Structure-Activity Relationships for Ethanol Dehydrogenation to Acetaldehyde by Silica-Supported Zinc Oxide Catalysts

Silica-supported ZnO efficiently catalyzes the nonoxidative dehydrogenation of ethanol to acetaldehyde, which is relevant for production of 1,3-butadiene from bioethanol. Characterization with in situ spectroscopies under dehydrated conditions (high sensitivity-low energy ion scattering (HS-LEIS), diffuse reflectance (DR) UV-vis, X-ray absorption spectroscopy (XAS), diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS), inelastic neutron scattering (INS), and UV Raman), and ammonia adsorption probed by temperature-programmed desorption followed by DRIFTS and mass spectrometry (DRIFTS-MS NH₃-TPD), and DFT calculations revealed that the supported ZnO _x phase was present as isolated surface ZnO _x sites on SiO₂, with the vast majority coordinated by two siloxane bonds and one silicon atom with two nonbridging oxygens ((SiO)₂Zn²⁺O₂Si=), anchored at 4-, 5-, and 6-membered siloxane rings. A minor fraction of surface ZnO _x sites possessed Lewis acidity, and even fewer sites possessed a Bro̷nsted acidic Zn-(OH)⁺Si moiety. Ethanol temperature-programmed surface reaction-mass spectrometry (TPSR-MS) with various oxidative or ethanol reaction pretreatments indicated that only sites with Lewis and Bro̷nsted acidic character (Zn-(OH)⁺Si) were active for ethanol dehydrogenation, while the majority surface (SiO)₂Zn²⁺O₂Si= sites were inactive. Greater heterogeneity among all surface ZnO _x sites, as assessed by in situ DR UV-vis spectroscopy, was associated with a greater number of ZnO _x sites that were active for ethanol dehydrogenation as well as lower enthalpic barriers for acetaldehyde production among the most active surface ZnO _x sites. Turnover frequencies and the apparent activation energy for ethanol dehydrogenation were determined from steady-state kinetics. Together, these findings suggested that anchoring inactive surface (SiO)₂Zn²⁺O₂Si= sites on the silica support caused a greater number of active surface ZnO _x sites to adopt a more strained configuration, promoting ethanol dehydrogenation catalysis. Pretreatments and catalysts that promoted desorption of ethanol during TPSR, taken as a marker of surface dehydroxylation, were associated with an increased number of the most active surface (Zn-(OH)⁺Si) sites. Such findings suggested that inactive surface ZnO _x sites were activated for ethanol dehydrogenation by dehydroxylation of the support and/or decreased coordination to hemilabile siloxane ligands.