Photodegradation of Lignin: A Photochemical Study of a Phenolic α-Carbonyl β—O—4 Lignin Model Dimer 4-Hy droxy-3-Methoxy-α- (2′-Methoxyphenoxy)-Acetophenone

Abstract

KeywordsLignin Photodegradation β-Ο-4 Lignin modcl compound Phenolic dimer Photochemical bond cleavage 5,5' -O-4Tetramer Frec-radicals α-Carbonyl-phenyl derivatives synthesis Bisulphite pulp (maritime pine) SummaryThe photoyellowing of bleached high yield wood pulp induced by lignin was studied using the title phenolic a-carbonyl -O-4 lignin model L Its photochemical reactivity was examined in both liquid (benzene and buffered aqueous ethanol) and solid (adsorbed on bleached chemical pulp) media and compared to the 4-O-methylated dimer 2. The influence of oxygen during the Irradiation was considered. Efficient photolysis of l (0 r ~ 0.25 in solution) yields monomers, dimers, a tetramer and a complex mixture of oligomers which include coloured substances.The structure and the titration of the photoproducts indicate that an intramolecular β C-O bond cleavage mechanism leading to phenacyl and guaiacoxy radicals is the major pathway of the photodegradative reaction. The phenolic hydrogen abstraction mechanism, evaluated by the formation of the 5-5' tetramer 14 contributes only for 10%. These results support the previous hypothesis ) concerning the -cleavage of α-carbonyl β-Ο-4 dimer units s the primary step in lignin photodegradation.

An Introduction to Model Compounds of Lignin Linking Motifs; Synthesis and Selection Considerations for Reactivity Studies

“…[3] It has been shown that "essentially all" 5-5 linking motifsi nl ignin are actually found as part of the dibenzodioxocin linking motif ( Figure 6). [3] The 5-5 linking motif can be modelled using biphenyl-type compounds,w hich are generally synthesized via dimerization reactions using sodium or potassium persulfate/iron(II) sulfate mixtures [91,132] or K 3 Fe(CN) 6 [133,134] as shown in Scheme12. Alternatively,c ommercially available biphenyl or 2,2' biphenol are often used.…”

“…Severals tepwise approaches begin with the synthesis of ad ifunctional, often as ymmetric 5-5 or a4 -O-5 motif, which can then be extended to as equentially symmetrical oligomer.T his approachh as been used to accesst etramers [132,146] As imilara pproachi st os ynthesize dimeric or trimeric sequences of linking motifs that are then subjected to radicald imerization reactions. This has been used to generate tetramers [147,148] with three linking motifs-(b-O-4) (5-5) (b-O-4) and (b-5) (5-5) (b-5)-and hexamers [149] These two related approaches can be very successful as they allow the swift building up of multi-linking motif model compounds in moderate-to-good overall yields.…”

13C CP/MAS NMR Study of Photodegraded Stoneground Wood Pulp

“…Brought to you by | University of Queensland -UQ Library Authenticated Download Date | 6/20/15 9:13 PM Castellan et al (1989) have shown in their studies on lignin model compounds that the cleavage of a-carbonyl-ß-O-aryl ether linkages is one of the main pathways of photodegradation of lignin. Holmbom et al (1989) have provided evidence for the cleavage of acarbonyl-ß-O-arylether linkage, by means of GC/MS analysis of the extractives of photodegraded alkaline hydrogen peroxide bleached and unbleached spruce groundwood.…”

Photochemistry of a model lignin compound. Spin trapping of primary products and properties of an oligomer

“…The objectives of this study are (1) to determine if spin trapping by phenyl tert-butylnitrone (PBN) can be used to intercept radicals initially formed such as 2 and 3 before radical rearrangements and (or) recombinations occur; (2) to study the properties of colored oligomers, which also form on photolysis of 1 (1)(2)(3), and (3) to synthesize a phenolic rearranged-recombination isomer of 1 and study how its free radical chemistry is related to colored oligomers and presumably to the photoyellowing of lignin itself. ated by radical reactions (8).…”