No abstract

No abstract

No abstract

Complexes of the general formula [MoO2X2L2] (X=Cl, Br, Me; L2=bipy, bpym) have been prepared and fully characterized, including X‐ray crystallographic investigations of all six compounds. Additionally, the highly soluble complex [MoO2Cl2(4,4′‐bis(hexyl)‐2,2′‐bipyridine)] has been synthesized. The reaction of the complexes with tert‐butyl hydroperoxide (TBHP) is an equilibrium reaction, and leads to MoVI η1‐alkylperoxo complexes that selectively catalyze the epoxidation of olefins. Neither the Mo−X bonds nor the Mo−N bonds are cleaved during this reaction. These experimental results are supported by theoretical calculations, which show that the attack of TBHP at the Mo center through the X‐O‐N face is energetically favored and the TBHP hydrogen atom is transferred to a terminal oxygen of the Mo=O moiety. After the attack of the olefin on the Mo‐bound peroxo oxygen atom, epoxide and tert‐butyl alcohol are formed. The latter compound acts as a competitive inhibitor for the TBHP attack, and leads to a significant reduction in the catalytic activity with increasing reaction time.

Microporous silicoaluminophosphates SAPO-5, SAPO-11 and SAPO-40 have been tested as solid acid catalysts in the dehydration of xylose into furfural (FUR) under biphasic aqueous-organic conditions, at 170°C. For all materials, no decrease in catalytic activity is observed after three consecutive recycling runs. Furfural yields at 4 h using SAPO-11 (34-38%) are comparable with that for HMOR zeolite with Si/Al * 6 (34%), under similar reaction conditions, while H 2 SO 4 (0.03 M) gives 2% FUR. Complete xylose conversion is reached within 16-24 h, with furfural yields of up to 65%. Brønsted and Lewis acidity of the silicoaluminophosphates was determined through FTIR analysis of adsorbed pyridine, and tentatively correlated with the catalytic performances.

No abstract

No abstract

The mesoporous silica MCM-41 functionalized with a chelating pyrazolylpyridine ligand (MCM-41-L2) was used as a support for the immobilization of tris(β-diketonate) complexes Ln(NTA) 3 [Ln ) Eu, Gd; NTA ) 1-(2-naphthoyl)-3,3,3-trifluoroacetonate]. The derivatized materials were characterized by powder X-ray diffraction, N 2 adsorption, FTIR and FT Raman spectroscopy, Eu L 3 -edge X-ray absorption fine structure (XAFS), diffuse reflectance, and photoluminescence spectroscopy. The spectroscopic studies, supported by ab initio calculations, provide strong evidence that the immobilized europium(III) complex is 8-coordinate, with a local coordination environment that is similar to that for a model complex containing the ligand ethyl[3-(2-pyridyl)-1-pyrazolyl]acetate (L1). No emission from the pyrazolylpyridine ligands is observed in the room-temperature emission spectrum of MCM-41-L2/Eu, despite the fact that only about one-third are engaged in coordination with Eu 3+ ions. In contrast, the pyrazolylpyridine groups in the precursor ligand-silica exhibit efficient emission. Furthermore, the radiance value measured for MCM-41-L2/ Eu (0.33 µW cm -2 ) is only about one-half of that measured for the complex Eu(NTA) 3 ‚L1 (0.73 µW cm -2 ), even though the concentration of emitting centers in the MCM material is much lower. The results point to the existence of an unusual two-step intermolecular energy transfer between "free" and complexed ligands in MCM-41-L2/Eu, culminating in the observation of enhanced Eu 3+ luminescence.