Improved performance of silica membranes for gas separation

Improved performance of silica membranes for gas separation

Journal of Membrane Science 143 (1998) 37±51 Improved performance of silica membranes for gas separation Renate M. de Vos, Henk Verweij* Laboratory f...

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Journal of Membrane Science 143 (1998) 37±51

Improved performance of silica membranes for gas separation Renate M. de Vos, Henk Verweij* Laboratory for Inorganic Materials Science, Faculty of Chemical Technology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands Received 12 September 1997; received in revised form 11 December 1997; accepted 12 December 1997

Abstract Silica membranes with extremely low defect concentrations have been prepared using sol±gel synthesis starting from tetraethyl orthosilicate (TEOS). An asymmetric structure is obtained by applying two silica layers on top of a g-Al2O3 layer, supported by an a-Al2O3 support. The selective silica top layers have a total thickness of 30 nm and micropores with a pore Ê , determined by physical adsorption on unsupported silica membrane material. The morphology of the diameter 5 A homogenous silica layer is analysed by FE-SEM and TEM. The transport properties of the membranes are measured in the temperature range of 50±3008C and at pressure differences ranging from 0.5 to 3 bar. The membranes have reproducible high permeances (210ÿ6 mol/m2sPa) for H2 and much lower permeances for CO2, N2, and O2 (10 lower), CH4 (500 lower). The silica membranes show a slight increase of permeance with increasing temperature for H2, CH4, N2, O2 and a slight decrease for CO2. For the time being the pore size of the microporous supported silica membranes can be estimated best from the relation between the permeance and kinetic diameter (dk) of the gases used. The pore size can be derived from the value of the dk of the smallest gas where permeance is no more observed. The silica membranes can be applied for different types of separations: CO2/CH4 with a permselectivity (Fa) of 75, O2/N2 with Faˆ4, H2/CH4 with Fa>500 and H2/CO2 with Faˆ70, all at 2008C. Separation factors obtained from gas separation experiments with 50/50 (vol.%) gas mixtures are very similar to permselectivities calculated from single gas permeance experiments. # 1998 Elsevier Science B.V. Keywords: Microporous membranes; Silica; Preparation; Characterisation; Gas separations; Hydrogen

1. Introduction Microporous inorganic membranes (rpore<1 nm) have great potential for gas separation [1,2]. Compared to polymeric membranes, microporous inorganic membranes with molecular sieve-like properties have relatively high gas permeances and a good stability towards higher temperatures and

*Corresponding author. Fax: 31 53 4894683. 0376-7388/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0376-7388(97)00334-7

corrosive atmospheres [3]. Moreover, inorganic membranes can be used in membrane reactors for conversion enhancement in, e.g., dehydrogenation reactions [4]. State-of-the-art microporous silica membranes consist of a microporous top layer on top of a supported mesoporous (1 nm
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membrane depend on, for instance, drying speed, amount and size of particles in the preparation atmosphere and the thermal processing schedule. Improving the membrane properties by lowering of defect size and density is currently one of the greatest challenges in inorganic membrane preparation and the subject of the present study. 2. Experimental 2.1. Support preparation Coherent and strong porous a-Al2O3 supports are made from granulated alumina powder (Type: CR-6, Baikowski Chimie, Annecy, France. Granulation was done at Philips Lighting, Uden, The Netherlands, in a dedicated spray-drying production facility). The supports are uniaxially pressed at 105 kPa, followed by isostatic pressing at 4000 kPa and sintering at 12508C with a heating rate of 18C/min from room temperature to 6008C and 28C/min to 12508C. This temperature is maintained for 3 h; cooling is done with rate of 58C/ min. The ®nal porosity was 40% with an average pore size of 160 nm, determined by mercury porosimetry (Micromeritics, Autopore II 9220). 2.2. -Al2O3 membrane preparation g-Al2O3 membranes are prepared by dip coating the sintered a-Al2O3 supports in a boehmite (g-AlOOH) sol followed by drying and calcining [5]. The dip coating is performed in a class 1000 clean room in order to minimise particle contamination of the membrane layer. After dipping, the membranes are dried in a climate chamber at 408C and 60% R.H. (VTRK300, Heraeus VoÈtsch, Balingen, Germany). The drying rate at such conditions is suf®ciently low to avoid any crack formation in the boehmite layer [6]. The g-Al2O3 membranes are formed by subsequently calcining at 6008C for 3 h in air with a heating and cooling rate of 18C/min. The whole process of dipping, drying and calcining is repeated once, to repair any defects in the ®rst g-Al2O3 layer. These defects may be caused by particle contamination, aggregates, micro air bubbles in the sol and irregularities in the a-Al2O3 support surface. The ®nal total g-Al2O3 layer thickness is in the order of 3±5 mm, with

