REVIEWS

Advances in Mesoporous Molecular Sieve MCM-41

Xiu S. Zhao,† G. Q. (Max) Lu,*,† and Graeme J. Millar‡

Department of Chemical Engineering and Department of Chemistry, The University of Queensland,Brisbane, Queensland 4072, Australia

The discovery of mesoporous molecular sieves, MCM-41, which possesses a regular hexagonalarray of uniform pore openings, aroused a worldwide resurgence in this field. This is not onlybecause it has brought about a series of novel mesoporous materials with various compositionswhich may find applications in catalysis, adsorption, and guest-host chemistry, but also it hasopened a new avenue for creating zeotype materials. This paper presents a comprehensiveoverview of recent advances in the field of MCM-41. Beginning with the chemistry of surfactant/silicate solutions, progresses made in design and synthesis, characterization, and physicochemicalproperty evaluation of MCM-41 are enumerated. Proposed formation mechanisms are presented,discussed, and identified. Potential applications are reviewed and projected. More than 100references are cited.

Contents1. Introduction 20752. General Background 20763. Chemistry of Surfactant/Silicate

Aqueous Solution2077

3.1. Behavior of SurfactantMolecules in an AqueousSolution

2077

3.2. Chemistry of Silicates/Aluminosilicates in an AlkalineAqueous Solution

2077

4. Synthesis of MCM-41 20784.1. Expanding Synthesis Conditions 20784.2. Synthesis of Aluminum-Rich

MCM-412079

4.3. Synthesis of Hybrid AtomMCM-41

2080

5. Formation Mechanism 20805.1. Liquid Crystal Templating (LCT)

Mechanism2080

5.2. Transformation Mechanismfrom Lamellar to HexagonalPhase

2081

5.3. “Folded Sheets” Mechanism 20835.4. Identifying the Formation

Mechanism2083

6. Characterization, PhysicochemicalProperties, and Structure Model

2083

6.1. Characterization and StructureModel

2083

6.2. Acidity 20846.3. Stability 20846.4. Interaction with Water 2085

7. Application 20857.1. Catalysis 20857.2. Ship in a Bottle 20867.3. Model Adsorbent 2086

8. Perspective 20868.1. Host -Guest Encapsulation 20868.2. Modification 20878.3. Adsorbent 20878.4. Environment 2087

9. Acknowledgment 208710. Literature Cited 2087

1. Introduction

Over the past 15 years, there has been a dramaticincrease in the literature of design, synthesis, charac-terization and property evaluation of zeolites and mo-lecular sieves for catalysis, adsorption and separation,environmental pollution control, and intrazeolite fab-ricating technology. Progress has been made in avariety of disciplines, including inorganic and materialschemistry (Davis and Lobo, 1992; Ozin, 1992), mineral-ogy and crystallography (Smith, 1988), petrochemistry(Chen et al., 1989; Corma, 1995a), environmental sci-ence (Armor, 1992; Iwamoto, 1994; Tabata, 1994), andeven biochemistry (Mann, 1993). As excellent examples,bicontinuous phases have been used to orient organicspecies which are then photopolymerized (Davis, 1993).After the extraction of the unpolymerized components,porous organic monoliths resulted. This process has ledto a new concept of self-assembled microstructureserving as structure-directing agents. Again, materialscientists have used large organic molecules to inter-calate into layered clays to generate novel materialswith controllable pore size (Yanagisawa et al., 1990).Biochemists have established a model that makes useof the cooperative organization of inorganic and organic

* Author to whom all correspondence should be addressedat the Department of Chemical Engineering, The Uni-versity of Queensland, Brisbane, Queensland 4072, Australia.Phone: 61 7 33653708. Fax: 61 7 33654199. E-mail: maxlu@cheque.uq.edu.au.

† Department of Chemical Engineering.‡ Department of Chemistry.

2075Ind. Eng. Chem. Res. 1996, 35, 2075-2090

S0888-5885(95)00702-0 CCC: $12.00 © 1996 American Chemical Society

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molecular species into three dimensionally structuredarrays for the synthesis of nanocomposite materials(Mann, 1993). Molecular sieve scientists have takenadvantage of this concept to generate novel inorganicmaterials, which resulted in the discovery of the newfamily of mesoporous molecular sieves M41S (Beck etal., 1992; Kresge et al., 1992).MCM-41, one member of the M41S family, possesses

a regular hexagonal array of uniform pore openings witha broad spectrum of pore diameters between 15 and 100Å (Beck et al., 1992a; Kresge et al., 1992). The meso-porous structure can be controlled by a sophisticatedchoice of templates (surfactants), adding auxiliary or-ganic chemicals (e.g., mesitylene) and changing reactionparameters (e.g., temperature, compositions). Hence,numerous studies of this novel materials have beenpublished since its discovery. However, there lacks amultiple review on MCM-41 except the brief report(Casci, 1994) which was aimed at summarizing the workon ultra large pore molecular sieves and related zeotypematerials (pillared layered clays).This paper is intended to provide a comprehensive

overview on synthesis, formation mechanisms, charac-terization, modification, and applications of MCM-41 inorder to identify significant trends.

2. General Background

Zeolite was first discovered in 1756 by the Swedishscientist Cronstedt when an unidentified silicate min-eral was heated in a blowpipe flame which fused readilywith marked fluorescence. He called, therefore, thismineral zeolite (in Greek, zeo ) boil and litho ) stone).The term molecular sieve was derived from McBain(1932) when he found that chabazite, a mineral, had aproperty of selective adsorption of molecules smallerthan 5 Å in diameter. Since then the nomenclature ofthis kind of porous material seems to be ambiguous. Thesuccess of synthetic crystalline aluminosilicates, inparticular the emergence of the new family of alumino-phosphates (Wilson et al., 1982) and silicoaluminophos-phates (Lok et al., 1984), made the concept of zeoliteand molecular sieves more intricate. In a broad sense,zeolites are molecular sieves. Strictly speaking, zeolitesare crystalline aluminosilicates with molecular sieveproperties. Hence, MCM-41 is called a molecular sieve.According to the definition of IUPAC, mesoporous

materials are those that have pore diameters between20 and 500 Å. Table 1 typically lists some zeolites andmolecular sieves with variable pore sizes, mostly in themicroporous range prior to the discovery of MCM-41.There has been, however, an ever growing interest

in expanding the pore sizes of zeotype materials from

the micropore region to mesopore region in response tothe increasing demands in both industrial and funda-mental studies. Examples are treating heavy feeds (e.g.,processing of tar sand from the high distillates of crudeoils to valuable low-boiling products), separating andsynthesizing large molecules (e.g., protein separationand selective adsorption of large organic molecules fromwaste water) and intrazeolite fabricating technology(e.g., supramolecular assembly of molecular arrays,metal complexes encapsulated in zeolite frameworks,and introduction of nanometer particles into zeolites andmolecular sieves for electronic and optical applications)(Davis, 1992; Mitchell, 1994; Ozin, 1992). Therefore,numerous works to create zeotype materials with porediameters larger than those of the traditional zeoliteswere carried out. It was not until 1982 (Wilson et al.,1982) that success was achieved by changing thesynthesis gel compositions when the first so-called ultralarge pore molecular sieve, AlPO4-8, which contains 14-membered rings, was discovered (yet its structure wasonly solved in 1990 (Dessau et al., 1990)). Indeed, thisnot only broke the deadlock of the traditional viewpointthat zeolite molecular sieves could not be constructedwith more than 12-membered rings, but also stimulatedfurther investigations on other ultra large pore molec-ular sieves, such as VPI-5 (Davis et al., 1988), cloverite(Estermenn et al., 1991), and JDF-20 (Jones et al.,1993). However, these materials have not found, up tillnow, any significant applications because of either theirinherently poor stability or their weak acidity. As aconsequence, they seem to be inferior compared topillared layered clays.In 1990, Yanagisawa et al. (1990) reported the

synthesis of the mesoporous materials characteristic ofMCM-41. Through intercalation of long-chain (typicallyC16) alkyltrimethylammonium cations into the layeredsilicate kanemite followed by calcination to remove theorganic species, a mesoporous material was obtained.The silicate layers condensed to form a three-dimen-sional structure with “nanoscale pores”. 29Si solid-stateNMR indicated that a large number of Q3 species wereconverted to Q4 species during the intercalation andcalcination process. X-ray powder diffraction gave onlyan uninformative intensity centered at extreme lowangles. Unfortunately, no further characterization datawere available, so Yanagisawa et al.’s results have beenignored.In 1992, researchers at Mobil Corporation discovered

the M41S family of silicate/aluminosilicate mesoporousmolecular sieves with exceptionally large uniform porestructures (Beck et al., 1992a; Kresge et al., 1992),which has resulted in a worldwide resurgence in this

