Studies on the Stability and Deactivation Mechanism of Immobilized Ionic Liquids in Catalytic Esterification Reactions

Solid-supported ionic liquid catalysts (SILs) are the simplest form of a heterogenized ionic liquid and have attracted soaring attention because of the high catalytic activity as well as separation. Unfortunately, instability severely hinders their practical application, and the reason for the deactivation of SILs has not been investigated in detail. In the present study, the immobilized ionic liquid catalysts MIL-101-[IA-SO3H][HSO4] and MIL-101-[IA-COOH][HSO4] were prepared and used to study the stability in the esterification reaction. The results show that compared with MIL-101-[IA-COOH][HSO4], MIL-101-[IA-SO3H][HSO4] has a higher catalytic activity and a lower stability. The deactivation mechanism is discussed based on experiments and theoretical analysis: the protons on -SO3H dissociate in a polar solvent and combine with anion HSO4-, and then, the formative H2SO4 molecule will leach out into the solvent. Our discussion indicates that the stability of immobilized ionic liquids is determined by the substituents of ionic liquid cations and becomes the significant factor controlling the stability limits. The study presented here would be important for understanding the deactivation reason and can help in choosing the suitable cation to avoid leaching of the active site during the reaction.