Rules and mechanism for the oxidation of lignin-based aromatic aldehyde under alkaline wet oxygen

Abstract

To explore the extensive oxidative mechanism of syringaldehyde, vanillin, and p-hydroxybenzaldehyde during the alkaline wet oxidation process (AWOP), the yield of these aromatic aldehydes from AWOP at various temperatures and reaction times was studied. The results showed that the aromatic aldehydes could not be converted into the corresponding aromatic acids during the AWOP, and that the aromatic aldehyde was stable when subjected to the oxygen-free AWOP. However, as the reaction temperature increased or the number of methoxyl groups on the aromatic ring increased, the yield of aromatic aldehyde sharply decreased during the AWOP. The reason for the decreased yield was that the aromatic aldehyde underwent ring-opening reactions. The possible mechanism of the degradation indicated that the number of methoxyl groups on the phenolic ring determined the stability of the key intermediate of this reaction. The conversion of isoeugenol to vanillin during the AWOP at 60 ºC was > 99%, and the vanillin yield and selectivity were both 69.8%, indicating that the decrease in yield of the product from this degradation of lignin substrate and the product can be reduced or avoided in the AWOP at suitable temperature.

Advanced Lignin Valorization for Biorefinery Application

“…These aldehyde fragments can further undergo oxidation and decarboxylation to form the final products, namely vanillin (from guaiacyl units in lignin) and syringaldehyde (from syringyl units in lignin). While most reaction pathways favor aldehyde formation, excessive oxidation (overoxidation) can break down the aldehyde chain further, leading to the corresponding aromatic acids [59,67,68]. Some intermediate hydroxy acids formed during lignin cleavage can also undergo decarboxylation to yield aromatic acids, and certain side reactions with intermediates can also form aromatic acids as minor products.…”