Table 1. Pore Size Definition of Zeolites and Molecular Sieves

pore size (Å) definition typical material ring size pore diameter (Å) reference

>500 macroporous20-500 mesoporous MCM-41 15-100 Beck et al., 1992a<20 microporous

ultralarge pore cloverite 20 6.0 × 13.2 Estermann et al., 1991JDF-20 20 6.2 × 14.5 Jones et al., 1993VPI-5 18 12.1 Davis et al, 1988AlPO4-8 14 7.9 × 8.7 Dessau et al., 1990

large pore faujasite 12 7.4 Olson, 1970AlPO4-5 12 7.3 Bialek et al., 1991ZSM-12 12 5.5 × 5.9 Fyte et al., 1990

medium pore ZSM-48 10 5.3 × 5.6 Schlenker et al., 1978ZSM-5 10 5.3 × 5.6 van Koningsveid et al., 1990

5.1 × 5.5small pore CaA 8 4.2 Meier and Olson, 1987

SAPO-34 8 4.3 Lok et al., 1987

2076 Ind. Eng. Chem. Res., Vol. 35, No. 7, 1996

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area. The template agent used is no longer a single,solvated organic molecule or metal ion, but rather a self-assembled surfactant molecular array as suggestedinitially (Beck et al., 1992a). Three different meso-phases in this family have been identified, i.e., lamellar(Dubois et al., 1993), hexagonal (Beck et al., 1992a), andcubic phases (Vartuli et al., 1994), in which the hex-agonal mesophase, MCM-41, possesses highly regulararrays of uniform-sized channels whose diameters arein the range of 15-100 Å depending on the templatesused, the addition of auxiliary organic compounds, andthe reaction parameters. The pores of this novel mate-rial are nearly as regular as, yet considerably largerthan those present in crystalline materials such aszeolites, thus offering new opportunities for applicationsin catalysis (Corma et al., 1995b; Kozhevnikov et al.,1995; Kloetstra and van Beckkum, 1995; Armengol etal., 1995), chemical separation (Thomas, 1994), adsorp-tion media (Rathousky et al., 1994, 1995; Llewellyn etal., 1995; Branton et al., 1993, 1994, 1995; Feuston etal., 1994), and advanced composite materials (Huber etal., 1994; Wu and Bein, 1994a-c; Abe et al., 1995).Accordingly, MCM-41 has been investigated extensivelybecause the other members in this family are eitherthermally unstable or difficult to obtain (Vartuli et al.,1994).

3. Chemistry of Surfactant/Silicate AqueousSolution

Reaction gel chemistry is considered to play animportant role in zeolite synthesis (Feijen et al., 1994).It is, therefore, not surprising that much effort has beenmade to elucidate the gel chemistry related to theformation mechanisms and eventually to the resultantproducts for MCM-41 synthesis (Beck et al., 1994;Vartuli et al., 1994; Chen et al., 1993; Huo et al.,1994a,b). In this sense, knowledge of the chemistry ofsurfactant/silicate solution is a prerequisite for under-standing the synthesis and mechanisms responsible forthe formation of MCM-41 from its precursors.3.1. Behavior of Surfactant Molecules in an

Aqueous Solution. In a simple binary system ofwater-surfactant, surfactant molecules manifest them-selves as very active components with variable struc-tures in accordance with increasing concentrations, asschematically shown in Figure 1. At low concentrations,they energetically exist as monomolecules. With in-creasing concentration, surfactant molecules aggregatetogether to formmicelles in order to decrease the systementropy. The initial concentration threshold at whichmonatomic molecules aggregate to form isotropic mi-celles is called cmc (critical micellization concentration).As the concentration process continues, hexagonal close

packed arrays appear, producing the hexagonal phases(Lawrence, 1994). The next step in the process is thecoalescence of the adjacent, mutually parallel cylindersto produce the lamellar phase. In some cases, the cubicphase also appears prior to the lamellar phase. Thecubic phase is generally believed to consist of complex,interwoven networks of rod-shaped aggregates (From-herz, 1981).According to Myers (1992), the particular phase

present in a surfactant aqueous solution at a givenconcentration depends not only on the concentrationsbut also on the nature of itself (the length of thehydrophobic carbon chain, hydrophilic head group, andcounterion) and the environmental parameters (pH,temperature, the ionic strength, and other additives).This is reflected by the effect of the aforementionedmatters on cmc. Generally, the cmc decreases with theincrease of the chain length of a surfactant, the valencyof the counterions, and the ion strength in a solution,respectively. On the other hand, it increases withincreasing counterion radius, pH, and temperature. Forexample, in an aqueous solution at 25 °C, the cmc isabout 0.83 mM for surfactant C16H33(CH3)3N+Br-;between the cmc and 11 wt %, small spherical micellesare present; in the concentration range of 11-20.5 wt%, elongated flexible rodlike micelles are formed (Chenet al., 1993b); hexagonal liquid crystal phases appearin the concentration region between 26 and 65 wt %,followed by the formation of cubic, lamellar, and reversephases with increasing concentration (Lawrence, 1995).At 90 °C, the hexagonal phase is observed at a surfac-tant concentration of more than 65% (Steel et al., 1994).Many characterization techniques can be employed

to identify the individual aggregate state, among whichthe 14N NMR is one of the most effective means. Figure2 shows the 14N NMR spectra for cetyltrimethyl-ammonium chloride (CTACl) in aqueous solution at 90°C (Steel et al., 1994). It is clear that with the increasein concentrations of surfactant CTACl the phase se-quence is isotropic micelles f hexagonal phase fhexagonal + cubic phase f cubic + intermediate phasef lamellar phase.3.2. Chemistry of Silicates/Aluminosilicates in

an Alkaline Aqueous Solution. As many as 25individual silicate structures in an alkaline solutionhave been identified so far, as illustrated in Figure 3(Swaddle et al., 1995), which are deemed as importantspecies for zeolite and molecular sieve synthesis. It hasbeen found that the distribution of these anionic silicatespecies are sensitive to pH, temperature, cation, and Siconcentration. A reduction in silicon concentration orincrease in temperature or pH favors the formation ofmonomer and small oligomers (Kinrade and Swaddle,

Figure 1. Phase sequence of surfactant-water binary system (Myers, 1992; Lawrence, 1995).

Ind. Eng. Chem. Res., Vol. 35, No. 7, 1996 2077

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1988). When the ratio of SiO2/Na2O is more than 1, forinstance, the major form of Si is Q3 [Si(OSi)3OH] andQ2 [Si(OSi)2(OH)2] connectivity. Small anions, such asthe cyclic trimer, are found at high concentrations of Siinstead of the larger anions (McCormick et al., 1989).Some other silicate species, such as the hexagonal prism(Q3

12) and the four-way connectivity Q4 [Si(OSi)4] spe-cies, characteristic of many zeolite structures are, onthe contrary, generally not observed in an alkalineaqueous solution.The aforementioned silicate species can react with

monomeric Al(OH)4- species (the only species in alkalinesolution with pH ) 7-13) to produce aluminosilicate

intermediates with the same structures responsible forthe formation of zeolites and molecular sieves. That is,the aluminosilicate species are much like those of thenumerous silicate oligomers shown in Figure 3. 27AlNMR has revealed that an increase in the SiO2/Al2O3ratio favors the reaction between these two species(Muller et al., 1981). This means that large oligomersof silicate easily react with aluminate species becauseincreasing silicate concentration results in the largeextent of oligomerization. It has also been found thatboth the solubility and the distribution of alumino-silicate structures can be altered upon adding organicalkylammoniums (Engelhardt and Mickel, 1987). Forinstance, when tetramethylammonium hydroxide(TMAOH) is used to dissolve SiO2, the predominantspecies are D3R (Q3

6), D4R (Q38), and D5R (Q3

10).Additionally, increasing the length of the alkyl chainsof organic species often results in the predominance ofthe D3R anionic species (McCormick et al., 1989). Thisreveals that the nature of the amine species exerts akey influence on the distribution of silicate/alumino-silicate species. Undoubtedly, the anion-cation pairingis one of several factors acting on the resulting speciesdistributions (Hendricks et al., 1991), as well as theresultant products when considering zeolite synthesis.

