Defect-Patching of Zeolite Membranes by Surface Modification Using

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Defect-patching of zeolite membranes by surface modification using siloxane polymers for CO2 separation Rongfei ZHOU, Huamei Wang, Bin Wang, Xiangshu Chen, Shiguang Li, and Miao Yu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b01034 • Publication Date (Web): 11 Jun 2015 Downloaded from http://pubs.acs.org on June 20, 2015

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Industrial & Engineering Chemistry Research

Defect-patching of zeolite membranes by surface modification using siloxane polymers for CO2 separation Rongfei Zhou,†,‡,* Huamei Wang,†,‡ Bin Wang,‡ Xiangshu Chen‡, Shiguang Li §, Miao Yuǁǁ †

State Key Laboratory of Material-oriented Chemical Engineering, Department of

Chemistry and Chemical Engineering, Nanjing Tech University, Nanjing, P.R. China, 210009 ‡

Department of Chemistry and Chemical Engineering, Jiangxi Normal University, Nanchang, P. R. China, 330022 §

ǁǁ

Gas Technology Institute, Des Plaines IL 60018, USA

Department of Chemical Engineering, University of South Carolina, Columbia, SC 29208, USA

* Corresponding author; Fax: +86-25-8317-2261; Phone: +86-25-6880-3018.E-mail: [email protected] (R.F. Zhou)

ABSTRACT Grain boundary defects are normally formed in zeolite membranes during membrane preparation and calcination processes. In this work, a siloxane polymer coating with the imidazole group was grafted on the surface of defective SSZ-13 membranes by chemical liquid deposition to seal the defects. The parameters such as silanization time, polymerization time, monomer type and concentration were optimized. Characterizations including Fourier transform infrared spectroscopy (FT-IR), field emission scanning electron microscopy (FESEM) and energy dispersive x-ray spectroscopy (EDX), showed that siloxane polymers were coated on the surfaces of SSZ-13 crystals and membrane. Six modified membranes showed decreased CO2 permeance by only 21±5% [average CO2 permeance of 1.9×10-7 mol/(m2 s Pa)] and increased CO2/CH4 selectivity by a factor of 9±3 (average CO2/CH4 selectivity of 108) for an equimolar CO2/CH4 mixture at 298 K. CO2/CH4 and CO2/N2 selectivities of the 1

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modified membrane decreased with pressure and temperature. Membrane stability was investigated for a long-time test and exposures to water vapor at temperatures up to 378 K and to some organic solutions. This modification method is also effective in sealing the defects of other zeolite membranes such as AlPO-18 membranes. Key words: zeolite membrane, CO2 separation, grain boundary, defect-patching.

1. INTRODUCTION Zeolite membranes have been widely studied for separations because of their molecular-sized pores, desirable adsorption/diffusion properties and high thermal and chemical stabilities1,2. Boundary defects in these polycrystalline membranes are always formed during hydrothermal growth and calcination processes3,4. These nonzeolitic pores that are larger than the zeolite pores are typically nonselective, resulting in low separation selectivity. Thus, elimination of defects in zeolite membranes is essential for their separation applications. Various post-treatment methods to eliminate or reduce these defects have been investigated in order to improve separation selectivities5-9. Hong et al.5 modified the B-ZSM-5 and SAPO-34 membranes by catalytic cracking deposition (CCD) of methyldiethoxysilane (MDES) to increase the separation selectivities at 773 K. Hydrogen/carbon dioxide separation selectivity of B-ZSM-5 membrane at 473 K increased from 1.4 to 37. However, the silanization process decreased H2 permeance by more than one order of magnitude. The small-pore SAPO-34 membrane after silanization showed increased CO2/CH4 selectivity from 73 to 110 and decreased CO2 permeance from 1.0×10-7 to 7.7×10-8 mol/(m2 s Pa). An on-stream CCD of MDES was used to modify the defects and pore sizes of MFI membranes by Gu’s group6 and Dong’s group7. In one work, H2/CO2 separation selectivity at 773 K increased from 3 to 42.6 and H2 permeance decreased by 12.5%6. Kanezashi et al

