Chromium(III) terephthalate metal organic framework (MIL-101): Hf-free synthesis, structure, polyoxometalate composites, and catalytic properties

Hybrid materials of the metal-organic framework (MOF), chromium(III) terephthalate (MIL-101), and phosphotungstic acid (PTA) were synthesized in aqueous media in the absence of hydrofluoric acid. XRD analysis of the MIL101/PTA composites indicates the presence of ordered PTA assemblies residing in both the large cages and small pores of MIL-101, which suggests the formation of previously undocumented structures. The MIL101/PTA structure enables a PTA payload 1.5-2 times higher than previously achieved. The catalytic performance of the MIL101/PTA composites was assessed in the Baeyer condensation of benzaldehyde and 2-naphthol, in the three-component condensation of benzaldehyde, 2-naphthol, and acetamide, and in the epoxidation of caryophyllene by hydrogen peroxide. The catalytic efficiency was demonstrated by the high (over 80-90%) conversion of the reactants under microwave-assisted heating. In four consecutive reaction cycles, the catalyst recovery was in excess of 75%, whereas the product yields were maintained above 92%. The simplicity of preparation, exceptional stability, and reactivity of the novel composites indicate potential in utilization of these catalytic matrices in a multitude of catalytic reactions and engineering processes.