Metal-organic frameworks with high tungstophosphoric acid loading as heterogeneous acid catalysts

• MIL-101 supported 12-tungstophosphoric acid prepared by two-pot synthesis strategy.
• The unique characteristics of MIL-101 enable high dispersion level of HPW.
• The prepared catalysts were used successfully in catalyzing some organic reactions.
• The catalysts showed high activity and reusability in the selected reactions.
• The prepared HPW/MIL-101 has the potential to be used as an industrial catalyst.

High loadings of 12-tungstophosphoric acid (H3PW12O40) were incorporated within highly porous metal-organic framework MIL-101. The samples have been characterized using different techniques such as XRD, TEM, FT-IR, TGA, N2 adsorption and Raman spectroscopy. XRD and TEM did not show any aggregation of the H3PW12O40 nanocrystals on the surface of MIL-101 crystals up to 70 wt%. Similarly, N2 sorption–desorption measurements reveal that samples with H3PW12O40 loading up to 70 wt% retain their mesoporosity. The incorporation of H3PW12O40 crystals gradually increases both the surface acidity and the acid strength of the H3PW12O40/MIL-101 catalysts up to 70 wt% loading. The catalytic performances of the H3PW12O40/MIL-101 solid catalysts were tested in three acid-catalyzed organic transformations, namely, the Pechmann, Esterification and Friedel–Crafts acylation reactions. In the three reactions, the catalytic activity attains the maximum value at 70 wt% of H3PW12O40 loading, thus demonstrating direct correlation with dispersion, mesoporosity and acid strength. The unique attributes of MIL-101 and a well-dispersed level of H3PW12O40 prohibit the conglomeration and deactivation of H3PW12O40, which leads to the enhancement of the catalytic properties. The H3PW12O40 catalysts supported on MIL-101 are potentially promising heterogeneous catalysts for acid-catalyzed organic transformations in environmental friendly processes, to supersede the use of conventional homogeneous H3PW12O40 catalysts.

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