an average pore width of about 2.5 nm, determined by permporometry [7]. 2.3. Silica membrane preparation The g-Al2O3 membranes, in turn, are used as a substrate for the preparation of microporous silica membranes. The microporous silica membrane toplayer is prepared by dipping in a polymeric silica dip solution, followed by drying and calcining. A silica sol is prepared ®rst by acid-catalysed hydrolysis and condensation of tetraethyl orthosilicate (TEOS, Aldrich, p.a. grade) in ethanol. A mixture of acid and water is carefully added to a mixture of TEOS and ethanol under vigorous stirring. During the addition of the acid/water mixture the TEOS/ethanol mixture is placed in an ice-bath to avoid premature (partial) hydrolysis. After the addition is complete the reaction mixture is re¯uxed for 3 h at 608C in a water bath under continuous stirring. The reaction mixture had a ®nal molar TEOS/ethanol/water/acid ratio of 1/ 3.8/6.4/0.085 in agreement with the ``standard'' recipe of silica sol preparation, as de®ned in [8]. The reacted mixture was cooled and diluted 19 times with ethanol to obtain the ®nal dip solution. After dipping, the membranes were calcined at 4008C for 3 h in air with a heating and cooling rate of 0.58C/ min. The whole process of dipping and calcining is repeated once to repair any defects in the ®rst silica membrane layer. These membranes are henceforth referred to as ``Si(400) membranes''. Another type of membranes were prepared by the same procedure as described above but with the only difference that the calcination temperature was 6008C. These membranes will be referred to as ``Si(600) membranes''. 2.4. Unsupported silica materials Unsupported microporous silica material was made for characterisation by means of physical adsorption measurements as follows: 60 cm3 of 19 ethanoldiluted, hydrolysed silica sol was allowed to evaporate in a 10 cm é petri-dish at room temperature, so that 0.1±0.3 mm thick silica gel ¯akes were obtained overnight. These ¯akes were calcined at 4008C or 6008C for 3 h with a heating and cooling rate of 0.58C/min, to produce unsupported silica material.

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2.5. Membrane characterisation Membrane gas permeance was measured in the pressure-controlled dead-end mode [9] in the temperature range of 50 to 3008C. Prior to the permeance measurements the membranes were dried for several hours at 2008C to remove adsorbed water from the micropores. The disk-shaped membranes were placed in stainless-steel permeance cells with the microporous top-layer at the feed side. The pressure difference over the membrane was adjusted by an electronic pressure controller (Type 5866, Brooks Instruments, USA). The gas ¯ow through the membrane was measured by mass ¯ow meters with maximum ¯ow ranges of 25 or 100 cm3/min (SPT). The pressure over the membrane was measured with an electronic pressure transducer (Validine Inc., Northridge, CA, USA). A schematic representation of the permeance set-up is given in Fig. 1. The permeance, F, is de®ned as ratio of the gas ¯ux, J (mol/m2s) and the pressure difference, P (Pa). Gas separation experiments were performed in the counter current mode [9]. The membranes were placed in stainless-steel cells with the microporous membrane top-layer at the feed side. The composition of the gas mixture at the feed side was controlled by mass ¯ow controllers. The feed pressure and the pressure difference over the membrane were measured by electronic pressure transducers. Argon was used as sweep gas with a ¯ow of 120 ml/min. The gas composition of the permeate and retentate were analysed by a gas chromatograph (Varian, Star 3400CX, with a

