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Dual-Surface-Modified Reverse-Selective Membranes Rajinder Pal Singh,† Praveen Jha,‡ Kerem Kalpakci,‡ and J. Douglas Way*,‡ Pall Corporation, Cortland, New York 13045, and Department of Chemical Engineering, Colorado School of Mines, Golden, Colorado 80401
The reverse selectivity of inorganic membranes can be enhanced by surface modification using long hydrocarbon chain silanes. Improvements in n-C4H10 selectivity of Vycor porous glass over N2 and CH4 were obtained by sequentially modifying it with a C18 monochlorosilane (OCS) followed by a C18 trichlorosilane (ODS). The membranes were tested for pure and mixed gases at different feed pressures. The dual-modified membranes have higher ideal, or pure-gas, selectivities than membranes modified using OCS alone. The n-C4H10/N2 and n-C4H10/CH4 ideal selectivities for the dual-modified membrane were 88 and 35, respectively, as compared to values of 13 and 7, respectively, for the OCS-modified membrane. Adsorption experiments showed an increase in the relative adsorption of n-C4H10 over N2 and CH4 after dual surface modification of Vycor glass. Nuclear magnetic resonance, X-ray photoelectron spectroscopy, and thermogravimetric analysis techniques were used to confirm the presence of silanes on the surface of the substrates. A high flux of n-butane was measured for a dual-modified membrane fabricated using a 5 nm pore diameter γ-alumina ultrafilter substrate. The mixed-gas n-butane permeance for this membrane was 2 × 10-4 cm3(STP) cm-2 cm Hg-1 s-1 (6.7 × 10-8 mol m-2 s-1 Pa-1). Introduction A membrane-based gas separation process can be more energy efficient compared to existing technologies such as pressure-swing adsorption, distillation and cryogenics, etc. A few gas separation processes are based on membranes at present in the chemical industry such as CO2 separation from natural gas, hydrogen/nitrogen separation, and air separation. This is because of the scarcity of membrane materials that can withstand the harsh industrial conditions and at the same time provide high selectivity and permeance. Considerable research efforts have been directed toward identification and development of novel materials for membranes. Some examples include organicinorganic hybrids,1,2 nanoporous inorganic materials,3 zeolites,4,5 metals and metal alloys,6,7 and crosslinked polymers,8 etc. The aim of this research was to develop reverse-selective membranes to remove heavier and larger gases such as n-C4H10 from gas mixtures containing smaller and lighter gases such as CH4 and N2. The potential applications were separation of natural gas liquids (C3+ hydrocarbon gases) from natural gas and CO2 separation from mixtures with CH4, N2, and H2. Hydrocarbon gases and CO2 are highly interacting or adsorbing gases and usually have high sorption-based selectivity. This is in contrast to smaller and lighter gases like N2 and CH4, which usually have high diffusivity-based selectivity because of their smaller size, compared to butane. An ideal reverse-selective membrane would have a high sorption-based selectivity and minimized diffusivity-based selectivity. In general, polymers like poly(dimethylsiloxane) and co-poly(dimethyl, phenyl methyl siloxane) have very high selectivities and permeances for condensable gases because of their high solubility. Unfortunately, most of the polymers either dissolve or plasticize in the presence of highly interactive gases, decreasing the selectivities. The essence of this research is to introduce polymer-type characteristics into the inorganic mem* Corresponding author. E-mail:
[email protected]. Phone: (303) 273-3519. Fax: (303) 273-3730. † Pall Corporation. ‡ Colorado School of Mines.
branes by attaching long-chain hydrocarbons, especially C18. The silane molecules having either alkoxy or halide reactive groups can covalently bond to the surface, replacing hydroxyl groups. In this manner, the functionality of the substrate could potentially be manipulated based on the separation application. Utilization of silane surface modification for the development of gas separation membranes is a relatively new area of research. Javaid et al.9 and McCarley and Way10 have modified γ-alumina supports with hydrocarbon silanes. Singh and co-workers11,12 have reported surface modification of Vycor glass substrate by hydrocarbon silanes, and zirconia substrates have been silane modified by Picard et al.13 These surface modification efforts have been made to improve the separation performance of the supports/substrates. Luebke et al.,14 in a recent study, have reported improved separation performances of CO2 over He by modification of γ-alumina supports with polar alkyltrichlorosilanes. The most common silane of interest has been octadecyltrichlorosilane (ODS) based on its vast literature thanks to researchers and industries involved in media for liquid chromatography. We have previously reported the gas separation properties of octadecyltrichlorosilane (ODS) and octadecyl dimethylchlorosilane (OCS) modified mesoporous Vycor glass.12 In brief, the ideal n-C4H10/N2 selectivity of the ODS-modified membrane was approximately 5 times higher than that of an unmodified membrane. On the other hand, the ideal n-C4H10/N2 selectivities of the OCS-modified membranes varied from 3 to 0.6 times the selectivity of an unmodified Vycor glass, depending on the synthesis conditions. In the mixed-gas experiments, an interesting reversal of high selectivity was observed when comparing these two membranes. For an 80/20 mixture of n-C4H10 and N2, the selectivity of the OCS membrane was 220 as compared to 50 for the ODS-modified membrane. The OCS membrane showed high mixed-gas selectivity due to preferential adsorption of n-C4H10 on the surface of the membrane. The highly interactive nature of n-C4H10 allows it to block the flux of N2 by adsorbing on the surface sites. No pronounced pore blocking by n-C4H10 was observed in the case of the ODS membrane, as pure- and mixed-gas selectivities were approximately the same.
