Zeolites properties

With regard to the structure, zeolites may have pores too small to allow the passage of even the smallest molecule, or large enough to adsorb relatively large molecules. They may also contain cavities and 

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Various analytical tests determine zeolite properties. These tests supply information about the strength, type, number, and distribution of acid sites. Additional tests can also provide information about surface area and pore size distribution. The three most common parameters governing zeolite behavior are as follows ... 

UCS, rare earth, and sodium are just three of the parameters that are readily available to characterize the zeolite properties. They provide valuable information about catalyst behavior in the cat cracker. If required, additional tests can be conducted to examine other zeolite properties. 

While Cordfuncke [997] believes that there are only four stable compounds in the U03—NH3—H20 system, the results of Stuart et al. [998, 999] indicate the existence of a continuous non-stoichiometric phase containing the NH4 ion and possessing zeolitic properties U02(0H)2 x (ONH4 )x yH20. 

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. 

The objective of this contribution is to investigate catalytic properties of zeolites differing in their channel systems in transformation of aromatics, i.e. toluene alkylation with isopropyl alcohol and toluene disproportionation. In the former case zeolite structure and acidity is related to the toluene conversion, selectivity to p-cymene, sum of cymenes, and isopropyl/n-propyl toluene ratio. In the latter one zeolite properties are... 

Two important zeolite properties are (1) the intra-pore electrostatic field, and (2) its acid-base character. As discussed below post-synthetic modifications of many zeolites to fine-tune these properties are possible and provide a unique opportunity to influence reaction outcome. 

The currently available quantum chemical computational methods and computer programs have not been utilized to their potential in elucidating the electronic origin of zeolite properties. As more and more physico-chemical methods are used successfully for the description and characterization of zeolites, (e.g. (42-45)), more questions will also arise where computational quantum chemistry may have a useful contribution towards the answer, e.g. in connection with combined approaches where zeolites and metal-metal bonded systems (e.g. (46,47)) are used in combination. The spectacular recent and projected future improvements in computer technology are bound to enlarge the scope of quantum chemical studies on zeolites. Detailed studies on optimum intercavity locations for a variety of molecules, and calculations on conformation analysis and reaction mechanism in zeolite cavities are among the promises what an extrapolation of current developments in computational quantum chemistry and computer technology holds out for zeolite chemistry. 

Another way to modify zeolite properties is to change the nature of the nonframework cations. Some examples of how cation exchange can affect properties are noted in Section 2.6. For catalytic applications it is often desirable to create active acidic sites. This can be done by exchanging the cations with NHJ and then calcining the zeolite, removing NH3 and leaving behind protons attached to framework oxygen atoms. 

An approximation of the extent of hydrogen transfer reactions occurring compared to cracking reactions and the net effect on product distribution can be initially seen by a consideration of the zeolite properties of the catalysts tested in the present study ... 

The NMR chemical shift of I29xe adsorbed on molecular sieves reflects all the interactions between the electron cloud of the xenon atoms and their environment in the intracrystalline void volume [1]. This nucleus therefore proved to be an ideal probe for investigating various zeolitic properties such as pore dimensions [2, 3], location of the countercations [4, 5], distribution of adsorbed or occluded phases [6-8] and framework polarisability [8, 9]. 

Xylenes. Because of the practical significance of xylenes, isomerization of xylenes over zeolites is frequently studied.348 The aim is to modify zeolite properties to enhance shape selectivity, that is, to increase the selectivity of the formation of the para isomer, which is the starting material to produce terephthalic acid. In addition, m-xylene isomerization is used as a probe reaction to characterize acidic zeolites.349,350... 

The influence of both heat treatment of decationized zeolites and the nature of cations on the proton-donating and electron-deficient zeolite properties has been studied (13-16). However, these works do not allow one to follow clearly the mutually dependent changes in proton-donating and... 

Investigation of methods to control zeolite properties and composition, to modify these properties to obtain the highest specificity in their application as catalysts for various processes. Development of new multi-component polyfunctional catalytic systems. 

Moreover, contrary to alkyne hydration where no adsorption of the carbonyl compound was detected, the problem is complicated here by the saturation of the strong acidic sites by the formed amide, the concentration of which shows a rapid stabilization against time (Fig.3). Consequently the reaction selectivity greatly depends on the ester percentage. The behaviour of the amide itself over the studied zeolites confirms this observation the conversion of the amide into ester goes faster on the HY2 g zeolite than on the Hg and on the HMg zeolites. This later point, together with the comprehension of the different mechanisms in relation with the zeolite properties, will be discussed in a further paper. 

The importance of recognizing and dealing with zeolite synthesis as a kinetic process that involves the isolation of metastable phases is pointed out in this book in a variety of ways. An examination of the extensive scientific and patent literature on zeolite synthesis rapidly convinces one that a lack of understanding of this point has been a major bottleneck in the characterization of zeolite chemical and physical properties. The zeolite properties are defined not only by synthesis parameters, but also by treatment following synthesis for example, most synthesis treatment of zeolites with fluorine can be used to modify hydrophobicity drastically and increase catalytic activity for n-butane cracking. 

The high degree of flexibility in the tailoring of zeolites properties partly explains why only a relatively small number of zeolites are used commercially today only a dozen of zeolites (and their modifications) are presently in use while more than 130 structures are known today (61). 

The rearrangement of cyclohexanone oxime to caprolactam is still an important step in nylon production, and the heterogeneously catalyzed Beckmann rearrangement has been extremely well investigated (4, 16-19). In order to obtain catalysts that couple a high lactam selectivity to long lifespan, careful tuning of the zeolite properties is required. Some important factors are ... 

The concept of porous solids originates from the discovery by Cronstedt [1], during the XVIIIth century, of the zeolitic properties of the mineral stilbite. Many natural zeolites were discovered later, but the importance of this family increased when chemists were able to synthesize them. The first success was due to Sainte Claire Deville as soon as 1862 [2] and most of the syntheses required organic molecules as templates. 

We mentioned that bonding in zeolites is dominated by covalent bonding. Therefore, zeolite properties can be described as being mainly locally dependent. For instance, it is known that Br0nsted acidic sites induce important distortions in the zeolite framework (see Figure 3). However, these distortions remain local. A direct application of this property is the validity of the use of small fragments to describe catalytic active sites (see Figure 5). 

Earlier discussion introduced the concept of using the characteristic truncated octahedral elements of the sodalite framework to explain the molecular architecture of the synthetic zeolites X and Y (see Section. 2.4.3). There are other structural correlations that can be drawn between felspathoids and zeolites, for example, that the cancrinite cage (11-hedron) is a face-sharing element seen in the LTL, ERI, OFF, and EOS frameworks. Furthermore, in nature, salt ion pairs are contained in felspathoid minerals and when these are removed the residual framework exhibits the zeolitic properties of ion exchange and reversible water loss. Other similarities arise in that zeolites can imbibe salt ion pairs, and isotypic structures... 

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