H2 Purification Durability of Dimethoxydiphenylsilane-Derived Silica

Nov 11, 2013 - Department of Environmental and Energy Chemistry, Faculty of Engineering, Kogakuin University, 2665-1 Nakano-machi, Hachioji-shi, Tokyo...
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H2 Purification Durability of Dimethoxydiphenylsilane-Derived Silica Membranes with H2−Toluene Mixtures Masahiro Seshimo,†,§ Kazuki Akamatsu,*,† Satoshi Furuta,‡ and Shin-ichi Nakao† †

Department of Environmental and Energy Chemistry, Faculty of Engineering, Kogakuin University, 2665-1 Nakano-machi, Hachioji-shi, Tokyo 192-0015, Japan ‡ Hydrogen & New Energy Research Laboratory, JX Nippon Oil & Energy Corporation, 8, Chidori-cho, Naka-ku, Yokohama 231-0815, Japan ABSTRACT: Long-term durability tests of dimethoxydiphenylsilane-derived silica membranes for purifying H2 from H2− toluene mixtures were carried out. The silica membrane maintained excellent H2-selective performance under H2−toluene mixture condition, despite H2 permeance was decreased from its initial value. Membrane performance changed in two steps. Performance initially decreased rapidly, and then decreased very slowly. We also examined membrane performance using H2− dehydrated toluene and H2−water mixtures, and it was indicated that adsorption of toluene molecules and water molecules dissolved in toluene on membrane surfaces was the predominant factor for the rapid initial decrease. Water molecules present within toluene also affected the subsequent slower decrease. These decrements were partially recovered by placing the membranes in an H2 stream. Incomplete recovery during regeneration was due to the strong effect of water molecules dissolved in toluene, even though the amount of water was quite small.

1. INTRODUCTION

Hydrogen purification from hydrogen/organic chemical hydrides gas mixtures using silica membranes has received little attention. Using membrane reactors to efficiently produce H2 by extracting produced H2 from dehydrogenating hydrides has received much interest.19−24 The focus of such studies has been the net performance of membrane reactors, but not the effect of organic chemical hydrides on such performance. There are also many reports on the hydrothermal stability of silica membranes,25−29 but they give little insight of their durability in H2/organic chemical hydride atmospheres. Herein, we investigate the durability of dimethoxydiphenylsilane (DMDPS)-derived silica membranes in purifying H2 from H2−toluene mixtures. Membranes were prepared by counter diffusion CVD. We previously demonstrated that DMDPS-derived silica membranes provided H2 of >99.99% purity, from H2 gas containing 2.0% toluene vapor.15 However, this was a short-term study. Herein, long-term tests of >1000 h were carried out to investigate the change in membrane performance with time. The membrane performance was also evaluated using H2−dehydrated toluene system and H2−water system to elucidate the influence of water within toluene. Finally, the regeneration of membranes was studied to elucidate if decreasing membrane performance could be recovered.

Organic chemical hydrides such as cyclohexane, methylcyclohexane, and decaline are promising hydrogen storage transport and production materials. Their dehydrogenation reactions have been extensively investigated in recent years.1−3 Highly pure hydrogen is essential, when supplied from organic chemical hydrides for polymer electrolyte fuel cells. Highly efficient hydrogen purification systems are thus required, and membrane separation technology is likely to be important in hydrogen purification from such hydrides. Inorganic membranes such as Pd-based, carbon, zeolite, and silica membranes are advantageous, because of their heat and chemical resistance and high mechanical strength. Pd-based membranes can be prepared by electroplating4,5 or electroless plating6,7 and, in principle, allow the permeation of only monatomic hydrogen. Carbon, zeolite, and silica membranes are considered molecular sieving membranes. Carbon membranes can be produced from polymeric precursors such as poly(phenylene oxide).8,9 Zeolite membranes can exhibit shape selectivity owing to their uniform micropores, which differentiates their characters from that of other inorganic membranes. Zeolite membranes are often applied in isomer separation.10,11 Mordenite framework inverted (MFI) zeolite membranes exhibit good hydrogen purification properties from various hydrogen/hydrocarbon mixtures.12 Silica membranes prepared from chemical vapor deposition (CVD) and sol−gel methods have also received much interest in hydrogen separation applications.13−18 There has been much recent development of H2-selective silica membranes, whose H2 permeation and selectivity are comparable to those of Pd-based membranes. Silica is advantageous in terms of material cost, so silica membranes are promising candidates in H2 separation and purification from organic chemical hydrides. © 2013 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Preparation and Evaluation of DMDPS-Derived Silica Membranes. A schematic of the experimental apparatus for preparing DMDPS-derived silica membranes and evaluating their permeation performance is available elsewhere.15 A Received: Revised: Accepted: Published: 17257

