Crystalline zeolite, study methods

In the following section, we will critically review representative methods for measuring surface acidity of solid catalysts. Recommendations will then be made of the most appropriate methods from the standpoint of the needs of the investigator. The final section is devoted to updating research activities dealing with individual solid catalysts. Particular attention will be devoted to studies of acidities of unusually active catalysts such as crystalline zeolites, synthetic clays, and chlorinated aluminas. 

A crystalline zeolite can also be studied by a variety of methods including crystallography and NMR but the intermediate phase, the gel, has proved very resistant to any type of study. The aim of this work, therefore, was to delay the formation of the gel long enough to investigate the precursor solution because solutions are generally easier to study than gels. 

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. 

Earlier studies of intracrystalline diffusion in zeolites were carried out almost exclusively by direct measurement of sorption rates but the limitations imposed by the intrusion of heat transfer and extra-crystalline mass transfer resistances were not always fully recognized. As a result the reported diffu-sivities showed many obvious inconsistencies such as differences in diffusivity between adsorption and desorption measurements(l-3), diffusivities which vary with fractional uptake (4) and large discrepancies between the values measured in different laboratories for apparently similar systems. More recently other experimental techniques have been applied, including chromatography and NMR methods. The latter have proved especially useful and have allowed the microdynamic behaviour of a number of important systems to be elucidated in considerable detail. In this paper the advantages and limitations of some of the common experimental techniques are considered and the results of studies of diffusion in A, X and Y zeolites, which have been the subject of several detailed investigations, are briefly reviewed. 

It is also possible to prepare crystalline electrides in which a trapped electron acts in effect as the anion. The bnUc of the excess electron density in electrides resides in the X-ray empty cavities and in the intercoimecting chaimels. Stmctures of electri-dides [Li(2,l,l-crypt)]+ e [K(2,2,2-crypt)]+ e , [Rb(2,2,2-crypt)]+ e, [Cs(18-crown-6)2]+ e, [Cs(15-crown-5)2]" e and mixed-sandwich electride [Cs(18-crown-6)(15-crown-5)+e ]6 18-crown-6 are known. Silica-zeolites with pore diameters of vA have been used to prepare silica-based electrides. The potassium species contains weakly bound electron pairs which appear to be delocalized, whereas the cesium species have optical and magnetic properties indicative of electron locahzation in cavities with little interaction between the electrons or between them and the cation. The structural model of the stable cesium electride synthesized by intercalating cesium in zeohte ITQ-4 has been coirfirmed by the atomic pair distribution function (PDF) analysis. The synthetic methods, structures, spectroscopic properties, and magnetic behavior of some electrides have been reviewed. Theoretical study on structural and electronic properties of inorganic electrides has also been addressed recently. ... 

The method proposed by Parrinello and Rahman is of special interest in studies of crystalline solids and zeolites in particular. The method allows a variation of both volume and shape of simulation box and therefore is suitable for studying phase transitions induced by sorption of molecules, isomorphic substitution, or change of temperature. In this method, the Lagrangian of a system with kinetic energy T and potential energy U... 

A series of crystalline gallosilicate molecular sieves with the zeolite P structure synthesised by a rapid method from alkali-free hydrogels have been studied by NMR methods, including Ga MAS NMR (Occelli et al. 1999). The Ga spectra (Figure 10.12B) show that most samples contain only tetrahedral Ga in framework... 

In this paper, oi and Al MASNMP. spectroscopy is used in conjunction with crystallinity, surface area and unit cell size measurements to study individual rare earth exchanged Y zeolites in order to determine the effect of individual rare earths cations on their structure and stability. These methods are used to further probe rare earth induced structural changes that occur during hydrothermal treatment of the zeolites. The studies were extended to also establish the effect of different lanthanum-cerium mixtures on zeolite stability. The data presented and discussed are for lanthanum, cerium, praseodymium and neodymium exchanged Y zeolites, as well as for zeolites exchanged with different lanthanum-cerium mixtures. 

Systematic studies carried out at C.F.R.I. pertainining to catalytic vapour phase synthesis of pyridine bases, namely 2 4-picolines through cyclodehydrogenation reaction of acetaldehyde and anunonia have been described. Metal oxide modified amorphous silica-aliunina and crystalline alumino-silicate namely ZSM-5 zeolites were found to be active and selective catalysts towards the formation of the lower pyridines bases. The method of preparation, pretreatment vis-a-vis the acidity have been found to affect the catalytic activity and selectivity. It is interesting to observe that metal modified crystalline alumino-silicate ZSM-5 is more selective than amorphous silica-alumina for the formation of 2 4-picoline. A plausible reaction mechanism based on the findings of this study has been proposed. 

The protonic sites at the surface of amorphous silica are weakly acidic compared to the acid centers in crystalline aluminosilicates (e.g., zeolites). On ZSM-5, Bronsted sites are formed by protons adjacent to aluminum atoms in the tetrahedral framework. The concentration of acid sites increases with the aluminum content. The total acidity as well as the acid strength distribution can be determined by using n-butylamine and Hammet or arylmethanol indicators (25). Depending on the pKa of the indicator, a relative scale of the strength distribution is obtained (54). Results for a series of amorphous porous silicas of graduated pore size are shown in Figure 10. The acidity varies between pKa -1 and +9 and is nearly the same for all silicas studied. The results are specifically valid for this method only and cannot be compared with those derived from other methods. 

One of the methods of synthesis of clusters of uniform size consisting of just several atoms is the intrusion of liquid phase (e.g., mercury) under high pressure into zeolites with voids of different volume. High pressure is necessary for overcoming the capillary pressure in order to achieve filling of small voids with a liquid. When the pressure drops, the column of liquid in the thin capillary ruptures, similarly to the column of mercury in the thermometer upon cooling, and monodispersed clusters become trapped in the zeolite voids. Computer modeling and experimental studies of such small clusters both indicated that they form unique crystalline structures, impossible in the case of macroscopic crystals. For example, such structures may contain the axes of symmetry of fifth order. 

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