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SiO₂ Immobilized Imidazolium Ionic Liquid as Acid Catalysts for Diacetins and Triacetins Synthesis in Glycerol Esterification

Yong Beom Park, Palraj Kasinathan*

Allyimidazolium based Bronsted acid ionic liquids with triflate ([ASBI][TfO]) and sulphate ([ASBI][HSO₄]) anions, and it’s lewis acid derivatives ([ASCBI][TfO]) ([ASCBI][HSO₄ ]) were synthesized to immobilize covalently on silica via anchoring thiol (-SH) functional group. The surface characterization of these silica immobilized ionic liquids was done by FTIR, TGA, elemental analysis, N2 surface area and acidity measurements. These high acidity SiO2 immobilized acid ionic liquids were used as bronsted and lewis acid catalysts in the esterification of glycerol with acetic acid, and found to have high selectivity for Diacetin (DAG) and Triacetins (TAG) compared with other established heterogeneous acid catalysts with excellent recyclability. To investigate the selectivity towards 1,3 diacetins and 1,2 diacetins, the acetylation reaction was studied at low conversions. The diacetin selectivity was also studied for different diols to compare with vicinal and terminal diols and found that diacetin selectivity is highly dependent on the chain length of the diols. The Bronsted acidic catalyst SiO₂-[ASBI][HSO₄], because of its high acidity was chosen to study the conversion and selectivity attributed to reaction parameters such as time, reaction temperature, catalyst loading and glycerol to acetic acid molar ratio.