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

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 Re(2)O(7) at 723 K led to a stoichiometric exchange with each ReO(x) species replacing one proton. Raman features for Re(2)O(7) disappeared during thermal treatment and Raman bands assigned to distorted-tetrahedral Si-O(f)ReO(3)-Al (O(f): 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 Re(2)O(7)/H-ZSM5 mixtures in air, and their subsequent reduction to Re(0) in H2 or CH4 to form encapsulated Re metal clusters similar in diameter (approximately 8 A) to the channel intersections in ZSM5. Si-O(f)ReO(3)-Al species in ReO(x)-ZSM5 catalyzed the oxidative conversion of C(2)H(5)OH to acetaldehyde, acetal, and ethyl acetate with very low selectivity to CO(x) (<1 %). Unprecedented turnover rates were exhibited at temperatures much lower than previously found for ReO(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-O(f)ReO(3)-Al precursors led to CH4 pyrolysis and C(3)H(8) 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 approximately 30 % higher than that of the best reported catalysts, based on encapsulated MoC(x) clusters, whereas C(2)H(4) and C6+ arene selectivities were similar. C(3)H(8) 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/Al(f) (Al(f): aluminum framework) ratio for both Re and ReO(x) species. This is consistent with the uniform character of the structures formed during grafting of the ReO(x) species through sublimation and their ability to retain their homogeneity even after their reduction to encapsulated Re-clusters.

Mechanistic study of the selective hydrogenation of carboxylic acid derivatives over supported rhenium catalysts

“…Although the earlier studies led to the development of a highly active and selective catalyst for hydrogenation of carboxylic acid derivatives, they did not reveal the precise nature of the catalytically active Re species. Moreover, little is known about the nature of the active species in a number of valuable Re promoted transformation [32][33][34][35][36][37][38][39][40][41][42] despite having been the subject of scientific investigations for years. [43][44][45][46][47][48][49][50][51][52][53][54][55] The main reasons for this are that Re often is highly dispersed on supports and is difficult to be reduced, and it can exist in a variety of oxidation states.…”

Ambient aging of rhenium filaments used in thermal ionization mass spectrometry: Growth of oxo-rhenium crystallites and anti-aging strategies

“…Measurements on small crystallites were difficult due to sublimation; however, spectra were collected successfully from crystallites larger than ∼1 μm. These spectra were compared with those reported for binary rhenium oxide species and were found to correlate with Re 2 O 7 [43] (Fig. 5).…”

Modern Spectroscopic Methods Applied to Nanoscale Porous Materials

“…Zeolite particles serve as a host matrix to encapsulate transition metal-oxide subnanoparticles. [50][51][52] Zeolites provide well defined and well-ordered porous system to confine the metal-oxygen clusters. Thus the size and configuration of the encapsulated clusters can be tailored using different microporous materials.…”

“…The method is especially advantageous in studying active centres in functionalized by appropriate doping porous materials; however, its proper utilization requires a synchrotron radiation. 15,39,52,60,61 Mößbauer spectroscopy is based on recoilless resonance absorption/emission of gamma-rays in solids. Its utilization for structural analysis stems from the fact that the principle energy levels of atomic nuclei are strongly influenced by the positions of the surrounding atoms.…”

“…[43][44][45][46][47][48][49] Another advanced application of Raman spectroscopy is the characterization of nanocomposites of carbon and metal-oxide wires and clusters encapsulated into the porous system of a zeolite-type host. [50][51][52][53][54][55][56][57] Visible/near-visible (optical absorption and laser-induced luminescence) spectroscopy, which probes valence-electron states, is an excellent tool to distinguish between cations in oxidation state and sub-nanometric metal-oxide clusters embedded into the framework of nanoscale porous materials. 10,17,33,50,51 The utilization of these spectroscopic methods for structural analysis is often hindered from the broad width and the overlapping of spectral signals; however, the combination of visible/ near-visible spectroscopy with vibrational and/or NMR spectroscopy is beneficial for characterization of photoactive materials.…”