Reaction kinetics of strong nucleophiles with a dimeric non-phenolic lignin model compound with α-carbonyl functionality (adleron) in aqueous alkali solution

The degradation kinetics of a non-phenolic lignin model compound with alpha-carbonyl functionality (adlerone) has been studied by varying temperature and concentrations of sodium hydroxide, sodium hydrogen sulfide, and sodium sulfite. The kinetics of adlerone degradation and formation of its reaction products were monitored by UV-Vis spectroscopy and their structures were analyzed by GC/MS. The two step degradation of adlerone was studied in two separate experimental setups. In the first alkali catalyzed step, adlerone is converted to a beta-elimination product that reacts further in the second step with hydrogen sulfide or sulfite ion. The Arrhenius kinetic parameters were derived by the KinFit software. The activation energy for the 1st step was 69.1 kJ mol(-1), and for the 2nd step with sulfide 42.4 kJ mol(-1) and with sulfite ion 35.8 kJ mol(-1). The reaction mechanisms presented are in line with those published earlier: beta-ether bonds of structures having alpha-carbonyl functionality do not cleave under soda pulping conditions, whereas in kraft and sulfite pulping the cleavage of beta-ether bonds proceeds via nucleophile attack and addition. The combination of hydroxyl and sulfite ions gives the fastest cleavage of beta-ether bonds in non-phenolic lignin structures with the a-carbonyl functionality.