Zeolite Types and Structures

In the isomerization of styrene oxides in a fixed bed reactor under gas phase conditions, the catalytic performance of various catalysts on the activity, selectivity and service time was screened at 300°C and WHSV = 2-3h" . As shown in Fig. 15.1, zeolites with MFI-structure are superior to other zeolite types and non zeolitic molecular sieves, as well as greatly superior to amorphous metal oxides. 

One of the main advantages of application of zeolitic or other porous materials is the shape-selectivity of this type of material, which arises due to differential diffusion of molecules with different sizes and shapes in the zeolitic or other porous materials. Iherefore, it is very instructive to monitor the pore architecture directly, with a molecule that "observes" the zeolitic type of structure. 

The addition of K+ cation to the synthetic system Na20-Al203-Si02-H20 significantly changed the type and structure of the zeolites crystallized from the parent mixture, which might be caused by the different templating ability of the cations on the SBUs of the zeolites. 

In the interior of these channels, which are characteristic of zeolites, are water molecules and mobile alkali metal ions, which can be exchanged with other cations. These compensate for the excess negative charge in the anionic framework resulting from the aluminum content. The interior of the pore system, with its atomic-scale dimensions, is the catalytically active surface of the zeolites. The inner pore structure depends on the composition, the zeolite type, and the cations. The general formula of zeolites is... 

Adsorbents Table 16-3 classifies common adsorbents by structure type and water adsorption characteristics. Structured adsorbents take advantage of their crystalline structure (zeolites and sllicalite) and/or their molecular sieving properties. The hydrophobic (nonpolar surface) or hydrophihc (polar surface) character may vary depending on the competing adsorbate. A large number of zeolites have been identified, and these include both synthetic and naturally occurring (e.g., mordenite and chabazite) varieties. 

Mono-dispersed silicalite and ZSM-5 type zeolite nanocrystals with a diameter of 80-120 nm were successfully prepared in a surfactant-oil-water solution. The ionicity of the surfactants used in the preparation affected the crystallinity and structure of the silicalite crystals, and silicalite nanocrystals could he obtained when using a nonionic sur ctant. By adding an A1 source into the synthetic solution, ZSM-5 type zeolite nanocrystals with strong acid sites could be obtained. 

Without any doubt, the zeolite framework porous characteristics (micropores sizes and topology) largely govern the zeolite properties and their industrial applications. Nevertheless for some zeolite uses, as for instance, host materials for confined phases, the zeolite inner surface characteristics should be precised to understand their influence on such low dimensionality sorbed systems. In that paper, we present illustrative examples of zeolite inner surface influence on confined methane phases. Our investigation extends from relatively complex zeolite inner surface types (as for MOR structural types) to the model inner surface ones (well illustrated by the AFI zeolite type). Sorption isotherm measurements associated with neutron diffraction experiments are used in the present study. 

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. 

We are studying the glycerol transformation in the presence of different catalysts of porous structure such as layered double hydroxides (hydrotalcites, HT and mixed oxides, MO), modified zeolites (ZSM5) and new type of Raney-Ni (RNi) to find optimal conditions for producing valuable components. 

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