Depolymerization of lignin via co-pyrolysis with 1,4-butanediol in a microwave reactor

Authors: Tarves, Paul; Mullen, Charles; Strahan, Gary; Boateng, Akwasi

Submitted to: ACS Sustainable Chemistry & Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/21/2016
Publication Date: 11/21/2016
Publication URL: https://handle.nal.usda.gov/10113/5601572
Citation: Tarves, P.C., Mullen, C.A., Strahan, G.D., Boateng, A.A. 2016. Depolymerization of lignin via co-pyrolysis with 1,4-butanediol in a microwave reactor. ACS Sustainable Chemistry & Engineering. 5: 988-994.

Interpretive Summary: Lignocellulosic biomass is primarily composed of cellulose, hemicellulose, and lignin. The cellulose and hemicellulose are used in the pulp and paper industries and, more recently, the cellulosic bio-ethanol industry. However, the leftover lignin is a low-value byproduct and is typically burned for energy. Therefore, lignin has potential as an abundant, low cost feedstock for the renewable production of biofuels and commodity chemicals. In order to do so, the lignin polymer must be depolymerized into smaller molecules for isolation and/or further processing. Current research efforts have focused on the thermochemical conversion of lignin to liquid products via pyrolysis (rapid heating in an oxygen-free environment) or hydrothermal liquefaction (heating in the presence of water in a high pressure environment). Unfortunately, these methods have provided only modest degrees of depolymerization and low liquid yields when compared to biomass. To further improve upon the aforementioned processes, we have performed laboratory scale microwave pyrolysis experiments with lignin in the presence of various amounts of butanediol as an additive. The addition of the butanediol provided higher liquid yields, smaller average molecular weight fragments, and higher concentrations of valuable phenolic compounds. The results of this work will allow for the further development of thermochemical lignin conversion methodologies and will be of interest to those looking to utilize residual lignin to produce renewable industrial chemical feedstocks from the aforementioned sources.

Technical Abstract: The production of valuable compounds from low cost but abundant residual lignin has proven to be challenging. The lack of effective biochemical lignin depolymerization processes has led many to focus on thermochemical conversion methods. Bench scale microwave pyrolysis of lignin has been performed at 1200 watts over the course of fifteen minutes in the presence of a microwave absorber (activated charcoal). The liquid products obtained are composed of smaller polymeric components and moderate yields of monomeric phenols. However, upon the addition of 1,4-butanediol, repolymerization reactions that limit the yield of monomeric and other reduced molecular weight products are inhibited. A great reduction in the average molecular weight (approximately 85-90% decrease) of the liquid products was observed as well as an overall increase in liquid yield. At the optimized ratio of 2:1 lignin to 1,4-butanediol (w/w), the yield of selected monomeric phenols increased three-fold to approximately 3.4 wt% (based on feedstock), while the yield of mono-aromatic hydrocarbons decreased by approximately 90%. The addition of the diol co-reactant also led to a significant shift in selectivity towards the production of methoxy-phenols (guaiacols, syringols) over non-methoxylated alkyl-phenols (phenol, cresols, etc.). The results obtained may help lead to the development of novel lignin co-processing methods.