Metal substitution and spectroscopy

In the development of zeolite science, infrared spectroscopy has been one of the major tools for structure and reactivity characterization. However, the field of zeolite Raman spectroscopy is gaining importance. The Raman effect is an intrinsically weak phenomenon, and Raman spectra of zeolites are often obscured by a broad fluorescence. Just like IR spectroscopy, Raman can detect small. X-ray amorphous zeolite particles. Therefore, Raman spectroscopy has been used to examine zeolite synthesis mixtures with ex-situ methods (with separation of solid and liquid) and in-situ methods. In this work we give an overview of the zeolite framework vibrations, zeolite synthesis, adsorption on zeolites and metal substitution and ion exchange in zeolites. 

The study of rapid, intermolecular ligand exchange between square-planar complexes trans-Ir(CO)L2X (X = C1 or Me, L - PPh3, P(p-tolyl)3, or PMePh2) by variable-temperature 31P NMR spectroscopy indicates that the reaction proceeds through dissociation of phosphine from the metal center and a subsequent associative substitution with other complexes 559,560 Ligand exchange between square-planar Ir and Pt complexes is slow. 

I propose to develop and apply such methods, based on ultrafast X-ray absorption spectroscopy, to study the ultrafast molecular motions of organometallics in solutions. In particular, initial studies will focus on photo-induced ligand dissociation and substitution reactions of transition metal carbonyls and related compounds in various solvent systems. 

Whatever the precursor, the formation of an intermediate solid phase was always observed. It can be inferred from X-ray diffraction (Fig. 9.2.7) and infrared spectroscopy that this intermediate phase shows a lamellar, incompletely ordered structure (turbostratic structure) built up with parallel and equidistant sheets like those involved in the lamellar structure of the well-crystallized hydroxides Ni(OH)2 or Co(OH)2, these sheets are disoriented with intercalation of polyol molecules and partial substitution of hydroxide ions by alkoxy ions (29). The dissolution of this solid phase, which acts as a reservoir for the M(I1) solvated species, controls the concentration of these species and then plays a significant role in the control of the nucleation of the metal particles and therefore of their final morphological characteristics. For instance, starting from cobalt or nickel hydroxide as precursor in ethylene glycol, the reaction proceeds according to the following scheme (8) ... 

The compound FeCo2(CO)9S thus prepared forms large black crystals. In this form it is virtually air stable (not so in solution). Its purity can be checked by IR spectroscopy [vco bands in cyclohexane solution at 2105 (w), 2069 (vs), 2055(vs), 2044(s), 2030(m), 1983(m), 1951 (vw)cm-1]. It is moderately soluble in all organic solvents to give reddish-brown solutions. It has been used for various ligand substitution and metal exchange reactions whereby the first optically active clusters were obtained.14,15... 

An obvious approach to the problem of conformational analysis in these complexes is the use of NMR spectroscopy. However, several general problems immediately arise, namely, that most metal ions have nuclear spin, many are labile to substitution of the ligands, and they are frequently paramagnetic. In addition the ligands often contain N14. All these properties potentially hinder the observation of fine structure and thus the analysis of the NMR spectra. However, in some complexes the problems are either eliminated or reduced. For example Co(III) compounds are usually diamagnetic and inert to substitution, and in some complexes the ligand-proton fine structure is observed to be relatively uncomplicated by Co or N quad-rupole relaxation effects, for example, (CoEDTA)", Fig. 18. 

Synthesis of transition metal containing molecular sieves (microporous as well as mesoporous) is one of the fastest developing areas in molecular sieve science, as evidenced by recent published reviews [1,2] Several transition metals have been substituted into crystalline silica or aluminophosphate frameworks to yield the corresponding metallosilicate or metalloaluminophosphate molecular sieves, However, the location of the metal species and their state always remain uncertain, despite the employment of numerous different characterization methods comprising IR, NMR and ESR spectroscopy. 

The photochemistry of benzene and of substituted benzenes has until recently received relatively little attention. Irradiation of benzene at wavelengths below 2000 A was said to produce small yields of hydrogen19,20 but other products were not positively identified although a polymer with the metallic luster and general insolubility of cuprene was reported.20 At wavelengths longer than 2000 A absorption spectroscopy indicated the formation of a product21 and a polymer, neither of which was satisfactorily identified. 

Transition metal dopants and impurities are probably incorporated substitutionally for Ti in BaTi03. Emission spectrographic analyses indicate that the distribution coefficients for Mn and Fe dopants are on the order of 1 to 2, i.e., the crystals are slightly enriched relative to the melt. Cr and Ni may have distribution coefficients slightly less than 1. For Co, the measured concentrations in the crystals display considerable scatter we estimate that the distribution coefficient is on the order of 4. Fe is the most prevalent transition metal impurity and is typically present at a concentration of 10-15 ppm by weight. Si, Al, Mg, and Cu are also typically present at 5-50 ppmw. Fe and Cr impurities have also been observed by EPR spectroscopy, although Cr could not be detected by emission spectroscopy, with a detection limit of 10 ppmw. 

In the dismutation of 02, two protons must be incorporated to form H2O2, but the reaction rate is independent of pH in a range between 5 and 9.5, indicating that the protons do not derive directly from H30+. The Cu+ in the reduced SOD is triligated, which is revealed by ESR and absorption spectrum of the metal substituted CuCo-SOD, H NMR, EXAFS and pulse radiolysis and rapid spectroscopy. These results indicate that the proton is donated from the protonated imidazole group of the bridging His-63 between the Cu and the Zn. His-63 shuttles a proton from medium to peroxide at a fast rate, probably not only from H30+ but also from the protonated buffers. 

The dimer [CpCr(CO)3]2 has a weak metal-metal bond and dissociates in solution at room temperature to a measurable extent to generate the 17-electron [CpCr(CO)3]. This permitted a detailed kinetic study of CO substitution in the radical, which established a rapid associative mechanism. 15,16 A comparison of these results with reactivity data17-47 for [Cp W(CO)3] suggests that CO substitution by PPh3 in the 17-electron radicals is faster for tungsten than chromium by a factor of ca. 106. Photolysis of [CpFe(CO)2]2 in conjunction with time-resolved infrared spectroscopy was... 

Even though the metal-substituted, mesoporous solids allow the oxidation of molecules that is not possible with zeolites, there are several issues that still need to be addressed. First, the activity of the metal-loaded catalysts decreases with increased metal loading, e.g. for Ti-MCM-41, the peak activity for alkene epoxidation is attained at 2 wt. % [44aj. Second, metal leaching can occur and care needs to be exercised in concluding that oxidation is taking place at the framework site rather than via metal ions leached into solution [184, 185]. Leaching has been shown to occur for V-substituted mesoporous materials in the oxidation of alkanes [184], X-ray absorption spectroscopy indicates that the inclination of the heteroatoms to remain in the MCM-41 framework after calcination follow the order Ti > Fe > V > Cr [56],... 

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