(625e) Understanding the Role of Framework Lewis Acid Sites in Rare Earth Element Containing Zeolites for Ethanol Upgrading Reactions.

Rare earth elements (REEs) dispersed in dealuminated Beta (deAlBeta) zeolites catalyze C-C coupling during ethanol upgrading to C₃₊ olefins over Cu-(Zn)-REE/deAlBeta catalysts. The proposed reaction pathway involves ethanol dehydrogenation to acetaldehyde (over Cu) and acetaldehyde self-aldol condensation to crotonaldehyde on REE sites. Under H₂-rich conditions, crotonaldehyde leads to butene via sequential hydrogenation (over Cu) and dehydration steps. In inert atmosphere the predominant product is butadiene formed via Meerwein-Pondorff-Verley (MPV) reduction of crotonaldehyde to crotyl alcohol (CA) and subsequent dehydration, each of which occurs on REE sites. For example, C₃₊ olefin yields were 88% at 100% ethanol conversion over Y-containing catalysts (e.g., CuZnY/deAlBeta; 7 kPa ethanol in balance H₂), and the rate of C-C coupling increased with Y loading for CuZnY/deAlBeta samples with Si/Y ratios ranging from 19-150. These findings motivated further kinetic and spectroscopic studies of monometallic REE/deAlBeta catalysts. Lewis acidic Y or La sites responsible for butadiene formation were quantified via ex situ probe molecule FTIR and in situ titration with pyridine during ethanol-acetaldehyde reaction. Across a series of Si/Y and Si/La ratios, La/deAlBeta samples had fewer such sites relative to Y/deAlBeta. The selectivities of crotonaldehyde, CA, and butadiene formation were distinct for Y- and La/deAlBeta. Y‑containing sites that form crotonaldehyde and CA are distinct from those that catalyze CA dehydration, based on in situ titration experiments and the deactivation profiles for each product. By varying residence time, we evince that crotonaldehyde and CA are intermediates enroute to butadiene. Using perdeuterated ethanol and acetaldehyde and ¹³C-labeled ethanol reagents we probed the various reaction fluxes occurring over these REE materials, and measured ratios of site time yields between reactions with deuterated and hydrogenated reactants. Taken together, these findings provide insight into the similarities and differences in the kinetics and active site requirements for ethanol conversion over Y- and La‑containing deAlBeta.