Pure Hydrolyzable Cellulose from Rice Straw, Wheat Straw and Sugarcane Bagasse by a Simple Scalable Two-Step Treatment

Abstract

Utilization of amply available lignocellulosic biomass for a cost-effective conversion to renewable chemicals has proven more difficult than anticipated. Sustainable and viable fractionation of any biomass to its individual monomeric components for their further conversion to products at commercial scale therefore remains elusive. A rapid and scalable multi-step pretreatment strategy for fractionation of rice straw using a combination of dilute aqueous acid and aqueous alkali treatment steps under subcritical conditions was investigated. The process steps and parameters were optimized for yield and purity of the resulting biomass components. Effects of acid and alkali concentrations on the fractionation efficiency were studied in the range of 0.2% to 12% w/v at temperatures ranging from 110°C to 200°C for time spanning from 15 to 30 min. The simple optimum sequence of operations and conditions was found to be a diluteacid hydrolysis step at 130°C for 15 min with 2% HNO₃ followed by the second treatment step at 130°C for 15 min with 2% NaOH. This combination gave 90% pure cellulose in more than 80% overall yield. Formation of furfurals in the hydrolysate was prevented significantly, and the cellulose obtained showed good amenability for enzymatic hydrolysis to sugars. The same process was applied to wheat straw and sugarcane bagasse, and the obtained results were found to be similar to those obtained for rice straw. The process was successfully scaled up to 50 L batch process with negligible deviations from smaller scale run results.