Understanding the impact of ionic liquid pretreatment on cellulose and lignin via thermochemical analysis

•IL pretreatment depolymerizes biomass lignocellulosic.•Extent of depolymerization depends on biomass, pretreatment time and temperature.•Temperature dependent crystallinity changes is observed.•The crystalline material formed at 160 °C undergoes exothermic decomposition.•Resultant lignin in pretreated biomass has a higher calorific value.

To date, there are few published reports on the fundamental physicochemical changes that occur in the biopolymers comprising lignocellulosic biomass during IL pretreatment. In particular changes to the degree of polymerization upon IL pretreatment although assumed have not been reported. In this study, we employ thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) to determine the impact of IL pretreatment on two of the primary biopolymers present in biomass – lignin and cellulose. IL pretreatment was carried out on technical biomass components (Avicel, Kraft lignin and low sulfonate alkali lignin) and on potential bioenergy feedstocks (switchgrass, pine and eucalyptus) at 120 and 160 °C for 1, 3, 6 and 12 h. The depolymerization and formation of lower molecular weight cellulose as a result of IL pretreatment were inferred from TGA/DSC results and were further confirmed by size exclusion chromatography (SEC). The results also confirm that there is a decrease in cellulose crystallinity in the biomass as a result of pretreatment. These results provide insight into the mode of depolymerization and breakdown of lignin and cellulose.