Methane reforming on molybdenum carbide on Al-FSM-16

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B. V. All rights reserved

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Methane reforming on molybdenum carbide on Al-FSM-16 Masatoshi Nagai, Toshihiro Nishibayashi, and Shinzo Omi Graduate School of Bio-applications and Systems Engineering, Tokyo University of Agriculture and Technology, 2-24 Nakamachi, Koganei, Tokyo 184-8588, Japan Methane reforming on the carbided 12% Mo/Al-FSM-16 catalysts with Si/Al ratios of 30, 50, and 80 was performed at 973 K under atmospheric pressure. The characterization was carried out by N2 adsorption, XRD, and ^^Al MAS NMR. AI-FSM-16 with a Si/Al ratio of 30 exhibited an implantation of aluminum into the Si02 structure of FSM-16. The 873 K-carbided 12% Mo/Al-FSM-16 catalyst was more active than the oxidized catalyst and the catalysts carbided at a higher carbiding temperature. The largest amounts of hydrogen and benzene were formed using the catalyst with the Si/Al ratio of 80. P-M02C on the catalyst was formed during the carbiding and methane reforming. 1. INTRODUCTION Recently, methane reforming has been extensively studied for effectively utilizing natural gas resources. Mo/ZSM-5 catalysts are very active in methane reforming. The implantation of aluminum into mesopore FSM-16 is expected to be used as a catalyst support by generating acid sites [1]. Mesoporous materials having a high surface area and heat tolerance promoted the reaction with a fast molecular diffusion in the mesopores. In this study, 12% M0O3/AI-FSM-I6 is carbided by a temperature-programmed reaction in a stream of 20% CH4/H2 [2], and analyzed by N2 adsorption, NMR, and XRD. The effects of the Si/Al ratio and preparation procedure on the structure were studied. The catalytic activity is determined during methane reforming using the 12% Mo/Al-FSM-16 catalysts with three different Si/Al ratios. 2. EXPERIMENTAL Sodium silicate and sodium aluminate (Si/Al=30, 50, and 80) were added to a small amount of water and the mixture was stirred at 333 K for 3 h. The solution was dried at 353 K (and 413 K) to yield a sodium aluminosilicate glass which was then calcined at 973 K for 3 h. The layered sodium silicates containing aluminum at three Si/Al ratios were dispersed in an aqueous solution of [Ci6H33N(CH3)3]Cl and stirred at 343 K. The solid products were separated from the solutions by suction filtration (or decantation), and dried at 353 K (Method II). The sample (Method I) was dried at 353 K and subsequently calcined at 813 K in air . The 12 wt% M0O3/AI-FSM-16 catalyst was prepared by an incipient wetness method after Al-FSM-16 (Method I or II) was added to an aqueous solution of (NH4)6Mo7024-4H20. The resulting product was dried at 353 K, calcined at 573 K, and carbided by the temperature-programmed reaction in 20% CH4/H2 at a flow rate of 66.7 ml min' from 573 to 873 (-1073) K at a rate of 1 K min''. The catalyst was maintained at this temperature for 3 h. The BET surface area of the samples was measured at 77 K using a Beckman-Coulter adsorption apparatus. The structure of the samples before and after pretreatment and carburidation was measured by XRD analysis. Diffraction patterns were determined using a RAD-II (Rigaku Co.) equipped with Cu-Ka radiation with slits of (ds) 1/2°, (rs) 0.3 mm, and (ss) 1/2°. The solid state MAS NMR spectra were measured on a JEOL JNM-EX400 spectrometer. ^^Al MAS NMR spectra were recorded at 6 kHz spinning. Methane reforming was carried out using a continuous-flow quartz reactor (0.03 g) in streams of methane and helium with a 15 mlmin"' rate at 973 K. The reaction mixtures were analyzed using a Balzer quadrupole mass spectrometer.

