university of groningen mesostructured sillicate-based ......ordered mesoporous materials (omms)...

University of Groningen

Mesostructured sillicate-based materialsZhang, Zheng

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Citation for published version (APA):Zhang, Z. (2014). Mesostructured sillicate-based materials: studies on mild detemplation methods andadvanced characterization. [S.n.].

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2

Thermal Detemplation of SBA-15 Mesophases. Effect of the Activation Protocol

on the Framework Contraction. §§

This chapter focuses the potential use of the solvothermal extraction (SE) as a preliminary step to calcination for detemplating SBA-15 mesophases. The purpose of this study to reduce the quantity of organics species and thereby the corresponding structural shrinkage due to calcination.

A systematic investigation was carried out on mesophases hydrothermally aged between 90-130 C. The mesophases containing variable amounts of template were then treated by calcination or pyrolysis/calcination. TGA was applied to quantify the template amount after the various treatments. The as obtained materials were characterized by SAXS and Ar ad/desorption for structural and textural information while 1H NMR gave information about the integrity of the as-recycled template.

The results shows that solvothermal conditions remove considerably amount of template, reducing from 50 (precursors) to 10-20 wt. % (extracted), mainly from the primary mesopores. Possible reuse of the extracted template is questionable as it is poor in polyethyleneoxide in comparison to the block-copolymer Pluronic P123 used in synthesis. For all thermal protocols applied (direct calcination, calcination after solvent-extraction or pyrolysis/calcination after solvent extraction), the thermal shrinkage was a decreasing function of the ageing temperature, which is consistent with the condensation degree of the Silica. It was observed that for each mesophase, the thermal shrinkage becomes much more pronounced when containing less residual template in the sample. Differences in mass transfer of O2 and thereby in the oxidation rates can explain this behaviour. The template can also serve as a structural support during calcination which is missing when calcining the solvent extracted (and pyrolyzed) materials.

This study shows that a mild method needs to be applied in order to obtain SBA-15 mesophases with enhanced textural properties.

§ This chapter has been published:

Thermal Detemplation of SBA-15 Mesophases. Effect of the Activation Protocol on the Framework Contraction, Z. Zhang, J. Yin, H.J. Heeres, I. Melián-Cabrera; Microporous and Mesoporous Materials, 176 (2013) 103–111.

Chapter 2

30

2.1 Introduction

Ordered mesoporous materials (OMMs) have drawn remarkable attention since their discovery in 19921

because of their potential application in heterogeneous catalysis 1-3, bio-adsorption 4, separation5,6, drug

delivery7, sensing 8 as well as optical and electronic uses.9 The core principle of OMMs is based on the

utilization of organic surfactants as templates in sol–gel process, which controls the interface between

inorganic–organic phases in basic or acid conditions. Since the earliest OMM 1,10, several synthesis

pathways have been reported.11,12

A breakthrough in OMMs synthesis was the discovery of SBA-15 13,14, by using amphiphilic triblock

copolymers as structure-directing agents in highly acidic aqueous media. This material possesses a

hexagonal structure with thick walls and large surface area. The thick and condensed walls provide high

thermal and hydrothermal stability to SBA-15.15 The pore diameter can be tuned between 5-30 nm by

means of the aging temperature, typically ranging 30-180 C.16,17 Especially interesting, SBA-15 possesses

secondary porosity in addition to the main mesopores. This porosity is visualized as interconnecting parts

between the main channels, whose existence was confirmed by carbon nano-replicas CMK-n

materials 18,19; also intra-wall micropores exist that were confirmed by gas physisorption 16,17,20-23 and

quantitative X-ray analysis.24 The total intra-wall porosity of SBA-15 contributes greatly to the total

surface area and the interconnecting secondary mesopores can reduce diffusion limitations that can

occur in one-dimensional structures. For a comprehensive discussion on block-copolymer-templated

mesoporous oxides, the reader is referred to Soler-Illia et al.25

OMMs are normally synthesized in the presence of surfactant-type organic templates; these organic

species need to be removed to generate the porosity. Generally, thermal calcination in air is used to

burn off the organic templates. This method completely removes the organic species; it can however

lead to significant framework contraction as a result of thermal silanols condensation. This phenomenon

can be particularly severe for less condensed SBA-15 materials; generally synthesized at low ageing

temperatures. Therefore, the development of alternative detemplating methods to calcination, such as

solvent extraction26-28,28-32, chemical and UV-Vis induced oxidation33-37 and microwave digestion37,38 have

been proposed. Among all those methods, solvent extraction is highlighted because of the potential

recovery and reuse of expensive organic templates. However, it was found that conventional solvent

extraction does not fully remove the template with an efficiency of 74 %.28 An additional calcination step

is often needed to fully remove the template residue. 38,39

Thermal Detemplation of SBA-15 Mesophases. Effect of the Activation Protocol on the Framework Contraction

