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.

Extractive-free pine wood was processed into vanillin (up to 18 wt.% of the initial lignin) and cellulose (typically 84–93% of the initial amount in the wood) by one-step catalytic oxidation followed by enzymatic hydrolysis of the resulting cellulose into glucose (reducing sugar yield up to 70% based on the post-oxidation cellulose). Correlation between the cellulose conversion in hydrolysis and the lignin content in the post-oxidation lignocellulosic material was established, which follows the general trend for the products of various delignification methods. The obtained results demonstrate the practical possibility of efficient two-step processing of wood into vanillin and glucose.

The capabilities of crystallization and extraction methods for separation of vanillin and syringaldehyde are described. Crystallization methods are based on differences in solubility of aldehydes ammonia derivatives in water, and solubility of their potassium salts in concentrated aqueous solutions of potassium carbonate. Temperature dependencies of solubility of potassium vanillate and potassium syringate in aqueous solutions containing 300–500 g/l of potassium carbonate are presented. The crystallization methods give separation factors of ca. 50 and allow to isolate syringaldehyde from its mixture with vanillin by one-stage crystallization with yield over 95% and purity over 98%. Extraction methods have lower separation factors of 2−2.5.

Flax shive is the main waste (up to 70 wt %) in the production of flax fiber. It represents the lignified parts of the flax stem mainly in the form of small straws. Complex processing of such wastes is a significant problem due to the heterogeneity of the chemical structure of lignin. This article presents the results of reductive catalytic fractionation (RCF) of flax shive in ethanol and isopropanol at elevated temperatures (225–250 °C) in the presence of a bifunctional catalyst (Ru/C) and molecular hydrogen. This provides solvolytic depolymerization of lignin and hemicelluloses presented in flax shive. Catalytic hydrogenation effectively stabilizes the formed lignin intermediates and prevents repolymerization reactions producing the lignin fraction with a high degree of depolymerization. RCF of flax shive produces solid products with a high cellulose content and liquid products consisting mainly of monophenolic compounds. Furthermore, the effect of different characteristics (the ruthenium content, particle size, and support acidity) of the bifunctional catalysts containing ruthenium nanoparticles supported on mesoporous, graphite-like carbon material Sibunit^®-4 on the yield and composition of the products of hydrogenation of flax shive in sub- and super-critical ethanol has been studied. Bifunctional catalysts Ru/C used in the RCF of flax shive increase its conversion from 44 to 56 wt % and the yield of monophenols from 1.1 to 10.2 wt % (based on the weight of lignin in the sample). Using the best Ru/C catalyst containing 3% of Ru on oxidized at 400 °C carbon support, the high degree of delignification (up to 79.0%), cellulose yield (up to 67.2 wt %), and monophenols yield (up to 9.5 wt %) have been obtained.

The influence of pine wood (Pinus Sylvestris) prehydrolysis on the consumption of main reagents (O₂ and NaOH) in the subsequent catalyzed oxidation of the obtained lignocelluloses into vanillin and pulp has been studied. Prehydrolysis of the feedstock was performed with HCl in the water and gas phases. Catalytic oxidation was performed in an autoclave in an aqueous–alkaline medium in the presence of CuO as a catalyst. Native lignin was shown to be inhomogeneous in the process of oxidation into vanillin, and in the first approximation lignin consists of two fractions: the first, which is easily and selectively oxidized to vanillin, and the second, which is oxidized slowly and non-selectively. Acid-catalyzed prehydrolysis of wood leads to further differentiation of the lignin into the selectively and non-selectively oxidizing fractions. The developed two-stage process (prehydrolysis of wood and oxidation of the resulting lignocelluloses) decreases the consumption of NaOH in the oxidation process by 2–5 times as compared to the known data down to the values of 3.7–5.2 g per g of vanillin obtained.

Текст статьи не публикуется в открытом доступе в соответствии с политикой журнала.Preparative amounts of furfuryl alcohol ethers and furfural acetals were prepared from renewable vegetable raw materials. The blending reseach octane numbers of mixing of furan derivatives in straight-run gasoline were estimated: butyl furfuryl ether, 97.8 ± 7; propyl furfuryl ether, 112 ± 6; furfural diethyl acetal, 105 ± 6, furfural ethylene glycol acetal, 108 ± 7; furfurylamine, 194 ± 4. These results demonstrate prospects for using furan derivatives as available biofuels