Studies On The Electrocatalytic Oxidation Mechanisms Of Lignin Model Compounds

Lignin consists of a network of highly-functionalized alkyl aryl ether monomers,which are connected by ?-O-4 bonds in a great measure(45%-60% of all lignin linkages).By interrupting such linkage through different depolymerization processes,lignin could be potentially converted into high value-added small molecular aromatic products.Many approaches have been tried to aim at breaking ?-O-4 bonds and yielding valuable aromatic products,such as chemical oxidation,photochemical oxidation,hydrogenolysis and reduction,and redox-neutral approach.In comparison with the above catalytic depolymerization strategies,electrochemical oxidation is a promising and effective method for lignin depolymerization owing to its selective oxidation capacity,the smaller size of reagent consumption,controllable reaction and environmental friendliness.However,common electrochemical techniques show a lack of qualitative judgement and are not able to provide direct evidence for any accompanying reactions or molecular transformations from intermediate products generated during oxidation reactions,resulting in difficulties in reaction mechanism elucidation and studies.So,there are very few reports for the mechanism exploration of the selective electrooxidation of lignin,especially on the cleavage of the predominant ?-O-4 linkages.As a consequence,our research idea turns to investigate the fragmentation pattern of alkyl aryl ether containing lignin model monomers to ?-O-4 bond containing model dimers with the help of in situ tracing technique.It potentially provides theoretical guidance and technical support for selective electro-oxidative depolymerization of native lignin.Involved the following major elements:(1)The electrochemical oxidation mechanism of 1-(4-ethoxyphenyl)ethanol(1)and its derivates(2-5)were experimentally(by CV,in-situ IR SEC,and UV-vis SEC)and theoretically(by DFT calculations)explored in details.For monomer 1-(4-ethoxyphenyl)ethanol(1),it exhibited a two-step oxidation.Firstly,Compound 1 underwent C?-carbonylation follow an ECEC mechanism and generated corresponding carbonyl compound 2.Subsequently,compound 2 underwent a bond cleavage on alkyl-aryl ether due to the delocalization of HOMO to ultimately produce a quinone compound.And the quinone compound can be reduced back to the phenol structure of 3 around-0.3 V.(2)As for 2-phenoxy-1-phenethanol(6),2-phenoxyacetophenone(7),and 2-phenoxy-1-phenylethane(8),the experimental research(CV,in-situ IR SEC,electrolysis and GC-MS)shows that the main cleavage of the C-O bond in ?-O-4 dimers of 6,7,and 8 was discovered,and the cleavage of C-C bond in dimer 6 and 7 was also discussed.The experimental result and proposed mechanism were supported by DFT calculations.In particular,the cleavages of C?-O and C?-C? bonds in dimers were ascribed to the delocalization of HOMOs.The most important consideration needs to be emphasized that the presence of the C?-hydroxyl group is the key factor to result in the formation of new aldehyde-containing products through a distinguish mechanism in contrast to the C?=O containing ?-O-4 dimer due to the electrochemical oxidation.