The Selectivity of the Hydrogenolysis of C₅ C₈ Normal Paraffins over Nickel-Alumina Catalyst

Abstract

The hydrogenolysis of pentane, hexane, heptane, and octane on nickel-alumina catalyst was studied in a flow system under hydrogen pressure of 10 atm and at the temperature ranging from 250° to 350°C.
The predomin ant reaction was successive degradation to methane and a hydrocarbon of o ne carbon atom less than the starting hydrocarbon. The probuct contained also relatively small amount of lighter hydrocarbons such as ethane and propane.
Distributions of products in the hydrogenolysis of heptane was studied with respect to the gas flow velocity. Mole fraction of hexane in product was decreased and that of pentane was increased with contact time, and those of ethane, propane, and butane were almost constant (Fig.3).
Rela tive amount of methane to other products given by
was estimated as 1.48 (pentane), 1.15 (hexane), O.97 (heptane), and O.68 (octane) at 260°C (Fig.4), where n is the carbon number of reactant and C_i is yield of the component contai ning i carbon atoms in product. The results sugge st a mechanism involving three types of hydrogenolysis reaction; a splitting of terminal carbon-carbon bonds to give methane and hydrocarbons of one carbon atom less than the starting hydrocarbon ( I ), a total fragmentation of an adsorbed molecule to give methane as the only product (II), and a rupture of other carbon-carbon bonds than terminal to give lighter hydrocarbons (III).
Reaction (II) was decreased, w hile reaction (III) was increased with the carbon number of the reactant.