Glycerol acetylation over new hybrid phosphate or sulphate silica catalysts

Biomass represents, nowadays, one of the most interesting alternatives to the fossil fuel as font of energy. Different platform molecules can be obtained from biomass and used, as it or as derivatives, as green and sustainable products. Glycerol, principal by-product (10% wt) in the transesterification of oils, represents an available platform molecule to achieve value added-products. Recently, several heterogeneous acid catalysts (Amberlyst-15, zeolites, zirconia derivatives, sulfonic hybrid silica) [2 and reference herein] were studied on esterification reaction of glycerol with acetic acid to achieve the corresponding mono-(MAG), di-(DAG) and tri-(TAG) acetins. The present work deals with the synthesis, characterization and catalytic properties of new hybrid organic-inorganic catalysts containing sulphate or phosphate functions on the glycerol acetylation. Fresh materials were characterized by TGA, XPS and acidity. The catalytic reactions were performed by using a mass ratio of catalyst/glycerol of 5 wt% and molar ratio acetic acid/glycerol corresponding to 3:1 [2]. In order to compare the activity of the new catalysts with commercial ones, sulfonic (22wt% H2SO4@SiO2) and phosphonic (0.7-1.3 mmol/g) silica were purchased by Aldrich. Recycling experiments were performed over the most active systems. Silica-bonded N-propyl sulfamic acid (SBNPSA) (1) showed the best catalytic behaviour towards the conversion of glycerol into TAG. The comparison between SBNPSA and the commercial H2SO4@SiO2 showed an increase of TAG when commercial catalyst was used, but a better stability of the synthetized material during the cycles. The analysis of the catalytic activities indicates that the best performance is achieved when sulfonic function is strictly linked to the amino-propyl silica. A leaching of the acid group is showed during the cycles, for these reasons further investigations are addressed toward the use of more stable mesoporous supports as SBA type.