A kinetic study of n-heptane catalytic cracking over a commercial Y-type zeolite under supercritical and subcritical conditions

Heptane catalytic cracking over a commercial Y-type zeolite (Promoted Octacat) was studied at 475 degrees C under subcritical and supercritical hydrocarbon conditions. Kinetic measurements showed that the activity of the catalyst was substantially higher during supercritical cracking, suggesting stabilization of the catalyst toward rapid deactivation. A mechanism based on kinetic and in situ cylindrical internal reflectance FTIR (CIR-FTIR) results was developed to explain the performance of the catalyst under supercritical conditions. The dense supercritical reaction medium, well known for its enhanced solubilization capabilities, continuously removes the newly formed coke through solubilization from the zeolite supercages, pore mouths, and the external surface. This results in a higher number of available active sites to perform catalytic cracking and in lower pore diffusion limitations due to the avoidance of pore blockage. The significantly higher yields of the olefinic and paraffinic reaction products and the higher paraffin/olefin ratios under supercritical cracking conditions are in agreement with the above mechanism. Finally, it was shown that a partial regeneration of a moderately deactivated cracking catalyst is possible by the supercritical reaction medium. Reactivation of a severely deactivated catalyst could not be achieved. (C) 1996 Academic Press, Inc.