Effects of catalyst regeneration with and without chlorine on heptane reforming reactions over Pt/Al2O3 and Pt-Sn/Al2O3

The reforming of heptane over chlorine-free Pt/Al2O3 and Pt-Sn/Al2O3 cataly sts has been studied as a function of Sn-content, coking, sequences of rege nerative oxidation/reduction treatments after catalytic use, and sequences of regeneration by oxychlorination, involving an air/1,2-dichloropropane mi xture, followed by reduction. Particle size effects on selectivities were d ominant for Pt/Al2O3, decreasing size favouring hydrogenolysis at the expen se mainly of aromatisation and favouring C-1 ring closure in cyclisation re actions. Electronic effects of added chlorine on metal catalysis were not i mportant, although, unlike the effects of repeated oxidation/reduction trea tments, regeneration involving chlorine gave reproducible Pt dispersions an d hence reproducible selectivity behaviour. Tin also induced reproducibilit y 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 great er intimacy between Sn and Pt in reduced catalysts. In particular oxychlori nated 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 isome risation were also important. The results are discussed in terms of Pt site character, and geometric and electronic effects of Sn, Cl, and coke on cat alyst structure and performance. The addition of Sn or Cl enhanced the occu rrence of central rather than terminal C-C fission due to cracking reaction s at acid sites on the oxide support. (C) 1999 Academic Press.