Hybrid Membranes for Selective Carbon Dioxide Separation from Fuel

1906

Energy & Fuels 2006, 20, 1906-1913

Hybrid Membranes for Selective Carbon Dioxide Separation from Fuel Gas David Luebke,* Christina Myers, and Henry Pennline National Energy Technology Laboratory, United States Department of Energy, M/S 84-206, P.O. Box 10940, Pittsburgh, PennsylVania ReceiVed February 13, 2006. ReVised Manuscript ReceiVed June 13, 2006

The potential of hybrid membranes as a CO2 capture technology for integrated gasification combined cycle applications was evaluated. Commercial γ-alumina supports were modified with a variety of trichlorosilanes intended to enhance the surface adsorption of CO2. The resulting hybrids were characterized using X-ray photoelectric spectroscopy and Fourier transform infrared spectroscopy and tested for performance in the separation of He and CO2. The silanization temperature was determined to be important because membranes fabricated at 273 K had substantially different performance properties than those fabricated at room temperature. Specifically, the permeances of membranes modified with alkyltrichlorosilanes at reduced temperatures were 1-2 orders of magnitude higher than those of membranes fabricated at room temperature, and the selectivities of these low-temperature silanized membranes were relatively similar to those expected from Knudsen diffusion. Supports modified with silanes containing one of a variety of functionalities were tested for CO2/He selectivity. Membranes modified with 2-acetoxyethyl, 2-carbomethoxyethyl, and 3-aminopropyl groups exhibited CO2 selectivity, with the highest values approaching 7 for 2-carbomethoxyethyl-silated membranes at 50 °C. Temperature dependences resulted in selectivity maxima for the 2-acetoxyethyl and 2-carbomethoxyethyl membranes. Mixed-gas selectivities were slightly higher than pure-gas selectivities because of a decrease in He permeance with a relatively minor reduction in CO2 permeance. Transport in the selective membranes is believed to occur by a combination of activated and solution diffusion for He and a combination of activated and surface diffusion for CO2.

Introduction Power generation based on the integrated gasification combined cycle (IGCC) has certain advantages over traditional pulverized coal (PC) power generation schemes in terms of both efficiency and pollution control. IGCC system conditions, while in some respects challenging, offer superior opportunities for CO2 capture in particular. Unlike PC flue gas streams, which generally contain less than 15 mol % CO2, shifted IGCC fuel gas may contain CO2 concentrations as high as 32 mol %.1 Though high temperatures and pressures, in excess of 250 °C and 20 atm, respectively, serve to limit the number of possible CO2 control technologies, the elevated pressure and CO2 concentration form a significant available driving force for membrane separations not found in flue gas. Relatively few studies have been conducted examining the feasibility of the post-shift, precombustion membrane capture of CO2 in IGCC systems. Bredesen et al. touched on the concept briefly in their examination of CO2-, O2-, and H2-selective membranes for gasification systems only to conclude that all of those technologies were too immature to form definite conclusions.2 More recently, Merkel et al. examined poly(dimethylsiloxane) and poly(1-trimethylsilyl-1-propyne) in simu* Author to whom correspondence should be addressed. E-mail: [email protected]. (1) EValuation of InnoVatiVe Fossil Fuel Power Plants with CO2 RemoVal Interim Report; Electric Power Research Institute: Palo Alto, CA, December 2000. (2) Bredesen, R.; Jordal, K.; Bolland, O. Chem. Eng. Proc. 2004, 43, 1129-1158.

10.1021/ef060060b

lated fuel gas.3 Both membranes were CO2-/H2-selective at room temperature but shifted to H2 selectivity as the temperature increased above 100 °C. Another approach recently explored by Lin et al. is that of developing H2-selective polymers.4 Selectivities (CO2/H2) for these materials as high as 10 were observed, but no attempt was made to examine their performance at temperatures above 35 °C. The polymer membranes are certainly interesting but lack the high permeance desirable in extremely high-throughput applications such as power generation. Another technology is required that is capable of sustaining increased gas flux through the membrane. Porous, inorganic membranes generally exhibit greater permeance than other types. In the most permeant among these membranes, the dominant mass transfer mechanism is Knudsen diffusion. The selectivity in a Knudsen diffusion membrane is determined by the inverse ratio of the square roots of the molecular weights of the diffusing gases, as shown in eq 1. Under these conditions, lighter gases will always pass through the membrane more quickly than heavier ones, leaving the permeate enriched in those lighter gases. In the case of H2 and CO2, 4.69 times as much H2 will permeate through the membrane, making Knudsen diffusion membranes useless for the selective transport of CO2.5 Molecular sieves, which selectively transport molecules on the basis of their shape or smaller size, are also not useful. (3) Merkel, T. C.; Gupta, R. P.; Turk, B. S.; Freeman B. D. J. Membr. Sci. 2001, 191, 85-94. (4) Lin, H.; Wagner, E. V.; Freeman, B. D.; Toy, L. G.; Gupta, R. P. Science 2006, 311, 639-642. (5) Geankoplis, C. J. Transport Processes and Unit Operations; Prentice Hall: Englewood Cliffs, New Jersey, 1993.

