Enhancement of Epoxidation Efficiencies for Ta-SBA15 Catalysts. The Influence of Modification with –EMe₃ (E = Si, Ge, Sn) Groups

Abstract

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.

Supported sub-nanometer Ta oxide clusters as model catalysts for the selective epoxidation of cyclooctene

“…Based on the number of molecules of cyclooctene converted by Ta 1 with respect to Ta n>1 (See also table S2) it is clear that the isolated atoms are more active than the tantalum clusters (about a factor four per catalytic entity). This result is consistent with previous observations in the literature 34, [39][40][41]42 and with a mechanism 43,44 in which individual tantalum centres are able to epoxidize olefins by formation of hydroperoxo, 45 peroxo and superoxo species after reaction with H 2 O 2 . 46 The conversion data allow to calculate a blank-corrected TOF of 1.72*10 5 h -1 for Ta 1 that is proof of the potential of the employed method for the preparation of the catalytically active isolated, ligand-free metal oxo moieties.…”

“…40,41,[47][48][49] The use of organic ligands to coordinate the tantalum centre has been proposed to play a role in increasing the reaction selectivity. 40,41,44 When Ta 1 was used for the epoxidation of cyclohexene, the reaction produced almost exclusively (about 90% of the C6 products) cyclohexene oxide and 1,2 cyclohexane diol (See Figure S7) in a 42:58 ratio (TON: 3*10 7 ; TOF: 6.2*10 5 h -1 ). The total cyclohexene conversion was 30 %.…”

Silica‐Grafted SnIV Catalysts in Hydrogen‐Transfer Reactions

“…For instance, the metal may form additional bonds to the surface during a thermal treatment (that is, multipodal anchoring) ,. All these synthetic steps can be monitored by various characterization techniques (FTIR, UV/Vis and solid‐state NMR spectroscopy) and often change the activity of the catalyst ,–…”

Modified Ta/MCM-41 catalysts for enantioselective oxidation of thioanisole

“…More recently, tantalum alkoxides on mesoporous silica SBA-15 have shown high efficiency in the epoxidation of alkenes with aqueous H 2 O 2 when the environment of the catalytic site is hydrophobic [29][30][31]. Recently, TaCl 5 and Ta(OEt) 5 have been described as excellent catalyst for sulfides oxidation with H 2 O 2 in homogeneous phase, with selectivity for sulfoxide or sulfone depending on the catalyst and the reaction conditions (solvent, temperature, time) [32].…”