10.1021/ie070765g CCC: $37.00 © 2007 American Chemical Society Published on Web 10/03/2007
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Figure 1. Hypothetical structure of a Vycor glass support sequentially modified with OCS-ODS.
NMR data obtained on silica gel modified with the membranes confirmed the formation of a cross-linked, polymerized silane layer by the ODS modification and a hydrocarbon polymer brush-type structure from the OCS treatment. This paper describes the effect of two-step modification of Vycor glass. First, it was modified with OCS, and then it was modified with ODS, shown schematically in Figure 1. For the readers’ convenience, sequentially modified will be referred to as OCS-ODS membrane in the paper. After modification with OCS, a hydrocarbon brush was formed by the reaction of the -Cl groups of the OCS molecules with a fraction of the surface silanols. Steric hindrance due to the two methyl groups in OCS limits the OCS coverage on the modified surface. One reactive group of the OCS molecule allows it to react with only one surface silanol or react with another OCS molecule, forming dimers. The dimers cannot react with the surface silanols. The remaining surface silanol groups of the OCS-modified substrate were then modified with ODS to create a polymerized silane layer on the surface of the membrane. Experimental Section This section describes the synthesis and characterization of the OCS-ODS membrane. Surface Modification. A symmetric, mesoporous Vycor glass membrane was used as a support for the synthesis of the OCSODS composite membranes. The average pore size of Vycor glass is 4 nm, and its wall thickness is ∼1 mm. To qualitatively study the chemistry of the surface treatment, silica gel was also modified at the same conditions in a separate container. The support was modified first with OCS and then immediately with ODS, according to a procedure described below. The procedure is based on the work of Sander and Wise,15 who reported that the addition of water to the silane reaction mixture could significantly increase the loading of trichlorosilanes on silica substrates. (1) The Vycor glass support was cleaned by boiling in 30% H2O2 and then boiled in deionized water. It was then stored in deionized (DI) water till ready for modification. (2) The procedure for modification with OCS is described in detail in a previous publication.12 Briefly, the support was dried at 423 K in vacuum for 1 h. The surface reaction was done in 5 wt % OCS solution in toluene for 24 h at 373 K under reflux. Pyridine was used as a nucleophilic catalyst. Then the membrane was rinsed with toluene. (3) The OCS-modified support was then dipped in a mixture of 1 mL of ODS and 34 mL of toluene for 2 h at ambient conditions.
(4) Water (0.5 mL) was then added to the mixture to facilitate polymerization of the ODS molecules, and the membrane was further kept in the solution for 1 h. (5) The modified support was rinsed and soaked in toluene for 24 h. It was dried in flowing helium for 24 h at 333 K. Membrane Characterization. Nuclear magnetic resonance spectroscopy (NMR), X-ray photoelectron spectroscopy (XPS), and thermogravimetric analysis (TGA) were used to characterize the OCS-ODS surface modification. Silica gel was used as a surrogate material for characterizing the microstructure of the silane layers using Si29 NMR. The species involved in the reaction between silanes and the surface are identical for both silica gel and Vycor glass. Hence, silica gel is an appropriate sample for qualitatively studying the surface modification using NMR. No quantitative analysis such as packing density, packing loading, etc. of silanes based on NMR data was done because of the difference in the porosity of silica gel and Vycor glass. The details of the NMR experiments are provided in a previous publication.12 XPS data were obtained on a small sample of γ-alumina membrane modified with the actual membrane. A Mg KR X-ray source was employed to generate the photoelectrons for analysis. A small piece of γ-alumina membrane was modified with the membrane. XPS data were obtained on the top layer of the support, which is on the inner, lumen side of the alumina membrane. Sputter depth profile analysis was performed using a beam of accelerated argon ions through 2 kV. This ion beam produced a sputter rate of 0.5 Å/s through thermally grown silica. Adsorption data were obtained on an unmodified and an OCS-ODS membrane using a volume-based adsorption apparatus. The details of the adsorption system are provided in a previous publication.16 Transport Measurements. Pure-gas permeance data were obtained in dead-end crossflow mode, using a bubble flow meter to measure the flow rates. The feed pressure was set at 72 psia (496 kPa) except for the n-C4H10 experiments, which were performed at 32 psia (221 kPa). The permeate side was open to the atmosphere (11.8 psia ) 81 kPa, atmospheric pressure in Golden, CO). All data reported in this paper were obtained at 295 K. Mixed-gas permeation data were obtained using a premixed cylinder of n-C4H10 and CH4 containing 10% n-C4H10, which was delivered using mass flow controllers in a crossflow mode. The permeate pressure during mixed-gas experiments was maintained at 12.8 psia (88 kPa), and the concentrations of both the retentate and permeate streams were obtained using a calibrated gas chromatograph (SRI Instruments GC 8610 C). Results and Discussion Structure of Membrane. Development of novel membrane materials requires extensive efforts on understanding the structure. This is necessary to optimize the material to obtain the best combination of separation and permeance. In this work, we have tried to understand the structure of the membrane after two-step modification. We were not able to obtain characterization data directly on the membrane because of very low signalto-noise ratio. Hence, it is inferred indirectly from data obtained on surrogate samples. NMR, XPS, and TGA were used for analysis, and the results are discussed in the following sections. NMR. Si29 NMR is extensively used in the literature to characterize the structure of various siliceous species attached to the surface of silica using a silane-coupling agent for liquid chromatographic applications. Figure 2 shows the NMR spectra