This article not subject to U.S. Copyright. Published 2006 by the American Chemical Society Published on Web 07/15/2006

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Energy & Fuels, Vol. 20, No. 5, 2006 1907

Ra/b )

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Reverse selectivity, selectivity for heavier molecules, is possible in membranes in which solution diffusion is the main mechanism, such as dense-polymer and immobilized-liquid membranes. Membranes of this type are able to separate gases on the basis of their relative solubility and diffusivity in the membrane material. Though diffusivity which tends to favor smaller molecules is generally more important, polymer synthesis is sufficiently developed to allow the design of reverse selective membranes for many gas pairs6 but with considerably lower permeances than those observed of porous membranes. Another mechanism which allows the selective separation of CO2 from smaller, lighter molecules is surface diffusion. Surface diffusion becomes important when one permeating gas has significantly stronger interactions with the surface than the others. These interactions allow a parallel, non-gas-phase permeation pathway for the adsorbing species. In cases where gas-phase diffusion becomes less favorable, such as effective pore shrinkage due to the accumulation of adsorbate species on the surface, surface diffusion is thought to become the dominant transport mechanism.7 Considerable efforts are underway to understand and optimize surface transport mechanisms in membranes primarily because of the potential enhancement in permeability over solution diffusion membranes. One method of fabricating membranes in which surface diffusion is the dominant transport mechanism is through the modification of inorganic Knudsen diffusion membranes with chlorosilanes. The synthesis process begins when chlorosilanes react with small amounts of water present in solution to eliminate HCl and form highly reactive hydroxysilanes. Hydroxyl groups on the inorganic surface can interact with these hydroxysilanes to eliminate water and form covalent bonds that anchor those silanes and any associated functionalities to the surface.8 The process occurs readily at room temperature for trichlorosilanes but more slowly for monochlorosilane species because of the steric hindrance associated with the additional organic (usually methyl) groups.9 In addition to offering greater reactivity, increased stability results because trichlorosilanes allow additional bonding to the surface and some degree of cross-linking within the silane monolayer.10 Silane monolayers on the substrate pore surface can facilitate the dominance of the surface diffusion transport mechanism in two ways. First, the effective pore size is reduced simply by the presence of the silane monolayer limiting gas-phase Knudsen diffusion. Second, silanes can be selected to produce a monolayer and, hence, a new pore surface, capable of adsorbing a particular gas more readily than others. Adsorbed gas molecules on the surface may then further restrict the pore and limit gasphase diffusion. A diagram can be seen in Figure 1. An excellent example of this type of modification is found in the work of McCarley and Way11 in which octadecyltrichlorosilane was grafted to γ-alumina to produce membranes selective toward hydrocarbons over lighter gases. Similar examinations of (6) Koros, W. J.; Coleman, M. R.; Walker, D. R. B. Annu. ReV. Mater. Sci. 1992, 22, 47-89. (7) Singh, R. P.; Way, J. D.; Dec, S. F. J. Membr. Sci. 2005, 259, 3446. (8) Silberzan, P.; Leger, L.; Ausserre, D.; Benattar, J. J. Langmuir 1991, 7, 7-1651. (9) Tripp, C. P.; Hair, M. L. J. Phys. Chem. 1993, 97, 5693-5698. (10) Fadeev, A. Y.; McCarthy, T. J. Langmuir 2000, 16, 7268-7274. (11) McCarley, K. C.; Way, J. D. Sep. Purif. Technol. 2001, 25, 195210.

Figure 1. Illustration of potential pore restriction mechanisms in a surface-selective membrane as seen in cross section including (A) the unmodified pore, (B) the pore with the addition of a silane monolayer, and (C) the pore with the addition of a silane monolayer and a layer of gas molecules adsorbed on the monolayer surface.

