Zeolite lattice

Besides stmctural variety, chemical diversity has also increased. Pure silicon fonns of zeolite ZSM-5 and ZSM-11, designated silicalite-l [19] and silicahte-2 [20], have been synthesised. A number of other pure silicon analogues of zeolites, called porosils, are known [21]. Various chemical elements other than silicon or aluminium have been incoriDorated into zeolite lattice stmctures [22, 23]. Most important among those from an applications point of view are the incoriDoration of titanium, cobalt, and iron for oxidation catalysts, boron for acid strength variation, and gallium for dehydrogenation/aromatization reactions. In some cases it remains questionable, however, whether incoriDoration into the zeolite lattice stmcture has really occurred. 

Zeolite lattices have a network of very small pores. The pore di uneter of nearly all of today s FCC zeolite is approximately 8.0 angstroms (°A). These small openings, with an internal surface area of roughly 600 square... 

In the above work quats are occluded in zeolite lattices in a molecular way. In the recently disclosed (ref. 8) superwide pore M41S-materials quats arranged in the... 

An in situ infrared investigation has been conducted of the reduction of NO by CH4 over Co-ZSM-5. In the presence of O2, NO2 is formed via the oxidation of NO. Adsorbed NO2 then reacts with CH4. Nitrile species are observed and found to react very rapidly with NO2, and at a somewhat slower rate with NO and O2. The dynamics of the disappearance of CN species suggests that they are reactive intermediates, and that N2 and CO2 are produced by the reaction of CN species with NO2. While isocyanate species are also observed, these species are associated with A1 atoms in the zeolite lattice and do not act as reaction intermediates. A mechanism for NO reduction is proposed that explains why O2 facilitates the reduction of NO by CH4, and why NO facilitates the oxidation of CH4 by O2. 

A zeolite lattice with micropores of specific sizes, useful... 

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. 

Work with the objective of comparing oxo-ions with oxide particles in order to test the validity of this reasoning has been reported by Chen et al. who used a catalyst that initially contains Fe oxo-ions, [HO-Fe-0-Fe-OH] +. These sites were first converted to Fc203 particies by a simpie chemical treatment. This was followed by another treatment, which redispersed these Fc203 particies back to oxo-ions. The change in particle size was monitored by a spectroscopic method based on the observation that in zeolites metal ions and oxo-ions, that are attached to the wall of a cage, give rise to a typical IR band caused by the perturbation of the vibrations of the zeolite lattice. 

Zeolite Type Zeolite Lattice Zeolite Content % Int./Na Y Internal Std. Nitrogen Method Kaolinite... 

The catalytic activity of zeolites has its origin in the fact that some of the silicon atoms in the crystalline framework of the solids are replaced by an aluminum atom. Since aluminum is trivalent, the replacement of the tetravalent silicon results in the introduction of a negative charge into the zeolite lattice. This negative charge has to be compensated by cations and particularly by protons, the latter resulting in the so-called Brpnsted acidity (Figure 13.2) that plays an important role in the catalytic activity of zeolites. 

The acidic/basic properties of zeolites can be changed by introdnction of B, In, Ga elements into the crystal framework. For example, a coincorporation of alnminnm and boron in the zeolite lattice has revealed weak acidity for boron-associated sites [246] in boron-snbstitnted ZSM5 and ZSMll zeolites. Ammonia adsorption microcalorimetry gave initial heats of adsorption of abont 65 kJ/mol for H-B-ZSMll and showed that B-substituted pentasils have only very weak acidity [247]. Calcination at 800°C increased the heats of NH3 adsorption to about 170 kJ/mol by creation of strong Lewis acid sites as it can be seen in Figure 13.13. The lack of strong Brpnsted acid sites in H-B-ZSMll was confirmed by poor catalytic activity in methanol conversion and in toluene alkylation with methanol. 

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. 

Zeolites are crystalline aluminosilicates whose primary structure is formed by Si04 and A104 tetrahedra sharing the edges . Their tertiary structure forms uniform channels and cavities of molecular dimensions that are repeated along the zeolite lattice. Due to the lower valence of the aluminium relative to silicon, the excess negative charge (one per A1 atom) is balanced by alkali metal cations, mainly Na". An important class of the zeolite family are the faujasites, known as zeolites X and Y, which have the typical composition for the unit cell as follows ... 

June et al. (12) used TST as an alternative method to investigate Xe diffusion in silicalite. Interactions between the zeolite oxygen atoms and the Xe atoms were modeled with a 6-12 Lennard-Jones function, with potential parameters similar to those used in previous MD simulations (11). Simulations were performed with both a rigid and a flexible zeolite lattice, and those that included flexibility of the zeolite framework employed a harmonic term to describe the motion of the zeolite atoms, with a force constant and bond length data taken from previous simulations (26). 

Bandyopadhyay and Yashonath (31), in an extension of their work on MD studies of noble gas diffusion, presented MD results for methane diffusion in NaY and NaCaA zeolites. The zeolite models were the same as those used in the noble gas simulations (13, 15, 17, 18, 20, 28, 29) and the zeolite lattice was held rigid. The methane molecule was approximated as a single interaction center and the guest-host potential parameters were calculated from data of Bezus et al. (49) (for the dispersive term) and by setting the force on a pair of atoms equal to zero at the sum of their van der Waals radii (for the repulsive term). Simulations were run for 600 ps with a time step of 10 fs. 

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