university of groningen mesostructured sillicate-based ......since the discovery of m41s type of...

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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4

Mesoporous Materials SBA-15 with enhanced hierarchical porosity: a combined

study of mild detemplation and structural preservation

In this work the feasibility of the Fenton detemplation on a series of SBA-15 mesophases has been

studied and compared to the conventional calcination. The as-synthesized and detemplated

materials were studied regarding their template content (TGA, CHN), structure (SAXS, TEM), surface

hydroxylation (Blin-Carteret’s approach) and texture (high resolution Argon physisorption). Fenton

detemplation achieves 99% of template removal, leading to highly hydroxylated materials. The

structure can also be better preserved when a second step is applied after the Fenton oxidation. Two

successful approaches are presented: drying in a low-surface-tension solvent (such as n-BuOH) and

hydrothermal stabilization. Both approaches gives rise to similar low structural shrinkage, lower than

calcination and the water-dried Fenton. Nevertheless, the textural features are remarkably different.

The n-BuOH exchange route gives rise to highly hierarchical structures with enhanced

interconnecting pores and the highest surface areas. The hydrothermal stabilization produces large-

pore SBA-15 structures with high pore volume, very low interconnectivity and micropores. Therefore,

the texture can be fine-tuned in this way while the template is removed.

Chapter 4

66

4.1 Introduction

Since the discovery of M41S type of materials in 19921, ordered mesoporous materials (OMMs) have

drawn remarkable attention because of their potential applications in heterogeneous catalysis2,3,

adsorption4, separation5 and as optical and electric devices6. A great breakthrough in OMMs is the

discovery of the SBA-15 structure.7,8 SBA-15 is defined by the same space group (P6mm) as MCM-41, but

is a very different structure in the aspects of synthesis mechanism and textural characteristics.

SBA-15 is synthesized by using amphiphilic triblock copolymers poly(ethylene glycol)-poly(propylene

glycol)-poly(ethylene glycol) (PEO-PPO-PEO) as structure-directing agent in highly acidic aqueous media;

the acid catalyzes the hydrolysis but it is also responsible of the interaction mechanism of the type

(S0H+)(X–I+); where S0 is the block copolymer, HX is the acid and I+ is the silica cationic species. The

hexagonal structure of SBA-15 holds thick walls, which provide better thermal and hydrothermal

stability9. Additionally, SBA-15 has a tunable pore diameter ranging 5-30 nm that can be easily 10,11, which give opportunities to obtain different textual

properties. Especially interesting, SBA-15 has a secondary porosity due to insertion of PEO units into the

silica wall during synthesis12, which is normally described as intra-wall porosity. This intra-wall porosity is

composed of interconnecting small mesopores between the main channels and micropores. The

interconnecting mesopores, sometimes defined as secondary mesopores, have been later confirmed by

making stable carbon CMK-n nano-replicas materials.13, 14 The existence of micropores has been proved

by gas physisorption10,11,15-17 and quantitative X-ray analysis12.

Besides the advantage to be used as hard template to synthesize replicas, the abundant intra-wall

porosity of SBA-15 also contributes greatly to the total surface area and can reduce diffusion limitations

which commonly occur to one dimensional structure, such as MCM-41. 18-21 Some studies have reported

the mechanism of intra-wall porosity formation during synthesis. It was found that SBA-15 synthesized

at intermediate ageing temperatures possesses an optimal intra-wall porosity. Materials aged at

tra-wall and microporosity, with the development of

large-pore diameters and improved hydrothermal stability of the walls.11,17

Two synthetic routes have been reported in order to tune the porosity of mesoporous silicas. One way

to go is to adjust the synthesis conditions to control the self-assembly pathway; the utilization of

swelling agents, such as mesityleen22-26, triisopropylbenzen27,28, alkanes29,30 and amines31 has been

reported in order to produce larger pores. Some of these approaches have been successfully applied to

SBA-15. By using swelling agent during SBA-15 synthesis, both the major mesopores and intra-wall pores

Mesoporous Materials SBA-15 with enhanced hierarchical porosity: a combined study of mild detemplation and structural preservation

67

are increased simultaneously.23,27-29 Nevertheless, the independent adjustment of the primary and

secondary porosity has seldom been reported by using micelle modifiers. Besides the utilization of

organic-based swelling agents, salts have also been reported. Yu et al.32 reports that KCl facilitates the

synthesis of highly ordered silica mesophase. Lately, efforts have been put into the selectively

adjustment of the secondary porosity, which could be very important for mass-transfer limited

applications, as mentioned earlier. Reichhardt et al. described the removal of intra-wall porosity of SBA-

15 by adding NaI during synthesis while maintaining major pore size.33 This procedure gives a similar 2-D

structure as MCM-41 however holding bigger pore diameters and thicker walls. On the other hand, Zhu

et al. published their efforts to fabricate SBA-15 with enhanced interconnectivity by the addition of Poly

(vinyl alcohol) without changing the mesopores.34

A different way to enhance the porosity can be the development of non-thermal detemplation methods.

