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

Silica-supported ZnO efficiently catalyzes the non-oxidative dehydrogenation of ethanol to acetaldehyde, which is relevant for production of 1,3-butadiene from sustainable 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 temperature-programmed desorption followed by DRIFTS and mass spectrometry (DRIFTS-MS NH3-TPD) revealed that the supported ZnOx phase was present as isolated surface ZnOx sites on SiO2, with the vast majority coordinated by two siloxane bonds and one silicon atom with two non-bridging oxygens ((≡SiO)2Zn2+O2Si=), anchored at 4, 5, and 6-membered siloxane rings. A minor fraction of surface ZnOx sites possessed Lewis acidity and even fewer sites possessed a Brønsted 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 Brønsted 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 ZnOx sites, as assessed by in situ DR UV-vis spectroscopy, was associated with a greater number of ZnOx sites that were active for ethanol dehydrogenation as well as lower enthalpic barriers for acetaldehyde production amongst the most active surface ZnOx 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 ZnOx 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 an increased number of the most active surface (Zn(OH)+Si) sites. Such findings suggested that inactive surface ZnOx sites were activated for ethanol dehydrogenation by dehydroxylation of the support and/or decreased coordination to hemilabile siloxane ligands.