 cm–1, which was not observed from the spectra, showing that the acidity of
both Fe-MCM-41 and Fe-SBA-15 originated mainly from Lewis acid sites instead of
Brønsted acid sites, which should be beneficial for their use as a catalyst in the Mukaiyama
aldol condensation reaction (see below).
4.4.3 Catalytic Tests
Effect of Catalyst Amount. The Mukaiyama aldol reaction of benzaldehyde with two
different substrates, 1-phenyl-1-(trimethylsiloxy)propene and 1-(trimethylsilyloxy)-
cyclohexene, was investigated over different amounts of Fe-MCM-41 (20%, pH 10) catalyst
and the results are given in Table 4.3 and Table 4.4. Fe-MCM-41 (20%, pH 10), which was
estimated to have a high catalytic ability because of its high iron content, was first chosen as
the catalyst. The final yield was calculated from the amount of aldol product after purification
by silica gel column chromatography. There was no product obtained in the absence of the
catalyst, while the target products, 3-hydroxy-2-methyl-1,3-diphenylpropan-1-one and 2-
(hydroxyphenylmethyl)-cyclohexanone, were afforded using catalysts. Two different
Figure 4.5 FT-IR spectra for Fe-MCM-41(20%) and Fe-SBA-15
(20%) materials with and without pyridine.
58
byproducts could be identified, 2-hydroxy-1-phenyl-1-propanone and benzoic acid arising
from the oxidation of 1-phenyl-1-(trimethylsiloxy)-propene and benzaldehyde, respectively.
As shown in the Table 4.3 and Table 4.4, the reactions were initially investigated with a
moderate amount (30 mg) of the catalyst, and the final yields were found to be 42% and 45%,
respectively. A higher yield was obtained when the catalyst amount was increased to 50 mg,
indicating that more active Lewis acid sites were introduced into the reaction. The product
yield was only slightly improved as the largest amount of catalyst (60 mg) was used, which
could be explained by the saturation of the active Lewis acid sites in the reaction mixture.
Table 4.3 Effect of Catalyst Amount for Mukaiyama Aldol Reaction over Fe-MCM-41 (20%, pH 10)
Catalyst.
Reaction conditions: benzaldehyde, 0.2 mmol; 1-(trimethylsilyloxy)cyclohexene, 0.4 mmol; water 0.25
mL, ethanol 0.25 mL, room temperature; time, 3 h.
a Yield of aldol product after purification by silica gel column chromatography.
Effect of the Solvent. Strictly anhydrous organic solvents and low reaction temperature (-
78 °C) were required since the Mukaiyama aldol reaction was first discovered.22 Currently,
aqueous media without any harmful organic solvents for organic reactions have been
attracting increasing interest because of environmental concerns. Table 4.5 presents the
catalytic behavior of Fe-MCM-41 (50 mg, Fe/Si = 20%, pH 10) for the Mukaiyama aldol
reaction of benzaldehyde (0.2 mmol) with 1-phenyl-1-(trimethylsiloxy)propene (0.4 mmol)
conducted in various typical organic solvents.
entry catalyst amount (mg) yield (%)a syn/anti
1 0 0 0
2 30 42 75:25
3 50 87 79:21
4 60 87 76:24
59
Table 4.4 Effect of Catalyst Amount for Mukaiyama Aldol Reaction over Fe-MCM-41 (20%, pH 10)
Catalyst.
Reaction conditions: benzaldehyde, 0.2 mmol; 1-phenyl-1-(trimethylsiloxy)-propene, 0.4 mmol; water
0.25 mL, ethanol 0.25 mL, room temperature; time, 24 h.
a Yield of aldol product after purification by silica gel column chromatography.
The catalyst had no or weak catalytic activity when the reactions were carried out in polar
(MeCN and THF) and nonpolar (DMC and CH2Cl2) solvents, while the hydrophobic
reactants can be easily dissolved in these solvents.166 In contrast, the yield of the reaction
carried out in water-SDS and water-ethanol systems reached up to 86% and 90%,
respectively, suggesting that they were both suitable media for the Mukaiyama aldol reaction,
which was also evidenced by other researchers with different catalysts.73,167 The surfactant
SDS was essential because the Mukaiyama aldol reaction involves hydrophobic molecules
requiring water, and thus the yield of the product could be enhanced by the presence of
SDS.166 Shintaku et al. reported that Ti-SBA-15 showed higher catalytic activity for the
Mukaiyama aldol reaction conducted in water-THF solutions, while no catalytic activity in
pure THF solvent was observed.166 As reported by Hatanaka et al., the transition state formed
between the aldehyde activated by the Lewis acid and the silyl enol ether could be stabilized
by water molecules, which could also contribute to the dissociation of trimethylsilyl groups;
therefore, the reaction rate can be increased.168
entry catalyst amount (mg) yield (%)a syn/anti
1 0 0 0
2 30 45 87:13
3 50 81 88:12
4 60 89 70:30
60
Table 4.5 Effect of the Solvent for Mukaiyama Aldol Reaction over Fe-MCM-41(20%, pH
10)
entry solvent yield (%)cd syn/anti
1ab H2O+SDS 86 90:10
2bc H2O+ethanol 81 88:12
3 THF 0 0
4 MeCN 0 0
5 CH2Cl2 18de 45:55
6 DMC 0 0
aReaction conditions: benzaldehyde, 0.2 mmol; 1-phenyl-1-(trimethylsiloxy)propene, 0.4mmol; solvent,
0.5 mL. Abbreviations: SDS, sodiumdodecyl sulfate; DMC, dimethylcarbonate.
abAdditional reaction conditions: water, 0.5 mL, SDS (11.5mg ); room temperature; time, 24 h.
bcAdditional reaction conditions: water, 0.25 mL; ethanol, 0.25 mL; room temperature; time, 24 h.
cd Yield of aldol product after purifcation by silica gel column chromatography.
deConversion of benzaldehyde determined by 1H NMR.
