Iron poisoning is a well known problem for fluid catalytic cracking (FCC) catalysts.  When the issue is serious, a dense layer of iron rich material with nodules forms on the catalysts’ surfaces, hindering the diffusion of large feed molecules into the inner structures of the catalysts for cracking. <br/> <br/>To better understand the iron poisoning process, several characterization techniques, including scanning electron microscope (SEM), frequency-dependent magnetic susceptibility measurement, and transmission electron microscope (TEM), were used to examine equilibrium catalysts from a commercial unit.  Several fractions were obtained by density separation, and the fraction corresponding to the most aged catalysts was extensively studied in this report.  Under the SEM, most cross sections of the catalyst particles exhibited iron nodule rims enriched with Fe, Mg and Ca.  This fraction had relatively high magnetic susceptibility.  More importantly, the magnetic susceptibility values were frequency-dependent, indicating the presence of superparamagnetic (SP) grains, and suggesting that the iron-bearing species should be in the nanometer size range.  In the TEM, the nodules have been found to consist of Fe nanoparticles, up to tens of nanometers in diameter, imbedded in a glassy matrix.  The Fe nanoparticles were enriched in Mg, Ni and Ca, while the matrix was dominated by Si and Al.  In addition, selected area electron diffraction (SAED) patterns indicated that these iron enriched nanoparticles had the spinel structure.  Due to their extremely small sizes and low volume fraction, these spinel nanoparticles have been undetected by conventional bulk methods such as X-ray powder diffraction.  Scanning TEM X-ray mapping further confirmed that, as compared with the catalyst interior material, the surface layers were indeed enriched in Fe and Si, while depleted in Al.  These results offer a new and fresh look at these iron nodules, and should help us better understand the mechanism of iron poisoning of FCC catalysts.

Characterization of iron contamination on equilibrium fluid catalytic cracking catalyst particles

254th National Meeting (2017)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.