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modified small-pore DDR membranes using an

on-stream counter diffusion chemical vapor deposition (CVD) modification technique at 773 K to eliminate the intercrystalline micropores. The carrier gas containing tetraethylorthosilicate (TEOS) vapor was fed to the membrane surface, while the 2


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carrier gas with water vapor to the alumina supports side. H2/CO2 selectivity of the membrane did not change after modification. Yu et al.9 developed a molecular layer deposition (MLD) technique for the surface modification of SAPO-34 membranes. The deposited microporous Al2O3 coating thickness was controlled at the subnanometer level. Hydrogen/nitrogen selectivity for the modified membrane increased from 11 to 550 with 53% loss of H2 permeance at 473 K. In contrast to the above vapor deposition techniques, which were generally operated at high temperatures and/or at vacuum, modification of zeolite membranes by liquid phase deposition/grafting only requires moderate operation conditions. Matsuda et al.10 dip-coated a silicate-1 membrane into a diluted (< 3 wt.%) silicone rubber solution to repair the defects of silicate-1 membrane under room temperatures. The ethanol/water selectivity increased from 51 to 125 with a low decrease in ethanol flux after reparation. Zhang et al.11 repaired silicate-1 membrane by counter-diffusion chemical liquid deposition (CLD) at room temperature. The hydrolysis of silicone precursors was carried out at the organic/aqueous interface in the defects. The modified membrane had an increased CO2/N2 separation selectivity from 1 to 15 and 1/3 loss of CO2 permeance. The post-treatments using liquid deposition under moderate conditions make the operation easy and are suitable to be scaled up. Various kinds of laboratory-scale zeolite membranes such as SAPO-3412-14, zeolite T15, DDR16,17, MFI18, FAU19,20, AlPO-1821,22 and SSZ-1323,24 membranes had been reported to achieve good CO2 separation performance for CO2/CH4 and CO2/N2 mixtures. However, preparation of highly CO2-selective zeolite membranes is a great challenge because the defects are hard to omit during the membrane preparation and calcination. In a previous study, we prepared 10-cm-long tubular SSZ-13 membranes with CO2/CH4 selectivities larger than 15024. When we attempted to scale up the synthesis to 40-cm-long membranes, typical CO2/CH4 selectivities of most membranes ranged from 4 to 39 since the non-selective defects were formed during the scale-up synthesis process. In the current work, we developed a facile CLD method for the surface modification of SSZ-13 membranes under moderate conditions in order to patch the 3



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defects. A siloxane polymer layer was obtained by silanization using a siloxane on membrane surface and subsequent polymerization using 1-allylimidazole on the siloxane. The polymer chain containing alkali imidazole group further increased the preferential adsorption to CO2. The method could be suitable to reduce the defects in other zeolite membranes.

2. EXPERIMENTAL SECTION 2.1 Membrane preparation SSZ-13 membranes were prepared on the outer surface of mullite tubes (Nikkato, 40-cm length, 12-mm OD, 8-mm ID and 1.3-µm average pore size) by secondary growth as reported previously.24 The membrane gel had a mole composition of 1.0 SiO2:

0.1Na2O:

0.025Al2O3:

0.05TMAdaOH:

0.05TEAOH:

80H2O,

where

TMAdaOH is N,N,N-trimethtyl-1-adamantammonium hydroxide, and TEAOH is tetraethylammonium hydroxide. Colloidal silica (TM-40, Aldrich) was used as silica source whereas Al(OH)3 (Wako) was used as alumina source. The hydrothermal process was carried out at 443 K for 48 h. The as-synthesized membranes were calcined at 753 K for 6 h in air to remove the template. The membranes were cut into 10-cm-long pieces for further modifications. Most of the SSZ-13 membranes used for the post treatments had low initial CO2/CH4 selectivities of 4-39.

