Vanillin oligomerization as a model of side reactions in lignin fragmentation

Vanillin (p-hydroxy, m-methoxybenzaldehyde) has been tested as a model molecule for the rearrangement of intermediate products of lignin fragmentation, an important step in the chemical valorisation of lignocellulosic biomass. The reactivity of vanillin has been studied in neutral aqueous media at 200 °C under controlled pressure of oxygen. The characterisation of hydrophobic and hydrophilic products by gas chromatography, size exclusion chromatography and FT-IR spectroscopy are provided. The presence of heterogeneous catalysts modifies conversion and selectivity of the oxidation reactions. The nature of the catalysts (nanosized Co3O4 and Mn3O4 and Pd/alumina) significantly affect the level of total oxidation as well as the nature and the yield of the products of mild oxidation. At moderate oxygen pressure, the main products are hydrophilic oligomers rich in free and bonded carboxylic groups. The products formed present similar structural features very similar to the products classically obtained in oxidative treatments of lignin. The results indicate that vanillin is a representative model molecule for studies of lignin valorisation.