(344c) Evaluation of Chromium Sites in Cr/ZSM-5 for Ethane Dehydrogenation

Zeolite-supported metal catalysts present opportunities to gain fundamental understanding of industrially relevant catalysts as well as to develop catalysts with new or optimized properties. Zeolite surfaces provide precise binding sites for supported metals, resulting in active sites that are amenable to precise characterization. Probe molecule Fourier-transform infrared spectroscopy (FTIR) is a powerful tool to provide information about the electronic and coordinative states of dispersed metals on zeolite surfaces. FTIR with nitric oxide (NO) and carbon monoxide (CO) probe molecules was effective in distinguishing various types of reduced Cr species present in Cr/H-ZSM-5 (Si/Al 15) samples with varying Cr loading (0.27-1.6 wt%). Characterization by FTIR and X-ray absorption near edge structure spectroscopy (XANES) was related to catalytic performance in ethane dehydrogenation to provide insight into design principles for improved Cr/zeolite catalysts for ethane conversion. It was found that in 0.5 wt% Cr/H-ZSM-5, reduction of Cr occurred under N₂ flow at dehydrogenation reaction temperature (650 ^oC), and that exposure to ethane at this temperature provoked no further change in the state of Cr. Correspondingly, this catalyst displayed stable ethane dehydrogenation activity under reaction conditions. The presence of zeolite framework Al sites was necessary to stabilize active Cr sites during ethane dehydrogenation. In the absence of available Al sites, Cr sites deactivated quickly. With increasing Cr/Al molar ratio, greater abundance of electron-rich Cr²⁺ species existed after reduction. The resulting catalysts were more active for ethane dehydrogenation but activity was less stable with time on stream. The accompanying Figure compares ethane dehydrogenation activity and NO-DRIFTS spectra for Cr/M-ZSM-5 samples differing in exchanged extra-framework cation (M = H⁺, Na⁺, Ca²⁺). The results suggest that not only the availability, but also the proximity to one another, of framework Al sites influences the state of reduced Cr and the activity of Cr/ZSM-5 catalysts.