Lignin to Adipic Acid

Second-generation lignocellulosic ethanol plants make a byproduct consisting primarily of lignin, a heterogeneous mixture of largely-aromatic polymers that provides plant cell walls their structural strength and recalcitrance to biologic attack. The chemical industry has not been able to make money from lignin because a boiler to generate steam and electricity from lignin is expensive. This capital would be avoided if an alternative use for lignin is found. An economical projection estimated that lignin, as raw material, would be available at a price of half of its fuel value (Humbird et al., 2011). The National Renewable Energy Laboratory (NREL) is working under this premise to make lignin a profitable byproduct. NREL has demonstrated an integrated process from corn stover-derived lignin to cis,cis-muconic acid, followed by high yield recovery and hydrogenation to adipic acid (Vardon et al., 2015). Adipic acid is the dicarboxylic acid produced industrially in the largest quantity (Chenier, 2002). Alternatively, adipic acid can be synthesized from the conventional petrochemical process. This process is well developed, however the high cost of the raw material (cyclohexane) as well as the environmental impact (CO2 and NOxgas emissions) leave room for improvement and present a business opportunity. The question then arises as to whether the NREL process is competitive when scaled up from laboratory to industrial basis, or if it needs significant new inventions to be economically viable in replacing the conventional approach. This project addresses the question by investigating the conversion of lignin to adipic acid alternative. Moreover, the project develops, industrial scale, the process demonstrated by NREL, laboratory scale, to identify limitations and opportunities of its implementation.