Role of Sulphur in Catalytic Reforming of Hydrocarbons on Platinum/Alumina

Abstract

Reforming reactions on 0.3% Pt/Al 2O 3—0.82% Cl catalyst (Akzo-Chemie, CK3O3) using cyclohexane, n-hexane and n-heptane were performed in the presence of sulphur under experimental conditions of 350–480°C, 10 atm (1 MPa) absolute, weight hourly space velocity (WHSV) =5 h −1 and hydrogen:hydrocarbon ratio = 10. Catalyst sulphidation was either via a doped liquid feed or presulphation with hydrogen sulphide in the reduction stage of catalyst pretreatment. The presence of irreversible sulphur (S i) was found to have no effect on the dehydrogenation activity of cyclohexane under pseudo steady state at 480°C compared to a lined out catalyst. When the reaction temperature is decreased to 350°C, a kinetic limitation appears and the irreversibly sulphided catalyst has a lower dehydrogenation activity compared to the lined out catalyst. At 480°C, the dehydrocyclization activity of n-heptane decreased by 50% in the presence of S i compared to the lined out catalyst. The behaviour of sulphided catalysts in the presence of reversible sulphur (S r) is explained in terms of the equilibrium between hydrogen sulphide and adsorbed S r. The mode of Si adsorption was shown to produce the same “pseudo” steady state activities for all the reforming reactions on the sulphided catalysts. A model was proposed to explain the behaviour of all the observed reactions in the presence of reversible coke, graphitic coke, S i and S r on the platinum surface. The spillover of adsorbed species to the alumina under periods of high S r levels was hypothesized. The optimal operating condition found for a sulphided Pt/Al 2O 3 catalyst was obtained by keeping the feed sulphur level at a minimum. This was because hydrogenolysis sites (unfavourable cracking reactions) are blocked during S i adsorption and any further addition as S r blocks the dehydrogenation and dehydrocyclization reactions.