Approaches to the Selective Catalytic Conversion of Lignin: A Grand Challenge for Biorefinery Development

Abstract

Lignin comprises 15-25% of terrestrial biomass and is the second most abundant source of renewable carbon after cellulose. However, its structural heterogeneity frustrates efforts for its selective conversion into biobased chemicals. Catalyst design for lignin transformation offers an opportunity to improve selectivity, and, hence, improve lignin's utility as a raw material in chemical production. Catalytic deconstruction and conversion of lignin has been examined using a variety of thermochemical treatments, analogous to those used in the petrochemical industry. However, the complex nature of these products limits their utility. More recently, greater focus has been given to an understanding of lignin's molecular level structure, and designing catalysts that can be targeted to key individual structural units within the biopolymer. This review gives a sense of the field by providing a representative description of recent developments in some of the primary technologies employed for lignin conversion and approaches that promise to improve selectivity.

Selective oxidative C–C bond cleavage of a lignin model compound in the presence of acetic acid with a vanadium catalyst

“…However, lignin is a three-dimensional structured macromolecule which leads to its resistance to chemical degradation. [1][2][3][4] Depolymerization of lignin selectively into smaller aromatic fragments is still rather challenging.…”

“…Typically lignin model compounds like 2-aryloxy-1-arylethanols are usually used to represent the most common substructure of lignin. 3,4 Many approaches have been explored to depolymerize lignin, 5 for example hydrogenolysis of lignin and model compounds has been studied extensively. However, selectively cleaving C-O or C-C bonds under harsh reaction conditions while preserving aromatic groups is rather challenging due to the facile hydrogenation of benzene rings.…”

Oxidative depolymerization of Kraft lignin to high-value aromatics using a homogeneous vanadium–copper catalyst

“…16,17 Despite the heterogeneity of lignin causing several difficulties, its unique chemical structure and the high bioavailability of this renewable, nonedible feedstock offers a huge potential towards its industrial implementation. 18 However, in pulp and paper mills as well as in lignocellulosic biorefineries, where lignin is generated in large quantities, the focus currently lies on the valorization of carbohydrates. In contrast, lignin is mainly utilized for heat and power generation.…”

Processes of catalytic oxidation for the production of chemicals from softwood biomass

“…Catalyst can increase by 1.5-2 times the yield of aromatic aldehydes in the processes of lignins oxidation by oxygen [64]. These catalysts are presented by oxides and hydroxides of Cu(II), Co (III), Ag (I), Mn (IV).…”