(169a) Novel Catalysts for Selective Oxidation of Lower Alkanes

Olefins are industrially important starting molecules for the production of many oxygenated compounds and nitriles, obtained respectively by the oxidation and ammoxidation reactions. However, the high cost of such feedstocks have stimulated research on catalytic oxidation processes based on C2-C4 alkanes. Among different catalysts investigated, Mo-V-Te-Nb-Ox mixed metal oxide catalysts are very promising for oxidation, ammoxidation and oxidative dehydrogenation of lower alkanes. Recently, the Mo-V-Te-Nb-Ox catalysts with orthorhombic M1 phase structure received much attention as catalysts for (amm)oxidation of propane to acrylonitrile. However, the current M1 phase catalysts suffer from low thermal stability due to Te volatility and limited selectivity to acrylonitrile.

In this study we investigated the use of Fe, Ru and Au as promoters of thermal stability and selectivity in (amm)oxidation of C2-C4 alkanes over the M1 phase catalysts. Trivalent Fe is well known as an active component of catalysts employed for partial oxidation for ethane and propane, whereas small Ru and Au nanoparticles (<10 nm) exhibit unexpectedly high catalytic activities towards many types of catalytic reactions, such as selective oxidation, steam reforming and water-gas shift reaction. Selective (amm)oxidation of lower alkanes were investigated over hydrothermally synthesized Mo-V-Nb-Te-Ox, Mo-V-Nb-Fe-Ox as well as Au- and Ru-doped Mo-V-Nb-Te-Ox M1 phase catalysts. Au ad Ru dopants were introduced to the Mo-V-Nb-Te-Ox M1 phase catalysts were by the incipient wetness impregnation. The structural characteristics of multi-component Mo-V-Nb-Te-Ox, Mo-V-Nb-Fe-Ox, Ru- and Au-doped Mo-V-Nb-Te-Ox catalysts were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and N2 adsorption at 77 K. Selective (amm)oxidation of lower alkanes were carried out in a tubular reactor at 300-4300C. The total flow rate as well as the He: C2-C4: O2: NH3 molar ratios were varied to optimize the conversion of feed, selectivity and yield of the desired products. The experimental results showed that the Mo-V-Nb-Fe-Ox catalysts was least active as compared to all other catalysts tested, which may be due to the presence of a different phase other than the M1 phase. The best results were obtained over Au-doped Mo-V-Nb-Te-Ox catalysts in terms of alkane conversion, selectivity and yield of desired products. The enhanced catalytic activity of the Au-doped Mo-V-Nb-Te-Ox M1 phase catalysts may be attributed to the presence of the supported Au nanoparticles.