Zeolites location

Rigoreau, J., Laforge, S., Gnep, N.S., and Guisnet, M. (2005) Alkylation of toluene with propene over H-MCM-22 zeolite, location of the main and secondary reactions. /. Catal., 236, 45-54. 

The accessibility of the protonic sites also plays a significant role in the catalytic activity of zeolites. Obviously this accessibility depends both on the location of the OH in the zeolite and on the size of the reactant molecules. Thus, the portion of protonic sites of HFAU zeolites located in the supercages is accessible to many organic molecules whereas the others, located in the hexagonal prisms, are inaccessible to all the organic molecules. HMOR also has protonic sites accessible (in the large channels) to many organic molecules and less accessible sites (in the... 

The key structural feature of the molecular sieves is the narrow, uniform, continuous channel system that becomes available after the zeolitic water has been driven off by heating and evacuation. Great thermal stability after dehydration has been observed in the rigid lattices of X- and Y-type faujasites, zeolite A, mordenite, and chabazite. The geometry of the internal channel and cavity system is characteristic of the individual zeolite. Entrance to the intracrystalline volume is through orifices (ranging from 3 to 9 A in the various zeolites) located periodically throughout the structure. It is thus apparent that access to the intrazeolitic environment is limited to molecules whose dimensions are less than a certain critical size. 

The well-known BE difference between XPS signals of metal ions in their oxides and in zeolites [33,35] has frequently been used to prove the intra-zeoHte location of species. Higher BE values observed with zeolites have been attributed to the influence of intra-crystalHne potentials, in some cases also to final-state effects (La, Cu, vide infra). In view of the considerable variation in the Na Is BE with the fi amework Al content [7-9,11,12] it may be expected that the binding energies of other metal ions will also depend on the Al/Si ratio. This has recently been confirmed for VO + ions [106], but a systematic study of this problem is not yet available. The BE of metal ions in zeoHtes are sometimes close to those in the corresponding hydroxides (e.g., Ni +, Cu +, Co +), so that discrimination between extra- and intra-zeolite locations on the basis of XPS binding energies alone is not safe. Thus, it has been found that the Cu 2p BE of Cu(II) oxide species well... 

Satellite structure maybe another spectral feature suited to demonstrate the intra-zeolite location of an element SateUites arise from interactions between imoccupied atomic levels and the valence band in the final-state wave fimction. 

For Cu-based systems (e.g., CuCl/Na-ZSM-5 [96]), the Cu Auger parameter (vide supra) is a powerful tool to prove intra-zeolite guest location. Special effects were reported for Co phthalocyanine, where an inequivalence of the N atoms, which is not observed in the XPS of solid phthalocyanine, can be detected when the molecule is dispersed in a zeolite [202]. With Ru and Os phthalocyanine (but not with the Co-, Ni-, and Fe-based complexes), the oxidation state of the metal and, consequently, its BE, has been reported to be higher in intra-zeolite locations than in the bulk solids [200,202]. 

By means of CO2 adsorption, the basicity of the so-called VHP hydroxyls in zeolites (located at about 3780 cm ) was revealed (486). 

The expense is justified, however, when tackling polymer chains, where reconstruction of an entire chain is expressed as a succession of atomic moves of this kind [121]. The first atom is placed at random the second selected nearby (one bond length away), the third placed near the second, and so on. Each placement of an atom is given a greater chance of success by selecting from multiple locations, as just described. Biasing factors are calculated for the whole multi-atom move, forward and reverse, and used as before in the Metropolis prescription. For fiirther details see [122, 123. 124. 125]. A nice example of this teclmique is the study [126. 127] of the distribution of linear and branched chain alkanes in zeolites. 

The zeolite frameworks are built up of SiO tetraliedra, which are neutral, and AlO tetraliedra, which have a charge of-1. The charge of tire AlO tetraliedra is balanced by tire charges of cations located at various crystallographically defined positions in the zeolite, many of tliem exposed at tire internal surface. The cations are... 

The diffusion, location and interactions of guests in zeolite frameworks has been studied by in-situ Raman spectroscopy and Raman microscopy. For example, the location and orientation of crown ethers used as templates in the synthesis of faujasite polymorphs has been studied in the framework they helped to form [4.297]. Polarized Raman spectra of p-nitroaniline molecules adsorbed in the channels of AIPO4-5 molecular sieves revealed their physical state and orientation - molecules within the channels formed either a phase of head-to-tail chains similar to that in the solid crystalline substance, with a characteristic 0J3 band at 1282 cm , or a second phase, which is characterized by a similarly strong band around 1295 cm . This second phase consisted of weakly interacting molecules in a pseudo-quinonoid state similar to that of molten p-nitroaniline [4.298]. 

