Solar-Driven Oxidative Cleavage of Lignin By Using Organic Photo-Catalysts

Abstract

Traditionally, Dye-sensitized photo electrochemical cells (DSPECs) have targeted solar-driven water splitting for the conversion of solar energy into fuels in aqueous media. Recently, dye-sensitized photo-electrochemical(HAT-DSPEC) which used photo and heterogeneous catalysts incorporating system was reported for carry out chemo-selective oxidation of benzylic alcohol. This study reported replacing Ru based sensitizer by organic dye(L1) which has high stable, high molar adsorption coefficient. Also for hydrogen atom transfer(HAT) mediator, three kinds of organic nitroxyl was used such as 4-Acetamido-2,2,6,6-tetramethylpiperidine 1-oxyl(ACT), 9-Azabicyclo-(3,3,1)nonan-3-one-9-oxyl(Keto-ABNO) and (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl(TEMPO). For lignin model compounds(LMCs), 1-ol and two kinds of 2-ol was used and each LMC has different subunit groups; 1-ol(H), 2-ol(S,G) and 2-ol(G,G). Among the LMCs, 2-ol(S,G) was mainly focused on in this system because of the similarity with real lignin. Solar driven oxidative cleavage mainly come from cleavage of β-O-4 linkages of LMCs and it is promoted by the oxidation of Cα−OH to the Cα=O with specific condition(0.4V vs Ag/AgCl bias under light(1-sun illumination). Each HAT mediators(ACT, Keto-ABNO and TEMPO) showed different reactivity and conversion efficiency(about 29%, 75% and 40%).In HAT-ODSPEC(organic dye-sesitized photo-electrochemical cell), Keto-ABNO showed highest conversion efficiency with 2-ol(S,G) over 74%. The results indicates HAT-ODSPEC system could be used as efficient bio-mass conversion materials with high selectivity and lignin cleavage reaction under mild condition.*1-ol(H) : 2-Phenoxy-1-phenylethanol*2-ol(S,G) : 1-(4-Hydroxy-3,5-dimethoxyphenyl)-2-(2-methoxyphenoxy)propane-1,3-diol*2-ol(G,G) : 1-(3,4-Dimethoxyphenyl)-2-(2- methoxyphenoxy)propane-1,3-diol