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Figure 3. Si atom depth profile obtained by XPS on an alumina membrane dual modified with silanes.
Figure 2. (a) 29Si NMR spectra obtained on silica gel before and after modification with OCS-ODS. Peaks labeled Q, T, and M are due to Si atoms of bulk SiO2, polymerized silanes (ODS), and polymer brush (OCS) molecules on the surface, respectively; (b) structures represented by the peaks are shown.
obtained on silica gel before and after the two-step OCS-ODS modification. For a semiquantitative comparison, the intensity of various peaks was plotted relative to the intensity of the SiO2 peak. The peaks labeled Q are due to Si atoms of bulk silica (SiO2), and subscripts 2, 3, and 4 represent groups having 2, 1, and 0 hydroxyl groups, respectively. After the modification with the silanes, the intensity of the Q3 peak decreased and peak Q2 was not observed within the sensitivity of the NMR instrument. The substitution of surface silanols with the OCS molecules is confirmed by peak M, which is due to the Si atom bonded to two methyl groups. The decrease in the relative intensities of peaks Q2 and Q3 and the presence of peak M confirm the reaction between surface silanols and OCS molecules. Peaks T3 and T2 confirm the presence of the polymerized ODS molecules on the surface of the substrate. T3 represents the Si atom bonded to two surface silanols and having one unreacted -OH group. T3 can be viewed as a measure of silane cross-linking, which only occurs from modification by trichlorosilane (ODS).12 T2 represents the Si atom bonded to one surface silanol and having two unreacted -OH groups. These data confirm the presence of polymerized ODS species forming a thin polymer layer on the outer surface of the substrate. XPS and Thermal Analysis. XPS data were obtained on an alumina membrane as shown in Figure 3. A depth of approximately 1 µm from the surface of the alumina substrate was modified with the silanes as shown by the Si atom profile obtained using XPS. This indicates that the surface modification
with silanes did not uniformly modify the entire thickness of the substrate and that penetration is limited by the diffusion of silane molecules in the pores. This is expected because of the similarity of pore size and length of the silane molecule. The thermogravimetric analysis of OCS-ODS-treated Vycor glass shows a weight loss of 10 wt %. The sample was heated at 5 °C/min in air with a final temperature of 650 °C (923 K). A corresponding weight loss of approximately 2 wt % was obtained by TGA of an untreated sample. These data provide additional evidence regarding the presence of silanes on the surface of Vycor porous glass. Structure of MembranesDiscussion. The reaction between silane molecules and the substrate is evident from NMR data obtained on silica gel. In this reaction, the species involved are the -Si-Cl group of the silane molecule and the -Si-OH group of the substrate. These species are common in Vycor glass and silica gel. Hence, it is safe to assume that a similar reaction should take place on the surface of Vycor glass. Quantitatively, the porosity, the pore size of the silica gel, and the density of the surface hydroxyl groups are different. This means that loading of silanes on silica gel and Vycor glass can differ significantly. Therefore, we have not done any quantitative analysis. The exact structure of the silane layer of an OCS-OCS membrane is difficult to predict at this stage. The pore size of Vycor glass is 4 nm, which is very similar to the size of silane molecules (∼2.5 nm). The high mass transfer resistance encountered by these molecules in diffusing the pores of Vycor glass will limit the depth of the surface modification of the substrate. This mass transfer resistance will be even higher after the modification with OCS molecules. Therefore, the presence of ODS molecules will be limited to the outer surface. On the basis of our indirect evidence from the analysis of the characterization data, the most likely structure of this membrane is a thin polymerized layer of ODS molecules followed by a thin section of OCS-modified Vycor glass. A pictorial presentation of this structure is shown in Figure 1. Gas Permeance. The pure-gas permeances of an unmodified substrate, an ODS membrane, an OCS membrane, and an OCSODS membrane for a range of penetrants are compared in Figure 4. Ideal separation factors are compared in Table 1. As compared to an unmodified Vycor glass membrane, the OCS-ODS membrane has 1-2 orders of magnitude lower permeance but higher ideal selectivity for n-C4H10 over permanent gases. The permeance of permanent gases such as N2 and He was reduced by 2 orders of magnitude by the dual-silane modification as compared to approximately 10 times drop in the permeance of
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Figure 4. Comparison of the pure-gas permeances of an unmodified, OCSmodified, ODS-modified, and OCS-ODS-modified Vycor porous glass, obtained at 295 K.
