Conversion of ethanol to longer chain molecules such as butanol and butadiene occurs in a cascade of catalytic reactions that likely includes dehydrogenation, aldol condensation, hydrogen transfer, and dehydration. Therefore, efficient catalysts for ethanol-to-butanol or ethanol-to-butadiene are necessarily multifunctional in nature. Hydroxyapatite, or calcium hydroxyphosphate, is a well-recognized catalyst for the ethanol-to-butanol reaction, with high selectivity achieved at low conversion of ethanol. Attempts to shift the selectivity of hydroxyapatite from butanol to butadiene by adjusting the number of acid sites through changes in composition and thermal treatment were unsuccessful because the resulting Brønsted acid sites dehydrated ethanol. The amphoteric oxide ZrO2 was therefore explored next. In particular, Ag-promoted, silica-supported ZrO2 catalysts were studied in the ethanol-to-butadiene reaction. This metal-promoted system effectively produced butadiene at high levels of ethanol conversion, as long as the amounts of the Ag and ZrO2 components are appropriately balanced in the reactor. Extensive characterization of bulk and silica-supported ZrO2 (IR spectroscopy of probe molecules, Zr K-edge X-ray absorption spectroscopy, UV-vis spectroscopy, and various catalytic probe reactions) revealed important structural features of the silica-supported ZrO2 species that correlate with high performance in the ethanol-to-butadiene transformation. Comparisons to zeolite-supported ZrO2 will be discussed.

Cascade reactions of ethanol over metal phosphates and oxides

ACS Spring 2022

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