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Synthesis and Permeation Properties of Silicon-Carbon-Based Inorganic Membrane for Gas Separation Lan-Luen Lee and Dah-Shyang Tsai* Department of Chemical Engineering, National Taiwan University of Science and Technology, 43 Keelung Road, Section 4, Taipei, Taiwan 106
The pyrolysis temperature is a critical factor in the synthesis of the silicon-carbon membranes which are derived from poly(dimethylsilane). These membranes show respectable molecular sieving capability when the pyrolysis temperature is 873 K or lower. Permeation of He, H2, N2, CH4, and i-C4H10 through these membranes is dominated by activated diffusion. Nonetheless, a pyrolysis temperature higher than 873 K sinters the micropores and reduces the integrity of the membrane. Consequently, both permeation and permselectivity decline. The 873 K pyrolyzed membrane exhibits H2 permeance of 2.7 × 10-9 mol Pa-1 s-1 m-2, H2/N2 selectivity of around 20, and H2/i-C4H10 selectivity of around 80, at permeation temperature 473 K. Introduction The potential advantages of high-temperature gas separations have stimulated considerable interests in the inorganic porous membrane. To achieve an applicable selectivity, the porous membranes need to have molecular sieving capabilities, which are rendered by micropores with dimensions near those of permeating gas molecules. It has been demonstrated in carbon membranes that such microporosity can be created by channeling molecular debris through a thermosetting polymer matrix during controlled pyrolysis of synthetic and natural hydrocarbon precursors.1,2 Carbon molecular sieve membranes can be synthesized via pyrolysis of polyimide,3-8 poly(furfuryl alcohol),9,10 and aromatic resin.11 Early developments in carbon membranes have found that the pyrolysis temperature has a strong influence on the membrane properties. Also the porous structure of the separation layer can be adjusted by oxidation to open, or sintering to seal, those microchannels.12 Although the carbon molecular sieve membrane has been studied for nearly 20 years and carbon is perhaps the most thermochemically stable material, its susceptibility to oxidation is an obvious shortcoming. Combining with silicon, which is the neighbor of carbon in the fourth column of the periodic table, seems to be a natural choice for the next step in expanding the membrane technology. Following the same methodology in preparing carbon membranes, combination of silicon and carbon in membrane synthesis can be carried out by carefully pyrolyzing a preceramic polymer that contains both elements of Si and C in its polymeric chain. A silicon-carbonbased membrane was prepared through pyrolysis of poly(silastyrene), which was cross-linked under 254 nm UV light by Shelekhin et al.13 The 743 K pyrolyzed membrane was supported on a porous Vycor glass and exhibited a considerably larger permselectivity than the unpyrolyzed membrane. The micropore volumes in the pyrolyzed residues, accessible to gas molecules of different sizes, were also measured.14 Another precursor, * To whom the correspondence should be addressed. Phone: 886-2-2737-6618. Fax: 886-2-2737-6644. E-mail: tsai@ ch.ntust.edu.tw.
poly(carbosilane), was used in preparing the Si-C-O membrane which was supported on an R-alumina porous tube by Morooka et al.15-17 The permeances of poly(carbosilane)-derived membranes seemed to be low; an addition of 1-5% polystyrene enhanced the membrane permeances because the polystyrene decomposition generated channels of micron sizes. The Si-C-O membrane exhibited H2 permeance of 4 × 10-8 mol Pa-1 s-1 m-2 and H2/N2 permselectivity of 20 at permeation temperature 773 K. The membrane synthesis via polysilazane, which contained Si-N bonds in its backbone, was studied on its pore structure evolution during pyrolysis.18 Unfortunately, no permeation properties were reported. This paper reports the synthesis and the permeation properties of an asymmetric membrane derived from poly(dimethylsilane) (PMS). The preceramic solution of PMS was coated on a porous silicon carbide support, thermally treated, cured, and finally pyrolyzed under various temperatures. The pyrolysis temperature is identified as a crucial processing factor. The physical and chemical changes in PMS during the thermal treatment and the pyrolysis are studied. The permeances and selectivities of membranes pyrolyzed at different temperatures are discussed. Experimental Section Silicon carbide porous supports were used to provide the mechanical strength of the membrane; their preparation procedures and mechanical strengths were reported elsewhere.19 A calculated amount of PMS (melting point 648 °C; PSS-1M01, Gelest Inc., PA) was dissolved in p-xylene to prepare a 20 wt % solution. An asymmetric porous SiC tube was dip-coated twice, using the PMS solution, to ensure a uniform coating of the inner wall. After overnight drying, the membrane was placed in a furnace to proceed thermolytic reaction at 733 K for 14 h under 1 atm of argon. The membrane was then oxygen-cured in air at 473 K for 1 h. The cured PMS was converted to a themosetting resin. Finally the membrane was pyrolyzed under argon at 573, 723, 873, 1023, and 1223 K. PMS lost 5-12% of its own weight in the thermolytic reaction step and the pyrolysis step and gained 0.14% of its weight in the curing step.20
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Figure 2. Schematic diagram showing the molecular debris of hydrogen and hydrocarbon channels through the preceramic SiC-Si skeleton.
