Efficient synthesis of lactic acid from cellulose through the synergistic effect of zinc chloride hydrate and metal salts

Abstract

The chemocatalystic conversion of cellulose, the main component of lignocellulosic biomass, to building-block chemicals in water under mild conditions, is an ideal but highly challenging process due to the robust crystal structure of cellulose. It is also the key to establishing a sustainable biomass-based chemical process. Here, we present a highly efficient and selective chemocatalytic hydrolysis of cellulose using ZnCl2·3H2O hydrate as the pretreatment reagent and water-compatible metal salts - ErCl3 as the catalyst, into lactic acid (LA), which is an important chemical building-block widely utilized in the food industry and in the production of chemicals and biodegradable plastic. With 94.0 % conversion of cellulose, an impressive LA yield of 84.6 % was achieved at 170 °C after 4 h under ambient air pressure in water. High yields of LA were also obtained from other carbohydrates, such as fructose (68.3 %), glucose (52.7 %), starch (54.4 %), and inulin (67 %). A series of experiments demonstrated that Er(III) combination with water catalyzed cascading steps of soluble cellulose into LA after ZnCl2·3H2O hydrate disrupted the hydrogen bonds in the cellulose, Zn(II) played an indirect role by promoting LA formation through inhibition of side reactions. A plausible mechanism was proposed for the chemocatalytic conversion of cellulose to LA.