Molecular Oxygen Lignin Depolymerization: An Insight into the Stability of Phenolic Monomers

Abstract

During oxidative depolymerization of lignin in aqueous alkaline medium using molecular oxygen as oxidant, the highly functionalized primary phenolic monomers are not stable products, owing to various not fully identified secondary reaction mechanisms. However, better understanding of the mechanisms responsible for the instability of the main part of the products of interest derived from lignin is of much interest. Evaluation of their individual reactivities under oxidative conditions should significantly help to find a better way to valorize the lignin polymer and to maximize the yields of target value‐added products. Consequently, the main objective of this study is to assess the individual stabilities of some selected lignin‐based phenolic compounds, such as vanillin, vanillic acid, and acetovanillone, together with some other pure chemical compounds such as phenol and anisole to give an insight into the mechanisms responsible for the simultaneous formation and repolymerization of those products and the influence of the oxidation conditions. Various complementary strategies of stabilization are proposed, discussed, and applied for the oxidative depolymerization reactions of a technical lignin extracted from pinewood with a high content of β‐O‐4 interconnecting bonds to try to obtain enhanced yields of value‐added products.

Catalytic oxidative conversion of C/G-type lignin coexisting in Tung nutshells to aromatic aldehydes and acids

“…48,49 We believed that an appropriate reaction temperature is essential for the oxidative depolymerization process to maintain the balance between the reactivity of the native lignocellulose substrate and the stability of protocatechuic aldehyde and protocatechuic acid products. 47,50 These thermal stability results helped us get insights into the characteristics of the C-type aromatic products despite their unsatisfactory yields, which also demonstrate the possibility of C-lignin valorization by the oxidation strategy under moderate conditions.…”

Cleavage of Organosolv Lignin to Phenols Using Nitrogen Monoxide and Hydrazine

“…To investigate this aspect, the oxidized organosolv lignin 22 was submitted to column chromatography. While the significant difficulties encountered with solubilization point to the fact that the oxidized lignin 22 has remained largely polymeric, two aldehydes, vanillin ( 23a ) and syringaldehyde ( 23b ), could be separated and identified. ,,,,, The oxidation step can thus be described as shown in Scheme , where the yields of 23a and 23b were calculated on the basis of the estimated average mass of 200 g/mol per lignin monomer (see above). As an example, the yields from an oxidation experiment with 1 g of organosolv lignin 21 refer to 5 mmol of starting material.…”

“…Regarding plausible pathways leading to the formation of phenols 17a – d from organosolv lignin (Scheme ), the intermediates vanillin ( 23a ) and syringaldehyde ( 23b ) are well known as possible cleavage products resulting from lignin under oxidative conditions (Figure ). ,,,,, …”

“…Regarding plausible pathways leading to the formation of phenols 17a−d from organosolv lignin (Scheme 5), the intermediates vanillin (23a) and syringaldehyde (23b) are well known as possible cleavage products resulting from lignin under oxidative conditions (Figure 1). 7a,b,9a,b,12b, 26 Control experiments revealed that 23a and 23b are even present in small amounts in the organosolv lignin 21 used as starting material (23a: 1.11%, 23b: 0.53%), but when treated separately under the oxidative conditions comprising nitrogen monoxide and DDQ, they turned out to be unstable, and full decomposition to a complex product mixture was observed. Accordingly, the aldehydes 23a and 23b are formed as well as partially decomposed in the oxidation step, leading from 21 to oxidized lignin 22, which is a plausible background for deviations in yields even in identical experiments.…”

Biobased Vanillin Production by Oxidative Depolymerization of Kraft Lignin on a Nitrogen- and Phosphorus-Functionalized Activated Carbon Catalyst

“…The stability of certain phenolic derivatives, produced by this route, has been studied to propose the corresponding oxidation mechanism. 26 For this purpose, the redox potential of the catalyst should be low but high enough to oxidize the phenolate anion. Thus, species such as Cu salts, which have a moderate redox potential, are good candidates for the biobased vanillin production from lignin.…”