2.2 Defect-patching for zeolite membranes Zeolite membranes were grafted by a siloxane polymer layer using a chemical liquid deposition method. The process includes the silanization step (condensation of zeolitic hydroxyl group with vinyltriethoxysilane) and the polymerization step (polymerization of the siloxane with 1-allylimidazole) as illuminated in Figure 1. The calcined SSZ-13 membranes were washed in 10 wt.% hydrochloric acid solution at 303 K for 1 h to increase the density of hydroxyl group on the membrane surface. The membranes after drying were immersed in a 10 wt.% vinyltriethoxysilane (VTEOS, >99%, Sinopharm) ethanol solution. The silanization was carried out at 353 K for 5 h with stirring. Both ends of the tubes were sealed with the solid Teflon inserts 4


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to eliminate the silanization reaction on the inner surface of the support. The silanized membrane was washed using ethanol for three times and then polymerized with 1-allylimidazole (AMD, 97%, Sinopharm) or 1-vinylimidazole (VMD, 99%, Alfa Aesar) on the siloxane group. The polymerization reaction was catalyzed using di-t-butyl peroxide (≥97.0%, Sinopharm) with a concentration of 0.2 wt.% in ethanol at 353 K for 3 h under N2 atmosphere. Finally, the resulting membranes were immediately washed with ethanol and then dried and stored at 423 K.

Figure 1 A schematic representation of a two-step defect-patching process: the first step is condensation of zeolitic hydroxyl group with vinyltriethoxysilane and the final step is polymerization of the siloxane with 1-allylimidazole.

2.3 Characterization and separation measurements Scanning electron microscopy images of surfaces and cross-sections of membranes were taken by a field emission scanning electron microscopy (Hitachi, SU8020), at an acceleration voltage of 5 kV. To determine the Si/Al ratio of fresh, modified zeolite crystals and membrane surfaces, the energy dispersive x-ray (EDX) spectroscopy was recorded on an EDX spectroscope (Bruker Quantax 200) at 20 kV. Fourier transform infrared spectroscopy (FT-IR) analysis was carried out on a spectrophotometer (Thermo, Nicolet 6700) in the range of 600-4000 cm-1. Thermogravimetric analysis (TG) is recorded on a Diamond TG/DTA thermal analyzer. Samples were heated from room temperature up to 1173 K with a heating rate of 10 K/min under nitrogen flow. Single-gas permeations through membranes were tested by changing pressure drop and temperature without sweep gas described 5



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previously22 without sweep gas. Mixed-gas separations were measured using the Wicke-Kallenbach method with helium as a sweep gas with a permeate pressure of 101 kPa (atmospheric pressure). Membranes were mounted in a stainless steel module, and each side was sealed by three silicone O-rings and two stainless steel rings in turn. The compositions of the feed and permeate streams were measured by a Shimadzu GC-2014C with a thermal conductivity detector. Mass flow controllers were used to control the molar ratio of each component. The flow rates for each composition in the binary mixtures and the sweep gas were 150 and 400 standard mL/min (SMLPM), respectively. The ideal and mixed-gas selectivities are the ratio of the single-gas permeances and the ratio of mixture permeances, respectively. Each data point was obtained after a 2-h stabilization time, and changes in permeances of each component were lower than 5% in 1 h. To study the separation performances of membranes at higher pressures, some modified membranes were tested using the device described previously22 without sweep gas.

3. RESULTS AND DISCUSSION 3.1Characterization of modified SSZ-13 crystals and membranes To investigate the modification efficiency by the siloxane polymers, SSZ-13 crystals and membranes were characterized by FT-IR, FESEM, EDX and TG. The modification procedures for the SSZ-13 crystals and membranes are the same. Typical treatment conditions are: 5-h silanization reaction followed by 3-h polymerization reaction and AMD as the monomer with 10 wt.% in ethanol. Figure 2 shows FT-IR spectroscopy of fresh SSZ-13 crystals, silanized zeolite (after the first modification step) and polymerized zeolite (after the second modification step). The broad peaks around 3460 cm-1 and the sharp peaks around 1630 cm-1 of all samples could be attributed to the stretching vibration Si-OH group and the H-O-H stretching mode of adsorbed water, respectively25. The broad peak at around 1100 cm-1 is the characteristic peak of the asymmetric stretching vibration of TO4 tetrahedron (T represents Si or Al) in zeolite26. Compared with the fresh SSZ-13 sample, silanized zeolite had stronger peaks at 1073 and 806 cm-1 for Si-O-Si 6


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vibrations and a sharp peak at 963 cm-1 for C=C-H vibration, suggesting that VTEOS was grafted onto the SSZ-13 zeolite surface by the covalent binding of Si–O–Si group. Polymerized SSZ-13 zeolite had bands at 2928, 2980 and 3114 cm-1 (C-H stretching and/or N-H stretching vibration27) and that at 1507 cm-1 (C=N and C=C vibrations of the imidazole ring27) as shown in Figure 2c, suggesting that the AMD group was functionalized with the siloxane group after the second modification step.