31

Even though there are remarkable non-thermal methods reported, calcination is still the most used

technique for detemplation. The main reason is that thermal treatment offers facile operation,

reproducibility and complete template removal, although in the expense of properties such as, pore size,

pore volume and surface hydrophilicity. In order to optimize the calcination conditions, the thermal

mechanism of different surfactant-templated mesostructured silicas has been studied in depth.40-44

Various studies point out that the surfactants decomposition involves a stepwise pathway for MCM-41

and MCM-48 40-42, including a Hofmann degradation followed by oxi C.

Concerning SBA-type mesoporous materials, the mechanism depends on the type of surfactant.42

Particularly for SBA-15 most of the block-copolymer template is removed at surprisingly low

temperature in a single oxidation step; the residual carbonaceous species and water from condensation

are released between 300-550 C.42 Additionally, the correlation between template removal efficiency

and calcination temperature has been studied in a recent study for a mesophase aged at 80 C.44 The

results show that full detemplation (i.e. carbon < 0.3 wt. %) is achieved at 575 C even though this

implies framework contraction.

Among the various textural features of SBA-15, the interconnectivity between the primary channels, also

termed secondary mesopores, has various positive effects. It provides stability to nanoreplicas 18,19,45-48

and can improve mass transfer in heterogeneous catalytic reactions, in particular at low temperature.

Therefore, avoiding the framework contraction in SBA-15 will produce a better interconnectivity and

superior materials for those applications. In this chapter, the effect of the solvothermal extraction was

investigated as preliminary step to calcination, aiming at reducing the amount of organics to be burnt

and thereby the corresponding thermal shrinkage. This was done systematically on SBA-15 mesophases

aged from 90-130 C.

2.2 Experimental

2.2.1 Synthesis of SBA-15 mesophases

Chemicals

All materials were used as received without further purification. Tetraethoxysilane (TEOS, 98 %, Aldrich),

poly(ethyleneoxide)20-poly(propyleneoxide)70-poly(ethyleneoxide)20 (EO20PO70EO20; Pluronic®-P123,

Sigma-Aldrich), hydrochloric acid (HCl, 37 wt. %), ethanol (99.99% EG, Merck). Argon were of analytical

purity (>99.9 %, Alpha gas).

Chapter 2

32

Synthesis

The mesophases were prepared by the surfactant assisted sol-gel procedure according to the method

reported elsewhere.13,14 In all experiments the hydrolysis step was identical and the condensation was

systematically varied by means of ageing temperature from 90- . In a typical experiment, 8 g of

P123 were mixed with 240 g HCl (2.0 M) and 60 g of Mili-Q water until a homogenous mixture was

obtained. This mixture was placed in water-bath at 42 C and 17 g of TEOS was added slowly. The molar

composition of the wet SiO2 gel was: 1.0 SiO2:0.017 P123:5.9 HCl: 204 H2O. The hydrolysis was

completed after 20 h at 42 C; the mixture was then transferred into a 500 ml Teflon bottle and aged

under static conditions at various temperatures: 90, 95, 100, 110 and 130 for 24 h. The slurry was

filtered and the as-obtained solid was washed with 2 litres of Mili-Q water and dried overnight at 85 C.

The corresponding SBA-15 materials are denoted as SBA/x, where x represents the aging temperature.

Template removal by solvent extraction

The materials were solvothermally extracted by means of a soxhlet extracting set-up. Typically, 0.5-2.0 g

of SBA/x precursor were loaded in a cellulose thimble (Whatman) and placed into the soxhlet chamber.

Absolute ethanol was poured into a 500 ml round bottom flask, together with a small amount of boiling

stone. The system was assembled and heated using a heating mantel until reflux. The soxhlet extraction

time was varied between 0.5-24 h in order to obtain SBA/x materials with various amounts of template

residues. “Sox t” is used as a suffix in the sample code, where t represents the soxhlet extracting time in

h. The structural study in section 3.3 makes use of the material after 24 h extraction time (Sox24); the

suffix was omitted for simplicity.

