The energy gradient selectivity concept and the routes of paraffin cracking in FCC catalysts

The conversion of n-hexadecane over steam dealuminated and equilibrium FCC catalysts of different types, containing Y zeolite, was studied in a discontinuous fluidized bed reactor at different reaction temperatures in the range of 440-550 degrees C and contact times from 1 to 60 s. The analysis of product distributions following the approach of energy gradient selectivity by assessing the ratios C-2/C-4 or C-3/C-4 showed that it is possible to track the changes in the relative extent of most important cracking reactions (monomolecular cracking via pentacoordinated carbonium ions as the initiation step, and bimolecular hydride transfer plus "classical" beta-scission of carbenium ions, as propagation steps), since these ratios also represent a relationship between the reactions. As conversion increases for a given catalyst, the magnitude of monomolecular cracking, and consequently the index, decrease until a stable relationship with bimolecular cracking is reached. The apparent energy of activation of monomolecular cracking is higher. Both the changes in zeolite framework electric charge density induced by dealumination and the procedure used for dealuminating the catalysts, which generates different degrees of heterogeneity, influence the relationship between cracking routes. When dealumination is higher, the effect of energy gradients, the protolytic cracking, and consequently, the index, increase; this approach also shows that steam dealumination produces more heterogeneous zeolite aluminum distributions than equilibration in commercial FCC units. (C) 1998 Academic Press