HYDROGENOLYSIS AND ISOMERIZATION OF ALKANES ON RU/AL2O3 CATALYSIS OF VARYING DISPERSIONS

The conversion of a series of hydrocarbons including n-hexane, 2-methylpentane, 2,2-dimethylbutane, 2,2,3,3-tetramethylbutane and methylcyclopentane has been investigated over a series of Ru/Al2O3 catalysts, in which the dispersion of ruthenium varied from 1% to 100%. The specific activity changes with the ruthenium dispersion, well dispersed catalysts being less active. The magnitude of the effect depends on the structure of the hydrocarbon: it reaches a factor of 10-20 for 2-methylpentane and > 103 for 2,2,3,3-tetramethylbutane. Large ruthenium particles (H/Ru < 0.4) favour deep hydrogenolysis, whereas small particles (H/Ru) catalyse single CC bond splitting, and to a lesser extent, isomerization. The type of CC bond which can be broken also depends on the surface structure: 2,2,3,3-tetramethylbutane is split into isobutane on ruthenium catalysts of intermediate dispersion (H/Ru = 0.6), whereas a substantial amount of demethylation to 2,2,3-trimethylbutane is observed at high dispersion. This last catalytic behaviour can be rationalized assuming the formation of two surface intermediates: αδ-species adsorbed at ruthenium atoms of high coordination, and αγ-species formed at ruthenium atoms of lower coordination. The kinetic study in function of reactant pressure shows different reaction orders versus hydrogen and hydrocarbon on large or small ruthenium particles. By increasing the hydrogen pressure, deep hydrogenolysis is inhibited. A clear effect of the particle size of ruthenium then appears on the turnover frequency and selectivity of single CC bond splitting. © 1990.