Figure 1. Thermal gravimetric analysis and differential thermal analysis curves of PMS. Thermolytic reaction of synthesis occurs in the thermal event around 700 K.
Hydrogen, helium, nitrogen, methane, and isobutane (purity > 99.9%) were used in the permeance measurement. Permeances of the tubular membrane were measured in a setup of a double-pipe configuration, in which the tubular membrane was connected to two quartz tubes and served as the inner tube. Epoxy was used in sealing the connections between the membrane tube and the quartz tube. The permeation setup was evacuated to 5 × 10-3 Torr before each measurement. The pure permeating gas flowed in the tube side, while the sweeping gas flowed in the shell side. Argon was the sweeping gas except in the permeance measurement of isobutane, in which helium was the sweeping gas. The total pressures of both the tube and shell sides were maintained at 1 atm. After the permeation rate reached a steady state at the preset temperature, the concentration of the permeate was measured by thermal conductivity detection-gas chromatography (molecular sieve 5 A and OV-10 packings) and the permeate flow rate was measured by two bubble flowmeters. Usually, the concentration and the flow rate were measured after 2 h of permeation, to ensure a steady-state permeation. The specific surface area of pyrolyzed residues was measured by the Brunauer-Emmett-Teller (BET) method (ASAP2000, Micromeritics). These specimens underwent the same preparation steps and were pyrolyzed under various temperatures. Part of the residues was mixed with KBr and subject to FTIR analysis (FTS40, Digilab). Another part of the residues was subject to elemental analysis of carbon and hydrogen (CHN2400, Perkin-Elmer). Results and Discussion Physical and Chemical Changes in Synthesis. In three successive preparation steps of 733 K thermolytic heat treatment, 473 K oxygen curing, and pyrolysis, the molecular structure of PMS has undergone drastic changes. Thermal analysis of PMS is illustrated in Figure 1. There are two major sigmoids in the weight loss curve: a low-temperature loss occurs around 700 K, and a high-temperature loss occurs between 773 and 873 K. The differential thermal analysis curve shows a small exothermic peak in the low-temperature event, yet a much larger and broader exothermic peak in the high-temperature event. According to Figure 1, the 733 K thermolytic treatment of the membrane is associated with vaporization of small molecules from PMS and the event is exothermic. Another important consequence of 733 K thermolytic reaction is the entanglement of molecular chains in
Figure 3. Infrared spectra of PMS and PMS after pyrolysis.
PMS. At 733 K, some of the methylene (CH3) groups insert themselves into the backbone of PMS. The insertion reaction not only turns a portion of the SiSi-Si chain into a Si-C-Si-Si chain but also establishes considerable networking (cross-linking) in PMS. An increase in the degree of cross-linking is evidenced in the solubility change of PMS in organic solvents. PMS is highly soluble in p-xylene before thermolytic reaction. After 733 K thermolytic reaction, only 24.2 wt % of dried PMS can still be dissolved in p-xylene; the remaining 75.8 wt % is so cross-linked that it is no longer soluble in p-xylene. In 473 K curing, a small amount of oxygen is incorporated in the polymer, and PMS gains weight by 0.14 wt %. After oxygen curing, PMS is further cross-linked so that it becomes totally insoluble in p-xylene. The cured PMS monolith appears hard and brittle. During pyrolysis, fragments of molecular size channel through the cross-linked skeleton and generate micropores in the PMS matrix, shown in Figure 2 schematically. The persistence of these micropores relies on the rigidity of the cured skeleton, which sustains the porous structure under high temperatures. Infrared spectra, illustrated in Figure 3, provide information about the functional groups cleaved from PMS. The IR spectrum of an as-received PMS precursor presents typical absorption bands due to C-H stretching (2900-2950 cm-1), Si-H stretching (2100 cm-1), CH2 deformation of Si-CH3 (1400 cm-1), Si-CH3 de-
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Table 1. Combustible Carbon and Hydrogen Contents in Pyrolyzed PMS, Measured in Elemental Analysis pyrolysis temp (K)
carbon (wt %)
hydrogen (wt %)
573 723 873 1023 1173
20.4 23.7 33.4 35.1 37.7
4.8 4.6 4.0 2.2 0.38
