Synthesis, modification, characterization and catalytic studies of zeolite based bifunctional catalysts for hydroisomerization reactions

Zeolite ZSM-12 was identified as a promising candidate for hydroisomerization of n-alkanes and a large part of the work was focused on the preparation of ZSM-12 and studying its catalytic performance. Reliable and efficient methods for synthesis of ZSM-12 with high levels of acidity were not available in the literature. Through systematic studies, the synthesis was greatly simplified and ZSM-12 was successfully crystallized at a relatively low Si/Al ratio of 30 using tetraethylammonium cations as the template. In the hydroisomerization of n-heptane as well as mixtures of n-alkanes, ZSM-12 catalysts provided a significantly higher selectivity to both monobranched and dibranched isomers, compared to other large pore zeolites. Kinetic studies carried out using individual octane isomers showed that the formation of certain dibranched and tribranched isomers was mildly hindered by the shape selective effect of the ZSM-12 pore, which resulted in the higher isomer yield.; It was found that the cracking activity of ZSM-12 could be increased by three to four times by carrying out the calcination to remove the template at temperatures higher than what is normally used. The variation of the cracking activity with respect to the calcination temperature was very unusual, only the samples calcined in two separate narrow temperature ranges showed substantially enhanced activity. Extensive FTIR characterizations, with pyridine adsorbed on the acidic zeolite, indicated that the enhancement in the cracking activity was due to a synergistic interaction between the framework Bronsted acid sites and a Lewis acidic extraframework aluminum (possibly cationic) species present in the zeolite. Hydroisomerization of n-heptane was studied over large pore zeolites with different acidity properties and wide range of Si/Al ratios. The performance of zeolite catalysts with enhanced activity were very sensitive to the platinum precursor used and the pretreatment conditions. The results suggested that zeolite acidity characteristics do not affect the isomer yield, provided the catalyst is carefully prepared to ensure good metal/acidity balance. Therefore, it appears that the only way to obtain isomer yields higher than that of Y zeolite is by employing shape selective zeolites.