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Aromatic Monomers by in Situ Conversion of Reactive Intermediates in the Acid-Catalyzed Depolymerization of Lignin
Peter J. Deuss, Martin Scott, Fanny Tran, Nicholas J. Westwood, Johannes G. de Vries, and Katalin Barta
Conversion of lignin into well-defined aromatic chemicals is a highly attractive goal but is often hampered by recondensation of the formed fragments, especially in acidolysis. Here, we describe new strategies that markedly suppress such undesired pathways to result in diverse aromatic compounds previously not systematically targeted from lignin. Model studies established that a catalytic amount of triflic acid is very effective in cleaving the Î²-O-4 linkage, most abundant in lignin. An aldehyde product was identified as the main cause of side reactions under cleavage conditions. Capturing this unstable compound by reaction with diols and by in situ catalytic hydrogenation or decarbonylation lead to three distinct groups of aromatic compounds in high yields acetals, ethanol and ethyl aromatics, and methyl aromatics. Notably, the same product groups were obtained when these approaches were successfully extended to lignin. In addition, the formation of higher molecular weight side products was markedly suppressed, indicating that the aldehyde intermediates play a significant role in these processes. The described strategy has the potential to be generally applicable for the production of interesting aromatic compounds from lignin.
Peter J. Deuss, Katalin Barta
In the coming decades major changes are expected in the chemical industry regarding the utilized raw material inputs. Depleting fossil resources will gradually be replaced by renewable feedstocks wherever possible. Because of this transition, new and efficient methodologies are required that enable depolymerization and defunctionalization of these complex, highly oxygenated biopolymers. Additionally, utilization of all components of lignocellulose is of great importance. In particular, depolymerization of lignin into its aromatic subunits or defined aromatic platform chemicals has proven challenging. Various approaches to overcome these difficulties have been attempted and resulted in new and exciting developments in many fields. In this review we will give an overview of bond cleavage strategies relevant for lignin depolymerization using homogeneous catalysts, focusing especially on reductive and hydrogen transfer methods.