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molecular sieve and Haysep column). A schematic diagram of the separation equipment is given in Fig. 2. The separation factor ( ) is calculated from the gas concentrations (x, y) at the retentate and the permeate side according to: yretentate xpermeate  ˆ ypermeate xretentate Morphological membrane characterisation was done by ®eld-emission scanning electron microscopy (FE-SEM Hitachi, Type S800) and transmission electron microscopy (TEM CM30 TWIN/(S)TEM, Philips Analytical, Eindhoven, The Netherlands). The FESEM recordings were made of a perpendicular fracture surface. TEM recordings were performed on a thin cross section of the membrane, made as follows: One silica membrane was cut in halves and both parts were glued together with the silica top layers facing each other. From this ``sandwich structure'' a small slab was cut, which was abraded to a thickness of 200 mm. The specimen was further thinned by making dimples on both sides, in the middle down to 15 mm using a Dimple grinder. Finally the thickness of the centre of the specimen was reduced further by ion milling until a centre hole had just appeared. During the thinning procedure the specimen was carefully positioned so that the opposing silica layers formed the centre of the thinned area. The TEM recording was made near the centre hole, at the thinnest part of the sample. The unsupported (microporous silica) membrane material was characterised by nitrogen physical

Fig. 1. Schematic diagram of the experimental set-up for permeance measurements.

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Fig. 2. Schematic diagram of the experimental set-up for separation measurements.

adsorption at 77 K to determine its micropore volume, porosity and pore size distribution (Sorptomatic 1900, Carlo Erba Instruments, Milan, Italy). The physical gas adsorption set-up was provided with a turbo molecular pump system (Turbotronik, NT50 Leybold, Germany) and an extra pressure transducer for the low pressure range (10ÿ3 Torr to 10 Torr) to be able to determine microporosity. This was checked with measuring zeolites [10]. All samples were degassed at 3008C for 24 h prior to the sorption experiments. The pore size distribution is calculated according to the HorvaÁth±Kawazoe method [11] combined with the 10:4 Lennard-Jones potential functions for adsorption of N2 on SiO2 [11±13]. 3. Results and discussion 3.1. Morphological characterisation of silica membranes The FE-SEM and TEM recordings, given in Figs. 3 and 4 reveal a very thin silica layer of 30 nm, obtained after 2 times dipping. This result is a little in contrast with earlier suggestions in the literature [8] in which a thickness of 100 nm is proposed. The TEM micrograph indicates that the silica layer is deposited on top of the g-Al2O3 layer as a distinct separation between the two layers is present. A clear division

between silica and g-Al2O3 is also visible on the FESEM recording. The g-Al2O3 layer is about 3 mm thick after 2 times dipping. The boundary between the ®rst and second g-Al2O3 layer at approximately 250 nm from the surface is clearly visible. The ``colour'' difference between the two g-Al2O3 layers, visible in the TEM micrograph, is very distinct. The second dipped layer has a lighter appearance than the ®rst dipped which can be caused by two effects: (i) The ®rst layer is calcined two times at 6008C. (ii) The ®rst layer is applied on the a-Al2O3 support which has a much coarser structure than the g-Al2O3 layer on which the second layer is applied. Since capillary forces play an important role in the layer formation this can result in a more dense second gAl2O3 layer. The colour difference is not caused by a difference in pore size. This was checked by permporometry: The one- and two-layer dipped membranes were found to have both a pore slit width of 2.5 nm; see Figs. 5 and 6. 3.2. Pore size characterisation of unsupported silica The physical adsorption isotherm of the unsupported Si(400) material, see Fig. 7, is type I, characteristic for microporous materials. The micropore volume is 0.28 ml/g, from which we can calculate the porosity to be 38%. The pore size distribution, calcu-

R.M. de Vos, H. Verweij / Journal of Membrane Science 143 (1998) 37±51

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Fig. 3. SEM micrograph of a Si(400) membrane cross section showing a part of the g-Al2O3 layer and the silica layer at high magnification (a) and the a-Al2O3 support and g-Al2O3 layers at lower magnification (b).

lated with the HorvaÁth±Kawazoe method [11] is given in Fig. 8. This plot shows that the pore size distribution of the unsupported silica material is narrow with a Ê . The unsupported Si(600) maximum at Deffective 5A material shows a type II isotherm, which is typical for nitrogen-dense materials. This result is in agreement with the pore size of the supported membranes, estimated from the permeance of various gases and their kinetic diameter. It should be mentioned though that these results for unsupported material can not be transferred quantitatively to the supported membrane situation. They can only be of qualitative use to show

trends in changes in pore structure with processing. It must be clear that the supported silica layer cannot be expected to have the same structure as similar processed unsupported silica material, since the forces present during the drying process of both materials are different. 3.3. Gas permeance Permeance through microporous amorphous silica membranes occurs by solid-state-type diffusion of molecules. The permeance ¯ux can be derived from

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Fig. 4. TEM micrograph of Si(400) membrane cross section showing a part of g-Al2O3 layer and silica layer.

