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

The catalytic properties of three supported nickel catalysts, 0.97 wt.% Ni/ SiO2, 0.95 wt.% Ni/Al2O3 and 0.54 wt.% Ni/MoO3, are reported for the hydrog enolysis of ethane, propane, n-butane and iso-butane. The reactions were ca rried out in a flow reactor at atmospheric pressure. The three nickel catal ysts had the following order of hydrogenolysis activity: Ni/SiO2 > Ni/Al2O3 > Ni/MoO3. 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 chara cteristics of the fresh catalysts (nickel dispersion, support etc.). The Ni /SiO2 catalyst showed the highest activity for the hydrogenolysis reactions of the alkanes tested. The higher specific rate of hydrogenolysis of the N i/SiO2 catalyst is likely to be an effect not only of the small particle si ze of the nickel but also the manner in which carbonaceous matter builds up on these particles. The Ni/MoO3 catalyst had a lower activity than would b e expected from its nickel dispersion. The suppression activity on the Ni/M oO3 sample may be related to a strong interaction between the metal and the support. The selectivity behaviour shown during hydrogenolysis by the Ni/S iO2 and Ni/Al2O3 catalysts was typical of that expected for nickel catalyst s (multiple hydrogenolysis, demethylation, low isomerisation). These select ivity features can be accounted for by a reverse Fischer-Tropsch synthesis mechanism that involves 1,1,2 adsorbed alkylidene intermediates on the cata lyst surface. The Ni/MoO3 catalyst demonstrated uncharacteristic isomerizat ion activity during the hydrogenolysis of n- and iso-butane. This can be ac counted for by a bifunctional mechanism involving acid sites on the MoO3 su pport.