Figure 2 FT-IR spectroscopy of (a) fresh SSZ-13, (b) VTEOS modified SSZ-13 and (c) VTEOS-AMD modified SSZ-13.

The morphologies of fresh SSZ-13 membrane and the membranes after polymerization with different monomer concentrations were shown in Figure 3. The modified membranes had similar crystal morphologies to the fresh one. However, their images became blurred as the AMD concentration increased, suggesting that an amorphous siloxane polymer coating was formed on the membranes surface, and the thickness increased with increasing monomer concentration. The Si/Al ratios in SSZ-13 crystals and membranes before and after silanization were detected by EDX. After typical silanization, the Si/Al ratios in the crystals and in the membrane increased from 15.7 to 32.7 and from 14 to 17.8, respectively.

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Figure 3 SEM images of (a) the surface of fresh SSZ-13 membrane, (b) cross section of fresh SSZ-13 membrane and the surface of SSZ-13 membranes treated in (c) 10 wt.% and (d) 15 wt.% AMD ethanol solutions, respectively.

Thermal resistance of the coated siloxane polymers on SSZ-13 crystals was investigated by TG analysis. TG curve of fresh SSZ-13 crystals (after template removal and the storage at room temperature for one month) showed the obvious weight loss when temperature increased from room temperature to 523 K, which was assigned the removal of adsorbed water molecules on the surface of the zeolite micropores30,31. Besides this obvious weight loss, the polymer-coated SSZ-13 had the second obvious weight loss from 523 K to 1123 K (curve b in Figure 4), which could be attributed to the decomposition of the polymers and adsorbed organics. TG analysis shows that the polymers start to decompose at 523 K.

Figure 4 TG curves for (a) fresh SSZ-13 and (b) VTEOS-AMD modified SSZ-13 crystals

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3.2 Optimization of surface modification The coating on membranes for defect-patching generally increases the selectivity but decreases the permeation flux. In order to obtain a large increase in selectivity and a low decrease in flux, the parameters such as silanization time, polymerization time, monomer type and concentration were optimized, as shown in Table S1. Seventeen SSZ-13 membranes were used for membrane modifications with different treatment conditions. These fresh membranes showed low CO2/CH4 selectivities (from 4 to 39). As either silanization time or polymerization time increases, the modified membranes (M1-M8) showed increased selectivities but decreased CO2 permeances compared with fresh membranes. Among these modification approaches, the most promising improvement was obtained for membrane M7 with a 5-h silanization and a subsequent 3-h polymerization. The CO2/CH4 selectivity increased by one order of magnitude, whereas the CO2 permeance of the membrane decreased by only 15% after defect-patching. Further increase in silanization time or polymerization time had little positive effect on selectivity, but decreased the membrane flux to a large extent. The compositions of the coating might affect the surface properties of the membrane. In this study, two monomers, AMD and VMD, were employed in the polymerization. They have the same functional group of imidazole but different alkyl groups. A number of base imidazole groups in the polymer coating could adsorb CO2 preferably over CH4. The increased CO2/CH4 selectivity for the modified membrane (M7) was attributed to the effects of defect-patching and the preferable adsorption of the functional group. The quality of the siloxane polymer coating was also determined by the type and the concentration of the monomers as shown in Table S1. The membranes (M9 and M10) using VMD had higher permeance loss and lower selectivities increase than those using AMD (M14-M17), which might be attributed to the lower quality of polymer coating resulted from the steric effect of VMD. The optimal AMD concentration was 10 wt.% in ethanol. When the concentration increased to 15 wt.%, a thicker coating (SEM observation in Figure 3) was formed, and thus the permeance of the membrane (M13) was reduced by 52%, larger than that of the membrane M7 using 10 wt.% AMD. 9



