Adsorption properties of zeolites

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. 

The study of zeolites as adsorbent materials began in 1938, when Barrer published a series of papers on the adsorptive properties of zeolites [97], In the last 50 years, zeolites, both natural and synthetic, have become one of the most important materials in modern technology [97-107], Today, the production and application of zeolites for industrial processes is a multimillion dollar industry. 

The study of zeolites as adsorbent materials began in 1938 when Professor Barrer published a series of papers on the adsorptive properties of zeolites [28], In the last 50 years, zeolites, natural and synthetic, have turned out to be one of the most significant materials in modem technology [27-37], Zeolites have been shown to be good adsorbents for H20, NH3, H2S, NO, N02, S02, C02, linear and branched hydrocarbons, aromatic hydrocarbons, alcohols, ketones, and other molecules [2,31,34], Adsorption is not only an industrial application of zeolites but also a powerful means of characterizing these materials [1-11], since the adsorption of a specific molecule gives information about the microporous volume, the mesoporous area and volume, the size of the pores, the energetics of adsorption, and molecular transport. 

Calculation of Some Adsorption Properties of Zeolites by Statistical Methods... 

TJecause of their regular structure and the presence of more or less separate cavities, the zeolites offer a very convenient system for the application of statistical methods. In this work, an attempt has been made to calculate some adsorption properties of zeolites using statistical thermodynamics. 

A full comparison of the two discussed zeolite structures and the adsorption properties based on parameters influencing the zeolite pore dynamics are given in Table 4 Table 4 Specifications, parameters of relevance to the deformation of framework, framework dynamics and adsorption properties of zeolite-types MFI and RHO ... 

Zeolites can be used as microreaetors in which the size of the reactor, i. e. the size of pores and cavities can be varied. The adsorption properties of zeolites can be modified and this, with their molecular sieve properties either for reactants, products, or transition states can be very useful for directing the reaction towards the desired products. 

The structure and properties of zeolites have attracted widespread attention in molecular modeling. In addition to catalytic properties, zeolites are valuable technically because they can adsorb other molecules, and these molecules can undergo reactions within the zeolite framework. Both the adsorption properties of zeolites and the behavior of other molecules in zeolites have been extensively studied. 

The polarizabilities of N2, O2, and Ar are nearly the same (1.74, 1.58, and 1.63 in units of 10 " cm, respectively), and are all nonpolar. Consequently, they adsorb nearly the same on all sorbents except zeolites. The fact that zeolites can distinguish between N2 and O2 was observed as early as 1938 (Barrer, 1937 1938). Barrer reported values for heats of adsorption of N2 on chabazite as high as 8 kcal/mol. The high heats of adsorption were subsequently explained quantitatively in terms of the quadrupole-electric field gradient interactions (Drain, 1953 Kington and Macleod, 1959). The unique adsorption properties of zeolites derive from the fact that their surfaces are composed of negatively charged oxides with isolated cations that are located above the surface planes. Despite... 

Lohse, U., et al., Adsorption properties of zeolites prepared by different processes, Adsorpt, Sci. Technol.. 3(3), 173-180 (1986). 

Another important physical property of the ash zeolites is their pore radius Rp. This parameter helps in studying the adsorption properties of zeolites as an adsorbent. Rp can be correlated with the specific surface area SSAbet, which can be determined by nitrogen adsorption technique (i.e., by employing BET method and the relationship, Rp = 2 VpISSAbet> where Vp is the pore volume) [44]. The pores are assumed to be cylindrical in shape for natural zeolites Clinoptilolite and Mordenite, for which SSAbet generally lies between 11-16 m and 115-120 m /g, respectively. The trend depicted in Fig. 2.4 exhibits an initial increase in Rp with an increase in SSAbet, up to 20 m /g, beyond which it decreases sharply [8]. This trend violates the inverse relationship between the two parameters as mentioned above. 

The papers by Conner, Kaneko, Rouquerol, Unger and their co-workers underline the importance now attached to the determination of physisorption isotherms at very low levels of surface coverage or fractional micropore filling, i.e. in the region of very low p/p . Such high resolution adsorption (HRADS) measurements have been shown to be especially useful for the characterization of the adsorptive properties of zeolites, aluminophosphates and molecular sieve carbons. 

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. 

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. 

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... 

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 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 properties of zeolites, most notably their stability, adsorptive capacity and catalytic activity, are strongly dependent on the precise location of Si and A1 in the emionic framework. This is one of the most challenging and debated problems in silicate crystal chemistry. 

Acidic properties of zeolite L were observed to correlate well with its structural disorders. The Si-MAS-NMR spectrum of zeolite L having a Si/AI ratio different from 3 revealed that Al distribution deviated from the ideal and suggested the presence of six different boat-shaped 8-ring patterns. Differential molar heats of adsorption of ammonia changed step-wise with the adsorbed amount, which reflects the difference in the acid strength of protons located in structurally different 8-rings. 

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. 

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. 

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... 

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-... 

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. 

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. 

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