Preventing self-poisoning in [Pt/Al2O3 + SO42−-ZrO2] mixed catalysts for isomerization-cracking of heavy alkanes by prereduction of the acid function

Abstract

The activity and stability of composite catalysts obtained by mixing Pt/Al2O3 with SO42−-ZrO2, were studied in the reaction of hydroisomerization-cracking of n-octane (300 °C, 1.5 MPa, WHSV=4 h−1, H2/n-C8=6) obtaining light isoalkanes (isobutane, isopentane, isohexane), very important components of the pool of reformulated gasoline. Both components of the composites were pretreated in H2 before mixing. Prereduction eliminated the fraction of SO42− reducible at low temperature, thought to produce SO2 during reaction, a poison for the metal function. A reduction in the poisoning of the metal, produced by adsorption of S compounds, was tried in order to get a higher metal activity for the production of atomic H, needed for the hydrogenation of coke precursors and the prevention of deactivation.

The pretreatment temperature was adjusted between 300 and 500 °C in order to keep a suitable amount of sulfate, needed for a sustainable isomerizing-cracking activity and for the stabilization of the tetragonal crystal phase of ZrO2, the most catalytically active crystal phase. When compared to non-reduced catalysts, composites reduced at 300–350 °C displayed improved activity and stability for benzene hydrogenation, a metal-catalyzed reaction, and the results were addressed to a decrease in S poisoning. In the pretreated composites the SO42−-ZrO2 active tetragonal phase was maintained in spite of the loss of S. For isomerization-cracking of n-octane, the pretreated catalysts showed a higher selectivity to isobutane and a fairly longer stability than non-pretreated catalysts, like Pt/SO42−-ZrO2 and SO42−-ZrO2 mixed with Pt/Al. TPO tests showed that the catalysts were essentially free of coke. This fact was addressed to a lower coking rate on prereduced sites and to a higher metal activity for hydrogenation of coke precursors.