Towards Quantitative Catalytic Lignin Depolymerization

Abstract

The products of base-catalyzed liquid-phase hydrolysis of lignin depend markedly on the operating conditions. By varying temperature, pressure, catalyst concentration, and residence time, the yield of monomers and oligomers from depolymerized lignin can be adjusted. It is shown that monomers of phenolic derivatives are the only primary products of base-catalyzed hydrolysis and that oligomers form as secondary products. Oligomerization and polymerization of these highly reactive products, however, limit the amount of obtainable product oil containing low-molecular-weight phenolic products. Therefore, inhibition of concurrent oligomerization and polymerization reactions during hydrothermal lignin depolymerization is important to enhance product yields. Applying boric acid as a capping agent to suppress addition and condensation reactions of initially formed products is presented as a successful approach in this direction. Combination of base-catalyzed lignin hydrolysis with addition of boric acid protecting agent shifts the product distribution to lower molecular weight compounds and increases product yields beyond 85%.

Characterisation of Sequential Solvent Fractionation and Base-catalysed Depolymerisation of Treated Alkali Lignin

“…22 and 0.19,respectively,lower than in OL (0.25 and 0.22,respectively) and F3 in OL (0.30 and 0.26,respectively). This suggests that dealkylation of the side chains of lignin phenyl-propane units occurred during the BCD treatment process (Roberts et al 2011). This finding was in agreement with results reported earlier in the previous section (i.e., yield and molecular weight distributions of lignin) in which the yield for F3 in combined log R0 6.72-treated OL was higher than that for F3 in OL.…”

“…The latter have a negative charge in ortho or para position to the phenolic hydroxyl group (McDonough 1993). According to Roberts et al (2011), the carbanion generated during the BCD treatment forms C-C bond with carbonyl group (ketone) i.e. 3,5-dimethoxy-4-hydroxyacetophenone through aldol addition.…”

Heterogeneously catalyzed lignin depolymerization

“…In base-catalyzed depolymerization, the choice of lignin influences the yield of bio-oil, but not the resulting composition [59]; however, the use of mineral bases may necessitate neutralisation of the subsequent bio-oil, in addition to reactor corrosion. Thring et al studied the effect of time and temperature on the depolymerization of Alcell lignin catalyzed by NaOH [57] obtaining guaiacol, syringol, and under certain conditions catechol and its derivatives [57,60,61]. Base-catalyzed depolymerization has proven an effective protocol in polar organic solvents, such as ethanol and methanol, suppressing char formation, and hence enhancing monomer and dimer production relative to solid acid catalysts.…”

Depolymerization Pathways for Branching Lignin Spirodienone Units Revealed with ab Initio Steered Molecular Dynamics

“…Thus, depolymerization of unprocessed lignin remains of significant interest 8,16 and has been pursued experimentally with polyoxometalate catalysts 17 , base-catalyzed hydrolysis [18][19] , acidolysis [20][21] , ionic liquid treatment 22 , reduction with hydrosilanes 23 , photochemical degradation 24 , and ball milling 25 . In addition to general depolymerization, efforts have also included selective strategies via homogeneous catalysts [26][27][28][29][30] and enzymes 31 .…”