Fick's ®rst law: dc (1) dx ~ is the chemical diffusion coef®cient, c the in which D local concentration of molecules and x the coordinate along the permeance direction. As microporous diffusion is assumed to be the rate limiting step, therefore gas-phase diffusion limitations can be neglected, and thermodynamic equilibrium can be assumed at the interface. The concentrations of molecules at both membrane surfaces can thus be derived from equilibrium gas-phase adsorption data. To our knowledge the only quantitative adsorption

~ J ˆ ÿD

data available for amorphous microporous silica in the literature is by de Lange et al. [14], but they did not determine the isosteric heat of adsorption for all gases used in this study. In addition the complication is met that it is not exactly known what type of amorphous silica best represents our silica layers. Hence we have to rely on data available for silicalite that can be regarded as a model system for the microporous membrane silica [15±18]. Silicalite is a pure silica Ê 5.1 A Ê) zeolite with elliptical straight channels (5.7 A Ê pore diameter) which and sinusoidal channels (5.4 A are comparable to our membrane material. See also the pore size estimation of silica membranes by means of gas permeance experiments further on in this sec-

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Fig. 5. Accumulated O2 permeance as a function of the relative vapour pressure of cyclohexane of a g-Al2O3 membrane measured by permporometry.

Fig. 6. Pore size distribution of a g-Al2O3 membrane measured by permporometry.

Fig. 7. Physical adsorption isotherm of Si(400) unsupported material.

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Fig. 8. Pore size distribution of Si(400) unsupported material by the HorvaÁth±Kawazoe method.

    Qst ÿ Em P Ea P ˆ ÿJ0 exp RT RT L L (6)

tion. From the silicalite adsorption data we conclude that under our measurement conditions, gas adsorption is mostly in the low-coverage Henry's law regime:

~ 0 K0 exp J ˆ ÿD

c ˆ KP

in which we de®ne a new temperature-independent ~ 0 K0 and an effective proportionality constant J0 ˆ D activation energy for permeance:

(2)

in which K is a proportionality constant that depends on temperature according to:   Qst (3) K ˆ K0 exp RT where K0 is a temperature-independent proportionality constant, Qst the isosteric heat of adsorption, R the gas constant and T the absolute temperature. The validity of Henry's law implies that the concentration of molecules absorbed in the microporous solid is small compared to the number of available sites (pores). In the Henry (and Langmuir) law adsorption ~ is independent of the concentration [19]. Its regime, D temperature dependence is given by:   Em ~ ~ (4) D ˆ D0 exp RT ~ 0 is a temperature-independent proportionin which D ality constant and Em is the positive mobility energy. According to atomic jump theory [20], Em, represents the energy barrier between two adjacent sorption sites. ~ is independent of c, Eq. (1) can be written as: Since D ~ c J ˆ ÿD L

(5)

in which c is the concentration difference between both membrane surfaces and L is the membrane thickness. Combination of Eqs. (2)±(5) yields:

Ea ˆ Em ÿ Qst

(7)

Ea can have any sign; we have used a sign convention such that J increases with temperature if Ea is positive. 3.3.1. Si(400) Permeance results for Si(400) membranes are presented as function of temperature in Fig. 9. The permeance data are not corrected for the support resistance since the permeance through the support is signi®cantly higher than through the silica layer. Table 1 shows that the H2, CO2 and O2 permeance are independent of the pressure for pressure differences (P) >0.5 bar. H2 and CO2 permeance are slightly dependent on pressure, if a smaller pressure difference than 0.5 bar is used. This effect is found over the whole temperature range. The experimental CH4 permeance shows an increase for a P of 2 bar, which is probably due to the fact that the low CH4 ¯uxes at lower pressures are dif®cult to measure and therefore not very accurate. Fig. 9 indicates a slight increase of permeance with temperature, which can be described as an apparent thermal activation (Ea) of the permeance of all gases, except CO2. This demonstrates that Ea is not necessarily always positive. The CO2

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45

Fig. 9. Temperature dependence of the permeance for the Si(400) membranes at Pˆ1 bar and a mean pressure of 1.5 bar.

