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Microporous and Mesoporous Materials 111 (2008) 97–103

Facile synthesis of hydrophobic microporous silica membranesand their resistance to humid atmosphere

Qi Wei a, Yan-Li Wang a, Zuo-Ren Nie a,*, Chun-Xiao Yu a, Qun-Yan Li a,Jing-Xia Zou a, Cong-Ju Li b

a College of Materials Science and Engineering, Beijing University of Technology, 100 Pingleyuan, Chaoyang District, Beijing, PR Chinab Beijing Key Laboratory of Clothing Material R&D and Assessment, Beijing Institute of Clothing Technology, Beijing 100029, China

Received 17 October 2006; received in revised form 10 July 2007; accepted 11 July 2007Available online 19 July 2007

Abstract

The ethylene-modified silica membranes were prepared by the acid-catalyzed co-hydrolysis and condensation reaction of tetraethyl-orthosilicate (TEOS) and ethylenetriethoxysilane (TEVS) in ethanol and the final materials were characterized by scanning electronmicroscope (SEM), water contact angle measurement, solid-state 29Si magic angle spinning nuclear magnetic resonance (29Si MASNMR), and N2 adsorption. The modification leads to a transform from superhydrophilicity for the unmodified silica membranes tohydrophobicity for the modified materials. The ethylene-modified silica membranes are much less water sensitive than the unmodifiedmaterials because the hydrophobic ethylene groups replace a portion of the hydrophilic hydroxyl groups on the pore surface. The mod-ified materials process a microporous structure with a narrow pore size distribution centered at 1.1 nm. Such a microporous structure canbe stabilized after exposured to humid atmosphere for 450 h, in intense contrast to the collapse of the micropores in the unmodified silicamembranes.� 2008 Published by Elsevier Inc.

Keywords: Silica membranes; Hydrophobic properties; Microporous structure; Humid atmosphere

1. Introduction

Microporous silica membranes are considered to be themost promising materials for the clean-energy system sincethey can separate hydrogen from gas mixtures produced bythe chemical processing such as coal gasification or steamreforming of methane [1–3]. Their unique pore structurewith a pore size close to the dynamic diameter of gas mol-ecules plays an important role in gas separation sincemolecular sieving mechanism can be realized in such a poresystem. Silica membranes with a high flux of H2 and a largeselectivity of H2 over CO2, CH4 or N2 have been success-fully synthesized by various research groups predominantlyvia the sol–gel and dip-coating process [4–9]. However, sil-ica membranes have been proved to be hydrothermally

1387-1811/$ - see front matter � 2008 Published by Elsevier Inc.

doi:10.1016/j.micromeso.2007.07.016

* Corresponding author. Tel./fax: +86 10 67391536.E-mail address: zrnie@bjut.edu.cn (Z.-R. Nie).

unstable since they suffer from water if exposed to thehumid atmosphere, especially at relatively low tempera-tures, leading to a considerable decrease of the H2 fluxand selectivity within the first few hours of testing in thepresence of steam [10–14]. This observation can be attrib-uted to the hydrophilic nature of silica membranes. It iswell known that water can be adsorbed on silica materialsvia the intermolecular hydrogen bonding between watermolecules and silanol(hydroxyl) groups of silica mem-branes. As reported previously [15], a neighboring siloxanetends to react with physisorbed water, leading to the for-mation of mobile silica. The mobile silica groups peel offfrom the surface of the larger pores and migrate into thehigher attraction regions in the smaller pores, resulting inthe widening of the larger pores and the closure of thesmaller pores, and further causing the collapse of themicropore system. It is also reported that the interactionof silica membranes with the water from process streams

98 Q. Wei et al. / Microporous and Mesoporous Materials 111 (2008) 97–103

at higher temperatures can result in serious degradationphenomena in the materials [16,17]. In order to obtain ahigher gas permeance and selectivity for silica membranesin the industrial applications, a long-term hydrothermalstability, in other words, stabilizing the pore structure inmicropore range, is required due to the presence of steamin the real application cases. Hydrophobic silica mem-branes are proposed to meet this requirement.

