Article link: Green Chem. 2018, 20, 735-744.
Catalytic transformation of non-edible cellulose into valuable chemicals such as lactic acid under mild conditions represents a promising route for utilization of renewable biomass. However, efficient catalytic systems for this reaction are rare. Moreover, some aspects of reaction mechanism are still unclear, limiting the rational design of highly selective catalysts. In this article, we report that the combination of Al(III) and Sn(II) cations can efficiently catalyze the conversion of cellulose and related carbohydrates into lactic acid in water. Al(III) and Sn(II) with a molar ratio of 1/1 work cooperatively, providing lactic acid with yields of 90%, 81% and 65% in the conversions of fructose, glucose and cellulose, respectively. The formation of lactic acid involves a series of tandem steps including the hydrolysis of cellulose to glucose, the isomerization of glucose to fructose, the retroaldol fragmentation of fructose to C3 intermediates and the subsequent conversion of the C3 intermediates to lactic acid. Our experimental and computational studies suggest that Al(III) mainly catalyses the isomerization of glucose or the C3 intermediates, whereas Sn(II) is primarily responsible for the retro-aldol fragmentation. The combination of the two cations enables the reaction to proceed smoothly with few side reactions, providing outstanding catalytic performances for lactic acid production from cellulose or the related carbohydrates.
The work is a continuation of our previous work published in Nature Communications in 2013, which reported the catalytic transformation of cellulose into lactic acid in water by Pb(II). The experiments were mainly conducted by Dr. Weiping Deng, Pan Wang, and Dr. Yanliang Wang. The paper was a result of collaboration with Dr. Binju Wang and Prof. Zexing Cao, who performed DFT calculations to understand the energy barriers of different fundamental steps. This work was financially supported by the National Natural Science Foundation of China (21690082, 91545203 and 21473141), the Research Fund for the Doctorial Program of Higher Education (20130121130001), the Fundamental Research Funds for the Central Universities (20720160029) and the Program for Innovative Research Team in University (IRT_14R31).