4. Synthesis of MCM-41As the traditional zeolite and molecular sieve syn-

thesis, preparation of MCM-41 materials is also madeby mixing organic amine (surfactants), silica, and/orsilica-alumina source to form a supersaturated solutionwhile maintaining the mixture at a temperature be-tween 70 and 150 °C for selected periods of time. Sincethe first report of MCM-41 (Beck et al., 1992a; Kresgeet al., 1992), a number of patents (see recent review ofCasci (1994)) and publications (see Table 2) on thesynthesis of MCM-41 have been reported. Table 2attempts to list the publications concerning synthesisof MCM-41, and it is shown that the majority ofinterests in the previous reports were aimed at expand-ing synthesis conditions, generating acidity, synthesiz-ing hybrid atom MCM-41, and identifying synthesismechanisms.4.1. Expanding Synthesis Conditions. It has

been demonstrated that the preparation of MCM-41materials can be achieved by using a broad spectrumof surfactants and a wide range of synthesis conditions(e.g., temperatures, pH, reaction time) (Huo et al.,1994a; Monnier et al., 1993; Tanev et al., 1994). Table3 lists four possible routes through which the hexagonalMCM-41 materials with various compositions have beenobtained. Route I, the initially discovered pathway, androute II, expanded by Huo et al. (1994a), are easilyimagined. In the case of route III, the cooperativeassembly of S+H-I+ complexes was suggested throughthe mediation of the counterion H- (Huo et al., 1994a,b). Route IV, postulated by Tanev and Pinnavaia(1995), was proposed to be templated by the neutralammine liquid crystal.The various routes for creating MCM-41 materials

offer the possibility to generate mesophases with variouscompositions and characteristics. Such materials areanticipated to find main applications in electrochromicor solid electrolyte devices (Lampert and Ganqvist,1990), high surface area redox active catalysts (Thomas,1994), as well as biochemical and pharmaceuticalseparation matrices (Fisher, 1995).Interestingly, the pore sizes or interlayer distances

can also be tailored by changing the chain length of

Figure 2. 14N NMR spectra of CTACl aqueous solution withdifferent concentrations at 90 °C (Steel et al., 1994).

Figure 3. Silicate species that have been identified by 29Si NMRin alkali aqueous solution (Swaddle et al., 1995). Filled circlesrepresent tetrahedral Si atoms; lines represent Si atoms linkedthrough O atoms.

2078 Ind. Eng. Chem. Res., Vol. 35, No. 7, 1996

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alkylhexadecyldimethylammonium, i.e., (C16H33)-(CnH2n+1)(CH3)2N+ (Sayari et al., 1995a). In this sys-tem, a hexagonal phase was obtained for n ) 1, 3, 5,and 7, while a lamellar phase was obtained for all othertemplates (see Figure 4). The d100 spacing of thehexagonal MCM-41 increases with the increasing car-bon number (n) at a rate of 2.45 Å/C atom, which ismuch higher than the 1 Å/C atom reported by Beck etal. (1992a) for a series of calcined samples. Surpris-ingly, the d100 spacing of the lamellar phases was almostconstant at ca. 32.5 Å regardless of the n value,indicating that this distance of lamellar phases iscontrolled by the C16 carbon chain only.Ryoo et al. (1995) reported the synthesis of highly

ordered MCM-41 by adding acetic acid to the reactionsystem to shift the equilibrium between the reactantsand the mesophases formed toward to the desireddirection. The authors suggested that MCM-41 phasewas in dynamic equilibrium with the reactants. Addi-tion of acid to the reaction mixture was proposed toneutralize the hydroxyl so as to shift the equilibriumtoward the positive direction.We have synthesized MCM-41 in an alkali-metal-free

system using aqueous ammonia solution to adjust thepH of the reaction gel Zhao et al. (1995a). The resultantproduct was found to exhibit higher catalytic activityfor n-heptane cracking compared to those obtained fromthe sodium-containing system. This was explained thatmore mild acidic sites could be generated from thesample synthesized in an alkali-metal-free system thanthose synthesized in a sodium-containing system.Other modified synthesis methods, such as low tem-

perature synthesis (Elder et al., 1995; Zhao and Gold-fard, 1995) and simplifying the reactants (Chen et al.,1993a), are also found in the literature.4.2. Synthesis of Aluminum-Rich MCM-41. Acid

sites, in particular the Brønsted ones (from the tetra-hedrally coordinated Al), are the active locus for mosthydrocarbon reactions. Therefore, incorporating tetra-hedrally coordinated aluminum to the framework ofMCM-41 has been the subject of much attention.Schmidt et al. (1994a) reported the synthesis of MCM-41 with Si/Al ratios as low as 8.5. No disorderedaluminum was found either in the as-synthesized or thecalcined sample (540 °C for 7 h) as characterized by 27AlMAS NMR. Borade et al. (1995) reported the synthesisof MCM-41 with a Si/Al ratio as low as 2, withoutobserving the presence of octahedral Al in the 27Al MASNMR using sodium aluminate as the aluminum source.However, no data concerning the acidity were given inthe above authors’ works. In fact, one is most interestedin acidity rather than the Si/Al ratios.Luan et al. (1995) have extensively investigated the

influence of the aluminum source upon the resultingcoordination state by examining the 27Al NMR and 29SiNMR spectra. They found that when Captal aluminumor sodium aluminate was used, virtually all Al in theproduct was 6-coordinate, whereas 4-coordinate alumi-num in the MCM-41 framework with a Si/Al ratio aslow as 10 could only be obtained by using aluminumsulfate as the aluminum source. On the other hand,Corma et al. (1994b) and Chen et al. (1993a) have foundthat the most active aluminum source for the formationof MCM-41 is sodium aluminate. In our laboratory(Zhao et al., 1994), we have also examined the effect ofaluminum source for synthesis of MCM-41 in a hydro-thermal system of 4.0Na2O:29.2SiO2:Al2O3:6.0CTACl:900H2O and discovered that highly ordered MCM-41T

able

2.Rep

orts

onMCM-41Syn

thesis

reference

molar

ratioofgelcom

positions

temperature

(°C)

time

(h)

summary

Kresgeetal.,1992

2.6O

H-:30SiO

2:Al 2O3:6.3C

TAa :8.4T

MAb :382H

2O150

48synthesisofMCM-41vialiquidcrystalm

echanism

Becketal.,1992a

2.6O

H-:30SiO

2:Al 2O3:6.3[CnH2n

+1(CH3)3N

+]c:8.4TMA:382H

2O150

48relationshipbetweensurfactantchainlengthandpore

diam

eter;