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porous α-alumina tube with pores of 70 nm was used as the membrane support. This membrane support had a length and outer diameter of 100 and 6.3 mm, respectively. The effective membrane area was 1.36 × 10−3 m2 on the center of the membrane support, and other parts were sealed with glass. A γalumina layer was coated on the α-alumina support, using the sol−gel method.15 An amorphous silica layer was deposited on the γ-alumina layer by counter-diffusion CVD. DMDPS (ShinEtsu Chemical Co. Ltd.) saturated vapor was supplied in a N2 (200 mL/min) carrier gas, from the outside of the γ-aluminacoated membrane support, and O2 (200 mL/min) was supplied from the inside. The CVD reaction temperature was 873 K, and the reaction time was 60 min. As a result, a silica layer derived by DMDPS formed in the pores of, and on the surfaces of, γalumina. Membrane performance was evaluated at 573, 473, and 373 K using single-component gases of H2, N2, and SF6. H2 and N2 permeation were measured with a bubble flow meter (SF 1U, Horiba Co.), and SF6 permeation was determined using the pressure difference method. 2.2. Durability for Purifying H2 from H2−Toluene. Durability tests of the membranes for purifying H2 from H2− toluene gas mixtures were carried out using the same apparatus as that for the permeation measurements. A mixture of H2 containing 2.0% toluene vapor was used as a continuous feed gas. Toluene was supplied to a vaporizer using a microfeeder (N-CL-100, Nihon Seimitsu Kagaku Co. Ltd.), and carried with H2 gas from outside the silica membrane. The transmembrane pressure difference was 0.20 MPa, and the membrane temperature was fixed at 473 K. To evaluate the purification performance, permeated gas was analyzed by gas chromatography (GC-2014, Shimadzu Corp.) to calculate the toluene concentration. Periodically, the feed gas was temporarily halted and membrane performance was evaluated by the permeation testing of single-component gases of H2, N2, and SF6. For the durability tests, two membranes (denoted membrane-1 and membrane-2) were used. Membrane-1 was employed for testing using toluene without pretreatment, and membrane-2 was employed for testing using toluene dehydrated through molecular sieves (3A 1/8, Wako Pure Chemical Industries Ltd., Japan), to investigate the effect of water dissolved in the toluene. 2.3. H2 Permeation Test under H2−Water Mixture Condition. For this test, another membrane (denoted membrane-3) was prepared, and a H2 permeation test containing water through membrane-3 was carried out to clarify the effect of water molecules on the membrane performance in a direct way. Water was fed using a microfeeder and carried with H2 gas to the membrane. The flow rates of water were 0.0, 0.1, 0.2, and 0.3 μL/min. The transmembrane pressure difference was kept at 0.20 MPa, and the membrane temperature was fixed at 473 K, which was the same condition in the durability tests using H2−toluene mixtures. 2.4. Regeneration of Membranes. Regeneration of membranes was carried out in an attempt to recover the performance. Membranes were placed in a H2 atmosphere at 573 K for 5 h. H2 gas (500 mL/min) was fed from outside the membrane, and the transmembrane pressure difference was kept at 0.20 MPa. Regeneration of membrane-1 was conducted twice in 608 h and also after the durability test (1314 h). Regeneration of membrane-2 was conducted once just after the durability test (480 h). After the regeneration procedure, the

membrane performance was again evaluated at 573, 473, and 373 K, using single-component gases of H2, N2, and SF6.

3. RESULTS AND DISCUSSION 3.1. Performance of DMDPS-Derived Silica Membranes. The permeation of H2, N2, and SF6 through membrane-1, membrane-2, and membrane-3 at 573, 473, and 373 K, is shown in Figure 1. All the membranes exhibited

Figure 1. Temperature dependence of (a) H2, (b) N2, and (c) SF6 permeation through membrane-1, membrane-2, and membrane-3 at 573−373 K.

similar performances. The H2-selective performances were excellent at the sampled temperatures, and gas permeation was little affected by temperature. The performances of these DMDPS-derived membranes were similar to those in previous reports.15,24 H2/SF6 selectivity is an indicator for H2/toluene selectivity because the kinetic diameter of toluene is similar to that of SF6, and much larger than that of H2. These membranes can be exactly used for the following durability tests of H2 purification. 3.2. Durability for Purifying H2 and Effect of Water. Long-term H2 purification tests from the H2−toluene mixture were carried out, using toluene without pretreatment (membrane-1) and dehydrated toluene (membrane-2). During the test, the toluene concentration on the permeation side was measured by gas chromatography 17 times (at 1, 2, 3, 4, 5, 40, 88, 228, 328, 461, and 608 h before the first regeneration and at 608 h after the first regeneration, and 712, 911, 1134, and 1314 h) when membrane-1 and toluene without pretreatment were used. And, when membrane-2 and dehydrated toluene were used, the toluene concentration on the permeation side was measured at 1, 2, 3, 4, and 5 h, and then every 50 h. All the measurements confirmed that H2 purity was >99.99%. Figures 2−4 show comparisons of the time evolution of H2, N2, and SF6 permeance through membrane-1 and membrane-2 at 473 K, respectively. In the durability test using toluene without pretreatment, H2 permeance rapidly decreased from its initial value, followed by a much smaller slower decline. The 17258

