Pt/MgO as catalyst for hydrogenolysis reactions of C₅ and C₆ hydrocarbons: Evidence for metal-support interactions

Pt/magnesia catalysts having high metal dispersion (av. dia. 21 Å) have been prepared employing nonaqueous impregnating solutions and an incipient wetness technique. Both chlorine-free and residual-chlorine-containing catalysts were studied, the latter being prepared from platinum (IV) chloride. Both types of catalyst are of negligible sulphur content (<2 ppm). Comparison of hydrogen chemisorption and transmission electron microscopy results for both “chlorine-free” and chlorine-containing Pt/MgO catalysts showed that (i) chemisorption capacity was substantially as expected from TEM-derived particle size following a reduction temperature of 593 K (LTR), and (ii) chemisorption was depressed to 40% or less of expectation when a reduction temperature of 753 K (HTR) was used. Reactions of saturated hydrocarbons in a flow of hydrogen (p_hydrocarbon = 8-16 Torr, balance to 760 Torr hydrogen) showed hydrogenolysis to be a prominent reaction. With LTR chlorine-free catalysts, hydrogenolysis of n-pentane was almost exclusively by central scission. The chlorine-containing HTR catalyst yielded largely terminal scission. HTR chlorine-free catalysts and LTR chlorine-containing catalysts gave an intermediate behaviour. Thus, both higher reduction temperature and presence of chlorine act to move scission of the C-C chain from the centre to the ends. Hydrogenolysis of neohexane showed increased direct demethanation to give n-butane with the presence of chlorine in the catalyst. Ring scission of methylcyclopentane gave an enhanced production of 2-methylpentane on all the Pt/magnesia catalysts. The considerably depressed hydrogen chemisorption and the carbanionic reactivity in these cracking reactions are interpreted in terms of a moderately strong metal-support interaction with a shift of negative charge from the magnesia O²⁻ ions to the platinum. This interaction is increased by the presence of chlorine in the catalysts.