Synthesis of methylal from methanol and formaldehyde catalyzed by Brønsted acid ionic liquids with different alkyl groups

The catalytic reaction of methanol with formaldehyde for the preparation of methylal was investigated in various Brønsted acid ionic liquids with different carbon chain length of alkyl groups. The structures, acidities, and properties of ionic liquids were experimentally characterized and theoretically analyzed. The Brønsted acidity-viscosity-activity correlation for the ionic liquids was studied. Among all these ionic liquids, [C₆ImBS][HSO₄] exhibited the best catalytic performance, which was ascribed to its strong Brønsted acidity and low viscosity. The catalytic activity of the ionic liquid was near that of concentrated sulfuric acid. The influences of ionic liquid dosage, reaction temperature and molar ratio of methanol to formaldehyde were explored using [C₆ImBS]HSO₄ as the catalyst. Under the optimal conditions of n(methanol):n(formaldehyde):n(ILs) = 2.5:1:0.0258, 60 °C, and 4 h, the conversion of formaldehyde can reach 63.37%. The ionic liquid [C₆ImBS]HSO₄ could be reused.