Highly stable chromium(III) terephthalate metal organic framework (MIL-101) encapsulated 12-tungstophosphoric heteropolyacid as a water-tolerant solid catalyst for hydrolysis and esterification

A heterogeneous catalyst, chromium(III) terephthalate metal organic framework (MIL-101) encapsulated 12-tungstophosphoric heteropolyacid (HPW), HPW@MIL-101, was prepared by a simple one-step hydrothermal reaction of aqueous mixtures of Cr(NO3)(3), terephthalic acid, and HPW. The obtained catalyst was systemically characterized by XRD, N-2 adsorption, FT-IR, acid-base titration, and P-31 MAS NMR techniques as well as by elemental analysis. The catalytic activity and reusability of HPW@MIL-101 were evaluated in the liquid-phase hydrolysis of ethyl acetate and in esterification of acetic acid with n-hexanol. In both reactions, water exists as a reactant/solvent and as a product. HPW@MIL-101 shows a high activity and excellent reusability in both of the reactions, behaving as a water-tolerant heterogeneous acid catalyst without agglomeration, leaching and deactivation of HPW during its repeated use. The unique characteristics of MIL-101 and the well-dispersed level of HPW confined as noncoordinating guests in the mesoporous cages of the MIL-101 matrix with the integrated protonic acidity of HPW may account for the high catalytic properties of HPW@MIL-101.