Research Note pubs.acs.org/IECR
Long-Term Stable H2 Production from Methylcyclohexane Using a Membrane Reactor with a Dimethoxydiphenylsilane-Derived Silica Membrane Prepared via Chemical Vapor Deposition Kazuki Akamatsu,* Toshiki Tago, Masahiro Seshimo,† and Shin-ichi Nakao Department of Environmental and Energy Chemistry, Faculty of Engineering, Kogakuin University, 2665-1 Nakano-machi, Hachioji-shi, Tokyo 1920015, Japan ABSTRACT: Continuous and stable operation of a membrane reactor for 1054 h to dehydrogenate methylcyclohexane for the purpose of producing high-purity H2, using a dimethoxydiphenylsilane (DMDPS)-derived silica membrane, is successfully demonstrated. The silica membrane used was prepared via a chemical vapor deposition method, using DMDPS as a precursor, and Pt/Al2O3 catalysts were employed and loaded inside the tubular membrane. During the 1054 h of continuous operation, an equilibrium shift was stably demonstrated, because of the stable extraction of the produced H2 from the reaction side to the permeate side, and, accordingly, the purity of H2 in the permeate gas was stably high. Although the DMDPS-derived membrane showed a slight decrease in performance after 1054 h of operation, followed by regeneration under an H2 atmosphere, this first successful demonstration of the long-term stable operation of a membrane reactor using silica membranes is of significance to its practical and industrial use.
1. INTRODUCTION With widespread attention to hydrogen energy, the dehydrogenation of organic chemical hydrides to obtain high-purity H2 has been extensively studied, because they are regarded as feasible H2 carriers. Among the organic chemical hydrides, the methylcyclohexane (MCH)−toluene (TOL) system is the most promising H2 carrier,1,2 because MCH and TOL are liquids at ambient temperature and pressure and have sufficiently low melting points that they will not freeze in cold regions. In addition, the equilibrium conversion of MCH is higher than for other hydride systems such as cyclohexane− benzene. From the viewpoint of developing a practical system, two possible methods of obtaining high-purity H2 from MCH are considered. One method consists of an MCH dehydrogenation reactor placed in series with a unit to purify H2 from unreacted MCH and TOL. In some cases, precise control of pressure and temperature is required to connect these two unit operations. The second method is to use a membrane reactor or catalytic membrane, which is quite simple and enables the dehydrogenation reaction and H2 purification simultaneously, to produce high-purity H2 in a single step. When H2-generating reactions that are limited by thermodynamic equilibrium are conducted using membrane reactors or catalytic membranes with H2-selective membranes, equilibrium shift to obtain higher conversion is easily achieved by extracting only the produced H2 through the H2-selective membranes. This also enables high-purity H2 to be obtained easily, without additional post-treatments. In fact, there have been many reports on the development of membrane reactors and catalytic membranes using palladium and its alloyed membranes aimed at steam reforming of hydrocarbons, such as methane.3−5 Notably, some researchers have already achieved stable and continuous operations for more than one month.6,7 There are also some papers on the development of the membrane reactors or catalytic membranes for dehydrogenating organic © 2015 American Chemical Society
chemical hydrides using not only palladium and its alloyed membranes,8−10 but also porous Vycor membranes,11 carbon membranes,12 and organosilica membranes.13,14 These studies successfully demonstrated the concept of equilibrium shift. The performances of the membrane reactors are strongly dependent on the performance of the membranes used. In particular, there are many reports on the silica-based membranes with high H2 permeance and high selectivity, which were prepared by sol−gel method15,16 or chemical vapor deposition (CVD) method.17,18 Some of these studies successfully demonstrated the long-term durability of membrane performance.19,20 It is notable that Diniz da Costa and co-workers demonstrated thermally stable and robust performances of cobalt oxide silica membranes for 2000 h.19 However, no researchers have attempted to demonstrate long-term performance durability of membrane reactor with silica-based membrane for dehydrogenating MCH, to our knowledge. The objective of this study was to demonstrate stable, longterm (more than 1000 h), continuous H2 production from MCH using a membrane reactor with a dimethoxydiphenylsilane (DMDPS)-derived silica membrane prepared by chemical vapor deposition (CVD). In our previous studies, we developed DMDPS-derived silica membranes that can be used to purify H2 from organic chemical hydrides,21−24 and successfully demonstrated the durability of DMDPS-derived silica membranes for purifying H2 from an H2-TOL mixture for over 1000 h.25 We also successfully developed membrane reactors with DMDPS-derived membranes for dehydrogenating cyclohexane26,27 and MCH28 to produce high-purity H2. However, in those studies, we carried out daily startup and shutdown Received: Revised: Accepted: Published: 3996
February 6, 2015 April 6, 2015 April 9, 2015 April 9, 2015 DOI: 10.1021/acs.iecr.5b00527 Ind. Eng. Chem. Res. 2015, 54, 3996−4000
Research Note
Industrial & Engineering Chemistry Research
Figure 1. (a) Schematic of the experimental setup for dehydrogenation reaction. (b) Schematic of the membrane reactor.
