Supported Keggin heteropolyacid catalysts for glycerol acetylation

Heteropolyacids (HPAs) are a class of molecular metal oxides called polyoxometalates, with a heteroatom in the cluster structure forming an oxo-metallic cluster with the addenda metal atoms. Due to the tunable acid-base and redox properties, HPAs have been used as catalysts both for homogeneous and heterogeneous reactions. Many efforts have been devoted to HPAs heterogeneization because, being soluble in polar solvents, their separation and recovery in liquid-solid regime reactors is a relevant practical application problem. Moreover, due to the low specific surface areas (1÷10 m2 g-1) their dispersion on supports with high surface area can increase the accessibility of the reacting species to their acidic sites and consequently their catalytic activity as Brönsted acid and oxidant catalysts. HPAs have been used in many acidic catalysed reactions, among them, the esterification of glycerol into acetins constitutes an important example due to the versatile industrial application of the products. Monoacetin (MAG) and diacetin (DAG) could be used as solvents, plasticizer, softening agent and solvent for dyes, whereas triacetin (TAG) is an antiknock additive for gasoline. The supported Keggin heteropolyacid PW12 has been prepared successfully avoiding leaching phenomena in liquid-solid regime. The composites resulted active as heterogeneous catalysts for glycerol acetylation. The hydrothermal conditions gave rise to a material where the PW12 structure and acidity was preserved giving rise to a glycerol conversion of ca. 75-80% to form mainly MAG and in less amount DAG.