Figure 5. Pressure dependence of the permeance of the OCS-ODS #1 modified membrane. Data were obtained at 295 K with a constant permeate pressure of 11.8 psia [square with diagonal line, n-C4H10 mixed gas (79% n-C4H10-21% N2); square with cross, n-C4H10 mixed gas (50% n-C4H1050% CH4); open right triangle, n-C4H10 mixed gas (80% n-C4H10-20% CH4); solid square, N2 mixed gas (79% n-C4H10-21% N2); half-filled square, CH4 mixed gas (50% n-C4H10-50% CH4); solid right triangle, CH4 mixed gas (80% n-C4H10-20% CH4)]. Table 1. Comparison of Ideal Selectivities of the OCS-, ODS-, and OCS-ODS-Modified Membranes; Average Permeance Values Obtained at Various Pressure Drops and 295 K Are Used to Calculated Selectivities
n-C4H10/He n-C4H10/N2 n-C4H10/CH4 n-C4H10/CO2 n-C4H10/C2H6
unmodified
OCS
ODS
3.96 9.67
9.91 13.4 6.93 6.47 4.05
31.7 52.7
8.58 8.52
6.66 7.35
OCS-ODS #1 59.7 88.1 35.1 10.4 7.24
OCS-ODS #2 23.0 10.5
interacting and condensable gases such as CO2, C2H6, and n-C4H10. The permeance of He dropped the most after silane modification. The ideal n-C4H10/N2 and n-C4H10/He selectivities increased by a factor of 9 and 15, respectively, for the OCSODS membrane as compared to an unmodified support. The permeances of n-C4H10 and CH4 were approximately 2 × 10-5 cm3(STP) cm-2 cm Hg-1 s-1 (6.7 × 10-9 mol m-2 s-1 Pa-1) and 8 × 10-7 cm3(STP) cm-2 cm Hg-1 s-1 (2.7 × 10-10 mol m-2 s-1 Pa-1), respectively. Figures 5 and 6 show the pressure dependence of the permeance at a constant permeate pressure for two OCS-ODS membranes. The permeances of N2 and CH4 are approximately constant, which is consistent with a Knudsen diffusion mech-
Figure 6. Pressure dependence of the permeance of the OCS-ODS #2 modified membrane. Data were obtained at 295 K with a constant permeate pressure of 11.8 psia (81 kPa). A binary mixture having 9-10% n-C4H10 in CH4 was used as a feed gas for the mixed-gas experiments.
