Methylcyclohexane transformation over dealuminated HBEA samples: Mechanisms and active sites

Methylcyclohexane transformation was carried out at 450 °C over HBEA samples resulting from dealumination of the same parent HBEA zeolite by three different methods: steaming, treatment with aqueous solutions of hydrochloric acid or ammonium hexafluorosilicate. With all the samples, methane, C2–C4 and C7 alkenes, isomers and toluene appear as primary products, the other products C2–C4 alkanes, C5 and C6 alkenes and alkanes, benzene, xylenes and trimethylbenzenes being secondarily formed. These products were shown to result from two mechanisms: carbenium ion chain, protolytic cracking and dehydrogenation. The activities of all the HBEA samples for each of the processes were estimated. A linear correlation was found between the activity of the samples for the carbenium ion chain process and the square of the concentration of protonic sites, which suggests a demanding process. Furthermore a close similarity could be observed between the change of the protolytic activity and Lewis acidity of the HCl treated samples with their Al content. Strong protonic sites resulting from interaction of bridging OH groups with neighbouring Lewis species: structure defects and monomeric extraframework Al species were proposed to be the active sites.