Effect of the surfactant (SDS). Table S4.2 (in the Supporting Information) shows the effect
of the surfactant for the Mukaiyama aldol reaction. It should be noted that both the water-
SDS system and the water-ethanol system have previously been reported as effective media
for Mukaiyama aldol reactions.126,169 Both SDS and ethanol were adopted as a surfactant or
a dispersing agent, to enhance the solubility of the hydrophobic reactants in aqueous media.
Fe-MCM-41 (10%, pH 10) and Fe-SBA-15 (10%, pH 10) were used as the catalysts under
different conditions to compare the water-SDS and water-ethanol systems. As one can
observe, there were no noticeable differences in the final yields; therefore, both were suitable
media for the Mukaiyama aldol reaction. However, a larger amount of the surfactant SDS,
which is not environmentally friendly, was always required.169 As water-ethanol is a clean,
environmentally benign medium and a surfactant-free aqueous system in comparison with
61
the water-SDS system, all further experiments in this study were carried out at room
temperature in water-ethanol.
Effect of the Iron Content (Fe-MCM-41, Fe-SBA-15) on the Reactivity. MCM-41 and
SBA-15 with two different pore sizes were chosen to probe the effects of the pore size, and
the effects of iron content of the catalysts of the catalysts for the Mukaiyama aldol reaction
of both 1-phenyl-1-(trimethylsiloxy)propene and 1-(trimethylsilyloxy)cyclohexene with
benzaldehyde were examined by using catalysts with various iron contents (Fe/Si = 1-20
mol%). As shown in Table 4.6, the lowest yields of 3-hydroxy-2-methyl-1, 3-
diphenylpropan-1-one were obtained when catalyst Fe-MCM-41 (1%, 3%) and Fe-SBA-15
(1%, 3%) were used, both with low iron content. This suggested that only minimal amount
of iron species was inserted into silica supports when the iron concentration of grafting gel
was too low. These low yields were also in agreement with the lower density of acid sites of
these two catalysts, as demonstrated above in the surface acidity tests. When catalysts Fe-
MCM-41 (5%) and Fe-SBA-15 (5%) exhibiting stronger acidity (i.e., higher density of acid
sites) were used, the yield substantially increased to 85% and 90%, respectively. This increase
in yield was clearly due to the higher iron loading of the catalysts, indicating an increasing
number of active Lewis acid sites in the catalysts. However, it seems that the catalytic activity
of catalysts reached a maximum when the Fe/Si molar ratio was 5%. The yield of product
fluctuated around 90% when Fe-MCM-41 (10%, 20%) and Fe-SBA-15 (10%, 20%) were
used as catalysts. Ammonia treatment was very efficient for increasing the iron loading, yet
it cannot improve the catalytic activity of the materials. Catalysts with ammonium treatment
apparently possess higher iron loading but barely promote the yields. Similar results have
been found in Mukaiyama aldol reaction of 1-(trimethylsilyloxy)cyclohexene and
benzaldehyde (Table 4.7). All of the catalysts except Fe-MCM-41 (1%, 3%) and Fe-SBA-15
(1%, 3%) possess high catalytic activities, with a product yield around 87%.
Iron-modified mesoporous silica had a high catalytic activity for the Mukaiyama aldol
reaction, owing to their highly dispersed active Lewis acid sites. In addition, the adsorption
and diffusion of the reactants could be favored by the mesoporous channels of the catalysts.
62
Table 4.6 Effect of the Iron Content in the Mukaiyama Aldol Reaction of 1-phenyl-1-
(trimethylsiloxy)propene with Benzaldehyde.
entry catalyst Fe/Si (mol %)ab yield (%)bc syn/anti
1 Fe-MCM-41 (1%) 0.4 50 77:23
2 Fe-MCM-41 (3%) 0.7 59 83:17
3 Fe-MCM-41 (5%) 0.6 85 70:30
4 Fe-MCM-41 (5%, pH 10) 4 90 85:15
5 Fe-MCM-41 (10%) 0.7 82 62:38
6 Fe-MCM-41 (10%, pH 10) 6 89 80:20
7 Fe-MCM-41 (20%) 0.7 90 70:30
8 Fe-MCM-41 (20%, pH 10) 10 81 88:12
9 Fe- SBA-15 (1%) 0.4 50 75:25
10 Fe- SBA-15 (3%) 0.8 56 79:21
11 Fe-SBA-15 (5%) 0.5 90 71:29
12 Fe-SBA-15 (5, pH 10) 3 70 86:14
13 Fe-SBA-15 (10) 0.7 86 80:20
14 Fe-SBA-15 (10, pH 10) 7 86 80:20
15 Fe-SBA-15 (20) 0.9 90 70:30
16 Fe-SBA-15 (20, pH 10) 10 93 79:21
aReaction conditions: benzaldehyde, 0.2 mmol; 1-phenyl-1-(trimethylsiloxy)propene, 0.4 mmol; water
0.25 mL, ethanol 0.25 mL, room temperature; time, 24 h.
bMeasured by ICP-MS.
c Yield of aldol product after purification by silica gel column chromatography.It can be postulated that
The results showed a distinct increase in yield as the iron content was raised from 1% to 5%.
Meanwhile, no significant difference in yield was observed for the catalysts with an iron
contents in the range of 5% to 20%. Therefore, the catalytic experiment can also lead to a
conclusion that the active sites density of catalysts with iron content of 5% (in the initial
grafting gel) was getting saturated, resulting in an unchanged catalytic activity of the catalysts
with increased iron content. However, no differences between the yield of reaction using
catalysts exhibiting different pore size were observed. This indicates that the pore size from
63
4 to 9 nm have the same effect for the Mukaiyama aldol reaction. The effects of pore size
beyond this range need further investigation.
Table 4.7 Effect of Iron Content for Mukaiyama Aldol reaction of 1-(trimethylsilyloxy)cyclohexene with
Benzaldehyde
a Reaction conditions: benzaldehyde, 0.2 mmol; 1-(trimethylsilyloxy)cyclohexene, 0.4 mmol; water 0.25
mL, ethanol 0.25 mL, room temperature; time, 3 h.
b Measured by ICP-MS.
c yield of aldol product after purification by silica gel column chromatography.
