Polyoxometalates confined in the mesoporous cages of metal-organic framework MIL-100(Fe): Efficient heterogeneous catalysts for esterification and acetalization reactions

12-Tungstophosphoric heteropolyacid (HPW) has been encapsulated in a mesoporous metal-organic framework (MOF), MIL-100(Fe), by a simple low-temperature (<100 degrees C) HF-free incorporation route via the reaction of ferric nitrate and trimesic acid in the presence of HPW. The resulting hybrid HPW@MIL-100(Fe) material was systemically characterized by XRD, N-2 adsorption, FT-IR, acid-base titration, P-31 MAS NMR, TEM, SEM, TGA, XPS techniques and elemental analysis, and then used as a heterogeneous catalyst in acid-catalyzed esterification and acetalization. The characterization results indicate that the HPW molecules were successfully incorporated within the mesoporous cages of the MIL-100(Fe) matrix as noncoordinating guests, thus maintaining the integrity of the protonic acidity of HPW. The resulting HPW@MIL-100(Fe) catalyst exhibits high activity and excellent reusability in the esterification reactions with no evidence for agglomeration, leaching, or deactivation of the HPW during several repeated uses of the catalyst. Similarly, the HPW@MIL-100(Fe) catalyst also shows excellent catalytic properties in the acetalization of benzaldehyde and ethanediol. The unique characteristics of MIL-100(Fe) and the uniform dispersion of HPW molecules within the mesoporous cages of the MIL-100(Fe) matrix may account for the high catalytic activity and recyclability of the HPW@MIL-100(Fe) catalyst. (C) 2015 Elsevier B.V. All rights reserved.