www.elsevier.com/locate/micromeso

Microporous and Mesoporous Materials 95 (2006) 180–186

Direct formation of hydrophobic silica-basedmicro/mesoporous hybrids from polymethylhydrosiloxane

and tetraethoxysilane

Dongjiang Yang a,b, Junping Li a, Yao Xu a,*, Dong Wu a, Yuhan Sun a,Huaiyong Zhu c, Feng Deng d

a State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, 27, Taoyuan South Road, Taiyuan 030001, PR Chinab Graduate School of the Chinese Academy of Sciences, Beijing 100039, PR China

c School of Physical and Chemical Sciences, Queensland University of Technology, Brisbane QLD 4001, Australiad State Key Laboratory of Magnetic Resonance and Atomic Molecular Physics, Institute of Physics and Mathematics, Chinese Academy of Sciences,

Wuhan 430071, PR China

Received 12 September 2005; received in revised form 12 March 2006; accepted 14 May 2006Available online 7 July 2006

Abstract

A facile pathway has been developed to prepare silica-based micro/mesoporous bimodal hybrids with super hydrophobicity. It is thespecial feature of this pathway that though there are no surfactants introduced during the hydrolysis and polycondensation process, theobtained materials exhibit high specific surface areas, high porosity and bimodal structure. Actually, polymethylhydrosiloxane (PMHS),one of the silica sources, assembles itself into a special conformation and plays the role of a structure-directing agent that leads to theformation of the micro/mesophases. Thus, it is thought this novel protocol might be a profitable complement to those methods previ-ously reported for the preparation of porous materials.� 2006 Elsevier Inc. All rights reserved.

Keywords: Silica-based; Hydrophobic; Bimodal; Hybrid; Polymethylhydrosiloxane

1. Introduction

Since the discovery of surfactant-templated mesoporousmaterials in the early 1990 [1–3], the organic–inorganicsilica-based mesoporous nanocomposite materials haveattracted much interest because of their wide applicationsin the fields of catalysis and environmental remediation[4]. Generally speaking, the family of organic–inorganic sil-ica-based mesoporous materials includes mesoporousmaterials with the Si–O–Si tetrahedron frameworks andsurfaces functionalized with chemically bonded organicgroups, mesophases with organicsilica frameworks, and

1387-1811/$ - see front matter � 2006 Elsevier Inc. All rights reserved.

doi:10.1016/j.micromeso.2006.05.022

* Corresponding author. Tel.: +86 351 4049859; fax: +86 351 4041153.E-mail address: xuyao@sxicc.ac.cn (Y. Xu).

mesoporous silicas with occluded organic materials suchas polymers [4,5].

Hierarchically structured porous materials containingboth micro and mesoporosity have attracted significantattention owing to their important role in the systematicstudy of structure–property relationship and their techno-logical promise in applications [6,7]. With the successfulsynthesis of zeolite faujasite (FAU) and MCM-41, Kloet-stra et al. [8] have obtained a composite of FAU andMCM-41 with the overgrowth of a thin layer of MCM-41 on FAU, and some good results were achieved using thiscomposite for vacuum gas oil cracking. Karlsson et al. [9]have prepared some composite materials by simultaneoussynthesis of MFI/MCM-41 phase by using two-templateapproach at the optimized template concentrations andreaction temperatures. Furthermore, Li et al. [10] prepared

D. Yang et al. / Microporous and Mesoporous Materials 95 (2006) 180–186 181

a MCM-41/b composite by a two-step crystallization,which has dual acidity and pore structure. Recently, severalmethyl-modified mesoporous materials have been success-fully synthesized with dual porosity using non-ionic surfac-tant [11] in our research group. Although significantprogress has been made on the synthesis of micro/mesopor-ous materials, to the best of our knowledge, there are noreports on the bimodal materials with micro/mesoporesusing non-surfactant route, not to mention preparinghydrophobic hybrids.

