Selective Fragmentation of Lignocellulosic Biomass with ZnCl2·4H2O Using a Dissolution/Precipitation Method

Lignocellulosic biomass dissolution in an inorganic salt hydrate (ZnCl2·4H2O) and its subsequent precipitation with water for the separation of its main compounds were investigated. For this purpose, different dissolution times and temperatures were studied, where 24 h and 70 °C were found to be the optimal choice. Three solids were obtained, which were analyzed and identified by XRD, SEM, NMR, and FTIR spectroscopy. Solid I is the undissolved part of the starting material, and it consists of lignin, which does not react with the inorganic salt hydrate and the unreacted cellulose. Solid II is a cellulose-rich solid with a low portion of hemicellulose and lignin, and Solid III is mainly pure lignin as the characterization results showed. Hemicellulose is mainly dissolved and hydrolyzed in the dissolution treatment and the amount present in all solids was very small. The reactivity of Solid I and Solid II in a hydrolysis reaction was tested (0.2 M/L H2SO4, 5 h, and 140 °C), where a significant improvement in the conversion and the yield of sugars was obtained with respect to the untreated samples in both cases. Solid II yields a large amount of total reducing sugars, with a % selectivity of 78–88%, depending on the starting biomass.