Isolation of Rhenium and ReOx Species within ZSM5 Channels and their Catalytic Function in the Activation of Alkanes and Alkanols

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Abstract
Synthesis protocols, structures, and reactivity of Re–oxo species grafted onto HZSM5, and their subsequent conversion to Re-clusters through contact with H2 or CH4 were studied by using Raman, infrared, and X-ray absorption spectroscopies. Reactivity measurements by using alkane and alkanol reactants were also examined. Sublimation of Re2O7 at 723 K led to a stoichiometric exchange with each ReOx species replacing one proton. Raman features for Re2O7 disappeared during thermal treatment and Raman bands assigned to distorted-tetrahedral Si-OfReO3-Al (Of: zeolite-lattice oxygen atoms) species emerged; infrared bands for acidic OH groups in HZSM5 weakened concurrently. X-ray absorption near-edge and fine-structure spectra detected the formation of distorted-tetrahedral Re7+–oxo species during thermal treatment of Re2O7/HZSM5 mixtures in air, and their subsequent reduction to Re0 in H2 or CH4 to form encapsulated Re metal clusters similar in diameter (≈8 Å) to the channel intersections in ZSM5. Si-OfReO3-Al species in ReOxZSM5 catalyzed the oxidative conversion of C2H5OH to acetaldehyde, acetal, and ethyl acetate with very low selectivity to COx (x–based catalysts, and without deactivation or sublimation processes ubiquitous in crystalline Re7+ compounds at temperatures required for catalysis. Encapsulated Re metal clusters formed by the reduction of Si-OfReO3-Al precursors led to CH4 pyrolysis and C3H8 dehydrocyclodimerization rates (per Re) that are higher than those previously reported for zeolite-based catalysts. The rate of CH4 conversion to benzene, by using ReZSM5, was ≈30 % higher than that of the best reported catalysts, based on encapsulated MoCx clusters, whereas C2H4 and C6+ arene selectivities were similar. C3H8 activity and selectivity of ReZSM5 was significantly higher than that of GaZSM5, the best reported catalyst for these reactions. Reaction rates (per Re) were independent of the Re/Alf (Alf: aluminum framework) ratio for both Re and ReOx species. This is consistent with the uniform character of the structures formed during grafting of the ReOx species through sublimation and their ability to retain their homogeneity even after their reduction to encapsulated Re-clusters.

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