Propene formation rates during propane conversion at 773 K on Zn/Na-ZSM5 are about ten times higher than on Zn/H-ZSM5 catalysts with similar Zn content. The total rate of propane conversion is also higher on Zn/Na-ZSM5 by a factor of four. Propane reactions lead to high propene selectivities ([50%) as protons are replaced by Na cations in Zn/H-ZSM5 catalysts. The titration of acid sites with Na`cations decreases the rate of acid-catalyzed chain growth reactions and the selectivity to aromatics. X-ray absorption studies at C 6 ÈC 9 the Zn-K edge showed that aqueous ion exchange of Na-ZSM5 with Zn cations leads to isolated (ZnOH)s pecies located at cation exchange sites. Unlike Zn species in Zn/H-ZSM5 (\1.0 wt.%), high temperature condensation reactions of (ZnOH)`species with neighboring zeolite OH groups are less likely to occur in Zn/Na-ZSM5 and most Zn species remain as (ZnOH)`. Temperature programmed reduction studies show that Zn species in Zn/Na-ZSM5 reduce at lower temperatures than the (O~ÈZn2`ÈO~) species present in Zn/H-ZSM5.exchange with surface OH groups showed that some protons are formed during ion D 2 exchange. Higher deuterium contents in products of mixtures on Zn/Na-ZSM5 suggest that C 3 H 8 ÈD 2 (ZnOH)`species in Zn/Na-ZSM5 catalyze rate-determining hydrogen desorption steps during propane conversion more e †ectively than (O~ÈZn2`ÈO~) sites present in Zn/H-ZSM5. The presence of (ZnOH)s pecies and a lower acid site density in Zn/Na-ZSM5 leads to much higher propane conversion rates than on Zn/H-ZSM5. As the acid site density decreases, propene aromatization rates decrease, which leads to less hydrogen to be disposed by a more efficient hydrogen recombinative desorption species (ZnOH)`.
Reactions of 13 C-labeled alkanes show that chain growth and cyclization reactions on H-ZSM5 require the initial formation of the corresponding alkene and its extensive participation in rapid oligomerization/ -scission reactions before cyclization occurs. The role of alkene intermediates was established by the initial formation of predominantly unlabeled products from mixtures of propene and propane-2-13 C reactants. Aromatic products of propane-2-13 C reactions on H-ZSM5 contain similar fractions of 13 C-atoms and binomial isotopomer distributions. Sequential formation, rapid intramolecular isomerization, and -scission reactions of long surface chains must occur during each aromatization turnover in order to form such binomial 13 C isotopomer distributions.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.