formation (1250 cm-1), and Si-CH2-Si (CH2 wagging at 1020 cm-1). IR spectra of cured PMS and 573 and 723 K pyrolyzed PMS show significant reductions in absorption bands at 2900-2950 and 1400 cm-1 and moderate reductions in the absorption band at 1250 cm-1. Comparison of these four IR spectra indicates that a major portion of hydrogen and a certain amount of methyl groups detach from PMS in the thermolytic reaction and curing. Gas chromatographic analysis confirms the high hydrogen contents of the evolved gases in these two heat treatments. When the pyrolysis temperature is raised to 873 K, more methyl groups disconnect from the skeleton and result in an obvious reduction in the absorption band at 1250 cm-1. When the pyrolysis temperature is further raised over 1023 K, nearly all attached functional groups disappear; only the skeleton of Si-C-Si-Si remains.21 The residues, after 1223 K pyrolysis, remain X-ray amorphous. Table 1 lists the combustible contents of carbon and hydrogen in pyrolyzed PMS, measured in elemental analysis. When the pyrolysis temperature increases from 573 to 1173 K, the combustible carbon content increases from 20.4 to 37.7 wt % and the combustible hydrogen content decreases from 4.8 to 0.38 wt %. The theoretical carbon content in PMS is 41.4 wt %, and the hydrogen content is 10.3 wt %. Both contents of combustible carbon and hydrogen in pyrolyzed PMS are lower than the theoretical values of PMS. A significant difference between the hydrogen contents of PMS and 573 K pyrolyzed PMS implies the relative ease of hydrogen detachment from PMS in the thermolytic reaction and curing. The decreasing hydrogen content in pyrolyzed PMS indicates the slow release of the remaining hydrogen with an increase of the pyrolysis temperature. Unlike hydrogen, carbon tends to be trapped in the cured PMS that was further oxidized in the elemental analysis. Pyrolysis of the cured PMS under high temperatures exposes more carbon for oxidation; therefore, the combustible carbon content increases with the pyrolysis temperature. Another indication of increasing free carbon is the increasing darkness in the cured PMS appearance with an increase of the pyrolysis temperature. Nitrogen adsorption results of 573, 723, and 873 K pyrolyzed PMS exhibit characteristics of Langmuir (or type 1) isotherms, which imply that they are microporous materials. The isotherm of PMS pyrolyzed at 1223 K shows the adsorption characteristics of type IV. The 1223 K isotherm undergoes a drastic increase at P/P0 > 0.6 and a large hysteresis, which indicate that a number of ink-bottle mesopores are generated at 1223 K. BET surface areas of PMS pyrolyzed under various temperatures are plotted in Figure 4. The BET surface area increases with the pyrolysis temperature, reaches a maximum value of 353 m2/g at 873 K, and then decreases rapidly. The cumulative pore volumes versus pore diameters are shown in Figure 5. Although those pores of diameters of less than 1.7 nm are not measured,
Figure 4. BET surface area of pyrolyzed PMS versus pyrolysis temperature.
Figure 5. Pore volume of pyrolyzed PMS versus pore diameter.
Figure 5 testifies that the pyrolysis operation both opens up and sinters the mesopores of PMS. The volatilization of small molecules increases the mesopore volume when the pyrolysis temperature increases from 573 to 873 K. A further increase in the pyrolyis temperature leads to a drastic reduction of the mesopore volume, which is caused by pore closure. Permeation Properties. The permeation properties of membranes are controlled by the pyrolyzed PMS layers, which offer the selectivity at the molecular level. Essentially, the consecutive steps of 733 K thermolytic reaction, 473 K curing, and pyrolysis all affect the permeation properties; for instance, the membrane after 733 K thermolysis is more permeable than that after 473 K curing. The pyrolysis temperature, nonetheless, is the most important factor in the synthesis. Parts a-c of Figure 6 are the permeances of the membranes, pyrolyzed under 723, 873, and 1023 K, versus the reciprocal permeation temperature. The permeances of these three membranes increase with the permeation temperature. Although membrane defects, such as pinholes and cracks, could exist, it is not wrong to say that activated gas transport plays a dominant role if the pyrolysis temperature does not exceed 1023 K. When the pyrolysis temperature is raised to 1223 K, the behavior of increasing permeance with respect to permeation temperature switches to a different trend, that is, a decreasing permeance versus increasing temperature, illustrated in Figure 6d. Because of the different temperature dependences, the 300 K permeances of the 1223 K membrane are higher than those of the 1023 K membrane, yet the 473 K permeances of the 1223 K membrane are lower than those of the 1023 K membrane. The decreasing permeance could result from certain cracks or pinholes that are caused by pyrolysis under 1223 K. Although such defects exist, they are not extensive in the microstructure of the top layer, judging from very low permeances of the 1223 K membrane.
