(635c) Synthesis of Artificial Lignin Polymers and Their Effects on Enzymatic Hydrolysis of Cellulose

Lignin is the second abundant polymer present in lignocellulosic biomass. Dimeric lignin model compounds have been often used to investigate lignin degradation of lignin and catalytic conversion. However, these models are difficult to represent the three dimensional complex structure of lignin polymers and their effects on enzymatic hydrolysis of cellulose.

The lignin model polymers were synthesized to contain β-O-4 and β–β linkages that are structural motifs ubiquitous in native lignin obtained from hardwood. The method mainly consists of 4 parts: 1) preparation of monomeric acetate, 2) multiple-step synthesis of β–β dimeric diacetate, 3) construction of β-hydroxyl ester-containing oligomers via lithium diisopropylamide (LDA)-promoted aldol type reaction of β–β dimeric diacetate with monomeric acetate, and 4) reduction of the ester moiety with LiAlH₄ to give the lignin model polymer. The combined yield of the last two steps is 27%. The β–β linkage is from β–β dimeric diacetate while the β-O-4 linkage is the repeating unit of this polymer.

The synthesized lignin polymers were characterized by gel permeation chromatography and HSQC NMR. The number average degree of polymerization (DP_n) of the polymer is 8.25 and the molecular weight of lignin polymers were 2378. These artificial lignin polymers decreased the 72 h hydrolysis yield of Avicel from 57% to 53%, which is similar to the effect of native lignin on Avicel.