Diffuse reflectance spectroscopy study of the thermal genesis and molecular structure of chromium-supported catalyst

The diffuse reflectance spectra of a series of chromium-supported silica aluminas, with varying SiO2 content, have been investigated before and after drying and after calcination, successive CO reductions, and recalcination. The molecular structure of chromium before drying is influenced by the isoelectric point (IEP) of the supports. It is concluded that the lower the IEP of the support, the higher the amount of surface dichromate. After calcination at 720 degrees C, Cr is anchored to the surface without changing the chromate:dichromate ratio. Reduction results in the formation of octahedral Cr3+, pseudooctahedral and pseudotetrahedral Cr2+. A higher silica content of the support, a higher reduction temperature, and a higher calcination temperature result in a deeper reduction of the supported Cr. Recalcination restores the initial chromate:dichromate ratio on each support; however, small quantities of Cr3+ remain in the samples. This deeper reduction and reversibility can be explained by the low OH content of the supports calcined at 720 degrees C

Zeolite-encapsulated copper (II) amino acid complexes: synthesis, spectroscopy, and catalysis

The spectroscopic properties and catalytic behavior of Cu(AA)(n)(m+) complexes (AA = amino acid (glycine, lysine, histidine, alanine, serine, proline, tyrosine, phenylalanine, glutamine, glutamic acid, cysteine, tryptophan, leucine, and arginine)) in faujasite-type zeolites have been investigated. Successful immobilization was achieved by a simple cation exchange procedure with aqueous solutions of preformed Cu(AA)(n)(m+) complexes. The best ion exchange results were obtained with lysine, arginine, proline (at pH = 10), and histidine (at pH =7.3) as ligands and rvirh a AA:Cu2+ ratio of 5. The internal surface and pore volume are drastically reduced by the uptake of the Cu(AA)(n)(m+) complexes, and no precipitation of Cu(AA)(n)(m+) crystals was observed by scanning electron microscopy. Both observations suggest the location of the complexes in the supercages of the faujasite-type zeolites. The composition of the first coordination sphere around Cu2+ can be designed from NNNN to NOOO by varying the type of amino acid. A free coordination site is available for catalysis, and the oxidation of alcohols, alkanes, and alkenes with peroxides was observed at low temperatures

Theoretical study of the structure and spectroscopic properties of cobalt(II) coordinated to six-rings in zeolites

The structure of the local Co(II) six-ring oxygen environment in zeolite A and the corresponding ligand field spectrum have been studied using large cluster models, including all six surrounding Si or Al tetrahedra terminated by either H or OH groups. Structures were optimized by means of density functional theory (DFT), using a nonlocal (BP86) approach and keeping the orientation of all dangling bonds frozen at the X-ray diffraction (XRD) positions. Electronic spectra were calculated using multiconfigurational perturbation theory based on a CASSCF wave function (CASPT2). It is shown that, in all cases, the presence of the Co(II) ion induces a local distortion of the zeolite surface, resulting in an oxygen coordination number of 3, 4, or 5, depending on the Si/Al ratio. This distortion is reflected in the calculated electronic spectra, showing an increased splitting of the Co2+ free-ion, F-4 and P-4 states as compared to the (average) XRD structures. A new I general assignment of the spectrum is proposed, different from earlier assignments based on ligand field theory. The calculated excitation energies of the optimized structures are in excellent agreement with the experimental band positions, thus proving the strength of the present combined DFT-CASPT2 approach. Our results further suggest that the experimentally observed splitting of the main band in the spectrum is due to the presence of asymmetric coordination sites, rather than to Jahn-Teller effects or spin-orbit coupling. The latter may, however, at least partly be responsible for the splitting of the weak feature at 25 000 cm(-1).((