In addition, for comparison, FeCl3 was chosen as a model homogeneous Lewis acid catalyst
and also applied in the Mukaiyama aldol reactions in similar conditions. As shown in Table
S4.3 (in the Supporting Information), surprisingly low yields of 27% and 42% were obtained
entry catalyst Fe/Si (mol%)ab yield (%)bc syn/anti
1 Fe-MCM-41 (1%) 0.4 38 80:20
2 Fe-MCM-41 (3%) 0.7 49 78:22
3 Fe-MCM-41 (5%) 0.6 85 86:14
4 Fe-MCM-41 (5%, pH 10) 4 78 78:22
5 Fe-MCM-41 (10%) 0.7 88 88:12
6 Fe-MCM-41 (10%, pH 10) 6 84 78:22
7 Fe-MCM-41 (20%) 0.7 84 86:14
8 Fe-MCM-41 (20%, pH 10) 10 87 79:21
9 Fe-MCM-41 (1%) 0.4 29 77:23
10 Fe-MCM-41 (3%) 0.8 56 75:25
11 Fe-SBA-15 (5%) 0.5 85 88:12
12 Fe-SBA-15 (5%, pH 10) 3 81 76:24
13 Fe-SBA-15 (10%) 0.7 81 78:22
14 Fe-SBA-15 (10%, pH 10) 7 85 77:23
15 Fe-SBA-15 (20%) 0.9 85 80:20
16 Fe-SBA-15 (20, pH 10) 10 82 75:25
64
when FeCl3 was used as catalyst, showing its distinctly lower catalytic activity and selectivity
in aqueous medium in comparison to the iron-modified mesoporous materials.
Substrate Scope. To verify the versatility of our system, a series of aldehydes were tested
with 1-phenyl-1-(trimethylsiloxy)propene to explore the scope of the Fe-modified
mesoporous materials (i.e., Fe-SBA-15 (5 % and 20 %)) in the catalytic Mukaiyama aldol
reactions. As shown in Table 4.8, the corresponding aldol products were obtained in moderate
to excellent yields in all cases. We first investigated the reaction of benzaldehyde with 1-
phenyl-1-(trimethylsiloxy)propene and excellent yields of 90% were observed.
Correspondingly, similar yields of coupling aldols were also achieved when 4-substituted
benzaldehydes with either an electron-donating group (-OCH3, entries 3 and 4) or an
electron-withdrawing group (-Cl, entries 5 and 6) were examined as substrates. Notably, high
yields around 90% were obtained when the less reactive 4-cyanobenzaldehyde, 4-
nitrobenzaldehyde and 2-nitrobenzaldehyde were used as substrates (entries 7-12), although
a reverse selectivity was observed; i.e., the anti-isomers were the major product. n-Butanal
as a representative example of an aliphatic aldehyde showed lower reactivity in comparison
to the benzaldehyde or benzaldehyde derivatives, with the aldol products obtained in 49%
and 53% yields. However, a similar yield of 64 % was previously observed by the Ollevier
group using the same aliphatic aldehyde (n-butanal) in pure water with a homogeneous
catalyst.170
65
Table 4.8 Catalytic Performances of Fe-SBA-15 (5%)(A) and Fe-SBA-15 (20%)(B) in Mukaiyama
Aldol Reactions (substrate Scope)
entry cat. aldehyde R’ yield
(%)ab
syn/anti
1 A
H 90 71:29
2 B H 90 70:30
3 A 4-OMe 87 79:21
4 B 4-OMe 90 83:17
5 A 4-Cl 88 76:24
6 B 4-Cl 86 88:12
7 A 4-CN 90 34:66
8 B 4-CN 93 33:67
9 A 4-NO2 91 40:60
10 B 4-NO2 92 39:61
11 A 2-NO2 90 34:66
12 B 2-NO2 91 31:69
13 A
49 67:33
14 B 53 70:30
a Yield of aldol product after purification by silica gel column chromatography.
b Reaction conditions: aldehyde, 0.2 mmol; 1-phenyl-1-(trimethylsiloxy)-propene, 0.4 mmol; water 0.25
mL, ethanol 0.25 mL, room temperature; time, 24 h.
Reusability Experiments. One of the most significant advantages of heterogeneous Lewis
acid catalysts is that they can normally be easily removed from the reaction medium and
recycled. For reusability tests, 400 mg of Fe-modified mesoporous silica (e.g., Fe-SBA-15
66
(5%) and Fe-SBA-15 (20%)) was used as catalyst for the water-medium Mukaiyama aldol
reaction between benzaldehyde and 1-(trimethylsilyloxy)cyclohexene. In each case, the
catalyst was recollected by filtration, washed with ethanol and subsequently dried overnight
at 150 ℃ in a vacuum oven. The amount of the reactants was adjusted to the same scale
according to the amount of catalyst. As presented in Table 4.9 and Table S4.4 (in the
Supporting Information), the recycled catalysts could be reused for at least nine times without
significant loss in the yield and no change in selectivity. It can thus be suggested that no
elution of Fe species was taking place during the reactions and, therefore, Fe-modified
mesoporous silica can be viewed as a highly stable, active and reusable heterogeneous
catalyst for the Mukaiyama aldol reaction.
