Synthesis of polyoxymethylene dimethyl ethers catalyzed by Brønsted acid ionic liquids with alkanesulfonic acid groups

Catalytic reaction of methylal (DMM) with trioxane (TOX) for preparation of polyoxymethylene dimethyl ethers (PODME_n, CH₃O(CH₂O)_nCH_3, where n > 1) was investigated in a series of Brønsted acid ionic liquids with alkanesulfonic acid groups. The structures, acidities, and properties of ionic liquids were experimentally characterized and theoretically analyzed. The Brønsted acidity-activity correlation for the ionic liquids was studied. Among all these ionic liquids, 1-(4-sulfonic acid) butylpyridinium hydrogen sulfate ([PY-BS][HSO₄^-]) exhibited the best catalytic performance, which was ascribed to its strong Brønsted acidity. The influences of ionic liquid dosage, molar ratio of DMM to TOX, and reaction temperature on the catalytic reaction were explored using [PY-BS][HSO₄^-] as the catalyst. Under the optimal conditions of n(methylal):n(trioxane):n(ILs) = 180:60:1, 170°C, and 10h, the conversion of trioxane and the selectivity of PODME_3-8 were as high as 91.18% and 70.90%, respectively. The ionic liquid [PY-BS][HSO₄^-] could be reused.