Ethane dehydrogenation over Cr/ZSM-5: characterization of active sites through probe molecule adsorption FTIR†
Dispersed Cr species supported on zeolite ZSM-5 were investigated in the context of catalytic ethane dehydrogenation. Probe molecule FTIR and XANES were used to characterize the nature of Cr sites across a range of Cr loadings and in the presence of various extra-framework cations on ZSM-5. These characteristics were related to catalytic performance in ethane dehydrogenation toward the goal of developing design principles for Cr/zeolite catalysts for improved alkane conversions. Diffuse reflectance infrared Fourier transform spectroscopy (FTIR-DRIFTS) with nitric oxide (NO) and carbon monoxide (CO) probe molecules was determined to be an effective tool for distinguishing various types of Cr species present after exposure to reducing conditions typical of alkane dehydrogenation catalysis. It was found that 0.5 wt% Cr/H-ZSM-5 is a stable, ethylene-selective catalyst whose Cr sites are reduced at reaction temperature (650 °C) in the presence of either ethane or N2. Zeolite framework Al sites were determined to be critical for the stabilization of active Cr sites during ethane dehydrogenation – in the absence of available Al sites, Cr sites quickly deactivated. It was also found that greater Cr/Al molar ratios yielded a greater abundance of electron-rich Cr2+ species after reduction. This trend correlated with increased ethane dehydrogenation activity and decreased stability in time-on-stream studies. Both high activity and enhanced stability were observed for Cr/Ca-ZSM-5. Taken together, these results indicate that ethane dehydrogenation over Cr/ZSM-5 is optimized when Cr preferentially situates adjacent to isolated framework Al sites.