hydrogen at 673 K. As feed materials, MCH dehydrated through molecular sieves (3A, Wako Pure Chemical Industries Ltd., Japan) was used. The dehydrated MCH was fed using a microfeeder and was immediately vaporized through heating by a ribbon heater that was maintained at 503 K, and the dehydrogenation reaction occurred in the reactor at 543 K. The pressure on the reaction side was maintained at 0.3 MPa. The dehydrogenation reaction was carried out continuously for 1054 h. To determine the conversion of MCH and the purity of H2 in the permeated gas, the concentrations of MCH, TOL in the retentate and permeate gases were measured by a gas chromatograph (Model GC-2014, Shimadzu Corp., Japan). Benzene, which can be produced as a byproduct, was not detected during the 1054 h of operation. The amount of H2 that permeated through the membrane was estimated with a film flow meter. Note that neither a carrier gas nor a sweep gas was used during operation. After the 1054-h run, regeneration of the DMDPS-derived membrane was carried out. The membrane was placed in an H2 atmosphere at 573 K for 5 h. A quantity of 400 mL min−1 of H2 gas was fed from outside the membrane, and the transmembrane pressure difference was maintained at 0.20 MPa. The permeances of H2, N2, and SF6 single gases through the regenerated DMDPS-derived silica membrane were again measured at 373, 473, and 573 K to compare their performance with the fresh membrane.
operations with total operating times of only a few tens of hours. Thus, successful demonstration of the continuous H2 production from MCH using a membrane reactor with a DMDPS-derived membrane for more than 1000 h is expected to provide a breakthrough from fundamental research to practical industrial applications.
2. EXPERIMENTAL SECTION Membrane Preparation. A porous α-alumina support (outer diameter = 6.3 mm, length = 100 mm) with 70 nm pores, kindly supplied by Noritake Co. Ltd., Japan, was used as the substrate. Fifteen millimeters (15 mm) on both ends were glazed with a sealant, and 70 mm at the center was used for depositing the silica layer. A γ-alumina layer was coated onto the α-alumina support using the sol−gel method to reduce the pore size to ∼4 nm.23 Briefly, the outer surface of the center area of the support was dipped in boehmite sol solution for 5 s, and then dried for 1 h in air and calcined at 873 K for 3 h. This coating process was repeated twice. The counter-diffusion CVD method was used to develop the silica membrane with DMDPS (purchased from Shin-Etsu Chemical Co. Ltd., Japan) as a precursor. The experimental details for preparing the silica membrane are described elsewhere.23 The saturated vapor of DMDPS was carried with 200 mL min−1 of N2 supplied to the outside of the support and 200 mL min−1 of O2 supplied to the inside. The CVD temperature was 873 K, and the CVD time was 1 h. Evaluation of the Membrane Performance. The permeances of H2, N2, and SF6 single gases through the prepared DMDPS-derived silica membrane were measured at 373, 473, and 573 K. A bubble flow meter was used to measure the permeances of H2 and N2, and the pressure difference method was used to measure the permeance of SF6. The experimental setup for the measurement was the same as that used for the preparation of the DMDPS-derived silica membrane. Development of a Membrane Reactor and 1054 h Continuous Operation. Figure 1 shows a schematic of the membrane reactor developed for this study. Pt/Al2O3 particles (1 wt %) were prepared according to our previous paper28 and used as catalysts. Al2O3 particles (reference catalyst ALO-6) were kindly supplied by the Catalysis Society of Japan. The prepared catalysts were loaded inside the DMDPS-derived membrane tube, as shown in Figure 1b, and reduced under