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Among the membranes listed in Table S1, six membranes (M7, M12, M14-17) modified under the optimized conditions (5-h silanization followed by 3-h polymerization) showed decreased CO2 permeance by only 21±5% [average CO2 permeance of 1.9×10-7 mol/(m2 s Pa)] and increased CO2/CH4 selectivity by a factor of 9±3 (average CO2/CH4 selectivity of 108). Such results suggested that the siloxane polymer coating patches effectively the defects with low loss of CO2 permeance and the modification operation was reproducible. Table 1 shows more details of separation performances of the original M14 and M15, and their separation performances after the silanized treatment and polymerized treatment. The silanization process normally increased membrane selectivity by a factor of 2-3, indicating that this treatment decreases the concentration and/or the size of the defects to a certain extent. After the subsequent polymer coating, the selectivities of the membranes were almost 10 times higher than those of the fresh membranes. Apparently, the follow up imidazole-functioned polymer coating further increased CO2/CH4 selectivity of the membrane. Meanwhile, it only slightly decreased the CO2 permeance.

Table 1 Separation properties of fresh SSZ-13 membranes and modified membranes by VTEOS and VTEOS-AMD defect-patching for an equimolar CO2/CH4 mixture at 298 K and 0.2 MPa feed pressure with sweep gas. CO2 permeance [×10-7 mol/(m2 s Pa)] Membr.

Original

Silanized

Polymerized

treatment

treatment

CO2/CH4 selectivity Original

Silanized

Polymerized

treatment

treatment

M14

2.2±0.1

1.8±0.1

1.6±0.05

14±1

44±2

119±6

M15

2.1±0.1

1.7±0.1

1.6±0.05

8±0.3

32±2

128±5

In addition, the defects of AlPO-18 membranes can also be sealed under the method established from SSZ-13 As shown in Table 2, carbon dioxide/methane selectivities of two AlPO-18 membranes after defect-patching increased from 16 and 20 to 60 and 79 for an equimolar CO2/CH4 mixture, respectively, indicating that the VTEOS-AMD coating reduces the defect concentration in AlPO-18 membrane. 10


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Carbon dioxide permeances of the modified membranes was 45% and 50% of the original values [~1.5×10-7 mol/(m2 s Pa)]. The reparation for AlPO-18 membrane might need to be further optimized because the concentration of hydroxyl group on AlPO-18 membrane differed from that on SSZ-13 membrane.

Table 2 Separation properties of fresh AlPO-18 membranes and modified membranes by VTEOS-AMD defect-patching for an equimolar CO2/CH4 mixture at 298 K and 0.2 MPa feed pressure with sweep gas. CO2 permeance [×10-7 mol/(m2 s Pa)] Membr. Original

Polymerized treatment


Polymerized treatment

AlPO-18-1

1.6±0.07

0.77±0.04

16±1

60±3

AlPO-18-2

1.4±0.05

0.72±0.03

20±1

79±4

3.3 Single-gas permeation Figure 5 shows the single-gas permeation through membrane (M7) before and after the VTEOS-AMD defect-patching as a function of kinetic diameter of the permeate gas at 298 K and 0.2 MPa feed pressure without sweep gas. Both curves show a similar tendency that permeances of gases decreased with the increase of their kinetic diameters, except for CO2. Carbon dioxide permeances are higher than H2 permeances, which is attributed to the preferential adsorption of CO2 in the fresh and modified membranes. The modified membrane shows a smaller decrease of CO2 permeance than other gases, resulting in the increase of ideal CO2/CH4 selectivity by almost one order of magnitude. The ideal H2/CH4 and H2/C3H8 selectivities increased by a factor of ~5 after defect-patching. Single-gas permeance of CO2 for the modified membrane (M14) decreased with temperature and pressure; but N2 and CH4 permeances of the modified membranes were almost independent of or changed slightly with temperature and pressure, as shown in Figures 6 and 7. Thus CO2/CH4 and CO2/N2 selectivities decreased with temperature and pressure. The results were consistent with those of our previous SSZ-13 membranes24 and other zeolite membranes1,22 11



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Figure 5 single-gas permeances of M7 before (dash line) and after (solid line) defect-patching at 298 K and feed pressure drop of 0.2 MPa without sweep gas; the inset shows ideal CO2/N2, CO2/CH4 and H2/CH4 selectivities before and after defect-patching.

