The catalytic and physical properties of supported tungsten carbide alkane conversion catalysts.
Choi, Saemin
1998
Abstract
The objectives of this research were to rationally synthesize supported tungsten carbide catalysts, characterize their structures and evaluate their catalytic properties. In the end, we expected to elucidate structure-function relationships for these new catalytic materials. A series of $\gamma$-$\rm Al\sb2O\sb3$ and SiO$\sb2$ supported tungsten carbides with tungsten loadings up to 19 wt% tungsten were prepared via the temperature programmed reaction of supported oxides or nitrides with a mixture of 49% CH$\sb4$ in H$\sb2.$ Thermal gravimetric analysis was employed to determine temperatures for carburization of the oxide without depositing excess carbon. The $\gamma$-$\rm Al\sb2O\sb3$ supported tungstates were more difficult to carburize than the SiO$\sb2$ supported materials. This result was attributed to differing interactions between the tungstate and support. Most of the catalysts were x-ray amorphous suggesting that the carbide domains were highly dispersed. Small crystallites of W$\sb2$C, WC and WC$\rm\sb{1-x}$ were observed on the highest loaded SiO$\sb2$ materials. The supported carbides were as active as a commercial Pt-Sn/$\gamma$-$\rm Al\sb2O\sb3$ catalyst for the dehydrogenation of butane and hexane. More importantly, the dehydrogenation selectivities were much higher for the supported carbides compared to their bulk counterparts. The butane and hexane conversion activities depended on the carbide phase and decreased in the following order; $\rm W\sb2C>WC>WC\sb{1-x}.$ The selectivities towards dehydrogenated products increased with decreasing loading regardless of the support type. Modifications in the catalytic activities on changing the loading can be explained in terms of variations in the dispersion of carbide domains and the degree of reduction. Alkane hydrogenolysis reactions are typically structure sensitive; the activities are influenced by the particle size. We have concluded that the small carbide domains on the supported materials were more selective for alkane dehydrogenation than the large domains in the bulk carbides. High temperature reduction may have converted the stoichiometric carbide domains into metallic and/or substoichiometric carbide sites. These sites could catalyze the hydrogenolysis reactions. In general, the $\gamma$-$\rm Al\sb2O\sb3$ supported carbides were more active than the SiO$\sb2$ supported carbides suggesting that domains on the $\gamma$-$\rm Al\sb2O\sb3$ were more highly dispersed. Furthermore, the $\gamma$-$\rm Al\sb2O\sb3$ supported carbides were active for the isomerization of hexane. Alkane isomerization typically requires a bifunctional catalyst. Infrared spectroscopy of adsorbed pyridine suggested that the $\gamma$-$\rm Al\sb2O\sb3$ supported carbides possessed Bronsted acidity while the SiO$\sb2$ supported carbides were slightly Lewis acidic. The hexane isomerization rates correlated well with the relative Bronsted acidity suggesting that acid sites contributed to the isomerization activity.Subjects
Alkane Carbide Catalytic Conversion Physical Properties Supported Catalysts Tungsten
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