Table 1 Microporous silica isosteric heats of adsorption, Qst (kJ/mol) at low coverage Source

CH4

CO2

H2

De Lange [14] Rees [15] Golden [16] Choudary [17] Dunne [18] Used in this study

10 20.0 18.6 28 20.9 20

22 24.6 24.0 20 27.2 24

6

permeance is nearly constant with T and even seems to decrease with T at higher temperatures. Using our experimental data for Ea together with typical Qst data from [14±18] and shown in Table 1 we may obtain values for Em according to Eq. (7), the results are given in Fig. 10 and Table 5. It is clear that CH4 has greater dif®culty moving through the pores, its mobility energy is much higher than for the other gases. This can be explained from the fact that its dk is much larger. In Fig. 11 the permeance is given as a function of the kinetic diameter, of the molecules. An almost exponential correlation is observed between permeance and dk for the Si(400) membranes. A summary of typical permeance data for Si(400) and Si(600) is presented in Tables 2 and 3 for several gases. The permeance and permselectivity (Fa) values obtained compare favourable with literature results. The H2 permeance at 2008C is 210ÿ6 mol/m2sPa with Fa>500 for H2/CH4. Previously reported values were 1.610ÿ6 mol/m2sPa and 43, respectively [21].

N2

6.0

17.3 15.0

6

17.6 17

O2

16.3 16

An Faˆ235 for H2/CH4 at 1508C was published by Hassan et al. [22] for a silica hollow ®ber membrane, but no single gas permeance values were speci®ed. Commercially available CVI silica membranes made by MPT are reported to have a Faˆ27 for H2/CO2 with a H2 permeance of 1.710ÿ7 mol/m2sPa at 4008C [23]. The Si(600) have a Fa ˆ70 for H2/CO2 with a H2 permeance of 610ÿ7 mol/m2sPa at 3008C. The permselectivities for H2/CO2, H2/N2 and N2/O2 increase with increasing temperature; the permselectivity for H2/CH4, however, decreases with increasing temperature. This can be explained by the high Em that we calculated for CH4 in the Si(400). Although the kinetic diameters of O2 and N2 are quite similar, Ê and 3.65 A Ê respectively, our microporous 3.46 A membranes have an O2 permeance that is 4 times higher than N2. Ê has some The fact that CH4, with a dkˆ3.8 A detectable permeance and that SF6, with a kinetic Ê , does not permeate at all leads us diameter of 5.5 A to an estimate of the membrane pore size between

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R.M. de Vos, H. Verweij / Journal of Membrane Science 143 (1998) 37±51

Fig. 10. Calculated mobility energy (Em) versus kinetic diameter.

Fig. 11. Kinetic diameter versus permeance for Si(400) and Si(600) membranes at 2008C, Pˆ1 bar and a mean pressure of 1.5 bar.

Ê . This result is in, a possibly fortuitous, 3.8<é<5.5 A agreement with the microporous sorption results for unsupported material. 3.3.2. Si(600) The Si(600) membranes show a different permeance behaviour when compared to Si(400). The permeance at 3008C of H2 is lower, 610ÿ7 mol/ m2sPa and CO2 permeance is even much lower, 910ÿ9 mol/m2sPa at 3008C, resulting in Faˆ70 for H2/CO2. The decrease of the permeance is a result of densi®cation of the structure and a smaller pore size. The large increase of Fa for H2/CO2 from 7 to about 70 can also be attributed to a decrease in the

amount of terminal hydroxyl groups at the internal surface of the silica since higher calcination temperatures lead to lower hydroxyl concentrations [24]. A decrease of the amount of hydroxyls makes the material more hydrophobic which may result in a lower (surface) occupation and hence, a lower CO2 permeance. Methane does not permeate at all through the Si(600) membranes, and the N2 permeance is below the lower limit where we can measure with a reasonable accuracy. Using a very sensitive qualitative soap-solution test we detected some N2 ¯ow but not for CH4. Determination of the pore size by size exclusion by means of the permeance experiments from kinetic diameters thus results in a pore size

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Table 2 Permeance (mol/m2sPa) of Si(400) and Si(600) Gas

P b

H2

0.5 1.0 1.5 2.0 2.5 3 0.5 1.0 1.5 2.0 2.5 3 0.5 1.0 1.5 2.0 2.5 3 0.5 1.0 1.5 2.0 2.5 3 0.5 1.0 1.5 2.0 2.5 3

