Ozone as Oxidant for Biomass Pretreatment and Nanocellulose Production

摘要

Ozone gas is a robust and easy to generate oxidant with proven efficacy in increasing enzymatic digestibility of lignocellulosic material. The motivation behind this study was a) elucidate the manner in which a packed column simulating a bale of biomass is delignified by ozone treatment b) develop a method to modify cellulose surfaces using ozone for production of value added byproducts from pulp. This information will be of value in determining ozone's potential as a versatile and portable oxidant for cellulosic ethanol and value added by-product production. Pretreatment of compacted switchgrass with ozone was carried out in a packed bed reactor. The material density and particle size was similar to that in a bale of biomass. Pretreatment was conducted using low ozone concentrations feasible on a commercial scale. Kinetic analysis demonstrated that lower ozone concentrations (3 L/min) are necessary to achieve a consistent increase in digestibility. A 58% increase in enzyme digestibility of the cellulose in switchgrass was achieved after treatment. Ozone transport in the reactor was modeled using combined reaction, diffusion and convection. The low effective reaction rate of 6.5e-4 s-1 and convective flow in the reactor were found to be the limiting factors. While ozone gas is an efficient oxidizing agent, ozone alone is relatively ineffective in oxidizing cellulose surfaces. The second study is motivated by the knowledge that radicals, such as hydroxyls, formed as a result of ozone reaction with the moisture in the biomass, are the main oxidants reacting with lignin. We demonstrate that lignin monomers formed as byproducts of pulping or bioprocessing of lignocellulosic biomass are an effective enhancer of ozone for oxidizing cellulose surfaces. This is demonstrated in the production of cellulose nanofibers (CNF) since the CNF films made by this method have carboxylate content similar to conventional, commercially carboxylated CNF prepared by TEMPO-mediated oxidation.