Diffusion in fluid catalytic cracking catalysts on various displacement scales and its role in catalytic performance

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.