Performance of basic mixed oxides for aqueous-phase 5-hydroxymethylfurfural-acetone aldol condensation

- Oligomerization reactions, strong adsorption, and reaction reversibility hinders the reaction.
- Extent of these undesirable processes is higher than for furfural-acetone condensation.
- Selectivity for C15 adduct only significant for Mg-Zr, but lower than for furfural reaction.
- Kinetic model considering consecutive reversible reactions yielding C9 and C15.
- Stability in aqueous phase conditioned by the oligomerization reactions, not by catalyst leaching.

This paper is focused on the study of the catalytic activity of MgAl and MgZr mixed oxides in the aqueous-phase aldol-condensation of 5-hydroxymethylfurfural (5-HMF) and acetone. The experiment is carried out in a stirred batch reactor. It considers the effect of reaction conditions (namely temperature, reactant concentration and catalyst loading) on catalyst performance. It also looks into the stability of the materials (catalyst reusability in subsequent reaction cycles), as well as into the development and validation of a simplified kinetic model. MgZr catalyst provides the best yield to aldol condensation products, especially for the formation of the second adduct (C15). This material high activity results from its optimum acid/basic sites ratio, mainly medium-strength. Retroaldolization, promoted by stronger basic sites, was identified as an important side-reaction, whereas adsorption effects and oligomerization reactions (evaluated using thermogravimetric analysis and temperature-programmed oxidation of the spent catalysts) determine catalyst stability. Concerning the effect of operation conditions, the best results were obtained working with 0.5 g of MgZr, at 323 K. The highest C15 yield was reached with 5-HMF excess (2:1) (16.1%), whereas the whole yield is maximized at equimolar conditions (37%).

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