The organic template species has to be removed to generate the porosity before putting into application.

2 combined with air/O2) is used to oxidize the organic

templates. Such a process completely removes the organic species; however it leads to significant

framework contraction and loss of hydrophilicity35 as a result of thermal condensation. We have shown

that this phenomenon is more severe for SBA-15 mesophases with less condensed structure, which are

aged at relatively lower temperature.36 Therefore, alternative non-thermal detemplation routes, other

than calcination, are still sought. This type of mild methods aims to preserve better the structure

without the need to modify the conventional synthesis protocols.

Several mild methods of detemplation have been reported including solvent extraction36-39, supercritical

fluid extraction40-42, chemically aided and UV-Vis stimulated oxidation43-51 , sonication52, microwave

digesting53 and combined methods54,55. Despite most of these techniques yield acceptable to good

detemplation yields and structural ordering, based on X-ray diffraction, the effect of these mild

approaches on the secondary porosity and surface properties, for the case of SBA-15 is hardly found.

Another mild detemplation method is based on the Fenton chemistry. This was applied to zeolite

BEA56,57 and FER58 (in this case by break down strong complexation equilibria) and some mesoporous

materials50,59,60. The approach serves as detemplation or Fe-incorporation technique, or both. Fenton

chemistry was originally applied in organic pollutant treatment in waste water since it was defined in

1894.61 In this process, extremely oxidizing hydroxyl radicals (OH ) are formed from hydrogen peroxide

catalyzed by Fe (III)/Fe(II) (Eqs. 1.-2; reactions 3 and 4 show two of the possible parallel reactions where

OH are wasted). These radicals can oxidize organics compounds in aqueous solution very effectively, as

Chapter 4

68

well as the block copolymer entrapped within the structure of SBA-15 (eq. 5), where SiOx(OH)2–x

represents an ill condensed network: + + + (1) + + + (2) + + (3) + + (4) ( ) ( 123) + ( ) ( ) + + (5)

We have recently reported the application of the mild Fenton detemplation method to a SBA-15

structure aged at 105 °C.62 The method renders a nearly pristine SBA-15 without structural shrinkage,

low residual template, improved surface area, pore volume and silanol concentration. In this work, we

have extended this approach to a series of SBA-15 mesophases that were aged in the temperature range

of 90 to 130 °C. It turned out that for the low-temperature aged samples, a complete template removal

and full structural preservation cannot be achieved if a second step is applied, in order to compensate

for the capillary tension exerted during drying. Two post-detemplation methods were applied. In one

method, a low-surface-tension solvent exchange before drying is applied, which was successfully applied

on soft MCM-4160. A second novel method was implemented as well, consisting of the application of a

hydrothermal treatment after Fenton detemplation. Both methods will be compared with the calcined

counterparts in aspects of detemplation level, surface hydrophilicity, ordering and textural

characteristics.

4.2 Experimental parts

4.2.1 Synthesis of SBA-15 mesophases

Chemicals

All chemical compounds were used as received without further purification: tetraethoxysilane (TEOS,

98%, Aldrich), poly(ethyleneoxide)20-poly(propyleneoxide)70-poly(ethyleneoxide)20 (Pluronic®-P123),

Sigma-Aldrich), hydrochloric acid (HCl, 37 wt. %, ACROS), Iron (III) nitrate nonahydrate (Fe(NO3)3·9H2O,

Riedel-de Haën), hydrogen peroxide (H2O2, 30%, Merck, 1.07209.1000).


69

Synthesis

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

reported elsewhere.7, 8 In all experiments the hydrolysis step was identical and the condensation was

systematically varied with the ageing temperature. 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- 2

gel was: 1.0 SiO2:0.017 P123:5.9 HCl: 204 H2

mixture was then transferred into a 500 ml Teflon bottle and aged under static conditions at various

iltered and the as-obtained solid

was washed with 2 litres of Mili- -15

materials are denoted as Sx-M, where x represents the aging temperature and M indicates mesophase.

Template removal by calcination

kept 6 h at constant temperature, and then cool down to room temperature. The calcination was

performed in a Nabertherm box furnace model LT9/11 equipped with a P330 temperature. The suffix C

is added to the samples code when those are calcined, e.g. S100-C.

Template removal by Fenton oxidation

Before performing the Fenton experiments, the materials were subjected to solvent extraction in

absolute ethanol for 24 h. The details of solvent extraction were described in a former work36. In a

typical Fenton detemplation protocol (method A), a suspension is made by mixing 1.5 g solvent

extracted SBA-15 material and 30 ml Mili-Q water in a round-bottom flask at room temperature. Then

1.5 ml of a Fe (NO3)3 solution (having 1000 ppm iron concentration) was added to the mixture. To this

suspension, 60 ml H2O2 was poured in slowly when stirring. The as-obtained mixture, having ~17 ppm

Fe (III) and 10 vol. % H2O2 -bath equipped with a

condenser. Various treatments were applied after the reaction ended as described in the following

section.

