Zeolite catalytic transformation

The first mode of the high resolution C-NMR of adsorbed molecules was recently reviewed Q-3) and the NMR parameters were thoroughly discussed. In this work we emphasize the study of the state of adsorbed molecules, their mobility on the surface, the identification of the surface active sites in presence of adsorbed molecules and finally the study of catalytic transformations. As an illustration we report the study of 1- and 2-butene molecules adsorbed on zeolites and on mixed tin-antimony oxides (4>3). Another application of this technique consists in the in-situ identification of products when a complex reaction such as the conversion of methanol, of ethanol (6 7) or of ethylene (8) is run on a highly acidic and shape-selective zeolite. When the conversion of methanol-ethylene mixtures (9) is considered, isotopic labeling proves to be a powerful technique to discriminate between the possible reaction pathways of ethylene. 

To develope the scientific bases of zeolite catalysts preparation for selective organic and petrochemical synthesis, the elaboration of methods of directed modification of acid properties of these systems seems to be very important, namely, the creation in them of acid sites of different nature and the determination of their irole in multi-route catalytic transformations. 

Whereas solid acid catalysts (zeolites) have been intensively studied, only recently has the use of basic solid catalysts received substantial interest for production of intermediate and fine chemicals [1], Heterogeneous catalytic transformations are environmentally friendly compared with processes requiring neutralization and... 

In addition, significant advances have been made in both basic and applied research which allow a smart and efficient solution to most of these problems. As an example, let us quote the development of the synthesis of novel catalytic materials with tailor-made and more suitable characteristics (stable nanocrystals, controlled hydrophobicity, better thermal and/or mechanical stability, etc.), the understanding of the complex phenomena involved in the catalytic transformation of polar molecules within zeolite micropores or the demonstration that fixed bed reactors, which have many advantages over conventional batch reactors, can be easily used, even for liquid-phase reactions and even for laboratory scale experiments. 

Synthetic zeolites are the most important materials used currently in industry for catalyst preparation. However, natural zeolites are not contemplated in catalyst manufacturing because of the impurities present in the natural raw materials nevertheless, in some reactions, such as the isomerization of hydrocarbons, this contamination does not affect the catalytic transformation therefore, acid natural zeolites can be used for this purpose [19]. Furthermore, acid clinoptilolites were tested for catalytic cracking with success [19,21,137-143], We have shown [19,21,138-143] that the acid clinoptilolite, used as catalyst in the reaction of ethanol dehydration, exhibits high selectivity for ethylene production due to steric restrictions imposed on the formation of diethyl ether. The scheme of the ethanol dehydration reaction is shown in Figure 9.18 [145],... 

During the last 20 years, great progress was made in the synthesis of molecular sieves (Table 1.1) and in the understanding of the catalytic transformation of organic molecules on zeolites. This fundamental knowledge was successfully... 

Zeolite Effects on Catalytic Transformations of Fine Chemicals... 

Catalytic transformation of dichloromethane over Y and X zeolites L. Pinard, J. Mijoin, R. Lapeyrolerie, P. Magnoux and M. Guisnet... 

The catalytic transformations of nitrogen oxides are consido ably affected by the oxidation state of the metal ions occupying exchange position in the zeolite. The ESR technique is a usefiil method to follow the oxidation states and changes of catalysts in a diffo ent way, howevo-, the real reactions could be characterized by measurements in actual conditions. 

Zeolites are microporous crystalline solids which find a wide variety of industrial applications in the fields of ion exchange and separation, purification and catalytic transformation of organic compounds. As heterogeneous catalysts, most of their uses have been as acid catalysts where the combination of high acidity, high specific surface area and the shape selectivity derived from the size and shape of their... 

Supported metal clusters play an important role in nanoscience and nanotechnology for a variety of reasons [1-6]. Yet, the most immediate applications are related to catalysis. The heterogeneous catalyst, installed in automobiles to reduce the amount of harmful car exhaust, is quite typical it consists of a monolithic backbone covered internally with a porous ceramic material like alumina. Small particles of noble metals such as palladium, platinum, and rhodium are deposited on the surface of the ceramic. Other pertinent examples are transition metal clusters and atomic species in zeolites which may react even with such inert compounds as saturated hydrocarbons activating their catalytic transformations [7-9]. Dehydrogenation of alkanes to the alkenes is an important initial step in the transformation of ethane or propane to aromatics [8-11]. This conversion via nonoxidative routes augments the type of feedstocks available for the synthesis of these valuable products. 

Complexes of guest species with active sites in zeolite pores are formed during various processes such as characterization of the material, catalytic transformation, selective sorption, and separation [124,125]. A guest molecule adsorbed in a zeolite can in general be involved in several types of interactions ... 

The effect of NH3 adsorption on catalytic activity and selectivity of cation-exchanged Y zeolite and natural clinoptilolite (deposit of Georgia) in ethanol transformation was investigated. The ODH of alcohol is accompanied by dehydration to ethylene and diethylether and deep oxidation to CO2 NH3 blocks the acidic active centers of dehydration of Y zeolites and promotes the oxidative activity of samples with transition metal cations. The influence of the ion nature for the catalytic transformation of ethanol was shown by a considerable increase of carbon dioxide formation after NH3 adsorption. The cation associated has been destroyed under NH3 influence, and NH3 formed with cations coordinated-unsaturated complexes. 

Catalytic transformation of 1-butene over acidic (low polarity) zeolites resulted in somewhat higher yield and selectivity toward isobutene. A single mode continuous-flow reactor was used and the results were explained by formation of hot spots [119]. 

There is a great similarity in the catalytic transformation of ethanol and of methanol on a HZSM-5 zeolite, either concerning the reaction mechanism [4], or in the spectra of products [5,6], Nevertheless, the high content of water in the feed in the BTG process markedly influences the distribution of products and catalyst deactivation [5-7], It has been proven that water attenuates deactivation by coke at moderate temperatures [6], but causes irreversible deactivation by dealumination of the zeolite at high temperatures. 

Spectroscopic methods in general play an expanding role in characterization of host/guest systems like, e.g., zeolite systems. They provide valuable microscopic information on symmetry, structure and bonding in such associations, on the location and motion of the guest, on the nature of active sites for catalytic transformations, on the thus-formed intermediates and products, as well as on the detection of poisons. 

The ability of microporous solids to act as high-capacity molecular sieves has long been exploited in a wide range of applications in adsorption and separation. The electrostatic interactions of the traditional cationic forms of aluminosilicates are well suited for the uptake of polar molecules (such as H2O) and are also able to separate oxygen from air. The development of microporous solids with varied chemistry has enabled adsorption and diffusion properties to be finely tuned for particular technologies. Pure silica zeolite polymorphs such as silicalite have particular importance, because they enable separation on the basis of a different range of polarity and on molecular size the absence of aluminium in the framework also prevents the presence of unwanted acidity, so adsorbed hydrocarbons do not undergo any catalytic transformation. 

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