roleofauxiliaryorganized

Chen

etal.,1993a

30SiO

2:0.4A

l 2O3:2.6C

TA:4.5TMAOH:500H

2O70

-150

24-240

synthesisandcharacterization

Vartulietal.,1994

30SiO

2/CTAOH

)2

100

48influence

ofsurfactant/silica

molar

ratioon

mesophase

Huoetal.,1994a

26.9H

+:30SiO

2:3.5C

TA:3800H

2Oam

bient

0.5-

24synthesisofMCM-41inan

acidicmedium(pH

)1-

5)Becketal.,1994

30SiO

2/15[C

nH2n

+1(CH3)3N

+]d

)2,pH

)10,

11wt%surfactantoftotalm

ixture

100-

200

influence

oftemperature

andsurfactantchainlengthon

mesophases

Tanev

etal.,1994

0.6H

+:30SiO

2:192C

2H5OH:8DDAe :1060H

2Oam

bient

18synthesisusingprimaryam

ineat

room

temperature

inacidicmedia

Reddy

etal.,1994

5.1N

a 2O:30SiO

2:0.6V

O2:15CTMA:900H2O

100

144

synthesisofV

-MCM-41

Cormaetal.,1994a

30SiO

2:0.5T

iO2:5.2C

TA:7.6TMAOH:715H

2O140

28synthesisofTi-MCM-41insodium-freesystem

Zhao

etal.,1995a

4.0(NH4)2O:30SiO

2:Al 2O3:6C

TA:700H

2O100

72synthesisofMCM-41inan

alkali-freesystem

Schmidtetal.,1995a

13OH

-:30SiO

2:xA

l 2O3:2.1C

14H29(CH3)3N

+:500H2O

100

24synthesisofMCM-41withhighlytetrahedralaluminum(Si/A

l)8.5)

Boradeetal.,1995

6.5O

H-:30SiO

2:Al 2O3:6.9C

TA:2.7TMA:3660H

2O100

16-70

synthesisofMCM-41withSi/A

lratioas

lowas

2Luan

etal.,1995

7.8O

H-:30SiO

2:Al 2O3:8C

TA:7.8TMA:1755H

2O150

48influence

ofaluminumsourceon

coordinationstate

Ryooetal.,1995

15OH

-:30SiO

2:Al 2O3:5C

TMA:1125H

2O100

72synthesisofhighlyorderedMCM-41by

addingaceticacid

Sayarietal.,1995a

2.3O

H-:30SiO

2:x(Ti,V

)O2:5.8[(C

16H33)(CnH2n

+1)-

(CH3)2N

+]f :7.6T

MAOH:2165H

2O100-

150

30-168

synthesisof(Ti)MCM-41inasodiumsystem

usingsurfactantwith

twoalkylchains

Yuan

etal.,1995

15OH

-:30SiO

2:0.3F

e 2O3:6C

TA

150

168-

240

synthesisofFe-

MCM-41

Sayarietal.,1995b

2.5N

a 2O:30SiO

2:(6.25-

∞)BO2:4.8C

TA:1890H

2O100

24synthesisofB

-MCM-41

Zhao

andGoldfarb,1995

(4.7

-14.3)OH

-:30SiO

2:(0.01-

2.6)MnO:3.5CTA:4000H

2O21

-100

72synthesisofMn

-MCM-41

aCTA

)cetyltrimethylam

monium[C

16H33(CH3)3N

+].

bTMA

)(CH3)4N

+.cn

)8,9,10,12,14,16.dn

)6,8,10,12,14,16.eDDA

)dodecylamine[CH3(CH2)11NH2].fn

)1-

12.

Ind. Eng. Chem. Res., Vol. 35, No. 7, 1996 2079

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can be synthesized from gels with a Si/Al ratio as lowas 14 by using sodium aluminate rather than aluminumsulfate. In contrast, when aluminum sulfate is used,highly ordered MCM-41 could only be obtained if theSi/Al ratio was as high as 40, which is in good agreementwith the results reported by Corma et al. (1994b) andChen et al. (1993a).4.3. Synthesis of Hybrid Atom MCM-41. Isomor-

phous substitution of T atoms (T ) Si, Al) by otherelements can generate a new hybrid atom molecularsieve with interesting properties. A typical example istitanium-containing ZSM-5 zeolite (TS-1), which is aneffective catalyst for selective catalytic oxidation oforganic compounds in the presence of hydroperoxide(Notari et al., 1988). The range of reactive compoundswas greatly limited by the pore sizes of the traditionalzeolites and molecular sieves prior to the invention ofMCM-41. Therefore, titanium-containing MCM-41 mo-lecular sieves have attracted much attention becausetheir large pore size can permit almost all organicreactants, intermediates, and products to easily pass inand out. Titanium-substituted MCM-41 has been syn-thesized in an acidic system by using either ionicsurfactants (CTA+) or primary amine (DDA) as template(Tanev et al., 1994). The Ti-containing mesophasesobtained from the two different templates differ greatlyin textural mesoporosity reflected by both the X-raydiffraction (XRD) patterns and the nitrogen adsorption-desorption isotherms. The catalytic activities of thesesamples were also remarkably distinct (vide infra).Corma et al. (1994a) have synthesized Ti-MCM-41 ina sodium-free system. A band at 960 cm-1 in the IRspectra and two bands at 210-230 nm in the UV-visspectra were taken as proof for Ti incorporation into theframework of MCM-41 without isolated Ti atom.V-MCM-41 (Reddy et al., 1994), Mn-MCM-41 (Zhaoand Goldfard, 1995), Fe-MCM-41 (Yan et al., 1995), and

B-MCM-41 (Sayari et al., 1995b) have also been suc-cessfully synthesized.

5. Formation Mechanism

The most outstanding feature of preparation of MCM-41, in contrast to the traditional single organic moleculeor metal ion templating preparation, is that the tem-plates used are surfactants, having an alkyl chainlength of greater than 6 carbon atoms (in most casesgreater than 10 carbon atoms). Therefore, the mecha-nisms responsible for the formation of MCM-41 from itsprecursors have attracted much attention. Two typicalmechanisms have been proposed so far (Beck et al.,1992a; Monnier et al., 1993), accompanied by othermodified routes (Inagaki et al., 1994; Steel et al., 1995).It is well-known that formation of the traditionalzeolites and molecular sieves is templated by singlemolecules or ions. Here we take ZSM-5 as an exampleas shown in Figure 5. The crystallization is via silicatecondensation around a tetrapropylammonium cation.The initial ordered species may consist of an aggregateof water molecules or silicate moieties (Kerr, 1966;Feijen, 1994). Subsequent growth proceeds because ofnucleation by this initial structure or assembly of anumber of such structures, but crystal growth is theresult of the initial silicate organization. In the case ofMCM-41 formation, however, Beck et al. (1992a) ini-tially proposed the liquid crystal templating mechanism;i.e., supermolecular arrays preformed are the templatesthat act as the structure-directing agents (see Figure6).5.1. Liquid Crystal Templating (LCT) Mecha-

nism. A key feature of the LCT mechanism is that theliquid crystalline mesophases or micelles act as tem-plates rather than individual single molecules or ions.Accordingly, the final product is a silicate skeletonwhich contains voids that mimics these mesophases.The silicate condensation is not the dominant factor inthe formation of the mesoporous structure. The wholeprocess may be via two possible mechanistic pathwaysas schematically shown in Figure 6: (1) the liquidcrystal mesophases may form prior to the addition ofsilicate species; (2) the silicate species added to thereaction mixture may influence the ordering of theisotropic rodlike micelles to the desired liquid crystalphase, i.e., hexagonal mesophase. Therefore, the meso-phase formed is structurally and morphologically di-rected by the existing liquid crystal micelles and/ormesophases.The influence of alkyl chain length and the addition

of mesitylene on the pore size have been taken as strongevidence for the LCT mechanism, since this phenom-enon is consistent with the well-documented surfactantchemistry (Winsor, 1966; Myers, 1992). The auxiliaryorganic species added to the reaction gel can be solu-bilized inside the hydrophobic regions of micelles, caus-ing an increase in micelle diameter so as to increasethe pore size of MCM-41. The observed pore size

Table 3. Five Possible Routes for Generating Mesophase

routea typical example pH resulting phase reference

I (S+I-) CTA + silicate species <7 or 10-13 hexagonal, cubic, and lamellar Beck et al., 1992a; Huo et al., 1994a,bII (S-I+) C16H33SO3

- + lead oxide hexagonal Huo et al., 1994a,bC12H25PO4

2- + ion oxide 1-5 lamellarIII (S+H-I+) CTABr + silicate species <2 hexagonal Huo et al., 1994a,b

CTABr + zincophosphate <3 lamellarIV (S0I0) C12H25NH2 + (C2H5O)4Si <7 hexagonal Tanev et al., 1995a S+ ) surfactant ions; I- ) anionic inorganic species; H- ) anionic halides; M+ ) cationic alkaline ions.