In an alternative method (method B), 1 g of solvent extracted SBA-15 material was mixed with 25 ml

H2O in a round bottom flask at room temperature. The mixture having an Fe(III) concentration of 140

Chapter 4

70

ppm was pre- 2O2 was added stepwisely (5, 10, 15 and finally 20 ml).

The final concentration is ~45 ppm for Fe(III) and 10 vol. % for H2O2..

All experiments were performed by using method A unless it is specified.

4.2.2 Post-detemplation treatments

Direct drying

The aqueous slurry after Fenton-detemplation is centrifu

overnight. The samples obtained in this manner are suffixed with FW, where W comes from water dried.

Hydrothermal treatment

The aqueous slurry after Fenton-detemplation was transferred into a 150 ml Teflon bottle and heated at

-generated pressure. The final silica product was collected by

where HT comes from hydrothermal treatment.

Solvent exchange

After Fenton-detemplation, water was firstly removed by centrifugation. Then around 20 ml n-Butanol

(anhydrous, 99.8%, Sigma-Aldrich) was added into the as-obtained wet solid and stirred for 15 minutes

to reach a good mixing. This solvent-exchange step was repeated for 5 times to reach a thorough

exchange of H2O with n-

overnight.

For the samples detemplated by method B, solvent exchange was carried on by adding 10 mL n-butanol

and stirring for 20 minutes at 70 °C in a test-tube. Subsequently, the mixture was centrifuged and the

24 h. Afterwards the

The samples obtained in this manner are suffixed with FB, where B comes from n-BuOH exchange.

The samples codes and treatments are summarized in Table 1.


71

Table 1. Summary of sample codes and their preparative conditions.

Material Method of preparation Sx-M Mesophase: SBA-15 aged at x °C for 24 h, washing and drying. Sx-C Calcination at 550 °C on the Sx-M mesophase. Sx-se Solvent extraction as described elsewhere.36 Sx-seFW Fenton oxidation, dried directly in H2O, on the Sx-se. Sx-seFBD Fenton oxidation, exchange in n-BuOH, drying, on the Sx-se. Sx-seFHT Fenton oxidation, hydrothermal treatment 100 °C 24h, drying, on the Sx-se.

4.2.3 Characterization

Small angle X-ray scattering (SAXS) measurements were performed using a NanoStar instrument (Bruker)

at room temperature. A ceramic fine-focus X-ray tube powered with a Kristallflex K760 generator at 35

kV and 40mA. The primary X-ray flux is collimated using cross-coupled Gobel mirrors and a pinhole of

at the sample position. The sample-detector distance was 104 cm. The scattering is registered by a Hi-

Star position sensitive area detector (Siemens AXS) in the range of 0.1-2.0 nm. After measurement SAXS

spectra are integrated with the Chi method.

The template content was firstly examined by TGA in a Mettler-Toledo analyzer (TGA/SDTA851e).

Typ - -Al2O3 crucible was filled 10 mg of sample. The decomposition of the

template was monitored in air flow of 100 ml/min NTP while the temperature was increased from 30 to

The template content was quantified by CHN analysis as well. Approximately 2 mg of sample was

decomposed into CO2, H2O, and N2. These are later separated in an online gas chromatograph equipped

TM software enables the integration of the chromatogram. The integrated peak height is used in the

calculations using acetanilide (puriss.

Transmission electron microscopy (TEM) images were obtained on a JEOL transmission electron

microscope equipped with a field emission gun operating at 200KV.

The Fe content was evaluated by inductively coupled plasma atomic emission spectroscopy (ICP-AES)

after dissolving the samples in 5 wt.% HF aqueous solution.

Chapter 4

72

Gas sorption isotherms were measured with an ASAP2420 adsorption analyzer (Micromeritcs) by using

for 4 hours. The total pore volume was calculated by

single point at 0.98 p/p0 relative pressure. Brunauer-Emmet-Teller (BET) method was applied for total

surface area. Pore size distribution (PSD), cumulative pore volume distribution (CVD), and cumulative

surface area distribution (CSD) were determined using non-local density functional theory (NLDFT)

developed for oxide surfaces for Argon adsorption.63

4.2.4 Definitions

Detemplation efficiency based on TGA:

= (1 - Sx-FW Sx-se

)×100 (6)

where TGA150-800 is the TGA weight loss between 150 and 800 oC.

Detemplation efficiency based on CHN:

= (1 - Sx-FW Sx-se

)×100 (7)

where CCHN is the carbon content as determined by CHN analysis.

Shrinkage level: a = ( ) ( ) ( ) × 100 (8)

where a0 is the hexagonal lattice parameter: = 100 (9)

Density of silanols:

For template-free materials (i.e. C, FW and FHT) the silanol group density was evaluated by the Blin-Carteret equation64 (10): OH(groups. nm ) = (10)

Where is the TGA weight loss (wt.%) between 150-800 oC, NA is Avogadro’s number, SBET is the specific surface area using Ar and MWH2O is the molecular weight of water.