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3. RESULTS AND DISCUSSION

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3.1. Preparation of Mo/Al-FSM-16 The N2 isotherms of the Al-FSM-16, 12% Mo/Al-FSM-16 (I), and 12% Mo/Al-FSM-16 (II) are shown in Figure 1. The predominant increase in the adsorptions of Al-FSM-16 and 12% Mo/Al-FSM-16 (II) were observed at P/Po = 0.25 ~ 0.4, which was Y^^ characteristic of the N2 adsorption in mesopores, while these characteristic 0.2 1.0 peaks were not observed for the 12% Mo/Al-FSM-16 (I) after molybdenum P/Po loading. This result indicated that the Fig. 1. N2 Adsorption/desorption is oth erms loading of molybdenum destroyed the of(#)Al-FSM-16, (x)Mo/Al-FSM-16(l), and structure of 12% Mo/Al-FSM-16 (I), while the structure of the 12% ( • ) Mo/Al-FSM-16(11). Volume(gas)l.56Xl0 3 Mo/Al-FSM-16 (II) was uniformly maintained even after calcination at 813 K. The BET surface areas of the Al-FSM-16 are 715, 799, and 1275 m^ Table 1 g"' for the Si/Al ratios of 30, 50, and 80, BET surface area, spacing dioo, and unit cell respectively, showing that the surface dimension ao (100) of each sample area increased with the increasing Si/Al ratio. After drying the sample (II), the XRD Surface area Sample surface area after loading the dioo/nm ao/nm /m^g' molybdenum species decreased from 715 to 514 m^ g ' in Table 1, but 1444 FSM-16 3.73 4.31 maintained the structure of the support. 3.84 4.43 Al-FSM-16 715 The surface area of the sample (I) l2%Mo/Al-FSM--16(1) 269 decreased much more than that of the 12%Mo/Al-FSM-16(1) 247 sample (II). The decrease in surface carbide at 973 K area of the sample (I) was due to l2%Mo/Al-FSM-16(II) 514 3.73 4.31 plugging of the molybdenum oxides in the micropores of Al-FSM-16 and destroying the FSM-16 structure. The XRD patterns of FSM-16, Al-FSM-16, and 12% Mo/Al-FSM-16 (I, II) are shown in Figure 2. The (100), (110), and (200) phases were observed for FSM-16, but only the (100) phase was seen for Al-FSM-16. The Al-FSM-16 exhibited the implantation of the aluminum atom into the SiOz structure of FSM-16 by having an irregular structure. The surface area of FSM-16 was two times greater than that of Al-FSM-16, supporting the result of the structure regularity by XRD. Thus, the structural regularity was likely to affect the surface area. The 12% Mo/Al-FSM-16 (II) was prepared by calcination after loading the molybdenum compound which resulted in retaining the structure of the (100) phase. Since carbization of the sample (II) slightly decreased the surface area, the structure of the support was not changed before and after the carbization. In Table 1, FSM-16 and Mo/Al-FSM-16 (II) had the same unit cell dimensions as the value (4.31 nm) in the literature [3]. This result showed that Al-FSM-16 and 12% Mo/Al-FSM-16 maintained the mesoporous structure of the 16 carbon chains. The pore sizes of Al-FSM-16 were 2.8 and 4.2 nm. The former pore size was due to FSM-16 and the latter due to the formation of bridging of the aluminum with silica in the preparation of Al-FSM-16. The 12% Mo/Al-FSM-16 (II) contained micropores of 2.8 nm more than Al-FSM-16. The XRD pattern of the impregnated 12% Mo/Al-FSM-16 is shown in Figure 2e. Al-FSM-16 had a peak of Si02. 12% Mo/Al-FSM-16 carbided at 973 K had the peak of P-M02C but no peaks for the oxide form. P-M02C had agglomerated outside the pores

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of the support in flowing 20% CH4/H2 at high carbiding temperatures. This result showed that molybdenum oxides on the surface of the Al-FSM-16 were loaded more than that inside of the micropores. 3.2. Properties and structure The ^^Al MAS NMR spectra for the Al-FSM-16 with the three Si/Al ratios are shown in Figure 3. The Al-FSM-16 sample (filtration) had a peak at 50 ppm, which is ascribed to four coordinated sites, while the Al-FSM-16 sample (decantation) had the peak at 8 ppm for the six coordinated sites. The formation of six-coordinated alumina is due to the more basic solution of sodium silicate and sodium aluminate at a pH of about 12.4. These compounds were precipitated and changed to the six-coordinated compounds containing aluminum sources. The ratio of the six-coordinated octahedral to four-coordinated tetrahedral aluminum increased with the decreasing aluminum content (high Si/Al ratio). This result suggested that the implantation of aluminum in the Si02 body required a certain amount of aluminum in the feed solution. The uniform implanting of aluminum into the SiOi structure needs an excess amount of aluminum. For Al-FSM-16 with Si/Al = 80, the large ratio of the hexahedral aluminum to the tetrahedral aluminum was observed more than those with Si/Al = 30 and 50 from the decantation preparation. The decantation cannot completely remove the dissolved feed (sodium aluminate). The XRD analysis confirmed maintaining of the hexagonal structure after the molybdenum oxides were loaded and subsequently carbided in a stream of 20%CH4/H2. Thel2%Mo/Al-FSM-16 (II) with good hexagonal structure exhibited a higher surface area than 12% Mo/Al-FSM-16(I).

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3.3. Methane reforming Figure 4 shows the methane reforming on the 973 K-carbided catalysts with the three different Si/Al ratios. The largest amounts of hydrogen and benzene were formed at the Si/Al ratio of 80. The catalyst with the Si/Al ratio of 80 was the most active based on the surface area. This is because the aluminum atom in the support was involved in the reforming reaction because of the high activity per surface area. Furthermore, the catalysts carbided at 873 K exhibited a high activity during methane reforming, or possibly the interaction of

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alumina with silica. Furthermore, the methane reforming was carried out over the oxidic catalyst with the Si/Al ratio of 80. This lag time is required for carbidization with methane in the feed. M0O3 on the surface was changed from a to P-M02C. The molybdenum species changed from molybdenum oxide to molybdenum carbide even after 10 min from the run-start since the molybdenum carbide was formed. l.UUh-UU/ i 8.00E-008 6.00E-008 4.00E-008 3

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The BET surface areas of Al-FSM-16 increased from 715 and 799 to 1275 m^g"' with the increasing Si/Al ratios from 30, 50, to 80, respectively, and those of the 12% M0O3/AI-FSM-I6 were 269, 306, and 735 m^g•^ The ^^Al MAS NMR spectra of Al-FSM-16 showed that the sample by filtration mainly had four coordinated sites and the sample by decantation contained six coordinated sites. The uniform implanting of aluminium into the SiOi structure needs excess aluminium. In methane reforming on the oxidized catalyst, the induction period was observed until the catalyst was carbided on the surface. This activation period was due to the time for changing M0O3 to P-M02C on Al-FSM-16.

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