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Figure 6. Permeance of the (a) 723 K, (b) 873 K, (c) 1023 K, and 1223 K pyrolyzed membrane versus reciprocal permeation temperature.
A comparison of hydrogen permeances of membranes pyrolyzed under different temperatures reveals the detrimental effect of pyrolysis. For instance, the 473 K H2 permeance of the membrane pyrolyzed under 723 K is 1.65 × 10-8 mol Pa-1 s-1 m-2. It decreases by 2 orders of magnitude, after 1023 K pyrolysis, to 1.64 × 10-10 mol Pa-1 s-1 m-2. The hydrogen permeance increases slightly after 1223 K pyrolysis, its order of magnitude remains at 10-10, yet its temperature dependence changes. The variation of the H2 permeance with pyrolysis temperature indicates the existence of a large amount of micropores in low-temperature pyrolyzed PMS. These micropores are responsible for the behavior of activated diffusion of the membrane, which is the dominant mechanism in the permeation properties of Figure 6a,b. Evidently, the amount of micropores decreases with an increase of the pyrolysis temperature and the membrane permeance decreases. While the membrane defects also develop with an increase of the pyrolysis temperature, they take over the permeation mechanism when the pyrolysis temperature is 1223 K. The selectivity of the membrane is denoted by the permeance ratio of two different gases. The plot of permeances versus kinetic diameters (Figure 7) indicates the significance of activated diffusion on the selectivity of the membrane. Large differences between the H2 permeance and the i-C4H10 permeance demonstrate the membrane capability in differentiating gases according to their molecular sizes. The membranes pyrolyzed at temperatures lower than 873 K exhibit permeance differences of 2-3 orders of magnitude between H2 and i-C4H10. The membrane pyrolyzed at 1023 K or above is much less permselective. Permeance and selectivity of the 873 K membrane are similar to
Figure 7. Permeances of various membranes at 473 K permeation temperature versus kinetic diameters of gases.
those of zeolite and carbon membranes and inferior to those of CVD-modified silica membranes.22,23 Summary The porous structure and the integrity of the PMSderived layer, formed in the synthesis of an inorganic membrane, are extensively altered in its thermal treatments. The understanding of physical and chemical changes of PMS is crucial in manipulating the mem-
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brane properties. The molecular sieving capability is controlled by microporosity which is created when the molecular debris penetrates through a preceramic skeleton primarily consisting of silicon and carbon. The cross-linked skeleton is forged in the thermolytic reaction and the curing steps. The channeling of evolved gas that creates micropores mainly occurs in the thermolytic reaction and pyrolysis. With increase of the pyrolysis temperature, the micropores in PMS begin to seal and the membrane defects develop. Therefore, the permeance and the permselectivity of the membrane decrease with the pyrolysis temperature. If the pyrolysis temperature does not exceed 873 K, the PMS-modified membrane demonstrates substantial permeances and applicable H2/N2, H2/CH4, and H2/i-C4H10 selectivities. Acknowledgment The authors are grateful to National Science Council of Taiwan for financial support of this work through Grant NSC88-2214-E011-019. Literature Cited (1) Foley, H. C. Carbogenic Molecular Sieve: Synthesis, Properties and Applications. Microporous Mater. 1995, 4, 407. (2) Koresh, J. E.; Soffer, A. Molecular Sieve Carbon Permselective Membrane. Part I. Presentation of a New Device for Gas Mixture Separation. Sep. Sci. Technol. 1983, 18, 723. (3) Geiszler, V. C.; Koros, W. J. Effects of Polyimide Pyrolysis Conditions on Carbon Molecular Sieve Membrane Properties. Ind. Eng. Chem. Res. 1996, 35, 2999. (4) Kusakabe, K.; Yamamoto, M.; Morooka, S. Gas Permeation and Micropore Structure of Carbon Molecular Sieving Membranes Modified by Oxidation. J. Membr. Sci. 1998, 149, 59. (5) Petersen, J.; Matsuda, M.; Haraya, K. Capillary Carbon Molecular Sieve Derived from Kapton for High-Temperature Gas Separation. J. Membr. Sci. 1997, 131, 85. (6) Kusuki, Y.; Shimazaki, H.; Tanihara, N.; Nakanishi, S.; Yoshinaga, T. Gas Permeation Properties and Characterization of Asymmetric Carbon Membranes Prepared by Pyrolyzing Asymmetric Polyimide Hollow Fiber Membrane. J. Membr. Sci. 1997, 134, 245. (7) Yamamoto, M.; Kusakabe, K.; Hayashi, J.; Morooka, S. Carbon Molecular Sieve Membrane formed by Oxidative Carbonization of a Copolyimide Film Coated on a Porous Support Tube. J. Membr. Sci. 1997, 133, 195. (8) Fuertes, A. B.; Centeno, T. A. Carbon Molecular Sieve Membranes from Polyetherimide. Microporous Mesoporous Mater. 1998, 26, 23.
