Spatial distribution of oxygen vacancy on ceria catalysts for chemoselective synthesis of lignin-derived cyclohexanol

Abstract

The synergy of metal/oxygen vacancy (O_v) pairs is critical in catalyzing activation of C–H, C=C, and C–O bonds. However, gaining fundamental understanding on spatial distance of metallic and O_v sites on catalyst surface would lead to unexpected chemoselectivity toward important and challenging reactions. In this work, we have proposed and validated unique Ni-O-Ce-O_v enriched Ni/CeO₂ catalysts prepared by a deposition-precipitation method, for the transfer hydrogenation of lignin-derived guaiacol toward cyclohexanol rather than benzene derivatives. The counter-intuitively designed high Ni loading Ni₂₀/CeO₂ catalyst (20 wt% Ni content) displays a distance of 0.5 nm for Ni/O_v pairs with a remarkable activity (TOF: 166.5 h⁻¹) and 90%+ selectivity for C_Ar=C_Ar bond saturation, outperforming better metal-dispersed Ni₅/CeO₂ catalyst with limited presence of Ni-O-Ce-O_v sites. The high hydrogenation activity against hydrogenolysis reactions on Ni₂₀/CeO₂ catalyst is attributed to tunable Ni/O_v distances, which constrain the cleavage of C_Ar–OH bond and deep deoxygenation. Such spatial distribution effect has also facilitated tandem dehydrogenation (O–H bond cleavage) and hydrogenation (C_Ar=C_Ar hydrogenation) reactions, leading to cyclohexanol as the target product in the absence of externally added H₂. Insights into spatial distribution of O_v sites open an alternative perspective in designing efficient catalysts toward producing value-added cyclic oxygenates through upgrading of lignin compounds.