alkyltrichlorosilanes grafted to γ-alumina membranes include the work of Leger et al.12 and Javaid et al.13 Later work has generally been carried out on more easily characterized Vycor glass substrates.7,14,15 A study has been conducted examining the effect of various silane-grafted functionalities on CO2/He selectivity and permeance in γ-alumina membranes. Helium was selected as a surrogate for hydrogen in this study to alleviate potential safety concerns. The membranes developed in this study operate by the interaction of CO2 with functionalized surfaces to the exclusion of gases incapable of interacting with those surfaces. Any inert gas could then replace H2 in the experimental work without significant alteration of the separation mechanism. To achieve a more realistic estimate of CO2/H2 separation properties, it is necessary to match as closely as possible the size and diffusivity of H2. Helium is a good match to both properties and does not interact with the surface, so it was chosen as the most appropriate surrogate. Experimental Section Fabrication. Membranes were prepared by modifying Pall Membralox membrane tubes consisting of a 2-5 µm layer of γ-alumina supported on the inner surface of 2.5 cm cylindrical segments of 1 cm diameter R-alumina tubes. The nominal pore diameter of the active γ-alumina layer was stated by the manufacturer to be 5 nm. In Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectric spectroscopy (XPS) studies, identically modified γ-alumina Whatman Anodiscs with a diameter of 13 mm and a nominal pore size of 20 nm were used as a surrogate for these membranes. To facilitate sealing, each tube was covered at the ends with Duncan GL 612 glaze and fired at 800 °C for 6 h, and the process was repeated. After glazing, the active membrane length was approximately 1 cm. The disks were heated in the same fashion, but no glaze was applied. The cooled tubes and disks were placed in a 500 mL round-bottomed flask with approximately 100 mL of (12) Leger, C.; Lira, H. D. L.; Patterson, R. J. Membr. Sci. 1996, 120, 187-195. (13) Javaid, A.; Hughey, M. P.; Varutbangkul, V.; Ford, D. M. J. Membr. Sci. 2001, 187, 141-150. (14) Kuraoka, K.; Chujo, Y.; Yazawa, T. J. Membr. Sci. 2001, 182, 139149. (15) Singh, R. P.; Way, J. D.; McCarley, K. C. Ind. Eng. Chem. Res. 2004, 43, 3033-3040.

1908 Energy & Fuels, Vol. 20, No. 5, 2006

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Figure 2. Schematic of the membrane performance testing apparatus equipped for examination of porous, tubular membranes in mixed or pure gas flows.

30% H2O2 (Fisher, 31.1%) and refluxed for 30 min. Afterward, the tubes and disks were rinsed in deionized water and returned to the flask to reflux in 100 mL of deionized water for 30 min. These steps were designed to clean and hydroxylate the alumina surface before silanization. After completion of this procedure, the tubes and disks were dried at 80 °C for 1 h in the air. To isolate the role of pretreatment, disks were prepared with pretreatment modifications including removal of the prefiring step, hydroxylation in deionized water instead of H2O2, and drying at 150 °C rather than 80 °C. Silanization was initially conducted by placing the membrane supports in a mixture containing 140 mL of anhydrous toluene (Aldrich, 99.8%) and 5 mL of one of various trichlorosilanes (Gelest, >95%) or 3-aminopropyltrimethoxysilane. A trichlorosilane was not available for modification with the 3-aminopropyl group; trialkoxysilane was substituted, because amines catalyze the selfreaction of trichlorosilanes.9 A magnetic stirrer was added, and mixing began at a slow speed. After a few seconds, the tube or disk was added. The reaction was allowed to continue for 6 h. The tube or disk was then removed, rinsed with additional toluene, and allowed to soak in fresh toluene for 24 h. The silanization procedure was conducted entirely in a glovebox under dry nitrogen. The silanized tubes and disks were then dried in an oven at 80 °C in ambient air for 24 h. A significant modification was later made in the silanization procedure; the reaction mixture was first exposed to the substrate at 0 °C. The reaction mixture was then treated as usual and allowed to come to room temperature over the course of the first hour of reaction time. The remainder of the fabrication procedure was carried out without modification. FTIR. A Thermo-Nicolet Nexus 670 FTIR spectrometer was used to conduct the transmission analysis of Anodisc samples. Samples were examined at room temperature under ambient pressures of N2 using a liquid-N2-cooled mercury cadmium telluride detector. The Anodiscs were held in the beam at an angle approximately 30° from the perpendicular to limit fringence effects. Samples were placed in the chamber under dry N2 and allowed to equilibrate for 1 h. A total of 200 scans were taken for each sample at a resolution of 4 cm-1. Each sample was analyzed before and after silanization, and the post-treatment spectrum was normalized to its own pretreatment spectrum in order to reduce the effect of minor differences in the thickness among the support. XPS. XPS measurements were performed on a PHI 5600ci spectrometer with a monochromatic Al X-ray source. Anodisc samples were examined with an analyzer pass energy of 58.7 eV. All measurements were taken at a takeoff angle of 90°. Sputter profiling to a depth of 1000 Å was performed using 3 kV argon ions at a sputter rate of approximately 235 Å/min. Performance Testing. Performance testing was conducted in the flow system shown in Figure 2. The membrane tubes were mounted in the system using high-temperature Teflon Swagelok fittings. A flow of 500 mL/min of He, CO2, or an equal mixture of the two was passed along the tube interior while 500 mL/min of Ar sweep gas was passed along its exterior in countercurrent flow. The pressure was varied, 0-350 kPa, on each side of the membrane

Figure 3. Transmission FTIR spectra for 0.2 µm Anodiscs after various pretreatments and silanization procedures.

using a needle valve downstream. The temperature was regulated between ambient and 150 °C by a Lindberg/Blue Mini-Mite tube furnace with a built-in proportional integral derivative controller. Both the permeate and retentate were analyzed using an HP 5890 gas chromatograph with twin thermal conductivity detectors and Alltech Hayesep D 100/120 packed columns.

