Zeolites active framework involvement

Treatments with hydrogen (at 480°C) resulted in complete and partial losses in activity (139) for impregnated and ion-exchanged catalysts, respectively. This could be correlated with the degree of reduction which was incomplete under these conditions for the ion-exhanged catalysts. It was therefore concluded that the active site involved Ni2+ ions bonded to the oxygen anions of the zeolite framework and that nickel metal was inactive in the temperature range 150-250°C. 

Although It Is accepted that the catalytic activity of these materials has to be ascribed to the presence of Ti In the sllicallte lattice, the exact nature of the active sites Involved still remains unclear. However, a common feature Is observed In all samples showing catalytic activity, that Is the presence of an adsorption band at 960 cm in the infrared spectra (9), its intensity increasing when increasing the titanium content of the zeolite. This band has been assigned to Ti=0 (10) or Si-0-(Ti) groups (11) in the zeolite framework, which will form the actual oxidant sites after the addition of hydrogen peroxide. 

There are two procedures used to prepare dealuminated zeolites. One procedure involves a rapid hydrothermal dealumination at a relatively high temperature of 500°C (10,11). The resulting material contains a network of larger zeolitic mesopores within the crystal and connecting directly to the zeolitic pore system. Another procedure uses silicon hexafluoride at a relatively low temperature, <100°C (12). The result is a slow dealumination with the formation of much less mesopore structure. The two procedures result in zeolites that differ in the distribution of active framework aluminum sites as well as in the presence or absence of mesoporosity. The liquid phase dealumination results in a zeolite with the active aluminum sites preferentially removed from the outside of the zeolite particle or crystal. Further, during the dealumination process, additional silicon is deposited on the zeolite surface (13). The exact opposite happens during hydrothermal dealumination. While the framework is more or less uniformly dealuminated, the aluminum atoms removed from the framework do not remain within the pore system, but are observed to migrate to the outside of the zeolite particle (14). Consequently, unless the alumina is removed, the outside of the zeolite particle is alumina rich. 

Zeolites are crystalline aluminosilicates that have exhibited catalytic activities ranging from one to four orders of magnitude greater than amorphous aluminosilicates for reactions involving carbonium ion mechanisms such as catalytic cracking (144). As a result extensive efforts have been undertaken to understand the nature of the catalytic sites that are responsible for the observed high activity. The crystalline nature of zeolites permits more definite characterization of the catalyst than is possible for amorphous acidic supports such as alumina and silica-alumina. Spectral techniques, in conjunction with structural information derived from X-ray diffraction studies, have led to at least a partial understanding of the nature of the acidic sites in the zeolite framework. 

The first two examples both involved the creation of cationic species on an acidic zeolite. In both cases we did not need to model the interaction of the cation with the zeolite framework good agreement was obtained with just calculation of the isolated cation. Apparently, the cation is not strongly perturbed by the presence of the zeolite. Such fortunate circumstances are rare. Here we show an example of how theoretical NMR calculations can help elucidate chemistry on a basic metal oxide surface, in particular, the adsorption of acetylene on MgO (26). For this study we needed to model the active sites of the catalyst, for which there are many possibilities. It is assumed the reactive sites are those in which Mg and O are substantially less coordinated than in the bulk. Comer sites are those in which Mg or O are three-coordinate, whereas Edge sites have four-fold coordination. These sites are where the strongest binding of the adsorbates are obtained. 

Clearly, with good evidence for Rh(I) as the active species in RhY, there is now a consistency between the homogeneous 141) and heterogeneous catalyst systems. A mechanism for the homogeneous dimerization reactions has been proposed by Cramer (141), which involves the formation of anionic Rh(I) complexes. The presence of such species inside the cavities of the anionic zeolite framework does not seem very probable. 

The catalytic activity of the zeolitic framework is strongly dependent on the Si Al ratio, i.e. the concentration of the potential catalytic sites. This structural feature, as well as the spectroscopic and energetic properties of the Br0nsted acid sites, has also been investigated by empirical force field techniques. However, in contrast to the adsorption and diffusion phenomena, the stability of the acid sites, and their acid strength is a result of a subtle balance of covalent and ionic bonding interactions, with an active involvement... 

Active involvement of zeolitic framework in electron-transfer chemistry... 

The demonstration by Enichem workers [1] that titanium silicalite (TS-1) catalyzes a variety of synthetically useful oxidations with 30% aqueous hydrogen was a major breakthrough in the field of zeolite catalysis [2], The success of TS-1 prompted a flourish of activity in the synthesis of other titanium-substituted molecular sieves, such as titanium silicalite-2 (TS-2) [3], Ti-ZSM-48 [4] Ti-Al-mordenite [5], Ti-Al-beta [6]and Ti-MCM-41 [7]. Moreover, this interest has also been extended to the synthesis of redox molecular sieves involving framework substitution by other metals, e.g. chromium, cobalt, vanadium, etc. [8]. 

In conclusion, it has been shown that zeolites are largely stabilised by the presence of rare earths in extra-framework positions. The acidity generated during the exchange yields catalysts that are active and selective, either by themselves or in combination with metals, for acid catalysed processes involving carbonium and carbenium ions. They are of practical importance in processes related with oil refining petrochemistry and also in the production of chemicals. 

One further, particularly informative experiment important to consideration of framework nuclei involves the detection and characterization of the proton sites within the zeolite structure, as developed by Freude et al. [25] (Fig. 11). Even when the protons are not major contributors to the overall lattice structure, they may be central to the catalytic reactions. Since they are relatively dilute in the lattice, a simple MAS experiment often yields spectra of sufficient resolution to identify the different functionalities. Spectra of this type will be critical in probing the catalytic natures of these systems and the optimization of techniques for their activation. Together with the use of DAS and DOR techniques (see Sec. V.C.), it should become possible to selectively obtain O spectra of the acid sites themselves. 

Zeolites are important refining catalysts for two reasons. The first, and most important, is the presence of strong acid sites. Two types of acid sites are present in zeolites. The first are Brpnsted acid sites, shown in Figure 10.6. These are protons that act as charge-compensating cations for framework aluminum. The second are Lewis acid sites, which are less well defined than Brpnsted sites and involve extra-framework aluminum species formed by removing framework aluminum (often due to steam). Both of these are extremely active catalytic centers, which can activate relatively inert substrates such as normal alkanes. 

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