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

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, N-2 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, N-2 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 70wt% 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. (c) 2014 Elsevier B.V. All rights reserved.