Template removal by calcination

The calcination was carried out by heating the material (mesophases or solvent extracted counterparts)

in static air at a rate of 1 C /min until 550 C and then kept for 6 h. The calcination was performed in a

box furnace Nabertherm equipped with P330 temperature controller. The samples names are suffixed

with C; i.e. SBA/x_C for directly calcined mesophases and SBA/x_Sox t_C for solvent extracted materials

that were subsequently calcined.

Two step pyrolysis and calcination

The materials after solvent extraction for 24 h were subjected to a two-step thermal activation. The

sample was first treated in a flow of N2 (100 mL/min STP) at 320 C for 1.5 h based on the TGA patterns.


33

Subsequently the samples were calcined in static air at a rate of 1

6h.

2.2.2 Characterization

Small angle X-ray scattering (SAXS) measurements were carried out at room temperature using a Bruker

NanoStar instrument. A ceramic fine-focus X-ray tube, powered with a Kristallflex K760 generator at 35

kV and 40 mA, has been used in point focus mode. The primary X-ray flux is collimated using cross

coupled Göbel mirrors and a pinhole of 0.1 mm in diameter providing a CuK radiation beam with a full

width at half-maximum of about 0.2 mm at the sample position. The sample-detector distance was 1.04

m. The scattering intensity was registered by a Hi-Star detector (Siemens AXS) in the q-vector range of

0.1–2.0 nm. The hexagonal a0 cell parameter was calculated as a0=2d100/ 3 , where d100 is the

interplanar spacing for the 100 reflection thas was derived from the q-vector value. The shrinkage was

calculated as:

Thermal shrinkage = 1 - (a0C

/ a0P ) (1)

The normalized shrinkage difference of a generic sample (q100S) with respect to the calcined material is

defined as:

Normalized shrinkage difference (-) = /q100C = ( q100

S – q100

P ) / q100C (2)

Where q100C is q-vector value of the 100 reflection for the calcined material, and q100

P is the scattering

vector for the corresponding precursor; 100S

– q100P.

Thermogravimetric analysis (TGA) was carried out in a Mettler-Toledo analyzer (TGA/SDTA851e). About

5-10 mg of sample was filled into fused 70 L -Al2O3 crucibles. The decomposition of the template was

monitored in a flow of synthetic air (100 mL/min) while the temperature was increased from 30 to 900

C at a rate of 10 C /min. Baseline subtraction was employed.

Liquid 1H NMR spectra of the fresh and as-recovered P123 template were recorded in a Varian AMX400

tetramethylsilane (TMS). The extracted template P123 was recovered in a rotary evaporator at 50 C

until all the ethanol was evaporated. The as-recycled template was dissolved in deuterated chloroform

(Chloroform-d, 99.5% atom D, Sigma-Aldrich). The ratio of polyethylene oxide to polypropylene oxide

(EO/PO) of the extracted polymer was calculated from the 1H NMR spectra according to equation (3) that

takes into account the overlapping of the methylene resonances from EO and PO :

Chapter 2

34

3/]2[4/)3/]2[2/])4[]1[((

3/][3/][2/][

3

32

IAIAIAIA

CHHCHHCHH

POEO (3)

In this equation, H[CH2] represents the hydrogen amount in methylene group existing in EO and PO

blocks; H[CH3] denotes the hydrogen amount in methyl groups existing only in the PO block. IA

represents the integrated area of the corresponding resonances that are identified in Figure 5 and

equation (4). The x value was calculated assuming that the length of PO block (y) does not change and is

equal to the starting P123; i.e. y=70. . The recycled template analysed by 1H NMR corresponds to 24 h

extraction time.

Gas sorption isotherms were measured in a Micromeritics ASAP 2420 adsorption analyzer by using Ar (87

K). Before analysis, a mild degasification was performed at 140 C for 10 h for the solvent extracted

samples while the calcined materials were degassed at 350 C for 6 h. The Brunauer-Emmet-Teller

method 49 was applied to obtain the surface area (SBET) and non-local density functional theory Kernel

(NLDFT) for cylindrical pores 50 was applied to calculate the pore size distribution (PSD) and the

cumulative pore volume for pores below 500 Å, while the pore size distribution (PSD) was calculated

from the BJH method from the adsorption branch. The single point total pore volume (VT) was calculated

at a relative pressure of 0.98 in the desorption branch.

