The hydrocracking of saturated hydrocarbons over evaporated metal films

Abstract

Rates have been measured for hydrocracking of the lower saturated aliphatic hydrocarbons in the presence of excess hydrogen over evaporated metal film catalysts of nickel, tungsten, rhodium and platinum and the distributions of products formed initially have been determined. Activation energies and frequency factors have been obtained over nickel and tungsten. With ethane, propane and neopentane the simultaneous adsorption of hydrocarbon and hydrogen has been measured at temperatures below but approaching the range used for hydrocracking. Hydrocarbon adsorption, probably involving carbon-carbon bond rupture, was rapid and the coverage of the surface by hydrocarbon residues approached or exceeded 50%. The measurement of hydrogen liberated during hydrocarbon adsorption gave values for the mean hydrogen/carbon ratio for the adsorbed residues from ethane and propane in the range 0.4 to ca. 2. For straight-chained hydrocarbons there was a general trend for reactivity to fall with decreasing carbon number. Reaction over nickel resulted in extensive production of methane with the formation of only relatively small amounts of products of higher carbon number. Over tungsten and rhodium fragmentation of the skeletal structure was also extensive but the production of methane was less dominant than with nickel. The product distributions formed over nickel from propane and n-butane, when considered in relation to the relative hydrocarbon reactivities cannot be reconciled with the rupture of a carbon-carbon bond as the rate-controlling step. A comparison of reaction rates with desorption rates estimated from deuterium/hydrocarbon exchange data leads to the conclusion that the rate of hydrocracking over nickel and probably over tungsten and rhodium is desorption controlled except for reaction within a quaternary group when the rate depends on the rate of carbon-carbon bond rupture. From a similar analysis of the data for platinum it is concluded that on this catalyst the rate of hydrocracking is in all cases dependent on the rate of carbon-carbon bond rupture, followed by fast product desorption and this agrees with the small tendency to extensive skeletal fragmentation. Particularly on platinum, but also on tungsten and rhodium, examples were observed of isomerization, formation of a product with carbon number in excess of the parent hydrocarbon and the formation of `abnormal' product distributions. These reactions have been attributed to recombination processes among surface residues.