Sofar, many different mechanistic studies of base-catalyzed depolymerization oflignin revealed that susceptible alkyl-aryl ether linkages (?-O-4) in lignin structurewere effectively broken down to LMW fragments between 200 and 400 ?C in BCD reaction.26 However,these reaction temperatures promote secondary reaction between the reactiveintermediates of phenols and aldehyde/ketone, consequently leading to totally decreasein monomer yields.27 A more recent study, additional capping agents suchas boric acid and phenols were applied to prevent repolymerization reaction of initiallyformed smaller phenolic products into oligomers.21,24 Robert et al.21employed the combination of NaOH and boric acid as catalysts and cappingagent to suppress further condensation reactions of the highly reactivesubstituted phenols, resulting in higher LMW product yields beyond 85%.
Inaddition, they also proposed a mechanism for base-catalyzed lignin ether bondcleavage which described the formations of syringol derivatives as well as monomeroligomerization between a phenolate and a ketone, proving that boric acid actsan outstanding protecting agent to reduce oligomerization and polymerization reactionsby forming esters with phenolic hydroxyl groups. Dabral and Barrett independentlyreported a presence of dimethyl carbonate (DMC) in bases/alcohol system.28-29They demonstrated that DMC assisted to increase the aromatic yields particular methylatedphenol and benzoic acid derivatives as products by O-methylation of thephenolic intermediates. The propose reaction pathways of capped-methoxy aromaticswere also observed during depolymerization. Lately, the mechanistic studies on degradationof lignin in isopropanol to aromatic monomers and oligomers using microwaveheating technique has been introduced by Liu et al.30 The possiblemechanism showed that oligomers were formed first and then broken down tomonomers via hydrogenation process for C? and C?. Condensationof monomers to oligomers could be promoted simultaneously when the reactiontemperature exceeded 120 ?C.
Nevertheless, understandingof mechanism for lignin depolymerization needs to be further investigated topromote the formation of LMW aromatic compounds.