Textural parameters:

VT (cm3/g): Total pore volume determined at p/po = 0.98 in the desorption branch. (11)


73

SBET (m2/g): Specific surface area determined by the BET model. (12)

DNLDFT (Å): Pore diameter based on the non-local DFT model (NLDFT) (13

V (cm3/g): Micropore volume obtained from the pore volume distribution (PVD) determined by the NLDFT model for pores 20 Å.

(14)

V IW (cm3/g): Intrawall porosity, determined as the pore total volume before the raising step in the pore volume distribution determined by the NLDFT model.

(15)

V IC (cm3/g): Interconnecting porosity defined as VIW – V . (16)

4.3 Results and discussion

4.3.1 Evaluation of the template removal efficiency

The mesophases, before applying the Fenton detemplation, were first subjected to ethanol extraction to

reduce the amount of template, which was found to be between 11.9 and 27.1 wt.% based on TGA after

Figure 1. TGA patterns and corresponding DTGA derivative plots for the samples Sx-FW, x =90-130, after Fenton detemplation.

the extraction. These correspond to 5.1 to 8.8 wt.% based on carbon (Sx-se samples in Table 2). Thus,

solvent extraction reduces the template content in 55-80% as compared to the mesophase (TGA basis).

S90 S95 S100 S110 S130Sx-seFW

200 400 80060070

80

90

100

-0.2

-0.1

0.0

Aging Temperature (x) / °C

Deriv

ate

/ % (°

C)-1

Rela

tive

wei

ght /

%

Chapter 4

74

The template was almost eliminated from the porous network after applying the Fenton treatment. The

carbon content (Table 2) was significantly reduced with a final carbon level of 0.08-0.12 wt.% This

implies that the Fenton detemplation efficiency is about 98-99% ( CHN). Application of harsher

conditions (method B, where the initial Fe concentration is ~8 times higher than in method A) does not

enhance the detemplation efficiency; the carbon contents lies in a comparable range than for method A

(Table 2).

Table 2. Composition of Sx mesophases after Fenton detemplation (method A), evaluated by TGA and CHN analysis, and Fe content.

Ageing T (x) Fe a Sx-se b Sx-seFW b TGA Sx-se c Sx-seFW c,d CHN d wt. % TGA wt.% TGA wt.% % carbon wt % carbon wt. % %

90 0.11 (80) 27.1 3.9 86 7.6 0.08 (0.17) 99 (98) 95 0.10 (83) 17.0 3.8 78 5.8 0.08 (0.03) 99 (99) 100 0.09 (79) 11.9 3.4 71 6.0 0.10 (0.07) 98 (99) 110 0.11 (89) 19.2 3.2 83 5.1 0.12 (0.17) 98 (97) 130 0.07 (61) 12.8 2.7 79 8.8 0.11 (0.16) 99 (98) a. ICP-AES. Values between parenthesis correspond to the percentage of Fe adsorbed from the solution; b. TGA

c. carbon content obtained by CHN elemental analysis (average value of two analyses); d. value in parenthesis corresponds to detemplation method B.

The TGA patterns of the Fenton detemplated materials (Fig. 1) are formed by two weight losses. The

first one accounting 5-10 wt.% is centered at 130 °C, due to the release of physisorbed water.60. A broad

weight loss occurs from 200 until 900 °C (accounting 3-4 wt.%). The detemplation efficiency based on

TGA weight loss was calculated as well ( TGA, Table 2), which was found to range 71-86%. These values

are much smaller than the carbon-based efficiencies. This implies that the broad weight loss at 200-

900 °C is not only due to the residual carbon decomposition but, to a large extent, to the water from the

silanols condensation. Thus, for this type of mild detemplated samples the use of the carbon-based

detemplation efficiency ( CHN) most accurate.

The Fe concentration in the final detemplated samples was determined by ICP-AES, for those treated by

method A. It turns out that the Fe from the solution is adsorbed on the samples. Between 60 to 90% of

the Fe from the Fenton-solution is adsorbed on the samples’ surface, with absolute values ranging 0.07-

0.11 wt.%. This result could explain why not all the template is removed at the applied conditions. Once

the Fe cations (FeIII, FeII) are adsorbed on the material’s surface, its intrinsic Fenton activity decreases; it

is well known that the intrinsic activity of grafted Fe-based Fenton catalysts is inferior to that in solution

using homogeneous-based catalysts. 65


75

4.3.2 Silanols concentration

The relatively high weight loss between 200 and 900 °C observed in the TGA pattern indicates that the

material’s surface is highly hydroxylated after Fenton detemplation. The silanols’ concentration can be

calculated from the Blin-Carteret equation,64 which relates the silanols concentration as a function of the

TGA weight loss and surface area. This method was first applied to the calcined (Sx-C) and Fenton

detemplated materials (Sx-seFW). Figure 2 shows the absolute values and trends with the ageing

temperature. There is a decrease of the Si-OH concentration with the ageing temperature. This is

related to the higher degree of network condensation with the increasing ageing temperature, which

implies that less silanols are available on the surface due to the condesantion, which makes the material

more polymerized. We recently proposed that such a higher condensation makes the structure more

resistant to thermal contraction.36 The improved condensation with the synthesis or ageing temperature

have been reported for MCM-4166,67 and JLU-2068-70.

