Hydroconversion of methylcyclohexane on bifunctional sulfated zirconia-supported platinum catalysts

A series of bifunctional catalysts prepared from sulfated zirconias (SZs) by deposition of platinum were studied in the conversion of methylcyclohexane (MCH). The main reaction observed is the isomerization to alkylcyclopentanes (ACPs), the slow step of which is the acid-catalyzed contraction of the ring. The positive order with respect to hydrogen pressure is tentatively assigned either to a nonclassical bifunctional mechanism where hydride transfer steps pray a key role or to the increased formation of active Bronsted sites, coming from the reduction of Lewis sites by hydrogen. Some ring-opening products, i.e., C-7 alkanes, which represent greater than or equal to 5% at high hydrogen pressure, are formed from ACPs either by hydrogenolysis on the Pt sites or more probably by bifunctional acid catalysis. The acidity of the samples was studied by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy and temperature-programmed desorption (TPD) of acetonitrile. Acetonitrile desorbs as two peaks, one around 410 K found on all the samples and a second at 670 K on SZ and at 500 to 570 K on Pt/SZ. Hence the introduction of Pt changes the distribution of acidity of the support. This modification is more important when chloroplatinic acid is used as precursor rather than Pt acetylacetonate. A rough correlation exists between the rate of MCH conversion on Pt/SZ and the amount of acetonitrile desorbed under the high-temperature peak. Pt/SZ catalysts doped with Fe and Mn show activities for MCH conversion close to those of unpromoted sulfated zirconias. This result can be interpreted using classical mechanisms through the charge transfer complex assumed in homogeneous catalysis. (C) 1997 Academic Press.