Table 4.9 Reusability of catalysts for Mukaiyama aldol reaction of 1-(trimethylsilyloxy)-cyclohexene
with benzaldehyde over Fe-SBA-15 (20%) catalyst
a Reagents and conditions: benzaldehyde, 1 equiv.; 1-(trimethylsilyloxy)cyclohexene, 2 equiv.;
water/ethanol = 1:1, room temperature; time, 3 h. b Yield of aldol product after purification by silica gel
column chromatography.
entry yield (%)ab syn/anti
first run 87 76:24
second 88 83:17
third 92 75:25
fourth 93 77:23
fifth 89 78:22
sixth 91 79:21
seventh 93 76:24
eighth 93 82:18
ninth 94 79:21
67
Catalytic Behavior of Passivated Fe-MCM-41 and Fe-SBA-15. The catalytic activities of
nonpassivated materials and passivated materials were compared to verify a possible effect
of the silanol groups present on the surface of silica for the activity (Table 4.10). A high yield
of the reaction could be obtained with the presence of all catalysts, among which the
passivated materials showed a slightly better catalytic activity with the yield increasing from
84 to 89% and from 85 to 92% for Fe-MCM-41 and Fe-SBA-15, respectively. It was expected
that the silylating layer on the silica surface could provide strong surface hydrophobicity,
which may facilitate the diffusion of the organic reactants, thus increasing the possibility of
contact between the hydrophobic reactants and the active Lewis acid sites, leading to a higher
yield of the final products.88,166
Table 4.10 Catalytic test of Fe-MCM-41-HMDS (20%) and Fe-SBA-15-HMDS (20%) catalyst
a Reagents and conditions: benzaldehyde, 0.2 mmol; 1-(trimethylsilyloxy)cyclohexene, 0.4 mmol; water
0.25 mL, ethanol 0.25 mL, room temperature; time, 3 h. b Yield of aldol product after purification by silica
gel column chromatography.
4.5 Conclusion
To summarize, a versatile and easy synthesis of iron-modified mesoporous silica using
acetylacetonate as a metal precursor was demonstrated. It was shown from XRD and TEM
that no iron oxide species with large crystal domain size were formed during the synthesis.
catalyst yield (%)ab syn/anti
Fe-MCM-41 84 86:14
Fe-MCM-41-HDMS 89 73:27
Fe-SBA-15 85 80:20
Fe-SBA-15-HDMS 92 80:20
68
Furthermore, only a fraction of the iron precursor can be grafted on the silica, with cationic
iron species preferentially dispersed on the surface of the silica support. The Lewis acidity
of the prepared catalysts was explored by titration method using various Hammett indicators.
The results of pyridine adsorption FT-IR revealed that the iron-modified mesoporous
materials exhibited a significant majority of Lewis acid sites in comparison to Brønsted sites.
Finally, in the catalytic tests, the iron species deposited on mesoporous silica could function
as highly active and selective sites for the Mukaiyama aldol reaction of various aldehydes
with 1-(trimethylsilyloxy)cyclohexene and 1-phenyl-1-(trimethylsiloxy)propene. The
experimental procedure was straightforward, and the ethanol-water solution used in this
study was a clean solvent system and environmentally benign; no harmful organic solvents
were used. The catalysts could easily be recovered and successively reused in the same
reaction without loss of catalytic activity. Therefore, it is believed that iron-modified
mesoporous silica is a highly active heterogeneous Lewis acid catalyst and could be further
applied not only for the Mukaiyama aldol reaction but also for various environmentally-
friendly chemical transformations in organic chemistry.
69
4.6 Supporting information
4.6.1 Materials and general procedure of catalytic reactions
All the solvents were ACS grade and used without purification. For silica synthesis,
cetyltrimethylammonium bromide, Pluronic P123 (Mw = 5800 g/mol) and tetraethylorthosilicate (98%)
were all purchased from Sigma-Aldrich Company. NH4OH (28%) and HCl (36.5%) were obtained from
Anachemia. Anhydrous ethanol (100%) was provided by Fisher Scientific. Iron(III) acetylacetonate (97%)
used in the grafting process was purchased from Sigma-Aldrich and used without further purification.
Pyridine (99.8%) for analysis of Lewis acid sites and benzaldehyde for catalytic tests were obtained from
Sigma-Aldrich. Hammet indicators methyl yellow (analytical grade), dicinnamalacetone (98%) and
phenylazodiphenylamine (97%) were purchased from Sigma-Aldrich and used as received. For catalytic
reactions, sodium dodecyl sulfate (>98.5%), iron(III) chloride (97%), benzaldehyde (99.5%) and n-
butanal (98%) were obtained from Sigma-Aldrich. 4-methoxybenzaldehyde (98%), 4-
cholorobenzaldehyde (99%), 4-cyanobenzaldehyde (98+%), 4-nitrobenzaldehyde (99%), and 2-
nitrobenzaldehyde were all purchased from Alfa Aesar. n-Butanal was distilled prior to use.
4.6.2 Catalytic reactions
Synthesis of silyl enol ethers. The general procedure for the preparation of silyl enol ethers was
performed by following the procedure reported in the literature.171 First, sodium iodide (5.9 g, 39 mmol,
Alfa Aesar) was dissolved in acetonitrile (Caledon Laboratories, 36 mL) in a one-neck round bottom flask
at room temperature. The ketone (30 mmol, Alfa Aesar), triethylamine (6.1g, 60 mmol, 99%, Alfa Aesar)
and trimethylchlorosilane (4.2 g, 39 mmol, 98+%, Alfa Aesar) were successively added into the system.
An exothermic reaction occurred with the mixture turning into a brown color, and a white precipitate
appeared. The mixture was continuously stirred for 1 h to get a complete reaction. Water was first added
into the final brown mixture to dissolve the precipitated salt, and hexanes (30 mL were then added until
organic-water layers were formed. The aqueous layer was extracted with hexanes at least 5 times, and the
combined hexanes-product solution was washed with sodium chloride solution. The water in the organic
solution was then removed using anhydrous sodium sulfate (99%, Alfa Aesar). After the evaporation under
reduced pressure, the final silyl enol ethers were obtained.