CO2

O2

N2

CH4

a b

Si(400) a

Si(600) a

258C

508C

1008C

1508C

2008C

1008C

1508C

2008C

2508C

3008C

7.33 5.91 5.31 5.03 4.85 4.72 2.51 2.28 2.14 2.06 2.01

9.65 8.19 7.7 7.35 7.21 7.04 2.94 2.76 2.65 2.62 2.61

12.7 11.6 11.5 11.3 11.3 11.4 2.64 2.91 3.07 3.19

17.7 16.7 16.4 16.4 16.4 16.4

18.5 17.4 17.3 17.4 17.2 17.3

2.32 2.40 2.52 2.49 2.56 2.60

3.01 3.11 3.20 3.28 3.37 3.38

3.97 4.02 4.11 4.03 4.15 4.12

5.13 5.22 5.38 5.32 5.40 5.37

6.05 6.28 6.32 6.41 6.47 6.48

2.72

2.31

0.84 0.88 0.89 0.89 0.90 0.90

0.061 0.097 0.11

0.065 0.091 0.10

0.046 0.078 0.092

0.57

0.83 0.83 0.81 0.81 0.81 0.80

0.11 0.13

0.31

0.77 0.74 0.74 0.73 0.72 0.72

0.094 0.13 0.14

0.03 0.033

0.044 0.048

0.043 0.055 0.060

0.053 0.059 0.070

0.058 0.067 0.073

0 0 0 0

0 0 0 0

0.10 0.10 0.10 0.09 0.09

0.15 0.14 0.13 0.13 0.12

0.19

0.22

0.27

0.007

0.010

0.015 0.015 0.026 0.03 0.025

0.027

0.031

in 10ÿ7 mol/m2sPa. in bar, pressure at permeate side is always 1 bar.

Ê which is signi®cantly smaller than that of 3.6<é<3.8 A the Si(400) membranes. The permeance results for the Si(600) membranes are presented as function of temperature in Fig. 12. In Table 2, for all gases the permeance increases with pressure. In Table 4 the corresponding permselectivities (Fa) are given. Ea of the permeance of H2, CO2 and O2 is lower than for the Si(400) membranes (Table 5). Since EaˆEmÿQst this observation may be explained by either a smaller Em or a higher Qst or both. For the time being this result cannot be explained. When taking into account the fact that the Si(600) is much denser than the Si(400) structure, a larger Em and a smaller Qst are both possible for

Si(600). The value of Qst is largely determined by the ``binding energy'' of the molecule and the pore. A denser structure with smaller pores and/or lower hydroxyl concentrations may lead to smaller Qst values. It is known that, for example, N2 interacts strongly with the hydroxyls at the silica surface [25]. Em can be thought of composed of the pore binding energy (Qst) and an additional energy, Ew, needed to pass the ``window'' between one pore and another. Hence Ea may be of the order of Ew. In the dense Si(600) structure Ew is expected to be signi®cantly larger than in the Si(400) structure since the ``window'' can be narrower and the effective average jump distance from pore to pore much larger. We do not

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R.M. de Vos, H. Verweij / Journal of Membrane Science 143 (1998) 37±51

Table 3 Permselectivity, Fa, and gas separation factors, a, for Si(400) (50/50) Mixture

H2/CO2

H2/N2

H2/CH4

CO2/CH4

H2/CO O2/N2 a

P a

0.5 1.0 1.5 2.0 2.5 3 1.0 1.5 2.0 2.5 3 1.0 1.5 2.0 2.5 3 1.0 1.5 2.0 3.0 1.0 2.0 3.0 1.0

258C

508C

1008C

Fa

Fa

Fa

2.9 2.6 2.5 2.4 2.3

3.3 3.0 2.9 2.8 2.8 2.7 55 55 57 55 59 819

4.8 4.0 3.8 3.8

59 53 50 54 52 844

326

3.1

276

3.8

61

773 767 435 377 456 194 205 123

3.9

1508C

2008C

a

Fa

Fa

a

3.9

6.1

7.5

6.8

3.4 3.2

533

6.5 76

64

619

561

507 528 118 107 132 36 38 40

6.4

321 334

100

3.8

75

3.26

in bar, pressure at permeate side is always 1 bar.

Fig. 12. Temperature dependence of the permeance of the Si(600) membranes at Pˆ3 bar and a mean pressure of 2.5 bar.