2.3 Results and Discussion

2.3.1 Direct mesophase calcination

The SAXS patterns of the as-synthesized mesophases are given in Figure 1-a. The mesophases aged up to

-defined main (100)

and secondary (110) and (200) reflections. At 130-180 C (Figure A.1-1, in Appendix I) the material is

transformed into a wormhole structure with a remarkable reduction of the SAXS intensity and the

disappearance of the 110 and 200 secondary reflections. The intensity of the secondary reflections is

maximized for the mesophases aged between 100-

reflected by the equal position of the 100-reflection.

Calcination always gives rise to an enhancement of the SAXS intensity as attributed to an improved

contrast between scattering walls and cavities after the removal of the template.51 Calcination produces

however a different degree of framework contraction (also called thermal shrinkage) as evidenced by the

shift towards higher q-vector values. The thermal shrinkage (TS hereafter) comes from the surface

dehydrolyation and formation of siloxane bonds.52 The TS has a dependency on the ageing temperature;


35

Figure 1. SAXS patterns for SBA-15 samples aged at various temperatures (90 a) mesophases and b)

-line corresponds to the q[100] position for the precursors that is equal for all of them.

this can be seen in Figure 2-a, where TS is represented as a function of the mesophase ageing

temperature. At low ageing temperatures the TS can be as high as 10 %. This is reduced with the ageing

temperature to a final value of ca. 3.6%. A higher degree of condensation with the increasing aging

temperature is expected, which makes the material more polymerized and therefore resistant to

thermal contraction. The improved condensation with the synthesis/aging temperature has not been

reported for SBA-15, to the best of our knowledge, but it was proven for related materials in

hydrothermal stability studies; MCM-41 53,54 and JLU-20 55-57. Figure 2b plots TS as a function of the total

amount of template that is burnt during the direct calcination, determined by TGA; TS appears to have

no relation to the total amount of template burnt in the direct calcination of the mesophases. It is worth

noting that for SBA/180 (not shown) the template content was remarkably low with only 1 wt.%, due to

the template decomposition during the synthesis.58

0.04 0.06 0.08 0.10 0.12 0.14 0.16

SBA/110

Inte

nsity

/ a.

u.

q-factor / Å

SBA/100

SBA/95

SBA/90

SBA/130

100

110

200

0.04 0.06 0.08 0.10 0.12 0.14 0.16

SBA/90C

SBA/95C

SBA/100CIn

tens

ity /

a.u.

q-factor / Å-1

SBA/110C

SBA/130C

(a) (b)

Chapter 2

36

Figure 2. Comparison of the degree of thermal shrinkage (TS) as a function of the aging temperature (a) and template content (b). TS is given as the relative difference in the hexagonal cell parameter (a0) between the calcined (SBA/n_C) and

Estimation of the wall thickness can be done by subtracting the unit cell and the mean pore size. The

values are compiled in Table 1 for materials aged between 90-130 C. It shows a maximum at 95 oC with

5.4 nm after which it decreases to 2.6 nm. This implies that the structure certainly lose potential

interconnectivity 17,21,22 after calcination owing to the shrinkage that is useful for providing stability to the

nanoreplicates. Thus, in order to maximize the interconnectivity it is desirable to minimize the thermal

shrinkage. Due to the relatively weak interaction of the P123 and the silica in these mesophases, the

template can be removed gradually by solvent extraction; controlled by operation time. In the next

sections, the solvothermal extraction efficiency using a soxhlet extractor was investigated and the effect

of template amount on structural damage after calcination or pyrolysis-calcination is presented.

90 100 110 120 1300

2

4

6

8

10

12

1-a 0C /a

0P / %

Aging temperature / C40 42 44 46 48 50 52 540

2

4

6

8

10

12

1-a 0C /a

0P / %

Template content / %

(a) (b)

Table 1. Structural (SAXS) and textural parameters derived from Argon (87 K) ad-desorption for the series of directly calcined and solvent extracted materials. a0

C WC a VT / cm3 ·g-1 b SBET / m2·g-1 b Dp

/ Å b

Materials Å Å C Sox24 C Sox24 C Sox24 SBA/90 114 48 1.026 0.833 678 450 66 72 SBA/95 118 53 1.039 0.857 684 567 70 67 SBA/100 120 48 1.017 0.795 622 377 72 81 SBA/110 120 43 1.215 1.046 641 452 77 86 SBA/130 122 26 1.264 1.182 509 422 96 102 a. Wall thickness as a0

C – DpC; b. suffices ”C” represents calcined and “Sox24” the solvent extracted

material after 24h extraction time.