Figure 6 single-gas permeances of CO2, N2 and CH4 through membrane M14 as a function of temperature at feed pressure of 0.2 MPa without sweep gas.

Figure 7 single-gas permeances of CO2, N2 and CH4 through membrane M14 as a function of feed pressure at 298 K without sweep gas. 12


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Bakker et al.28 explained the behavior of single-gas diffusion through zeolite membranes by a model that assumes the flux is the sum of surface diffusion and gas translational diffusion. The permeation of the adsorbed gas through zeolite pores decreased with temperature until it is primarily dominated by gas translation diffusion, so that the permeance has a minimum. However, the minimum was always observed at temperatures higher than 423 K.28,29 The model could explain the decrease of the three gas permeances through our membrane with temperature in the range of 298 to 373 K. For a real membrane with defects, the flux (J) of a gas component through a zeolite membrane, which is attributed to the surface diffusion, viscous flow and Knudsen diffusion, is related to the feed and permeate pressure, Pf and Pp, by  1 + KPf J = Αqs ln   1 + KPp 

 2 2  + Β ( Pf − Pp ) + C ( Pf − Pp )LLLL (1) 

The first term in Eq. (1) is the surface diffusion contribution, and the second and third terms are Darcy’s law for viscous flow and Knudsen diffusion contributions, receptively.29 The surface diffusion term in Eq. (1) decreases with pressure, but the viscous flow and Knudsen diffusion increase with pressure. The kinetic diameters of CO2 and N2 molecules are 0.33 and 0.364 nm, respectively, which are smaller than the pore size of SSZ-13 crystals. Therefore, the two molecules could permeate through both zeolite pores and the defects. In contrast to N2 permeance that was independent of pressure, CO2 permeance decreases significantly with pressure. The lower CO2 permeance at higher feed pressures is a typical surface diffusion behavior that is due to the fact that CO2 transport mainly through the SSZ-13 pores and it approaches to saturation in the feed side as pressure increases.

The CH4 permeance increased

slightly with pressure, mainly because that CH4 molecule (0.38 nm) diffuses in the SSZ-13 pores (0.38 nm) very slowly and the viscous flow and Knudsen diffusion dominates the permeation of CH4.

3.4 CO2/CH4 and CO2/N2 separations Figure 8 shows selectivity and permeances of the modified membrane M7 for an 13



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equimolar CO2/CH4 mixture as a function of temperature at 0.2 MPa feed pressure with sweep gas. Single CO2 permeance decreased from 4.2×10-7 to 1.6 ×10-7 mol/(m2 s Pa), while mixture CO2 permeance decreased from 2.1×10-7 to 1.1×10-7 mol/(m2 s Pa), as temperature increased from 298 to 373 K. Single and mixture CH4 permeances are close to each other and are independent of temperature. And thus, mixture CO2/CH4 selectivities were lower than ideal selectivities and both of them decreased with temperature. Temperature dependence of single and mixture permeances for the modified membranes was similar to our pervious higher quality SSZ-13 membranes24. At 298 K, our modified membrane had a little higher flux and lower selectivity than those of our previous as-synthesized SSZ-13 membrane24. Note that CO2/CH4 selectivities of our modified membranes were still as high as 100, although the quality of the initial untreated membranes was poor. This result shows our coating method dramatically decreases the contributions of non-zeolitic defects (poor selectivity) on permeance but slightly affected permeances through zeolite pore (high selectivity). The simple and reproducible post treatment method described in this study is a powerful tool to increase the quality of SSZ-13 membrane. Selectivity and permeances of the modified membrane M18 for an equimolar CO2/CH4 mixture as a function of feed pressure at 298 K were shown in Figure 9. This membrane was tested without sweep gas. CO2 permeance decreased and CH4 permeance was almost independent of pressure (no shown) when the feed pressure from 0.5 MPa to 3 MPa (atmosphere pressure was 0.101 MPa and permeate absolute pressure was 0.103 MPa), resulting into CO2/CH4 selectivity decreased. The similar results were found for the previous pure and best SSZ-13 membranes24. The decreases of CO2 permeance and CO2/CH4 selectivity were mainly due to that the coverage of CO2 molecule on membrane surface decreased as pressure increased. Besides, the flow through the defects had some negative effects on the separation selectivity. The CO2/CH4 selectivity of the modified membrane at 3 MPa was 45, indicating that the modified membranes have potentials for high-pressure separations such as natural gas purification. Note that this modified membrane had CO2 permeance of 2.0×10-7 mol/(m2 s Pa) and CO2/CH4 selectivity of 150 when it was tested at 0.2 MPa with 14


