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

Abstract

Abstract:The degradation kinetics of a non-phenolic lignin model compound with α-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 β-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 1 st step was 69.1 kJ mol -1 , and for the 2 nd 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: β-ether bonds of structures having α-carbonyl functionality do not cleave under soda pulping conditions, whereas in kraft and sulfite pulping the cleavage of β-ether bonds proceeds via nucleophile attack and addition. The combination of hydroxyl and sulfite ions gives the fastest cleavage of β-ether bonds in non-phenolic lignin structures with the α-carbonyl functionality.

Lignin for Bioeconomy: The Present and Future Role of Technical Lignin

“…During the pulping process, the dissolution of phenolic lignin occurs via ionization of phenolic groups (due to alkaline pH) and an unstable intermediate quinone methide (Figure 5). It is followed by the reaction of bisulfide ions (strong nucleophiles) with quinone methide and finally cleaving the βaryl ether linkage [83,84]. Common nucleophiles participating in the kraft process are HO − < HS − <S 2− in increasing order of nucleophilicity that attack the electron-deficient centers of lignin molecules and lead to the dissolution of lignin in processing liquor [85].…”

Detailed Modeling of Kraft Pulping Chemistry. Delignification

“…The non-phenolic lignin units are difficult to degrade; however, the alkaline cleavage of β-ether bonds in non-phenolic units can be facilitated by the presence of α- or γ-carbonyl functionality . The cleavage of β- O -4 bonds in non-phenolic units with α-carbonyl functionality takes place under ambient conditions . The reaction mechanism is presented in Figure c.…”

“…The kinetic parameters for S9 formation were published by Miksche and Gierer . Recently we conducted additional experimental work in order to obtain kinetic parameters for the second step and more precise data on the first step of the reaction (Table , reactions 4 and 5, respectively).…”