Zeolites adsorption properties

One of the most signiflcant variables affecting zeolite adsorption properties is the framework structure. Each framework type (e.g., FAU, LTA, MOR) has its own unique topology, cage type (alpha, beta), channel system (one-, two-, three-dimensional), free apertures, preferred cation locations, preferred water adsorption sites and kinetic pore diameter. Some zeolite characteristics are shown in Table 6.4. More detailed information on zeolite framework structures can be found in Breck s book entitled Zeolite Molecular Sieves [21] and in Chapter 2. 

Though as yet in its infancy, the application of laser Raman spectroscopy to the study of the nature of adsorbed species appears certain to provide unusually detailed information on the structure of oxide surfaces, the adsorptive properties of natural and synthetic zeolites, the nature of adsorbate-adsorbent interaction, and the mechanism of surface reactions. 

The reaction of tin organometallic complexes with the external surface of zeolites can modify the adsorption properties.325... 

The low silica zeolites represented by zeolites A and X are aluminum-saturated, have the highest cation concentration and give optimum adsorption properties in terms of capacity, pore size and three-dimensional channel systems. They represent highly heterogeneous surfaces with a strongly hydrophilic surface selectivity. The intermediate Si/Al zeolites (Si/Al of 2-5) consist of the natural zeohtes eri-onite, chabazite, clinoptilolite and mordenite, and the synthetic zeolites Y, mordenite, omega and L. These materials are still hydrophilic in this Si/Al range. 

Given the complex nature of the crystal structure and small crystal size with an anisotropic morphology of UZM-5, the normal X-ray diffraction patterns were not sufficient to deduce an unambiguous structure. Thus a multi-technique approach was required to successfully solve the structure, to explain the adsorption properties and by analogy to the structure of other zeolites in order to assess potential applications. 

The chemical composihons of the zeolites such as Si/Al ratio and the type of cation can significantly affect the performance of the zeolite/polymer mixed-matrix membranes. MiUer and coworkers discovered that low silica-to-alumina molar ratio non-zeolitic smaU-pore molecular sieves could be properly dispersed within a continuous polymer phase to form a mixed-matrix membrane without defects. The resulting mixed-matrix membranes exhibited more than 10% increase in selectivity relative to the corresponding pure polymer membranes for CO2/CH4, O2/N2 and CO2/N2 separations [48]. Recently, Li and coworkers proposed a new ion exchange treatment approach to change the physical and chemical adsorption properties of the penetrants in the zeolites that are used as the dispersed phase in the mixed-matrix membranes [56]. It was demonstrated that mixed-matrix membranes prepared from the AgA or CuA zeolite and polyethersulfone showed increased CO2/CH4 selectivity compared to the neat polyethersulfone membrane. They proposed that the selectivity enhancement is due to the reversible reaction between CO2 and the noble metal ions in zeolite A and the formation of a 7i-bonded complex. 

The concept zeolites conventionally served as the synonym for aluminosilicates with microporous host lattice structures. Upon removal of the guest water, zeolites demonstrate adsorptive property at the molecular level as a result they are also referred to as molecular sieves. Crystalline zeosils, AlPO s, SAPO s, MAPO s (M=metal), expanded clay minerals and Werner compounds are also able to adsorb molecules vitally on reproval of any of the guest species they occlude and play an Important role in fields such as separation and catalysis (ref. 1). Inclusion compounds are another kind of crystalline materials with open framework structures. The guest molecules in an inclusion compound are believed to be indispensable to sustaining the framework structure their removal from the host lattice usually results in collapse of the host into a more compact crystal structure or even into an amorphous structure. 

Hi. Zeolites exchanged with transition metal ions. In the first row, scandium-, titanium-, cobalt-, and nickel-exchanged zeolites have been the most studied. Cobalt-exchanged zeolites are discussed in Section IV,E since they lead to oxygen adducts on adsorption of oxygen. There are several cases where copper and particularly iron ions are found as impurity cations which affect the oxygen adsorption properties of the zeolite. 