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sweep gas.

Figure 8 Selectivity and permeances through membrane M7 for an equimolar CO2/CH4 mixture as a function of temperature at 0.2 MPa feed pressure with sweep gas.

Figure 9 Selectivity and CO2 permeance through membrane M18 for an equimolar CO2/CH4 mixture as a function of feed pressure at 298 K without sweep gas.

Similar to CO2/CH4 separation, the single and mixture CO2 permeances in CO2/N2 separation through the modified membrane (M14) also decreased with temperature, as shown in Figure 10. At 298 K, the membrane (M14) before treatment showed ideal and mixture CO2/N2 selectivities of 4 and 2, respectively. The ideal and mixture CO2/N2 selectivities increased by a factor of 3.5 and 3.3 after modification, respectively, indicating that the coating is also effective to reduce the N2 permeation through the membrane. The modified membrane had a 13% decrease in CO2 permeance. The smaller N2 molecule permeates faster than the bigger CH4 molecule through either SSZ-13 pore or the defects. As a result, CO2/N2 selectivity is lower 15



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than CO2/CH4 selectivity even after the current modification. Separation of CO2/N2 mixture requires fewer defect concentrations. Therefore, our current modification conditions used for CO2/CH4 separation could not be suitable to improve CO2/N2 selectivity. To prevent N2 molecules permeating through the pores (zeolite pore and defects), the density of siloxane grafted on membrane surface in the proposed first step could be focused in the future study. The improvement for the density of surface siloxane directly reduced the defect size and supplied the possibility to obtain denser polymer layer.

Figure 10 Selectivity and permeances through membrane M14 for equimolar CO2/N2 mixture as a function of temperature at 0.2 MPa feed pressure with sweep gas.

3.5 Membrane stabilities Membrane M14 after modification using the siloxane polymer was tested for 7 days in our lab, as shown in Figure 11. During this period, CO2 permeance had a little increase but CH4 permeance was independent of test time for an equimolar CO2/CH4 mixture at 298 K after the stabilization time of 1 d, resulting in a little increase in separation selectivity with the increase of test time. Similar stable performances [CO2 permeance of ~1.0×10-7 mol/(m2 s Pa) and CO2/CH4 selectivity of ~20] for 72-h test at 393 K were obtained through the siloxane-polymer coated membranes.

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Figure 11 Selectivity and permeance through membrane M14 for equimolar CO2/CH4 mixture as a function of test time at 298 K at 0.2 MPa feed pressure with sweep gas.

Carbon dioxide separations from natural gas and flue gas require good stabilities under water vapor at room temperature or higher temperatures since these mixtures contain water vapor. In our previous work, the as-synthesized SSZ-13 membrane was stable in pure water vapor at 378 K for 4 days24. In the current study, one of two membranes after coating were exposed to water vapor at room temperature for 1d or at 378 K for 1d, and the treatments were repeated for 3 times. After each treatment, the membrane was dried at 423 K overnight and then tested using an equimolar CO2/CH4 mixture at 298 K and 0.2 MPa feed pressure. Table 3 compares the separation performance of the siloxane-polymer coated membranes before and after water vapor treatment. Carbon dioxide permeances and CO2/CH4 selectivities of the modified membrane (M16) after water vapor treatments at room temperature were similar to the original value. Meanwhile, the modified membrane (M17) after water vapor treatment at 378 K showed no significant change in permeance and selectivity. The results showed that the coating on the membrane surface are resistant to water vapor even at 378 K and the modified membranes have potentials for CO2 separations from natural gas and flue gas.

