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H2-Selective Carbon Membranes Prepared from Furfuryl Alcohol by Vapor-Phase Synthesis Yong-Rong Dong, Norikazu Nishiyama,* Yasuyuki Egashira, and Korekazu Ueyama DiVision of Chemical Engineering, Graduate school of Engineering Science, Osaka UniVersity, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan
Microporous carbon membranes were prepared by a vapor-phase synthesis using furfuryl alcohol (FFA). An R-alumina support was coated with a sulfonic acid solution and then exposed to an FFA vapor at 180 °C in a closed vessel. The deposited FFA polymer was carbonized under a N2 atmosphere. Under carbonization at elevated temperature (200-500 °C), micropores larger than 0.33 nm were generated. The pores shrunk at higher temperatures of 600-1000 °C. The pore size of the membrane carbonized at 800 °C is thought to be about 0.30 nm according to the gas permeation results. The FFA carbon membrane showed high separation factors for H2/CO2 (>22 900 at 150 °C), H2/N2 (>2480 at 150 °C), and H2/CO (>2510 at 150 °C) binary gas mixtures. 1. Introduction Microporous carbon membranes have been widely used in gas separation industries such as natural gas processing, landfill gas recovery, olefin/paraffin separation, and air separation. Carbon molecular sieve membranes with a pore size distribution of 0.3-0.5 nm can separate gas mixtures such H2/CO, O2/N2, and CO2/CH4 according to the molecular sieving mechanism. Many polymers such as polyimide,1-4 phenolic resins,5-7 poly(furfuryl alcohol) (FFA),8-10 and poly(2,6-dimethyl-1,4-phenylene oxide)11 have been used as a carbon precursor to prepare carbon molecular sieve membranes. The pore size distribution has been controlled by carbonization conditions and posttreatments. Recently, development of polymer electrolyte membrane fuel cells (PEMFC) has become very interesting. A fuel for the fuel cells, hydrogen-rich gas, can be produced from methanol or gasoline by reformers.12 However, carbon monoxide is formed as a byproduct at concentration levels of 1-25%, and severely poisons the Pt electrocatalyst in the fuel cell.13 In general, CO content in the product hydrogen has to be below 20 ppm in order to be used as an anode gas for the PEMFC.14 Use of compact membrane modules is becoming very attractive to remove CO in reformed gas to the order of several parts per million.15 In this study, we have prepared H2-permeable carbon membranes using an FFA vapor. The conditions of deposition and carbonization were optimized to a control pore size. Single gas permeations and mixed gas separations were performed using the carbon membranes. 2. Experimental Section 2.1. Preparation of Carbon Membranes by Vapor-Phase Synthesis. A porous R-Al2O3 (NGK Insulators, Ltd.) with an average pore size of 100 nm was used as a support. The preparation procedure of microporous carbon membranes is illustrated in Figure 1. A sulfuric acid solution was prepared from sulfuric acid (H2SO4, 1 mol/L), ethanol (EtOH), and deionized water with mass ratios of 1H2SO4:1EtOH:xH2O (x * To whom correspondence should be addressed. Tel.: +81-66850-6256. Fax: +81-6-6850-6256. E-mail: nisiyama@ cheng.es.osaka-u.ac.jp.
Figure 1. Preparation procedure for microporous carbon membranes.
) 0, 0.5, 2, and 10). Sulfuric acid acts as a catalyst for the polymerization of FFA. All the chemicals were purchased from Wako Pure Chemical Industries. The solution was stirred at 30 °C for 1 h and then deposited on the alumina support by spin coating. The H2SO4/alumina support was exposed to an FFA vapor at 180 °C for 10, 30, and 60 min by the following method. The support and a small vessel filled with FFA were separately put into a closed vessel. The closed vessel was placed in an oven at 180 °C. After cooling to room temperature, the product was dried at 90 °C for 12 h to promote polymerization. The FFA vapor-phase synthesis and drying procedure were repeated twice to get a crack-free polymer membrane. The membrane was then carbonized at 200-1000 °C for 2 h under a N2 atmosphere with a heating rate of 1 °C/min. The cooling rate to room temperature was 2 °C/min. The carbonized membrane will be called the FFA carbon membrane, hereafter. 2.2. Characterization. The structure of the membranes was observed with a scanning electron microscope (SEM; Hitachi S-2250). The chemical components (C and S) were measured with an energy-dispersive X-ray analyzer (EDX). 2.3. Gas Permeation. Permeation of pure gases (H2, CO2, CH4) and separations of equimolar gas mixtures such as H2/
10.1021/ie060910a CCC: $37.00 © 2007 American Chemical Society Published on Web 12/08/2006
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Figure 2. Effect of concentration of H2SO4 on mass of FFA polymer deposited on alumina support. The H2SO4/alumina support was treated with an FFA vapor for 30 min.
