Highly Selective Oxidation and Depolymerization of α,γ‐Diol‐Protected Lignin

Abstract

Lignin oxidation offers ap otential sustainable pathway to oxygenated aromatic molecules.H owever,c urrent methods that use real lignin tend to have low selectivity and ay ield that is limited by lignin degradation during its extraction. We developed stoichiometric and catalytic oxidation methods using 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) as oxidant/catalyst to selectively deprotect the acetal and oxidizethe a-OH into aketone.The oxidized lignin was then depolymerized using af ormic acid/sodium formate system to produce aromatic monomers with a36mol %(in the case of stoichiometric oxidation) and 31 mol %(in the case of catalytic oxidation) yield (based on the original Klason lignin). The selectivity to as ingle product reached 80 %( syringyl propane dione,and 10-13 %toguaiacyl propane dione). These high yields of monomers and unprecedented selectivity are attributed to the preservation of the lignin structure by the acetal. 1015 Lausanne( Switzerland)

Recent Advances in the Catalytic Depolymerization of Lignin towards Phenolic Chemicals: A Review

“…It has been demonstrated that the oxidation of benzylic hydroxyl could both weaken the C β −O bonds and suppress the condensation reactions induced by the reactive benzylic carbocations (C α + ) . The earliest work on the depolymerization of oxidized lignin was proposed by Stahl group in 2013 and 2014 (Figure ).…”

Metal‐Free Photochemical Degradation of Lignin‐Derived Aryl Ethers and Lignin by Autologous Radicals through Ionic Liquid Induction

“…In the subsequent experiments we turned our attention toward the photochemical degradationo f2-phenoxy-1-phenylethanol (3), as shown in Figure S7 (Supporting Information). From the GC-MSa nalysis, 3 first converted to ketone( 1), and then underwent similarc leavage of CÀC/CÀOt o1.F urther,a ttention was paid to the photochemical degradation of a-O-4 types of lignin-derived aryl ethers, benzyl phenyle ther (4), and 4-(benzyloxy)phenol (5), as shown in Figure S8 (Supporting Information).F rom the results, aromatic aldehydes originating from oxidative processes were detected, indicatingt he implementation prospect of the photochemical system to CÀO bond cleavage in lignin a-O-4 linkages.…”

“…[2] However,u ntil now, this large amount of lignin has been underexploited chemically,a nd less than 5% of lignin is used as low-grade fuels or concrete additives. [4] In addition, lignin-derived compounds containing a b-aryl ether bond, the most abundant linkageo f native lignin, [5] are often adopted to study lignin conversion processes. [4] In addition, lignin-derived compounds containing a b-aryl ether bond, the most abundant linkageo f native lignin, [5] are often adopted to study lignin conversion processes.…”

“…[3] The heterogeneousa nd highly cross-linked recalcitrant structure impedes the upgrading of lignin to valueadded fine chemicals. [4] In addition, lignin-derived compounds containing a b-aryl ether bond, the most abundant linkageo f native lignin, [5] are often adopted to study lignin conversion processes. [4b, 6] Strategies such as hydrogenolysis, catalytic cracking, and oxidation [7] have been developed to exploit the aromaticity of lignin and lignin-derived compounds effectively.…”

Selective Depolymerisation of γ‐Oxidised Lignin via NHC Catalysed Redox Esterification

“…Thus, a beech butanosolv lignin was prepared according to previous reports [18] and the oxidation of the primary alcohol groups to aldehydes in Scheme 1. Previously reported two-step oxidative approaches to lignin depolymerisation via a) secondary [6][7][8][9][10][11] or b) primary alcohol [12][13][14] oxidation of native type β-aryl ether linkages lignin compared to this work c) employing selective oxidation and NHC catalysis for the depolymerisation of butanosolv lignin βaryl ether linkages. the β-aryl ether linkages was attempted using the biphasic TEMPO/NCS catalytic system developed by Einhorn et al [22] In contrast to our previous reports on the direct TEMPO-catalysed oxidation of this alcohol to carboxylic acids, [23] attempts to perform this reaction on the butanosolv lignin failed to provide the expected aldehyde product.…”