The combined effect of sodium and vanadium contamination upon the catalytic performance of USY zeolite in the cracking of n-butane: Evidence of path-dependent behavior in Constable–Cremer plots

NaCl and [VO(C5H7O2)2] were impregnated on a commercial ultra-stabilized Y zeolite (USY). Samples were calcined in dry air at 700 °C and examined by X-ray diffraction (XRD), N2 adsorption, isopropylamine decomposition and the n-butane cracking reaction. Sodium concentration was kept constant but in excess related to a variable vanadium concentration. Dry thermal treatment in the presence of metal contamination was found to induce framework destruction by sodium but not by vanadium. Vanadium was found to neutralize preferentially the Brønsted acid sites (BASs) responsible for alkane cracking. Selectivity to dehydrogenation increases with vanadium loading due to its intrinsic redox activity. No evidence of a vanadium–sodium compound was found; sodium reacts preferentially with the framework than with vanadium. Arrhenius plots rendered quite linear relations, and isokinetic behavior was established for cracking and dehydrogenation reactions. The findings clearly show that the slopes in Constable–Cremer plots are sensitive to the reaction mechanism.