Effect of Substituents on Molecular Reactivity during Lignin Oxidation by Chlorine Dioxide: A Density Functional Theory Study

Abstract

Lignin is a polymer with a complex structure. It is widely present in lignocellulosic biomass, and it has a variety of functional group substituents and linkage forms. Especially during the oxidation reaction, the positioning effect of the different substituents of the benzene ring leads to differences in lignin reactivity. The position of the benzene ring branched chain with respect to methoxy is important. The study of the effect of benzene substituents on the oxidation reaction’s activity is still an unfinished task. In this study, density functional theory (DFT) and the m062x/6-311+g (d) basis set were used. Differences in the processes of phenolic oxygen intermediates formed by phenolic lignin structures (with different substituents) with chlorine dioxide during the chlorine dioxide reaction were investigated. Six phenolic lignin model species with different structures were selected. Bond energies, electrostatic potentials, atomic charges, Fukui functions and double descriptors of lignin model substances and reaction energy barriers are compared. The effects of benzene ring branched chains and methoxy on the mechanism of chlorine dioxide oxidation of lignin were revealed systematically. The results showed that the substituents with shorter branched chains and strong electron-absorbing ability were more stable. Lignin is not easily susceptible to the effects of chlorine dioxide. The substituents with longer branched chains have a significant effect on the flow of electron clouds. The results demonstrate that chlorine dioxide can affect the electron arrangement around the molecule, which directly affects the electrophilic activity of the molecule. The electron-absorbing effect of methoxy leads to a low dissociation energy of the phenolic hydroxyl group. Electrophilic reagents are more likely to attack this reaction site. In addition, the stabilizing effect of methoxy on the molecular structure of lignin was also found.

Furfural production from lignocellulosic biomass: one-step and two-step strategies and techno-economic evaluation

“…14 Maleic anhydrides, such as succinic, malic, and fumaric acids, can be obtained by selectively oxidizing furfural. 28 Furan, which is the simplest five-membered heterocyclic compound containing oxygen can be obtained by decarboxylation of furfural, which can be further hydrogenated to tetrahydrofuran. 29 Fuels and fuel additives, such as methylfurans, can be obtained via the etherification of furfural derivatives.…”

“…30 Furan alcohol, the most important chemical derived from furfural, is primarily used in the production of resins in the foundry industry, and as a chemical building block for the synthesis of tetrahydrofurfuryl alcohol, ethyl furfuryl ether, levulinic acid, and γ-valerolactone (GVL). 28 Tetrahydrofuran is polymerized to polytetramethylene oxide and is also used as an industrial solvent for the production of polyvinyl chloride. 31 2-Methylfuran and 2-methyltetrahydrofuran are good liquid fuel additives and solvents in organic and polymer chemistry, as well as chemical intermediates for drug synthesis.…”

“…35 Other countries, such as South Africa (20 000 t) and the Dominican Republic (32 000 t) contribute significant furfural production. 28 China and EU represent the largest and the second-largest markets with 55% and 15% of the global demand of furfural globally, respectively, which is expected to increase to 200 000 tons per year by 2030. 36 In 2004, furfural was listed as one of the 12 most promising platform chemicals by the U.S. Department of Energy.…”