Results and Discussion FTIR. The FTIR spectra of Anodisc samples treated with n-butyltrichlorosilane are shown in Figure 3. The two distinct bands observed for all samples between 2800 and 3000 cm-1 can be assigned to the symmetric and asymmetric stretching modes of the C-H bonds of methyl groups.16 These bands have been taken as evidence of bound silanes by McCarley and Way11 and Leger et al.12 Progressively greater absorbance is observed when the post hydroxylation drying temperature is reduced from 150 to 80 °C, when the support is hydroxylated with H2O2 instead of with water, and when the initial silanization temperature is reduced from room temperature to 0 °C. The reductions in bound silane species associated with the lack of H2O2 hydroxylation and post hydroxylation drying at increased temperatures are easily understood in terms of surface hydroxyl coverage on the support at the time of silanization. After initial firing at 800 °C, the number of surface hydroxyl groups would be considerably depleted.17 Restoration of those groups is accomplished more efficiently by H2O2 than water, and fewer of the newly reattached groups are removed by drying at 80 rather than 150 °C. XPS. The atomic fraction of Si present in a γ-alumina Anodisc, as observed by XPS sputter profiling while monitoring changes in the Si 2p peak (Figure 4), decreases to less than 1% within the first 20 nm below the surface of samples treated at room temperature with n-butyltrichlorosilane. This finding is in reasonable agreement with the results of McCarley and Way,11 who observed the presence of Si to a depth of approximately 10 nm below the surface after room-temperature treatment of the Anodiscs with octadecyltrichlorosilane. The result, along with the presence of a Si 2p spectral shift not consistent with a silane at the alumina surface, was interpreted to indicate the presence of a thin polymeric layer deposited on the alumina. (16) Silverstein, R. M.; Webster, F. X. Spectrometric Identification of Organic Compounds, 6th ed.; John Wiley & Sons: New York, 1998. (17) Bottero, J. Y.; Cases Dabrowski, J. M. Surface Chemistry and Adsorption Properties of AlO3 Colloids. In Adsorption on New and Modified Inorganic Sorbents; Dabrowski, A., Tertykh, V. A., Eds.; Elsevier: New York, 1996.

Hybrid Membranes

Figure 4. XPS sputter profiles of Pall Membralox tubes silanized with n-butyltrichlorosilane at room temperature (b) and 0 °C (9).

In a similar sputter profile of an Anodisc sample treated at 0 °C with n-butyltrichlorosilane, the Si atomic fraction remained greater than 2% to a depth of at least 100 nm. The result is consistent with the FTIR spectra shown above, suggesting the presence of more silane in samples silanized at reduced temperatures. Though this finding seems initially counterintuitive, others have observed the formation of more uniform monolayers at lower temperatures in reactions of di- and trichlorosilanes with silica surfaces.8,18-19 Brzoska et al. went so far as to calculate a threshold temperature below which silanization would achieve the most tightly packed and ordered monolayer possible for any given silane molecule. This threshold temperature was thought to increase with increasing alkyltrichlorosilane chain length, but no detailed explanation was provided of the physical phenomena associated with the property. It has generally been accepted7-8,13,19 and some evidence has been shown18 that trace amounts of water, either adsorbed on the support surface or in solution, have a substantial effect on the nature of the silane layer in reactions of trichlorosilanes with inorganic surfaces. This effect is explained by the requirement that the chlorosilanes react with water to eliminate HCl and form silanol intermediates before reacting with surface hydroxyl groups to achieve binding or with other silanols resulting in polymerization. A reduction in the reaction temperature would result in the presence of more water adsorbed on the support at the expense of the water concentration in the silanization solution. Silberzan et al. theorized that this change in water distribution would result in the formation of the majority of silanol intermediates from trichlorosilane molecules already adsorbed on the support surface, leading to a more uniform layer in which each silane molecule is bound at two points to the surface and at one point to another silane molecule.8 Extrapolation of these phenomena to a porous support can only lead to a more pronounced effect. Free water in the silanization solution, as would be present at higher reaction temperatures, could result in the formation of partially polymerized agglomerates with a reduced ability to migrate into the pores. Once these agglomerates reacted with the external support surface, they could further reduce the uniformity of the monolayer by blocking other, unpolymerized silanes from diffusing into the pore openings. Restricting access to the internal support surface would then encourage reaction at the surface to form a thin polymeric layer like the one observed by (18) Brzoska, J. B.; Ben Azouz, I.; Rondelez, F. Langmuir 1994, 10, 4367-4373. (19) Fadeev, A. Y.; McCarthy, T. J. Langmuir 1999, 15, 3759-3766.


