(618c) Monitoring the Catalytic Conversion of Methane to Benzene By Supported MoOx/ZSM-5 Catalysts With In Situ and Operando Molecular Spectroscopy

The catalytic conversion of methane to benzene represents a highly desirable route for conversion of natural gas into liquid fuels.^1-3 The supported MoO_x/ZSM-5 catalyst system has been reported to be the most promising catalyst system for methane dehydroaromatization (DHA) to aromatics (mostly benzene).^4-6 The structure of the Mo species present in the supported MoO_x/ZSM-5 catalysts and its catalytic properties for methane DHA have been the focus of many publications,but the nature of the supported Mo species and their anchoring sites on the ZSM-5 support (Si sites or Al sites) are still not resolved.^1-6 For the first time, this catalytic system has been investigated with in situ and operandomolecular spectroscopy during methane conversion to allow establishment of molecular structure-activity relationships that can guide the design of improved catalysts.

In the present study, supported Mo/ZSM-5 catalysts (Mo=0.67-3.33wt % and Si/Al=15-140) were examined with in situ FTIR (ZSM-5 surface hydroxyls), UV-vis (domain size of surface MoOx) and Raman (molecular structure) spectroscopy under dehydrated conditions and by operando Raman-MS spectroscopy during methane DHA. In situ FTIR indicates that the supported MoO_x species preferentially anchor at the surface Brønsted acid sites (3610 cm^-1) of the ZSM-5 support. In situ UV-vis and Raman reveal that the supported MoO_x phase is 100% dispersed on the ZSM-5 support as isolated dioxo (O=)₂MoO₂ and mono-oxo O=MoO₄ surface species. Operando Raman-MS result suggests that reduction of MoO_x is required but not the rate determining step for the reaction. The nature of reduced Mo species will be examined by in situ EXAFS and XANES. The methane DHA reaction pathway by the supported Mo/ZSM-5 catalysts proceeds by initial reduction of the surface MoO_x species that forms CO/CO₂ and transformation to carburized molybdenum species (Mo₂C, Mo₂C_1-x or MoO_xC_y). The intermediate [CH₂] species initially oligomerize to C₂H_x products that further oligomerize to form aromatic compounds. Carbonaceous deposits, however, also form during methane DHA that lead to catalyst deactivation by coking. These new fundamental insights provide strategies for the molecular design of improved supported MO_x/ZSM-5 catalysts for the methane DHA reaction.

(1)   Choudhary, T. V.; Aksoylu, E.; Goodman, D. W., Catal. Rev. 2003,45, 151-203

(2)   Burns, S.; Hargreaves, J. S. J.; Pal, P.; Parida, K. M.; Parija, S., J. Mol. Catal. A: Chem 2006,245, 141-146

(3)   Ismagilov, Z. R.; Matus, E. V.; Tsikoza, L. T., Energy Environ. Sci. 2008, 1, 526–541

(4)   Borry, R. W.; Kim, Y. H.; Huffsmith, A.; Reimer, J. A.; Iglesia, E., J. Phys. Chem. B 1999, 103, 5787–5796

(5)   Zheng, H.; Ma, D.; Bao, X.; Hu, J. Z.; Kwak, J. H.; Wang, Y.; Peden, C. H. J. Am. Chem. Soc. 2008, 130, 3722-3723

(6)   Kung, H. H.; Kung, M. C., Adv. Catal. 1985, 33, 159-19