Structure and Catalytic Function of Re-Oxo Species Grafted onto H-MFI Zeolite by Sublimation of Re₂O₇

Abstract

The formation of single-site heterogeneous catalysts remains a formidable challenge. 1 Sites with uniform structure and catalytic function seldom prevail in microporous inorganic solids prepared by aqueous exchange of high-valent cations because of the size and charge of aqueous metal-oxo oligomeric precursors and their diverse structures at conditions of pH and temperature consistent with the structural integrity of aluminosilicates. Anchoring of high-valent metal-oxo species leads to structures that are less reducible than the corresponding bulk oxides and to relatively unreactive oxidation catalysts. 2 Here, we report the exchange of isolated and uniform Re 7+-oxo cations of significant catalytic relevance for oxidation of alcohols via reactions of Re 2 O 7 (g) with acidic protons in H-MFI. The structure and catalytic function of these species do not depend on Re content, a hallmark of single-site catalysts with isolated and uniform active sites. Re 2 O 7 and organometallic Re-oxo species form volatile species at conditions required for oxidation catalysis. As a result, they cannot be used for oxidation reactions at elevated temperatures despite their high reactivity in alkene epoxidation. 3 Re 2 O 7 , NH 4-ReO 4 , and MTO (methyltrioxorhenium) have been grafted onto several inorganic oxide supports in order to stabilize active sites and allow their use as catalysts. 3,4 The structures of ReO x species during synthesis, thermal treatment, and catalysis, and even their presence at exchange sites in H-MFI, remain uncertain. Raman spectra were used to probe the structure of Re-oxo species in H-MFI (Figure 1) after removing fluorescing compounds using a novel UV irradiation treatment (Supporting Information). Raman spectra for Re 2 O 7 /H-MFI (0.4 Re/Al f) mixtures resembled those for crystalline Re 2 O 7 6,7 before thermal treatment. Re 2 O 7 bands disappeared during treatment in dry air at 823 K for samples with Re/Al f ratios of 0.3-0.8; broad bands emerged concurrently at 1020, 980, and 347 cm-1 , similar to those for ReO 4-(aq), but shifted to slightly higher frequencies, consistent with the formation of Si-O f ReO 3-Al (O f is framework O) centers distorted from perfect tetrahedra. Changes in local Re-O coordination upon heating Re 2 O 7 /H-MFI were also evident in Re L I edge X-ray absorption spectra (Figure 2A). The intensity of the pre-edge feature increased after treatment at 723 K in dry air because of an increase in the tetrahedral character of Re centers as in distorted tetrahedral NH 4-ReO 4 , compared with Re 2 O 7 , in which 50% of the Re exhibits distorted octahedral symmetry. Metal-oxo cations with M 7+ centers can exist as either monomers (Structure I, Scheme 1) or dimers (Structure II) at exchange sites. Dimers might resemble a distorted C 2V symmetry with two bridging O atoms and two terminal RedO bonds at each Re center as depicted in Scheme 1; there are, however, no precedents for di-µ-O linkages in Re-oxo complexes. Re-ORe stretches in dimers, expected at 456 and 185 cm-1 (from bands in mo...

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

“…[54] Raman and infrared spectroscopy: HÀZSM5 samples were exposed to UV light at ambient temperature, a procedure that led to the removal of unsaturated adsorbed organics, which fluoresce strongly and decrease the quality of Raman spectra. [16] HÀZSM5 was then physically mixed with Re 2 O 7 , pressed into self-supporting wafers, and rotated at % 7 Hz during the acquisition of Raman spectra to minimize laser heating. Wafers were heated to 823 K (0.17 K s À1 ) for 1 h in dry air and cooled to ambient temperature before measuring Raman spectra by using a HoloLab 5000 Research Raman spectrometer (Kaiser Optical Systems) equipped with a 532 nm laser.…”

“…[4,9,10,12,13] We recently reported the vapor-phase exchange of Re 2 O 7 (sublimes at 535 K; 100 Pa Re 2 O 7 (g) at 496 K) [14,15] with HÀ ZSM5. [16] Similar methods were previously used to graft (Mo 2 O 5 ) 2 + species prepared from MoO 3 [17,18] (sublimes at 1425 K, 4.9 Pa at 973 K). [15] These protocols led to uniform and highly-dispersed Re 7 + -oxo species that were amenable Abstract: Synthesis protocols, structures, and reactivity of Re-oxo species grafted onto H À ZSM5, and their subsequent conversion to Re-clusters through contact with H 2 or CH 4 were studied by using Raman, infrared, and X-ray absorption spectroscopies.…”

Zirconium phenylphosphonate-anchored methyltrioxorhenium as novel heterogeneous catalyst for epoxidation of cyclohexene

“…This paper is also supported by the "Chemical Grid Project" of Beijing University of Chemical Technology. 24 …”

“…Various inorganic and organic materials, such as acidic metal oxides (niobia [18], [19], zeolites [20]- [24], or alumina [25]- [27]), polymers [28], [29], natural polysaccharides [30] and functionalized silica matrices [31]- [33] have been employed as supports for the immobilization of MTO. Meanwhile, several important methods for improving the catalyst performance have been revealed by surface chemistry and computational studies, for instance, the structural feature and bonding mode of the MTO species on the support surface, the beneficial pyridine effect manifested in both homogeneous and heterogeneous MTO-catalyzed epoxidation, and the modification of support surface with organic or polymeric functional groups.…”

Immobilization of organorhenium(VII) oxides

“…Another study shows that thermal treatment of Re 2 O 7 / H-MFI mixtures, leads to the selective grafting of Re-oxo species onto the zeolite via sublimation by the reactions of formed -OH groups to give Si-O f ReO 3 -Al [39]. The formed catalyst shows high reactivity, particularly for the oxidation of C 2 H 5 OH with turnover rates independent of Re loading.…”