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Figure 4. Time evolutions of SF6 permeance during the H 2 purification tests using toluene without pretreatment (membrane-1) and dehydrated toluene (membrane-2) at 473 K.

Figure 2. Time evolutions of H2 permeance during the H2 purification tests using toluene without pretreatment (membrane-1) and dehydrated toluene (membrane-2) at 473 K.

Figures 2 and 3 suggested two different reasons for the permeance decrease during each stage. Physical adsorption of toluene on the membrane surface was considered to be responsible for the rapid initial H2 and N2 permeance decreases. Physical adsorption of water would also occur,30 and there would be this effect. Such adsorption would result in a blocking effect. And the water dissolved in the toluene was also responsible for the smaller subsequent permeance decreases, compared with these two results. In Figure 4, plots of SF6 permeance showed significant fluctuation, attributed to the difficulty in measuring such low values. Little conclusions could be drawn from the SF6 data, and some decrement in SF6 permeance occurred in both toluene without pretreatment and dehydrated toluene. To clarify the effect of water on the membrane performance in a direct way, H2 purification test from H2−water mixture was carried out using membrane-3. Figure 5 shows time evolution of H2 permeance at 473 K when the flow rate of water varied from 0.0 to 0.3 μL/min. When no water was accompanied with H2, H2 permeance showed no decrease and kept its initial value. However, 0.1 μL/min of water was fed with H2, the decrease of

Figure 3. Time evolutions of N2 permeance during the H2 purification tests using toluene without pretreatment (membrane-1) and dehydrated toluene (membrane-2) at 473 K.

first membrane regeneration was carried out at 608 h, by which time H2 permeance had decreased to 5.78 × 10−7 mol m−2 s−1 Pa−1. It recovered to 7.76 × 10−7 mol m−2 s−1 Pa−1 after the regeneration, but a rapid initial decrease in permeance was again observed. After that, H2 permeance decreased slowly with increasing time. At 1314 h, H2 permeance was 5.17 × 10−7 mol m−2 s−1 Pa−1, which was almost half of its initial value. On the other hand, in a durability test using dehydrated toluene, a rapid initial H2 permeance decrease was still observed, but no further permeance change was observed. The ratio of H2 permeance decrease at 460 h compared with the initial value for dehydrated toluene was 21.7%. This was smaller than that with toluene without pretreatment at 461 h (38.1%). Similar decreasing trends were observed in the N2 permeation, shown in Figure 3. When toluene without pretreatment is used, an initial rapid decrease was observed as well as another immediately after regeneration, after which the extent of subsequent decreases became smaller. On the other hand, a rapid initial decrease was observed, and no further change in N2 permeance occurred during the test using dehydrated toluene.

Figure 5. Time evolution of H2 permeance through membrane-3 under H2−water mixture condition at 473 K. 17259

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H2 permeance was observed. And the H2 permeance decreased more when the flow rate of water increased to 0.2 μL/min, and to 0.3 μL/min. After the regeneration procedure, membrane-3 was again used for H2 purification test from H2 with 0.3 μL/ min of water. The result is also shown in Figure 5 as closed keys. In this case H2 permeance also decreased. Additionally, the decreasing ratio of H2 permeance was similar. The water concentrations in toluene without pretreatment and in dehydrated toluene measured were 258 and 16 ppm, respectively. And 0.3 μL/min of water in this test almost corresponded to the water content in the purification test using H2−toluene without pretreatment system. On the basis of this result, we can conclude that this level of water concentration in toluene surely affected the membrane performance, and accordingly water molecules dissolved in toluene surely affected the membrane performance. As for the H2 selectivity in the H2 purification test from the H2−toluene system using membrane-1 and membrane-2, the time evolutions of H2/N2 and H2/SF6 selectivity are shown in Figure 6. That of H2/N2 using toluene without pretreatment

Figure 7. Comparison of temperature dependence of permeance through fresh membrane-1 (as shown in Figure 1) and the regenerated silica membrane at 573−373 K.

Figure 6. Time evolution of H2/N2 and H2/SF6 selectivity during the H2 purification tests using toluene without pretreatment (membrane1) and dehydrated toluene (membrane-2) at 473 K.

