From bifunctional site to metal-proton adduct site in alkane reforming reactions on sulphated-zirconia-supported Pt or Pd or Ir catalysts

Isomerization reactions of n-heptane, n-octane and n-nonane are studied on sulphated-zirconia-supported 0.2 wt% Pt, Pd or Ir catalysts. Evolutions of isomer selectivity versus total conversion and reaction temperatures are analysed. When total conversion (alpha (T)) is increased, isomer selectivity (%S-isom) is decreased and the slope of the curve %S-isom = f(alpha (T)) is more pronounced when the carbon number in the alkane is more important. At isoconversion, around 20%, below 473 K, cracking is favoured over isomerization reaction, and above 473 K it is the reverse. Moreover, with n-heptane, when the catalytic reaction occurred at 423 K and at low conversion, alpha (T) less than or equal to 20%, we observed a large decrease in the isomer selectivity percentages on Pd/SZ and Ir/SZ compared to Pt/SZ. What is remarkable is that, at this low temperature, both metals are inactive in the carbon-carbon bond rupture. To explain these results the following points are raised: (i) an associative mechanism is proposed for the adsorption step of the alkane involving an agostic intermediate species where the carbon-hydrogen bonds act as ligands to the transition metal centres forming covalent C-H... M systems, and (ii) a metal-proton adduct site, which gathers metallic and acidic sites is suggested. This approach seems to better explain our results than the "traditional" bifunctional mechanism.