N-Heptane transformation over a HMCM-22 zeolite: Catalytic role of the pore systems

Abstract

n-Heptane transformation was carried out at 350 °C over a HMCM-22 zeolite (Si/Al = 14.5) previously characterized by various techniques: X-ray diffraction, nitrogen adsorption, scanning electron microscopy, pyridine and 2,4-dimethylquinoline (2,4-DMQ) adsorption followed by FTIR. A pronounced deactivation was shown to occur in the first 10 min reaction, due to a very fast initial coke formation, followed by a quasi-plateau in activity. Cracking was the main reaction. The role played by each of the three pore systems was established by selectively deactivating the supercage sites by coking then by selectively poisoning the protonic sites of the external cups with a bulky base molecule (2,4-DMQ). The supercage sites (∼70% of the inner ones) were found to be responsible for 97% of n-heptane transformation, those of the sinusoidal channels (∼20%) for only 3%, which means that these latter sites were ∼16 times less active probably because of pronounced steric constraints. Unexpectedly, the protonic sites of the external cups, which were demonstrated as able to catalyse efficiently various reactions including methylcyclohexane cracking, were found to be completely inactive.