Figure 5. CO2 (b) and He (9) permeance as a function of the alkyl chain length of trichlorosilanes used to treat Pall Membralox tubes at 0 °C compared to CO2 permeance after treatment at room temperature by ourselves (-) and that in the literature (2).

McCarley and Way and as would be consistent with the above XPS analysis of samples silanized at room temperature.11 It is then reasonable to interpret the sputter profile of samples silanized at 0 °C as evidence of a more uniform, deeply penetrating silane layer. Performance Testing Alkylsilanes. Silanization with n-butyltrichlorosilane at room temperature reduces the CO2 permeance significantly below that of the unmodified γ-alumina support from 1.87 × 10-2 to 3.92 × 10-5 cm3 (STP) s-1 cm-2 cm Hg-1. Similar CO2 permeances were observed by Leger et al.12 and McCarley and Way11 for the room-temperature silanization of the same-type support with octadecyltrichlorosilane, 1.14 × 10-5 and 5.50 × 10-5 cm3 (STP) s-1 cm-2 cm Hg-1, respectively. Samples treated below room temperature with alklytrichlorosilanes of varying chain lengths showed a gradual reduction in permeance with increasing chain length to minimum values of 7.77 × 10-4 and 8.95 × 10-4 cm3 (STP) s-1 cm-2 cm Hg-1 for hexadecyltrichlorosilane and octadecyltrichlorosilane, respectively. Selectivity for He over CO2, ranging from 2.29 to 2.77, was reduced from the theoretical Knudsen value of 3.31. These values are in contrast to Leger et al., who observed a He/CO2 selectivity of 1.1, and McCarley and Way, who reported a selectivity of 0.80 for a H2 over CO2 theoretical Knudsen selectivity of 4.69. Figure 5 indicates the advantage of low-temperature silanization with respect to permeance. The performance characteristics of the γ-alumina membranes after treatment with straight-chain alkyltrichlorosilanes are distinct from those of the previous work of Leger et al. and McCarley and Way in both CO2 permeance and selectivity relative to lighter molecules. It has often been difficult to reproduce the properties of alkyltrichlorosilane monolayers deposited on inorganic surfaces, with differences in silanization temperature and trace water content playing a significant role in determining the homogeneity of surface coverage, the packing density of the chains, and the degree of cross-linking within the monolayer as well as allowing the formation of thin polymeric layers.8-10,18 In this case, the silanization procedure seems to have resulted in modification of the alumina surface without a change in the dominant transport mechanism. Knudsen diffusion, which is characterized by an enhanced selectivity for the lighter molecule and the absence of pressure dependence


Luebke et al.

Figure 6. Room-temperature CO2 permeance as a function of the mean pressure for unmodified Membralox tubes ([) and tubes silanized at 0 °C with n-butyltrichlorosilane (9) and n-octadecyltrichlorosilane (2).

for the permeance (Figure 6), seems to remain important regardless of the alkyl chain length, though the decreased He selectivity in the membrane over the untreated support is evidence that surface and solution diffusions are being enhanced. These enhancements are, however, not remotely as pronounced as those observed in either of the literature studies. In both of those cases, convincing evidence is shown that non-gas-phase mechanisms are dominant. CO2-Philic Groups. The treatment of γ-alumina tubes with trichlorosilanes containing nonalkyl groups intended to enhance CO2 surface interactions resulted in a reduction in CO2 permeance in all cases other than 3-(p-methoxyphenyl)propyltrichlorosilane and an increase in CO2/He selectivity in all cases except 3,3,3-trifluoropropyltrichlorosilane and 3-(methacryloxypropyl)trichlorosilane as compared to tubes treated with octadecyltrichlorosilane (Table 1). Of the silanes tested, only treatment with phenyl- and 2-carbomethoxyethyl-trichlorosilane and 3-aminopropyltrimethoxysilane resulted in substantial improvement of the CO2/He selectivity at room temperature, with the latter two membranes actually selective toward CO2. The membranes varied widely in CO2 permeance between the lower sensitivity limit of the instrument, approximately 5.0 × 10-6, and 2.5 × 10-3 cm3 (STP) s-1 cm-2 cm Hg-1. In general, reductions in He permeance correlate roughly to the steric bulk associated with the attached groups. Two membranes revealed surprising performance properties. The membrane treated with 3-(p-methoxyphenyl)propyltrichlorosilane has a CO2 permeance, 2.5 × 10-3 cm3 (STP) s-1 cm-2

