Enzymatic Hydrolysis of Switchgrass and Tall Wheatgrass Mixtures Using Dilute Sulfuric Acid and Aqueous Ammonia Pretreatments

Abstract

Combinations of biomass feedstocks such as perennial grasses or agricultural residues need to be evaluated to ensure maximum flexibility and the long-term viability of the cellulosic ethanol industry. This article reports the enzymatic hydrolysis of solids resulting from dilute sulfuric acid (H2SO4) and aqueous ammonia (NH3OH) pretreatments of switchgrass, tall wheatgrass, and their mixtures. To determine the effectiveness of pretreatment and enzymatic hydrolysis on mixtures of two different feedstocks, switchgrass and tall wheatgrass were used individually and mixed at 1:2 and 2:1 ratios (w/w, dry basis). These four different feedstock combinations were pretreated with either H2SO4 (1.5%, 140°C, 20 min) at a solid loading rate (SLR) of 1:10 (w/v) or soaked in NH3OH (15% w/v, 60°C, 24 h) at SLR of 1:6. The pretreated and untreated (control) samples were subjected to enzymatic hydrolysis by adding cellulase (NS50013, 25 FPU g-1 glucan) and s-glucosidase (Novozyme 188, 50 CBU g-1 glucan) at an initial glucan loading of 1% (w/v). Delignification of 40% and 57% to 60%, respectively, was achieved with all materials during acid and aqueous ammonia pretreatments. Glucose yields of 84% to 98% and 70%, respectively, were achieved from enzymatic hydrolysis of all acid-pretreated and alkali-pretreated samples. Our results suggest that feedstock mixtures can be processed as readily as individual feedstocks, producing glucose yields comparable to those of the individual feedstocks. The results also demonstrate that wheatgrass has a similar composition to switchgrass and has potential as a feedstock for ethanol production.