Fibrous hydrogenation catalysts derived from NiAl eutectic alloys

Abstract

The directional solidification (DS) of eutectic Ni-Al alloys was investigated as a new route for the synthesis of catalytic materials of the Raney (trade-mark) type with improved properties. Critical surface properties of the caustic-activated alloys, such as specific surface area, pore structure and available catalytic nickel surface area for active nickel derived from DS precursor alloys, were found to be superior to those of commercial bulk-cast Raney alloys activated in the same manner. Microporosites derived from low temperature nitrogen adsorption isotherms on caustic-activated alloys are significantly greater and pore distributions occur not only in both the small pore sizes with radii of less than about 10 Å and the sizes with radii of about 40–70 Å, which typical of Raney nickles, but also in the coarse pore sizes, with radii of about 90 Å, which are not typical of commercial activated Raney nickels. The enhanced pore structure of these DS caustic-activated alloys was demonstrated to be quite stable toward sintering by air oxidation at temperatures of about 500 °C. The Brunauer-Emmett-Teller surface area and available nickel metal surface area were found also to provide promising stability toward sintering in hot (about 350 °C) reducing atmospheres typical for methanation catalysis. The catalytic performance of caustic-activated DS Ni-Al eutectic alloys, compared with commercial Raney alloys, was found to be consistently superior by factors of 1.14–12.9 for initial reaction rates at low temperature (22 and 80 °C) for six selected liquid phase hydrogenation reactions, i.e. hydrogenation of acetone to isopropanol, nitrobenzene to aniline, toluene to methyl cyclohexane, butyronitrile to butylamine, itaconic acid to methyl succinic acid and dextrose to sorbitol. This superiority was due principally to the greater available catalytic nickel surface but in two instances (butyronitrile and nitrobenzene) the hydrogenation rates on a specific area basis are superior by factors of about 7 and 3 respectively, indicating that selectivity also plays a role.