Biomass Pretreatment using Ionic Liquid and Glycerol Mixtures
AuthorLynam, Joan Goerss
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Lignocellulosic biomass is a renewable, sustainable resource that can replace or supplement fossil fuels use for liquid fuels and chemicals. However, its recalcitrant structure including interwoven cellulose, hemicelluloses, and lignin biomacromolecules is challenging to deconstruct. Pretreating biomass so that it can be converted to useful liquids dominates process economics. Many pretreatment methods exist, but most require hazardous chemicals or processing conditions. Many ionic liquids (ILs), salts molten below 100 °C, can be used to deconstruct lignocellulosic biomass and are less hazardous than the volatile organic compounds typically used.While effective, relatively safe, and recyclable, ILs are expensive. To reduce costs, dilution with other safe compounds is desirable, if there is no impact on deconstruction efficiency. Glycerol, a food additive, is inexpensive and becoming even more so since it is a by-product of the burgeoning biodiesel industry. Use of glycerol as an additive or diluent for ILs is extensively evaluated in this work. Rice hulls are an abundant biomass, with over 100 million tons produced per year, but with little practical use. The IL 1-ethyl-3-methylimidazolium formate ([C2mim][O2CH] or EMIM Form) when mixed with an equal amount of glycerol has been shown to be effective in pretreating rice hulls. Ambient pressure, a pretreatment temperature of 110 °C, and a reaction time of three hours produced rice hulls that could be enzymatically hydrolyzed to give reasonably good glucose and xylose yields considering the recalcitrance of this silica-armored biomass. The IL [C2mim][O2CH] was also effective when mixed with an equal amount of glycerol to pretreat loblolly pine, a fast-growing softwood. Loblolly pine was pretreated at 140 °C for three hours to produce a solid rich in cellulose and hemicelluloses, while a lignin-rich product could be precipitated from the IL. Similar products were obtained from pretreatment with a mixture of 75% 1-ethyl-3-methylimidazolium acetate ([C2mim][OAc] or EMIM Ac) and 25% glycerol. Enzymatic hydrolysis of the pretreated solids gave glucose, mannose, and xylose yields up to 18 times that of the raw pine.Viscosity measurements of pure glycerol, [C2mim][O2CH], [C2mim][OAc], and IL-glycerol mixtures were very different at ambient temperature, but were similar at typical biomass pretreatment temperatures. Biomass pretreated by mixtures with higher viscosity tended to give better carbohydrate yields after enzymatic hydrolysis. Higher excess molar volumes, VmE, tended to align with better carbohydrate yields after enzymatic hydrolysis. This phenomenon may relate to more energy put into shearing flow of the IL-glycerol-biomass system resulting in biomass particle shearing or stretching that allowed better solvent access into the biomass.