Figure 2. Silanols concentration based on the Blin-Carteret’s approach as a function of the ageing temperature for the calcined (Sx-C), Fenton-water-dried (Sx-seFW) and Fenton plus hydrothermal treatment (Sx-seFHT). Raw data are given in Table A2-2.

In terms of absolute values, the Fenton-derived materials have between 60 and 120 % higher silanol

concentration than the directly calcined counterparts, with a maxium obtained at 100 °C ageing

temperature, of the studied samples.

90 100 110 120 130 1400

2

4

6

8

Si-O

H co

ncen

tratio

n /

wt.%

Aging Temperature (x) / CAging Temperature (x) / °C

Si-O

H c

once

ntra

tion

/ wt.%

0

2

4

6

8

90 100 120110 130

Sx-C

Sx-seFWSx-seFHT

Chapter 4

76

4.3.3 Structural and textural characterization. Effect of the post-detemplation treatment.

Influence on structure and texture after Fenton detemplation.

After detemplation, the as-obtained template-free mesophases were characterized structurally and

texturally and compared to calcined ones and mesophases. Characterization by SAXS of the mesophases

(Fig. 3, A2-1 and A2-3) reveals a hexagonally packed cylindrical morphology characterized by a distance

Table 3. Argon-based textual properties of SBA-15 obtained under different detemplation routes.a

Material Treatmentsa VT / cm3 ·g-1 SBET / m2·g-1 DNLDFT

/ Å V / cm3 ·g-1 VIW / cm3 ·g-1 VIC

/ cm3 ·g-1

S90 C 1.034 678 106 0.054 0.263 0.209 seFHT 1.242 698 129 0.039 0.246 0.207 seFB 1.265 698 112 0.114 0.352 0.238 seFW 0.910 717 96 0.094 0.268 0.174





a. The nomenclature for the different treatments are given in Table 1 and the textural parameters are defined in the experimental part.

After detemplation, the as-obtained template-free mesophases were characterized structurally and

texturally and compared to calcined ones and mesophases. Characterization by SAXS of the mesophases

(Fig. 3, A2-1 and A2-3) reveals a hexagonally packed cylindrical morphology characterized by a distance


77

Figure 3. SAXS patterns for the SBA-15 mesophase aged at 90 oC (S90) after various treatments. Two detemplation protocols were applied: A) method A and B) method B. Additional SAXS patterns are given in Fig. A2-1 and A2-2.

between the cylinders of ~12.4 nm, from the position of the 100 reflection, and well-defined secondary

110 and 200 reflections with p6 mm symmetry. Figure 3-a illustrates one example, where several

treatments were applied to the S90 mesophase. The directly calcined material also reveals a similar

0,04 0,06 0,08 0,10 0,12 0,14

Inte

nsity

/ a.

u.

q-factor / Å-1

0,04 0,06 0,08 0,10 0,12 0,14

Inte

nsity

/ a.

u.

q-factor / Å-1

A

B

S90

S90

FW

M

FB FHT

C

seFBseFHTseFWCM

0.04 0.06 0.100.08 0.12 0.14

q-factor / Å-1

Inte

nsity

/a.u

.In

tens

ity/a

.u.

seFBseFWM

Chapter 4

78

Figure 4. A) Argon sorption isotherms at 87 K, B) corresponding pore size distribution (NLDFT); C) cumulative pore volume (NLDFT) and D) cumulative surface area (NLDFT), for S90 after different treatments. Additional graphs can be found in Fig. A2-3.

hexagonal ordering, though the entire pattern shifts towards higher angles due to thermal shrinkage.

The SAXS pattern of the Fenton-derived material (seFW) also has a hexagonal arrangement but the

pattern shifts to higher q-values and the intensity of peaks are remarkably reduced. In the case of

50 100 1500

2

4

6

8

10

PSD NL

DFT/

cm3 g-1

DNLDFT / Å

0.0 0.2 0.4 0.6 0.8 1.00

10

20

30

40

50

Qua

ntity

Ads

orbe

d/m

mol

·g-1

Relative Pressures p/p0

50 100 1500

200

400

600

800

DNLDFT / Å

SAD NL

DFT/

cm2 g-1

50 100 1500.0

0.2

0.4

0.6

0.8

1.0

1.2

PVD

NLDF

T/ cm

3 g-1

DNLDFT / Å

VIW

VICV

CseFWseFBseFHT

S90

B

C

D

A


79

detemplation method B, that is harsher than method A, the effect is even more catastrophic since the

100-reflection almost flattens (Fig. 3-b), and the complete disappearance of 110 and 200 secondary

reflections was observed, indicating a fully collapsed structure with severe shrinkage.

