Alternatives for a better performance of Pt in SO42--ZrO2 catalysts for n-octane hydroisomerization-cracking.: Selective adsorption of Pt over composites of SO42--ZrO2 mixed or supported onto Al2O3 and SiO2

The addition of Pt over various pure and heterogeneous supports containing zirconia promoted by the SO42- oxoanion was studied. The supports used were: (i) pure SO42--ZrO2; (ii) mechanical mixtures of gamma-Al2O3 or SiO2 with SO42--ZrO2; and (iii) SO42--ZrO2 supported onto gamma-Al2O3 or SiO2. Variations in the particle size of SO42--ZrO2 and the technique of Pt deposition were performed in order to meet two seemingly contradictory objectives: (i) avoid a harmful physicochemical association of Pt and the SO42- containing acid function; and (ii) reduce the mean free path between them. For the alumina- or silica-containing catalysts, a selective adsorption technique - based on electrostatic repulsion/attraction between different Pt precursors and the supports - was employed in order to avoid the anchorage of Pt onto sulfated zirconia during impregnation. Supported sulfate-zirconia was synthesized as a means of obtaining highly dispersed small particles of the acidic function. A new method of sulfation with a previous reducing treatment in hydrogen of the sintered and crystalline supported zirconia was found to be superior to the common technique of sulfation of the amorphous dispersed hydroxide. The catalysts were tested in the reaction of hydroisomerization-cracking of n-octane (300 degrees C, 1.5 MPa, WHSV=4 h(-1) and molar ratio H-2/nC(8)=6), trying to obtain light branched paraffins. Different reactivities and stabilities were analyzed. Pt deposited on a support obtained by dispersing SO42--ZrO2 over SiO2 and Pt/(SO42-)-ZrO2/SiO2, was the most stable catalyst with a high value of conversion. Hydrogenation of benzene was also used for the assessment of the hydrogenation capabilities of the metal function. A strong poisoning of Pt was detected in all sulfated catalysts. The results encourage the use of supported sulfate-zirconia as an alternative to common bulk catalysts. Supported zirconia brings about the possibility of the anchoring of other promoters (in this case Pt) on the free area of the support, to produce new SO42--ZrO2 catalysts. (C) 1998 Elsevier Science B.V. All rights reserved.