Figure 4. Influence of another alkyl chain length (n) of (C16H33)-(CnH2n+1)(CH3)2N+ on the mesophase and the d100 spacing (Sayariet al., 1995a).

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increase of the aluminosilicates compared to the sili-ceous MCM-41 was due to the replacement of theshorter Si-O bonds (1.6 Å) by the longer Al-O bonds(1.75 Å) (Beck et al., 1992a).The LCT mechanism has been further confirmed by

subsequent reports (Chen et al., 1993a; Beck et al., 1994;Vartuli et al., 1994). Vartuli et al. (1994) have studiedthe effect of surfactant/silica molar ratio on the resultantphases in a simple system containing alkali metal,tetraethylorthosilicate, water, and CTAOH at 100 °C.They found that as the surfactant/silica molar ratioincreased from 0.5 to 2, the siliceous products obtainedcould be classified into four separate groups: MCM-41(hexagonal), MCM-48 (cubic), thermally unstable lamel-lar phase, and the cubic octamer [(CTMA)SiO2.5]8. Thedata are in excellent agreement with the behavior ofsurfactants in solution as mentioned above (Myers,1992). The influence of alkyl chain length (C6-C16) andthe reaction temperatures (100-200 °C) on the resultingproducts was also extensively investigated by Beck etal. (1994). Figure 7 shows the XRD patterns of theproducts synthesized with different surfactants and atdifferent temperatures. At 100 °C, with the increaseof the surfactant alkyl chain length, the products arein the sequence of amorphous materialsf poorly definedMCM-41f well-defined MCM-41. At 150 °C, ZSM-5zeolite with high crystallinity was generated from boththe C6 and C8 surfactants. The C10 surfactant prepara-tion produced less well-defined MCM-41 compared withthe C12, C14, and C16 preparation. At 200 °C, ZSM-5was synthesized from C6; a mixture of ZSM-5, ZSM-48,and dense phases from C8-14 surfactants; and amor-phous phases from C16. These data are also in goodagreement with the surfactant solution chemistry(Winsor, 1968; Myers, 1992). Another interesting de-duction from their study was that mesoporous andzeolite materials were not coproduced in the investi-gated temperature range using C6-16 surfactants astemplates. This behavior was explained in terms of the

formation mechanism of MCM-41 which is quite distinctfrom that of the traditional zeolite templating mecha-nism.Chen et al. (1993b) have investigated the formation

mechanism by employing XRD, 29Si NMR, in situ 14NNMR, and thermogravimetric analysis (TGA) tech-niques. No hexagonal liquid crystalline mesophaseswere detected either in the synthesis gel or in thesurfactant solution used as template in their study. Thishas also been confirmed by a later study (Steel et al.,1994). It was, therefore, concluded that formation ofMCM-41 phase is possibly via pathway 2 (see Figure 6)rather than pathway 1. That is, the randomly orderedrodlike micelles interact with silicate species by Cou-lombic interactions in the optimal reaction mixture toproduce approximately two or three monolayers ofsilicate encapsulation around the external surfaces ofthe micelles. These randomly ordered composite speciesspontaneously pack into a highly ordered mesoporousphase with an energetically favorable hexagonal ar-rangement, accompanied by silicate condensation. Withthe increase in heating time, the inorganic wall contin-ues to condense.It must be noted that the majority of reports regard-

ing LCT mechanism have been investigated in a systemcontaining relatively large amounts of surfactant (gen-erally more than 10 wt % of the total mixture). It isclear that liquid crystal micelles, even crystalline meso-phases, indeed exist under such circumstances, andtherefore, the LCT mechanism seems to be plausible.5.2. Transformation Mechanism from Lamellar

to Hexagonal Phase. Another formation mechanismproposed by Stucky and co-workers (Monnier et al.,1993; Huo et al., 1994b; Firouzi et al., 1995) is thetransformation mechanism from lamellar to hexagonalphase. The central tenet of their conclusion is that themesophase formed is governed by charge density, coor-dination state, and steric requirements of the inorganicand organic species at the interface and not necessarily

Figure 5. Mechanism of the formation model for ZSM-5 templating by single TPA molecule.

Figure 6. Schematic model of liquid crystal templating mechanism via two possible pathways (Beck et al., 1992a).

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by a preformed liquid crystal structure. In contrast, amicellar assembly of organic molecules will be brokenup and rearranged upon addition of inorganic speciesto form a new phase often with lamellar morphologies(Huo et al., 1994b; Firouzi et al., 1995). As reportedpreviously (Monnier et al., 1993), a lamellar phase wasindeed isolated between the reaction period of 1-20 minat 75 °C, and after 20 min the hexagonal mesostructureswere simultaneously detected (see Figure 8). When thelamellar phase was hydrothermally treated at 100 °C,it was converted to the hexagonal phase over 10 dayswith intermediate, and the final X-ray patterns areshown in Figure 8. Therefore, the authors proposedthat in a surfactant/silicate aqueous mixture withrelatively low pH, a low degree of polymerization ofsilica species, and low temperatures, small silica oligo-mers (three to eight silicon atoms, e.g., S3R, D4R) playan important role in the formation of mesoporous phase

(Monnier et al., 1993). In part, this is because they caninteract with surfactant cations by Coulombic interac-tions at the interfaces, leading to a strong interactionby multidentate binding between them. This multi-dentate binding of silicate oligomers to the cationicsurfactant can subsequently further polymerize to formvery large ligands, and enhance the cooperative bindingbetween the surfactant and silicate species. It is thesemultidentate surfactant-silicate ligands that lead to alamellar biphase governed by the optimal surfactantaverage head group area (A0) (Monnier et al., 1993). Aspolymerization of the silicate species proceeds, theaverage head group area (A) of surfactant assemblyincreases due to the decrease of the charge density oflarger silicate polyanions. This leads to the corrugationof the silicate layers and ultimately results in thehexagonal mesophase precipitation. Figure 9a sche-matically shows the transformation mechanism fromlamellar to hexagonal phase through charge matching.Alternatively, Huo et al. (1994b) and Firouzi et al.

(1995) suggested that ion exchange between surfactantanions (OH-, Br-, Cl-) and multiply charged anionicsilica oligomer (D4R, D3R) may take place in a surfac-tant-silicate aqueous solution because the high anioniccharge densities of the oligomer enhance Coulombicattractions (Figure 9b). This multidentate bonding canscreen the intraaggregate electrostatic head grouprepulsions so as to reduce the average head group areaof the surfactants. The collective results can furtherdecrease the local curvature of the aggregate, whichimplies that the cooperative assembly of inorganic-organic species is formed with structures different fromthat of the precursor micelles.This transformation process and the final mesophase

with hexagonal pore structures (rather than cylindricalpore structures) was proved by high-resolution trans-mission electron microscopy (Alfredsson et al., 1994).Interestingly, Steel et al. (1995) suggested that MCM-

41 materials are indeed transformed from a lamellarmesophase. However, it is the sandwiched micellesaround which silicate species assemble into lamellarmesophase rather than the coorganization process.

Figure 7. X-ray powder diffraction patterns of calcined products obtained using different surfactants at (A) 100, (B) 150, and (C) 200 °C(Beck et al., 1994).

Figure 8. X-ray powder diffraction patterns of (A) the initiallylamellar phase from the reaction mixture of 3.4Na2O:29.2SiO2:0.7Al2O3:8.3CTACl:2.6TMAOH:3650H2O, (B) an intermediate ma-terial of A after hydrothermal treatment, and (C) the hexagonalphase transformed from A after hydrothermal treatment for 10days (Monnier et al., 1993).