Polymethylhydrosiloxane (PMHS) is a silicon industryby-product, which is cheap, nontoxic and stable to airand moisture, and it can transfer its hydrides to a varietyof metal catalysts (including Sn, Ti, Zn, Cu and Pd) thatcan then participate in a wide range of reduction [12]. Atpresent, PMHS has mainly been employed as a reducingagent to reduce halogen, ketones, ethers, imines, and phos-phine oxides. For example, ketones can be reduced intochiral alcohols with PMHS in the presence of zinc-chiraldiamines, cadmium/bisoxazoline catalysts or copper/chiraldiphosphines [13,14]. Furthermore, chiral imines can beobtained from imines using PMHS as a reducing agentwith the appearance of some catalysts such as titanocenecomplex, ZnCl2, Ti(OiPr)4, and zinc-diamine complex[14–16].

Here, we report the synthesis of organic–inorganicmicro/mesoporous xerogels using mixed TEOS and PMHSas silica sources without additional introduction of any sur-factants. To our interest, though neither solvent extractionnor calcination was utilized, the present amorphous sam-ples exhibited high specific surface areas, pore volumes,bimodal pore structure and super hydrophobicity. Further-more, the relative content of micropore and mesoporecould be finely tuned by adjusting the mass ratio of PMHSto TEOS. It can be deduced that PMHS not only is onepart of silica sources, but also plays a more important roleof structure-directing agent.

2. Experimental

All the chemical agents were used as received withoutfurther purification. Polymethylhydrosiloxane (PMHS)(99%, MW ¼ 2700–5400) was supplied by Acros. Tetraeth-oxysilane (TEOS, 99%), ethyl alcohol (anhydrous) andsodium hydroxide (NaOH) were provided by Tianjinchemical corporation.

In a typical synthesis, 1.57 mL (1.5587 g), 2.82 mL(2.8056 g), and 4.70 mL (4.676 g) PMHS were dripped intothree typical flasks containing 70 ml ethanol, respectively.The resultant solutions were further stirred for 48 h atroom temperature in order that PMHS can react with partof ethanol and release hydrogen gas under the catalysis ofNaOH (0.008 g). Then 5 mL (4.676 g) TEOS and 2 mLdeionized water were added into the three systems and keptunder vigorous stirring for 3 h. The sols were preparedfrom a TEOS: ethanol: water molar ratio of 1:60 :6. Theformed sols were statically aged under ambient conditions

for 4–5 d. When these sols turned into gels, they were putinto an oven to remove the solvent at 80 �C and finallyhard glass solids were obtained. The solids were grindedinto powder and the powder was washed with deionizedwater three times to remove a trace amount of sodiumhydroxide. The washed powder was dried for 24 h at80 �C for characterization. Corresponding to the PMHS/TEOS mass ratio of 1:1, 1:2, and 1:3 (PMHS (per Siatom)/TEOS molar ratio of 4:3, 5:2, and 5:1), threeobtained silica materials were designated as sample M1,M2 and M3, respectively. The whole preparation processwas carried out in the open systems.

All the samples were characterized without any solventextraction and calcination. Powder X-ray diffraction pat-terns were measured using Ni filtered Cu Ka radiation(k = 1.5404 A) on a Rigaku D Max III VC diffractometerequipped with a rotating anode operated at 40 kV and30 mA. N2 adsorption/desorption isotherms were obtainedat �196 �C on a Tristar 3000 Sorptometer, using staticadsorption procedures. Samples were degassed at 150 �Cand 10�6 Torr for minimum 12 h prior to analysis. BETsurface areas were calculated from the linear part of theBET plot according to IUPAC recommendations. Pore sizedistributions of the samples were calculated via the conven-tional BJH model and DFT model, respectively. TEMimages were recorded using a JEOL 100CX microscopewith a CeB6 filament and an accelerating voltage of200 kV. 29Si MAS NMR spectra were obtained on aUNITY INOVA-500 Spectroscopy using a DOTY Scien-tific multinuclear probe and 5 mm zirconia rotors: 29Si res-onance frequency, 99.745 M Hz; pulse width, 4 ls; recycledelay time, 400 s; spinning speed, 8 kHz; reference to tetra-methylsilane (TMS) assigned 0 chemical shift. The contactangle for water of the tableted sample M3 was measured bycontact-angle meter

PCA-A (Kyowa, Japan).

