Co/H-ZSM5 catalyzes propane dehydrogenation and aromatization reactions. Initial product selectivities, product site-yields, and the 13 C content and distribution in the products of 2-13 C-propane show that propane undergoes two primary reactionssdehydrogenation to propene and H 2 and cracking to methane and ethene. Propene and ethene then form aromatics via oligomerization-cracking reactions and both ethene and propene hydrogenate to form ethane and propane, respectively, via both hydrogen transfer from coadsorbed intermediates and dissociative adsorption of H 2 . These reaction pathways resemble those occurring on H-ZSM5, but Co cations provide an alternate pathway for the removal of hydrogen atoms in adsorbed intermediates as H 2 . A kinetic model was used to describe experimental rates based on these observations and to obtain rate constants for individual reaction steps as a function of Co content and H 2 concentration. H 2 inhibits propane dehydrogenation to propene and alkene conversion to aromatics, and increases the rate and the rate constant for ethene hydrogenation. Rate constants were obtained for propane reactions on H-ZSM5 and Co/H-ZSM5 (Co/Al ) 0.05-0.22). Propane dehydrogenation (k 1 ), ethene hydrogenation (k 3 ), and alkene dehydrocyclization (k 4 ) rate constants increased with increasing Co/Al ratio, because Co cations catalyze both the recombinative desorption of H 2 and its microscopic reverse, the dissociative adsorption of H 2 . Co cations increase the reversibility of hydrogen adsorption-desorption steps, and in this manner, they increase the deuterium content in all products of C 3 H 8 -D 2 reactions. Propane cracking rate constants (k 2 ) were not influenced by Co cations, because cracking occurs on acid sites and its stoichiometry and mechanism do not require hydrogen.

Co/H-ZSM5 catalysts with Co/Al ratios of 0.09-0.22 were prepared by aqueous exchange. Turnover rates for propane conversion to propene and to C 6 -C 8 aromatics on these catalysts are about 10-fold higher than on H-ZSM5. The selectivities to propene, aromatics, and H 2 are also higher on Co/H-ZSM5 than on H-ZSM5. The rate of D 2 exchange with OH groups increases with increasing Co/Al ratio, suggesting that Co cations catalyze D 2 dissociative chemisorption steps that limit the rate of isotopic exchange. Co cations also catalyze hydrogen recombinative desorption steps, which limit the rate of propane dehydrogenation and aromatization reactions. The density of residual zeolitic hydroxyls was measured by D 2 -OH isotopic exchange and by changes in the intensity of OH infrared bands as a function of Co content. D 2 -OH and infrared measurements showed that Co 2+ cations replace 1.1-1.3 zeolitic protons, suggesting the predominant presence of Co 2+ -O-Co 2+ dimers, with some Co 2+ monomers, each bridging two next-nearest neighbor Al sites. The location and structure of exchanged Co cations were probed using X-ray absorption spectroscopy (XAS) and temperature-programmed reduction (TPR). No H 2 consumption was detected up to 1273 K during TPR in any of the Co/H-ZSM5 samples, consistent with the absence of CoO x crystallites, which reduce at ∼800 K. In situ near-edge X-ray absorption studies confirmed that Co species remain as divalent cations during exposure to H 2 or C 3 H 8 at 773 K. Near-edge and fine structure analysis detected Co 2+ cations with similar structure in all Co/H-ZSM5 samples (Co/Al < 0.22), and Co coordination changes from octahedral to tetrahedral upon sample dehydration at 773 K in He. Radial structure functions showed weak contributions from the first and second shells around Co. This reflects the nonuniform nature of the distance and orientation in Al-Al nextnearest neighbor sites in ZSM5.

Temperature programmed desorption and infrared and X-ray absorption near-edge spectroscopies were used during adsorption and reactions of thiophene in order to probe adsorbed intermediates and catalytic structures responsible for thiophene reactions with propane or H 2 on H-ZSM5 and Co/H-ZSM5. Infrared spectra showed that thiophene interacts with acidic OH groups in H-ZSM5 via hydrogen bonding at ambient temperature. No additional bands were detected on Co/H-ZSM5, suggesting the absence of specific interactions with Co cations. During adsorption at ambient temperatures, infrared bands assigned to CH 2 groups near C=C bonds or Satoms and to S-H species were detected and H-ZSM5 and Co/H-ZSM5 acquired colors typical of thiophene oligomers. Slightly above ambient temperatures, benzene and H 2 S formed from pre-adsorbed thiophene. These results indicate that hydrogen-bonded thiophene undergoes ring opening or oligomerization near ambient temperature on acidic OH groups in H-ZSM5. Some of the adsorbed thiophene (20-50%) interacts weakly with channel walls or with residual Na cations and desorbs unreacted. The remaining adsorbed thiophene desorbs as H 2 S, aromatic hydrocarbons, and organosulfur compounds, such as methylthiophene and benzothiophene, or forms irreversibly adsorbed unsaturated organic deposits. In situ infrared studies during thiophene and thiophene-propane reactions at 773 K on H-ZSM5 and Co/H-ZSM5 showed that surface coverages of thiophene-derived intermediates were low on acidic OH groups and Co cations. Co K-edge X-ray absorption near-edge spectra measured during these reactions confirmed that Co 2+ cations do not reduce or sulfide; their local environment, however, changes slightly, apparently because of interactions of strongly adsorbed species with Co cations. Sulfur K-edge X-ray absorption spectra detected small amounts of organosulfur species, but no inorganic sulfides, after thiophene, thiophene-H 2 , and thiophene-propane reactions, consistent with the observed stability of exchanged cations against reduction and sulfidation. S : Al ratios were less than 0.04 at. on all samples; these amounts represent less than 1% of the S-atoms removed from thiophene as H 2 S during catalytic propane-thiophene reactions.

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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Stoichiometric reactions can be used to remove H2 and the associated kinetic and thermodynamic barriers that lead to low dehydrocyclodimerization selectivity during alkane reactions on cation-modified H-ZSM5. O2 co-reactants can form H2O in exothermic reactions that balance the enthalpy of endothermic dehydrogenation steps. O2 reacts preferentially with H2 via homogeneous and heterogeneous pathways, but also with hydrocarbons as H2 is depleted; thus, it must be gradually introduced as H2 forms in dehydrogenation reactions. Staged O2 protocols significantly increased aromatics yields during C3H8 reactions on unexchanged and on Ga- and Zn-exchanged H-ZSM5. On Ga/H-ZSM5, the maximum aromatic yields increased from 54% to 68% and aromatization/cracking selectivity ratios increased from 2.1 to 3.9 when O2 was introduced gradually into a gradientless batch reactor as H2 formed. O2 introduced was converted to H2O with >95% selectivity; an equivalent amount of O2 initially added with C3H8 led to low H2O selectivities (<60%). Similar effects of O2 addition and of staging protocols were observed for alkane reactions on H-ZSM5 and Zn/H-ZSM5. Staging strategies led to selective use of O2 to remove thermodynamic and kinetic bottlenecks and to unprecedented aromatics yields during alkane reactions on cation-exchanged H-ZSM5.