Platinum/alumina. III. Influence of Chloride on Methylcyclopropane Hydrogenolysis at Moderate Temperatures

Abstract

The hydrogenolysis of methylcyclopropane (MCP) was investigated over a series of Pt/Al2O3 catalysts containing different levels of chloride at atmospheric pressure and 0-100°C. Catalysts were prepared by chloriding a set of chloride-free Pt/Al2O3 catalyst that encompassed a wide range of percentage metal exposed. Both HCl and CCl4 were used as the chloride source, and both low (0.3 wt.-%) and high (1.0 wt.-%) levels of chloride were studied. MCP hydrogenolysis on the metallic function takes place via a metallocycle intermediate producing mainly i-butane (in excess H2) with a selectivity (ratio of i-butane to n-butane formation rates) of 10-15. On the acidic function the reaction goes via a carbenium ion mechanism producing an increased amount of n-butane with selectivities on the order of 1-5. While HCl chloriding increases catalyst activity, the selectivities of the series remain comparable to those on non-chlorided catalysts. However, the Pt/Al2O3-Cl ex CCl4 produces a large increase in n-butane and some increase in i-butane, with corresponding selectivities of 5-7. This increase in n-butane production is lost upon dilution of the Pt/Al2O3-Cl with additional Al2O3, presumably by Cl migration, and is indication of the participation of a localized PtClAl complex in the overall reaction. Apparent activation energies for both i- and n- butane production are about 20% higher for chlorided catalysts ex-HCl (ca. 50 and 69 kJ/mol, respectively) than those for corresponding non-chlorided materials in the mid-range of metal dispersion. Corresponding catalysts prepared ex-CCl4 show a sharp decrease in the apparent activation energy for n-butane formation, to about 30 kJ/mol, indicative of a possible shift away from metal function hydrogenation in this case. © 1990.