37

2.3.2 Solvent extraction studies

Extraction efficiency

Solvent extraction is in general the preferable detemplation technique when the silica-template

interactions are weak. This would allow in theory the full recovery and possible reuse of the template 59.

Pluronic P123 in SBA-15 mesophases can be partly washed away without any significant structural

damage by using supercritical fluids, water and organic solvents. Ethanol, an amphiphilic solvent, has

shown to be superior to hydrophilic or hydrophobic solvents for the removal of surfactants from as-

synthesized mesoporous materials 60,61 ; it is therefore used here in this chapter.

The various SBA-15 mesophases were subjected to solvent extraction using a Soxhlet setup and absolute

ethanol (these samples are denoted as Sox). This technique allows a continuous reuse of pure solvent at

its boiling point on the material to be extracted.62 Consequently, it is more effective than a traditional

one-pot solvent washing. The detemplation efficiency using this approach was investigated by collecting

Figure 3. Top) TGA patterns for SBA/95 after solvent extraction at various times 0.25-24 h and bottom) corresponding TGA derivative; inset: magnification of the derivative curves.

200 400 600 800

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

Deriv

ativ

e / %

(°C)-1

Temperature / oC

100 200 300-0.8

-0.6

-0.4

-0.2

0.0

()

Sox0.00Sox0.25Sox0.50Sox1.00Sox3.00Sox24.0

200 400 600 80040

50

60

70

80

90

100 SBA/95

Rela

tive

weig

ht /

%

Chapter 2

38

samples after time intervals and evaluating its template content by TGA. Figure 3-a illustrates the study

for the mesophase SBA/95; including the TGA and DTGA profiles for the starting and solvent extracted

(Sox) samples. The Sox samples display two major weight losses; the first one is small at 50-100 C

attributed to physisorbed water and a second one happens between 160-4 C due to the P123

decomposition (Figure 3, bottom). It was observed that with less template content in the Sox materials,

the light-off temperature of the thermal decomposition shifts to higher temperatures, as can be deduced

in the magnification of DTGA graph in Figure 3, bottom (inset). This implies that the template residues

after longer extraction time are more stable and require higher burn off temperatures. This behaviour

indicates that those residues have a strong adhesion and interaction with the inorganic framework. It

can be expected that such a residues will have a strong impact on the thermal behaviour as it is directly

attached to the wall.

The extraction experiments were extended to the other mesophases; the TGA results are compiled in

Figure A1-2 (in Appendix I). The TGA profiles are very similar to those for SBA/95 (Figure 3, top). The

amount of template as a function of the extraction time was represented for all the mesophases in

Figure 4. A rapid extraction was found for all mesophases since most of the template removal took place

within 1 h; differences in template content between 1 and 24 h are small. It was found that the final

template content can be reduced between 13 and 27 wt.% after 24 h of extraction but not all, in

agreement with van Grieken et al.28 and Bae et al.60 Interestingly, the amount of template in the sample

can be easily adjusted by the extraction time.

Nature of the extracted template

When mechanism of template removal is under consideration, two possible modes of Pluronic removal

by solvent extraction can be mostly possible. One implies that the majority of P123 is removed without

loss of its integrity while a small amount is left inside the framework. Another possibility is that certain

parts of the block copolymer are removed preferentially. As a consequence, the long-chain polymer is

Figure 4. Solvent extraction of various mesophases. TGA profiles for the template decomposition (left) and evolution of the template content with the extraction time (right).

0

10

20

30

40

50

60

0.00 0.25 0.50 1.00 3.00 5.00 24.00

Tem

plat

e co

nten

t / w

t %

t / hours

SBA/90_Sox t

0

10

20

30

40

50

60

0.00 0.25 0.50 1.00 3.00 5.00 24.00

Tem

plat

e co

nten

t / w

t %

t / hours

SBA/130_Sox t

0

10

20

30

40

50

60

0.00 0.25 0.50 1.00 3.00 5.00 24.00

Tem

plat

e co

nten

t / w

t %

t / hours

SBA/95_Sox t

0

10

20

30

40

50

60

0.00 0.25 0.50 1.00 3.00 5.00 24.00

Tem

plat

e co

nten

t / w

t %

t / hours

SBA/110_Sox t

0

10

20

30

40

50

60

0.00 0.25 0.50 1.00 3.00 5.00 24.00

Tem

plat

e co

nten

t / w

t %

t / hours

SBA/100_Sox t


39

broken and partly washed away by ethanol. This is in contradiction with the potential reuse of the

recycled P123 as its block composition would change.