3. Results and discussion

3.1. Bimodal structure determination

The nitrogen adsorption isotherms for the series of xero-gels all clearly exhibit a resolved type IV isotherm witha steep desorption branch and a type H2 (type Ein deBoer classification) hysteresis loop [17] (see Fig. 1(a)),and Fig. 1(b) is the corresponding Barrett–Joyner–Halenda(BJH) pore size distributions. It can be clearly seen that thethree distributions all mainly center at �3.9 nm. Taking thetensile strength effect (TSE) [18] into consideration, that is,the current pore data �3.9 nm might be the combined effectof the micropores and mesopores, BJH model is not com-pletely suitable for the samples. Moreover, from Fig. 1 (a),it can been found that the desorption inflection points allare very smooth and the desorption hysteresis loops atlow P/P0 values (<0.2) are not closed, strongly indicatingthe possibility that the nitrogen fills in the micropores[19,20]. Therefore, it is necessary to introduce another poresize distribution calculation model, Density Functional

0 10 15 20

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Pore diameter (nm)

M1

M2

M3

c 1.8 nm4 nm

0.2 0.4 0.6 0.8 1.0 5 10

Fig. 1. Nitrogen adsorption and desorption isotherms (a), BJH pore size distributions (b) and DFT pore size distributions (c) for M1, M2 and M3.

Fig. 2. TEM images of the representative sample M3: (a) Region ofwormhole pore structure; (b) small part of ordered-like pore arrays.

182 D. Yang et al. / Microporous and Mesoporous Materials 95 (2006) 180–186

Theory (DFT) method that usually is employed to charac-terize mesoporous materials containing micropores (meso-porous carbons) [21,22], to illustrate the particular poresize distributions (see Fig. 1(c)). The DFT pore size distri-butions show obvious existence of the bimodal pore struc-ture, including some super micropores with radius less than2 nm and mesopores around at 4 nm. Simultaneously, itcan be clearly observed that the amount of mesoporesdecreases and the pore size is reduced with the increaseof the PMHS/TEOS ratio. The detailed structure informa-tion of the samples is listed in Table 1.

To further elucidate the micro/mesoporous bimodalcharacteristic of the obtained samples, detailed pore struc-ture was studied by HRTEM technique, and Fig. 2 showsthe representative TEM images of sample M3 taken in dif-ferent regions. Clearly, Fig. 2(a) depicts a direct image ofthe 3D wormhole-like pore frameworks (�4 nm), whichare regular in diameter. The network is very similar tothose pore arrays of MSU-X type mesosilicas withoutlong-range ordering prepared using a neutral surfactantpathway [23–28]. Fig. 2(b) shows a small lamellar regionwhere all layers are parallel to each other with �2.0 nmspacing. From the selected-area electron diffractionpattern of the microphase (inset), the well-ordered porearrangement is further confirmed. At the same time, itcan also be clearly observe that other parts in thisimage appear to be disordered as shown in Fig. 2(a).The bimodal structure from TEM images is in good agree-ment with the results obtained by nitrogen adsorptionmeasurement.

Table 1The surface area data and pore parameters of the obtained xerogels measured

Sample PMHS:TEOS(mass ratio)

BET surfacearea (m2 g�1)

Microporea

(surface area, m2 g�1)Total porevolume (S

M1 1:1 490 282 0.452M2 1:2 599 307 0.530M3 1:3 488 265 0.654

a Obtained by Dubinin–Astakhov model.

