Metallo‐Deuteroporphyrin as a Biomimetic Catalyst for the Catalytic Oxidation of Lignin to Aromatics

Abstract

A series of metallo-deuteroporphyrins derived from hemin were prepared as models of the cytochrome P450 enzyme. With the aid of the highly active Co(II) deuteroporphyrin complex, the catalytic oxidation system was applied for the oxidation of several lignin model compounds, and high yields of monomeric products were obtained under mild reaction conditions. It was found that the modified cobalt deuteroporphyrin that has no substituents at the meso sites but does have the disulfide linkage in the propionate side chains at the β sites exhibited much higher activity and stability than the synthetic tetraphenylporphyrin. The changes in the propionate side chains can divert the reactivity of cobalt deuteroporphyrins from the typical CC bond cleavage to CO bond cleavage. Furthermore, this novel oxidative system can convert enzymolysis lignin into depolymerized products including a significant portion of well-defined aromatic monomers.

Redox Catalysis Facilitates Lignin Depolymerization

“…The analysis of the structure of I and III shows that they can only arise from the cleavage of two consecutive β-O-4 linkages under the applied reaction conditions, which demonstrates that the developed electrocatalytic oxidation–photocatalytic cleavage is not only highly effective in fragmenting β-O-4 bonds but also highly selective, leaving intact other linkages found in native lignin. Compared to other methods reported in the literature for native lignin depolymerization, 6,8,11,13,15,16 the yields reported for the observed monomeric units using our conditions might seem to be low. However, it is important to notice that due to the high selectivity of this method, nonconsecutive β-O-4 linkages that are also being cleaved are not being taken into account in the overall yield for the monomeric species (Figure S34).…”

Review on Catalytic Cleavage of C–C Inter-unit Linkages in Lignin Model Compounds: Towards Lignin Depolymerisation

“…Among these reports, catalytic depolymerisation of lignin has received greater attention recently. In the literature, six major strategies have been reported for the depolymerisation of lignin namely pyrolysis, hydrolysis, oxidation, hydrogenolysis, photocatalytic and enzyme catalysis (biocatalysis) [50]. Some of these routes like pyrolysis are thermal and non-catalytic while others are catalytic.…”

“…Behling lignin including some oxidative C-C cleavages [55]. Paper and pulping industries use some of the most advanced oxidative routes for the depolymerisation and eventual removal of traces of lignin present in cellulosic materials [50,55,56]. Oxidative depolymerisation of lignins typically result in monomeric oxygenates like carbonyl compounds and carboxylic acids.…”

Origins of Catalyst Inhibition in the Manganese‐Catalysed Oxidation of Lignin Model Compounds with H2O2

“…The β‐O‐4 linkage (Figure ) is the most abundant (≈55 %) linkage in lignin polymers . Hence, the oxidation of the functional groups adjacent to this linkage and particularly at benzylic positions represents an attractive starting point for lignin depolymerisation …”

“…[2,3] Hence, the oxidation of the functional groups adjacent to this linkage and particularly at benzylic positions represents an attractive starting point forlignin depolymerisation. [1,2,[12][13][14][15] Selective oxidative depolymerisation of lignin with homogeneous catalysts is ap romising approachi nt erms of energy efficiency and offerso pportunities to make use of aw ide range of ligands and complexes already availablef or small-molecule oxidation. Given the scale of the process, catalysts based on first-row transition metals together with simple ligands are especially relevant.…”