Effects of Catalyst Regeneration with and without Chlorine on Heptane Reforming Reactions over Pt/Al2O3 and Pt–Sn/Al2O3

Abstract

The reforming of heptane over chlorine-free Pt/Al2O3 and Pt–Sn/Al2O3 catalysts has been studied as a function of Sn-content, coking, sequences of regenerative oxidation/reduction treatments after catalytic use, and sequences of regeneration by oxychlorination, involving an air/1,2-dichloropropane mixture, followed by reduction. Particle size effects on selectivities were dominant for Pt/Al2O3, decreasing size favouring hydrogenolysis at the expense mainly of aromatisation and favouring C1 ring closure in cyclisation reactions. Electronic effects of added chlorine on metal catalysis were not important, although, unlike the effects of repeated oxidation/reduction treatments, regeneration involving chlorine gave reproducible Pt dispersions and hence reproducible selectivity behaviour. Tin also induced reproducibility of catalytic behaviour for Cl-free catalysts, but the presence of both Sn and Cl showed additional effects due to the promotion by chlorine of greater intimacy between Sn and Pt in reduced catalysts. In particular oxychlorinated Pt–Sn/Al2O3 gave low or zero aromatisation selectivity, and strongly promoted cyclisation and isomerisation reactions involving C2C6 rather than C1C5 or C1C6 cyclic intermediates. However, bond-shift mechanisms of isomerisation were also important. The results are discussed in terms of Pt site character, and geometric and electronic effects of Sn, Cl, and coke on catalyst structure and performance. The addition of Sn or Cl enhanced the occurrence of central rather than terminal C–C fission due to cracking reactions at acid sites on the oxide support.