(187m) Microwave Assisted Lignin Depolymerization Using Deep Eutectic Solvents

Lignin, the second most abundant polymer, is greatly underutilized. Recent research has shown that lignin has a great potential for synthesizing wide variety of value added chemicals and renewable materials. However, one of the major hurdles in utilization of lignin is its complex polymeric structure. Lignin is a complex hetero-biopolymer composed of three different monomers (monolignols; H, G and S) connected through as many as eight different bonding motifs. The most common bonding motif in soft (~50%) and hardwood (~65%) lignin is the β-O-4 linkage. Lignin depolymerization is a crucial step in conversion of lignin to chemicals. Thermochemical and biochemical pathways for lignin depolymerization have numerous disadvantages such as high energy requirement to achieve high temperatures and limited product selectivity. Ionic liquids (IL’s) have also been tested for lignin fractionation. While IL’s have high efficiency, they are costly and sometimes toxic. A new line of solvents known as Deep Eutectic Solvents (DES) are a promising alternative for ionic liquids that can be used in lignin fractionation. DES’s are cheaper than most ILs due to low precursor cost, simple synthesis and recyclability. DES is a mixture of two or more chemicals acting as either hydrogen-bond donors (HBD) or hydrogen-bond acceptors (HBA). The interactions between HBD and HBA of the DES provide a dual acid–base catalysis mechanism which will facilitate controlled cleavage of labile ether linkages among phenylpropane units and thus lead to lignin depolymerization. This interaction can be exploited to fractionate lignin and generate a low molecular weight lignin products. Due to rapid and volumetric heating, microwave based chemistry has significant advantages over conventional heating techniques. In this report we investigate the application of microwave heating for lignin depolymerization using various deep eutectic solvents. In this study we investigate three different DES’s namely; oxalic acid: choline chloride (1:1), lactic acid: choline chloride (2:1) and Formic acid: Choline chloride (2:1). The DES’s are synthesized in a 2450MHz microwave batch reactor. 10% by weight lignin sample is mixed with 30 g DES and the mixture is heated at different temperatures (110, 120, 130, 150, 170 °C) and times (1, 5, 10, 15, 20 min). The optimum time and temperature conditions will be determined and these conditions will be tested on conventional heating for comparison. The dissolved lignin is precipitated and characterized using GPC to compare the average molecular weight of the fractionated lignin products with original sample. GCMS and NMR analysis will also be conducted to understand the bond cleavage and product composition.