Catalytic hydrogenation of crude hexanoic acid, easily obtained by anaerobic fermentation of grape pomace

The transition from fossil resources to renewable ones represents an urgent need. Biomasses are promising feedstocks, potentially exploitable through novel bio-catalytic processes, such as acidogenic fermentation to carboxylic acids, which can be further converted into more value-added bio-products through cascade chemical approaches, such as hydrogenation to corresponding alcohols/esters. In this work, the optimization of the hydrogenation of commercial hexanoic acid to 1-hexanol and hexyl hexanoate was first investigated. For this purpose, 5 wt% Re/C resulted active and selective towards 1-hexanol production. The same catalyst was further tested for the hydrogenation of crude hexanoic acid, obtained by fermentation of red and white grape pomaces. Hydrogenation of these crude hexanoic acid mixtures confirmed the promising performances of 5 wt% Re/C, achieving the complete substrate conversion with a prevailing selectivity to 1-hexanol (~58 mol%), rather than to hexyl hexanoate (~30 mol%). Moreover, the use of an acid support, such as Al2O3, markedly shifted the selectivity towards hexyl hexanoate (~51 mol%). This observation was further demonstrated by testing physical mixtures of 5 wt% Re/C and different amounts of acidic niobium phosphate. Based on these promising results, exploitation of grape pomace for 1-hexanol/hexyl hexanoate production, to use as bio-fuels or bio-solvents, represents a smart possibility.