Selective cleavage of lignin-derived diphenyl ether C-O bond over weakly acidic Ni/Nb2O5 catalyst

Abstract

Selective cleavage of stable ether C-O bonds is particularly of significance for the valorization of lignin and designing high active and cheap catalysts attracts researchers to conduct a lot of investigations. The niobium species containing both Brønsted and Lewis acid sites can be applied in hydrodeoxygenation of diphenyl ether (DPE). It is important to regulate the acidity of niobium to achieve the selective cleavage of DPE without hydrodeoxygenation. Herein, the weakly acidic Ni/Nb2O5 prepared by an incipient wetness impregnation method could efficiently cleave C-O bonds of DPE without hydrodeoxygenation. DPE was completely converted and the yields of target products (cyclohexane and cyclohexanol) reached 90% under the coexistence of H2 and isopropanol. It was ascribed to the common hydrogen supply between H2 and isopropanol which could produce more active hydrogen. Acetone was detected in the solution after the reaction, demonstrating that a dehydrogenation reaction of isopropanol provided hydrogen. In addition, the turnover frequency (TOF) and the apparent activation energy (Ea) for catalytic hydrogenolysis of DPE revealed that Ni/Nb2O5 possessed higher activity than Co/Nb2O5 under the same conditions. The possible reaction pathways were also summarized and analyzed.