Mo L3-XANES Study of Silylation Effect for Methane Dehydroaromatization over Mo/H-MFI Catalysts

A Mo-modified H-MFI zeolite (Mo/H-MFI) catalyst is one of a typical catalyst for direct GTL (Gas to liquid), because it shows high activity for dehydroaromatization of methane in absence of oxygen. In this reaction, reduction of Mo species is brought about in contact with methane in initial step, and reduced Mo ions react methane to form carbide and/or oxycarbide species in next step. It has been accepted that the carbide and/or oxycarbide species are the active center for dehydroaromatization of methane. However, deactivation cannot be avoided by carbon deposition. Hydrogen co-feed with methane is one of an effective method for suppression of coking. However, excess hydrogen may affect a reduction of Mo species during the reaction, and decrease of active Mo-oxycarbide species may give low reactivity. On the other hand, strong acid sites in external H-MFI surface possibly relates to the coking directly. Thus, selective covering of the coking site may be effective for durable activity [1]. Silylation of one of a unique method for selective coverage of external acid sites onto H-MFI. This study focused on the effect of silylation on methane dehydroaromatization over Mo/H-MFI catalysts. Mo LIII-edge XANES studies were introduced to characterize the active Mo species on bare and silylated H-MFI. Catalysts were prepared by impregnation of H-MFI (Si/Al2=67) synthesized hydrothermally with MoO2(acac)2 CHCl3 solution, and followed by drying overnight and calcination at 773 K. MoO3-loading amount is 5.0 wt% in whole catalysts. For silylation of Mo/H-MFI catalysts, heptane solution of teiethoxyvinylsilane (Mo/Si=10) was employed for impregnation, and followed by drying and calcination at 873 K. Mo LIII-edge XANES spectra were operated in BL1A of UVSOR-IMS in a top-up mode. InSb double crystal monochromator was used for the XANES measurements in a total-electron yield mode. The value of photon energy was calibrated by using Mo metal-foil at Mo LIII-edge, and normalized XANES spectra and their second derivatives were obtained for characterization. Catalytic activity was evaluated in a fixed bed flow reactor. Each catalyst (0.250 g) was placed in a quartz-tube reactor, and pretreated in He CO(0-2%) flow (30 mL min) at 973 K for 1 h. Then CH4(20%) H2(1%) He reactant gas was fed at 973 K (30 mL min: SV = 7.2 L g h). Products were analyzed by online GC. Figure 1 shows the Mo LIII-XANES spectra of silylated/unsilylated Mo/H-MFI catalysts after dehydroaromatization at 973 – 1073 K for 3 h. As shown in Fig. 1 (A-1 and B-1), According to a higher reaction temperature, the edge energy of XANES threshould becomes lower. It indicates the reduction and/or carbonization of MoOx species to MoOxCy ones. Excess reduction of Mo species brings about the deactivation of the catalysts because of excess carbon deposition [1, 2]. In case of silylated Mo/H-MFI [Fig. 1 (B-1) and (B-2)], the reduction degree of Mo species is almost as same as bare Mo/H-MFI [Fig. 1 (A-1) and (A-2)]. On these catalysts, high and durable activity for methane dehydroaromatization is significantly shown by silylation. The effect of silylation is possibly due to the depression of coking from methane, and the reduction of Mo species is almost independent. For Mo/H-MFI catalysts, formation of active MoOxCy species (partially carbonized MoOx) is one of a key role for high activity. And thus, deactivation of coking sites in H-MFI extrapores is very effective to obtain durable activity. In this study, it is suggested that silylation by using teiethoxyvinylsilane act as the depression of activity on the coking sites because coking amount becomes low. Detailed study about the silylation effect on Mo species is now in progress.