Vanadium poisoning of FCC catalysts: A quantitative analysis of impregnated and real equilibrium catalysts

[Display omitted] •A quantitative method to measure the extent of vanadium oxidation in FCC catalysts was developed, which is able to distinguish equilibrium catalysts from FCC units operating at partial or full combustion mode.•Impregnation of fresh FCC catalysts with aqueous solution of VOSO4 followed by steam deactivation resulted in a proportion of V4+/Vtotal of nearly 10%, similar to that of Ecats sampled from FCC units.•Apart from vanadium trapped at the acidic sites of the FCC catalysts, other part is stabilized in a ferri-/ferromagnetic surface phase, which is detected by EPR as a broad unstructured line.•The new phase is likely formed by induced oxygen vacancies at the surface under hydrocarbon cracking conditions and is quite resistant to the regeneration step. Amongst the contaminant transition metals Fe, Ni, V that deposits onto the fluid catalytic cracking (FCC) catalyst and promote unwanted secondary reactions, vanadium is the most deleterious one because not only acts as undesired dehydrogenation site but also migrates deeper inside the catalyst particles under the FCC regeneration conditions, stabilizing in different phases that attack the crystalline structure of the catalyst, thus resulting in its irreversible loss of activity. The mechanisms through which vanadium affect the catalyst during its use in the FCC process remain under debate and the accurate determination of the chemical species involved and of their concentration is of paramount importance for the investigations in this field. In this work, a quantification methodology for contaminant vanadium in FCC catalysts was developed based on X-ray fluorescence (XRF) and electron paramagnetic resonance (EPR). A commercial FCC catalyst was artificially contaminated with vanadium at different loads by pore volume impregnation with aqueous vanadyl sulfate solution, in the presence or not of a complexing agent, followed by steam deactivation at 788 °C/5 h, and the samples were compared to spent FCC catalysts from different Brazilian refineries. It has been shown that EPR is a very sensitive and reliable technique to quantify vanadyl ions (V4+) in FCC catalysts while lower valence states such as V3+ have not been detected in any of the samples studied. Vanadyl ions (VO2+) deposited onto the catalyst are mostly oxidized to higher valence state species and the extent of this oxidation in equilibrium catalysts from FCC units was calculated and related to the combustion mode at which the FCC r