(9) Acharya, M.; Raich, B. A.; Foley, H. C.; Harold, M. P.; Lerou, J. J. Metal-Supported Carbogenic Molecular Sieve Membranes: Synthesis and Applications. Ind. Eng. Chem. Res. 1997, 36, 2924. (10) Chen, Y. D.; Yang, R. T. Preparation of Carbon Molecular Sieve Membrane and Diffusion of Binary Mixtures in the Membrane. Ind. Eng. Chem. Res. 1994, 33, 3146. (11) Kusakabe, K.; Gohgi, S.; Morooka, S. Carbon Molecular Sieving Membranes Derived from Condensed Polynuclear Aromatic Resins for Gas Separations. Ind. Eng. Chem. Res. 1998, 37, 4262. (12) Koresh, J.; Soffer, A. Study of Molecular Sieve Carbon. J. Chem. Soc., Faraday Trans. 1 1980, 76, 2457. (13) Shelekhin, A. B.; Grosgogeat, E. J.; Hwang, S. T. Gas Separation Properties of a New Polymer/Inorganic Composite Membrane. J. Membr. Sci. 1991, 66, 129. (14) Grosgogeat, E. J.; Fried, J. R.; Jenkins, R. G.; Hwang, S. T. A Method for the Determination of the Pore Size Distribution of Molecular Sieve Materials and Its Application to the Characterization of Partially Pyrolyzed Polysilastyrene/Porous Glass Composite Membranes. J. Membr. Sci. 1991, 57, 237. (15) Li, Z.; Kusakabe, K.; Morooka, S. Pore Structure and Permeance of Amorphous Si-C-O Membranes with High Durability at Elevated Temperature. Sep. Sci. Technol. 1997, 32, 1233. (16) Li, Z.; Kusakabe, K.; Morooka, S. Preparation of Thermostable Amorphous Si-C-O Membrane and Its Application to Gas Separation at Elevated Temperature. J. Membr. Sci. 1996, 118, 159. (17) Kusakabe, K.; Li, Z. Y.; Maeda, H.; Morooka, S. Preparation of Supported Composite Membrane by Pyrolysis of Polycarbosilane for Gas Separation at High Temperature. J. Membr. Sci. 1995, 103, 175. (18) Tsubaki, J.; Mori, H.; Ayama, K.; Hotta, T.; Naito, M. Characterized Microstructure of Porous Si3N4 Compacts Prepared Using the Pyrolysis of Polysilazane. J. Membr. Sci. 1997, 129, 1. (19) Lin, P. K.; Tsai, D. S. Preparation and Analysis of a Silicon Carbide Composite Membrane. J. Am. Ceram. Soc. 1997, 80, 365. (20) Lee, L. L.; Tsai, D. S. A Hydrogen-Permselective Silicon Oxycarbide Membrane Derived from Polydimethylsilane. J. Am. Ceram. Soc. 1999, 82, 2796. (21) Hasegawa, Y.; Iimura, M.; Yajima, S. Synthesis of Continuous Silicon Carbide Fibre, Part 2. J. Mater. Sci. 1980, 15, 720. (22) Morooka, S.; Kusakabe, K. Microporous Inorganic Membranes for Gas Separation. MRS Bull. 1999, 24, 25. (23) Vroon, Z. A. E. P.; Keizer, K.; Burggraaf, A. J.; Verweij, H. Preparation and Characterization of Thin Zeolite MFI Membranes on Porous Supports. J. Membr. Sci. 1998, 144, 65.
Received for review January 3, 2000 Revised manuscript received April 26, 2000 Accepted October 20, 2000 IE000009+
