Catalytic Oxidation of Lignin in Solvent Systems for Production of Renewable Chemicals: A Review

Abstract

Lignin as the most abundant source of aromatic chemicals in nature has attracted a great deal of attention in both academia and industry. Solvolysis is one of the promising methods to convert lignin to a number of petroleum-based aromatic chemicals. The process involving the depolymerization of the lignin macromolecule and repolymerization of fragments is complicated influenced by heating methods, reaction conditions, presence of a catalyst and solvent systems. Recently, numerous investigations attempted unveiling the inherent mechanism of this process in order to promote the production of valuable aromatics. Oxidative solvolysis of lignin can produce a number of the functionalized monomeric or oligomeric chemicals. A number of research groups should be greatly appreciated with regard to their contributions on the following two concerns: (1) the cracking mechanism of inter-unit linkages during the oxidative solvolysis of lignin; and (2) the development of novel catalysts for oxidative solvolysis of lignin and their performance. Investigations on lignin oxidative solvolysis are extensively overviewed in this work, concerning the above issues and the way-forward for lignin refinery.

Matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry analysis for characterization of lignin oligomers using cationization techniques and 2,5‐dihydroxyacetophenone (DHAP) matrix

“…Lignin is the second most abundant naturally occurring polymer and is composed of three monolignol units, coumaryl alcohol (H), coniferyl alcohol (G), and sinapyl alcohol (S). Lignin chemistry has increased in relevance for its use in biorefineries, bioplastics, and renewable sources of carbon for fine materials . While gains have been made in lignin depolymerization chemistry, comprehensive analytical techniques are still needed to effectively characterize the resulting products …”

State-of-the-art catalytic hydrogenolysis of lignin for the production of aromatic chemicals

“…[7][8][9][10][11][12][13][14] As the price of fossil-based fuels goes down, lignocellulose represents a preferrable source for producing valueadded platform chemicals. 15 Lignocellulosic biomass composed of semi-crystalline polysaccharide cellulose (38-50%), amorphous multicomponent polysaccharide hemicellulose (23-32%) and amorphous phenylpropanoid polymer lignin (15-25%). [16][17][18] The conversion of cellulose and hemicellulose with repetitive structural units into biofuels and valuable platform chemicals (e.g.…”

Ozone mediated depolymerization and solvolysis of technical lignins under ambient conditions in ethanol

“…Furthermore, repolymerization pathways usually observed in oxidative processes due to the formation of radicals 30,81 seem to be suppressed as no molecular weight increase was observed at elongated reaction times. This positive observation is likely a result of the acidic reaction medium and solvent system used, as ethanol is reportedly an efficient radical scavenger able to quench reactive lignin fragments 70,82,83 under such conditions. Accordingly, while the mixture of ethanol and KL before reaction was slightly acidic (average pH of 6, standard error of 0.1), the lignin oil solution aer ozonation for 20 minutes had a substantially higher acidity (average pH of 2.3, standard error of 0.1).…”