Intensification Mechanism of Acidic Imidazole-Based Ionic Liquids on the Synthesis of 1,3,5-Trioxane Catalyzed by Sulfuric Acid

The production of 1,3,5-trioxane (TOX) from formaldehyde (FA) solution is seriously limited by the reaction equilibrium. 1-Propyl-3-methylimidazolium bisulfate ([PMIm]HSO4), 1-(3-sulfonic acid)propyl-3-methylimidazolium bisulfate ([PSMIm]HSO4), and 1,3-bis(3-sulfonic acid)propyl imidazolium bisulfate ([PS2Im]HSO4) were, respectively, used as cocatalysts of H2SO4 to intensify the TOX synthesis from FA solution. The results show that the intensification performance of ILs on the TOX synthesis was according to the following order: [PS2Im]HSO4 > [PSMIm]HSO4 > [PMIm]HSO4. The yield of TOX was increased from 13.84 to 16.95% with the addition of 0.5 mol [PS2Im]HSO4 per kilogram of FA solution. DFT calculation and MD simulation were conducted, and the activity of water was measured to clarify the intensification mechanism. The results illustrated that the water activity had a strong influence on the TOX yield. In the three ILs, [PS2Im]HSO4 has the highest dipole moment and the largest overall charged surface areas. It can form the most stable H-bonds with H2O molecules, resulting in the biggest decrease in the activity of water, thereby showing the best performance to shift the reversible reaction of TOX synthesis to right forward. The result provides a guidance for the development of new catalysts for TOX synthesis.