PNNL

Abstract

Yttrium-modified dealuminated beta-zeolite (Y-BEA) represents a type of Lewis acid zeolite that has gained attention for its potential to efficiently catalyze the conversion of biomass-derived oxygenates. The structure of the Y active sites and their dynamics during biomass conversion reactions, which normally involve substantial amounts of water, necessitates thorough investigation for the rational design of more active and stable catalysts. Here, we conducted a study where a series of Y-BEA catalysts with different yttrium loadings (1-7 wt.%) were subjected to hydrothermal treatment (450 °C, 20% water) and investigated for its structural and catalytic activity change through a combination of multiple characterizations and kinetic measurements. The acid sites of Y-BEA decreased in number without a change in acid strength following the hydrothermal treatment, which was confirmed by the results of acid site titration, infrared spectroscopy of probe molecules, and kinetic measurements for probe reactions (acetone aldol condensation). Structural analysis using X-ray diffraction (XRD), specific surface area measurement, X-ray absorption spectroscopy (XAS), and X-ray photoelectron spectroscopy (XPS) demonstrated that both the zeolite structure and the isolation status of the Y site remain intact after the hydrothermal treatment. Further, the Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) spectra, thermogravimetric analysis (TGA), and operando 1H and 29Si magic angle spinning (MAS) nuclear magnetic resonance (NMR) revealed the dehydroxylation of Y-BEA induced by hydration-rearrangement-condensation restructuring during the high-temperature steam treatment. Dehydroxylation impacts the structure of Y sites by reducing their vicinal silanol sites. This conversion of Lewis acidic Y sites into non-acidic sites is the primary factor behind the change in acid site quantity and catalytic activity on Y-BEA.