Iron contamination of FCC catalysts: Quantification of different crystalline phases and valence states

Abstract

Amongst the contaminant transition metals that deposit onto the fluid catalytic cracking (FCC) catalyst, iron is well-known for its accumulation on the outer surface, negatively affecting accessibility of reactants to the interior of the porous particles and even causing fluidization problems in the circulating unit. Earlier studies have been dedicated to understand the complex deposition mechanism of iron in FCC particles, but only a few was devoted to determine the crystalline phases and the valence states of the deposited Fe during its operation under alternate redox conditions. Besides, the co-existing structural iron in the manufactured catalyst (fresh) makes such studies even more challenging. To better elucidate this field, we present in this paper a combined investigation using X-ray fluorescence (XRF), electron paramagnetic resonance (EPR) and Mössbauer spectroscopy (MS) in fresh, artificially metalized samples and deactivated catalysts collected from the industrial FCC process (E-cat). Using as subsidiary information the characterization of fresh and laboratory impregnated samples submitted to different redox environments at 750 °C, the investigation of E-cats from two refineries working in partial and full burn regeneration mode allowed us to conclude that contaminant iron is mostly deposited as maghemite (γ-Fe2O3) with spinel structure in both its forms: the ferrimagnetic and the superparamagnetic phase, this latter associated to nanosized crystals. The structural Fe(III) in meta-kaolinite from the original catalyst is mostly transformed to Fe(II), indicating that in real FCC unit the regeneration conditions are not severe enough to fully oxidize the Fe species formed in the reductive conditions of the FCC cracking step. Moreover, evidences suggested that the ferrimagnetic form of maghemite is preferentially formed in the reaction zone and it is more or less converted in the regenerator into the superparamagnetic species, more resistant to the oxidizing condition.