Simultaneous deactivation by coke and sulfur of Pt-Re(Ge,Sn)/Al2O3 catalysts for n-hexane reforming

Abstract

The simultaneous deactivation by coke and sulfur of monometallic Pt/Al2O3 and bimetallic Pt–Re(Ge, Sn)/Al2O3 catalysts was studied using n-hexane reforming as bifunctional test reaction and thiophene as poisoning molecule. The residual activities in the activity decay curves were used for measuring the catalyst sensitivity to coke formation and sulfur poisoning. Sulfur and carbonaceous deposits accumulated essentially on the metallic fraction and affected the catalyst activity for both monofunctional metallic and bifunctional metal–acid catalyzed reactions. The overall deactivation rate for n-hexane conversion increased in the order Pt–Ge<Pt⪡Pt–Sn≤Pt–Re. This deactivation trend resulted from the combination of the catalyst resistance to each individual deactivation process. Pt–Ge/Al2O3 was the most stable catalyst essentially because of its high thiotolerance for n-hexane transformation reactions and also because it showed low activity for dehydrogenation reactions leading to the formation of coke precursors. Sulfur poisoning on Pt/Al2O3 decreased monofunctional metal-catalyzed reactions but concomitantly increased the activity for acid-controlled skeletal rearrangement reactions; as a result, n-hexane conversion was only slightly diminished by the addition of sulfur. Pt–Sn/Al2O3 showed high resistance to coke deactivation but was severely poisoned by the addition of sulfur. The Pt–Re/Al2O3 activity was significantly decreased by both deactivation processes. Changes in catalyst selectivity are interpreted in terms of selective deactivation by coke and sulfur of individual reaction pathways involved in the n-hexane reforming mechanism.