Since the hydroxyl groups on the pore surface are themost active sites for the water–membrane interaction,replacing the hydroxyl groups with hydrophobic groups isa facile strategy to reduce the water adsorption and retainthe micropore system for silica membranes [18]. A largenumber of functional groups have been used to improvethe hydrophobic properties of areogel and silica glass [19–25], but scarce studies on hydrophobic silica membraneshave been reported. de Vos et al. are the first authors to pre-pare hydrophobic silica membranes by adapting the synthe-sis of the sol and reported the results of the structure andproperties of the methyl-modified membranes [18]. In ourprevious works, we reported the hydrogen transport inthe microporous system and the hydrogen separation fromgas mixtures in the presence of steam for the silica mem-branes modified by hydrophobic methyl groups [26]. Inthe present paper, an ethylene-modified silica membranewas synthesized by a facile sol–gel method and character-ized by means of SEM, water contact angle measurement,solid-state 29Si MAS NMR, and N2 adsorption. The hydro-phobic properties and the pore structure change of the mod-ified silica membranes exposured to the humid atmospherewere investigated in detail. As far as we know, no similarresearch has been reported so far.

2. Experimental

2.1. Preparation of silica membranes

Silica sols and membranes were prepared by the acid-catalyzed hydrolysis and condensation of tetraethylortho-silicate (TEOS, Acros) in an ethanol medium as describedin the previous literature [6]. Ethylenetriethoxysilane(TEVS, Acros) was used as the source of hydrophobicgroups, in which the ethoxy groups hydrolyzed in waterand the unreacted ethylenes behaved as hydrophobicagents. Modified membranes were obtained by incorporat-ing hydrophobic groups into silica sol at a certain stage ofpreparation as described below. A mixture of water andHNO3 was added drop-wise into a solution of ethanol(EtOH, Acros) and TEOS with a TEOS/EtOH/H2O/HNO3 molar ratio of 1/3.8/6.4/0.085, which was mixedhomogenously under ice-bath previously. Then the mixturewas heated to 60 �C and refluxed for 2.5 h to obtain solu-tion A. A solution B was achieved by mixing TEVS withethanol with a TEVS/EtOH molar ratio of 0.4–0.8 to 3.8.The solution B was then added to solution A and reactedfor 30 min at 60 �C under stirring to get a final sol with aTEOS/TEVS/EtOH/ H2O/HNO3 molar ratio of 1/0.4–

0.8/7.6/6.4/0.085. The pure silica membranes were denotedas SiO2, and the modified membranes as (0.4TEVS)SiO2

and (0.8TEVS)SiO2 hereafter, with a TEVS:TEOS molarratio of 0.4 and 0.8, respectively. The supported silicamembranes were deposited on the top of c-Al2O3 mem-branes supported by a-Al2O3 ceramic by dip-coating witha computer-controlled dip-coater. After coating, the sam-ples were dried and then calcined at 400 �C for 5 h with aheating and cooling rate of 1 �C/min under N2 atmosphere.The rest of dipping sol was poured into a petri-dish anddried at room temperature, after gelation, the gel was cal-cined and used for NMR and N2 adsorption measurementfor the unsupported samples.

2.2. Membrane characterization

The morphology of silica membranes was observed bySEM(JEOL JSM 6500F). Water contact angles(CAs) weremeasured on a Dataphysics OCA20 video-based contactangle system at ambient temperature. Water droplets(about 1 ll) were dropped carefully onto the coating silicamembranes. In order to eliminate the 3D capillary effect ofthe porous c-Al2O3/a-Al2O3 supports on the real waterCAs values [27,28], silica membranes were coated on asmooth glass substrate for the water CAs measurement.Solid-state 29Si MAS NMR spectra were recorded on aBruker AV300 spectrometer according to the followingmeasurement conditions: 5 mm MAS probe, 59.62 MHzresonance frequency, p/2 pulse, 6.0 ls pulse width, 5 kHzmagic-angle spin, 600 s delay time, and 200 scans. Chemi-cal shifts were referenced to tetramethylsilane (TMS) at0 ppm. Nitrogen adsorption measurements were carriedout at 77 K on a Micromeritics ASAP 2020M volumetricadsorption analyzer. Before the measurements, the sampleswere outgased under vacuum at 300 �C for 12 h. The totalpore volume was calculated at the saturated pressure. Themicropore volume was determined from the t-plot, and themicropore size distribution was calculated according tothe Horvath–Kawazoe (HK) method, using zeolite as a ref-erence adsorbent to determine the interaction parameterbetween N2 and silica membranes, and assuming cylinderalpore geometry.