Fig. 3 shows characteristic XRD patterns of the micro/mesostructured materials prepared at the different PMHS/TEOS mass ratios. It is clearly seen that all the diffractionpatterns only consist of a broad diffraction peak (100reflection) corresponding to the d-spacing of 6.8 nm, whichcould possibly be from overlapping 110 and 200 reflections.The broad peak is in contrast to MCM-41 or MCM-48mesoporous molecular sieves, where further diffractionlines observed indicate hexagonal or cubic symmetry. Anal-ogous broad XRD patterns have been reported by Weiet al. [29] and other scientists [23,24] for some mesostruc-tured silica materials. It is possible that the bimodal poredistributions and the introduction of large amount of–CH3 organic group (will be discussed later) directly leadto the formation of the broad peak. Furthermore, unfortu-nately, we cannot find the diffraction peaks corresponding

by different adsorption models

TP) (cm3 g�1)Limitinga microporevolume (cm3 g�1)

BJH porediameter (nm)

DFT porediameter (nm)

0.124 �3.9 �2, �40.137 �3.9 �2, �40.122 �3.9 �2, �4

0

20 30 40 50 60

Inte

nsity

2 θ / degrees

M1

M2

M3

Inte

nsity

2 θ /degrees

M1

M2

M3

2 4 6 8 10

Fig. 3. XRD patterns of bimodal mesophases prepared at the differentPMHS/TEOS mass ratios. Inset: The diffraction patterns at high 2hvalues.

Scheme 1. Formation of silica-based bimodal hybrids from PMHS andTEOS in EtOH.

D. Yang et al. / Microporous and Mesoporous Materials 95 (2006) 180–186 183

to the micropores that have been observed from HRTEMimages. Our tentative explanation to the unusual XRD pat-terns is that the very small regions of the lamellar micropo-rous structure in the bimodal hybrids may be the obstaclesto the presence of the diffraction peaks of the microphases.The presence of a single broad peak centered at a 2h valueof �23� (Fig. 3, inset) is attributed to the amorphous struc-ture of silica framework [29].

3.2. Formation mechanism

One of the outstanding features of the present synthesisroute is that no templates are introduced in the synthesis ofmesoporous framework and removed after the synthesis.TEOS is a widely used silicon source without any structuredirecting function for forming mesostructures. Therefore,PMHS is not only a silicon source, but it also plays a cru-cial role in structure directing. In the synthesis of mesopha-ses using block copolymers (BC) as templates, the phasesegregation of hydrophobic block and hydropholic blockleads to the formation of mesostructure [30,31]. For theobserved microporsity of SBA-15, Imperor–Clere et al.[31,32] attribute it to the partial occlusion of the hydro-philic PEO chains into the silica matrix, which becomesmicroporous upon calcination. However, Haddad et al.[31,33] suggest that microporsity is rather related to incom-plete silica condensation. In this work, PMHS is very differ-ent in structure compared with those BC templates, whosehydrophobic fraction and hydrophilic fraction are not longchain. Therefore, we attempt to present a brand-new modelto explain the bimodal pore structure formation mecha-nism. As illustrated in Scheme 1, in the presence of NaOHas catalyst, the acitve hydrogen atoms in PMHS arereplaced by ethoxy groups and hydroxys, leading to the

formation of new polymethylhydroxysiloxane (PMHOS).In the EtOH system, the long PMHOS molecule assemblesitself into a helix with several loops whose both sides areoccupied by hydroxys and methyls, respectively. Thenumerous methyls should be inside the loop because oftheir hydrophobic trait and outside the loop is the hydro-philic hydroxys. Once the hydroxys outside the loops reactwith TEOS, these loops are fixed at molecule scale and theregion inside the loops leads to the formation of mesop-ores. Considering that the hydrophilic section in PMHSis a hydroxy group bonded with silicon atom, it can directlyreact with another silicon source unlike those hydrophilicPEO chains occluded into the silica matrix [32]. Further-more, the high condensation degree between PMHS andTEOS (see the following 29Si MAS NMR data) indicatesthat the microporosity should not be related to the incom-plete silica condensation [33]. Here, the formation ofmicroporosity is assigned to the existence of the unreacted

Scheme 2. PMHS and TEOS hydrolysed and condensed in hexane.

