Esterification via saccharine mediated silica solid catalyst

Saccharine (Sac) was immobilized onto rice husk ash (RHA) silica via 3-(chloropropyl)triethoxysilane (CPTES) to form a new catalyst, RHAC-Sac. The saccharine loading on the silica was found to be ca. 82%. The 29Si MAS NMR showed the presence of T2, T3, Q3 and Q4 silicon centers. The 13C MAS NMR showed that RHAC-Sac had three chemical shifts consistent with the three carbon atoms of the propyl group and a series of chemical shifts consistent with the presence of the aromatic ring. RHAC-Sac had a specific surface area of 250 m2 g−1 and a narrow average pore size of 3.28 nm. RHAC-Sac was used as a catalyst in the esterification reaction between ethyl alcohol and acetic acid. A 66% conversion was achieved at 85 °C with 100% selectivity for the ester. The stoichiometric composition of acid:alcohol = 1:1 was found to give the highest conversion. The use of Sac as a homogeneous catalyst also gave similar results to that of RHAC-Sac. RHAC-Sac could be reused several times by regenerating at 150 °C.

The weakly basic catalyst RHAC-Sac is believed to use the amine group in the catalytic esterification of carboxylic acids. It forms a strong conjugate acid pair through resonance with the SO2 group. This can capture a proton from ethyl alcohol to form an alcoholic-ethoxide which can act as a nucleophile. The reaction takes place on the catalyst surface continuously with elimination of water.Figure optionsDownload as PowerPoint slide