Figure 3. Time on stream of mass of FFA polymer deposited on alumina support. The alumina support was pretreated with a H2SO4 solution (0.4 mol/L).
CO2, H2/CO, H2/N2, H2/He, and H2/Ar were performed using the FFA carbon membranes at 23-150 °C. In both the single and mixed gas permeation tests, the total pressure on the feed and permeate sides was kept constant at atmospheric pressure (0.1 MPa). Argon was used as the sweep gas with a flow rate of 20 cm3 min-1 on the permeate side. Only in the case of the separation of the H2/Ar mixture was He used as the sweep gas. The feed and the permeate streams were analyzed with a quadrupole mass spectrometer (GSD301, Preiffer Vacuum Omnistar). The carbon layer on the support was always facing the permeate side. We always kept the membranes under dry air at 150 °C before permeation tests. Steady-state permeation fluxes were obtained after 6-24 h. 3. Results and Discussion 3.1. Formation of FFA Polymer Layers. In this study, an FFA polymer was deposited on the alumina support coated with H2SO4. On the other hand, FFA polymer was not deposited on a bare alumina support, indicating that FFA could be deposited and polymerized in the presence of H2SO4, which acts as a catalyst for the polymerization. When other acids such as HCl and HNO3 were used, FFA polymer was not formed on the support. A nonvolatile acid, H2SO4, is an appropriate catalyst in this vapor-phase synthesis. We coated the alumina support with diluted H2SO4 solutions of different concentrations to control the deposition rate of the FFA polymer and film thickness. Figure 2 shows the mass of the FFA polymer deposited for 30 min on the alumina support per unit membrane area as a function of the concentration of H2SO4. The deposition rate of the FFA polymer increased with increasing concentration of H2SO4. Apparently, the deposition of the FFA polymer was strongly promoted by the presence of H2SO4. When the mass of FFA polymer was 0.27 kg/m2, the thickness of the FFA polymer on the support was 2.5 µm. To obtain a high gas flux through the membrane, a layer thickness should be as thin as possible. However, it was difficult to prepare FFA polymer layers with a thickness less than 2 µm. For further experiments, the concentration of the H2SO4 solution was 0.4 mol/L. A vapor-phase synthesis was carried out at 180 °C for 10, 30, and 60 min. Figure 3 shows the effect of synthesis time on the mass of the FFA polymer per unit membrane area. The mass of the FFA polymer proportionally increased with increasing synthesis time. The amount of H2SO4 was sufficiently present in the membrane within 1 h. Figure 4 shows the SEM images of the cross section of the FFA polymer membranes deposited
Figure 4. SEM images of cross section of FFA polymer membranes deposited for 10, 30, and 60 min.
for 10, 30, and 60 min. Uniform layers were not observed on the support for the membrane synthesized for 10 min. After 30 min, however, the membrane has a uniform top layer with a thickness of about 2.5 µm. After 60 min, the membrane has a top layer with a thickness larger than 5-10 µm (Figure 4d). As shown in Figure 4e, the membrane partly has a lumpy surface. We consider that FFA molecules were deposited and polymerized around a H2SO4 catalyst and small particles were formed at the beginning of the vapor-phase synthesis. The particles grew with increasing synthesis time, resulting in the formation of a top layer on the support after 30 min. After a prolonged synthesis time, H2SO4 must have been inhomogeneously dispersed over the layer. The layer with a high concentration of H2SO4 would grow fast. Based on these results, the vapor synthesis time was set to 30 min to prepare carbon membranes. 3.2. Formation of Carbon Membranes. The molar ratios of sulfur to carbon in the membranes measured by EDX are plotted in Figure 5 as a function of carbonization temperature
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Figure 5. Molar ratios of S/C in the FFA polymer during carbonization.
Figure 7. Effect of carbonization temperature on permeances of H2 and CO2 and permselectivity of H2/CO2. The gas permeation was performed at 23 °C. Figure 6. Mass remaining of FFA polymer during carbonization.
