Synthesis of polyoxymethylene dimethyl ethers from methylal and trioxane catalyzed by Brønsted acid ionic liquids with different alkyl groups

Brønsted acid ionic liquids with different alkyl group carbon chain lengths and an alkane sulfonic acid group were synthesized through bromoalkane, imidazole and 1,4-butane sultone as raw materials. The structures and properties of the ionic liquids were experimentally characterized. Catalytic reaction of methylal (DMM) with trioxane (TOX) for preparation of polyoxymethylene dimethyl ethers (PODME_n, CH₃O(CH₂O)_nCH₃, where n > 1) was investigated in various Brønsted acid ionic liquids with different carbon chain length of alkyl groups. The carbon chain length of alkyl groups and activity correlation for the ionic liquids was studied. It was found that the structures of ionic liquids were consistent with the designed structure and their purities were high. They possessed high thermal stability and wide liquid range. The hydrophobicity of ionic liquids became stronger with the increase of carbon chain length. With increasing the carbon chain length of ionic liquids, the selectivity of PODME_3-8 is increased at first and then decreased. Among all the ionic liquids, [C₆ImBS][HSO₄] shows the best catalytic performance and the selectivity of PODME_3-8 is 57.85%.