Alkaline Pretreatment of Switchgrass

Alkaline pretreatment using sodium hydroxide offers a means to extract lignin and acetate from lignocellulosic biomass, in turn enabling higher enzymatic digestibility of the remaining polysaccharides and production of a lignin-enriched stream for potential valorization. Key criteria for alkaline pretreatment processes, which are important for commercial feasibility, include the minimization of carbohydrate loss during pretreatment and the ability to capture carbon lost to the liquor stream, much of which will be feedstock dependent. Here, we present a comprehensive study of alkaline pretreatment of switchgrass over NaOH loadings from 35 to 140 mg NaOH/g dry switchgrass and with a constant charge of 0.2% anthraquinone for pretreatment temperatures between 100 and 160 °C for 30 min. Full compositional analysis of the pretreated solids are reported as a function of pretreatment severity, along with the yields of each biomass component present in the process streams generated during pretreatment (pretreated solid, liquor, and wash fraction). The pretreated solids are further characterized through crystallinity measurements and electron microscopy. Additionally, enzymatic digestions of the residual solids are performed over a range of enzyme loadings for varying pretreatment severities. These results are compared to our recent work with alkaline pretreatment of corn stover using the ratio of lignin fractionation to carbohydrate retention (in the solids after pretreatment), which highlights the greater recalcitrance of switchgrass relative to corn stover. Specifically, compared to corn stover, switchgrass requires approximately twice the NaOH loading to achieve identical delignification and high enzymatic digestibility. From this work, the optimal pretreatment conditions for switchgrass are suggested to be 154 mg NaOH/g dry switchgrass at 130 °C for 30 min at temperature. The results from these bench-scale experiments will serve as a guide to scale up processes for the optimization of lignin removal while minimizing carbohydrate loss during alkaline pretreatment.