Zeolites pore distribution

While our discussion will mainly focus on sifica, other oxide materials can also be used, and they need to be characterized with the same rigorous approach. For example, in the case of meso- and microporous materials such as zeolites, SBA-15, or MCM materials, the pore size, pore distribution, surface composition, and the inner and outer surface areas need to be measured since they can affect the grafting step (and the chemistry thereafter) [5-7]. Some oxides such as alumina or silica-alumina contain Lewis acid centres/sites, which can also participate in the reactivity of the support and the grafted species. These sites need to be characterized and quantified this is typically carried out by using molecular probes (Lewis bases) such as pyridine [8,9],... 

However, for the heavier resides, zeolite pore structure may preclude their use in HCK. We have introduce the effect of the pore size and distribution on the conversion and coke formation of asphaltene containing feeds (Section 5.2.1), but we should also point out that they also affect the dispersion of the hydrogenation metals on the catalyst surface. A poor dispersion will also lead to poor hydrogenation and indirectly favor coke formation. 

In aqueous solutions, the high negative surface charge of a zeolite must be neutralized by binding counterions, such as Na+, K+, and Ca +. The distribution of zeolite pore size can be modified, such that small molecules are included in the pores those too large to diffuse into the pores are excluded. Structure types are named by a three-letter lUPAC code, based in part on the name of the zeolite first used to identify the specific type. They are also classified by pore size, framework density, and/or symmetry. 

The product distribution in the zeolite-catalysed alkylation of polynuclear aromatics depends on the structure of zeolite pores. High regioselectivities were observed in the HM catalysed isopropylation of polynuclear aromatics, such as biphenyl, naphthalene, p-terphenyl, and dibenzofuran, to yield predominantly the least bulky products e.g., 4,4 -DIPB for biphenyl, and 2,6-DIPN for naphthalene. These reactions are controlled by steric restriction at the transition state inside the pores and by the entrance of intermediate products molecules into the pores. On the other hand, the catalysts with large-pore HY and HL zeolite are controlled at low temperature by the electron density of the reactant molecule and at higher temperature by the stability of the product molecules because their pores have enough space for a transition state, which allow the formation of all corresponding isomers. 

Therefore, it can be concluded that 1- and 2-AMN can be formed within the zeolite micropores of large and even medium pore zeolites, their distribution... 

The calculation methods for pore distribution in the microporous domain are still the subject of numerous disputes with various opposing schools of thought , particularly with regard to the nature of the adsorbed phase in micropores. In fact, the adsorbate-adsorbent interactions in these types of solids are such that the adsorbate no longer has the properties of the liquid phase, particularly in terms of density, rendering the capillary condensation theory and Kelvin s equation inadequate. The micropore domain (0.1 to several nm) corresponds to molecular sizes and is thus especially important for current preoccupations (zeolites, new specialised aluminas). Unfortunately, current routine techniques are insufficient to cover this domain both in terms of the accuracy of measurement (very low pressure and temperature gas-solid isotherms) and their geometrical interpretation (insufficiency of semi-empirical models such as BET, BJH, Horvath-Kawazoe, Dubinin Radushkevich. etc.). 

J. Turkevich We have been presented a series of correlations on variation with temperature of adsorption isotherms for single adsorbate on a given adsorbant. Until the discovery and development of crystalline zeolite adsorbants, charcoal, silica gel, and alumina were not well characterized as to pore distribution and homogeneity of surfaces. Now we have well defined adsorbants. Are they so well defined that we can characterize their adsorbant power by an isotherm, for any adsorbate, by one or two numbers ... 

Zeolites, crystalline alumina silicates with open regular structure, oiFer unusual opportunities for carrying out catal3rtic studies. Their well-defined crystalline structure and their regular pore distribution permit a better description of the surface than that offered by alumina-silica gel catalysts. Consequently, in recent years they have been the object of many scientific investigations. In addition, zeolites of a variety of types have shown highly desirable properties in industrial catalysis (1.2). This chapter is a review of the work on zeolites carried out at Princeton University and is not intended to be an exhaustive review of the topic. 

The narrowness of the zeolite pores also influences their catalytic nature. A faujasite in the K" form, even though it is not acidic, catalyzes the cracking of alkanes. The product distribution in cracking is consistent with free-radical intermediates and the induced homolytic rupture of C—H or C—C bonds. The activity arises from the strong electric flelds of the ions in the confining pores. 

Zeolites, as previously described, are crystalline microporous solids with well-defined structures made up of interlocking microporous Si04 and A104. Microporous means that the pores have dimensions of less than 20 A, on the order of the size of many petroleum-related molecules, and their crystalline nature means that they have a narrow pore distribution (mesoporous materials have pore sizes between 20 A and 500 A, macroporous materials have pores larger than 500 A). This combination of features not only restricts the size of molecules that can enter the pores, but also the dimensions of the transition state and of the molecules that can successfully leave. For these reasons, zeolites have been termed shape selective. ... 

The weak difference of coke composition observed from propene and isobutene (Table 1) confirms that at low temperature (T=100°C), the more pronounced effect of coke firom isobutene on the adsorption capacity (Fig.Sa) is due to the difference in coke location inside the zeolite pores. The distribution of ol omer molecules inside the zeolite crystallites would be more heterogeneous from isobutene. Indeed, owing to the olefinic character of isobutene and this slower diffusion rate in the zeolite pores, oligomers will be preferentially formed in the a cages close to the outer surface of the crystallites. 

The X rays diffraction techniques such as the X rays Extended Fine Structure and Electron Radial Distribution which permit the elucidation of the structure of possible cluster compounds within the zeolite pores and/or channels might be used. Unfortunatly these techniques are only applicable to high symmetry zeolites. 

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