General Procedure for the Mukaiyama Aldol Reaction. The Lewis acid catalytic behavior of the
prepared iron-containing mesoporous silica was examined by the Mukaiyama aldol condensation o
70
benzaldehyde with two different enol silyl ethers in aqueous conditions.166 The reaction was performed
in a glass tube. For this, the Lewis acid catalyst (50 mg) was added to a mixture of aldehyde (0.2 mmol)
and the silyl enol ether (0.4 mmol). Water (0.5 mL) or a mixture of ethanol (0.25 mL) and water (0.25
mL) was used as a solvent. Magnetic stirring was used for this reaction to get a good contact between
reactants and catalyst. During the reaction process, thin layer chromatography (TLC) was performed every
2 h to monitor the progress of the reaction. In the workup process, the catalyst was isolated by filtration
and washed with ethyl acetate (10 mL, 99%, Fisher Chemical) after the completion of the reaction. The
resulting filtrate was either evaporated directly or extracted at least three times with ethyl acetate (10 mL),
and the gathered organic layer was dried with anhydrous sodium sulfate. After the removal of the solvent
with a rotary evaporator under reduced pressure, the crude product was purified to discard the by-products
by silica-gel column chromatography (hexanes/ethyl acetate)
71
4.6.3 Schemes of Mukaiyama aldol reactions
Scheme S4.1 Mukaiyama aldol reaction of 1-(trimethylsilyloxy)-cyclohexene and benzaldehyde.
Scheme S4.2 Mukaiyama aldol reaction of 1-phenyl-1-(trimethylsiloxy)-propene and different
aldehydes.
72
4.6.4 Characterization of the catalysts
Figure S4.1 Low-angle XRD patterns for Fe-MCM-41 (A) and Fe-SBA-15 (B) with
various iron contents, as indicated.
A
B
73
Figure S4.2 Wide-angle XRD patterns for Fe-MCM-41 (A) and Fe-SBA-15 (B), with
various iron contents, as indicated, and the reference pattern of Fe2O3 is also shown.
A
B
74
Figure S4.3 SEM images of (A) MCM-41, (B) Fe-MCM-41 (10%), (C) Fe-MCM-41 (10%, pH=10), and (D) SBA-15, (E)
Fe-SBA-15 (10%), (F) Fe-SBA-15 (10%, pH=10), and their corresponding EDX spectra, with surface Fe/Si molar ratio of (G)
2%, (H) 13%, (I) 1.7% and (J) 11%, for samples in (B), (C), (E) and (F), respectively.
75
Figure S4.4 UV–vis diffuse reflectance (DR-UV–vis) spectra of calcined
Fe-MCM-41 and Fe-SBA-15 samples.
76
4.6.5 Lewis acidity tests
Table S4.1 Results of titration of Fe-modified mesoporous materials with Hammet indicators.
entry catalyst methyl
yellow
phenylazodiphenyl-
phosphine
dicinnamal-
acetone
1 HCl pink light purple –
2 H2SO4 pink light purple –
3 AlCl3•6H2O orange purple red
4 FeCl3 red purple red
5 MCM-41 – – –
6 Fe-MCM-41 (1%) red – –
7 Fe-MCM-41 (3%) red – –
8 Fe-MCM-41 (5%) red light purple orange
9 Fe-MCM-41 (5%, pH 10) red purple red
10 Fe-MCM-41 (10%) red purple orange
11 Fe-MCM-41 (10%, pH 10) red dark purple dark orange
12 Fe-MCM-41 (20%) red purple orange
13 Fe-MCM-41 (20%, pH 10) red dark purple red
14 SBA-15 – – –
15 Fe-SBA-15 (1%) red – –
16 Fe-SBA-15 (3%) red – –
17 Fe-SBA-15 (5%) red purple –
18 Fe-SBA-15 (5%, pH 10) red dark purple dark orange
19 Fe-SBA-15 (10%) red purple orange
20 Fe-SBA-15 (10%, pH 10) red dark purple red
21 Fe-SBA-15 (20%) red purple orange
22 Fe-SBA-15 (20%, pH 10) red dark purple red
77
4.6.6 Catalytic tests
Table S4.2 Effect of the addition of the surfactant (SDS) in the Mukaiyama aldol reaction
Entry Catalyst Solvent Surfactant Yield
(%)a
syn/anti
1 Fe-MCM-41 (10%, pH = 10) H2O SDS 80 89:11
2 Fe-MCM-41 (10%, pH = 10) H2O+ethanol – 89 80:20
3 Fe-MCM-41 (20%, pH = 10) H2O SDS 86 90:10
4 Fe-MCM-41 (20%, pH = 10) H2O+ethanol – 81 88:12
5 Fe-SBA-15 (10%, pH = 10) H2O SDS 87 82:18
6 Fe-SBA-15 (10%, pH = 10) H2O+ethanol – 86 80:20
7 Fe-SBA-15 (20%, pH = 10) H2O SDS 90 81:19
8 Fe-SBA-15 (20%, pH = 10) H2O+ethanol – 93 79:21
Reaction conditions: benzaldehyde, 0.2 mmol; 1-phenyl-1-(trimethylsiloxy)-propene, 0.4 mmol; water
0.25 mL, ethanol 0.25 mL, room temperature; time, 24 h. a Yield of aldol product after purification by
silica gel column chromatography.
Table S4.3 Mukaiyama aldol reaction using FeCl3 as catalyst
entry catalyst aldehyde silyl enol ether yield (%)a syn/anti
1 FeCl3
27 56:44
2 FeCl3
42 70:30
Reaction conditions: benzaldehyde, 0.2 mmol; silyl enol ether, 0.4 mmol; catalyst, 0.2 mmol, water 0.25
mL, ethanol 0.25 mL, room temperature; time, entry 1, 3 h; entry 2, 24 h. a Yield of aldol product after
purification by silica gel column chromatography.
78
Table S4.4 Reusability of catalysts for Mukaiyama aldol reaction of 1-(trimethylsilyloxy)-cyclohexene
with benzaldehyde over Fe-SBA-15 (5%) catalyst
entry yield (%)a syn/anti
1st 87 83:17
2nd 87 76:24
3rd 92 87:13
4th 90 81:19
5th 90 79:21
6th 93 78:22
7th 92 76:24
8th 93 78:22
9th 93 75:25
Reagents and conditions: benzaldehyde, 1 equiv.; 1-(trimethylsilyloxy)cyclohexene, 2 equiv.;
water/ethanol = 1/1, room temperature; time, 3 h. a Yield of aldol product after purification by silica gel
column chromatography.
