Supported nickel catalysts: Hydrogenolysis of ethane, propane, n-butane and iso-butane over alumina-, molybdena-, and silica-supported nickel catalysts

摘要

The catalytic properties of three supported nickel catalysts, 0.97 wt.% Ni/SiO_2, 0.95 wt.% Ni/Al_2O_3 and 0.54 wt.% Ni/MoO_3, are reported for the hydrogenolysis of ethane, propane, n-butane and iso-butane. The reactions were carried out in a flow reactor at atmospheric pressure. The three nickel catalysts had the following order of hydrogenolysis activity: Ni/SiO_2 > Ni/Al_2O_3 > Ni/MoO_3. The active site for the hydrogenolysis reactions over the three nickel catalysts is formed in situ and is likely to contain a carbonaceous component. Any carbonaceous component that is formed will not necessarily be the same in each catalyst as the laydown will be a function of the characteristics of the fresh catalysts (nickel dispersion, support etc.). The Ni/SiO_2 catalyst showed the highest activity for the hydrogenolysis reactions of the alkanes tested. The higher specific rate of hydrogenolysis of the Ni/SiO_2 catalyst is likely to be an effect not only of the small particle size of the nickel but also the manner in which carbonaceous matter builds up on these particles. The Ni/MoO_3 catalyst had a lower activity than would be expected from its nickel dispersion. The suppression activity on the Ni/MoO_3 sample may be related to a strong interaction between the metal and the support. The selectivity behaviour shown during hydrogenolysis by the Ni/SiO_2 and Ni/Al_2O_3 catalysts was typical of that expected for nickel catalysts (multiple hydrogenolysis, demethylation, low isomerisation). These selectivity features can be accounted for by a reverse Fischer-Tropsch synthesis mechanism that involves 1,1,2 adsorbed alkylidene intermediates of the catalyst surface. The Ni/MoO_3 catalyst demonstrated uncharacteristic isomerization activity during the hydrogenolysis of n- and iso-butane. This can be accounted for by a bifunctional mechanism involving acid sites on the MoO_3 support.