Zeolites aluminum distributions

Aluminum distribution in zeolites is also important to the catalytic activity. An inbalance in charge between the silicon atoms in the zeolite framework creates active sites, which determine the predominant reactivity and selectivity of FCC catalyst. Selectivity and octane performance are correlated with unit cell size, which in turn can be correlated with the number of aluminum atoms in the zeolite framework. ... 

The major effect of new advanced techniques on catalyst structure is found in zeolite catalysis. NMR techniques, especially MASNMR, have helped to explain aluminum distribution in zeolites and to increase our understanding of critical parameters in zeolite synthesis and crystallization. MASNMR, combined with TEM, STEM, XPS, and diagnostic catalytic reaction probes, has advanced our knowledge of the critical relationship between the structure and reactivity patterns of zeolites in the chemical fuels industry. Throughout the symposium upon which this book is based, many correlations were evident between theoretical quantum mechanical calculations and the structures elucidated by these techniques. 

The main result of extensive simulations of A1 placement in the FAU-framework topology is that random insertion of A1 into the structure, subject to Loewenstein s rule and to a weaker second neighbor Al-Al repulsion term, does not reproduce the measured Si-nAl distribution patterns [4]. The details of the aluminum distributions are therefore determined by additional or different factors. This is consistent with Melchior s model of FAU-framework construction from pre-formed 6-iing units [47,48], The simulation results also highlight the likely limitations of quantum mechanical studies of aluminum T-site preferences. If the factors controlling the aluminum distributions in zeolites X and Y are also at work in other systems, purely energetic arguments will likely have limited direct relevance for application to real materials. 

Molecular dynamics calculations were carried out for Na-Y zeolite with Si/Al ratio of 2.43. Different possible aluminum distributions were considered and their lattice energy values were computed based on simple force-field described in ref 20. It was possible to derive the exact distribution of aluminum, which satisfy the locations of extra framework Na ... 

Wang, Q. L., Torrealba,M.,Giarmetto,G., Guisnet,M.,Perot, G., Cahoreau, M., and Casso, J. 1990. Dealumination of Y zeolite with ammonium hexa-fluorosilicate A SIMS-XPS study of the aluminum distribution. Zeolites 10 703-706. 

In contrast to the stabilization of monovalent exchange cations in zeolites the structural aspects of stabilization of multivalent metal exchange cations are not quite clear. For example, the ion exchange position for bivalent metal cations should be formed by two lattice Al atoms. According to the traditional point of view the aluminum distribution over zeolite lattice is predominantly stochastic. It creates a variety of mutual localization of two nearest lattice Al atoms and results in a number of possible ion exchange structures for the bivalent cation stabilization. On the other hand, it is evident that structural peculiarities of bivalent metal cations stabilization influence the adsorption ability and catalytic reactivity of the cation. 

Kennedy, G.J., Afeworki, M., and Hong, S.B. 2002. Probing the non-random aluminum distribution in zeolite merlinoite with ultra-high-field (18.8 T) A1 and Si MAS NMR. Micropor. Mesopor. Mater. 52 55-59. 

H-ZSM-5 zeolite modified by phosphorus was studied by means of adsorption microcalorimetry of ammonia for acidity characterization [255]. It was foimd that phosphorus neutralizes acidic sites primarily at the entrance of the channels of the zeolite particles. However, the strongest acid sites remained immodified, which suggested that the aluminum distribution and consequently the distribution of acid site strengths along the zeolite channels was heterogeneous. 

Strong acids are able to donate protons to a reactant and to take them back. Into this class fall the common acids, aluminum hahdes, and boron trifluoride. Also acid in nature are silica, alumina, alumi-nosihcates, metal sulfates and phosphates, and sulfonated ion exchange resins. They can transfer protons to hydrocarbons acting as weak bases. Zeolites are dehydrated aluminosilicates with small pores of narrow size distribution, to which is due their highly selective action since only molecules small enough to enter the pores can reacl . 

The location of boron or aluminum sites in zeolites is of utmost importance to an understanding of the catalytic properties. Due to the inherent long-range disorder of the distribution of these sites in most zeolites, it is difficult to locate them by diffraction methods. The aforementioned methods to measure heteronuclear dipolar interactions can be utilized to determine the orientation between the organic SDA and A1 or B in the framework. The SDA location may be obtained by structure refinement or computational modeling. For catalytic reactions, the SDA must be removed from the pores system by calcination. 

Zeolites are aluminosilicates characterized by a network of silicon and aluminum tetrahedra with the general formula Mx(A102)x(Si02)Y. The M are cations that are necessary to balance the formal negative charge on the aluminum atoms. The tetrahedra are linked to form repeating cavities or channels of well-defined size and shape. Materials with porous structures similar to zeolites but with other atoms in the framework (P, V, Ti, etc.), as a class are referred to as zeotypes. The structure committee of the International Zeolite Association (IZA http //www.iza-online.org/) has assigned, as of July 1st 2007, 176 framework codes (three capital letters) to these materials. These mnemonic codes do not depend on the composition (i.e. the distribution of different atom types) but only describe the three-dimensional labyrinth of framework atoms. 

The preparation methods of aluminum-deficient zeolites are reviewed. These methods are divided in three categories (a) thermal or hydrothermal dealumination (b) chemical dea-lumination and (c) combination of thermal and chemical dealumination. The preparation of aluminum-deficient Y and mordenite zeolites is discussed. The structure and physico-chemical characteristics of aluminum-deficient zeolites are reviewed. Results obtained with some of the more modern methods of investigation are presented. The structure, stability, sorption properties, infrared spectra, acid strength distribution and catalytic properties of these zeolites are discussed. 

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