anism or surface diffusion controlled by a linear Henry adsorption isotherm. It also confirmed the absence of any viscous flow, in which permeance varies linearly with the pressure drop. The influence of partial pressure driving force on the n-C4H10 permeance with pressure was different for the two membranes. For OCS-ODS #1, it increased slightly with an increase in the pressure drop, as shown in Figure 5. On the other hand, the permeance of n-C4H10 for OCS-ODS #2 sharply increased with an increase in the pressure and then became constant (Figure 6). It also showed a hysteresis upon lowering the feed pressure. The absolute value of the n-C4H10 permeance was higher for the OCS-ODS #1 membrane and CH4 permeance was lower than that compared to the OCS-ODS #2 membrane. These kind of differences in the membranes synthesized by surface modification of inorganic membranes using silanes was also reported by Javaid and Ford.17 In their work, n-C4H10 permeance showed different trends with increasing pressures in three membranes synthesized using the same recipe. One membrane showed an increase in n-C4H10 permeance, whereas two membranes had approximately constant permeances of n-C4H10. These variations in the membranes can be due to different silane loadings, packing densities, and penetrations of silanes in the pores, etc. Also, the silanes can slowly react among themselves in the presence of atmospheric water, forming dimers (in the case of monochlorosilanes) or polymerized species (in the case of trichlorosilane), deteriorating the quality of the silanes. To avoid this, the silanes were kept refrigerated. All these factors probably contributed to the differences in the performance of the dualmodified membranes. Despite the discrepancies in the pure-gas data of the two membranes, the mixed-gas n-C4H10/CH4 separation factors were approximately the same as those shown in Table 2. A n-C4H10/ CH4 separation factor of approximately 20 was obtained for both the membranes at feed pressures ranging from 60 to 99 psig with a 10 (vol)% n-C4H10/CH4 mixture. Table 2 also shows the mixed-gas experiments with ODS-OCS #1 at different feed concentrations containing n-C4H10, CH4, and N2. It was found that increasing the n-C4H10 concentration in the feed resulted in an increase in the n-C4H10/CH4 and C4H10/N2 mixture separation factors. The n-C4H10/CH4 and n-C4H10/N2 mixedgas separation factors for 80% n-C4H10 in feed mixtures are 61 and 181, respectively, at 295 K. The separation factors are higher than the ideal separation factors at 295 K. The pure- and mixedgas permeances of n-C4H10, CH4, and N2 for OCS-ODS #1 are compared in Figure 5, which suggests blocking of light gases
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Table 2. Mixed-Gas Separation Factors for Dual-Modified Membrane Obtained Using a Binary Feed Mixture at 295 K feed pressure (psia)
n-C4H10 partial pressure drop (psia)
62
1.25
78
1.38
99
3.65
28
2.32
28
9.84
27
8.30
feed mixture n-C4H10/CH4 10:90 n-C4H10/CH4 10:90 n-C4H10/CH4 10:90 n-C4H10/CH4 50:50 n-C4H10/CH4 80:20 n-C4H10/N2 79:21
OCS-ODS #1
OCS-ODS #2
20.6 20.1 21.3 38.9 60.8 182
such as CH4 and N2 by n-C4H10 in mixed-gas experiments performed with n-C4H10 feed compositions greater than 10 volume %. We draw this conclusion by our observation of lower permeances of CH4 and N2 in the mixed-gas experiments as compared to pure-gas experiments, whereas the n-C4H10 permeance is approximately the same in pure- and mixed-gas experiments at a similar partial pressure drop of n-C4H10. For the OCS-ODS #2 membrane, the permeance of CH4 for the 10% n-C4H10 in CH4 feed mixture was slightly lower than the pure-gas value. This difference between the two membranes might be due to the differences in the morphology of the silane layers. At this stage, no data pertaining to silane loading, packing density, and depth profile, etc. for two membranes have been obtained. Also, it was observed that the permeance of gases, especially lighter gases, changed with time and the order in which the gases were tested. The OCS-ODS #1 membrane was first tested for pure gases starting with the least interactive gas, N2. However, mixed-gas experiments were performed on OCSODS #2 before pure-gas experiments. Residual adsorbed n-C4H10 from the mixed-gas experiment could be acting as a plasticizer for the silane layer, increasing the permeances of small and lighter gases. Enhanced mixed-gas n-C4H10/N2 selectivity was observed for the OCS membrane as compared to pure-gas data as reported in our previous work.12 This was attributed to pore blocking by n-C4H10 decreasing the permeance of N2. A similar phenomenon was observed in this work for the case of n-C4H10/CH4 and n-C4H10/N2 at high feed volume fractions of n-C4H10 in the mixed-gas experiments. Adsorption Data. The surface modification of ceramic mesoporous membranes is expected to alter both diffusion and adsorption properties. Adsorption experiments were conducted on Vycor glass samples before and after modification with silanes, which are shown in Figures 7 and 8. The data shown in this paper were obtained using the OCS-ODS #1 membrane. The adsorption isotherms were adequately fitted (r2 g 0.98) by Henry’s law for both the modified and unmodified membranes for all gases except for n-C4H10 adsorption on unmodified Vycor glass. A Langmuir-type adsorption isotherm was used to fit the n-butane adsorption of n-C4H10 for the unmodified Vycor glass substrate. The adsorption coefficients are given in Table 3. In general, the adsorption of gases decreased after the modification. The silane molecules filled up some of the pore volume of the mesoporous supports. As shown by the XPS data, the presence of silane molecules was detected up to 1 µm from the modified surface. This decreased the number of gas molecules, which can fit the remaining porosity or pore volume of the membrane. The adsorption selectivity was obtained by calculating the ratio of adsorption coefficients. It increased after the dual
Figure 7. Gas adsorption data on an unmodified Vycor glass membrane obtained at 295 K. Lines are shown to guide the eyes.