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Table 3 Separation properties of VTEOS-AMD modified membranes before and after the treatment by water vapor at 0.2 MPa feed pressure and 298 K with sweep gas. Membr .

CO2 permeance [×10-7 mol/(m2 s Pa)] Original

Treatment repeat 1

2

3


Treatment repeat 1

2

3

M16*

1.5±0.06

1.4±0.06

1.3±0.05

1.4±0.06

81±3

75±3

82±3

73±3

M17**

1.3±0.06

1.6±0.08

1.7±0.08

1.7±0.08

96±4

107±5

111±6

110±5

* Water vapor for 1 d at room temperature for each treatment; ** Water vapor for 1d at 378 K for each treatment.

The chemical tolerance of our modified membrane to some typical organic solutions were investigated and shown in Table S2. The results indicates that the modified membrane have the good resistances in the different chemical solutions. Four modified membranes (M19-M22) were treated by four typical organic solvents: n-hexane, diethyl ether, dimethylformamide and acetone, respectively. It was done for 1 d at 298 K, and the treatments were repeated for 3 times. After each treatment, the modified membranes were put into the vacuum oven at 473 K overnight prior to separation test. All membranes after each soaking treatment by the organic solutions and each circle still showed the similar high CO2 permeances and CO2/CH4 selectivities to the original value (after grafted). It suggests that the siloxane polymer coating on the membrane surface is resistant to the investigated organic solvents.

3.6 Comparison to the literatures The comparison of defect-patching efficiencies of our present method with those in the literatures was shown in Table S3. The decomposition of silicon and aluminum precursors by CCD5-7, CVD8 and MLD9 was reported to effectively modify the non-zeolitic pores and zeolite pores; and thus H2/CO2 selectivities of MFI and SAPO-34 membranes increased quickly. After CCD modification, SAPO-34 membrane showed only 51% increase in CO2/CH4 selectivity9 and 23% loss in CO2 permeance. Those results showed that the above coating of amorphous silica and alumina nanoparticles are more effective to increase H2/CO2 selectivities rather than 18


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CO2/CH4 selectivities. The decomposition of these silicon and aluminum precursors was always required to be performed at high temperatures and/or high vacuum, which increased the difficulty for scaling up the modification. In this work, we designed a siloxane polymer coating with alkali imidazole functional group for CO2 capture. Carbon dioxide/methane selectivity increased by one order of magnitude and CO2 permeance decreased by only 15% for a modified membrane. The CLD grafting of the siloxane polymer coating were operated under moderate temperatures and atmospheric pressure, which makes the operation feasible to scale up.

4. CONCLUSIONS A new and facile chemical liquid deposition method was developed to effectively patch the defects for CO2 separation at lower temperatures and atmospheric pressure. A siloxane polymer layer was obtained by silanization using a siloxane on membrane surface and subsequent polymerization using 1-allylimidazole on the siloxane. The second polymer coating is the crucial process for the increase of separation selectivity. Six modified membranes showed decreased CO2 permeance by only 21±5% [average CO2 permeance of 1.9×10-7 mol/(m2 s Pa)] and increased CO2/CH4 selectivity by a factor of 9±3 (average CO2/CH4 selectivity of 108) for an equimolar CO2/CH4 mixture at 298 K and 0.2 MPa feed pressure. The coating on the membrane surface is resistant to water vapor at 378 K and some typical organic solutions. This modification method is promising to seal the defects of other zeolite membranes.

ACKNOWLEDGMENTS This work was supported by National Scientific Foundation of China (NSFC, Grant nos.20906042 and 21366013) and A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). Supporting Information Available: Three tables: Table S1-S3. This information is available free of charge via the Internet at http://pubs.acs.org/.

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Defect-Patching of Zeolite Membranes by Surface Modification Using

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