Ethane-bridged hybrid mesoporous functionalized organosilicas with terminal sulfonic groups and their catalytic applications

Abstract

The synthesis of a new class of sulfuric acid-functionalized ethane-bridged, mesoporous hybrid organosilicas is reported where the propylsulfonic groups attached with ethane-bridged silica exhibited reasonable catalytic activity in esterification of acetic acid with ethanol as well as the hydrolysis of cyclohexylacetate. Materials were synthesized by co-condensation of ethane-bridged organosilane (MeO)₃SiCH₂CH₂Si(OMe)₃ with 3-mercaptopropyltrimethoxysilane (MeO)₃SiCH₂CH₂CH₂SH in the presence of octadecyltrimethylammonium chloride surfactant. Powder X-ray diffraction patterns and nitrogen sorption analysis reveal the formation of mesoporous material with uniform porosity. The maximum content of incorporated mercaptopropylsilane in the mesoporous framework obtained was 3.45 mmol g⁻¹. The thiol (–SH) moieties of mercaptopropyl groups that protrude into the pore channels are readily accessible for further oxidation. The surface moieties functionalized with propylsulfonic groups (–CH₂CH₂CH₂–SO₃H) were generated by the controlled oxidation of propylthiol surface groups using concentrated HNO₃. A maximum acid exchange capacity (acid–base titration methods) of 1.38 H⁺ mmol g⁻¹ was achieved after oxidation. Further, the materials were characterized using elemental analysis, FT-IR, ²⁹Si and ¹³C MAS-NMR, transmission electron microscopy and TGA. Two probe reactions, the esterification of acetic acid with ethanol and its reverse the hydrolysis of cycloacetonate, showed the significance of the material in catalytic applications. The catalytic results showed the esterification activity is comparable to that of commercial Nafion-H and also the involvement of hydrophobic nature of the material in the hydrolysis reaction.