Properties of Zeolites

Dehydration of Zeolites Most zeolites can be dehydrated to some degree without major alteration of their crystal structure they may subsequently be rehydrated, that is, absorb water from the vapour in the liquid phase (Table 6.3). However, many zeolites undergo irreversible structural changes and suffer total structural collapse. 

Thermal expansion About 6.9 X 1(H (zeolite A) coefficient compared to 5.2 x Iff (quartz) 

Optical property Coloiuless (presence of other metals in the zeolites adds colour) 

Dielectric Varies with temperature and water content over a wide range For example, for zeolite A , it varies from 0.01 to 0.3. The electrical conductivity depends on a number of factors such as temperature, SiO,/ AI2O3 ratio, AH, number of cations per unit ceU, cation radius, AG, water molecules per unit cell, molecules per unit cell and so on 

Zeolitic water NMR spectrum interprets the interchange of water molecules between sites and the transit time is small compared to residence time. Water molecules in larger zeolite cavities exhibit the same properties of isolated liquid, but for smaller zeolite cavities, they appear to cluster aroimd anions. During dehydration, it appears that water molecules line the inside of the zeolite super cages. Cation-dipole interactions play an important role in the nature and structure of zeolitic water 

Though, zeolites were first identified by Cronsted in 1756 their molecular sieve properties remained untouched until mid 1920s and a lack of development for commercial use of natural zeolites remained for some time more [2, 15]. With this in view, researchers attention turned to the synthesis of zeolites, popularly known as synthetic zeolites, by employing pure chemicals [15] and/or minerals present in natural resources or their by-products like coal fly ash [3, 4, 16-29]. 

Analeime Analeime, pollueite, wairakite, bellbca gite, bildtaite, boggsite, biewsterite Cubic/tetrahedral 

Chabazite Chabazite, willhendcasonite, eowlesite, dachiaidite, edingtonite, epistilbite, caionite, faujasite, ferrierite Rhombohedral 

Heulandite QinoptiloUte, heulandite, laumontite, levyne, mazzite, merUnoite, montesommaite, moidenite MonocUnic/orthogonal 

Natrolite MesoUte, natroUte, scolecite, offretite, paranatrolite, paulingite, perUalite Orthogonal/tetrahedral 

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Characterization of zeolites is primarily carried out to assess tire quality of materials obtained from syntliesis and postsyntlietic modifications. Secondly, it facilitates tire understanding of tire relation between physical and chemical properties of zeolites and tlieir behaviour in certain applications. For tliis task, especially, in situ characterization metliods have become increasingly more important, tliat is, techniques which probe tire zeolite under actual process conditions. 

Zeolites form a unique class of oxides, consisting of microporous, crystalline aluminosilicates that can either be found in nature or synthesized artificially [J.M. Thomas, R.G. Bell and C.R.A. Catlow in Handbook of Heterogeneous Catalysis (Ed. G. Ertl, H. Knbzinger and J. Weitkamp) (1997), Vol. 1, p. 206, VCH, Weinheim.]. The zeolite framework is very open and contains channels and cages where cations, water and adsorbed molecules may reside and react. The specific absorption properties of zeolites are used in detergents, toothpaste, and desiccants, whereas their acidity makes them attractive catalysts. 

While the discovery of the catalytic properties of zeolites was driven by the desire to improve industrial prcKessing, the development of emission control catalysts was necessitated by governmental fiat. The first requirement was for 90+% removal of CO and of hydrocarbons, a goal which could not be met by oxidation with base metal oxides. To achieve the required spedfications during automobile operations, it was necessary to develop supported platinum catalysts. Originally the support was alumina in pellet form. Later platinum on cordierite was used in honeycomb form, containing 200-400 square channels per square inch. 

Barthomeuf, D., Coudurier, G. and Vedrine, J.C. (1988) Basicity and basic catalytic properties of zeolites, Mater. Chem. Phys., 18, 553. 

The objective of this contribution is to investigate catalytic properties of zeolites differing in their channel systems in transformation of aromatics, i.e. toluene alkylation with isopropyl alcohol and toluene disproportionation. In the former case zeolite structure and acidity is related to the toluene conversion, selectivity to p-cymene, sum of cymenes, and isopropyl/n-propyl toluene ratio. In the latter one zeolite properties are... 

Table 1. Physicochemical properties of zeolite Beta samples.

