Green solvent-enabled biomass conversion for renewable chemicals and lignin upgrading

Abstract

Lignocellulosic biomass, which is mainly composed of cellulose, hemicellulose, and lignin, is a promising feedstock for producing renewable chemicals and fuels. To make lignocellulosic biomass-based refinery (biorefinery) competitive with petroleum refinery, maximizing the utilization of the three major components is critical. This requires the efficient fractionation of lignocellulose into different streams amenable to further upgrading. To this end, this work investigated the use of quaternary ammonium salt-based green solvents for lignocellulose fractionation and upgrading into renewable chemicals. First, a platform solvent system based on choline chloride and ethylene glycol was developed for switchgrass fractionation. Tailor-made lignin can be produced by using this platform solvent system, while the delignification and cellulose digestibility were not compromised. Highly concentrated sugar hydrolysate from pulp (cellulose) streams can be obtained via high solid loading enzymatic hydrolysis. The hemicellulose streams can be facilely upgraded into furfural via a novel biphasic system-acetone and aqueous choline chloride: ethylene glycol (ChCl:EG). Secondly, switchgrass can be fractionated effectively at high solid loading using either aqueous ChCl:EG or a series of ternary deep eutectic solvents. The cellulose stream can be converted into highly concentrated hydrolysate for high titer platform chemical production via fermentation. The hemicellulose and lignin streams can also be converted into other value-added products via other upgrading pathways. Thirdly, aqueous ChCl can also be a good solvent for switchgrass fractionation at mild conditions. The resultant lignin presented a very similar structure to native lignin, and thus, a great potential to be valorized into different products. The hemicellulose and cellulose streams can also be converted into valuable chemicals via biological and chemical routes. Lastly, a one-pot process capable of directly converting the untreated lignocellulosic biomass into furfural, digestible cellulose pulp and high purity lignin was developed. Furfural can be produced with high yield, while cellulose and lignin with appealing properties can be obtained via a one-pot process.