Abundance and Reactivity of Dibenzodioxocins in Softwood Lignin

Abstract

To define the abundance and comprehend the reactivity of dibenzodioxocins in lignin, model compound studies, specific degradation experiments on milled wood lignin, and molecular modeling calculations have been performed. Quantitative (31)P NMR measurements of the increase of biphenolic hydroxyl groups formed after a series of alkaline degradations in the presence of hydrosulfide anions (kraft conditions) showed the presence of 3.7 dibenzodioxocin rings/100 C9 units in milled wood lignin. The DFRC degradation protocol (Derivatization Followed by Reductive Cleavage) was chosen as an independent means to estimate their abundance. Initial experiments with a dibenzodioxocin model compound, trans-6,7-dihydro-7-(4-hydroxy-3-methoxyphenyl)-4,9-dimethoxy-2,11-dipropyldibenzo[e,g][1,4]dioxocin-6-ylmethanol, showed that it is not cleaved under DFRC conditions, but rather it isomerizes into a cyclic oxepine structure. Steric effects precluded this isomerization from occurring when DFRC was applied to milled wood lignin. Instead, monoacetylated biphenolic moieties were released and quantified by (31)P NMR, at 4.3 dibenzodioxocin rings/100 C9 units. The dibenzodioxocin content in residual lignins isolated from kraft pulps delignified to various degrees showed that during pulp delignification, the initial rate of dibenzodioxocin removal was considerably greater than the cleavage rate of arylglycerol-beta-aryl ether bonds. The activation energy for the degradation of dibenzodioxocins under kraft conditions in milled wood lignin was 96 +/- 9 kJ/mol, similar to that of arylglycerol-beta-aryl ether bond cleavage.

Lignin Biosynthesis and Structure

“…This mode of action, in which a monomer (radical) adds to the growing polymer, is termed endwise coupling: the polymer grows one unit at a time. Coupling of two lignin oligomers is rare in S/Glignins but relatively common in G-lignins, where 5-5 coupling accounts for approximately 4% of the linkages (Argyropoulos et al, 2002;Wagner et al, 2009). During each coupling reaction, two radicals are "consumed" (in a so-called "termination reaction") as each single electron contributes to the newly formed bond, making this type of radical polymerization intrinsically different from the radical chain reactions that occur in the polymerization of several industrial polymers, such as polyethylene, polypropylene, and polystyrene.…”

Enhancing Aromatic Production from Reductive Lignin Disassembly: in Situ O-Methylation of Phenolic Intermediates

“…8 Other linkages are present in smaller amounts with abundances being highly dependent on the plant species, growth conditions, and lignin isolation technique (Figure 1). [9][10][11] Various lignin disassembly methods utilizing acids, bases, and transition metal based hetero-and homogeneous catalysis have been described, [12][13][14][15][16] including catalytic hydrogenolysis to produce aromatic compounds from several types of lignin. 17,18 Hydrogenolysis (HDG) uses hydrogen to cleave C-X (X= O, S, Cl, and F) bonds.…”

Wood Based Lignin Reactions Important to the Biorefinery and Pulp and Paper Industries

“…"Important reactions of lignin," BioResources 8(1), 1456-1477. The dominant linkages between the phenylpropane units as well as the abundance and frequency of some functional groups have been determined and are shown in Table 1 ( Argyropoulos et al 2002;Froass et al 1996;Kukkola et al 2004). Linkages between the phenylpropane units and the various functional groups on these units give lignin a unique and very complex structure.…”