A novel insight of the catalytic mechanism of ionic liquids and H2SO4 on the synthesis of 1,3,5-trioxane

Abstract

Clarifying the catalytic mechanism is essential to intensify the production of 1,3,5-trioxane (TOX). The equilibrium distribution of linear oligomers (MG_n) in formaldehyde aqueous solution (FA-AS) has an important effect on the TOX yield. The equilibrium distributions of MG_n in FA-AS at 25 °C with the addition of 1,3-bis-(3-sulfonic acid)propyl imidazolium bisulfate ([PS₂Im]HSO₄), 1-(3-sulfonic acid)propyl-3-methylimidazolium bisulfate ([PSMIm]HSO₄), 1-propyl-3-methylimidazolium bisulfate ([PMIm]HSO₄), and H₂SO₄ were systematically studied respectively. The results show that the reaction from MG₁ to MG_n (n > 1) was promoted by the above-mentioned additives. The performance of these catalysts in changing the equilibrium content of MG_n was in the order [PS₂Im]HSO₄ > H₂SO₄ > [PSMIm]HSO₄ > [PMIm]HSO₄. This order was consistent with the experimental results about the intensification of ILs to the synthesis of TOX catalyzed by H₂SO₄. Furthermore, the mechanism was also studied by DFT calculation and experimental verification. The results show that the H-bonds interaction between H₂SO₄/ILs and H₂O reduces the water activity, and then changes the equilibrium distribution of MG_n. With the increase of MG₃ concentration and the acidity in FA-AS, the formation reaction of TOX was promoted. This study provided a theoretic guidance for the development of catalysts or cocatalysts of the TOX synthesis.