Synthesis and Catalytic Activity of Novel Heternuclear Dication Bronsted Acidic Ionic Liquids

Three new Brosted-acidic task-specfic ionic liquids (TSILS) that bear two butane sulfonic acid groups based on 1-(morpholinyl-4-yl)-3-(imidazolyl-1-yl)propane (mip) heternuclear dication: 1-[4-(4-sulfobutyl)morpholinium-4-yl]-3-[3-(4-sulfobutyl)-1H-imidazolium-1-yl)propane bis(trifluoromethane sulfonate) {A, [(C4SO3H)m(C4SO3H)i]p(OTf)(2)}, 1-[4-(sulfobutyl)morpholinium-4-yl]-3-[3-(4-sulfobutyl)-1H-imidazolium-1-yl]propane bis(hydrogen sulfate) {B, [(C4SO3H)m(C4SO3H)i]p(HSO4)(2)} and 1-[4-(4-sulfobutyl)morpholinium-4-yl]-3-[3-(4-sulfobutyl)-1H-imidazolium-1-yl]propane bis(bihydrogen phosphate) {C, [(C4SO3H)m(C4SO3H)i]p(H2FO4)(2)} were synthesized and characterized by NMR and FT-IR. The imidazolium-based and morpholinium-based single-SO3H functionalized acidic ionic liquids were used for comparison. Their catalytic activities were investigated in crossed-aldol condensation between cycloalkanone and various aromatic aldehydes. Hammett method has been used to determine the acidity order of these ionic liquids (ILs). The experiment results have shown that the catalytic activities of the three bi-SO3H-functionalized ILs were stronger than those of single SO3H functionalized ILs. Their acidity orders were consistent with their catalytic activities. Good adaptability to the reaction substrate and maximum product yield of 90% was observed when [(C4SO3H)m(C4SO3H)i]p(OTf)(2) was used as catalyst and the catalyst can be reused at least ten times without obvious activity loss.