Figure 8. Gas adsorption data on a dual-modified Vycor glass membrane obtained at 295 K; sample 1 from Table 1. Lines are shown to guide the eyes.
modification. Fifty and 120% increases in adsorption selectivity were obtained for n-C4H10 over N2 and CH4, respectively, after the dual modification. The adsorption of light gases decreased to a larger extent than that of n-C4H10, resulting in an increase in adsorption selectivity of n-C4H10 over the light gases. Compared to the pure-gas permeance selectivities, the adsorption selectivities were lower. The n-C4H10/N2 and n-C4H10/CH4 permeance selectivities were 88.1 and 35.1, respectively, as compared to adsorption selectivities of 29.1 and 22.3, respectively. The transport of gases in unmodified Vycor glass is composed of gas-phase flow through open pores and surface diffusion due to interaction of gases with the membrane surface. Smaller and lighter gases have higher gas-phase flows and lower surface diffusions than condensable gases. As reported in Table 1, Vycor glass itself is selective for n-C4H10 over all other gases. This is due to greater adsorption of n-C4H10, leading to a higher surface diffusion flux. The adsorption selectivity of n-C4H10 over N2 and CH4 increased after the modification. This suggests that an increase in the surface diffusion of n-C4H10 compared to those of the less interactive gases after OCS-ODS modification is one of the reasons for increased selectivity. In the case of the OCS-ODS #1 membrane, the pure-gas permeance selectivity is higher than the adsorption selectivity. The anticipated reason for this is the decreased pore volume after the modification. A smaller pore volume decreased the gas-phase flow contribution to the total flux. Less interactive gases had higher gas-phase flow compared to larger and more
Ind. Eng. Chem. Res., Vol. 46, No. 22, 2007 7251 Table 3. Adsorption of Gas on Unmodified and Dual-Modified Vycor Glass Substrate Obtained at 295 K; Adsorption Selectivity Is for n-C4H10 with Respect to Other Gases unmodified
n-C4H10 N2 CH4 a
modified
adsorption coefficient (scc (gas) cm-3 (membrane) atm-1)
adsorption selectivity (toward n-C4H10)
adsorption coefficient (scc (gas) cm-3 (membrane) atm-1)
adsorption selectivity (toward n-C4H10)
15.7a 0.81 1.56
19.4 10.1
13.4 0.46 0.60
29.1 22.3
Calculated using Langmuir adsorption isotherm.
Figure 9. Pure and mixed-gas permeances of a dual-modified γ-alumina membrane obtained at 295 K. The mixed-gas data were obtained with 10% (vol) n-C4H10/CH4 mixture.
interactive n-C4H10. A decrease in the pore volume has a greater affect on their permeances than that compared to n-C4H10. Consequently, the increased selectivity of the OCS-ODS membrane was due to the combined effects of increased relative adsorption of n-C4H10 and a drop in the gas-phase flow increasing the relative significance of the surface diffusion in the total flux. Contrary to the pure-gas data, the mixed-gas n-C4H10/CH4 selectivity at the 10% n-butane feed composition was approximately 20, which is very close to the adsorption selectivity as shown in Table 3. This suggests that surface diffusion could be the controlling transport mechanism. γ-Alumina Substrate. Vycor porous glass is an ideal support for fundamental research and development work, but it is not an industrially optimized substrate. Because of its symmetric structure, Vycor glass has a high resistance to the gas flow. On the other hand, an asymmetric γ-alumina substrate (Pall Exekia T1-70) has much lower hydraulic resistance, having a 5 nm pore size γ-alumina top layer that is 5 µm thick as compared to Vycor glass, which is 1 mm thick. The alumina substrate was modified similarly to the Vycor glass substrate to show that surface-modified membranes with industrially viable gas permeation can be obtained. The low permeance of Vycor glass modified with silanes could discourage a reader interested in this technology. It is beyond the scope of this current paper to comment on the effect of the substrate chemistry on the surface modification, although alumina substrates have been used in prior research.10,17 Figure 9 presents the pure- and mixed-gas permeances of n-C4H10 and CH4 at different partial pressures. Pure-gas n-butane permeance values ranged from 10-4 to 3 × 10-3 cm3(STP)/ cm2‚s‚cm Hg. These values are 10-100 times larger than those measured for dual-silane-modified Vycor glass. The permeance of n-C4H10 increased sharply with pressure and showed a hysteresis at lower feed pressure. A similar hysteresis was also observed in the permeance of n-C4H10 on Vycor glass membrane
