(175d) Stability and Reactivity of Re- and Mo- Zsm5 Catalysts for Ch4 and C3h8 Conversion

Carbides of Mo and W within medium pore zeolites are highly selective (~ 80% benzene Sel.) and stable catalysts for the non-oxidative conversion of CH₄. The materials resist agglomeration at CH₄ reaction temperatures (~ 1000 K) due to the high melting points (2960 K and 3140 K for Mo₂C and WC) and low vapor pressures of carbides. Catalysts based on metal clusters instead of carbides with similar reactivity and selectivity can be prepared with Rhenium (using Re₂O₇ as a precursor) which has a higher melting point (3450 K) than Mo₂C and WC.

Near edge X-ray absorption and fine structure spectra confirm the formation of 4-coordinate Re-oxo species after thermal treatment of Re₂O₇/H-ZSM5 in air, and complete reduction in H₂ to Re metal occurs at 723 K. Simulations of the X-ray absorption fine structure of Re⁰-ZSM5 in H₂ at 723 K or CH₄ at 950 K indicated the presence of 0.82 nm Re metal clusters (undetected by XRD) below Re/Al_f of 0.4 similar in dimensions to the channel intersection diameter in H-ZSM5. Higher Re content led to the formation of large Re clusters ~ 10 nm detectable by XRD. CH₄ pyrolysis and C₃H₈ dehydrocyclodimerization were studied on Re⁰-ZSM5. Benzene forward rates in 91 kPa CH₄ at 950 K were 4.1 x 10^-3 mol g-atom Re^-1 s^-1, over 30% higher than Mo-ZSM5 at 3.0 x 10^-3 mol g-atom Mo^-1 s^-1 with similar selectivity to C₂H₄ and C₆₊ arenes. C₃H₈ turnover rates at 773 K in 20 kPa C₃H₈ were 170 x 10^-3 mol C₃H₈ g-atom Re^-1 s^-1 which were more selective to C₃₊ hydrocarbons and more active than those for Ga-ZSM5, the best literature catalyst, at 93 x 10^-3 mol C₃H₈ g-atom Ga^-1 s^-1. First order rate constants for dehydrogenation, cylcization, and cracking were calculated, and it was found that Re-ZSM5 had a dehydrogenation rate constant that was 150% higher than Ga-ZSM5, but both catalysts had similar cyclization rates since the latter reaction occurs over acid sites.

Methods that improve the stability of the above materials were also studied. The presence of a co-reactant, such as CO₂, has been shown by Ichikawa, et al.¹ to improve catalyst stability during non-oxidative CH₄ conversion. We used transient kinetic studies and in situ spectroscopy to determine the nature of active sites during exposure to CH₄ with CO₂ and proposed a rational mechanism for the observed reaction products.² There are two reactor zones in plug flow conditions for CO₂-CH₄ mixtures: a reforming region at the front of the reactor where CH₄ is reacted exclusively with CO₂ to form synthesis gas, and a pyrolysis region where CH₄ reacts in the absence of CO₂ to form primarily C₂H₄, C₂H₆, and aromatic C_12- products. The pyrolysis region only occurs after complete CO₂ conversion. Moreover, there is less observed deactivation in the region then in a reactor without CO₂ cofeed; the improved catalyst stability is attributed to the presence of H₂ formed in the reforming region and was confirmed by cofeeding H₂ with CH₄.

1. R. Ohnishi, S. Liu, Q. Dong, L. Wang, and M. Ichikawa J. Catal. 182 (1999) 92.

2. H. Lacheen and E. Iglesia J. Catal. 230 (2005) 173.