Kinetic and mechanistic study of triose sugar conversion on Lewis and Brønsted acid solids

Abstract

The effect of catalyst acid site nature on the reaction kinetics of the liquid-phase conversion of dihy-droxyacetone with ethanol was investigated using Brønsted and Lewis acid solids. The reaction proceedsthrough a complex reaction network involving a sequence of consecutive and parallel reaction steps.Main final products were ethyl lactate and pyruvic aldehyde diethyl acetal. Different catalysts such asthe cesium salt of tungstophosphoric acid (Cs-HPA), Amberlyst resin and alumina-supported tin and zincoxides were used. Catalysts were characterized by ICP, X-ray diffraction, N2physisorption, TPD of NH3and FTIR of adsorbed pyridine. Cs-HPA contains only Brønsted acid sites and alumina-supported tin andzinc samples show exclusively Lewis acidity. During dihydroxyacetone conversion, the selectivities tothe final products strongly depend on the nature of the surface acid sites present on the solids. Brønstedacid solids favor the synthesis of pyruvic aldehyde diethyl acetal with final yields of up to 93%, whereasthe ethyl lactate is the main product on Lewis acid solids, reaching final yields of about 51%. A kineticmodel based on a pseudohomogeneous mechanism was proposed to interpret the experimental catalyticdata. The comparison of the kinetic rate constants obtained on both kinds of acid solids confirms that onBrønsted catalysts the route toward pyruvic aldehyde diethyl acetal is clearly favored. On Lewis solids,the synthesis of ethyl lactate occurs selectively via isomerization of the pyruvic aldehyde hemiacetalintermediate.