Institute of Chemistry and Chemical Technology SB RAS, Federal Research Center "Krasnoyarsk Science Center SB RAS", Akademgorodok 50/24, Krasnoyarsk 660036, Russia.
Int J Mol Sci. 2017 Nov 15;18(11):2421. doi: 10.3390/ijms18112421.
This review discusses principal patterns that govern the processes of lignins' catalytic oxidation into vanillin (3-methoxy-4-hydroxybenzaldehyde) and syringaldehyde (3,5-dimethoxy-4-hydroxybenzaldehyde). It examines the influence of lignin and oxidant nature, temperature, mass transfer, and of other factors on the yield of the aldehydes and the process selectivity. The review reveals that properly organized processes of catalytic oxidation of various lignins are only insignificantly (10-15%) inferior to oxidation by nitrobenzene in terms of yield and selectivity in vanillin and syringaldehyde. Very high consumption of oxygen (and consequentially, of alkali) in the process-over 10 mol per mol of obtained vanillin-is highlighted as an unresolved and unexplored problem: scientific literature reveals almost no studies devoted to the possibilities of decreasing the consumption of oxygen and alkali. Different hypotheses about the mechanism of lignin oxidation into the aromatic aldehydes are discussed, and the mechanism comprising the steps of single-electron oxidation of phenolate anions, and ending with retroaldol reaction of a substituted coniferyl aldehyde was pointed out as the most convincing one. The possibility and development prospects of single-stage oxidative processing of wood into the aromatic aldehydes and cellulose are analyzed.
这篇综述讨论了木质素催化氧化成香草醛（3-甲氧基-4-羟基苯甲醛）和丁香醛（3,5-二甲氧基-4-羟基苯甲醛）的主要规律。它考察了木质素和氧化剂性质、温度、传质以及其他因素对醛收率和过程选择性的影响。综述表明，各种木质素的催化氧化过程组织得当，在香草醛和丁香醛的收率和选择性方面，仅略逊于硝基苯氧化（10-15%）。该过程中氧气（以及相应的碱）的高消耗（每摩尔获得的香草醛超过 10 摩尔）被突出为一个未解决和未探索的问题：科学文献几乎没有研究致力于减少氧气和碱的消耗的可能性。讨论了不同的木质素氧化成芳香醛的机制假设，并指出了包括酚盐阴离子单电子氧化步骤，以及取代松柏醛的反醛缩合反应的机制是最有说服力的机制。分析了将木材氧化成芳香醛和纤维素的单阶段氧化处理的可能性和发展前景。