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Furthermore, the silicate layers pack between twolamellar biphases due to condensation. The sandwichedmicelles finally turn back to randomly isotropic surfac-tant molecules.5.3. “Folded Sheets” Mechanism. Although the

“folded sheets” mechanism, proposed by Inagaki et al.(1994), is based on the intercalation of surfactant tolayered silicates (kanemite) process, it should be eluci-dated here in order to give a comparison. Figure 10 isa schematic model representing the “folded sheets”mechanism. Initially, the surfactant cations intercalateinto the bilayers of kanemite via an ion exchangeprocess. As the ion exchange proceeds, the interlayercross-linking occurs by condensation of silanols. Thesilicate sheets of kanemite have the required flexibilityto be folded due to its single sheet structure.5.4. Identifying the FormationMechanism. Stud-

ies concerning MCM-41 formation mechanisms have

been performed under various experimental conditions(e.g., temperature, pH, compositions of reaction gel).These parameters are found to exert key influence onboth the surfactant behavior and the distribution ofsilicate species (Myers, 1992; McCormick, 1992). Ex-trapolation of such results to a detailed mechanism thusseems impossible. However, it is worth noting that ahigh surfactant concentration, high pH, low tempera-tures, and low degree of silicate polymerization alwaysfavor the formation of cylindrical micelles, as well asthe hexagonal mesophases. In fact, the majority ofMCM-41 syntheses were carried out using a precursorconsisting of a surfactant solution with concentrationsof ca. 25 wt %. In this way, it is inevitable that thedominate aggregates of surfactants are cylindrical mi-celles which are in equilibrium with the monomericmolecules. Upon combining with silicate species, thereaction between them will take place via ion exchangeto form ion parings due to Coulombic interactions at theinterfaces, leading to the precipitation of inorganic-organic complexes. The equilibrium between micellesand monomeric molecules is shifted toward the micelledirection. This is indeed the case when a reaction gelcontains a relatively high surfactant concentration(Chen et al., 1993b). On the other hand, in a reactiongel containing relatively low surfactant concentrations,the isotropic surfactant molecules or micelles may bedisassociated and reorganized upon the adding of sili-cate species to form lamellar inorganic-organic com-plexes in order to precipitate (Steel et al., 1995; Fironizet al., 1995; Huo et al., 1994b).

6. Characterization, PhysicochemicalProperties, and Structure Model

6.1. Characterization and Structure Model.Proper preparations of MCM-41 will always result inproducts with high surface areas (more than 1000 m2/g), large adsorption capacity of benzene (more than 60wt %), narrow pore size distribution (centered at ca. 35-38 Å using CTA+ as template), and X-ray diffractionpatterns with a few distinct maxima in the extreme lowangle region. X-ray diffraction patterns give only hkoreflections, and no reflections at diffraction angles largerthan about 6° 2θ can be observed (Behrens and Stucky,1993). The position of these peaks approximately fitsthe positions for the hko reflections from a hexagonallattice. However, only a single peak at ca. 2° 2θ can bedetected in most cases; other reflections are virtuallyabsent (Zhao et al., 1994). Preparations with only oneremarkable peak have also been found to containsubstantial amounts of MCM-41 (Borade et al., 1995;Tanve et al., 1994). Nitrogen adsorption-desorptionisotherms of MCM-41 materials exhibit a sharp stepcommencing at ca. p/p0 ) 0.38, as shown in Figure 11(Zhao et al., 1994), and the reversible type IV isothermswithout hysteresis (Kresge et al., 1992; Schmidt et al.,1995). The inflection point is attributed to the com-mencement of pore filling from which the pore diametercan be roughly estimated.Figure 12 typically shows the relationship between

XRD d100 spacing, Ar pore size, wall thickness (t), andthe alkyl chain length (n) for siliceous MCM-41 preparedusing alkyltrimethylammonium surfactants with dif-ferent chain lengths (Beck et al., 1992a). It exhibits thatthe wall thickness for siliceous MCM-41 is approxi-mately constant at about 10 Å. Considering a Si-Obond length of 1.6 Å and a Si-O-Si bond angle of 150°(Chen et al., 1993a), a wall thickness of about 10 Å can

Figure 9. Schematic model for transformation mechanism fromlamellar to hexagonal phase via (a) charge matching and (b) ionexchange pathways (Monnier et al., 1993; Huo et al., 1994; Feustonet al., 1994).

Figure 10. Model schematically representing “folded sheets”mechanism (Ingaki et al., 1994).

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be calculated which is in good agreement with theexperimental data. Combining with other characteriza-tion techniques, such as transmission electron micros-copy (Kresge et al., 1992; Beck et al., 1992a), electrondiffraction pattern (Kresge et al., 1992), lattice images(Feuston et al., 1994), 27Al and 29Si MAS NMR (Chenet al., 1993a; Firouzi et al., 1995), Fourier transforminfrared spectroscopy (Chen et al. 1993), temperature-programmed desorption of ammonia (NH3-TPD), andTGA (Chen et al., 1993a), two structural models forMCM-41 with an amorphous wall have been constructedas shown in Figure 13. Model A, a cylindrical porestructure with lattice constant a ) 44.6 Å and a wallthickness of 8.4 Å, based on the LCT mechanism, wasproposed by Feuston et al. (1994) through the classicalmolecular dynamics simulation approach. Model B, ahexagonal pore structure with an interpore distance ofca. 35 Å, based on the lamellar to hexagonal phase

transformation mechanism, was proposed by Behrenset al. (1993). Interestingly, both the cylindrical andhexagonal pore structures have been visualized throughtransmission electron microscopy by Chenite and LePage (1995) and Walker et al. (1995), respectively.6.2. Acidity. Figure 14 shows the pyridine adsorp-

tion IR (A) spectra (Corma et al., 1994b) and NH3-TPDprofiles (B) (Zhao et al., 1994) of H-form MCM-41 withother samples for comparison. In contrast to ZSM-5,MCM-41 possesses only some weak- and middle-strength acid sites that are similar to amorphousaluminum-silica. The pyridine adsorption spectra alsoreveal that both the Brønsted acid sites (band at 1540cm-1) and the Lewis acid sites (band at 1450 cm-1) arecomparable to amorphous silica-alumina (Si/Al ) 2.5)This is in good agreement with NH3-TPD results. Boththe NH3-TPD and IR data show that MCM-41 materialsare aluminosilicates with weak- and middle-strengthacid sites similar to amorphous silica-alumina.6.3. Stability. It has been found that MCM-41 is

thermally stable, but the hydrothermal stability isrelatively poor, in particular for the as-synthesized

Figure 11. Nitrogen adsorption-desorption isotherms of MCM-41 (Zhao et al., 1994).

Figure 12. Relationship between surfactant alkyl chain length(n) and argon pore size, a0 (a0 ) 2d100/x3), and wall thickness (t).

Figure 13. Structural model of MCM-41 with cylindrial pore (A)(Feuston et al., 1994) and hexagonal pore (B) (Behrens and Stucky,1993).

Figure 14. (A) Pyridine adsorption IR spectra for (a) amorphoussilica-alumina (Si/Al ) 2.5) and (b) MCM-41 (Si/Al ) 14) invacuum at (1) 423, (2) 523, and (3) 623 K (Corma et al., 1994b).(B) NH3-TPD profiles for MCM-41 (Si/Al ) 15), amorphous silica-alumina (Si/Al ) 3), and ZSM-5 (Si/Al ) 25) (Zhao et al., 1995a).