308 45 45 30 33 37

R.M. de Vos, H. Verweij / Journal of Membrane Science 143 (1998) 37±51 Table 4 H2/CO2 permselectivity, Fa, for Si(600) P a

1008C

1508C

2008C

2508C

3008C

2.0 2.5 3

26 20 19

30 26

66 42 37

81 59 52

139 83 70

a

in bar, pressure at permeate side is always 1 bar.

Table 5 Activation energies, Ea, and diffusion mobility energies, Em, for Si(400) and Si(600) Gas

Ea (kJ) Si(400)

Ea (kJ) Si(600)

Em (kJ) (Si400)

Em (kJ) (Si600)

H2 CO2 N2 O2 CH4

8 ÿ2 6 7 10

7.6 ÿ4 * 7 *

14 22 23 23 30

13.6 20 23

* Below detection limit.

exactly know if surface transfer plays an important role but we assume it is not very important. This assumption is based on the fact that we occasionally observed an in¯uence of membrane thickness on transport and that with an irregular amorphous structure surface transport limitations are less likely to occur. If Qst is the same for both membrane materials, Em, is expected to be higher for Si(600) than for Si(400),

49

because of the denser structure. The Em-values, however, that are calculated for Si(600) with the Qst-values of Table 1, are lower or equal to those calculated for Si(400), see also Fig. 10. This discrepancy may be explained by actual Qst-values which are for Si(600) membrane materials, signi®cantly lower than for Si(400). More accurate sorption studies of silica membrane material under relevant conditions are expected to lead to a better quantitative understanding of the relation between molecular transport and membrane structure. Additionally, the lower effective activation energies observed will make it necessary to perform a more complete description of sorptions thermodynamics in which weak temperature dependencies of standard gas-phase and solid-state enthalpies and entropies can no longer be neglected. This is a subject of future studies. 3.4. Gas separation results Gas separation results for a typical Si(400) membrane are given in Fig. 13 as a function of pressure. The separation factor was measured with a feed composition of 50% H2 and 50% CH4 or CO2. As can be seen in Fig. 13, the separation factor is nearly pressure-independent like the permselectivities. This result can be expected assuming that under the separation conditions used, there is only a limited mutual in¯uence of the gas ¯ows, due to the relatively low concentrations in the membrane phase (Henry

Fig. 13. Pressure dependence of the separation factor for the Si(400) membranes at 2008C.

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regime). This is supported by the fact that the separation factors are of the same order of magnitude as the permselectivities. The fact that the gas separation factors are lower than the permselectivities of the single gas measurements is for a large extent caused by a limitation in the maximum possible sweep ¯ow. If a higher sweep ¯ow is used, the partial pressure of the components in the permeate will decrease, as a result the mutual interactions of the components will be diminished and the separation factor will increase. Most separation factors increase with temperature, except for H2/CH4, again in agreement with the permeance results. In Table 3 separation factors are given for several binary gas mixtures. De Lange et al. [21] previously found for Si(400)-type membranes a separation factor for H2/CH4 of 18 at 2008C. From this result it may be concluded that the defect-density of the membranes (Si(400)) has largely decreased. In addition it is likely that the pore size distribution of the membranes is very narrow, since the relation between kinetic diameter of the gas molecules and the separation performance is quite outspoken. The pores are small enough to retard methane with a kinetic diaÊ (almost) completely, but suf®ciently meter of 3.8 A Ê big to pass easily CO2 with a kinetic diameter of 3.3 A Ê and H2 with a kinetic diameter of 2.9 A. Gas separation experiments were not performed for the Si(600) membranes because the separation factors are expected to be of the same orders as the permselectivities like the Si(400) membranes. 4. Conclusions Silica membranes with very low defect concentration, and thus good properties, can be prepared via a TEOS-based sol±gel synthesis. Homogeneous and clean synthesis results in reproducible high quality membranes. The Si(400) membranes have a homogeneous 30 nm thick silica layer, which results in high permeances, e.g. for H2 210ÿ6 mol/m2sPa at 2008C. They show good permselectivities and separation factors, H2/CH4 for example has Fa>500. Increasing the sintering temperature from 400 to 6008C results in a much denser membrane structure with smaller pores. From the relation between kinetic diameter and permeance results, the Si(600) mem-

