Trimetallic naphtha reforming catalysts II. Properties of the acid function and influence of the order of addition of the metallic precursors on Pt-Re-Sn/ -Al2O3-Cl

Abstract

Mono, bi- and trimetallic catalysts were prepared by successive impregnation of -Al2O3 with Pt, Re and Sn precursors, with intermediate drying, calcination and reduction steps. The catalysts were tested in the reactions of n-pentane (500 ◦C, WHSV = 4.5, H2:n-C5 = 6) and n-octane (500 ◦C, WHSV = 1.8, H2:n-C8 = 10). Concerning the preparation parameters it was found that the addition of Sn first favorably affects the catalytic acid function, increasing the activity for alkane isomerization and cyclization and decreasing cracking and coking. The addition of Re first has opposite effects. It increases the rates of undesirable reactions which require strong acidity, i.e. cracking and coking. Basic SnOx species are responsible for a decrease in the number and strength of acid sites while highly dispersed ReOx-AlOy surface species increase the relative concentration of strong acid sites. These facts have been confirmed by pyridine TPD and TPO of coke deposits.