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Oxidative Cleavage Studies - by Antonio Martinez-Pascual, Utrecht University

Submitted on: 10 October 2016

Antonio Martínez-Pascual, Pieter C.A. Bruijnincx, Bert M. Weckhuysen

Inorganic Chemistry and Catalysis, Debye Institute for Nanomaterials Science, Faculty of Science, Utrecht University

The valorization of lignin is challenging since its chemical structure is complex and depends of the source and the method used1. It is nonetheless essential to develop methods to efficiently use it as feedstock in biorefineries, to realize the potential that this aromatic biopolymer can have. Such methods should effectively depolymerize the lignin macromolecule by selective cleavage of the inter-unit linkages, preferably under mild conditions. The most common linkage in native lignin and in lignins isolated via mild pretreatment methods is the β-O-4 linkage, which can comprise up to 50% of the inter-unit bonds in native lignin. Here, the aim is to cleave the β-O-4 linkage by a two-step oxidative cleavage using green and mild conditions. First, the α-OH is oxidized to activate the β-O-4 linkage for cleavage, followed by a second oxidation step, with alkaline hydrogen peroxide. The oxidation in mild and green conditions in hydrogen peroxide (7 equivalents) and aqueous sodium hydroxide (0.5 M) in dioxane at room temperature has been studied first with oxidized guaiacyl-guaiacyl and syringyl-guaiacyl lignin model compounds (Figure 1) as function of time and reaction conditions. At full conversion, the G-G model compound gave a 95% and 84% of yield of dimethoxybenzoic acid and guaiacol respectively, in 32 h (with a 90% mole balance based on aromatic rings). For the S-G model compound, similar yields are obtained already after 3 h of reaction: 98% yield of trimethoxybenzoic acid and 85 % of guaiacol (mole balance 92%).

The two-step oxidation strategy is then extended to a set of real lignins. The influence of free phenolic end groups on cleavage efficiency is assessed by comparing as isolated lignins with fully methylated ones. The lignins were extracted by an organosolv process (poplar and walnut lignin) and by cellulytic enzyme hydrolysis (poplar) after ball milling2 to retain the high β-O-4 linkage content required for this oxidative cleavage. The lignins were characterized before and after reaction by HSQC, GPC and GC analysis.

 

Figure 1. Oxidative cleavage of an activated β-O-4 S-G model compound.

 

  1. (a) Lancefield, C. S.; Ojo, O. S.; Tran, F.; Westwood, N. J., Isolation of functionalized phenolic monomers through selective oxidation and CO bond cleavage of the β-O-4 linkages in Lignin. Angew. Chem. Int. Ed. 2015, 54 (1), 258-262; (b) Rahimi, A.; Ulbrich, A.; Coon, J. J.; Stahl, S. S., Formic-acid-induced depolymerization of oxidized lignin to aromatics. Nature 2014, 515 (7526), 249-252.
  2. Wu, S.; Argyropoulos, D. S., An improved method for isolating lignin in high yield and purity. J Pulp Pap Sci 2003, 29 (7), 235-240.