Aqueous-phase furfural-acetone aldol condensation over basic mixed oxides

Catalytic aqueous-phase aldol-condensation of acetone and furfural has been studied in this work. Three different mixed-oxides catalysts (Mg–Zr, Mg–Al and Ca–Zr, with different basic sites distribution) were studied, their activity and selectivity being correlated with their physico-chemical properties. Catalysts with the highest concentration of basic sites (especially medium-strength basic sites) are the most active and selective for the C13 fraction, whereas molar ratios of 1:1 yield the highest selectivities for C13 fraction (more than 60% atomic yield for the Mg–Zr mixed oxide). Concerning to reaction mechanism, cis isomers are the first ones formed, whereas trans isomers are the most abundant at higher reaction times. The main causes of catalysts deactivation are the modification of the physico-chemical properties of catalysts – because of the interaction with water – and in minor extent, the leaching effect; although homogeneous catalytic effects are discarded at studied conditions.The products formed, and the kinetic dependence on reactant concentration, are consistent with a catalytic mechanism in which the rate-determining step is the formation of the enolate species. The model considers consecutive reversible reactions yielding C8, in the first step, and C13, in the second step; with a first-order dependence on the species with α-hydrogen (acetone and C8), and zero-th order on those species without it (furfural and C13).

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► Studied reaction strongly depends on the concentration of medium-strength basic sites.
► Selectivity towards C13 depends on the molar ratio of reactants, being 1:1 the optimum ratio.
► Rates and selectivities are consistent with consecutive reversible reactions yielding C8 and subsequently C13.
► Catalyst stability in aqueous phase is a key issue in the process development.