Site-isolated Ta(V) centers were introduced onto the surface of a mesoporous SBA-15 support via the thermolytic molecular precursor method. After thermal treatment under oxygen, the resulting Si-OH and Ta-OH sites of TaSBA15-O(2)were modified with a series of trimethyl group 14 species, Me(3)E-, by treatment with Me(3)E-NMe(2) (E = Si, Ge, Sn) reagents. The resulting surface-modified catalysts (Me(3)E)(cap)TaSBA15 exhibit a significantly increased rate of cyclohexene epoxidation with H(2)O(2) as an oxidant, and provided a decreased amount of allylic oxidation products with respect to the unmodified material, TaSBA15-O(2). The rate of nonproductive H(2)O(2) decomposition, as monitored via (1)H NMR spectroscopy, significantly decreased after the surface modification. The structure of the TaSBA15 catalysts and potential Ta(V) epoxidation intermediates (formed upon treatment of Ta(V) materials with H(2)O(2)) were probed using UV-visible absorbance and diffuse-reflectance UV-visible spectroscopy. A Ta(V)(η(2)-O(2)) intermediate species is proposed for the TaSBA15-O(2), (Me(3)Si)(cap)TaSBA15, and (Me(3)Ge)(cap)TaSBA15 catalysts, while intermediate species for the (Me(3)Sn)(cap)TaSBA15 catalysts could not be characterized.

The complex Ti[OGe i Pr 3 ] 4 (1), prepared via the reaction of Ti(O i Pr) 4 with i Pr 3 GeOH, represents a useful structural and spectroscopic model for titanium−germanium species dispersed onto silica. This precursor was used to introduce site-isolated Ti(IV) centers onto the surface of a mesoporous SBA15 support via the thermolytic molecular precursor method. The local environments of the supported materials (TiGe 3 SBA15 and calcined TiGe 3 SBA15-O 2 ) were studied by various spectroscopic methods, including X-ray absorption spectroscopy. These materials are active catalysts for the epoxidation of cyclic and terminal olefins with alkyl hydroperoxides under anhydrous conditions. Compared to catalysts synthesized from siloxide-only precursors, the new catalysts produce 2−3 times more product after 9 h under identical reaction conditions for the epoxidations of cyclohexene and 1-octene. The new materials did not significantly leach under reaction conditions.

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...

Site-isolated Ti(IV) centers were introduced onto the surface of a mesoporous SBA-15 support via the thermolytic molecular precursor method. Prior to thermal treatment to generate Ti−OH sites, residual silanol groups were capped via reaction with Me2N-SiMe3 to give TiMe cap SBA15. After low temperature treatment in oxygen, the resulting Ti−OH sites of TiMe cap SBA15-O 2 were modified by reaction with a series of protic reagents: phenol, pentafluorophenol, acetic acid, and trifluoroacetic acid. The structure of the resulting TiSBA15 catalysts and the Ti(IV) epoxidation intermediates (formed upon treatment of Ti(IV) materials with TBHP or H2O2) were probed using diffuse-reflectance UV−visible spectroscopy and infrared spectroscopy. A titanium-hydroperoxo species similar to that found in TS-1 is proposed for all catalysts. Samples modified with phenol and pentafluorophenol exhibited conversions of 1-octene that are 20 to 50% higher than those for TiMe cap SBA15-O 2 , without a significant drop in selectivity for the epoxide product, 1,2-epoxyoctane, when TBHP was used as the oxidant. With aqueous H2O2 as the oxidant, the phenol-treated materials exhibited 1-octene conversions that are 15 to 50% greater than those observed for TiMe cap SBA15-O 2 , and an increased selectivity for 1,2-epoxyoctane of 10 to 30%. Additionally, the efficiency of H2O2 usage, as monitored via 1H NMR spectroscopy, increased by a factor of 2 to three for catalysts modified with phenol and pentafluorophenol, with respect to the efficiency observed over TiMe cap SBA15-O 2 . Catalysts modified with acetic acid and trifluoroacetic acid displayed decreased catalytic turnover numbers and epoxide selectivities when TBHP was used as the oxidant, but exhibited catalytic turnover numbers and epoxide selectivities similar to TiMe cap SBA15-O 2 when H2O2 was used as the oxidant. After treatment of TiMe cap SBA15-O 2 with acetic acid, the H2O2 efficiency decreased by a factor of 2 for the epoxidation of 1-octene with H2O2.