Loadings in zeolites

To prepare alkali- or alkaline earth-modified zeolites or mesoporous moleeular sieves, identieal general methodologies are used. Thus, alkaline earth eation-exehanged zeolites are prepared by exehange of the zeolite in the sodium form in aqueous solution of alkaline earth metal salts, followed by washing and ealeination. Alkaline earth metal oxides loaded in zeolites are also prepared by impregnation of alkaline earth metal salts sueh as nitrates, aeetates, or ethoxides followed by ealeination (70,215,216). 

Figure 9.23 Variation of the luminescence property for silicon nanoclusters loaded in zeolite Y. Reproduced with permission from [107], Copyright (1996) Wiley-VCH...

Besides cadmium sulfide, other semiconductor particles can also be loaded in zeolite microporous crystals. Moller et al. prepared cadmium sulfide nanoparticles in zeolite Y through a similar approach.[117] Nevertheless, structural analysis indicates that the formed cluster particles are actually rather complex, and apart from cadmium sulfide clusters, there exist other nanoclusters such as Cd404 or Cd202Se in the channels of zeolite Y. These nanoclusters are not isolated, and they strongly interact with the framework oxygen of the zeolite. 

A dramatic increase of the reaction rates with lowering the apparent activation energies evidently demonstrates a promoting effect of Zn on the reaction of H/D hydrogen exchange for C-n-C alkanes. Also, the regioselectivity of H/D exchange for propane and n-butane is provided evidently by an assistance of the Zn species loaded in zeolite. 

Deuterium NMR is used to study the molecular mobility of benzene-de in Na and Cs forms of zeolite X. The systems studied were prepared with loadings in the range 0.7 molecules/supercage to 5.6 and 5.0 mole-cules/supercage for (Na)X and (Cs,Na)X, respectively. 

Lowering the temperature has a similar effect on the deuterium spectra as does increased loadings. In Figure 3, spectra for benzene-d6/(Na)X at 0.7 molecules/supercage over the temperature range 298 to 133 K are shown. It is observed that both benzene species are detected simultaneously between 228 and 188 K. Below this temperature the oriented benzene species becomes the predominant form. A similar situation occurs for polycrystalline benzene-dg in which two quadrupole patterns, one static and the other motionally narrowed due to C rotation, are observed to coexist at temperatures between 110 and 130 K (7). This behavior has been attributed to sample imperfections (8) which give rise to a narrow distribution in correlation times for reorientation about the hexad axis. For benzene in (Na)X and (Cs,Na)X such imperfections may result from the ion/benzene interaction, and a nonuniform distribution of benzene molecules and ions within the zeolite. These factors may also be responsible for producing the individual species. However, from the NMR spectra it is not possible to... 

Capek, L., Kreibich, V., Dedecek, J. et al. (2005) Analysis of Fe species in zeolites by UV-VIS-NIR, IR spectra and voltammetry. Effect of preparation, Fe loading and zeolite type, Microporous Mesoporous Mater., 80, 279. 

It is well known that Rh(I) complexes can catalyze the carbonylation of methanol. A heterogenized catalyst was prepared by ion exchange of zeolite X or Y with Rh cations.126 The same catalytic cycle takes place in zeolites and in solution because the activation energy is nearly the same. The specific activity in zeolites, however, is less by an order of magnitude, suggesting that the Rh sites in the zeolite are not uniformly accessible. The oxidation of camphene was performed over zeolites exchanged with different metals (Mn, Co, Cu, Ni, and Zn).127 Cu-loaded zeolites have attracted considerable attention because of their unique properties applied in catalytic redox reactions.128-130 Four different Cu sites with defined coordinations have been found.131 It was found that the zeolitic media affects strongly the catalytic activity of the Cd2+ ion sites in Cd zeolites used to catalyze the hydration of acetylene.132... 

Figure 4 Methane (CD4) neutron diffractograms measured at T = 82 K for different loadings in AlP04-5 zeolite.

Figure 5. Evolution of the main neutron diffraction peak intensity versus methane confined phase loading in AlP04-5 zeolite.

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. 

Selectoforming A process for increasing the octane rating of a petroleum fraction by selectively cracking the n-pcntanc and n-hcxanc in it. The catalyst is a metal-loaded synthetic zeolite. Developed by Mobil Corporation and first commercialized in the mid-1960s. 

Na - K - Co exchange in Y zeolite. Heterovalent exchange reactions in zeolites generally show an even more pronounced dependency on loading (116-118). Rees (116) observed variations of the selectivity coefficient by a factor 1000 for the Na-Ca and Na-llg exchange in zeolite A at 25 °C. An+exajnple of e treipe variations is shown in fig. 9 for the K -Co and Na -Co selectivities in zeolite Y at 45 °C (117). The exchange... 

The conformation of ethylene glycol occluded in high silica sodalite has been determined [25]. In the category of zeolite-hydrocarbon complexes, we have seen completion of studies at relatively low loading levels of benzene in zeolites L [26,27] and Na-Y [28]. Benzene has also been studied in... 

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