(4)

In order to characterize the nature of template that has been removed by solvent extraction, the

extracted templates have been analysed by 1H NMR. Representative spectra are given in Figure 5 where

four resonances are visible; ascribed to methylene CH2 (1, 4), methyl CH3 (2) and methine proton CH (3);

identification of the resonance was done based on (4) 60:

Figure 5. Liquid-phase 1H NMR spectra of a) fresh P123, and b) recycled template (from SBA/x, x=100 °C as example).

Table 2 contains the estimated EO/PO ratio determined from equation 3. It can be seen that the recycled

template is always deficient in EO with values for x ranging from 6.2 to 17.7. In the case of SBA/130 the

lower value (6.2) is likely due to partial thermal decomposition. In terms of template’s nature, there exist

an optimal recovery for C where the EO loss is minimal with 17% and

15%, respectively. The results suggest in general that the template left in the silica after solvent

extraction must be rich in EO as long as it is not decomposed; that is the case for aging temperatures

below 130 C. Hence, the interaction of the EO-rich template to the wall is that strong that solvent

washing is not effective enough to remove it. These results indicate that the possible recycle of P123 by

CH2CH2OH CHCH2O

CH3

CH2CH2O Hx'

[1] [2] [3] [4]

yx

0.00.51.01.52.02.53.03.54.04.5Chemical Shift/ ppm

[2]

[1]

[4][3] DMSO

DMSO[4]

[2]

[3]

[1]

(a.)

(b.)

Chapter 2

40

solvent extraction is questionable as its composition changes. The following section discusses the textual

features of the Sox materials to study the location of the residual template and, afterwards its influence

on the structure after calcination or pyrolysis-calcination.

Table 2. Composition of the pluronic precursor and recycled templates a Composition of EOxPOyEOx x/y b x (y=70) EO loss /%

P123 experimental 0.25 17.7 c -

SBA/90 a 0.13 8.8 50 SBA/95 a 0.21 14.6 17 SBA/100 a 0.21 15.0 15 SBA/110 a 0.11 7.9 55 SBA/130 a 0.09 6.2 65

a. after 24 h extraction time; b. Estimated from equation 3; c. Theoretically x = 20.

The following section discusses the textual features of the Sox materials to study the location of the

residual template and, afterwards its influence on the structure after calcination or pyrolysis-calcination.

Textural properties of the solvent extracted materials

The texture of the solvent extracted materials after 24 h extraction time (Sox24) was investigated by Ar

physisorption at 87 K. The isotherms are given in Figure 6, which also includes the calcined counterparts

for comparison. The shape of the isotherms was similar to the calcined materials; type IV with H1

hysteresis, representing solids with cylindrical pore geometry with relatively high pore size uniformity

and facile pore connectivity.63 The derived surface area and pore volume are given in Table 1. It was

generally found that the Sox materials have substantial pore volume and surface area despite the fact

that the template content varied between 13 and 27 wt. %. This means that the residual template does

not block the main channels. A comparative study is given in Figure 6 (bottom) where the cumulative

pore volume is plotted for the Sox24 and calcined counterpart. The inset represents the pore size

distribution using the NLDFT model; the pore size of the maximum is given in Table 1. It was found that

the pore size is slightly higher than the calcined materials because the structure does not shrink upon

solvent extraction (see example in Figure 6-c) while the calcined one does.

Figu

re 6

Top

) Arg

on so

rptio

n iso

ther

ms a

t 87

K; b

otto

m) c

orre

spon

ding

NLD

FT c

umul

ativ

e po

re v

olum

e (in

set r

epre

sent

s the

por

e siz

e di

strib

utio

n) fo

r cal

cine

d (C

) and

solv

ent e

xtra

cted

(Sox

24) s

ampl

es a

ged

at: a

) 90,

b) 9

5, c)

100

, d) 1

10 a

nd e

) 130

°C.

2424

2424

24

Chapter 2

42

The cumulative pore volume shows that the Sox materials contain less intrawall porosity; this intrawall

porosity is defined as the pore volume below the capillary condensation (bear in mind that the start of

the capillary condensation varies among the samples). The fact that the Sox materials contain less

intrawall porosity implies that at least part of the remaining template is located in the walls. However,

the difference in the intrawall porosity between the calcined and Sox24 materials cannot explain the

difference in the total pore volume, that is normally higher (an exception was found for SBA/100

sample). This means that another part of the template occupies the main channel; resembling a coating

as the pore size distribution was very narrow (insets Figure 6, bottom). Hence, the remaining template is

either inside the wall or closely attached to it. This model seems to be applicable for the samples aged at

90, 95 and 110 C. This cannot be concluded for the sample SBA/100 where the template is reasonably

tend to be located in the wall rather than in the major mesopores since the total pore volume is even

higher than the calcined material. The sample SBA/130 has a tiny amount of intrawall template and the

majority in the main channel.