cm Hg -1, more closely resembling that of short-chain alkyltrichlorosilane-treated membranes than those of other membranes treated with silanes containing complex substituents. The CO2/He selectivity is also similar to that of alkyltrichlorosilanetreated membranes. These properties tend to indicate that surface affinity has been enhanced slightly, Knudsen diffusion is still the dominant mechanism, and the large majority of the pores remain open to gas-phase diffusion. One explanation of this result is reaction of the methoxy group with a silanol group on an adjacent silane molecule or a surface hydroxyl group, resulting in the phenyl moiety becoming bound parallel to the surface. Such a configuration would not only fail to obstruct the pore and limit gas-phase diffusion, it would also block reaction sites on the surface and reduce the density of the monolayer. The membrane treated with 3,3,3-trifluoropropyltrichlorosilane shows another unique result among the samples. The membrane displays a CO2/He selectivity of 0.33, which is indicative of Knudsen diffusion. However, the CO2 permeance, 2.5 × 10-5 cm3 (STP) s-1 cm-2 cm Hg-1, is much lower than would be expected for that mechanism, and the relatively small size of the 3,3,3-trifluoropropyl group does not seem to indicate a particularly great ability to block pores limiting gas-phase diffusion. The low permeance/low selectivity can be explained in one of two ways. The deposition of silane on the support surface could result in most pores becoming plugged while a few remain open to gas-phase diffusion, or a polymeric layer could form in which the high diffusivity He dominates the greater solubility of CO2. The reduction in CO2 and He permeance for nearly all of the membranes silanized with substituents intended to increase CO2 selectivity is also a somewhat surprising result. Elucidation can be found in the commonality of these substituents, chosen in part for their hydrophilic character, the sharp contrast of this character to the hydrophobicity of alkyl chains, and the relative hydrophobicity of the silanization solvent, toluene. With the exceptions of cyclohexyl and phenyl, which show relatively minor reductions in permeance, the CO2-phillic groups could have a tendency to self-associate in solution. Under these conditions, the opportunity for polymerization is enhanced. If the average silane molecule reaches the surface having connected to a greater number of other silane molecules in cases where a hydrophilic group is present, the resulting agglomerates would have a proportionally greater ability to block pores and limit gas-phase diffusion mechanisms, resulting in lower permeance. Effect of Temperature. Membranes treated with acetoxyethyl, carbomethoxyethyl, and aminopropyl-trichlorosilanes

Table 1. Pure Gas Permeance and Selectivity at Room Temperature, 1 atm, and No Total Pressure Differential for Pall Membralox Tubes Treated with Various Trichlorosilanes at 0 °C

Knudsen theoretical unmodified n-butyln-octadecylcyclohexylphenyl3-(methacryloxypropyl)3-(pentafluorophenyl)propyl3-(p-methoxyphenyl)propyl3,3,3-trifluoropropyl2-acetyloxyethyl2-carbomethoxyethyl3-aminopropyl-*

CO2 permeance scc s-1 cm-2 cm Hg-1

He permeance scc s-1 cm-2 cm Hg-1

CO2/He selectivity

1.9 × 10-2 9.4 × 10-3 9.0 × 10-4 6.5 × 10-4 2.4 × 10-4 2.8 × 10-4 low 2.5 × 10-3 2.5 × 10-5 7.5 × 10-5 4.2 × 10-5 3.5 × 10-4

5.3 × 10-2 2.6 × 10-2 2.1 × 10-3 1.2 × 10-3 3.0 × 10-4 6.7 × 10-4 low 5.6 × 10-3 7.5 × 10-5 1.5 × 10-4 1.9 × 10-5 2.6 × 10-4

0.30 0.35 0.36 0.43 0.54 0.80 0.42 N/A 0.45 0.33 0.50 2.1 1.4

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Figure 7. Temperature dependence of CO2 permeance of Membralox tubes silanized at 0 °C with 2-acetoxyethyltrichlorosilane ([), 2-carbomethoxyethyltrichlorosilane (9), and 3-aminopropyltrichlorosilane (2).


Figure 8. Temperature dependence of He permeance of Membralox tubes silanized at 0 °C with 2-acetoxyethyltrichlorosilane ([), 2-carbomethoxyethyltrichlorosilane (9), and 3-aminopropyltrichlorosilane (2).