modified in a similar fashion as shown in Figure 6. This type of variation is usually observed in adsorption of gases on mesoporous materials (Type IV adsorption isotherm). The hysteresis may be due to the condensation of n-butane in the 5 nm pores of the support top layer. The pure-gas n-C4H10/N2 and n-C4H10/CH4 separation values varied from 8 to 193 and 4 to 92, respectively, as the pressure drop of n-C4H10 increased from 1 to 19 psi (from 6.9 to 131 kPa). In comparison to the pure-gas data, the mixed-gas n-C4H10/ N2 and n-C4H10/CH4 separation factors were 6.5 and 5.5 for feed pressures of 60 and 100 psig, respectively. The mixed-gas data were obtained using the 10% n-C4H10 and 90% CH4 precalibrated mixture at 295 K. The mixed-gas permeance of n-C4H10 was approximately 2 × 10-4 cm3(STP) cm-2 cm Hg-1 s-1 (6.7 × 10-8 mol m-2 s-1 Pa-1). A possible reason for the lower value of mixed-gas selectivity is defects in the γ-alumina layer. These Membralox supports are fabricated for ultrafiltration (UF) and nanofiltration (NF) applications. The quality of the membranes required for UF and NF is not that stringent, and a few defects do not have a tremendous effect on the filtration performance. Contrary to this, a very good quality substrate is required for gas separation supports with possibly no defects (pores having larger diameters than the majority of the pores). Smaller and lighter gases have higher flow through these larger pores than heavier or larger gases. Even a few such defects can affect the separation factor tremendously. Comparison of Alkylsilane-Modified Membranes. The Vycor porous glass membranes were previously modified with monochlorosilane (OCS) and trichlorosilane (ODS).11,12 In this section, the dual-modified membranes (OCS-ODS) are compared to the OCS and ODS membranes. The permeances and ideal selectivities of these three membranes are compared in Figure 4 and Table 1, respectively. All three membranes have the same order of selectivity. The OCS-ODS membrane has the highest selectivity for n-C4H10 over N2, He, and CH4. The n-C4H10/N2 and n-C4H10/CH4 ideal selectivities were, respectively, 6.6 and 5.0 times higher in the case of the OCS-ODS membrane as compared to that for the OCS membrane.12 The n-C4H10/N2 selectivity was 1.7 times higher for the OCS-ODS #1 membrane than that for the ODS membrane. However, OCS-ODS #2 showed a factor of 1.7 times lower n-C4H10/N2 separation factor than that for the ODS membrane. The discrepancy in these two similarly modified membranes was discussed above. The n-C4H10 ideal selectivities over CO2 and C2H6 did not show any significant differences among the three membranes. This indicates that the surface modification of the Vycor glass support with silanes is not affecting the relative affinity of interactive gases with the membrane. However, this situation can change in mixed-gas experiments where n-C4H10 can suppress the flux of relatively less interactive gases like C2H6 or CO2. This has not been investigated in this paper. We have not studied the thermal, chemical, or mechanical stabilities of these membranes in this work. Previous work done in our group10,12 showed that these membranes were stable at
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least up to 373 K. A decrease in the selectivity was observed as permeation temperature increased. This is expected for any surface diffusion/solution diffusion-based separation because of the decrease in interaction of condensable gases as temperature increases. Angst and Simmons18 specifically studied water adsorption characteristics of silane-modified silicon wafer. They reported that water adsorption took place at the silane-oxide interface and that it has minimal effect on silane chain packing. They also reported that hydrophobicity increased after curing the silane layers at 423 K by increased cross-linking between unreacted hydroxyl groups of ODS molecules. No such increase in hydrophobicity was observed for OCS molecules after curing. A further understanding of the stability of silane-functionalized surfaces can be obtained from the liquid chromatography literature.15,18,19 Conclusions A Vycor glass membrane, with an average pore size of 4 nm, was sequentially modified with C18 monochlorosilanes (OCS) and trichlorosilanes (ODS). The OCS was used to modify the surface inside the pores, and the ODS was used to form a polymerized silane layer. The presence of two different silanes on the surface of the membrane was confirmed by NMR analysis of OCS-ODS-modified silica gel. XPS data show the presence of silane up to 1 µm deep from the surface of an alumina substrate based on the Si signal. The ideal selectivity of n-C4H10 over N2 and He increased by 9 and 15 times after the dual-silane