Improved production of methyl levulinate from catalytic conversion of cellulose over cobalt sulfide by nickel doping

Transition metal sulfides have rich Bronsted/Lewis's acid sites and various metal valence states, which have great influence on their catalytic performance. In this study, the nickel cobalt sulfides with different molar ratios of Ni to Co were synthesized by one-step solvothermal method for the highly efficient catalytic conversion of cellulose into methyl levulinate (ML). Interestingly, with the continuous incorporation of Ni, the catalytic efficiency of the catalysts increased first and then decreased. It was worth mentioning that when the ratio of Co/Ni is 1, the ML yield was the highest (39.6 wt%), much higher than CoS2 and NiS2 (30.5 and 30.1 wt%, respectively). Based on the characterization of the prepared catalysts, it was found that the improved catalytic performance of Ni0.50Co0.50S2 was potentially due to the synergistic effect of lower Co3+/Co2+ ratio and larger Ni3+/Ni2+ ratio. Furthermore, the Ni0.50Co0.50S2 catalyst had a great recycling performance, which still maintained 78% of the initial catalytic efficiency after reused for 5 times.