3. RESULTS AND DISCUSSION Performance of the DMDPS-Derived Membrane for the Membrane Reactor. Figure 2 shows the permeances of H2, N2, and SF6 at 373, 473, and 573 K. At 573 K, the permeances were 1.2 × 10−6, 3.8 × 10−8, and 1.3 × 10−10 mol m−2 s−1 Pa−1, respectively and the ideal selectivity for H2/SF6 was 9.6 × 103. Permeances were not dependent on temperature over the range tested. The performance of the DMDPS-derived membrane was comparable with the performances of the membranes developed in our previous work21−28 and we have developed membrane reactors for dehydrogenating cyclohexane26,27 and MCH28 using such membranes. Thus, we concluded that this membrane could be used as the membrane reactor in this study. Continuous H2 Production Using the Membrane Reactor. Figure 3a shows the time course for the conversion of MCH, and Figure 3b shows the time courses for the purity 3997
DOI: 10.1021/acs.iecr.5b00527 Ind. Eng. Chem. Res. 2015, 54, 3996−4000
Research Note
Industrial & Engineering Chemistry Research
The permeances of H2, N2, and SF6 single gases through the regenerated DMDPS-derived membrane measured at 373, 473, and 573 K are shown in Figure 4. (Permeances through the
Figure 2. Temperature dependence of gas permeation through the DMDPS-derived membrane at 573−373 K: (●) H2, (■) N2, and (◆) SF6.
Figure 4. Temperature dependence of gas permeation through the membrane regenerated after 1054 h of operation. For easy comparison, permeances through the fresh membranes are also shown: (○) H2, (□) N2, and (◇) SF6 (closed symbols represent fresh membrane data, open symbols represent regenerated membrane data).
fresh membrane, the same as those shown in Figure 2, are also plotted for easy comparison.) All permeances, under every temperature condition, through the regenerated DMDPSderived membrane were lower than those through the fresh membrane. At 573 K, the permeances were 9.2 × 10−7 mol m−2 s−1 Pa−1 for H2, 1.7 × 10−8 mol m−2 s−1 Pa−1for N2, and 8.6 × 10−11 mol m−2 s−1 Pa−1 for SF6. Compared to their permeances through the fresh membrane, each gas permeance decreased, by 25%, 56%, and 32%, respectively. As a result, the ideal selectivity of H2/SF6 changed to 1.1 × 104. Despite the observed decreases in permeance, no temperature dependencies in gas permeance were observed over the temperature range tested. When DMDPS-derived membranes are used, the permeance of N2 is often affected by various factors. However, this phenomenon does not have a significant impact, because only H2, MCH, and TOL exist in the reaction system and their kinetic diameters are very different from that of N2. In addition, we note that complete recovery of permeance was not achieved for any gas after the regeneration procedure. There are two major reasons for this incomplete recovery. One reason is the effect of the MCH and TOL molecules that coexist with H2. MCH and TOL molecules would adsorb onto the membrane surface and inside the membrane pores, and the regeneration procedure would not achieve complete desorption. Previously, a comparative study on the effects of TOL and MCH on the performance of DMDPS-derived silica membranes was carried out.24 In that study, permeation tests were conducted in H2/ TOL containing 5.0%, 10%, and 20% TOL, and H2/MCH containing 1.0%, 5.0%, and 10% MCH, with the results indicating that the effect of TOL on the DMDPS-derived membrane performance was more severe than that of MCH. In the present study, the conversion of MCH achieved by the membrane reactor was ∼40%, and thus the average concentrations of MCH and TOL on the reaction side were ∼27% and 18%, respectively. Although the previous study indicated that the effect of TOL was larger than that of MCH, we cannot disregard the effect of MCH in this case, because the
Figure 3. Time course of (a) the conversion of MCH and (b) the purity and flow rate of H2 in the gas permeate.
