Regulatory effects of water in two-phase protic ionic liquid-mediated catalytic conversion of non-edible lignocelluloses to biofuel precursors

Abstract

This work employs experiments and statistical modelling to characterize three water-regulated regimes (limited/sufficient/inhibited) in two-phase catalytic conversion of lignocelluloses to biofuel precursors. CuCl2-catalyzed conversion of two non-edible lignocelluloses (June grass, Sunn hemp) is performed for 24 h using a protic ionic liquid (Triethylammonium hydrogen sulfate) at 110 °C, wherein the temporal dynamics of glucose, 5-Hydroxymethylfurfural (5-HMF), Levulinic acid (LA), and Formic acid (FA) are measured by varying the water-addition. Water, being a substrate in cellulose hydrolysis to glucose (isomerized to fructose), and an inhibitor in fructose dehydration to 5-HMF, regulates product yields through interactions between transport (heat conduction and viscosity) and reaction (non-linear kinetics and activation energy). The yields of glucose and 5-HMF peak at 6 h to 60.2% and 10.2%, respectively, from Sunn hemp, and at 4 h to 62.7% and 12.8%, respectively, from June grass, while those of LA and FA peak at 8 h to 23% and 9.1%, respectively, from Sunn hemp, and at 5 h from 19.3% to 7.7%, respectively, from June grass. The protic ionic liquid is recovered (91.4%) through biphasic separation, glucose is separated (75%) using an anti-solvent, 5-HMF is separated (54%) by liquid-liquid extraction, and LA (58%) and FA (42%) are separated by reactive extraction. The regulatory effects of water on cellulose hydrolysis from Sunn hemp and June grass are quantified (R2 > 0.92) by a linear superposition of two Gaussian distributions representing ‘ignited’ and ‘extinguished’ reaction states that delineate the three water-regulated regimes, and allow us to quantify the water-sufficient regime for maximizing product yields.