29Si and27Al MAS NMR spectroscopy has shown that on calcination cracking components such asHY, Si-enriched HY, REHY, and CREY undergo dealumination and that steam-aging increases the presence of extra-framework Al in these zeolites. Dealumination is more severe in HY-type materials. The zeolite V resistance seems to decrease when RE ions are present and to increase with increasing extra-framework Al (generated during steam-aging). At the high temperatures used for FCC regeneration, oxycations of vanadium (VO2+or VO2+could attack the Al−O−Si bond in HY and cause lattice collapse. In REHY and CREY crystals, it is believed that Ce4+ions, present as an oxycerium complex, undergo a redox reaction with oxyvanadyl cations (VO2+), and form a stable orthovanadate. Removal of other charge-compensating cations (such as Na+ions) in the form of vanadates further destabilizes the crystal lattice, thus promoting zeolite destruction. Atomic Force Microscopy (AFM) can provide details of the surface topography of an FCC with unprecedented resolution, and can be used to rationalize the deleterious effects that metals such as Ni and V have on the properties of FCC. The deleterious effects of V deposits on zeolite-containing FCC can be greatly reduced by addition of certain materials (metal scavengers) capable of minimizing direct V-FCC interactions by selectively (and irreversibly) sorbing migrating V compounds such as H4V2O7. Dual-function cracking catalyst (DFCC) mixtures have been prepared that can retain most of their useful cracking activity (at MAT condition), even in the presence of 1.0% to 1.5% V. Thus, DFCC systems appear to have the necessary metal tolerance to crack residual oil as well as heavily V-contaminated crudes and may offer cost (as well as coke selectivity) advantages over conventional FCC. © 1996 Elsevier B.V.
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