Tuning the crystallite size of monoclinic ZrO2 to reveal critical roles of surface defects on m–ZrO2 catalyst for direct synthesis of isobutene from syngas

Abstract

• A series of monoclinic ZrO 2 catalysts with different crystalline size was prepared. • ZrO 2 with small crystalline size owned higher activity and isobutene selectivity. • Higher concentrations of surface defects on ZrO 2 was vital for isobutene synthesis. The effects of crystallite size on the physicochemical properties and surface defects of pure monoclinic ZrO 2 catalysts for isobutene synthesis were studied. We prepared a series of monoclinic ZrO 2 catalysts with different crystallite size by changing calcination temperature and evaluated their catalytic performance for isobutene synthesis from syngas. ZrO 2 with small crystalline size showed higher CO conversion and isobutene selectivity, while samples with large crystalline size preferred to form dimethyl ether (DME) instead of hydrocarbons, much less to isobutene. Oxygen defects (O Defects ) analyzed by X-ray photoelectron spectroscopy (XPS) provided evidence that more O Defects occupied on the surface of ZrO 2 catalysts with smaller crystalline size. Electron paramagnetic resonance (EPR) and ultraviolet–visible diffuse reflectance (UV–vis DRS) confirmed the presence of high concentration of surface defects and Zr 3+ on m -ZrO 2 -5.9 sample, respectively. In situ diffuse reflectance infrared Fourier transform spectroscopy ( in situ DRIFTS) analysis indicated that the adsorption strength of formed formate species on catalyst reduced as the crystalline size decreased. These results suggested that surface defects were responsible for CO activation and further influenced the adsorption strength of surface species, and thus the products distribution changed. This study provides an in-depth insight for active sites regulation of ZrO 2 catalyst in CO hydrogenation reaction.