For the non-oxidative activation of light alkanes, the direct alkylation of toluene with ethane was chosen as an industrially relevant model reaction. The catalytic performance of ZSM-5 zeolites, which are good catalysts for this model reaction, was compared to the one of zeolite MCM-22, which is used in industry for the alkylation of aromatics with alkenes in the liquid phase. The catalytic experiments were carried out in a fixed-bed reactor and in a batch reactor. The results show that the shape-selective properties of zeolite ZSM-5 are more appropriate to favor the dehydroalkylation reaction, whereas on zeolite MCM-22 with its large cavities in the pore system and half-cavities on the external surface the thermodynamically favored side reaction with its large transition state, the disproportionation of toluene, prevails. 

Some manufacturing processes use the special properties of zeolites, inorganic substances that have interconnected cavities of well-defined size and shape. Reactions can be catalyzed inside the cavities, and they are normally selective for the shape of the substrate that can enter the cavities, and for the shape of the... 

The analysis of the structural properties of zeolitic acid sites based on dipolar interactions has further improved the understanding of acidity. Grey and Vega were the first to apply the 1H 27A1 TRAPDOR technique [36]. The REAPDOR method was first applied by Kalwei and coauthors [236-238] on bare acid sites and also on zeolites loaded with probe molecules. These methods allow one to distinguish... 

Using the more advanced quantum chemical computational methods it is now possible to determine the fundamental electronic properties of zeolite structural units. The quantum chemical basis of Loewenstein s "aluminum avoidance" rule is explored, and the topological features of energy expectation value functionals within an abstract "nuclear charge space" model yield quick estimates for energy relations for zeolite structural units. 

Acid properties. The acid properties of zeolites, including those of aluminum-deficient zeolites, have been described in several reviews (e.g. 33-35). The methods used to study the acidity of aluminum-deficient Y zeolites include infrared spectroscopy (primarily pyridine and ammonia sorption studies), n-butylamine titrations in the presence of Hammett or arylmethanol indicators, and to a lesser extent potentiometric titrations and calorimetric measurements. 

Acidic properties of zeolitic materials have widely been investigated in the past fifteen years ( 1), particularly for Y-type zeolites. The presence of strong acid sites, as it is usually the case for acid zeolites, results in the presence of only weak basic sites, if any. Therefore, up to now, majority of the studies has dealt with the characterization of acid+rather than basic properties. The acid sites (Bronsted s H, Lewis Al) and basic sites (0, OH ) may be characterized directly by using physical... 

Considering all we know up to now, the specific properties of zeolites can be summarized as follows. Zeolites are aluminosilicates with defined microporous channels or cages. They have excellent ion-exchange properties and can thus be used as water softeners and to remove heavy metal cations from solutions. Furthermore, zeolites have molecular sieve properties, making them very useful for gas separation and adsorption processes, e.g., they can be used as desiccants or for separation of product gas streams in chemical processes. Protonated zeolites are efficient solid-state acids, which are used in catalysis and metal-impregnated zeolites are useful catalysts as well. 

Thus, by the mid-1930s the literature described the ion exchange, adsorption, molecular sieving and structural properties of zeolite minerals as well as a number of reported syntheses of zeolites. The early synthetic work remains unsubstantiated because of incomplete characterization and the difficulty of experimental reproducibility. 

Sherry, H.S. (1966) The ion-exchange properties of zeolites. I. Univalent ion exchange in synthetic faujasite. f Phys. Chem., 70 (4), 1158-1168. 

Katada, N., Igi, H., Kim, J.H., and Niwa, M. (1997) Determination of the acidic properties of zeolite by theorecti-cal analysis of temperature programmed desorption of ammonia based on adsorption equilibrium. J. Phys. Chem. B, 101, 5969-5977. 

The foundation of equilibrium-selective adsorption is based on differences in the equilibrium selectivity of the various adsorbates with the adsorbent While all the adsorbates have access to the adsorbent sites, the specific adsorbate is selectively adsorbed based on differences in the adsorbate-adsorbent interaction. This in turn results in higher adsorbent selectivity for one component than the others. One important parameter that affects the equilibrium-selective adsorption mechanism is the interaction between the acidic sites of the zeolite and basic sites of the adsorbate. Specific physical properties of zeolites, such as framework structure, choice of exchanged metal cations, Si02/Al203 ratio and water content can be... 

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