and the flow rate of H2 in the permeated gas. Under these operating conditions, the equilibrium conversion of MCH was calculated to be 0.27. In contrast, the initial conversion in the membrane reactor was actually 0.41. The reason for this is that hydrogen was extracted through the DMDPS-derived membrane, which means the membrane reactor worked well. During the 1054 h of operation, the conversion ranged from 0.37 to 0.42, and no marked decrease in conversion was observed. As for the permeate, MCH and TOL were detected by a GC, because we carried out the dehydrogenation reaction of MCH (C6H11−CH3 ⇔ C6H5−CH3 + 3 H2). Indeed, the side reaction to form methane and benzene would occur;1 however, no benzene was detected during the 1054-h operation. Thus, we estimated the purity of H2 in the permeate by subtracting the concentrations of MCH and TOL. As shown in Figure 3b, the purity and the flow rate of H2 in the permeate gas ranged from 98.5% to 99.1% and from 5.3 to 5.7 mL min−1, respectively, and no marked changes were observed. The recovery rate of H2 ranged from 50% to 57%. It is industrially significant that this membrane reactor, which consists of a DMDPS-derived membrane and Pt/Al2O3 catalysts, was successfully demonstrated to exhibit stable equilibrium shifts to produce highpurity H2 for over 1000 h, and it thus shows potential for practical application. 3998
DOI: 10.1021/acs.iecr.5b00527 Ind. Eng. Chem. Res. 2015, 54, 3996−4000
Industrial & Engineering Chemistry Research
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ACKNOWLEDGMENTS The catalyst Al2O3 support was kindly supplied by the Catalysis Society of Japan. We also thank Noritake Co. Ltd., Japan, for kindly supplying the α-alumina tube.
previous study did not involve such a high concentration of MCH. The effect of TOL may have been larger, even though the concentration of TOL was smaller than that of MCH in this case; however, we now regard the physical adsorption of MCH and TOL molecules as one major reason for the incomplete recovery of membrane performance. With regard to the second reason, we must consider the effect of a small amount of water molecules dissolved in MCH, which would result in an irreversible (nonregenerable) change in membrane performance. In our previous study,25 we demonstrated that (i) the performance of a DMDPS-derived membrane decreased slightly during 1314 h of operation when purifying H2 from H2/TOL containing 2.0% TOL, and (ii) this decreased performance could not be recovered completely, probably because an irreversible change in membrane structure would occur during operation. The present study involved MCH dehydrated using a molecular sieve, but the gas fed to the membrane reactor was 100% MCH, not 2%. Thus, we cannot neglect the effect of quite a small number of water molecules in the feed gas. However, we can state with confidence that we have demonstrated stable and continuous H2 production at high purity for more than 1000 h through achieving an equilibrium shift by H2 extraction using the DMDPS-derived membrane.
