Molecular modeling on pulping of wheat straw with sulfite and formaldehyde.

摘要

Non-wood fibers currently constitute 60% of the raw materials for China's pulp and paper industry, and will continue to be the predominant pulping material for the foreseeable future. Wheat straw is the most common of the non-wood fibers in China. Based on the characteristics of China's pulping of wheat straw, sulfite-formaldehyde pulping with anthraquinone (SFP-AQ) has been developed by the author. Due to the sulfonation at side-chains of lignin and sulfomethylation at the aromatic rings, this process can accelerate delignification rates and selectivity, increase pulp yields, improve bleachability and, produce sulfite-formaldehyde lignosulfonate (SFL) exhibiting good solubilities and surface activities.; In order to obtain a better understanding of reaction mechanisms in the SFP process and the surface properties of lignosulfonates in aqueous solution, the following projects have been completed in this dissertation: Firstly, molecular orbital methods have been used to investigate the reaction mechanisms of major lignin units in three types of reactions, sulfomethylation, condensation and sulfonation occurring during the SFP process. The results show that the two-step sulfomethylation is preferred over one-step process from standpoints of thermodynamic and kinetic control, and coulombic interactions. According to the thermodynamic results, the main condensation reaction, leading to the formation of diphenylmethane, results from the condensation between lignin models and their hydroxymethylated derivatives. The molecular orbital calculations indicated that the sulfonation at the α-carbon of quinone methide by the attack of SO₃– should be under orbital control rather than coulombic attraction between nucleophiles and electrophiles.; Secondly, molecular dynamics and force field calculations have been employed in conjunction with previous experimental results to study the physicochemical properties of lignosulfonate molecules in aqueous solution. Molecular modeling shows that the SFL monomer exhibits lower potential energy and is more flexible in aqueous solution as compared with the traditional alkaline sulfite fignosulfonate (ASL) under the same simulation conditions. Furthermore, both SFL and ASL monomers can form aggregates in aqueous solution. The intermolecular forces holding them together result from hydrogen bonding, van der Waals and electrostatic forces. Finally, the addition of nonpolar n-heptane molecules to solvated SFL and ASL aggregates has a more detrimental effect on the formation of ASL aggregates in comparison with SFL aggregates.