Ê branes are expected to have pores with 3.6<é<3.8 A and the Si(400) membranes to have pores with 3.8< é Ê . These results are in agreement with physical <5.5 A adsorption experiments, performed on unsupported Ê for material, which reveal a pore size of 5 A Si(400) while the Si(600) samples were N2-dense. Si(400) and Si(600) membranes show a different permeance behaviour. All permeances are lower in the Si(600) membranes, e.g. H2 is 610ÿ7 mol/m2sPa. The permselectivities, though, for H2/CO2 increased from about 7 to 70. The H2/CH4 and CO2/CH4 permselectivities are very large, effectively in®nite, because CH4 does not permeate at all. It is dif®cult to quantify the diffusion mobility energy of gases through the microporous structure if accurate Qst-values are unavailable. The gas separation factors are of the same order of magnitude as the permselectivities for single-gas permeance experiments which indicates that in mixtures the gases have only limited mutual in¯uence. This is in agreement with the expected Henry sorption behaviour. The pore size distribution determined by physical adsorption is narrow which is in agreement with a narrow range of kinetic diameters in which permeance is found to decrease drastically in both permeance and separation experiments. 5. Symbols Fa J ~ D c Qst Em Ea L

separation factor [ÿ] permselectivity [ÿ] flux [mol/m2s] chemical diffusion coefficient [m2/s] concentration [mol/m3] isosteric heat of adsorption [J/mol] mobility energy [J/mol] apparent activation energy [J/mol] membrane thickness [m]

Acknowledgements Mr. M. Smithers and Dr. E. Keim from the Centre for Materials Research are gratefully acknowledged for FE-SEM and TEM analysis; ECN, Petten, the Netherlands for mercury porosimetry measurements

R.M. de Vos, H. Verweij / Journal of Membrane Science 143 (1998) 37±51

and ®nancial support and Mr. N.E. Benes for discussions on transport properties. References [1] D.E. Fain, Inorganic membranes: The new industrial revolution, in: Y.H. Ma (Ed.), Proceedings of the Third International Conference on Inorganic Membranes, 10±14 July 1994, Worcester, MA, USA, pp. 365±380. [2] A.J. Burggraaf, L. Cot, General overview, Trends and prospects, in: A.J. Burggraaf, L. Cot (Eds.), Fundamentals of Inorganic Membrane Science and Technology, Membrane Science and Technology Series, Vol. 4, 1996, pp. 1±19. [3] A.J. Burggraaf, Important characteristics of inorganic membranes, in: A.J. Burggraaf, L. Cot (Eds.), Fundamentals of Inorganic Membrane Science and Technology, Membrane Science and Technology Series, Vol. 4, 1996, pp. 21±34. [4] H.M. van Veen, M. Bracht, E. Hamoen, P.T. Alderliesten, Feasibility of the application of porous inorganic gas separation membranes in some large-scale chemical processes, in: A.J. Burggraaf, L. Cot (Eds.), Fundamentals of Inorganic Membrane Science and Technology, Membrane Science and Technology Series, Vol. 4, 1996, pp. 641±676. [5] R.J.R. Uhlhorn, M.H.B.J. Huis in 't Veld, K. Keizer, A.J. Burggraaf, Synthesis of ceramic membranes, Part I, Synthesis of non-supported and supported g-alumina membranes without defects, J. Mater. Sci. 27 (1992) 527±537. [6] K.-N.P. Kumar, Nanostructured ceramic membranes, layer and texture formation, Ph.D. thesis, University of Twente, 1993. [7] G.Z. Cao, J. Meijerink, H.W. Brinkman, A.J. Burggraaf, Permporometry study on the size distribution of active pores in ceramic membranes, J. Membr. Sci. 83 (1993) 221±235. [8] R.S.A. de Lange, J.H.A. Hekkink, K. Keizer, A.J. Burggraaf, Formation and characterization of supported microporous ceramic membranes prepared by sol-gel modification techniques, J. Membr. Sci. 99 (1995) 57±75. [9] W.J. Koros, Y.H. Ma, T. Shimidzu, Terminology for membranes and membrane processes, Pure Appl. Chem. 68 (1996) 1479±1489. [10] R.S.A. de Lange, K. Keizer, A.J. Burggraaf, Characterisation of mircoporous non-supported membrane top-layers using physisorption techniques, J. Porous Mat. 1 (1995) 139±153.

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