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REFERENCES
(1) Okada, Y.; Sasaki, E.; Watanabe, E.; Hyodo, S.; Nishijima, H. Development of dehydrogenation catalyst for hydrogen generation in organic chemical hydride method. Int. J. Hydrogen Energy 2006, 31, 1348. (2) Alhumaidan, F.; Cresswell, D.; Garforth, A. Hydrogen storage in liquid organic hydride: Producing hydrogen catalytically from methylcyclohexane. Energy Fuels 2011, 25, 4217. (3) Tong, J.; Matsumura, Y.; Suda, H.; Haraya, K. Experimental study of steam reforming of methane in a thin (6 μm) Pd-based membrane reactor. Ind. Eng. Chem. Res. 2005, 44, 1454. (4) Gallucci, F.; Paturzo, L.; Famà, A.; Basile, A. Experimental study of the methane steam reforming reaction in a dense Pd/Ag membrane reactor. Ind. Eng. Chem. Res. 2004, 43, 928. (5) Chen, Y.; Wang, Y.; Xu, H.; Xiong, G. Efficient production of hydrogen from natural gas steam reforming in palladium membrane reactor. Appl. Catal., B 2008, 80, 283. (6) Shirasaki, Y.; Tsuneki, T.; Ota, Y.; Yasuda, I.; Tachibana, S.; Nakajima, H.; Kobayashi, K. Development of membrane reformer system for highly efficient hydrogen production from natural gas. Int. J. Hydrogen Energy 2009, 34, 4482. (7) Li, H.; Pieterse, J. A. Z.; Dijkstra, J. W.; Haije, W. G.; Xu, H. Y.; Bao, C.; van den Brink, R. W.; Jansen, D. Performance test of a benchscale multi-tubular membrane reformer. J. Membr. Sci. 2011, 373, 43. (8) Ali, J. K.; Baiker, A. Dehydrogenation of methylcycloheacne to toluene in a pilot-scale membrane reactor. Appl. Catal., A 1997, 155, 41. (9) Ferreira-Aparicio, P.; Rodriguez-Ramos, I.; Guerrero-Ruiz, A. Pure hydrogen production from methylcyclohexane using a new high performance membrane reactor. Chem. Commun. 2002, 18, 2082. (10) Itoh, N.; Watanabe, S.; Kawasoe, K.; Sato, T.; Tsuji, T. A membrane reactor for hydrogen storage and transport system using cyclohexane−methylcyclohexane mixtures. Desalination 2008, 234, 261. (11) Ferreira-Aparicio, P.; Rodriguez-Ramos, I.; Guerrero-Ruiz, A. On the performance of porous Vycor membranes for conversion enhancement in the dehydrogenation of methylcyclohexane to toluene. J. Catal. 2002, 212, 182. (12) Hirota, Y.; Ishikado, A.; Uchida, Y.; Egashira, Y.; Nishiyama, N. Pore size control of microporous carbon membranes by post-synthesis activation and their use in a membrane reactor for dehydrogenation of methylcyclohexane. J. Membr. Sci. 2013, 440, 134. (13) Li, G.; Niimi, T.; Kanezashi, M.; Yoshioka, T.; Tsuru, T. Equilibrium shift of methylcyclohexane dehydrogenation in a thermally stable organosilica membrane reactor for high-purity hydrogen production. Int. J. Hydrogen Energy 2013, 38, 15302. (14) Niimi, T.; Nagasawa, H.; Kanezashi, M.; Yoshioka, T.; Ito, K.; Tsuru, T. Preparation of BTESE-derived organosilica membranes for catalytic membrane reacots of methylcyclohexane dehydrogenation. J. Membr. Sci. 2014, 455, 375. (15) Boffa, V.; Blank, D. H. A.; ten Elshof, J. E. Hydrothermal stability of microporous silica and niobia−silica membranes. J. Membr. Sci. 2008, 319, 256. (16) Kanezashi, M.; Yada, K.; Yoshioka, T.; Tsuru, T. Design of silica networks for development of highly permeable hydrogen separation membranes with hydrothermal stability. J. Am. Chem. Soc. 2009, 131, 414. (17) Gu, Y.; Oyama, S. T. Permeation properties and hydrothermal stability of silica−titania membranes supported on porous alumina substrate. J. Membr. Sci. 2009, 345, 267. (18) Choi, H.-S.; Ryu, C.-H.; Hwang, G.-J. Obtention of ZrO2−SiO2 hydrogen permselective membrane by chemical vapor deposition method. Chem. Eng. J. 2013, 232, 302.
4. CONCLUSION Continuous H2 production by dehydrogenating MCH was successfully demonstrated for more than 1000 h using a membrane reactor consisting of a DMDPS-derived membrane and Pt/Al2O3 catalysts. Under the demonstration conditions, the conversion of MCH ranged from 0.37 to 0.42, whereas the equilibrium conversion was 0.27, and no marked change in the membrane reactor’s performance was observed during operation. The H2 purity of the produced gas was also stable and ranged from 98.5% to 99.1%. After 1054 h of continuous operation, regeneration of the DMDPS-derived membrane was carried out under an H2 atmosphere. However, complete recovery was not achieved: H2, N2, and SF6 permeances decreased by 25%, 56%, and 32%, respectively, compared with their permeances through the fresh membrane. The reasons for incomplete recovery of performance, based on our previous studies, would involve the physical adsorption of MCH and TOL molecules on and within the membrane, and irreversible changes in membrane structure induced by small amounts of water dissolved in the MCH feed.23−25 This demonstration of 1054 h of continuous operation provides a successful breakthrough toward the practical industrial application of membrane reactors with H2-selective silica membranes.