184 D. Yang et al. / Microporous and Mesoporous Materials 95 (2006) 180–186

Si–OH on the loops which leads to the seperation of someadjacent loops, the limited space between these seperatedloops may finally result in the formation of the orderedmicropores.

To verify the above conjecture, determined 1,3,5-trim-ethylbenzene (TMB) was introduced to the M3 system.After solvent extraction with EtOH, the correspondingxerogel was designated as Ms. Fig. 4 displays the N2

adsorption/desorption isotherms and pore size distribu-tions obtained by different models for M3 and Ms. Thedesorption hysteresis loop of the treated sample Ms shiftsobviously to relatively higher P/P0 range (see Fig. 4(a)),corresponding to the BJH pore diameter increase from�3.9 nm to 7.5 nm (see Fig. 4(b)). The result suggests thatthe hydrophobic TMB molecules automatically insertthemselves into these PMHOS loops leading to the meso-pore expansion, which effectively implies the existence ofthose PMHOS loops. Furthermore, DFT pore size distri-butions (Fig. 4(c)) display that, owing to the introductionof the swelling agent, the diameter of mesopores shifts toaround 9 nm and the micropores diameter has not changedat all. Though TMB molecules can congest the PMHOSloops and result in the swelling of mesopores, it couldnot change the limited space between the separatedPMHOS loops, which can explain why the microphaseshave no any change in diameter size.

In order to further test our hypothesis, another experi-ment was carried out. As we know, hexane is a nonpolarsolvent when compared with EtOH. According to our con-jecture, in the hexane system, although the long PMHOSalso assembles itself into several loops as that in the EtOHsystem (see Scheme 1), the numerous methyls should beoutside the loop due to their hydrophobic trait and insidethe loop is the hydrophilic hydroxys (see Scheme 2). In thissystem, these hydroxys inside the PMHOS loops will reactwith TEOS leading to the disappearance of mesophasesand retaining only micropores. Fig. 5 shows the DFT poresize distributions of M3 and Mh, which is obtained fromthe hexane system with the same mass ratio of PMHS/

0

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Vol

ume

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3 g-1 S

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0.2 0.4 0.6 0.8 1.0 5

Fig. 4. Nitrogen adsorption and desorption isotherms (a), pore size distributionMs.

TEOS as M3. Obviously, compared with sample M3, themesoporous characteristic of Mh completely disappears,

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Mh

10

Fig. 5. Pore size distributions obtained by DFT model for M3 and Mhsynthesized in hexane.

0 -100 -150

-110

Chemical shift (ppm)

M1

M2

M3

-103

-65

-56

12

-50

Fig. 6. 29Si MAS NMR spectra of the studied xerogels prepared fromdifferent PMHS/TEOS ratios.

D. Yang et al. / Microporous and Mesoporous Materials 95 (2006) 180–186 185

but it still displays evident microphases with about �2 nmpore diameter. Moreover, the surface area data and poreparameters of M3 and Mh are compiled in Table 2. Asexpected, the surface areas and micropore volume of Mhdecrease intensively, which can be absolutely attributedto the disappearance of the mesophases.

As for the fact of the increasing total pore volume andpore size with higher TEOS/PMHS ratio, we can put for-ward a tentative explanation to the unusual influence ofthe PMHS concentration in the synthesis of the pore struc-ture of the product silica. At high concentration, the hydro-lyzed PMHOS forms helix springs with more loops due tointensive intermolecular repelling, leading to the formationmore microporosity. Conversely, at low concentration, thehydrolyzed PMHOS molecules experience relatively weakintermolecular repelling force, forming helix springs withfewer large loops and subsequently fewer microporosityand the increasing total pore volume and pore size.

