Structural contributions of Zn in enhancing CO2 hydrogenation to methanol over ZnxZrOy catalysts†

Abstract

Single-reactor CO₂ conversion to light olefins via methanol is currently obstructed by the incompatible reaction conditions for the CO₂ to methanol and methanol to olefin steps. The conventional Cu/ZnO–Al₂O₃ CO₂ hydrogenation catalysts produce excessive CO and rapidly deactivate at the high temperatures preferred for methanol to olefins with zeolite or SAPO catalysts. Zn_xZrO_y catalysts are a promising alternative to Cu/ZnO–Al₂O₃. We studied Zn_xZrO_y with varying Zn doping levels, using XRD, XPS, H₂-TPR, CO₂-TPD, N₂-physisorption, DRIFT, and Raman spectroscopy, along with CO₂ conversion and methanol selectivity measurements, to examine structure-performance relationships in CO₂ hydrogenation to methanol. The interplay between dopant concentration, calcination temperature, and crystal structure dictates the catalyst's phase composition, which correlates with catalytic performance. The pristine ZrO₂ is a mixture of tetragonal and monoclinic phases. At Zn/Zr = 0.01, the tetragonal phase is dominant, while for Zn/Zr = 0.07–0.28, the cubic phase is obtained. Above Zn/Zr = 0.28, phase separation of ZnO occurs. For CO₂ hydrogenation to methanol, a Zn/Zr = 0.07–0.28 performs best. Zinc addition increases catalyst surface area, pore volume, basicity, and reducibility. XPS analysis reveals zinc enrichment near the surface and the formation of Zr–O–Zn species upon Zn incorporation into ZrO₂. A clear correlation between Zn content and catalyst activity is generally absent, but this relationship becomes evident in cubic-phase materials. At least in part, the relevance of zinc doping for CO₂ to methanol lies in its ability to distort the structure of zirconia, creating a cubic phase, with implications for selectivity that correlate with the adsorption of CO₂ and H₂.