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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.

Hydrogen removal steps limit alkane dehydrogenation reactions on cation-exchanged H-ZSM5 and cause desorption bottlenecks and the formation of hydrogen-rich reaction intermediates and products. For this reaction, the catalytic surface acts for all kinetic purposes as if it were in equilibrium with a H 2 pressure greater than in the prevalent gas phase. The hydrogen chemical potential within adsorbed intermediates is described rigorously by a virtual H 2 pressure, defined as that required to achieve the prevalent surface hydrogen content if adsorption-desorption steps were equilibrated. These virtual pressures can be measured from the deuterium content in products formed from C 3 H 8 /D 2 reactants. H 2 virtual pressures are high during propane reactions on H-ZSM5, because recombinative desorption of hydrogen atoms formed in C-H activation steps is slow. H 2 virtual pressures decrease as Zn cations replace protons in H-ZSM5, because cations catalyze hydrogen adsorption-desorption steps and provide a kinetic path for communication between H 2 in the gas phase and propane-derived reaction intermediates. As a result, the addition of H 2 to C 3 H 8 reactants decreases propane reaction rates and aromatics selectivity on Zn/H-ZSM5, causing it to resemble kinetically H-ZSM5. The hydrogen chemical potential on these catalytic surfaces and the hydrogen content within reactive intermediates reflect the rate at which hydrogen is formed in either C-H bond activation steps or in the dissociative chemisorption of gas-phase H 2 . The reaction pathways for species derived from each of these two H-sources are kinetically indistinguishable. Both sources contribute hydrogen atoms or hydrogen-rich species to the prevalent pool of reactive intermediates, the hydrogen content of which determines the rate and selectivity of all surface chemical reactions.

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