It should be noted that the formation mechanism for thebimodal structure is based on the presumption though thetest experiments have been carried out. The detailed hydro-lysis and condensation process, such as the investigation onthe configuration of the PMHOS molecules in EtOH sys-tem via 29Si MAS NMR technology, and the role of PMHSare being studied and the results will be reported in the nearfuture.

Table 2The surface area data and pore parameters of M3 and Mh measured by differ

Sample Solvent BET surfacearea (m2g�1)

Microporea

surface area (m2 g�1)Total porevolume (STP)

M3 EtOH 488 265 0.654Mh Hexane 227 135 –

a Obtained by Dubinin–Astakhov model.

3.3. 29Si MAS NMR characterization

Fig. 6 shows the 29Si MAS NMR spectra of the sampleM1, M2, and M3. Noticeably, all the spectra exhibit five dis-tinct signals at �110, �103, �65, �56, and 12 ppm, respec-tively. The two resonance peaks at�110 and�103 ppm canbe attributed to Q4ðSi�ðOSiÞ4 and Q3ðOHÞSi�ðSiOÞ3 [26],respectively. The signals at�65 and�56 ppm are correspond-ing to T3ððSiOÞ3Si�CH3Þ and T2ððOHÞðSiOÞ2Si�CH3Þ [14]environments of silicon atoms, respectively, and the mosthigh-field at 12 ppm can be assigned to the M1 peak [34]which is consistent with the SiOSi* (CH3)3 environmentlocating at two ends of PMHS. With the increase of thePMHS/TEOS mass ratio, T2, T3 and M1 peaks all becomemore intense, indicating PMHS and TEOS have jointly con-densed and –CH3 groups have been introduced into thematerials. Furthermore, the absence of Q0ðSiðOHÞ4Þ,Q1ðSi�ðOSiÞðOHÞ3Þ, Q2ðSi�ðOSiÞ2ðOHÞ2Þ and the existenceof weak T2 and Q3 peaks suggest that there is only a fewof the –OH group remaining after hydrolysis and polycon-densation, illuminating a high condensation degree betweenPMHS and TEOS.

3.4. Hydrophobicity measurement

The photographs in Fig. 7 show a water drop on thesample tablets. The water droplet with a diameter of

ent adsorption models

(cm3 g�1)Limitinga microporevolume (cm3 g�1)

BJH porediameter (nm)

DFT porediameter (nm)

0.122 �3.9 �2, �40.063 – �2

Fig. 7. Photograph of a water droplet on a tablet of M3.

186 D. Yang et al. / Microporous and Mesoporous Materials 95 (2006) 180–186

�2 mm on the tablets looks like a round ball. The contactangles for the three samples all exceed 160�, indicating thesuper hydrophobic nature of the micro/mesoporoushybrids. The high hydrophobicity can be attributed totwo reasons. Firstly, the introduction of a lot of methylgroups from PMHS improves the materials’ hydrophobic-ity. On the other hand, a high condensation degree makesthe residual hydrophilic –OH groups reduce to the least onthe surface of these materials.

4. Conclusions

This preliminary research work shows that silica-basedbimodal micro/mesoporous hybrids with high content oforganic functionality could be easily prepared withoutadditional introduction of any surfactants. Furthermore,characterization results indicate that the physico-chemicalproperties such as specific surface area, pore volume, poresize, relative content of micropore and mesopore could befinely controlled, and they all exhibit super hydrophobicity.More detailed investigation suggests that although thereare no surfactants introduced, PMHS, one of the silicasources, actually plays a more important role of a struc-ture-directing agent and leads to the formation of micro/mesophases. Therefore, the current method can be usedto prepare methyl modified porous silica-based films, andrelated advanced functional materials.

Acknowledgements

Financial supports from the national key native sciencefoundation (20133040) and State Key Program for Devel-opment and Research of China (2005CB221402) and Aus-tralian Research Council are gratefully acknowledged.

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