Texture characteristics of fluid catalytic cracking (FCC) catalysts were evaluated from N-2 adsorption and mercury porosimetry measurements. A hierarchy of four levels of voids were found and quantified in the beds of FCC particles. Diffusion coefficients of intraparticle diffusion for n-octane were measured in the catalyst beds by PFG NMR technique. The diffusion coefficients exhibit a linear dependence oil parameters xi that involve the mean Value of the pore radii (r) over bar in the pertinent porosity region and a function of the corresponding porosity. A strong linear correlation was also found between the parameter xi for macropores and that for mesopores. The mean sizes of macro- and mesopores thus become key parameters in tuning the FCC catalyst properties.

Pulsed-field gradient nuclear magnetic resonance (PFG NMR) has been applied to study molecular diffusion in industrial fluid catalytic cracking (FCC) catalysts and in USY zeolite for a broad range of molecular displacements and temperatures. The results of this study have been used to elucidate the relevance of molecular transport on various displacements for the rate of molecular exchange between catalyst particles and their surroundings. It turned out that this rate, which may determine the overall rate and selectivity of FCC process, is primarily related to the diffusion mode associated with displacements larger than the size of zeolite crystals located in the particles but smaller than the size of the particles. This conclusion has been confirmed by comparative studies of the catalytic performance of different FCC catalysts.

Diffusivities of n-octane in particles of industrial fluid catalytic cracking (FCC) catalysts and in zeolite USY, which is the main zeolitic component of the particles, are reported. Diffusion measurements have been performed by using pulsed field gradient (PFG) NMR for a broad range of molecular displacements and temperatures. The recorded diffusivities are used to evaluate the relevance of various transport modes in the particles of FCC catalysts, such as diffusion in the micropores of the zeolite crystals located in the particles, diffusion through the surface layer of these crystals, and diffusion in the meso- and macropores of the particles, for the rate of molecular exchange between catalyst particles and the surrounding atmosphere. This rate is shown to be primarily related to the diffusion in the meso- and macropores of the particles under the condition of fast molecular exchange between these pores and the zeolite crystals located in the particles. The diffusivity associated with this type of diffusion (i.e., the intraparticle diffusivity) is found to correlate well with the catalytic performance of FCC catalysts having the same fractions of the same zeolite USY but different systems of meso- and macropores.

Universitat Leipzig, Fakultat fur Physik und Geowissenschaften, Linnestr. 5, D-04103 Leipzig, Germany. Cepsa Research Center, Picos de Europa, 7 Poligono Industrial San Fernando de Henares II, 28850 San Fernando de Henares, Spain, SINTEF Materials and Chemistry, Department of Hydrocarbon Process Chemistry, P.O.Box 124 Blindern Forskningsveien 1, N-0314 Oslo, Norway, National Technical University of Athens, School of Chemical Engineering, 9 Heroon Polytechniou, Zografou Campus, Athens, GR-157 80, Greece, Grace GmbH & Co. KG, In Der Hollerhecke 1, D-67545 Worms, Germany, Institute of Chemical Technology, Technicka 5, 166 28 Prague 6, Czech Republic, J. Heyrovský Institute of Physical Chemistry, Dolejskova 3, 182 23 Prague 8, Czech Republic, Universitat Stuttgart, Institut fur Technische Chemie, Pfaffenwaldring 55, D-70550 Stuttgart, Germany, Department of Chemistry, University of Oslo, P.o. Box 1033 Blindern, N-0315 Oslo, Norway

PFG NMR has been applied to study intracrystalline diffusion in USY zeolite as well as in the parent ammonium-ion exchanged zeolite Y used to produce the USY by zeolite steaming. The diffusion studies have been performed for a broad range of molecular displacements and with two different types of probe molecules (n-octane and 1,3,5-triisopropylbenzene) having critical molecular diameters smaller and larger than the openings of the zeolite micropores. Our experimental data unambiguously show that, in contrast to what is usually assumed in the literature, the intracrystalline mesopores do not significantly affect intracrystalline diffusion in USY. This result indicates that the intracrystalline mesopores of USY zeolite do not form a connected network, which would allow diffusion through crystals only via mesopores.