Thermally stable supported metal catalysts and inorganic membranes prepared by sol-gel processing
Description
The application of sol-gel processing to the synthesis of supported metal catalysts, inorganic membranes and catalytic membranes was studied. Noble metals were supported on SiO2 and gamma-Al2O 3. These materials have high surface areas, well-defined pore size distributions, and average pore diameters around 4 nm. The effects of preparative variables such as pH, gelation temperature, metal precursor, metal loading and binder addition on the final properties of the materials were studied. The structure of the sol-gel derived support materials was used to stabilize the dispersion of the supported metal at elevated temperatures and enhance activity and selectivity of the catalyst The thermal stability of a sol-gel Rh/SiO2 catalyst was tested at 650°C in flowing O2 and compared to the stability of a similar sample prepared by the traditional method of ion-exchange. The ion-exchanged sample had an average pore diameter of 24 nm. The metal particle size distribution of the sol-gel catalyst was more stable due to the smaller pore diameter of that catalyst. The sol-gel samples were more active in the dehydrogenation of n-butane than the ion-exchanged sample. A similar effect of pore size was found for Pd/gamma-Al2O3 catalysts sintered in H 2 at 650°C The Pd/gamma-Al2O3 catalysts were found to be very active hydrogenation catalysts but were not selective to partially hydrogenated products. A boehmite sol was used as a coating on a porous alpha-Al 2O3 substrate to form a gamma-Al2O3 membrane that could act as a diffusion barrier for gases. A catalytic membrane was formed by adding a soluble palladium compound to the sol. The Pd/gamma-Al 2O3 membrane was used successfully in the selective hydrogenation reactions of acetylene and 1,3-butadiene to produce ethylene and butenes. This was accomplished by premixing the alkyne or diene with H2 and passing it through the membrane wall, therefore reducing the contact time between the reactants and the catalyst