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Research Note
AUTHOR INFORMATION
Corresponding Author
*Tel.: +81-42-628-4584. Fax: +81-42-628-4542. E-mail:
[email protected]. Present Address †
Department of Applied Chemistry, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan. Notes
The authors declare no competing financial interest. 3999
DOI: 10.1021/acs.iecr.5b00527 Ind. Eng. Chem. Res. 2015, 54, 3996−4000
Research Note
Industrial & Engineering Chemistry Research (19) Yacou, C.; Smart, S.; Diniz da Costa, J. C. Long term performance cobalt oxide silica membrane module for high temperature H2 separation. Energy Environ. Sci. 2012, 5, 5820. (20) Yoshino, Y.; Suzuki, T.; Taguchi, H.; Nomura, M.; Nakao, S.; Itoh, N. Development of an all-ceramic module with silica membrane tubes for high temperature hydrogen separation. Sep. Sci. Technol. 2008, 43, 3432. (21) Ohta, Y.; Akamatsu, K.; Sugawara, T.; Nakao, A.; Miyoshi, A.; Nakao, S. Development of pore-size controlled silica membranes for gas separation by chemical vapor deposition. J. Membr. Sci. 2008, 315, 93. (22) Saito, T.; Seshimo, M.; Akamatsu, K.; Miyajima, K.; Nakao, S. Effect of physically adsorbed water molecules on the H2-selective performance of a silica membrane prepared with dimethoxydiphenylsilane and its regeneration. J. Membr. Sci. 2012, 392−393, 95. (23) Seshimo, M.; Saito, T.; Akamatsu, K.; Segawa, A.; Nakao, S. Influence of toluene vapor on the H2-selective performance of dimethoxydiphenylsilane-derived silica membranes prepared by the chemical vapor deposition method. J. Membr. Sci. 2012, 415−416, 51. (24) Seshimo, M.; Akamatsu, K.; Furuta, S.; Nakao, S. Comparative study on the influence of toluene and methylcyclohexane on the performance of dimethoxydiphenylsilane-derived silica membranes prepared by chemical vapor deposition. Sep. Purif. Technol. 2015, 140, 1. (25) Seshimo, M.; Akamatsu, K.; Furuta, S.; Nakao, S. H2 purification durability of dimethoxydiphenylsilane-derived silica membranes with H2−toluene mixtures. Ind. Eng. Chem. Res. 2013, 52, 17257. (26) Akamatsu, K.; Ohta, Y.; Sugawara, T.; Hattori, T.; Nakao, S. Production of hydrogen by dehydrogenation of cyclohexane in highpressure (1−8 atm) membrane reactors using amorphous silica membrane with controlled pore sizes. Ind. Eng. Chem. Res. 2008, 47, 9842. (27) Akamatsu, K.; Ohta, Y.; Sugawara, T.; Kanno, N.; Tonokura, K.; Hattori, T.; Nakao, S. Stable high-purity hydrogen production by dehydrogenation of cyclohexane using a membrane reactor with neither carrier gas nor sweep gas. J. Membr. Sci. 2009, 330, 1. (28) Oda, K.; Akamatsu, K.; Sugawara, T.; Kikuchi, R.; Segawa, A.; Nakao, S. Dehydrogenation of methylcyclohexane to produce highpurity hydrogen using membrane reactors with amorphous silica membranes. Ind. Eng. Chem. Res. 2010, 49, 11287.
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DOI: 10.1021/acs.iecr.5b00527 Ind. Eng. Chem. Res. 2015, 54, 3996−4000
