Esterification of hexanoic acid to hexyl hexanoate with heterogeneous acid catalysts: preliminary evaluations with Amberlyst and niobium- zirconium-phosphates

Hexyl hexanoate (HexHex) is an ester of potentially relevant industrial interest, actually exploited as fragrance and lubricant, but also as an oxygenated additive for diesel fuels. It can be synthesised by esterification between hexanoic acid and 1-hexanol, a reaction occurring in the presence of an appropriate Brønsted acid catalyst. In this work, commercial heterogeneous Amberlyst resins and niobium- and zirconium-phosphates, have been tested for the synthesis of HexHex, optimized with the aim of reducing both the catalyst loading and the excess of the alcohol. After univariate screening of the reaction parameters, Amberlyst- A 35 was identified as the most interesting catalyst, identifying the best reaction conditions (catalyst loading: 0.5 wt%, temperature: 120°C, time: 8h, alcohol/acid molar ratio = 2), leading to a maximum hexanoic acid conversion of about 93 mol%, with a complete selectivity to the target ester. This promising achievement encourages further deepening of the kinetic aspects, to further improve its sustainable and clean production for energy uses.