In Figure 5 the relative shrinkage in the unit cell ( a0), derived from the SAXS patterns (Fig. 3, A2-1 and

A2-2) is represented as a function of the ageing temperature for various treatments. The relative

shrinkage for the calcined materials decreases with the ageing temperature which is consistent with a

former study; 36 this was explained by the enhanced condensation of the network that makes it more

resistant to thermal deformation. It was remarkable to find that the Fenton–derived materials (series

SeFW) show higher shrinkage than the calcined counterparts in the series, with shrinkages as high as

13%. This effect was noticeable at lower ageing temperature while it disappears at high ageing

temperatures, 110 °C for the series under investigation.

Figure 5. Relative shrinkage ( a0, %) as a function of the ageing temperature for various treatments: calcination (Sx-C), Fenton-warer-dried (Sx-seFW), Fenton and n-BuOH exchange (Sx-seFB) and Fenton and hydrothermal treatment (Sx-seFHT). SAXS patterns are given in Fig. A2-1 and A2-2. Raw data are given in Table A2-2. Fenton protocol: method A.

The overall textural properties using Ar including isotherms, pore size distribution, cumulative pore

volume and cumulative surface area, based on the NLDFT model, are given in Figure 4, Fig. A2-3, and

raw data are compiled in Table 3. It is generally found that the SeFW materials show reduced total pore

90 100 110 120 1300

5

10

15

a0 /

%

Aging Temperature (x) / C

Sx-CSx-seFWSx-seFBSx-seFHT

Sx-CSx-seFWSx-seFBSx-seFHT

Aging Temperature (x) / °C90 100 120110 130

a 0/%

0

5

10

15

Chapter 4

80

volume and decreased mesopore’s diameter compared to the calcined counterparts. This trend is

consistent with the shrinkage of the silica framework with the formation of smaller pore that

contributes less to the pore volume. An exception is found for the S130-seFW that have similar textual

properties than the calcined counterpart due to relatively higher hydrothermal stability. The

microporosity of SeFW samples has increased in all cases due to shrinkage from the structural collapse.

Therefore, these textural trends are in line with the structural changes concluded from SAXS.

The possible explanation for the structural and textual modifications upon Fenton detemplation can be

ascribed to the drying conditions. Figure 6 shows the capillary forces applied on the walls of a

hexagonally arranged mesostructured material during drying. A calculation of the capillary forces

present on the material’s walls under water can be done by means of the Laplace equation; assuming

the major mesoposre diameter is 120 Å, pressures as high as ~150 bar can be generated during drying,

n SBA-15, which

is also known as intra-wall porosity, could suffer even more pressure due to smaller radii according to

Laplace equation. Additionally, the Fenton derived materials have a low degree of polymerization, since

they have not undergone further condensation except under the synthesis steps. All these factors favor

structural damage on the Fenton-derived mesophases, and even collapse when the structure aged at

low temperatures holds smaller pores and low hydrothermal stability.

Figure 6. Illustration of the capillary forces applied on the walls of a hexagonally arranged mesostructured material during drying. The pressure increase ( P) is given by the Laplace equation: r is the radii (half of the pore diameter), is the surface tension and is the contact angle.

Based on this hypothesis, the first apparent solution is reducing the capillary force to a minimum extent.

Basically, there is three main factors in the Laplace equation that can be changed: as radii, surface


81

tension constant and contact angle. Among these three factors, the major mesopore diameter is

dependent on aging temperature which has been adjusted at the range of

process. The simplest choices left are to adjust the value of surface tension constant and contact angle.

Both factors are physical properties that heavily depend on the applied solvent. So there is no doubt

that a solvent with smaller surface tension is highly desirable; as it was applied recently for soft MCM-

41.60 Another possible solution is to improve hydrothermal stability of silica mesophases to resist better

the damaging pressure from drying. Both approaches will be discussed in the following sections.

Approach I: Influence on structure and texture by solvent exchange after Fenton detemplation.

The main cause of structural and textual damage appears to be the capillary force as we discussed in the

former section. Therefore, solvent exchange has been performed immediately after Fenton oxidation to

the wet sample before drying. Among possible options, n-butanol (n-BuOH) was chosen as it is an

optimal solvent for this study; the reason for this choice is due to low surface tension ( n-BuOH =24.6

dyn/cm) which is only one-third of this value for water ( water =72.8 dyn/cm). Also, the relatively high

boiling-point of n-BuOH

solvents co-exist during the drying process. After the solvent is settled, two methods have been applied

in the subsequent solvent-exchange step corresponding to detemplation methods A and B respectively.

In method A, n-butanol exchange is performed as 5 washing steps at room temperature while method B

involves an extra equilibration st

guarantee a thorough exchange.

