Zeolite lattice, stabilization

MOFs can be considered as organic zeolite analogs, as their pore architectures are often reminiscent of those of zeolites a comparison of the physical properties of a series of MOFs and of zeolite NaY has been provided in Table 4.1. Although such coordinative bonds are obviously weaker than the strong covalent Si-O and Al-O bonds in zeolites, the stability of MOF lattices is remarkable, especially when their mainly organic composition is taken into account. Thermal decomposition generally does not start at temperatures below 300 °C [3, 21], and, in some cases. 

In the following we shall briefly review some of the recent applications of computational quantum chemistry to zeolites, in particular, some studies on the quantum chemical origin of Loewenstein s aluminum avoidance rule, and on the role of counter ions in stabilizing various structural units in zeolite lattices. These calculations are often extremely time consuming, nevertheless, the scope of their application is continuously expanding. 

The reduction of Cu to Cu in the zeolite lattice is more difficult than reduction of platinum and palladium ions but easier than that of other transition metal ions.25 The resulting Cu" " ion in the zeolite is fairly stable both in a reductive atmosphere and imder degassing treatment at elevated temperatures, wh eas the precious metal ions are easily reduced to the respective metals and collect to yield metal particles. Die easy reducibility of Cu and the stability of Cu" " lead to a reversible redox behaivor betweoi Cu and Cu and result in the iqipearance of the specific catalytic activity. 

Two adsorption complexes were found for the interaction of the neutral water and methanol molecules with the methoxy-zeolite lattice, as shown in Fig. 15. The water molecule essentially is a spectator during the formation of DME, although it does stabilize the DME once it is formed. The largest activation energy of the elementary steps is that for methoxy formation (160 kJ/mol). This is similar to the barrier to DME formation via the... 

The use of zeolite clusters in quantum chemical calculations has now progressed to quite a sophisticated level. Elementary steps of reaction mechanisms can now be characterized and the results used to distinguish which steps are the most plausible. Computational power is such that clusters and methods can avoid obvious pitfalls (too small a cluster, basis set, etc.). Several key concepts that have arisen from theoretical studies are illustrated in the preceding discussion. These include the following carbo-cations exist as parts of transition state structures, rather than as stable intermediates, and their stabilization is controlled by the zeolite lattice. The transition states are very different from the ground states to either side of them, and each different reaction has been shown to proceed via a different transition state. 

Zeolites (Pr4N+)-ZSM-5 and (Bu4N+)-ZSM-ll are among the most famous examples illustrating this concept (5—5). Perfectly stabilized by the template, the zeolitic lattice is little influenced by the gel chemistry and can be built over a wide range of Si/Al ratios. 

Isomorphic substitution of cations of a lower valency for Si in the tetrahedral zeolite framework causes large-ring structures to stabilize with respect to dense structures. In small- to medium-pore zeolites, the cations which will have to be introduced in the micropore channels in order to compensate for the negative charge on the zeolite lattice and to maintain charge neutrality will interact with each unfavorably other if the concentration of low-valency cations in the lattice is high. In wide-pore zeolites the repulsion is less moreover, more favorable channel positions may be available than in the more dense zeolites. 

Lattice energy calculations can assist in assessment of which cations are to be chosen in the zeolite lattice or channels in order to stabilize the structure. 

There have been several attempts to understand the synthesis mechanism of zeolites but still a complete understanding is yet to emerge. The causes for formation of different topology of silica lattice, the mechanism of incorporation of A1 in place of Si and the variations in the stability of zeolite lattices are certain intriguing questions questions remaining unanswered. There are two widely accepted proposals for the synthesis ... 

Zeolite beta was among the first zeolites which underwent successful replacaaoent of boixrn for aluminium (1). The main grx>und for inserting boron in zeolitic frameworks is the modulation of the strength of the acid sites (2-5), but structural boron proved to be less stable than aluminium In the activation treatments, especially in hydrothermal conditions (6, 7). Ihis drawback may be turned into advantage Wien a network quite unstable under dealuminating conditions is concerned, as in the case of zeolite beta (8). The milder conditions required for deboration are likely to affect to a lesser extent the lattice stability. B-beta could then represent a suitable precursor of the activated form of the zeolite (9). Moreover the different kinetics of incorporation of boron and aluminium are likely to influence other properties of the solid, like the size and habit of the crystals and the defect patterns (10-12). 

It has been shown by XPS that the electron density of zeolite lattice oxygen (Oz) depends on the Si/Al ratio [16.17]. The electron density increases or the basic strength of the Ifamework oxygen increases as the Al content of zeolite increases. It is considered that high stability of the clusters results from... 

Radiolytic spin labeling of molecules adsorbed in zeolites occurs by ionization to form radical cations and by formation of H-adduct radicals by H atom addition. Ionization of adsorbed molecules is a two-step process, equations (1) and (2). Because the adsorbate loading used in experiments is low (typically one percent or less by weight), energy is absorbed by the matrix and not directly by the adsorbate. Holes (Z" ) created in the zeolite lattice migrate to adsorbate (A) by charge transfer. Stabilization of radical cations is made possible at low temperature by sequestration in the zeolite pores and by trapping of electrons by the matrix. 

Note that adsorption of water water and ammonia on cations located in the channels of the zeolite faujasite, depicted in Fig.(5.3), shows the logaritmic dependence predicted by Eq,(5.27). 7 — 70 is the surface or lattice stabilization energy. The corresponding changes in enthalpy or entropy are ... 

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