Figure 7. SAXS patterns of SBA/95: a -24

y -lines indicate the position of the (100) reflection upon various treatments.

0.04 0.06 0.08 0.10 0.12 0.14

( e.) SBA/95_Sox24_pyro320_C

( d.) SBA/95_Sox t_C

( c.) SBA/95_Sox t

( b.) SBA/95_C

( a.) SBA/95

Inte

nsity

/ a

.u.

q-factor / Å-1


43

2.3.3 Thermal treatments of the solvent extracted mesophases

The solvent extracted samples were subjected to calcination at 550 C and the resulting materials were

characterized by SAXS. Figure 7 compares the different materials derived from one mesophase to

illustrate the changes (SBA/95). It can be seen that the solvent extracted samples (Figure 7c) having

different template contents ranging between 10 and 52 wt.% (Figure 4), show no thermal shrinkage,

although the peak width changes slightly. Thermal shrinkage was observed when these samples were

calcined at 550 C (patterns d in Figure 7). It was found that the shrinkage was independent of the

template content, although it would be reasonable to assume that the structural shrinkage is a function

of organic content. This phenomenon was generally observed for the other mesophases as well; Figure 8

represents the shrinkage difference with respect to the precursor ( q = q100S

– q100P; where q100

P is

constant as shown in Figure 1-a) as a function of the template content. It can be seen that q is

Figure 8. Shrinkage difference ( q-vector, Å-1) after calcination of the solvent-extracted materials as a function of template content for mesophases aged at: a) 90, b) 95, c) 100, d) 110 and e) 130 °C (the open arrow indicate the starting mesophase while the black one corresponds the solvent extracted mesophase after 24 h, used in section 3.3). Y-axes have the same scale, i.e. 0 – 0.01 Å-1.

15 20 25 30 35 40 45 50 55600.000

0.005

0.010

q-ve

ctor

/ Å-1

Template content / wt. %

(a.)

(b.)

(e.)

(c.)

(d.)

Chapter 2

44

independent of the template amount for each mesophase. The shrinkage difference becomes lower and

it is nearly constant at higher aging temperatures. The q for the Sox24 samples (i.e. after 24 h extraction

time, indicated as black arrows in Figure 8) was taken for further discussion. This was normalized to the

shrinkage of each calcined counterpart and represented in Figure 9 for each mesophase. It can be seen

that the normalized shrinkage difference is a function of the aging temperature, with the highest value at

the lowest aging temperature while at 130 C this quantity achieves a minimal plateau value. This trend

is consistent with the degree of polymerization of the silica, which is low at low aging temperature; a

situation that generates considerable shrinkage. When the aging temperature is higher, the condensed

structure suffers less contraction. This is justified by the 29Si NMR studies on the effect of the

synthesis/aging temperature on the silica condensation of related materials.53-57

This quantity was also calculated for the directly calcined materials that contain considerable template,

ranging between 40 and 50 wt.% (Sox0 in Figure 2.4). It was found that the normalized shrinkage

difference for the directly calcined materials is lower compared to the solvent extracted ones. This

implies that the solvothermal extraction prior to calcination is detrimental from a structural stand point.

Figure 9. Normalized shrinkage difference, relative to the calcined counterpart, as a function of the ageing temperature for three series of materials: direct calcination at 550 °C ( ), calcination (550 °C) after 24 h solvent-extraction (x) and combined pyrolysis (320 °C) plus calcination (550 °C) after 24 h solvent extraction ( ).

90 100 110 120 1300.02

0.04

0.06

0.08

0.10

0.12Directly-calcinedSox24_ CalcinedSox24_Pyrolysed_C

q / q

C (-

)

Directly calcined

Solvent extracted-pyrolyzed-calcined

NO

RMAL

IZED

SHR

INKA

GE

Solvent extracted-calcined

Ageing temperature / oC


45

Additionally, the Sox24 materials were calcined in two steps; they were first pyrolyzed at 320 C to

reduce the template content even further and afterwards calcined. The pyrolyzed samples show a small

weight loss between 200 and 900 C (Figure A1-3, left, in Appendix I) that can mainly be ascribed to

residual carbon from P123; this can be better seen in the TGA derivative overlayer (Figure A1-3 right, in

Appendix I); although part of the weight loss is due to water release from the Si–OH dehydroxylation.