79
1H NMR and 13C NMR data of reagents and aldol products
1-(Trimethylsilyloxy)-cyclohexene. 171,172
1H NMR (400 MHz, Chloroform-d) δ 4.86 (t, J = 3.8 Hz, 1H), 2.00 (m, 4H), 1.69-1.62 (m,
2H), 1.55-1.47 (m, 2H), 0.18 (s, 9H).
1-Phenyl-1-(trimethylsiloxy)-propene.172
1H NMR (400 MHz, Chloroform-d) δ 7.48– 7.22 (m, 5H), 5.34 (q, J = 6.9 Hz, 1H), 1.74 (d,
J = 6.9 Hz, 3H), 0.14 (s, 9H).
2-(Hydroxyphenylmethyl)-cyclohexanone.173,174
The product was obtained as white solid. 1H NMR (500 MHz, Chloroform-d) δ 7.39-7.23
(syn+anti, m, 10H), 5.41 (syn, s, 1H), δ 4.80 (anti, dd, J = 8.8, 2.6 Hz, 1H), 3.96 (anti, d, J =
2.8 Hz, 1H), 3.02 (syn, dd, J = 3.2, 1.6 Hz, 1H), 2.67-2.59 (syn+anti, m, 2H), 2.51-2.43
(syn+anti, m, 2H), 2.44-2.32 (syn+anti, m, 2H), 2.10 (syn+anti, m, 2H), 1.90-1.83 (syn+anti,
m, 2H), 1.79-1.64 (syn+anti, m, 6H), 1.56-1.48 (syn+anti, m, 2H). 13C NMR (126 MHz,
80
Chloroform-d, syn+anti) δ 215.5, 214.7, 141.6, 141.0, 128.4, 128.1, 127.9, 127.1, 127.0,
125.8, 74.7, 70.5, 57.4, 57.2, 42.6, 30.8, 27.9, 27.8, 26.0, 24.8, 24.7.
3-Hydroxy-2-methyl-1,3-diphenylpropan-1-one.175,176
The product was obtained as colorless oil. 1H NMR (400 MHz, Chloroform-d) δ 7.99-7.92
(syn+anti, m, 4H), 7.62-7.55 (syn+anti, m, 2H), 7.51-7.24 (syn+anti, m, 14H), 5.25 (syn, m,
1H), 5.00 (anti, dd, J = 8.1, 4.4 Hz, 1H), 3.71 (syn, dq, J = 7.2, 3.6 Hz, 1H), 3.84 (anti, p, J =
7.4 Hz, 1H), 3.69-3.66 (syn, d, J = 2.1 Hz, 1H), 3.03 (anti, d, J = 4.5 Hz, 1H).1.20 (syn, d, J
= 7.2 Hz, 3H), 1.07 (anti, d, J = 7.2 Hz, 3H). 13C NMR (75 MHz, Chloroform-d, syn+anti) δ
205.8, 204.9, 142.2, 141.8, 136.7, 135.6, 133.6, 133.3, 128.8, 128.7, 128.5, 128.4, 128.3,
128.1, 127.9, 127.3, 126.7, 126.0, 76.8, 73.1, 48.0, 47.0, 15.7, 11.2.
3-Hydroxy-3-(4-methoxyphenyl)-2-methyl-1-phenyl-1-propanone7
The product was obtained as colorless oil. 1H NMR (400 MHz, Chloroform-d) δ 8.00 (anti,
d, J = 7.3 Hz, 2H), 7.93 (syn, d, J = 7.3 Hz, 2H), 7.61-7.55 (syn+anti, m, 2H), 7.50-7.44
(syn+anti, m, 4H), 7.37-7.29 (syn+anti, m, 4H), 6.92- 6.84 (syn+anti, m, 4H), 5.18 (syn, d,
J = 3.4 Hz, 1H), 4.96 (anti, d, J = 8.3 Hz, 1H), 3.81 (syn, s, 3H), 3.81 (anti, s, 1H), 3.80 (anti,
s, 3H), 3.68 (syn, qd, J = 7.2, 3.6 Hz, 1H), 3.55 (syn, s, 1H), 2.90 (anti, s, 1H), 1.22 (syn, d,
J = 7.2 Hz, 3H), 1.04 (anti, d, J = 7.2 Hz, 3H). 13C NMR (75 MHz, Chloroform-d, syn+anti)
81
δ 205.6, 205.0, 159.3, 158.8, 136.8, 135.8, 134.4, 134.1, 133.5, 133.3, 128.9, 128.8, 128.6,
128.4, 127.9, 127.3, 113.8, 113.6, 76.4, 73.0, 55.30, 55.26, 48.1, 47.3, 15.7, 11.6.
3-Hydroxy-3-(4-chlorophenyl)-2-methyl-1-phenyl-1-propanone7
The product was obtained as colorless oil. 1H NMR (400 MHz, Chloroform-d) δ 7.96-7.93
(syn+anti, m, 4H), 7.63-7.56 (syn+anti, m, 2H), 7.51-7.45 (syn+anti, m, 4H), 7.37-7.30
(syn+anti, m, 8H), 5.22 (syn, d, 1H), 4.98 (anti, d, J = 2.8 Hz, 1H), 3.80 (anti, s, 1H), 3.76
(syn, s, 1H), 3.64 (syn, qd, J = 7.2, 3.0 Hz, 1H), 3.12 (anti, brs, 1H), 1.16 (syn, d, J = 7.3
Hz,3H), 1.08 (anti, d, J = 7.2 Hz, 3H). 13C NMR (75 MHz, Chloroform-d, syn+anti) δ205.7,
204.8, 140.8, 140.4, 136.6, 135.6, 133.8, 133.7, 133.6, 133.1, 128.9, 128.8, 128.7, 128.6,
128.5, 128.1, 127.5, 76.1, 72.5, 47.97, 46.9, 15.8, 11.2.