The samples after n-BuOH exchange were measured by SAXS as shown in Figure 3, for the S90

mesophase. By using method A, S90-seFB shows a q[100] value clearly smaller than directly dried (S90-

seFW) and calcined samples, but still the position of the q-factor for the 100 reflection is somewhat

higher than the precursor. That indicates that S90-seFB possess less shrinkage than the S90-seFW due to

the reduced pressure from the capillary forces. Nevertheless, the slight difference of q [100] peak

position, when compared with corresponding precursors (P in Fig. 3) demonstrates that the framework

contraction still exists. The calculated cell parameters (Table A2-2) shows that the shrinkage level of S90-

seFB is much less ( a0=5.8 %) than either calcined (10.1%) or S90-seFW (13.0%) samples.

On the other hand, exchange method B resulted in a better structural preservation for the same

mesophase. As shown in Figure 3-b, S90-seFB gives a nearly identical SAXS pattern as the precursor with

well-defined 100, 110 and 200 reflections. This improvement is not surprising since 24-hours equilibrium

Chapter 4

82

step for method B possibly promotes the efficiency of solvent exchange greatly. Consequently, water in

silica mesophases was more thoroughly exchanged by n-BuOH in method B rather than method A; and

the existence of water during drying, as we stated earlier, is the main cause of structural and textual

damage. So the structural damage after a more thorough solvent-exchange in method B becomes more

negligible.

When we extend the method A to four additional mesophases (Figure A2-1a and A2-2), similar

improvement is observed compared to the corresponding seFW and calcined counterparts; however,

the improvement is less prominent than for S90 since the mesophases get stable with the ageing

temperature. So for samples aged at relatively higher temperatures, S110 and S130, the n-BuOH shows

less visible improvement compared to the water drying (seFW) according to SAXS patterns. The effect of

the n-BuOH exchange is clearly demonstrated in Figure 5, where the relative shrinkage of the seFB series

is much lower than the seFW and inferior than the calcined materials, as a result of the better drying

conditions. The effect disappears with the ageing temperature, and eventually the sample S130 is

insensitive to calcination, FW or FB due to its high hydrothermal stability. It is noteworthy that method B

provided spectacular results in the sense that no shrinkage was obtained for the complete series (Fig. 3

and Fig. A2-1, b). However, from a practical stand point, we opted to study the texture of samples

derived from method A, since it is a much faster protocol. TEM pictures (Figure 7) for the S100-C and

S100-seFB shows a similar porous structure which is consistent with SAXS patterns.

Detailed textual information for these materials can be found in Table 3 and Fig. A2-2. The shape of

the isotherms was in all cases similar; type IV with H1 hysteresis, representing solids with cylindrical

pore geometry with relatively high pore size uniformity and facile pore connectivity (Fig. A2-2, a).71 The

pore sizes for the seFB were always larger than for the calcined and seFW, in that order (Fig. A2-2, b);

though the differences disappears at higher ageing temperatures. This indicates a less contracted

structure for the seFB series. The cumulative pore volume curves are consistent with the total pore

volume, seFB > C > seFW.

Approach II: Influence on structure and texture of in-situ hydrothermal treatment after

Fenton detemplation.


83

Another method developed to avoid the structural damage is a post hydrothermal treatment. The

purpose of this step is to obtain a structure with further condensation; in other words, with better

hydrothermal stability. Amorphous silica bears silanol groups on the surface as well as exposed siloxane

(Si-O-Si) bonds.72 The former one is hydrophilic and dominates the silica surface properties while the

latter one is normally considered as hydrophobic. The condensation level was evaluated by the Blin-

Carteret’s approach (Fig. 2). The condensation extent of hydrothermally treated samples (Sx-seFHT) is

intermediate between the Sx-seFW and Sx-C; which indicates that the hydrothermal step reduces the

surface Si-OH and condense the structure further by creating siloxane bonds.

Figure 7. TEM pictures for S100 after: a) calcination (S100-C); b) Fenton-detemplation via method A plus n-butanol exchange (S100-seFB).

From the ordering point of view, the hydrothermal treatment preserves the hexagonal structure (Fig. 3,

A2-1 and A2-2) and the observed relative shrinkage is also low, in the same order as that found for the

Sx-seFB series (Fig. 5). A similar dependency with the ageing temperature could be appreciated for this

treatment. Therefore this approach seems to be effective in avoiding capillary-tension induced

shrinkage. The shape of the adsorption isotherms are also similar, of the type IV with H1 hysteresis but

20 nm20 nm

20 nm20 nm20 nm20 nm

(a.)

(b.)

20 nm20 nm

Chapter 4

84

the hydrothermal treatment gave rise to an enlargement of the pore size with a decrease of the

micropore volume. This is consistent with the well-known trends of the effect of the ageing temperature

as reported by Galarneau et al.11 Thus, bigger pores are formed at expense of eliminating microporosity.

Figure 8. Textural parameter derived from Ar physisoprtion: A) total pore volume; B) specific surface area; C) micropore volume and D) Interconnecting porosity.

