Anthraquinone polymer catalysts for alkaline delignification of lignocellulosic matter. Part 3. Delignification of softwood and action on a phenolic β-O-4 lignin model in kraft pulping conditions

Abstract

Abstract The delignification action of four new anthraquinone (AQ) polystyrene catalysts – three of them linear (CP-A, CP-B, CP-C) and one cross-linked (CP-3D) – were investigated on softwood (Pinus pinaster) chips under kraft pulping conditions, and their catalytic efficiency gains (EGs) were compared to that of AQ, with EGAQ=100. The catalytic EG of the hydrophobic polymers CP-B and CP-3D was 96%, while the inherent hydrophilic catalyst CP-C showed an EG of 30%. Interpretation: hydrogen sulfide anions presented in the kraft pulp liquor might participate in the delignification mechanisms as redox active mediators in the case of the hydrophobic polymers. Moreover, the latter could be recovered after pulping, and delignification tests with the recovered materials revealed that they preserved their catalytic properties. Mechanistic studies were carried out with a phenolic β-O-4 lignin model compound in the presence or absence of glucose (mimicking the carbohydrate end groups) and/or NaSH. It was shown that the cross-linked CP-3D polymer reacted with the lignin model through electron transfer reactions, a process most likely mediated by glucose and HS- redox active species. The concept of grafting AQ into polymeric substrates is discussed in terms of reusable delignification catalysts for large-scale pulping processes under kraft conditions.