Egerton and Stone (29), taking into account that synthetic sodalite zeolites did not adsorb CO molecules, concluded that CO does not enter the sodalite cages of the Y zeolites. However, the strong electric fields present in zeolites could also produce changes in the adsorptive properties of the solids thus the energies associated with the cationic sites in crystalline zeolites must be considered. From our IR results, we concluded that CO molecules were located in the volume of the sodalite cages. Thus, the steric effect alone cannot explain the different adsorptive properties exhibited by sodalite and faujasite. 

TTigh silica zeolites attract great attention since they are characterized by relatively high thermal stability and considerable acid resistance. Physicochemical properties of high silica zeolites, despite a number of investigations, have not been sufficiently studied. The same is true for L- and clinoptilolite zeolite. The data on synthesis, structure, adsorption properties, decationization, dealuminization, adsorption heats, and other properties of the above-mentioned zeolites have been given (1-15). Results of studies of physicochemical properties of L zeolites and of natural and modified clinoptilolite are given here. 

Adsorption properties were studied with a microbalance at 20° 0.05°C. Zeolites were dehydrated at 300°-400°C until a residual pressure of 10 6 torr and a constant sample weight were reached. 

We studied adsorption properties of L-zeolites with respect to water and benzene vapors. The experimental data are given as isotherms for... 

New applications of zeolite adsorption developed recently for separation and purification processes are reviewed. Major commercial processes are discussed in areas of hydrocarbon separation, drying gases and liquids, separation and purification of industrial streams, pollution control, and nonregenerative applications. Special emphasis is placed on important commercial processes and potentially important applications. Important properties of zeolite adsorbents for these applications are adsorption capacity and selectivity, adsorption and desorption rate, physical strength and attrition resistance, low catalytic activity, thermal-hydrothermal and chemical stabilityy and particle size and shape. Apparent bulk density is important because it is related to adsorptive capacity per unit volume and to the rate of adsorption-desorption. However, more important factors controlling the raJtes are crystal size and macropore size distribution. 

Vne of the major industrial applications of zeolites is in the area of ad-sorption processes. Zeolite adsorbents are not only the most important adsorbents today, but their importance is increasing, mainly because of the following unique adsorptive properties (a) selective adsorption of molecules based on molecular dimensions, (b) highly preferential adsorption of polar molecules, (c) highly hydrophilic surface, and (d) variation of properties by ion exchange. 

Tn presenting the adsorptive properties of molecular sieve zeolites, most authors (1, 2) report isosteric heats. These are obtained from the application of the thermodynamically derived Clausius-Clapeyron type equation to experimentally measured equilibrium data. At a constant... 

The computed values of WQ lie near those calculated from crystallographic data for synthetic zeolites. The value Wo = 0.195 cm3/gram estimated here for zeolite 5A also compares favorably with the mean value Wo = 0.20 cm3/gram previously obtained 21) for the adsorption of various adsorbates on the same adsorbent, without reference to any isotherm equation. For the synthetic zeolite, the preparation method may lead to variations in adsorption properties, and this may explain the difference between values of W0 shown in the Table I. Finally, for Cecalite, where no theoretical value is known, the values obtained here for W0 with two different adsorbates are consistent with each other. Thus, the proposed method gives realistic values for W0. 

A composite material (denoted as Y/MCM-41) composed of a core of zeolite Y particle and a thin layer of MCM-41 have been prepared by the crystallization of the reaction mixture of MCM-41 and zeolite Y particles. The Y/MCM-41 particle size increases with the increase of the Si02/Al203 ratio of MCM-41. Introduction of hydroxymethyl fiber into the zeolite Y particle favors the significant increase of its strength, but zeolite p easily formed. The adsorption property of Y/MCM-41 is different from those of zeolite Y and MCM-41. H(Y/MCM-41) as a catalyst is highly selective to C4-C5 hydrocarbons and slowly deactivated in the cracking of n-heptane compared to the mechanical mixture particles of HY and HMCM-41 (designated as H(Y+MCM-41)). 

However, when small size molecules, which can freely diffuse either in Beta zeolite and MCM-41, are used a lower turnover is observed on Ti-MCM-41 catalysts compared to that obtained on Ti containing Beta zeolite [13]. This low activity can be attributed to that some of the Ti sites in MCM-41 type of catalysts are buried on the silica walls, being non-accessible to the reactants and also, to the very different adsorptive properties of Ti-MCM-41 and Ti-... 