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samples (Chen et al., 1993a; Corma et al., 1994b). Wehave also studied the various stabilities of MCM-41 (seeTable 4). When MCM-41 was hydrothermally treatedat 450 °C for 2 h, considerable losses of both the BETsurface area and the benzene sorption capacity occurred.Calcination of MCM-41 in dry air resulted in slight lossof BET surface area, indicating that the thermal stabil-ity was satisfactory. When MCM-41 was impregnatedin 20% nitric acid overnight, both the BET surface areaand the benzene sorption amount were slightly in-creased. This observation may be associated with theremoval of some blockages in the channels of MCM-41.In sharp contrast with the acid treatment, the basictreatment in 5% potassium hydroxide bought aboutalmost complete destruction of the MCM-41 structuredemonstrated by both the nitrogen and benzene adsorp-tion and XRD spectra (only a broad intensity at ex-tremely low angle regions was detected). These dataindicate that MCM-41 material is thermally stable withhigh acid resistance, but hydrothermally unstable andwith low base tolerance.6.4. Interaction with Water. The adsorption of

water steam over MCM-41 characterized by a type Visotherm reveals an initially repulsive character fol-lowed by a capillary condensation step of water, indicat-ing that MCM-41 possesses both hydrophobic andhydrophilic properties (Llewellyn et al., 1995). Threedistinct stages were obtained in the TGA curves (Figure15) (Zhao et al., 1994). The first stage at 25-150 °C isassociated with the desorption of physically adsorbedwater and other gases; the second stage at 150-380 °Cis attributed to the decomposition and combustion oforganic species, and the third stage at 380-800 °C maybe related to the water losses due to condensation ofsilanol groups to form siloxane bonds (Chen et al.,1993a). In addition, with the increase of the aluminumcontent in MCM-41 framework the amount of waterdesorbed increases and the organic species decrease.

Thus, the conclusion could be drawn that siliceousMCM-41 is hydrophobic, while aluminosilicate MCM-41 is slightly hydrophilic.

7. Application

7.1. Catalysis. A US patent (Pelrine et al., 1992)claimed good catalytic activity of MCM-41 impregnatedwith Cr for olefin oligomerization to produce lube oiladditives. Both the pour points and the viscosityindexes were improved on Cr-MCM-41 catalyst com-pared with the commercial catalyst (Cr-SiO2). Thesupport (MCM-41, alumina-silica, and USY) effect onthe catalytic activity of hydrodesulfurization (HDS),hydrodenitrogenation (HDN), and mild hydrocracking(MHC) was also investigated by Corma et al. (1995b).A very high activity was observed on Ni,Mo-MCM-41catalyst which was attributed to a combination of highsurface area and large pore size that favors a highdispersion of the active species coupled with easyaccessibility of the large feedstock molecules.Use of MCM-41 as an acid catalyst for Friedel-Crafts

alkylation of 2,4-di-tert-butylphenol (bulky aromatic)with cinnamyl alcohol (Kloetstra et al., 1995a) and forthe tetrahydropyranylation of alcohol and phenol (Ar-mengol et al., 1995) was reported where the advantagesof MCM-41 were manifested. Besides being an acidcatalyst, Na-MCM-41 and Cs-MCM-41 catalysts ex-hibit satisfactory performance in base catalysis (Kloet-stra et al., 1995b). For example, in the Knoevenagelcondensation of benzaldehyde with ethyl cyanoacetate,81% conversion of benzaldehyde and 75% selectivity tothe desired product were obtained at 150 °C within 7h, and 90% conversion of benzaldehyde and ca. 100%selectivity were observed at 100 °C within 3 h in watersolvent.Ti(V,Cr)-MCM-41 materials exhibit excellent cata-

lytic oxidation performance in the presence of hydrogenperoxide or even the bulky oxidant THP (terbutylhydroperoxide), as reported by Tanev et al. (1994) andCorma et al. (1994a). A typical example from Tanev etal. (1994) is shown in Table 5. Both Ti-MCM-41 andTS-1 are effective catalysts for the hydroxylation ofbenzene, but for substrate 2,6-DTBP, this is not the casebecause the medium-sized pore structure of TS-1 (MFI)cannot permit such a large molecule to diffuse into the

Table 4. Thermal/Hydrothermal and Acid-Base Proof Stability of MCM-41 (Zhao et al., 1994)

samplea a sample b sample c sample d sample e

treatment method calcination in N2 at 540°C for 1 h and then inair for 8 h, 1 °C/min

calcination inair at 540 °C for8 h, 1 °C/min

hydrothermallytreated at 450°Cfor 2 h

20% nitric acid,room temperature,overnight

5% KOH solution,room temperature,overnight

XRD intensity 100 95 55 100BET surface area (m2/g) 1016 940 670 1114 3.5benzene uptakeb (%) 65.5 54.0 33.3 65.5

a Sample a as the batch material for other treatments. b At 25 °C, 65 Torr.

Figure 15. TGA curves of MCM-41 with different Si/Al ratios(Zhao et al., 1994).

Table 5. Catalytic Activity of Ti-MCM-41 Comparedwith TS-1 and TiO2

a (Tanev et al., 1994)

2,6-DTBP oxidation benzene oxidation

catalystconversion

(%)selectivity toquinone (%)

conversion(%)

selectivity tophenol (%)

Ti-MCM-41 83 >95 37 >95TS-1 6.5 >95 31 >95TiO2 14.5 >95 - -

a Conditions: 0.1 g of catalyst, 80 °C, 2 h, 5 mmol of 2,6-DTBPor 10 mmol of benzene, 0.13 mol of acetone as solvent, 29 mmol of30% aqueous H2O2.

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internal surface where the active sites (isolated Tispecies) are located.Molecular sieves have drawn much interest in envi-

ronmental catalysis in the past few years (Dartt andDavis, 1994; Iwamoto, 1994), in particular in decompo-sition of nitrogen oxides (Iwamoto, 1994; Tabata, 1994).The activity of selective catalytic reduction of NO onTi,V/MCM-41, Ti,V/SiO2, and a commercial catalystV,W/TiO2 has been compared (Beck et al., 1992b) asshown in Table 6. Ti,V/MCM-41 catalyst exhibits ahigher NOx conversion than the silica-based catalyst butis less active than the commercial one. This wasexplained in terms of the fact that although MCM-41is capable of supporting more active components thansilica due to its high BET surface area it has a low redoxperformance. In a recent study, we have demonstratedthat MCM-41 can be a potential NO decompositioncatalyst support with considerably higher activity thanZSM-5 (Zhao and Lu, 1995).7.2. Ship in a Bottle. The large cavities and

uniformed pore structures provide zeolites and molec-ular sieves with many interesting characteristics. Forexample, they can act just as molecular “factories”(bottles) where quantum-sized particles (ships) can bemanufactured inside (Ozin, 1992; Dag and Ozin, 1995;Mitchell, 1991). The huge “workshop” and broad chan-nels (without “traffic block”) for MCM-41 are expectedto manifest itself with specified characteristics in thearea of “ship in a bottle” as Fisher (1995) pointed out.Nanosized catalysts, such as low-valent transition metalmoieties, e.g., Me3SnMo(CO)3(η-C5H5), have been intro-duced into the channels of MCM-41 by combiningcalcined MCM-41 with Me3SnMo(CO)3(η-C5H5) in hex-ane solvent for 18 h under stirring, and importantlythese larger ligands seem to be very stable (Huber etal., 1994). When the attached complexes were heated,they were converted into nanometer bimetallic clustersin MCM-41 channels, which displayed high catalyticactivity. The fabrication of stable carbon wires in thechannels of MCM-41 was reported by Wu et al. (1994).The MCM-41 host was contacted with acrylonitrilevapor at room temperature for 4 h and then brieflyevacuated, resulting in a large amount of adsorbedacrylonitrile. Polymerization of acrylonitrile in thechannels of MCM-41 was carried out through a free-radical reaction process initiated by adding K2S2O8 andNaHSO3 at 40 °C. Finally, the sample was thermallytreated in a nitrogen flow between 350 and 1000 °C for24 h. The final nanosized carbon wires exhibited a highthermal stability (more than 800 °C) and low-fieldconductivity.7.3. Model Adsorbent. As shown in Figure 11,

MCM-41 materials exhibit an interesting nitrogenadsorption-desorption isotherms without hysteresisloop. However, it was found that this is not the casefor other adsorptives (Branton et al., 1995) and atdifferent temperatures (Rathousky et al., 1995). Thisreflects the importance of the pore shape and thenetwork of adsorbent. The well-controlled uniform pore

structures of MCM-41, thus, offer an opportunity toinvestigate this fundamental problem. Reports regard-ing this issue are readily available (Branton et al., 1993,1994, 1995; Rathousky et al., 1994, 1995).

