Selectivity Control for Cellulose to Diols: Dancing on Eggs

Catalytic conversion of cellulose to ethylene glycol (EG) or 1,2 propylene glycol (1,2-PG) represents an attractive approach in the valorization of biomass, due to the high atom economy of the reaction process and large market demand of the diol products. The one-pot catalytic conversion of cellulose is a complex reaction network, comprising hydrolysis, retro-aldol condensation, hydrogenation, isomerization, dehydrogenation, thermal side reactions, etc. In addition to EG and 1,2-PG, a variety of byproducts such as sorbitol, mannitol, erythritol, 1,2butanediol, and glycerol may be coproduced. The key point for obtaining high selectivity for EG or 1,2-PG lies in effective control of the major reaction steps in the reaction network proceeding at matching rates. In this Perspective, we depict the general reaction route for glycol production from cellulose and summarize the active elements for the retro-aldol condensation reaction, which is the determinant step for the formation of C-2 and C-3 intermediates. In situ or operando methods for the catalyst characterization are discussed. Then the reaction kinetics for one representative example, i.e. the tungstenic catalyst, is summarized briefly and approaches to control the product selectivity are suggested. After an overview of the progress and challenges in catalytic conversion of lignocellulose for applications, we present an outlook for cellulose conversion to diols from the aspects of catalyst development, reaction mechanism study, and practical applications.