(200-1000 °C). The mass of the FFA polymer/carbon at different carbonization temperatures (200-1000 °C) is plotted in Figure 6. The molar ratio of S to C decreased from 0.024 to 0.007 in the temperature range of 300-400 °C, indicating that H2SO4 decomposed at 300-400 °C. The weight loss was around 60% at 400 °C. By a gasification of FFA polymer, micropores may be generated in this temperature region. A detailed pore formation mechanism will be discussed in the next session, based on gas permeation results. The gasification continued at elevated temperature up to 1000 °C, and the final weight loss was about 70%. 3.3. Effect of Carbonization Temperature on Gas Permeances. Figure 7 shows the permeances of H2 and CO2 and the permselectivity of H2/CO2 through the FFA carbon membranes carbonized at different temperatures. All the membranes, hereafter, were prepared by a repeated vapor-phase synthesis before carbonization as shown in Figure 1. The detection limit of the permeance is about 10-12 mol m-2 s-1 Pa-1 under the permeation conditions in this study. No permeations of H2 and CO2 through the FFA polymer membrane were observed, indicating that the FFA polymer has no micropores before carbonization. The permeances of both H2 and CO2 through the membrane were increased with increasing carbonization temperature in the temperature range of 200-500 °C and exhibited maxima at 500 °C. However, the resulting membranes showed no permselectivity of H2/CO2. We consider that micropores appeared at 200-300 °C and the micropore volume increased at elevating temperature to 500 °C due to the gasification of the FFA polymer. This result is consistent with the results of the mass measurements shown in Figure 6. Considering the
molecular sizes of H2 (0.28 nm) and CO2 (0.33 nm), the pore size of the generated micropores must be larger than 0.33 nm. Above 600 °C, the permeances of both H2 and CO2 through the FFA carbon membranes were decreased. The permeance of CO2 through the FFA carbon membrane was significantly reduced at 800 °C, although the permeance of H2 was still higher than 10-8 mol s-1 m-2 Pa-1 at 600-800 °C. Finally, no permeation of H2 was observed at 1000 °C. It has been reported that gas permeances decreased with increasing pyrolysis temperature although higher permselectivities were achieved,1,6 suggesting that carbonization at higher temperatures causes pore shrinkage. The permeances of CO2 through the FFA carbon membranes carbonized at 800 °C ranged from 10-12 (detection limit) to 8 × 10-10 mol s-1 m-2 Pa-1. Furthermore, after the second coating, CO2 did not permeate all the FFA carbon membranes carbonized at 800 °C. Detailed results will be shown in the next section. In this study, a high permselectivity of H2/CO2 was achieved at 800 °C, suggesting that the pore size reduced to about 0.30 nm. The carbonization at 800 °C seems to be an appropriate condition to prepare carbon membranes derived from the FFA polymer. 3.4. Single Gas Permeation. Next, the coating process shown in Figure 1 was repeated twice to prepare a crack-free FFA carbon membrane reproducibly. As a result, the vapor deposition process was performed four times to obtain a final FFA carbon membrane. Hereafter, the single and mixed gas permeation results for the membrane prepared by this method will be shown. Figure 8 shows the SEM image of the cross section of the final FFA carbon membrane carbonized at 800 °C. The thickness of the carbon top layer on the support is about 3 µm. Table 1 lists the single gas permeation results through the FFA carbon membrane carbonized at 800 °C. The permeations
Ind. Eng. Chem. Res., Vol. 46, No. 12, 2007 4043 Table 1. Permeation and Separation Properties of FFA Carbon Membrane for H2/CO2 (50/50) Gas Mixture and for Single Gases single gases temp [°C] 23 50 100 150 a
flux of H2
[10-5
mol
284 363 473 519
m-2 s-1]
mixed gases a
permselectivity H2/CO2 >28 400 >36 300 >47 300 >51 900
flux of H2
[10-5
mol
m-2 s-1]
59.3 116 197 229
separation factor H2/CO2a >5 930 >11 600 >19 700 >22 900
Fluxes of CO2 for both the single and mixed gas permeations are below the detection limit (0.01 × 10-5 mol m-2 s-1).
Figure 8. SEM image of cross section of the FFA carbon membrane. The carbonization temperature was 800 °C.
Figure 9. Temperature dependence of gas permeances through FFA carbon membrane in single gas permeation experiments.
of CO2 and CH4 through the FFA carbon membrane were not observed. According to the kinetic diameters of CO2 and H2, the pore size of the membrane is thought to be about 0.30 nm. The permselectivity of H2/CO2 through the FFA carbon membrane at 23 °C exceeded 28 400. The value of the H2 permeance at 23 °C was smaller than that shown in Figure 7. This membrane was prepared by a repeated coating. The membrane must be thicker and more compact than the membrane in Figure 7. Temperature dependence of the permeances of H2 and CO2 through the FFA carbon membrane is shown in Figure 9. The H2 permeance increased with increasing temperature, indicating that the gas transport was governed by an activated process. The apparent activation energy was 5.1 kJ/mol for the permeation of H2. The CO2 permeation was not observed even at 150 °C. We consider that the FFA carbon membrane has a very narrow pore size distribution with a pore size less than the kinetic diameter of CO2 molecule (0.33 nm). 3.5. Separation of H2/CO2, H2/CO, H2/N2, H2/Ar, and H2/ He Mixtures. H2/CO2 binary mixed gas separation results at
Figure 10. Fluxes of gases through FFA carbon membrane as a function of partial pressure of H2 in H2/CO2 binary gas permeation. The total pressure of the feed side was 101 kPa. Detection limit is 10-7 mol m-2 s-1.
