Significant effect of Ca modification on improving catalytic stability of Cu-catalyst in gas-phase furfural hydrogenation to furfuralcohol

The gas-phase hydrogenation of furfural to furfuralcohol over Cr-free Cu-based catalysts has attracted increasing attention due to its environmentally friendly nature and mild operating conditions. Although reduced pure nano-sized CuO exhibits complete furfural hydrogenation and nearly 100% furfuralcohol selectivity, it suffers from rapid deactivation caused by sintering. In this study, we conducted comparative investigations on the catalytic performance and stability of two Cu-based catalysts: 90%CuO-10%SiO₂ and 90%CuO-5%CaO-5%SiO₂, in the gas-phase furfural hydrogenation. The reaction is carried out under various conditions, including temperatures ranging from 120 to 170 ℃, LHSVs of 1 to 2.2 h⁻¹, and H₂ to furfural molar ratios of 3.5 to 12.5. The results indicate that under optimal conditions, the Ca-modified catalyst achieves nearly complete furfural conversion and almost 100% furfuralcohol selectivity for a test duration of 31 h. In contrast, the unmodified catalyst exhibits stable performance for only seven hours despite the similar initial performance. XRD analysis confirms that the gradual deactivation of both catalysts is attributed to the oxidation of reduced metallic Cu sites to Cu oxides. Further characterizations of the two spent catalysts using HRTEM and XPS analyses, along with DFT calculations, suggest that the presence of Ca in Cu lattices prevents the loss of electrons from low-valence Cu sites or the reduced metallic Cu sites, thus inhibiting their oxidation to high-valence Cu oxides. This phenomenon contributes to suppressing the deactivation of Cu-catalysts in the gas-phase furfural hydrogenation process.