The dehydrogenation of alkanes to alkenes is one of the most investigated areas in organic synthesis and has received considerable attention from the petrochemical industry because of the increasing global demand for light alkenes. The chemical reaction of alkane to alkene (CnH2n+2 → CnH2n + H2) is highly endothermic at room temperature. Therefore, the majority of research has focused on thermal alkane dehydrogenation with metal and metal oxide catalysts, which suffer from harsh reaction conditions. The photochemical dehydrogenation of alkanes has been proposed as an alternative method to greatly lower the reaction temperature and supply the additional energy needed to drive this uphill reaction. Polyoxometalates (POMs), which are metal-oxo clusters of high-valent early-transition metals (V, Mo, W, Nb, and Ta), are promising catalysts for photochemical alkane dehydrogenation. Our group first reported that decatungstate, i.e., W10O324-, one of myriad possible POMs, catalyzes alkane photodehydrogenation with a platinum metal cocatalyst. We have prepared the binary system, Na4W10O32-stabilized Pt(0) nanoparticles (NPs) for the first time by a facile one-pot photochemical synthesis. Specifically, 38 μL of isopropanol was added to a 5 mL of a 0.8 mM Na4W10O32 and 0.2 mM H2PtCl6 aqueous solution, which was then irradiated with a 365 nm LED lamp for 20 minutes. The solution turned blue then finally black, the characteristic color of platinum nanoparticles. These black particles were collected via centrifugation at 5,000 rpm, then analyzed by TEM which showed the size of these Na4W10O32-stabilized NPs was ca. 20 nm. The NPs were redissolved in a mixed CH3CN/H2O (19:1 v/v) solution and tested for the photochemical dehydrogenation of 2,3-dimethylbutane. Based on the product analysis by GC and NMR, the Na4W10O32-stabilized Pt(0) NPs display high photocatalytic efficiency (turnover numbers ~ 100) and high selectivity (~ 80%) for 2,3-dimethyl-1-butene (Scheme 1). The TEM images and FT-IR spectra before and after catalysis indicated that these binary, composite NPs largely remain intact.

Polyoxometalate-stabilized platinum nanoparticles for photochemical dehydrogenation of alkanes at room temperature

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