Silica-Gel-Supported Dual Acidic Ionic Liquids as Efficient Catalysts for the Synthesis of Polyoxymethylene Dimethyl Ethers

The supported ionic liquids were prepared by anchoring 3-sulfobutyl-1-(3-propyltriethoxysilane) imidazolium hydrogen sulfate onto silica gel of different types. The properties of the catalysts were characterized by elemental analysis, Fourier transform infrared (FT-IR) spectra, scanning electron microscopy (SEM), thermogravimetry/differential scanning calorimetry (TG/DSC), and X-ray photoelectron spectroscopy (XPS). The catalytic activities of both the immobilized ionic liquids and the free one (1-butylsulfonate-3-methylimidazolium bisulfate, [MIM-BS][HSO4–]) were investigated by the synthesis of polyoxymethylene dimethyl ethers (DMMn) from methylal (DMM) and trioxane (TOX), which suggested that all covalently anchored ionic liquids have better catalytic effects than [MIM-BS][HSO4–]. The optimization experiment of catalyst dosage demonstrated that the acid amount of ionic liquid is a major factor for the catalytic activity.