The use of nuclear magnetic resonance, microcalorimetry, and atomic force microscopy to study the aging and regeneration of fluid cracking catalysts

The accelerated steam-aging procedure (with 100% steam at 760 degreesC for 5 h) used in this study reduces the surface area (SA) of a fresh fluid cracking catalyst (FCC) to a value close to that measured in the corresponding equilibrium sample. In addition, Si-29 nuclear magnetic resonance (NMR) spectra of steam-aged and equilibrium FCC samples are practically indistinguishable, However the steam-aged and equilibrium FCCs have different pore structures, initial heats of ammonia and pyridine chemisorption as well as different distributions of Al(IV), Al(V), and Al(VI) species. The Si-29 NMR spectrum of the fresh or steam-aged FCC can be considered a superposition of the spectra of its components (the HY zeolite, kaolin, and aluminosilicate matrix). After aging, the Si-29 NMR spectrum of zeolitic component is reduced to one main dominant resonance near the -107 ppm representative of T(4Si, 0Al) sites. Thus extensive dealumination of the cracking component in FCCs occurs during recirculation in a fluid cracking catalyst unit (FCCU). Similar conclusions have been obtained from Al-27 magic angle spinning (MAS) NMR spectra. Microcalorimetry experiments with ammonia and pyridine have shown that after aging either under microactivity test (MAT) conditions or in a FCCU, the fresh FCC undergoes severe losses in acid site density while retaining mast of the strength of its strongest Lewis acid sites. These sites, and the retention of an open micro- and mesoporosity, are believed to be responsible for the aged FCCs' cracking activity. Atomic force microscopy images have shown that the equilibrium FCC contains on its surface debris attributed to nickel and vanadium oxides and that most of this debris disappears from the surface of the regenerated catalyst. Al-27 and Si-29 MAS NMR spectroscopy results have revealed that the demetallation (DEMET) process, in addition to removing most of the metal contaminants, leaves a completely dealuminated zeolitic component. These results indicate that cracking activity in these FCCs does not depend on the presence of framework Al in the faujasite crystals present. The increase in acidity and microporosity is consistent with the observed enhanced cracking activity, under MAT conditions, of the regenerated FCC. (C) 2000 Academic Press.