3. Results and discussion

Fig. 1a is the SEM image of the membrane cross-sec-tion. It indicates that a silica top layer with a thickness ofabout 200 nm has been successfully deposited on the c/a-Al2O3 substrates. No significant pinholes and cracks arefound at the top layer according to the surface SEM image(Fig. 1b). Fig. 2 shows the wettability for silica membranes.The water CA of the pure silica membranes is about 0�(Fig. 2a) showing superhydrophilicity. The water dropsspread rapidly on silica membranes (Fig. 2b). The waterCA is 97.0 ± 1.8� (Fig. 2a) for (0.4TEVS)SiO2, indicatinga hydrophobic property compared to the superhydrophilic-ity for the pure silica membranes. Furthermore, the water

Fig. 1. SEM photo of silica membranes. (a) Cross-section, and (b) Surface.

Q. Wei et al. / Microporous and Mesoporous Materials 111 (2008) 97–103 99

CA increases to 105.4 ± 2.0� (Fig. 2a) for (0.8TEVS)SiO2,which means a higher hydrophobic property is achievedwith increasing the TEVS/TEOS ratio from 0.4 to 0.8.The hydrophobic surface also can be verified with the waterdrops standing on the membranes without spreading out(Fig. 2b). The enhancement of hydrophobic propertiescan be explained by the surface model of silica membranesas shown in Fig. 3. It should be stressed that this is just asimple model and just helps us imagine what happens whenethylene groups replace some of the surface silanol groups.In fact, it is very difficult to give a perfect surface model ofthe functionalized silica membranes since the type of termi-nating chemical bonds/groups on the surface, as well astheir concentrations, remain unclear. The surface silanol(hydroxyl) groups can physically adsorb water via intermo-lecular hydrogen bonding between the hydroxyl groupsand water (Fig. 3a), so the large amount of surface silanolgroups are predominantly responsible for the hydrophilic-ity of silica membranes. When the silica membranes aremodified by the organosilane TEVS, most silanol groupsare replaced by hydrophobic ethylene groups (Fig. 3b),causing a considerable decrease of the binding sites forwater molecules. Additionally, the steric hindrance owing

to the ethylene groups prevents water molecules to beingadsorbed on the surface.

The surface chemical properties of silica membranes canbe revealed by the solid-state 29Si MAS NMR experiments.Fig. 4 shows the 29Si MAS NMR spectra for the pure silicamembranes and those modified with TEVS (TEVS/TEOS = 0.4 and 0.8). The spectra are fitted according toGaussian approach in order to quantificationally determinethe concentration of the silnol groups and ethylene groupsand the results are shown in Table 1. The resonances at achemical shift from –110 to –90 ppm, assigned toQ4[Si(OSi)4], Q3[Si(OSi)3(OH)] and Q2[Si(OSi)2(OH)2] sili-con atoms, respectively, are observed for the pure silicamembranes [29,30]. For the modified silica membranes,the signals of Q4 and Q3 silicon atoms occur at the samechemical shifts as those of the pure silica membranes, butthe Q2 species is absent for the (0.8TEVS)SiO2 sample. Inaddition to Qm peaks, two additional resonances can befound for organosiloxane [Tn = RSi(OSi)n(OH)3�n,n = 1�3, and R = C2H3; T3 at –78 ppm and T2 at –65 ppm] species in the case of the modified materials [29–31]. From these data listed in Table 1, the concentration ofhydroxyl groups in per gram of the pure silica membranes

(0.8TEVS)SiO2

CA=105.4 ± 2.0°

(0.4TEVS)SiO2

CA=97.0 ± 1.8°

SiO2

CA≈0°

0 500 1000 1500 20000

20

40

60

80

100

120

140

(0.8TEVS)SiO2 SiO2Con

tact

ang

le/ o

a

b

Fig. 2. Wettability for silica membranes with variation of TEVS concen-tration. (a) Change of water drop profile, and (b) Water spreadingbehavior.

100 Q. Wei et al. / Microporous and Mesoporous Materials 111 (2008) 97–103

and modified materials can be calculated according to thefollowing formula, respectively,

½OH� SiO2¼

IQ3 þ 2� IQ2

60� IQ4 þ 69� IQ3 þ 78� IQ2

½OH�ð0:8TEVSÞSiO2

¼IQ3 þ IT2

60� IQ4 þ 69� IQ3 þ 78� IQ2 þ 79� IT3 þ 88� IT2

The degree of ethylene groups per gram of silica materials(mol/g) can be obtained from the formula,