Figure 11. Fluxes of gases through FFA carbon membrane as a function of partial pressure of H2 in H2/Ar binary gas permeation. The total pressure of the feed side was 101 kPa. Detection limit is 10-7 mol m-2 s-1.
different temperatures for the FFA carbon membrane are summarized in Table 1, together with the single gas permeation results. The separation factor of H2/CO2 is higher than 22 900 at 150 °C. The flux of CO2 was not detected even at 150 °C. These high separation factors imply that the FFA carbon membrane has very uniform pores. The flux of H2 in the binary mixture is less than half that in single gas measurements. This result indicates that the presence of CO2 partly block the permeation of H2. Figure 10 shows the fluxes of gases through the FFA carbon membrane in the H2/CO2 binary mixture permeation. The flux was plotted as a function of partial pressure of H2. The flux of H2 decreased with decreasing the H2 partial pressure from 100 to 30 kPa and became 0 at a partial pressure below 30 kPa. We consider that a pore blocking of micropores of the FFA carbon membrane by CO2 molecules adsorbed on the membrane top surface resulted in a large reduction of the H2 flux. Figures 11 and 12 show fluxes of gases through the FFA carbon membrane in H2/Ar and H2/He binary mixture permeations, respectively. The flux was plotted as a function of partial pressure of H2. In the H2/Ar binary mixture permeation, the
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Table 2. Permeation and Separation Properties of FFA Carbon Membrane for H2/CO (50/50) and H2/N2 (50/50) Gas Mixtures H2/CO temp [°C] 23 50 100 150 a
flux of H2
[10-5
mol
135 149 201 251
m-2 s-1]
H2/N2 separation factor H2
/COa
flux of H2
[10-5
>1350 >1490 >2010 >2510
mol
105 169 198 248
m-2 s-1]
separation factor H2/N2a >1050 >1690 >1980 >2480
Fluxes of CO and N2 are below the detection limit (0.01 × 10-5 mol m-2 s-1).
carbonized at 800 °C has uniform pores with a size of 0.30 nm. The FFA carbon membrane showed very high separation factors for H2/CO2 (>22 900), H2/N2 (>2480), and H2/CO (>2510) at 150 °C. Acknowledgment We thank the GHAS laboratory at Osaka University for the SEM measurements. Literature Cited
Figure 12. Fluxes of gases through FFA carbon membrane as a function of partial pressure of H2 in H2/He binary gas permeation. The total pressure of the feed side was 101 kPa. Detection limit is 10-7 mol m-2 s-1.
flux of H2 decreases with decreasing the H2 partial pressure from 100 to 30 kPa. The permeation of Ar was not observed because the molecular size of Ar (0.34 nm) is larger than the pore size of the FFA carbon membrane (0.30 nm). The permeation results of H2/Ar showed a trend similar to the results of the H2/CO2 binary mixture permeation. A pore blocking of micropores of the FFA carbon membrane by Ar molecules on the membrane top surface seems to reduce the permeation of H2. On the other hand, in the H2/He binary mixture permeation, the flux of H2 was not affected by the presence of He. Instead, the permeation of He was depressed by the presence of H2. The single gas permeance of H2 was larger than that of He even though the molecular size of He (0.26 nm) is smaller than that of H2 (0.28 nm). These results indicate that the FFA carbon can adsorb H2 molecules and the permeation of H2 is affected by the adsorption of H2 in the micropores. The activation energy for the permeation calculated from Figure 9 (5.1 kJ/mol) seems to be smaller than the activation energy for diffusion of H2 through the membrane due to a heat of adsorption. The binary mixed gas fluxes and separation factors of H2/ CO and H2/N2 at different temperatures through the FFA carbon membrane are summarized in Table 2. The FFA carbon membrane shows high separation factors for H2/CO (>2510) and H2/N2 (>2480) at 150 °C. No fluxes of CO and N2 were detected even at 150 °C. The FFA carbon membrane is expected to be used for hydrogen purification. Further studies on the H2 separation at high temperatures (200-600 °C) and long-term stability of the FFA carbon membranes under steam will be reported in the future. 4. Conclusions H2-selective carbon membranes were prepared by a deposition of an FFA vapor followed by a carbonization of the FFA polymer. The deposited FFA polymer was gastight. It seems that pores larger than 0.33 nm began to be generated under the carbonization above 200 °C and the pore size decreased at high temperatures of 600-1000 °C. The FFA carbon membrane
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ReceiVed for reView July 13, 2006 ReVised manuscript receiVed October 18, 2006 Accepted October 20, 2006 IE060910A