8. Perspective

As indicated above, the marked characteristics ofMCM-41 are as follows: (1) well-defined pore structurewith apertures in the range of 15-100 Å which can becontrolled by careful choice of surfactants, auxiliarychemicals, and reaction parameters, (2) high thermalstability, (3) mild acidity, (4) large BET surface area andpore volume, and (5) hydrophobic/hydrophilic propertywhich can be modified by changing Si/Al ratios. There-fore, MCM-41 materials have a promising potential incatalysis, adsorption, and advanced molecular sieve-based materials.8.1. Host-Guest Encapsulation. Semiconductor

clusters anchored in molecular sieves have receivedmuch attentions as advanced composite materials (Stuckyand Mac Dougall, 1990; Ozin, 1992) and photocatalysts(Liu et al., 1993; Tanguay et al., 1989). In this case,the uniform pore structures of molecular sieves can actas solid solvents to control both the size and topologyof the particles encapsulated inside. Nevertheless, itwas found that introduction of such species, in particu-lar large transition metal species (e.g., MoO3) couldresult in partial collapse of the host structures (Fierroet al., 1987) and ultimately lead to the decrease insurface area as well as the catalytic activity. From thisconsideration, MCM-41 should exhibit superior proper-ties compared to other common molecular sieves be-cause of its mesopore structure. As an example, wehave successfully encapsulated nanosized TiO2 or MoO3particles in the MCM-41 channels through either ionexchange or impregnation approaches (Zhao et al.,1995b). X-ray powder diffraction patterns show that nostructure collapse occurred. Furthermore, no diffractionpeaks attributed to these metal oxides at the externalsurface of MCM-41 could be observed for carefullypurified samples. This result was also confirmed byX-ray photoelectron spectroscopy. A remarkable in-crease in the band gaps of these composite materialswas observed in the UV-vis absorption spectra due toquantum size effect (Brus, 1986), indicating that nano-meter TiO2 clusters were indeed formed in the MCM-41 channels.Metals with two different, stable oxidation states,

such as Co(II)/Co(III), Fe(II)/Fe(III), Cr(II)/Cr(V), andCu(I)/Cu(II), were found to be effective oxygen carriersfor catalysis (Imamura and Lunsford, 1985; Mortier andSchoonheydt, 1985; Thomas, 1994). It was found,however, that the oxygen-carrying ability of theseligands was due to the formation of M-O-M (M ) Cu,Cr, Fe, etc.) bridges in the cubooctahedra. This wouldlead to diffusion limitations of reactants and productsinto and out of the cubooctahedra. On the other hand,metal-organic supermolecule complexes encapsulatedin zeolite cavities were also found to be effective gascarriers and reaction centers for catalysis (Ozin and Gil,1989; Mallouk and Lee, 1990; Mitchell, 1991). It waspointed out that the degree of [CoII(bpy)(terpy)]2+ com-plex formation inside a NaY zeolite was very low, hencethe efficiency for the utilization was low. In addition,some complexes are impossible to be encapsulated insidethe cages of the traditional zeolites because of their toolarge dynamic diameters (e.g., the ligand of [Co(phthalo-

Table 6. Catalytic Activity of MCM-41 in SCR of NOx

NOx conversion (%)

catalyst 350 °C 450 °C

6.1%Ti, 2.5%V/MCM-41 81 413.0%Ti, 2.6%V/SiO2 70 22.0%V2O5, 8.0WO3/TiO2 95 63a Conditions: 2.0 g of catalyst, WHSV ) 400 mL/min, 125 ppm

NO, 125 ppm NH3, 0.12% O2, helium balance.

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cyanine)]2+ with a diameter of 15 Å cannot be encap-sulated by NaY zeolite). All of these drawbacks can nowbe overcome by taking advantage of MCM-41 materials,as shown in Figure 16, because the advantages of MCM-41 (high surface area and mesoporous structure) canoffer the possibility to support a large amount ofchemical ligands with a large dynamic diameter.Attempts have also been made to make zeolite-based

solids capable of generating nonlinear optical propertieswhich are dramatically different from those of eitherhost or guest in recent years. For example, p-nitro-aniline (pNA), a centrosymmetric crystal, does notexhibit second harmonic generation (SHG), nor doesAlPO4-5 molecular sieve. However, when pNA mol-ecules were introduced into the one-dimensional chan-nels of AlPO4-5, a high SHG signal will be detectedbecause the polar host crystal forces alignment of theguest molecules so as to enhance the SHG (Marlow etal., 1994). If one imagines the p-nitroaniline moleculesbeing aligned in the MCM-41 channels with a 40 Å porediameter as indicated in Figure 17, the SHG signalgenerated might be expected to be 5-7 factors morethan that for AlPO4-5.8.2. Modification. From the consideration of the

uniformmesoporous structure, mild acid properties, andacid proof stability, introduction of superacid (e.g.,SO4

2-, F-) or heteropoly acid into the MCM-41 channelsto modify the acidity seems to be both practical andpromising. Kozhernikov et al. (1995) recently reportedresults for the introduction of tungstophosphoric acid(H3PW12O40) into the MCM-41 pores. By shaking MCM-41 with H3PW12O40 solution overnight, finely dispersedH3PW12O40 species on the surface of MCM-41 wereobtained, which exhibited strong Brønsted acid sitessimilar to those of H3PW12O40 supported on amorphoussilica. The catalytic activity of this composite materialwas tested for the alkylation of TBP with isobutene asa model reaction. H3PW12O40/MCM-41 shows a highcatalytic activity which is 3-4 times higher than that

for the bulk H3PW12O40 catalyst. Supported SO4- and

SO4--ZrO2 over MCM-41 may be a promising approach

to get highly dispersed superacids.The pore sizes of MCM-41 materials were claimed to

be distributed in the range of 15-100 Å. It generallyranged from 20 to 40 Å. The gap between 13 and 20 Åmakes it inconvenient to use this material. Althoughmesoporous materials with pore sizes in the range of14-22 Å have been reported through the gallery-templated route (Galarneau et al., 1995), this is notsatisfactory to zeolite and molecular sieve workers.Therefore, precise control of the pore structures byeither hydrothermal synthesis or postmodified approach(Beck et al., 1993) in order to make this novel materiala true molecular recognizer is an urgent need.8.3. Adsorbent. MCM-41 may be expected to find

application as an adsorbent since it exhibits bothhydrophobic and hydrophilic character depending uponthe exact composition and/or postmodification. Removalof hydrocarbons from water, storage of gases (e.g., H2,O2, CH4), adsorptive xylene separation, and separationof biological and pharmaceutical compounds now seemsto be potential areas for developing MCM-41 applica-tions.8.4. Environment. MCM-41 may find wide ap-

plications in environmentally safe processes, i.e., re-placement of environmentally hazardous catalysts inexisting processes. Titanium-containing MCM-41, Ru(II),Co(II), and Ni(II) complexes and Mo(CO)6 ligandsencapsulated within the intrasurface of MCM-41, etc.are very promising heterogeneous catalysts to replacetraditional homogeneous catalysts because of theirready regenerability, shape selectivity, and easy separa-tion and recovery.

9. Acknowledgment

The authors are grateful to Dr. D. Y. Zhao, theWeizmann Institute of Science, Israel, for some valuablediscussions. We also thank Mr. David Page, Center forMicroscopy and Microanalysis at the University ofQueensland, for his help in XRD characterization.X.S.Z. wishes to thank Dalian Institute of ChemicalPhysics, Chinese Academy of Sciences, where part ofthe work on MCM-41 was conducted, for permitting asabbatical leave.

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Received for review November 21, 1995Revised manuscript received March 25, 1996

Accepted March 27, 1996X

IE950702A

X Abstract published in Advance ACS Abstracts, June 1,1996.

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