Insight into the Effect of Dual Active Cu/Cu+Sites in a Cu/ZnO-Al2O3Catalyst on 5-Hydroxylmethylfurfural Hydrodeoxygenation

The production of biofuel 2,5-dimethylfuran (DMF) from an important biomass platform of 5-hydroxymethyl furfural (HMF) involves a complicated reaction pathway due to the competition of C=O hydrogenation and C-O hydrogenolysis; however, the dependence of formed intermediates and catalytic sites needs to be further identified. Herein, we constructed a highly efficient and selective Cu/ZnO-Al2O3 catalyst via reduction of a CuZnAl-layer double hydroxide (LDH) precursor, aimed for studying the effect of Cu/Cu+ species on forming different intermediates and their synergetic promotional effect on HMF hydrodeoxygenation. As expected, the dependence of formation of different intermediates on the Cu+/Cu ratio was observed; notably, Cu/ZnO-Al2O3 with higher Cu+/Cu ratio finally gave 90.1% DMF selectivity at 100% HMF conversion, more selective than CuNi/Al2O3 (59.6%) and Cu/MgO-Al2O3 (75.8%) references. By a combined study of in situ IR experiments, the observed correlation between the reaction intermediates and the Cu/Cu+ site suggests that Cu tends to adsorb the C=O bond as well as hydrogen molecule, while the Cu+ site is preferable to adsorb and activate the C-O bond. With the synergy of dual active sites, the Cu/ZnO-Al2O3 catalyst showed both high activity and selectivity to DMF. This work provides deep insight into the nature of active Cu species in multiple functional group biomass platform transformation.