½C2H4� ¼IT2 þ IT3

60� IQ4 þ 69� IQ3 þ 78� IQ2 þ 79� IT3 � 88� IT2

where Ii represents the intensity of various silicon lines inthe 29Si MAS NMR spectra. It can be seen from Table 1that ethylene groups have been incorporated onto the poresurface of the modified silica membranes, and their concen-tration increases with increasing the amount of TEVS inthe initial mixtures. On the contrary, the concentration ofsurface silanol groups decreases with the increase of theTEVS amount in the samples (8.77 mmol/g for the pure sil-ica, 6.78 mmol/g for (0.4TEVS)SiO2 and 4.84 mmol/g for(0.8TEVS)SiO2). As a result, the presence of hydrophobicethylene groups and the decrease of hydrophilic silanolgroups enhance the hydrophobicity of the modified mem-branes. The absence of Q2 signal in the sample (0.8TEVS)-SiO2 indicates a higher condensation degree, which makesthe surface residual hydrophilic –OH groups reduce to theleast [32].

It can be observed from Fig. 5 that all samples exceptthe pure silica membranes aging for 450 h display a typicaltype I isotherms, characteristic of microporous materials[33–36]. These samples exhibit a high uptake at the verylow relative pressure (close to zero), and then the adsorp-tion is rapidly saturated, with the curve changing into aplatform parallel to the x-axis. The pure silica membranesaging for 450 h also exhibit a large amount of adsorbednitrogen at a very low relative pressure, suggesting the pres-ence of micropores in the samples, but their isotherm shapeis quite different compared to other samples. The saturatedadsorption occurs at a medium relative pressure of 0.4rather than at the pressure close to zero, implying thatmulti-layers adsorption and capillary condensation takeplace in sequence with the increase of relative pressure.This adsorption behavior usually occurs in mesoporousmaterials, indicating that the membranes possess mesopo-rosity in addition to microporosity. It is well known thattypical mesoporous materials display a type IV isothermwith a hysteresis loop, however, it is not the case for the sil-ica membranes aging for 450 h in our work. It has beenexplicitly reported that mesoporous materials with a diam-eter less than 4 nm provide no adsorption hysteresis, so theabsence of an obvious adsorption/desorption hysteresis inthese samples suggests that the mesopores may be rathersmall [37,38].

To determine the microporosity for all samples, t-plotsare calculated from the isotherms as shown in Fig. 6, inwhich the amount of nitrogen adsorbed is plotted against

Fig. 3. Surface model of silica membranes and a proposed mechanism for the effect of surface functional groups on the wettability and pore structure ofthe materials (a) Unmodified membranes [15], and (b) Modified membranes.

-20 -40 -60 -80 -100 -120 -140 -160 -180

Chemical shift/ppm

(0.8TEVS)SiO2

(0.4TEVS)SiO2

SiO2

Fig. 4. Solid-state 29Si MAS NMR spectra of unsupported silicamembranes, which have been fitted according to Gaussian approach.

Q. Wei et al. / Microporous and Mesoporous Materials 111 (2008) 97–103 101

the statistical thickness t calculated from Harkins–Juraequation as follows using relative pressure (P/P0) data [39]:

logðP=P 0Þ ¼ 0:034� 13:99=t2

From the y-intercept of the lines fitted at the properrange of relative pressure, micropore volume can be calcu-lated [40], and the results are listed in Table 2. For theunmodified silica membranes aging for 450 h, 13% of thetotal pore volume is contributed by micropores, indicatingthat the samples should be considered as mesoporousmaterials rather than microporous materials. For the otherthree samples, more than 71% of the total volume is asso-ciated with micropores, which is the typical characteristicof microporous material and in good agreement with whatimplied in the N2 adsorption isotherms.

The micropore size distributions calculated by the HKmethod [41], assuming a cylindrical geometry (Saito-Foley), are shown in Fig. 7. HK analysis indicates thatthe pure silica without aging in humid atmosphere has anarrow pore size of 0.8 nm, and the modified samples, nomatter whether exposed to humid atmosphere, have a nar-row pore size distribution centered at 1.1 nm. Such a porestructure is of particular significance for the applications

Table 129Si MAS NMR fitting results

Membranes Q4 (mol%) Q3 (mol%) Q2 (mol%) T3 (mol%) T2 (mol%) [OH] (mmol/g) [C2H4] (mmol/g)

SiO2 49.1 44.6 6.3 – – 8.77 –(0.4TEVS)SiO2 47.5 16.2 5 6.3 19.8 6.78 3.85(0.8TEVS)SiO2 49.9 15.6 – 16.4 18.1 4.84 4.95

0.0 0.2 0.4 0.6 0.8 1.0

0

30

60

90

120

150

180

210

SiO2 SiO2 Aging for 450 h (0.8TEVS)SiO2 (0.8TEVS)SiO2 Aging for 450 h

Qua

ntity

ads

orbe

d/cm

3 g-1 S

TP

Relative pressure

Fig. 5. N2 adsorption isotherms at 77 k for unsupported silicamembranes.