This limited residual carbon is expected as P123 does not polymerize on a neutral silica surface. The

pyrolyzed samples were subsequently calcined at 550 C and analyzed by SAXS (see example in Figure 7e

for SBA/95). The normalized shrinkage difference was calculated and plotted in Figure 9 for all samples.

The shrinkage for the Sox24/pyrolyzed/calcined samples was comparable to the Sox24/calcined

counterparts and higher than direct calcination. Therefore, pre-pyrolysis does not help in reducing the

thermal shrinkage. The overall trend indicates that the lower template present in the sample the thermal

shrinkage is more pronounced, opposite to the original hypothesis. Possible reasons to explain this

Figure 10. Top) Argon sorption isotherms at 87 K; bottom) corresponding NLDFT cumulative pore volume (inset represents the BJH pore size distribution) for the directly calcined (C), solvent extracted/calcined (Sox_C) and solvent extracted/pyrolysed/calcined samples (Sox_Pyro_C) materials, for the mesophases aged at: a) 95 and b) 100 °C.

0.0 0.2 0.4 0.6 0.8 1.00

10

20

30

40SBA/95_CSBA/95_Sox_CSBA/95_Sox_Pyro_C

Qua

ntity

Ads

orbe

d/m

mol

•g-1

Relative Pressures (p/p0)

10 1000.0

0.2

0.4

0.6

0.8

1.0

Cum

ulat

ive P

ore

Volu

me/

cm

3 g-1

Pore Width/ Angstroms

10 1000

2

4

6

8

PSD BJ

H / c

m3 g-1

Pore Width / Angstroms

0.0 0.2 0.4 0.6 0.8 1.00

10

20

30

40SBA/100_CSBA/100_Sox_CSBA/100_Sox_Pyro_C

Qua

ntity

Ads

orbe

d/m

mol

•g-1

Relative Pressures (p/p0)

10 1000.0

0.2

0.4

0.6

0.8

1.0

Cum

ulat

ive P

ore

Volu

me/

cm

3 g-1

Pore Width/ Angstroms

10 1000

2

4

6

8

10

PSD DF

T / c

m3 g-1

Pore Width / Angstroms

(a1.)

(a2.)

(b1.)

(b2.)

Chapter 2

46

behaviour are that the template acts as support to avoid excessive shrinkage during the calcination or

the template can control the diffusion of O2 and the rate of oxidation or both. The solvent extracted

(Sox) and Sox/pyrolyzed samples are highly porous materials with the main pores fully accessible, where

O2 can diffuse much faster than the template constricted mesophases. A situation that can promote a

fast oxidation, possible hot-spots and therefore the thermal shrinkage can be more pronounced than for

direct calcination. This phenomenon seems to be consistent with the well-known mass transfer

limitations during the combustion of carbons.64

In the case of SBA-15 the presence of the template is therefore beneficial to minimize thermal shrinkage.

The effect of the thermal activation protocol on the textural properties was analyzed by Ar physisorption

for two mesophases, 95 and 100 C (that showed the largest differences in Figure 9). The isotherms,

cumulative and PSD graphs are given in Figure 10. It can be seen that the direct calcination gives higher

pore volumes that is in line with the lower normalized shrinkage discussed in Figure 9. The impact on the

interconnectivity is less pronounced and only noticeable for the SBA/100 mesophase. Hence, the thermal

shrinkage affects more to the mesochannel void space than the intrawall porosity.

2.4 Conclusions

In this chapter, it is found out that application of ethanol solvothermal conditions to SBA-15 mesophases

can remove a substantial part of the template from the main mesopores, but not completely. The

extracted template is poor in EO, which is left inside the walls or in the main channel resembling a

coating. Possible reuse of the extracted template is questionable as its nature changes from the original

P123.

For all thermal protocols applied, the thermal shrinkage was a decreasing function of the aging

temperature; a trend that is consistent with the degree of condensation of the silica. When the aging

temperature is higher, the condensed structure suffers less contraction. For each mesophase, the

thermal shrinkage becomes less pronounced when the material is fully templated and this mainly

influences positively the pore volume. Differences in mass transfer of O2 and thereby in the oxidation

rate can explain this behaviour. The template can also have a structural support effect that is missing

when calcining the solvent extracted (and pyrolyzed) materials.


47

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