showed increases in CO2 (Figure 7) and He (Figure 8) permeance with increasing temperature in the range of 298423 K. An exponential fit of the He permeance versus inverse temperature showed a good correlation with coefficients of determination of 0.999, 0.978, and 0.910 for the acetoxyethy, carbomethoxyethyl, and aminopropyl membranes, respectively. The lower coefficient of determination for aminopropyl is partially related to the permeance value at 423 K. At that temperature, the aminopropyl membrane shows unstable permeance with time, which is most likely due to degradation of the silane layer. The removal of that point from the correlation results in a coefficient of determination of 0.960. An exponential correlation is not observed for CO2 permeance. Exponential correlations accurately model a variety of transport mechanisms including molecular sieving, solution diffusion, and Henry’s regime adsorption-driven (low surface concentration) surface diffusion with the apparent activation energy having a slightly different meaning in each case. McCarley and Way successfully modeled the behavior of C4H10 in their octadecyltrichlorosilane-modified membrane by incorporating a Langmuir adsorption isotherm into the transport equations.11 For the special case of low coverage of a weakly interacting gas, the model reduces to a Henry’s regime exponential dependence with the apparent activation energy equal to the difference of isosteric adsorption energy and the intrinsic diffusional activation energy. Henry’s assumption holds where values of apparent activation energy remain negative, indicating that diffusional activation energy overpowers mechanisms related to adsorption. This behavior is observed in our experimental observations of He permeance. Apparent activation energies calculated from the exponential correlations described above are -2.4, -2.8, and -2.4 kcal/mol for the acetoxyethyl, carbomethoxyethyl, and aminopropyl membranes, respectively. Apparent activation energies are highly reproducible with uncertainty less than 0.1 kcal/mol. Similar apparent activation energies have been derived for other noninteractive gases in membranes in which surface diffusion is thought to be important.11,20-22 As the apparent activation energy becomes positive because of stronger surface/gas interaction, surface coverage of the

diffusing molecule reaches a point where it can only be accurately modeled by the Langmuir isotherm. This condition reflects a more significant contribution of surface diffusion to membrane transport and is characterized in some cases by maxima in the permeance of interacting gases as they vary with temperature. Physically, it has been suggested that the maxima result from increases in temperature, lowering coverage and making surface diffusion less important.11 No maximum was observed in the permeance of any of the CO2-selective membranes tested, but all three showed a considerable decrease in slope at higher temperatures, which could lead to a maximum if testing were conducted at temperatures above 423 K. In two cases, the reduction in the rate of increase in permeance was sufficient to cause a maximum in CO2/He selectivity (Figure 9). The maxima appear at 373 and 323 K for the acetoxyethyl and carbomethoxyethyl membranes, respectively. The trend indicates that a similar peak selectivity would have been observed for the aminopropyl membrane had measurements been made below room temperature. That the functionalization of the surface with acetoxy and carbomethoxy groups should increase the surface affinity and, hence, the selectivity of the membranes for CO2 is not surprising. It has been shown that carboxylic groups are the most important moieties in CO2 adsorption on coal.23 The acetoxy group differs from carbomethoxy only in the location of the in-chain oxygen atom. It is reasonable to suggest that this oxygen atom is important to the binding of CO2. Being further from the alumina/ silane interface in membranes modified with carbomethoxy than in those with acetoxy, the in-chain oxygen would likely be more available as a binding site. This hypothesis is consistent with our finding that the carbomethoxy-modified membrane has greater selectivity for CO2 than the acetoxy-modified membrane. The selectivity offered by the aminopropyl-modified membrane is also easy to accept. Bound amines have been well-studied as sorbents for CO2 though usually in moist atmospheres. It would be informative to compare the binding energy of CO2 to acetoxy, carbomethoxy, and amino groups and determine if a correlation exists between adsorption energy and selectivity. Unfortunately, a complication arises. In the presence of gasphase water and in aqueous systems, it is known that two primary or secondary amines are required to bind a single CO2.24

(20) Kusakabe, K.; Gohgi, S.; Morooka, S. Ind. Eng. Chem. Res. 1998, 37, 4262-4266. (21) Fuertes, A. B.; Centeno, T. A. J. Membr. Sci. 1998, 144, 105-111.

(22) Yampolskii, Y.; Shishatskii, S.; Alentiev, A.; Loza, K. J. Membr. Sci. 1998, 148, 59-69. (23) Nishino, J. Fuel 2001, 80, 757-764.


Luebke et al.

Table 2. Comparison of Room Temperature, 1 atm, No Total Pressure Differential Pure and Mixed Gas Performances for Membranes Showing CO2/He Selectivity Produced by Silanization of Pall Membralox Tubes at 0 °C pure gas

