The intrinsic kinetics of n-hexane hydroisomerization catalyzed by platinum-loaded solid-acid catalysts

The hydroisomerization of n-hexane on a number of platinum-loaded acidic zeolites and Mg saponite was studied at atmospheric pressure, hydrogen to n-hexane molar ratios of 14-56, and temperatures of respectively 513-543 K (zeolites) and 583-613 K (saponite). Care has been taken to obtain initial data of fresh catalysts so that in comparisons between zeolites differences in deactivation rate can be excluded. By fitting the measured kinetic data to an equation based on the bifunctional mechanism and using independently obtained dehydrogenation and adsorption data it was possible to simultaneously determine the energy of protonation of hexene (DeltaH(prot)) and the activation energy of the elementary isomerization step (E-act,E-isom) as well as the corresponding preexponential factors. The range of observed values of both DeltaH(prot) and E-act,E-isom are in agreement with results of quantum-chemical calculations. Even so, the variation in the Eact,isom values was quite large (96-182 kJ/mol) in spite of the apparent uniformity in acid strength of the catalysts. This implies that lattice distortions required to accommodate the transition state depend sensitively on the structural and electronic environment of the acid site, which is different for each type of zeolite. A compensation effect was observed between the preexponential factor nu(isom) of the isomerization rate constant and Eact,isom, as was shown by the fact that a plot of ln(nu(isom)) versus E-act,E-isom was linear. Therefore differences in overall kinetics between different zeolites are determined mainly by differences of adsorption constants of the reacting molecules. (C) 2002 Elsevier Science (USA).