Metal Microstructures in Zeolites. Preparation - Properties - Applications. Proceedings of a Workshop, Bremen, September 22-24,1982 edited by P.A. Jacobs, N.l. Jaeger, P. Jiiu and G. Schulz-Ekloff Adsorption on Metal Surfaces. An Integrated Approach edited by J.B6nard... 

Zeolites. Porous structure and good adsorption properties make them usable in gas-solid chromatography. They are a specific type adsorbent, with cavities allowing sieving action for molecules able to enter "holes" (windows). 

To achieve a significant adsorptive capacity an adsorbent must have a high specific area, which implies a highly porous structure with very small micropores. Such microporous solids can be produced in several different ways. Adsorbents such as silica gel and activated alumina are made by precipitation of colloidal particles, followed by dehydration. Carbon adsorbents are prepared by controlled burn-out of carbonaceous materials such as coal, lignite, and coconut shells. The crystalline adsorbents (zeolite and zeolite analogues are different in that the dimensions of the micropores are determined by the crystal structure and there is therefore virtually no distribution of micropore size. Although structurally very different from the crystalline adsorbents, carbon molecular sieves also have a very narrow distribution of pore size. The adsorptive properties depend on the pore size and the pore size distribution as well as on the nature of the solid surface. 

Secondly, the product distribution for the reaction of 3-phenylpropanoyl chloride with anisole catalyzed by zeolite beta (Table 4) is very similar to that found for acid faujasites and quite different to the AICI3 catalysis in which the ratio of 3 to 4 obtained is 7.0. Taking into account the adsorption properties of zeolites, their enhancement of the intermolecular reaction could be attributed to a high concentration of both reagents inside the cavities, thus promoting more efficiently the formation of the propiophenone 4 than a conventional AICI3 catalyst. 

Separation of gas streams by adsorption is becoming increasingly popular as improved technology comes on the market. Some examples of commercially practiced adsorption processes are shown in Table 1. These processes take advantage of the selective adsorption properties of a number of microporous adsorbents, including activated carbon, silica, alumina, and various synthetic and natural zeolites. 

Although zeolites have been known for their adsorption properties for over a century, it was not until 1952, when the first synthetic zeolite was prepared, that their utility in chemical transformations was explored. Since that time, zeolites have been used for a multitude of purposes, and to this day, they are essential catalysts in the petroleum industry, converting large and small hydrocarbons into high-octane compounds. As an outgrowth of this work, zeolites have found utility in industrial fine chemical synthesis for the construction of aromatics, heterocycles, aliphatic amines, and ethers, and the photochemistry within zeolites has already grown out of its infancy. 

Cation-exchanged KX and CaY zeolites are also known to be used for the separation of glucose and fructose on the basis of the selective adsorption properties of that kind of material.122-251 Some experiments have then been performed in the presence of Ca- and Ba-exchanged A, X and Y zeolites. Unfortunately, the CaY zeolite claimed for the separation of glucose and fructose was not as efficient as expected for a two-stage process involving isomerization followed by separation on the same type of material. 

Zeolites exhibit many unique adsorption properties, mainly because of their unique surface chemistry. The surface of the framework is essentially oxygen atoms, because Si and A1 are buried or recessed in the tetrahedra of oxygen atoms, so they are not exposed and cannot be accessed by adsorbate molecules. Also, the anionic oxygen atoms are much more polarizable... 

The transport and adsorption properties of hydrocarbons on microporous zeolites have been of practical interest due to the important properties of zeolites as shape-selective adsorbents and catalysts. The system of benzene adsorbed on synthetic faujasite-type zeolites has been thoroughly studied because benzene is an ideal probe molecule and the related role of aromatics in zeolitic catalysts for alkylation and cracking reactions. For instance, its mobility and thermodynamic properties have been studied by conventional diffusion 1-6) and adsorption 7-9) techniques. Moreover, the adsorbate-zeolite interactions and related motion and location of the adsorbate molecules within the zeolite cavities have been investigated by theoretical calculations 10-15) and by various spectroscopic methods such as UV (16, 17), IR 17-23), neutron 24-27), Raman 28), and NMR 29-39). 

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