Figure 8. Comparison of temperature dependence of permeance through fresh membrane-2 (as shown in Figure 1) and the regenerated silica membrane at 573−373 K.

slightly increased from its initial value, because the decreasing ratio of N2 permeance was slightly larger than that of H2 permeance. That of H2/N2 using dehydrated toluene was almost constant with time. The selectivity of the H2/SF6 mixture was almost constant throughout the test in both cases. This was because the decreasing ratio of SF6 permeance was similar to that of H2 permeance. Despite H2 permeance decreasing by around 50% from its initial value, the DMDPSderived silica membrane maintained excellent H2 selectivity under H2−toluene gas mixture conditions, even though toluene contained a little amount of water. 3.3. Gas Permeation Performances through Regenerated DMDPS-Derived Silica Membranes. Figures 7 and 8 show the temperature dependence of gas permeance through the regenerated membrane after durability test using toluene without pretreatment and dehydrated toluene, respectively. In Figure 7, the performance of the regenerated membrane was worse than that of its initial state. At 573 K, H2 permeance decreased from 1.09 × 10−6 to 7.14 × 10−7 mol m−2 s−1 Pa−1, and N2 permeance decreased from 1.90 × 10−8 to 6.82 × 10−9 mol m−2 s −1 Pa −1. The recovery ratios of H2 and N2

permeances were 65.5% and 35.9%, respectively. The H2/N2 selectivity of fresh and regenerated membranes was 57 and 105, respectively. On the other hand, in Figure 8, H2 permeances through the fresh and regenerated membranes at 573 K were 8.58 × 10−7 and 7.99 × 10−7 mol m−2 s−1 Pa−1, respectively. The recovery ratio of H2 permeance was as high as 93.1%. N2 permeance through the fresh and regenerated membranes was 2.09 × 10−8 and 2.22 × 10−8 mol m−2 s−1 Pa−1, respectively, and the recovery ratio of N2 permeance was 106%. The H2/N2 selectivity of fresh and regenerated membranes was 41 and 36, respectively. From these results, we can say that the second slow decrease in membrane performance when H2 is purified from the H2− toluene mixture is an irreversible change and cannot be recovered by the regeneration procedure, and that the first rapid decrease was a reversible change and can be recovered. When the dehydrated toluene was used, the effect of water was absent. It means that the irreversible and nonregenerable decrease was exactly due to water molecules. The long-term 17260

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durability of the DMDPS-derived membranes under hydrothermal conditions has not been investigated yet, although there are reports on the densification of silica membranes prepared by CVD or sol−gel methods.27 Decreased membrane performance due to densification is generally considered irreversible, even though it has only been reported at high temperatures. At this stage it is not clear whether this irreversible and nonregenerable decrease in the membrane performance was really due to densification, but this irreversible change was exactly induced by water present in toluene. On the other hand, the reversible and regenerable decrease was mainly due to the adsorption of toluene molecules. Water molecules in toluene were only 258 ppm, which meant a much smaller amount compared with toluene, but water molecules may also physically adsorb onto the membrane surface, and into the pores.

REFERENCES

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4. CONCLUSIONS Long-term durability tests of DMDPS-derived silica membranes for purifying H2 from H2−toluene mixtures were carried out. To clarify the effect of water molecules dissolved in toluene, this test was also carried out using a H2−dehydrated toluene system and a H2−water system. When toluene without pretreatment was used, performances changed in two steps. Membrane performance initially rapidly decreased and then subsequently decreased more slowly. On the other hand, the performance also rapidly decreased initially but no further changes were observed, when toluene dehydrated through molecular sieves was used. And the performance also decreased when the H2−water mixture was fed. From these results, the predominant factor affecting membrane performance differed in each step. Toluene adsorption on the membrane surface resulted in the initial rapid decrease, which sterically prohibited the permeation of small gas molecules. Water molecules in toluene may do. As for the subsequent smaller permeation decrease, water dissolved in toluene was responsible. Membrane regeneration was carried out in a H2 stream, to recover the decrease in performance after the long-term tests. Performance could not be completely recovered when toluene without pretreatment was used but was almost entirely recovered when dehydrated toluene was used. The regeneration procedures acted to physically remove adsorbed toluene and water molecules. However, the effect of water in toluene, whose concentration was as small as 258 ppm, was quite strong, and it would also induce irreversible change that could not be recovered by the regeneration procedure.

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AUTHOR INFORMATION

Corresponding Author

*E-mail, [email protected]; tel, +81-42-628-4584; fax, +81-42-628-4542. 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.

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ABBREVIATIONS DMDPS, dimethoxydiphenylsilane CVD, chemical vapor deposition 17261

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