(560af) Ionic Liquid-Mediated Rapid Catalytic Conversion of Lignocellulosic Grass to Biofuel Precursors in Microwave Reactors

The Biofuel Policy of India (2018) recommends the use of non-edible lignocelluloses for producing second generation bioethanol to meet the country’s blending targets for petrol set by the Paris Climate Accord (2011), and solve the connected problems of climate change and India’s dependence on foreign oil. Lignocelluloses are composed of three different polymers – cellulose, hemicelluloses, and lignin – bonded to each other by hydrogen and covalent bonds that make the lignocellulose structure recalcitrant. Switchgrass, Napier grass, Miscanthus, etc., are among the non-food lignocelluloses commonly used for producing 2G ethanol through the process of pretreatment (that breaks the inter-polymeric bonds), hydrolysis (that ruptures the β,1-4 glycosidic bonds linking the monomeric units in cellulose and hemicelluloses), and microbial fermentation (that converts the monomers to bioethanol).

This study uses a tall, thin, locally available, cellulose-rich grass – which qualifies neither as a food source nor as cattle-feed – as feedstock for biofuel production. The June grass used in this work is extremely rich in cellulose (82.3%), and has small amounts of hemicelluloses (1.3%), lignin (9.5%), ash (1.9%) and, moisture (4.5%), with an overall crystallinity index of 48%. We use a microwave reactor that provides an ionic liquid-mediated one-pot synthesis system for converting lignocelluloses to biofuel precursors [1]. The raw untreated grass is converted to biofuel precursors such as glucose, fructose, 5-Hydroxymethylfurfural (HMF), Levulinic acid (LA) and, Formic acid (FA), in only 36 minutes, when catalyzed by copper chloride in a BmimCl (ionic liquid) medium at temperatures ranging from 120-200ºC. When both the reactor temperature and the water concentration are used as reactor-scale parameters to control the product distribution, the maximum product yields are obtained as 88%, 25%, 18% and, 7%, for glucose, HMF, LA and FA, respectively.

The aprotic ionic liquid (BmimCl) rapidly ionizes to form a Lewis acid catalyst with copper chloride and water, and creates a supramolecular complex with cellulose. This polar supramolecule aligns itself along the microwave’s rapidly alternating electromagnetic field, resulting in frequent dipole rotations that lead to molecular collisions, and dissipation of electromagnetic energy as friction and heat. This increases the reaction temperature and accelerates the reaction rate, thus rapidly rupturing the glycosidic bonds in the cellulose to produce glucose monomers that are further converted to HMF, LA and FA [1].

Reference: [1] Paul, S. K., Chakraborty, S. (2018). Microwave-assisted ionic liquid-mediated rapid catalytic conversion of non-edible lignocellulosic Sunn hemp fibers to biofuels. Bioresource Technology, 253, 85-93.