2-acetoxyethyl2-carbomethoxyethyl3-aminopropyl-

mixed gas


CO2/He selectivity


CO2/He selectivity

7.5 × 10-5 4.2 × 10-5 3.5 × 10-4

0.50 2.1 1.4

4.9 × 10-5 4.1 × 10-5 3.3 × 10-4

0.59 2.7 3.6

The mechanisms associated with adsorption on a dry surface are less clear. If it is assumed that the 2:1 binding relationship holds, considerably less overall surface affinity would be observed for CO2 on a surface covered with amino groups than on a surface covered with a 1:1 binding group, particularly at lower coverage. Not only is the number of adsorption sites reduced by half at equal, high coverage in a 2:1 sorbent versus a 1:1 sorbent but, at lower coverage, it is reduced to a considerably greater degree because not all functionalities have an adjacent group with which to participate in binding. This aspect of surface adsorption makes it difficult to compare membrane behavior on the basis of the adsorption energy of CO2 on the modifying group because performance depends not only on coverage but also on the geometry and homogeneity of the surface. Mixed-Gas Selectivity. Membrane performance is altered slightly when measurements are made with an equal molar mixture of CO2 and He rather than the pure gases separately (Table 2). In all three of the CO2-selective membranes, CO2 permeance decreased, with the greatest decrease, 35%, observed in the acetoxy-modified membrane. Selectivity for CO2 over He increased in all cases. The increase in selectivity agrees with the work of McCarley and Way11 on similar membranes and Rao and Sircar,25 who studied microporous carbons. Both sets of authors attributed the increase in selectivity to competitive adsorption on the membrane pore surface, resulting in a decrease in permeance of the less interactive gas. With He, a gas which is expected to interact little with the surface, that explanation does not seem entirely satisfactory. Another possible mechanism by which the presence of CO2 could limit the He permeance is the obstruction of pores by increasing the size of surface functionalities. Acetoxy, carbomethoxy, and amino groups would all be substantially larger with CO2 adsorbed on them. Even at reasonably low surface

Figure 9. Temperature dependence of CO2/He pure gas selectivity of Membralox tubes silanized at 0 °C with 2-acetoxyethyltrichlorosilane ([), 2-carbomethoxyethyltrichlorosilane (9), and 3-aminopropyltrichlorosilane (2).

concentrations, the change in size could be sufficient to close pores to gas-phase He diffusion. All of these observations are consistent with the migration of He and CO2 through the modified membranes by hybrid mechanisms. In the case of He, the important mechanisms are Knudsen diffusion and solution diffusion. In the case of CO2, they are surface diffusion and solution diffusion. The results presented here are most reasonably explained by a membrane where small pores with surface affinity for CO2 are blocked at intervals by thin polymeric layers. Conclusion The modification of commercial γ-alumina membranes with trichlorosilanes results in substantial changes in performance. The silanization temperature is critical to the properties of the final membrane. Evidence has been presented that a reduction in the membrane fabrication temperature results in greater penetration of the silane into the support pores and a corresponding increase in overall silane loading on the membrane. Alkyltrichlorosilane-modified membranes fabricated in this way exhibit much higher permeance and lower selectivity than similar membranes fabricated at room temperature largely independent of alkyl chain length. The membranes fabricated at reduced temperatures also have CO2/He selectivities similar to those expected for membranes in which transport is dominated by Knudsen diffusion. For reasons that are not entirely clear but can be theorized to involve interaction of silane species with one another in solution prior to and during surface attachment, modification with silanes containing functionalities designed to enhance CO2 adsorption results in a substantially lower permeance for both He and CO2. In some cases, CO2/He selectivity is also significantly enhanced. Transport through the CO2-selective membranes is characterized by some form of activated or solution diffusion for He and a more complex mechanism probably involving activated and surface diffusion for CO2. Functionally, the result is a class of membranes with optimum operating temperatures at which maximum selectivity is achieved. Although the high permeance of the membranes is attractive, none of the samples approached the selectivity deemed necessary to be competitive as a CO2 capture technology for IGCC processes. The technique remains interesting because functionalities with greater CO2 adsorption strength, higher coverage of those functionalities, and more fully optimized fabrication could result in higher selectivity maximums. These avenues of advance have hard limits because, as the strength of interaction with the surface increases, so too must the operating temperature to remain within the optimal range and prevent permanent CO2 adsorption and poisoning. The limit arrives when that optimal operating temperature exceeds the decomposition temperature (24) Chang, A. C. C.; Chuang, S. C.; Gray, M. L.; Soong, Y. Energy Fuels 2003, 17, 468-473. (25) Rao, M. B.; Sircar, S. J. Membr. Sci. 1993, 85, 253-264.

Hybrid Membranes

of the CO2-philic functionality or its link to the surface. Further, the development of such membranes has the potential to be hindered by the difficulties of placing large functionalities at high coverage on the interior pore surfaces and by the likely expense of membranes which require the multistep fabrication procedures by which such coverages could be produced. If these difficulties could be overcome, however, a CO2-selective membrane of uniquely high permeance would result.

Energy & Fuels, Vol. 20, No. 5, 2006 1913 Acknowledgment. The authors gratefully acknowledge John Baltrus of the National Energy Technology Laboratory, Department of Energy (NETL DOE), for aid in conducting the XPS analysis; Angela Goodman also of NETL DOE for her advice and assistance with FTIR studies; and Michael Ciocco, Bryan Morreale, and the HMT crew of Parsons for helping to carry out preliminary performance testing. EF060060B

Hybrid Membranes for Selective Carbon Dioxide Separation from Fuel

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