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

Authors: 
Lacheen, H. S., University of California, Berkeley
Iglesia, E., University of California at Berkeley
Cordeiro, P. J., University of California, Berkeley


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/Alf 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 C12- 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.