Polychromates structures

C4H4N2H]2Cr30io Dipyrazinium trichromate, structure (20). An excellent review of chromate and polychromate structures. 

The molecular structure of the anchored Cr(VI) has been a strong point of discussion in the literature, and several molecular structures (monochromate, dichromate, polychromates) have been proposed (see Scheme 3). The nature of the silica support, the chromium loading, and the activation method can all influence the chemical state of the supported chromium. 

Chromium zeolites are recognised to possess, at least at the laboratory scale, notable catalytic properties like in ethylene polymerization, oxidation of hydrocarbons, cracking of cumene, disproportionation of n-heptane, and thermolysis of H20 [ 1 ]. Several factors may have an effect on the catalytic activity of the chromium catalysts, such as the oxidation state, the structure (amorphous or crystalline, mono/di-chromate or polychromates, oxides, etc.) and the interaction of the chromium species with the support which depends essentially on the catalysts preparation method. They are ruled principally by several parameters such as the metal loading, the support characteristics, and the nature of the post-treatment (calcination, reduction, etc.). The nature of metal precursor is a parameter which can affect the predominance of chromium species in zeolite. In the case of solid-state exchange, the exchange process initially takes place at the solid- solid interface between the precursor salt and zeolite grains, and the success of the exchange depends on the type of interactions developed [2]. The aim of this work is to study the effect of the chromium precursor on the physicochemical properties of chromium loaded ZSM-5 catalysts and their catalytic performance in ethylene ammoxidation to acetonitrile. 

Figure 141 shows the EL spectra from a microcavity (a) and conventional LED (b) based on the emission from an NSD dye forming a thin emitting layer of a three-organic layer device. It is apparent that the half-width of emission spectra from the diode with microcavity is much narrower than those from the diode without cavity. With 0 = 0°, for example, the half-width of the spectrum of the diode with cavity is 24 nm whereas that of the sample without cavity increases to 65 nm. According to Eq. (275), the resonance wavelength, A, decreases with an increase of 0 in agreement with the experimental data of Fig. 141. We note that no unique resonance condition in the planar microcavity is given due to broad-band emission spectrum of the NSD emission layer. Multiple matching of cavity modes with emission wavelengths occurs. Thus, a band emission is observed instead a sharp emission pattern from the microcavity structure as would appear when observed with a monochromator the total polychromic emission pattern is a superposition of a range of monochromatic emission patterns. The EL spectra...

One old and difficult issue pertaining to Phillips catalysts is the nature of the surface Cr(VI) species. Cr atoms could conceivably be attached in isolated chromate species, or they could bind in pairs as dichromate (or perhaps as higher polychromates). The question is important to our understanding of the catalyst because some researchers have suggested that polymerization requires two Cr centers. Thus, polymerization mechanisms involving paired Cr atoms have sometimes been proposed [76-92]. It was even suggested that unsupported organic Cr(VI) esters exhibit no activity precisely because they are chromate-like structures [76]. 

Szydlowska, H. Orange-staining structures in glial tumors stained with polychromes. Neuropatol. Polska 1966, 4, 187-202 Chem. Abstr. 1968, 68, 67202. 

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