An active matrix provides the primary cracking sites. The acid sites located in the catalyst matrix are not as selective as the zeolite sites, but are able to crack larger molecules that are hindered from entering the small zeolite pores. The active matrix precracks heavy feed molecules for further cracking at the internal zeolite sites. The result is a synergistic interaction between matrix and zeolite, in which the activity attained by their combined effects can be greater than the sum of their individual effects [2J. 

Electrochemical studies, in combination with EPR measurements, of the analogous non-chiral occluded (salen)Mn complex in Y zeoUte showed that only a small proportion of the complex, i.e., that located on the outer part of the support, is accessible and takes part in the catalytic process [26]. Only this proportion (about 20%) is finally oxidized to Mn and hence the amount of catalyst is much lower than expected. This phenomenon explains the low catalytic activity of this system. We have considered other attempts at this approach using zeolites with larger pore sizes as examples of cationic exchange and these have been included in Sect. 3.2.3. 

It is not the catalytic activity itself that make zeolites particularly interesting, but the location of the active site within the well-defined geometry of a zeolite. Owing to the geometrical constraints of the zeolite, the selectivity of a chemical reaction can be increased by three mechanisms reactant selectivity, product selectivity, and transition state selectivity. In the case of reactant selectivity, bulky components in the feed do not enter the zeolite and will have no chance to react. When several products are formed within the zeolite, and only some are able to leave the zeolite, or some leave the zeolite more rapidly, we speak about product selectivity. When the geometrical constraints of the active site within the zeolite prohibit the formation of products or transition states leading to certain products, transition state selectivity applies. 

Because the pore dimensions in narrow pore zeolites such as ZSM-22 are of molecular order, hydrocarbon conversion on such zeolites is affected by the geometry of the pores and the hydrocarbons. Acid sites can be situated at different locations in the zeolite framework, each with their specific shape-selective effects. On ZSM-22 bridge, pore mouth and micropore acid sites occur (see Fig. 2). The shape-selective effects observed on ZSM-22 are mainly caused by conversion at the pore mouth sites. These effects are accounted for in the hydrocracking kinetics in the physisorption, protonation and transition state formation [12]. 

Several previous studies have demonstrated the power of AEH in various catalyst systems (1-11). Often AEM can provide reasons for variations in activity and selectivity during catalyst aging by providing information about the location of the elements involved in the active catalyst, promoter, or poison. In some cases, direct quantitative correlations of AEM analysis and catalyst performance can be made. This paper first reviews some of the techniques for AEM analysis of catalysts and then provides some descriptions of applications to bismuth molybdates, Pd on carbon, zeolites, and Cu/ZnO catalysts. 

The location or distribution of the Mo sulfide species, that is, inside or outside the zeolite cavities, was examined by HREM, XRD [17], and pore volume measurements by using benzene as adsorbate [18]. HREM observations for MoSx/NaY possessing 2Mo/SC obviously demonstrated that no Mo sulfide spiecies were formed on the outside of the zeolite and that the framework structure of the zeolite was not destroyed at all on the accommodation of Mo sulfide species. The XRD and pore volume measurements confirmed the HREM observations. It is concluded that highly dispiersed intrazeolite Mo sulfide species are produced by using Mo(CO),. 

Based on previous studies [15, 22-25], the band at 1941 cm-i is assigned to Co2+(NO), and the pair of bands at 1894 and 1815 cm-i, to Co2+(NO)2- The shoulders at 1874 and 1799 cm may be due to a second dinitrosyl species. While little is known about the location and coordination of the Co 2+ in ZSM-5, it is likely that cobalt ions are associated with both [Si-0-Al]- and [Al-0-Si-0-AI]2- structures in the zeolite. In the former case, the cobalt cations are assumed to be present as Co2+(OH-) cations and in the latter case as Co2+ cations. The presence of cobalt cations in different environments could account for the appearance of two sets of dinitrosyl bands. The band at 2132 cm-> is present not only on Co-ZSM-5 but also on H-ZSM-5 and Na-ZSM-5, and has been observed by several authors on Cu-ZSM-5 [26-28]. 

The presence of methylenic bands shifted at higher frequency in the very early stages of the polymerization reaction has also been reported by Nishimura and Thomas [114]. A few years later, Spoto et al. [30,77] reported an ethylene polymerization study on a Cr/silicalite, the aluminum-free ZSM-5 molecular sieve. This system is characterized by localized nests of hydroxyls [26,27,115], which can act as grafting centers for chromium ions, thus showing a definite propensity for the formation of mononuclear chromium species. In these samples two types of chromium are present those located in the internal nests and those located on the external surface. Besides the doublet at 2920-2850 cm two additional broad bands at 2931 and 2860 cm are observed. Even in this favorable case no evidence of CH3 groups was obtained [30,77]. The first doublet is assigned to the CH2 stretching mode of the chains formed on the external surface of the zeolite. The bands at 2931 and... 

As an example, Figure 3.1.10 illustrates the use of this procedure for elucidating the location of coke deposits on zeolite catalysts [62]. Samples of zeolites H-ZSM-5... 

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