Evaluation of commercial FCC catalysts for hydrocarbon conversion I. Physicochemical characterization and n-hexane conversion

The physicochemical properties of six steam-stabilized, commercial FCC catalysts were compared in respect of their catalytic activity for n-hexane conversion. The conversion of n-hexane over these catalysts could be fully explained by three reaction pathways: protolytic cracking, protolytic dehydrogenation and hydride transfer. Matrix components did not contribute to the n-hexane conversion. A correlation of the acid strength distribution, measured by pyridine TPD showed that nearly all sites with high acid strength are located in the micropores of the zeolite component. ²⁷Al-MAS-NMR distinction between octahedral-, tetrahedral- and pentacoordinated species seems to be unsuitable for the determination of different tetrahedral species in this kind of catalysts. The introduction of rare-earth metals into the zeolites increases the acid strength of the active sites manifested in sequential reactions of the primary formed alkyl surface species, i.e. β-scission and hydride-transfer reactions. The addition of water to the reactant stream decreases the conversion due to competitive adsorption, but does not change the amount, nor the nature of the active site.