Homolytic Scission of Interunitary Bonds in Lignin Induced by Ultrasonic Irradiation of MWL Dissolved in Dimethylsulfoxide

Abstract

An electron spin resonance (ESR) method combined with a spin trapping technique was applied to trap and characterize unstable radicals which were generated by ultrasonic irradiation of the dimethylsulfoxide (DMSO) solution of a softwood, Yezo Spruce (Picea jezoensis carr.) lignin. It was found that an unstable secondary carbon radical,~CH • in the solution was trapped as the stable nitroxide spin adduct when the DMSO solution was subjected to ultrasonic irradiation in the presence of a spin trapping reagent: 2,4,6-tri-tert-butylnitrosobenzene (BNB) at 50°C for 30 min. This means that the alkyl phenyl ether bonds,~CH-O-phenyl, known as interunitary bonds in lignins were homolytically cleaved by the ultrasonic irradiation, although the phenoxy radical Ph-O •, called guaiacoxy radical, i.e. the counter radical of the secondary carbon radical, was not trapped by the BNB spin trap. This suggests that the trapping of the guaiacoxy radical, having a methoxy group in an ortho-position, by the BNB molecule, carrying two bulky butyl groups in the ortho-positions, is sterically hindered. KeywordsYezo spruce (Picea jezoensis carr.) Milled wood lignin (MWL) Electron spin resonance (ESR) Spin trapping Phenoxy radicals Alkyl phenyl ether bonds Secondary carbon radical Ultrasonic irradiation Brought to you by | University of Queensland -UQ Library Authenticated Download Date | 6/23/15 7:21 AM

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