Selective catalytic oxidation of humins to carboxylic acids using the H4[PVMo11O40] Keggin-type polyoxometalate enhanced by alcohol doping and solubilizer

Abstract

Oxidative valorization of humins is one promising approach for establishing carbon efficient biomass valorization pathways. In the present contribution, the development of an optimized reaction system for the selective catalytic oxidation (SCO) of water-insoluble and highly complex humins to short-chain carboxylic acids like formic acid (FA) and acetic acid (AA) using Keggin-type polyoxometalates is presented. In detail, the monovanadium-substituted polyoxometalate H₄[PVMo₁₁O₄₀] catalyst exhibited a considerable selectivity advantage in the aqueous phase over state-of-the-art catalysts. More specifically, the yield of the desired products FA and AA (esters) could be drastically improved up to 30%, while undesired side products resulting from thermal-induced decarbonylation and decarboxylation were drastically reduced down to a third. Hereby, alcoholic additives like methanol show a remarkable inhibiting effect on CO₂ formation. It was shown that a temperature of 120 °C represents an optimum where methanol can still inhibit CO₂ formation even at a low alcohol content of 5 vol%. Furthermore, the use of para-toluenesulfonic acid (pTSA) as solubilizer has been investigated to circumvent the water-insoluble character of humins and to optimize humin conversion. On the one hand, pTSA could efficiently promote the activity in the SCO of humins. On the other hand, methanol was far more efficient in inhibiting CO₂ formation, especially at elevated reaction temperatures. The results were reproducible and could even be transferred to various humins based on different sugars. Using the combination of methanol and pTSA as additives as well as a singly vanadium-substituted polyoxometalate catalyst provides a highly promising approach for the valorization of complex humins.