Zeolite selective sorption properties

Quite apart from this molecular sieving effect, zeolites are also effective in selectively sorbing particular components from a mixture of molecules all individually capable of penetrating the entire zeolite. Some liquid phase sorption equilibria studies have been reported for both the small-pore 5A molecular sieve (1 ) and the large-pore faujasite NaY zeolite (2). With the recent synthesis of intermediate pore sTze zeolites such as ZSM-5 and ZSM-11(3), a study of the selective sorption properties of these zeolites was initiated. 

Sorption Properties. Sorption isotherms were determined of n-hexane and 2,3-dimethylbutane on variously pretreated samples of zeolite by a gravimetric method using a Cahn electrobalance. No shape-selective sorption was observed for these sorbates, which bespeaks a pore size greater than about 0.5 nm. The sorption capacity of S2 was appreciably lower than that of zeolite X, Y, or mordenite. Routine sorption capacities were determined by a simple procedure of pore filling with benzene at room temperature after calcination of the samples at various temperatures. 

Linear paraffins are sorbed at substantially higher rates than branched hydrocarbons. For example, n-hexane sorbs at a much higher rate than even 3-methylpentane, which in turn sorbs more rapidly than 2,3-dimethylbutane. Molecules larger than trimethylbenzene are not sorbed. Clearly ZSM-5 zeolite has properties which make it unique for shape-selective sorption and catalysis. ... 

In addition to size exclusion and steric inhibition, the intermolecular forces between the zeolite and sorbate molecules offer opportunities to achieve unique selectivity based on competitive sorption properties of various zeolites. Variables such as silica to alumina ratio, the nature of the cation species and the geometry of the channels have been shown to be important factors for consideration (13-14). They also can contribute to catalyst stability and reduced coking propensity, two important characteristics of commercially useful catalysts. 

Intensive research on zeolites, during the past thirty years, has resulted in a deep understanding of their chemistry and in a true zeolite science, including synthesis, structure, chemical and physical properties, and catalysis. These studies are the basis for the development and growth of several industrial processes applying zeolites for selective sorption, separation, and catalysis. 

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. 

In view of the large number of new zeolites recently synthesized, considerable effort has been expended in their physical characterization, in particular, via their sorption capacities for various organic substrates. The molecular exclusion properties of these zeolites have been used to estimate their pore-opening catacteristics and shape-selective properties (6). In contrast to the molecular sieving... 

Cation exchanged zeolites are successfully applied as catalysts or selective sorbents in separation technologies. " For both catalytic and sorption processes a concerted action of polarizing cations and basic oxygen atoms is important. In addition, transition metal cation embedded in zeolites exhibit peculiar redox properties because of the lower coordination in zeolite cavities compared to other supports." " Therefore, it is important to establish the strength and properties of active centers and their positions in the zeolite structure. Various experimental methods and simulation techniques have been applied to study the positions of cations in the zeolite framework and the interaction of the cations with guest molecules.Here, some of the most recent theoretical studies of cation exchanged zeolites are summarized. 

The fascinating and wonderfully exploitable properties of zeolitic materials, such as their ion-exchange properties, their sorption capacity, their shape selectivity, their catalytic activity or their role as hosts in advanced materials, arc essentially determined by their structures. For example, sorption characteristics depend upon the size of the pore openings and the void volume ion-cxchangc selectivity upon the number and nature of the cation sites and their accessibility catalytic behavior upon the pore openings, the dimensionality of the channel system, the cation sites, and the space available for reaction intermediates and host applications on the size and spacing of the cages. Consequently, structural analysis is a fundamental aspect of zeolite science. 

The osmotic model has been shown by Barrett, Marinsky, and Pave-lich (2) to be applicable as well for the interpretation of the solvent-selectivity properties of the synthetic A-zeolite in mixed media. They studied the competitive sorption of several alcohols and water by the A-zeolite from two-component mixtures. The results of these solvent distribution studies are reported in Table II as stoichiometric distribution coefficients, 8K21, defined by Equation 4... 

Zeolites are widely used as acid catalysts, especially in the petrochemical industry. Zeolites have several attractive properties such as high surface area, adjustable pore size, hydrophilicity, acidity, and high thermal and chemical stability. In order to fully benefit from the unique sorption and shape-selectivity effects in zeolite micropores in absence of diffusion limitation, the diffusion path length inside the zeolite particle should be very short, such as, e.g., in zeolite nanocrystals. An advantageous pore architecture for catalytic conversion consists of short micropores connected by meso- or macropore network [1]. Reported mesoporous materials obtained from zeolite precursor units as building blocks present a better thermal and hydrothermal stability but also a higher acidity when compared with amorphous mesoporous analogues [2-6]. Alternative approaches to introduce microporosity in walls of mesoporous materials are zeolitization of the walls under hydrothermal conditions and zeolite synthesis in the presence of carbon nanoparticles as templates to create mesopores inside the zeolite bodies [7,8]. 

The predominant importance of the cations in zeolites is that they form so-called active sites for selective interaction with guest molecules in sorption and catalytic processes. From the point of view of advanced material science [47] they play a significant role in the formation of quantum-sized clusters with novel optical or semiconducting properties. As they give rise to cationic conductivity, zeolites can be used as solid electrolytes, membranes in ion-selective electrodes and as host structures in solid-state batteries. Organometallic compounds and coordination complexes can be readily formed on these cations within the larger cages or channels and applied to gas separation, electron-transport relays and hybrid as well as shape-selective catalysis [48]. 

Yang et al. (2001) studied PV properties of different zeolite-filled PDMS membranes. They reported that the incorporation of hydrophobic zeolites into PDMS enhances the permeation selectivity toward the VOCs, but decreases the permeation rate in the corresponding membranes. The decrease in the permeation rate results from the cross-linking effect of the zeolite particles and also from the increase in the diffusion path the zeolite particles act as solvent reservoirs in the sorption, but as obstacles for the permeation diffusion. Table 9.5 shows the flux and selectivity of filled and unfilled PDMS membranes in the PV of 1.23% EA-water mixture at 50°C. 

Adnadjevid et al. (1997) studied the effect of three different types of hydrophobic zeolites (ultrastable zeolite type Y, pentasyl-type zeolite (ZSM-5), and ALPO-5 type zeolite) on the PV properties of zeolite-tilled PDMS membranes. The physiochemi-cal properties of the zeolite used, primarily the degree of hydrophobicity, as well as the sorption capacity for EtOH, the specific pore volume, specific area, and mean crystallite size of the zeolite, significantly influence the membrane s PV properties. An increase in the zeolite content results in an increase in both membrane permeability and membrane selectivity, while an increase in the PV temperature results in an increase in the permeability and a decrease in the selectivity, as opposed to the effect of membrane thickness. 

Yang et al. (1999) prepared two types of hydrophobic zeolites Y by treating the NaY-type zeolite with SiCl4, with or without subsequent hydrothermal treatment. It was reported that the hydrophobic zeolite Y as filler had a significant effect on the silicone membrane properties, even at a filler content level as low as 5 wt%. In addition, the ester sorption, permeation selectivity, and flux of the filled membranes increased with the filler Si/Al ratio in the EA extraction from water by PV. 

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