A silicododecamolybdate/pyridinium-tetrazole hybrid molecular salt as a catalyst for the epoxidation of bio-derived olefins

[Display omitted] •Synthesis of a new hybrid salt (2-(tetrazol-5-yl)pyridinium)4[SiMo12O40]∙nH2O (1).•Crystalline 1 is an extended 3D hydrogen-bonding array mediated by H2O molecules.•Catalyst 1 outperforms (Bu4N)4[SiMo12O40] in homogeneous olefin epoxidation.•Catalyst 1 promotes the epoxidation of dl-limonene and fatty acid methyl esters.•The bio-olefin reactions catalyzed by 1 lead to useful (di)epoxide and diol products. The hybrid polyoxometalate (POM) salt (Hptz)4[SiMo12O40]∙nH2O (1) (ptz = 5-(2-pyridyl)tetrazole) has been prepared, characterized by X-ray crystallography, and examined as a catalyst for the epoxidation of cis-cyclooctene (Cy) and bio-derived olefins, namely dl-limonene (Lim; a naturally occurring monoterpene found in the rinds of citrus fruits), methyl oleate and methyl linoleate (fatty acid methyl esters (FAMEs) obtained by transesterification of vegetable oils). The crystal structure of 1 consists of α-Keggin-type heteropolyanions, [SiMo12O40]4-, surrounded by space-filling and charge-balancing 2-(tetrazol-5-yl)pyridinium (Hptz+) cations, as well as by a large number of water molecules of crystallization (n = 9). The water molecules mediate an extensive three-dimensional (3D) hydrogen-bonding network involving the inorganic anions and organic cations. For the epoxidation of the model substrate Cy in a nonaqueous system (tert-butylhydroperoxide as oxidant), the catalytic performance of 1 (100% epoxide yield at 24 h, 70 °C) was superior to that of the tetrabutylammonium salt (Bu4N)4[SiMo12O40] (2) (63% epoxide yield at 24 h), illustrating the role of the counterion Hptz+ in enhancing catalytic activity. The hybrid salt 1 was effective for the epoxidation of Lim (69%/85% conversion at 6 h/24 h) and the FAMEs (87–88%/100% conversion at 6 h/24 h), leading to useful bio-based products (epoxides, diepoxides and diol products).