Enhanced Cleavage of β-Aryl Ether Bonds in Lignin Model Compounds During Sulphite-Anthraquinone Pulping

Abstract

Abstract Under sulphite-anthraquinone cooking conditions reduced anthraquinones, such as anthrahydroquinone (AHQ) or oxanthrone, promote cleavage of the β-ether bond in the free phenolic β-ether lignin model 1. This enhanced cleavage, and the products formed, can be rationalised by a mechanism involving formation of an adduct between the reduced quinone and the quinone methide 3. This adduct then fragments to afford the observed products. Wood sugars are capable of reducing AQ to the catalytically active form during sulphite-AQ cooking. Consequently, AQ can catalyse the cleavage of free phenolic β-aryl ether linkages in lignin when sugars are present. This finding explains why AQ is able to accelerate delignification during the initial phase of sulphite-AQ pulping. However, it does not provide the full story since it cannot explain why AQ catalyses delignification during the bulk phase of sulphite-AQ pulping. Mixtures of low-molecular-weight sodium lignosulphonates, such as those produced during sulphite cooks of 1, can be readily analysed by 1H nmr spectroscopy. This analytical method has two advantages over alternative methods: one it is simpler to carry out, and two, for the first time, approximate product ratios can be determined from model cooks.