Evidence of different reaction mechanisms during the cracking of n-hexane on H-USY zeolite

Experimental data were collected for n-hexane cracking on acidic ultrastabl e Y (H-USY) zeolite at 673 K which clearly document a change from monomolec ular cracking to bimolecular/oligomeric cracking as the reactant partial pr essure and contact time (or conversion) increased. Under conditions of low pressure (<0.1 kPa) and low conversion (ca. 0%), the characteristic product distribution of monomolecular cracking, which contained relatively high am ounts of methane and ethane, was observed. The rates of methane and ethane production depended only on the hexane partial pressure and remained unaffe cted by alkene partial pressure over the entire range of conditions studied (0.1-6.2 kPa, 0.3-35% conversion). However, the presence of alkenes, eithe r as product or added in the feed, resulted in bimolecular and oligomeric c racking becoming more important contributors to hexane conversion. Correspo nding to this transition, the product distribution showed increased amounts of propane, propene, and higher branched products. Using a new methodology to account for varying adsorption properties with hydrocarbon chain length , the observed reaction rates conformed to a simple kinetic rate expression that incorporated both monomolecular and bimolecular/oligomeric cracking. The results showed that, for H-USY zeolite, the observed rate constant for bimolecular/oligomeric cracking was about 75 times greater per kPa of equiv alent propene partial pressure in the gas phase than that for monomolecular cracking at 673 K. This observation emphasizes the importance of making co mparisons between solid acids under conditions where the same mechanism dom inates. (C) 2000 Elsevier Science B.V. All rights reserved.