3-Hydroxy-3-(4-cyanophenyl)-2-methyl-1-phenyl-1-propanone7
The product was obtained as white solid. 1H NMR (400 MHz, Chloroform-d) δ 7.92-7.86
(syn+anti, d, J = 7.5 Hz, 4H), 7.63-7.53 (syn+anti, m, 6H), 7.53-7.41 (syn+anti, m, 8H), 5.26
(syn, s, 1H), 5.02 (anti, d, J = 7.4 Hz, 1H), 4.02 (syn, brs, 1H), 3.79(syn+anti, m, 4H), 3.68
(anti, dd, dd, J = 7.0, 2.6 Hz, 1H), 1.14 (syn, d, J = 7.1 Hz, 3H), 1.08 (anti, d, J = 7.0 Hz,
3H). 13C NMR (75 MHz, Chloroform-d, syn+anti) δ 205.1, 204.3, 147.8, 147.3, 136.2, 135.3,
133.8, 133.6, 132.2, 132.1, 128.9, 128.8, 128.5, 128.4, 127.4, 126.9, 118.8, 118.7, 111.5,
111.0, 75.9, 72.5, 47.6, 46.64, 15.6, 11.2.
82
3-Hydroxy-3-(4-nitrophenyl)-2-methyl-1-phenyl-1-propanone177
The product was obtained as white solid. 1H NMR (400 MHz, Chloroform-d) δ 8.26-8.19
(syn+anti, m, 4H), 7.97-7.90 (syn+anti, m, 4H), 7.64-7.56 (syn+anti, m, 6H), 7.54-7.44
(syn+anti, m, 4H), 5.36 (syn, d, J = 2.3 Hz, 1H), 5.10 (anti, d, J = 6.8 Hz, 1H), 4.02 (syn, brs,
1H), 3.82 (anti, m, 1H), 3.69 (syn, qd, J = 7.4, 2.5 Hz, 1H), 3.50 (anti, s, 1H) 1.19 (anti, d, J
= 7.3 Hz, 3H), 1.16 (syn, J = 7.3 Hz, 3H). 13C NMR (75 MHz, Chloroform-d, syn+anti) δ
205.4, 204.5, 149.7, 149.3, 147.6, 147.3, 136.1, 135.2, 134.1, 133.9, 129.0, 128.9, 128.6,
128.5, 127.5, 127.0, 123.7, 123.6, 75.9, 72.4, 47.6, 46.6, 15.9, 11.1.
3-Hydroxy-3-(2-nitrophenyl)-2-methyl-1-phenyl-1-propanone
The product was obtained as white solid. 1H NMR (400 MHz, Chloroform-d) δ 8.05-7.96
(syn, m, 4H), 7.91-7.78 (anti, m, 4H), 7.58-7.49 (syn+anti, m, 4H), 7.49-7.34 (syn+anti, m,
6H), 5.81 (syn, d, J = 1.5 Hz, 1H), 5.56 (anti, d, J = 4.7 Hz, 1H), 4.42 (anti, brs, 1H), 4.23
(syn, brs, 1H), 4.11 (anti, qd, J = 7.1, 5.1 Hz, 1H), 4.01 (syn, dd, J = 7.3, 2.1 Hz, 1H), 1.39
(anti, d, J = 7.2 Hz, 3H), 1.17 (syn, d, J = 7.4 Hz, 1H). 13C NMR (75 MHz, Chloroform-d) δ
206.5, 205.6, 148.3, 147.4, 138.1, 137.1, 136.3, 135.5, 133.9, 133.8, 133.5, 133.3, 130.2,
129.8, 129.0, 129.8, 128.7, 128.4, 128.4, 128.3, 124.9, 124.6, 72.5, 68.7, 45.4, 44.2, 16.5,
11.02. HRMS-ESI: m/z 286.1060 ([C16H15NO4+H]+ calcd. 286.1074).
83
3-Hydroxy-2-methyl-1-phenyl-1-hexanone132
The product was obtained as yellow oil. 1H NMR (400 MHz, Chloroform-d) δ 7.99-7.91
(syn+anti, d, J = 7.2 Hz, 4H), 7.62-7.55 (syn+anti, t, J = 7.3 Hz, 2H), 7.52-7.45 (syn+anti, t,
J = 7.6 Hz, 4H), 4.10-4.02 (syn, m, 1H), 3.87 (anti, brs, 1H), 3.55 (anti, m, 1H), 3.48 (syn,
qd, J = 7.2, 2.9 Hz, 1H), 3.10 (syn, s, 1H), 2.92 (anti, s, 1H), 1.68-1.46 (syn+anti, m, 4H),
1.45-1.33 (syn+anti, m, 4H), 1.26 (syn+anti, d, J = 7.2 Hz, 6H), 0.94 (syn+anti, t, J = 7.2
Hz, 6H). 13C NMR (75 MHz, Chloroform-d, syn+anti) δ206.04, 205.98, 136.8, 136.1, 133.6,
133.5, 128.9, 128.85, 128.58, 128.5, 73.9, 71.2, 45.86, 44.68, 37.20, 36.6, 19.4, 19.1, 15.7,
14.2, 11.2.
84
Figure S4.5 1H NMR spectrum of 1-(trimethylsilyloxy)-cyclohexene.
Figure S4.6 1H NMR spectrum of 1-phenyl-1-(trimethylsiloxy)-propene.
85
Figure S4.7 1H NMR spectrum of 2-(hydroxyphenylmethyl)-cyclohexanone.
Figure S4.8 13C NMR spectrum of 1-(trimethylsilyloxy)-cyclohexene.
86
Figure S4.9 1H NMR spectrum of 3-hydroxy-2-methyl-1,3-diphenylpropan-1-one.
Figure S4.10 13C NMR spectrum of 3-hydroxy-2-methyl-1,3-diphenylpropan-1-one.
87
Figure S4.11 1H NMR spectrum of 3-hydroxy-3-(4-methoxyphenyl)-2-methyl-1-phenyl-1-propanone.
Figure S4.12 13C NMR spectrum of 3-hydroxy-3-(4-methoxyphenyl)-2-methyl-1-phenyl-1-propanone.