However, the comparison of the textural features reveals differences between the n-BuOH and

hydrothermal treatment. This can be appreciated in Fig. 8 where the surface area, total pore volume,

micropore volume and interconnecting pores are compared among the various detemplation

approaches, for the series of mesophases. In terms of total pore volume both post-detemplation

approaches produces the highest values with a maximum for S130. The highest microporosity was found

at low ageing temperatures for the SeFW due to the significant collapse and formation of micropores;

SeFB possesses a better defined structure with an intermediate fraction of microprores while those are

severely depleted after the hydrothermal treatment. The specific surface area for all the treatments do

have a maximum at around 100 °C ageing temperature; the SeFB shows the highest value with 822 m2.g-


V μ/ c

m3 g

-1

0

0.04

0.08

0.12C

0

4

8

2


V IC

/ cm

3 g-1

0

0.1

0.2

0.3

0.4D

90 100 110 120 130

V T /

cm3 g

-1

0

0.4

0.8

1.2

Sx AS B

ET /

m2 g

-10

200

400

600

800

CFWFBFHT

CseFWseFBseFHTB

90 100 110 120 1300

0

0

0

0


85

1 that was minimal for the seFHT series. The interconnecting pores, defined as those mesopores smaller

than the main channel, was estimated and it turns out that the optimal interconnecting porosity is

maximized for the S100-seFB with a value of 0.366 cm3.g-1 which represents 27% of the total pore

volume. A second candidate is the S110-seFHT with 0.359 cm3.g-1 with 27% of the total pore volume as

well.

4.3.4 Mechanistic insights about the structural changes upon the various post-treatments.

When we take a look at all methods including calcination and the Fenton related routes, it is clear that a

mild detemplation itself is not enough to obtain well-preserved structures despite of the full template

removal. The two post treatments we developed can greatly preserve the structures. Most interestingly,

the porosity is improved preferably in different fashions.

The mechanistic pathways including all the situations presented in this study are summarized in Figure 9.

When calcination is performed (process ), the structure is highly condensed and gets contracted

while having a good ordering and hierarchical porosity; the template is fully oxidized in this process. The

Fenton detemplation is quite effective in removing the template with only trace amount iron left on the

surface, likely as FeOx oxides species. The drying process can be devastating to the structure when the

SBA-15 mesophases after Fenton-detemplation are dried in water (process ). Therefore a less

ordered structure, or even totally disordered, is obtained in case of drying directly after Fenton

detemplation (Material E). The reason for this damage is because of the large capillary forces generated

during water evaporation within the pores; the effect is more pronounced for low-ageing temperature

materials while it disappears at high ageing temperatures. After exchanging water with n-butanol

(Material D), the capillary forces can be significantly reduced, thus the final dried material holds a better

preserved hierarchial structure (Material G), including quasi-pristine cell parameter, with enhanced

interconnecting pores and the highest specific surface area. The hydrothermal treatment is another

method to avoid the damage. The as-obtained SBA-15s are more condensed with higher cell parameter,

enhanced diameter of major mesopores and minimized secondary porosity, consequently having the

smallest surface area (material F).

Chapter 4

86

Figure 9. Transformation pathways of the SBA-15 mesophase after various detemplation routes discussed in this study: SBA-15 mesophase after solvent extraction (A, Sx-se); A after calcination (B, Sx-C); A after Fenton oxidation containing water (C); C after drying (E, Sx-seFW); C after n-BuOH exchange (D); D after drying (G, Sx-seFB); C after hydrothermal ageing and drying (F, Sx-seFHT). Nomenclature: CAL (calcination), FD (Fenton detemplation), SE (solvent exchange), D100 (dried at 100 oC), HT100/24 (hydrothermally treated at 100 oC for 24 h).

4.4 Conclusions

Full template removal (99%) of SBA-15 mesophases without thermal calcination is achieved in this work

after applying a Fenton-chemistry based detemplation method. The structural preservation can only be

achieved if a post-detemplation methodology is applied, due to the intense capillary forces when drying

in a water-based medium. Two routes have been successfully applied to overcome this: n-BuOH

exchange to dry the mesophase in a low-surface tension medium or the application of a hydrothermal

step after the Fenton detemplation in order to condense further the structure and become more

resistant to the capillary forces. Both approaches preserve better the structure in terms of the relative

shrinkage that is smaller than calcination, and Fenton water dried, but give rise to different type of

porosities. The n-BuOH exchange produces high surface areas and hierarchical porous structures with

enhanced interconnecting pores while the hydrothermal treatment yields materials with less

interconnectivity, larger pore sizes and consequently high pore volumes.

seF BE

H2O

a0

DNLDFT

C.

n-BuOH

D.

B. E. F. G.

CAL D100HT100/24,

D100 D100

Template Pluronic P123

A.


87

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university of groningen mesostructured sillicate-based ......since the discovery of m41s type of...

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