Reaction of hexane, cyclohexane, and methylcyclopentane over gallium-, indium-, and thallium-promoted sulfated zirconia catalysts

Sulfate-zirconia catalysts were prepared via the colloidal-sol-gel method using ZrOCl2 as precursor. After precipitation and washing, the solids were peptized with a 1 : 1 mixture of CH3COOH and H2SO4. The promoters were introduced as gallium, indium, or thallium nitrates into the solution containing the zirconium sols. After a gel formation period, the samples were dried and calcined. The catalysts were characterized by chemical analysis, nitrogen sorption isotherms, FTIR analysis of adsorbed pyridine and deuterated acetonitrile, XRD, laser Raman and FTIR spectroscopy, and XPS. The catalytic tests were performed in a fixed-bed quartz microreactor using as reactant molecules hexane, cyclohexane, and methylcyclopentane. Characterization of the samples by the different techniques allowed us to propose that the main factors controlling the observed properties are the high sulfur content and the promoter effect. Low surface areas of the catalysts were related to the high sulfur contents. Sulfate species in different coordinations were indentified : isolated sulfate groups, polynuclear sulfate species, and sulfuric acid. Lewis acid sites as well as the polynuclear sulfate species determined the acidity of these solids. The presence of promoters contributed not only to the increase of the redox behavior of the catalysts, but also to a decrease of the population of polynuclear sulfates, in the order Tl > In > Ga. In the reaction conditions adopted, on unpromoted catalysts and (especially) Ga-promoted catalysts, dehydroizomerization and ring isomerization were the principal reaction pathways for hexane and methylcyclopentane, and for cyclohexane, respectively. Tl was found to promote ring cleavage. These data are consistent with previous studies pointing to the dehydrogenation activity of Ga embedded in zeolite matrices. (C) 1998 Academic Press.