88
Figure S4.13 1H NMR spectrum of 3-hydroxy-3-(4-chlorophenyl)-2-methyl-1-phenyl-1-propanone.
Figure S4.14 13C NMR spectrum of 3-hydroxy-3-(4-chlorophenyl)-2-methyl-1-phenyl-1-propanone.
89
Figure S4.15 1H NMR spectrum of 3-hydroxy-3-(4-cyanophenyl)-2-methyl-1-phenyl-1-propanone.
Figure S4.16 13C NMR spectrum of 3-hydroxy-3-(4-cyanophenyl)-2-methyl-1-phenyl-1-propanone.
90
Figure S4.17 1H NMR spectrum of 3-hydroxy-3-(4-nitrophenyl)-2-methyl-1-phenyl-1-propanone.
Figure S4.18 13C NMR spectrum of 3-hydroxy-3-(4-nitrophenyl)-2-methyl-1-phenyl-1-propanone.
91
Figure S4.19 1H NMR spectrum of 3-hydroxy-3-(2-nitrophenyl)-2-methyl-1-phenyl-1-propanone.
Figure S4.20 13C NMR spectrum of 3-hydroxy-3-(2-nitrophenyl)-2-methyl-1-phenyl-1-propanone.
92
Figure S4.21 1H NMR spectrum of 3-hydroxy-2-methyl-1-phenyl-1-hexanone.
Figure S4.22 13C NMR spectrum of 3-hydroxy-2-methyl-1-phenyl-1-hexanone.
93
Chapter 5
Conclusions
5.1 General conclusions
In this thesis, we have achieved a versatile, straightforward and reproducible method to
synthesize iron-functionalized ordered mesoporous silicas. This method allows us to
precisely control and finely tune the pore size and modify the surface properties of the
resulting materials using a suitable metal precursor. The potential catalytic properties of the
final materials exhibiting active sites have been demonstrated by performing Mukaiyama
aldol reactions.
First, two different Fe-modified mesoporous silicas Fe-MCM-41 and Fe-SBA-15 were
synthesized by using two typical mesoporous silicas MCM-41 and SBA-15 with different
pore sizes, based on the modified post-grafting method by using Fe(acac)3 as metal precursor
and 1-propanol as the grafting solvent. According to the results obtained from the nitrogen
adsorption isotherms, the mesostructure of the support is preserved and relatively high BET
surface area and large pore volume are obtained after the inorganic grafting process. The
amount of the iron loaded in the final materials was determined by the initial concentration
of the grafting gel containing the iron precursor. Moreover, it was found that a pH adjustment
during the grafting process could strongly improve the retention of the iron species on the
surface of the silica through ligand exchange. However, the pH adjustment may induce the
formation of the small iron oxide clusters on the silica surface.
This kind of material is of significant importance because its Lewis acid active sites that
make it a promising heterogeneous catalyst for various important organic reactions.
Therefore, we demonstrated the acidity of the resulting Fe-MCM-41 and Fe-SBA-15 using
different Hammett indicators and distinguished the Lewis acid site from Brønsted sites by
pyridine adsorption accompanied with FT-IR measurement.
To further probe the application of Fe-MCM-41 and Fe-SBA-15 in catalysis, both materials
were used as solid catalysts in the next step of the thesis. The Mukaiyama aldol reaction, as
94
a typical Lewis acid catalyzed reaction, was chosen as the representative reaction. A wide
variety of reactions of benzaldehyde with different silyl enol ethers at ambient temperature
in an aqueous environment using Fe-modified mesoporous silicas as catalysts were carried
out under different conditions to investigate the effect of catalyst amount, solvent, surfactant
and iron content. It was shown that Fe-modified mesoporous silica exhibits good catalytic
activity and selectivity towards Mukaiyama aldol reaction under aqueous conditions. More
importantly, it could be easily separated from the reaction mixture and reused as a catalyst
for at least nine times without loss of activity and selectivity, indicating a satisfying stability.
Finally, the effect of the silanol groups present on the surface of silica for the activity was
verified by coating a silylated layer on the silica surface. It was expected that the silylating
producing strong surface hydrophobicity, which may facilitate the diffusion of the organic
reactants, increasing the possibility of the contact between the hydrophobic reactants and the
active Lewis acid sites, leading to a higher yield. In summary, Fe-modified mesoporous silica
is an efficient, easy-removable and highly active solid Lewis acid catalyst for the Mukaiyama
aldol reaction.
5.2 Future prospect
The study of iron-modified mesoporous materials for Mukaiyama aldol reactions with
various substrates provided very promising results and opens the gate to this kind of Lewis
acid solid catalysts, since nowadays, research on solid acids is one of the hottest topics in
Green Chemistry and the application of supported catalysts. Therefore, there is still some
work that needs to be done in the future. First, the catalyst activity and the reaction kinetics
should be further investigated. For example, the reaction rates can be obtained at various
temperatures and concentrations. It is appropriate to determine the effect of the mass transfer
limitation or the diffusion in different phases by performing the kinetics study. Another way
to measure the catalyst activity is to determine the Lewis acidity or the density of the Lewis
acid sites of the catalyst. TON of the reaction can provide a straightforward measurement of
the catalytic activity of the catalyst. In addition, the catalytic scale of the catalyst should be
further explored through application to various chemical transformations catalyzed by Lewis
acids in organic chemistry, Friedel–Crafts alkylation, and carbonyl-ene reactions. Other
reactions such as azidolysis reactions, Diels-Alder reactions, Michael reactions and allylation
95
reactions could also be tested as probe catalytic reactions in aqueous media, since the iron-
modified mesoporous material has been proven to be a water-tolerant catalyst in this thesis.
Furthermore, more complicated architectures of the catalysts can be considered. For example,
the core-shell materials of iron-modified mesoporous materials (Metal-modified mesoporous
materials) coating a microporous material on the outer shell can be prepared to explore if the
diffusion of the reagents could be controlled by the specific pore size of the outer shell, how
the pore size of the outer shell affects the catalytic activity and if it can possess the selectivity
for different reactions.
96
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