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

0

50

100

150

200

250

300 SiO2

SiO2,Aging for 450 h (0.8TEVS)SiO2

(0.8TEVS)SiO2,Aging for 450 h

Qua

ntity

ads

orbe

d/cm

3 g-1 S

TP

Statistical thickness/nm

Fig. 6. t-plots for unsupported silica membranes. The micropore volumeand surface can be determined from the y-intercept and slope of fittedlines, respectively.

Table 2Pore structure data for unsupported silica membranes

Membranes Vt Vmicro Vmicro/Vt

SiO2 0.28 0.21 0.75SiO2–450 0.31 0.04 0.13(0.8TEVS)SiO2 0.09 0.07 0.77(0.8TEVS)SiO2–450 0.07 0.05 0.71

Vpore: total pore volume (cm3/g); Vmicro: micropore volume calculatedfrom t-plot (cm3/g); SiO2–450 and (0.8TEVS) SiO2–450: the correspondingmaterials aging at humid atmosphere for 450 h.

0 2 4 6 8 10

0.0

0.1

0.2

0.3

0.4

0.5

SiO2

(0.8TEVS)SiO2

(0.8TEVS)SiO2,Aging for 450 h

dV/d

W(c

m3 g-1

nm-1)

Pore width/nm

2 4 6 8 10 12 14 16 18 20

0.0

0.2

0.4

0.6

0.8

1.0

1.2

SiO2 Aging for 450 h

dV/d

(logD

) (c

m3 g

-1)

Pore diameter/nm

Fig. 7. Pore size distribution (PSD) for unsupported silica membranes,calculated according to the Horvath–Kawazoe method from N2 adsorp-tion isotherms at 77 K, insert is PSD of pure silica membranes aging for450 h, obtained from desorption isotherm by BJH approach.

102 Q. Wei et al. / Microporous and Mesoporous Materials 111 (2008) 97–103

such as gas separation. For the mesporous silica mem-branes aging for 450 h, however, the pore size distributiondetermined by BJH approach is more accurate than that byHK method, and the result is shown in the insert of Fig. 7.The membranes display a rather broad pore size distribu-tion with a pore diameter ranging from 2 to 4 nm, muchlarger than those of the samples not contacting steam. Sucha pore structure is not suitable for gas separation becausethe gas transport in mesoporous materials is mainly con-trolled by Knudsen diffusion and the permselectivity is con-fined to the Knudsen limit [42]. It is obvious that theethylene-modification helps to retain the microporousstructure of the silica membranes even exposed to a humidatmosphere. However, for the unmodified materials, expo-sure to steam leads to a considerable expansion of micro-pore to mesopore. This observation seems to beinconsistent with some previous reports that silanol groupswithin the silica structure react with water leading to struc-tural densification, but a recent report presented a similarresult as those described in our wok [15]. The wideningof pores after exposure to humid atmosphere may be basedon the mechanism that mobile silica species, derived fromreaction between a nearby siloxane with physisorbed water,peel off from the wall of the pores and locate in the necks ofthe pores, leading to the expansion of pore space (Fig. 3a)[15]. For the modified materials, because the hydrophobicsurface significantly reduces the water adsorption and then

Q. Wei et al. / Microporous and Mesoporous Materials 111 (2008) 97–103 103

prevents the formation of mobile silica species, the poresize can be stabilized within the micropore range (Fig. 3b).

4. Conclusions

A transform from superhydrophilicity to hydrophobic-ity for silica membranes is obtained by modifying silicamaterials through the co-hydrolysis and condensation reac-tion of tetraethylorthosilicate and ethylenetriethoxysilane.The hydrophobic surface tends to resist water attack andsignificantly reduce the physical water adsorption on silicamembranes. The modified silica membranes process amicroporous structure with a narrow pore size distributioncentered at 1.1 nm, and such a microporous structure canbe retained even exposed to the humid atmosphere for450 h. However, the microporous structure changes to mes-oporous structure for the unmodified membranes ifexposed to humid atmosphere for 450 h.

Acknowledgments

The financial support of National Natural ScienceFoundation of China (Granted No. 50502002, 50525413,50503001) and Scientific Research Common Program ofBeijing Municipal Commission of Education (GrantedNo. KM200610005016) is gratefully acknowledged.

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