modification. As hypothesized, based on the analysis of the OCS- and ODSmodified membranes, the dual modification was more effective in increasing the pure-gas selectivity than the single OCS or ODS modifications. Better blocking of the gas-phase flow by the dual-silane layers was probably the reason for an increase in the ideal selectivity. The mixed-gas selectivity of n-C4H10/CH4 for a feed stream of 9-10% n-C4H10 was approximately 20 at a feed pressure of 60-100 psig. The n-C4H10/CH4 and n-C4H10/N2 mixed-gas separation factors for 80% n-C4H10 in feed mixtures were 61 and 181, respectively, at 295 K. These factors are higher than the ideal separation factors at 295 K; therefore, blocking of light gases by n-C4H10 is observed in the mixed-gas experiments. There was also an increase in the relative adsorption of n-C4H10 over N2 and CH4. Acknowledgment The authors gratefully acknowledge financial support from the Department of Energy Office of Science, Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division under Grant DE-FG03-93ER14363. The authors thank Dr. Steve Dec of Chemistry Department at Colorado School of Mines for his help in conducting the NMR experiments, as well as undergraduate researcher Dan Steele of the Chemical
Engineering Department at Colorado School of Mines for his contributions to the experimental work. Literature Cited (1) Mahajan, R.; Koros, W. J. Factors controlling successful formation of mixed matrix gas separation materials. Ind. Eng. Chem. Res. 2000, 39, 2692. (2) Merkel, T. C.; Freeman, B. D.; Meakin, P.; Hill, A. J.; He, Z.; Pinnau, I. Effect of nanoparticles on gas sorption and transport in poly(1trimethylsilyl-1-propyne). Macromolecules 2003, 36, 6844. (3) Vos, R. M. D.; Verweji, H. High-selectivity, high-flux silica membranes for gas separation. Science 1998, 279, 1710. (4) Guan, G.; Tanaka, T.; Kusakabe, K.; Sotowa, K.; Morooka, S. Characterization of AlPO4-type molecular sieving membranes formed on a porous alpha-alumina tube. J. Membr. Sci. 2003, 214, 191. (5) Li, S.; Falconer, J. L.; Noble, R. D. SAPO-34 membranes for CO2/ CH4 separation. J. Membr. Sci. 2004, 241, 121. (6) Freemantle, M. Membranes for gas separation. Chem. Eng. News 2005, Oct 3, 49. (7) Roa, F.; Way, J. D.; McCormick, R. L.; Paglieri, S. N. Preparation and characterization of Pd-Cu composite membranes for hydrogen separation. Chem. Eng. J. 2003, 93, 11. (8) Wind, J. D.; Staudt-Bickel, C.; Paul, D. R.; Koros, W. J. The effects of crosslinking chemistry on CO2 plasticization of polyimide gas separation membranes. Ind. Eng. Chem. Res. 2002, 41, 6139. (9) Javaid, A.; Hughey, M. P.; Varutbangkul, V.; Ford, D. M. Solubilitybased gas separation with oligomer-modified inorganic membranes. J. Membr. Sci. 2001, 187, 141. (10) McCarley, K. C.; Way, J. D. Development of model surface flow membrane by modification of porous γ-alumina with octadecyltrichlorosilane. Sep. Purif. Technol. 2001, 25, 195. (11) Singh, R. P.; Way, J. D.; McCarley, K. C. Development of a model surface flow membrane by modification of porous Vycor glass with a fluorosilane. Ind. Eng. Chem. Res. 2004, 43, 3033. (12) Singh, R. P.; Way, J. D.; Dec, S. F. Silane modified membranes: Effects of silane surface structure. J. Membr. Sci. 2005, 259, 34. (13) Picard, C.; Larbot, A.; Guida-Pietrasanta, F.; Boutevin, B.; Ratsimihety, A. Grafting of ceramic membranes by fluorinated silanes: Hydrophobic features. Sep. Purif. Technol. 2001, 25, 65. (14) Luebke, D.; Myers, C.; Pennline, H. Hybrid membranes for selective carbon dioxide separation from fuel gas. Energy Fuels 2006, 20, 1906. (15) Sander, L. C.; Wise, S. A. Synthesis and characterization of polymeric C18 stationary phases for liquid chromatography. Anal. Chem. 1984, 56, 504. (16) Jha, P.; Mason, L. W.; Way, J. D. Characterization of silicone rubber membrane materials at low temperature and low pressure conditions. J. Membr. Sci. 2006, 272, 125. (17) Javaid, A.; Ford, D. M. Solubility-based, gas separation with oligomer-modified inorganic membranes. Part II. Mixed gas permeation of 5 nm alumina membranes modified with octadecyltrichlorosilane. J. Membr. Sci. 2003, 215, 157. (18) Angst, D. L.; Simmons, G. W. Moisture absorption characteristics of organosiloxane self-assembled monolayers. Langmuir 1991, 7 (10), 2236. (19) Ducey, M. W.; Orendorff, C. J.; Pemberton, J. E.; Sander, L. C. Structure-function relationships in high-density octadecylsilane stationary phases by raman spectroscopy. 1. Effects of temperature, surface coverage, and preparation procedure. Anal. Chem. 2002, 74 (21), 5576.
ReceiVed for reView June 3, 2007 ReVised manuscript receiVed July 31, 2007 Accepted August 2, 2007 IE070765G
