Zeolite sorption properties

Even if 5A zeolite is widely used in iso-paraffin separation from an n/iso paraffin mixture, the adsorbent is affected by a slow deactivation mainly due to coke formation inside the molecular sieve porosity. Its aging phenomenon decreases its sorption properties. According to previous studies, 5A zeolite deactivation results essentially from heavy carbonaceous compound formation in a-cages blocking the 5A zeolite microporosity [1-2]. 

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. 

Sorption. Lohse et al. (36,47,48) have studied the sorption properties of acid-dealuminated Y zeolites that contain about 99% SiC, and compared them with those of the... 

Beyer and Belenykaia (27) have investigated the sorption properties of DAY zeolites prepared from Y zeolite and SiCl vapors. They reported a very low adsorption capacity for water and ammonia, similar to that of the almost aluminum-free silicalite (49). The low adsorption capacity for water is indicative of a hydrophobic zeolite surface. The adsorption isotherms for n-butane, benzene and n-hexane obtained on the aluminum-deficient zeolite have a shape similar to those obtained on NaY zeolite and are characteristic for micropore structures. They show the absence of secondary pores in this DAY zeolite. 

Sorption. The sorption properties of aluminum-deficient mordenite are strongly affected by the dealumination procedure used and by the degree of dealumination. Materials prepared by procedures that do not involve high temperature treatments show a relatively high sorption capacity for water (15,70), due to the presence of silanol groups, which are hydrophilic centers. However, aluminum-deficient mordenite zeolites prepared by methods requiring heat treatment show a lower sorption capacity for water due to fewer silanol groups. This was shown by Chen (71), who studied the sorption properties of aluminum-deficient mordenite prepared by the two-step method. 

Ab initio methods, 147-49 Acetate ion, decomposition, 135 Acetylene, interaction with palladium, tunneling spectroscopy, 435,437f Acid-dealuminated Y zeolites catalytical properties, 183 sorption, 175-78 Acid sites, on zeolites, 254 acidification effects, 266 Acoustic ringing, in NMR, elimination, 386 Active sites, nature, 104 Activity measurements, Co-Mo catalysts, 74 Adsorbed molecules,... 

Dispersion of POMs onto inert solid supports with high surface areas is very important for catalytic application because the surface areas of unsupported POMs are usually very low (—10 m2g). Another advantage of dispersion of POMs onto inert supports is improvement of the stability. Therefore, immobilization of POMs on a number of supports has been extensively studied. Silica and active carbon are the representative supports [25], Basic supports such as MgO tend to decompose POMs [101-104], Certain kinds of active carbons firmly entrap POMs [105,106], The maximum loading level of POMs on active carbons is 14 wt% [107], Dispersion of POMs onto other supports such as zeolites, mesoporous molecular sieves, and apatites, is of considerable interest because of their high surface areas, unique pore systems, and possibility to modify their compositions, morphologies, and sorption properties. However, a simple impregnation of POM compounds on inert supports often results in leaching of POMs. 

The adaptation of zeolites to a particular purpose can be done by ion exchange and by different chemical and physical treatments. Physicochemical characteristics of zeolites often reflect the modifications introduced in the structure. Different methods are used to study the modifications and their correlations with sorption properties and catalytic activity. In this section G. T. Kerr reviews the chemistry involved in the thermal activation of NH4Y zeolites. 

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. 

C. Yang, Q. Xu and C. Hu, Aluminated Zeolites p and Their Properties. Part 3. Sorption Properties and Porous Characteristics, (submitted to Microporous and Mesoporous Materials)... 

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. 

USY (ultra-stable type Y) is a good material which has served us well but which has probably been pushed to its limit (10). In simplified terms, as Al3 is eliminated from the T-positions in the structure by thermal treatment in the presence of H2O, they are replaced by Si4 from some other portion of the crystal. Table n compares a typical USY (LZ-Y82) to the parent material, NaY. The Si02/Al203 ratio (5.77) probably understates the transformation because of non-framework alumina retained in the structure. Reduced crystallinity is evidence of structural damage this same effect would be expected to reduce the zeolite character of its sorption properties. The reduction in cation content (0.38 Na/Al) renders it unsuitable for an alkaline application such as the ELF-Aquitaine aromatization catalyst... 

Pressure Swing Adsorption (PSA) unit is a dynamic separation process. In order to create a precise model of the process and thus an accurate design, it is necessary to have a good knowledge of the mixture s adsorption behaviour. Consequently, the dilAision rates in the adsorbent particles and the mixture isotherms are extremely vital data. This article intends to present a new approach to study the adsorption behaviour of isomer mixtures on zeolites. In a combined simulation and experimental project we set out to assess the sorption properties of a series of zeolites. The simulations are based on the configurational-bias Monte Carlo technique. The sorption data are measured in a volumetric set-up coupled with an online Near Infra-Red (NIR) spectroscopy, to monitor the bulk composition. Single component isotherms of butane and iso-butane were measured to validate the equipment, and transient volumetric up-take experiments were also performed to access the adsorption kinetics. 

Molecular sieve zeolites have become established as an area of scientific research and as commercial materials for use as sorbents and catalysts. Continuing studies on their synthesis, structure, and sorption properties will, undoubtedly, lead to broader application. In addition, crystalline zeolites offer one of the best vehicles for studying the fundamentals of heterogeneous catalysis. Several discoveries reported at this conference point toward new fields of investigation and potential commercial utility. These include phosphorus substitution into the silicon-aluminum framework, the structural modifications leading to ultrastable faujasite, and the catalytic properties of sodium mordenite. 

The systematic investigations of the structure and sorption properties of natural and synthetic zeolites are certainly central to the body of knowledge that will aid in the future research and utilization of zeolites in various possible ways. 

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. ... 

Micro SXRF (Synchrotron-based micro-X-ray fluorescence) spectroscopy is an excellent tool to study elemental distributions on microscopic level. This method helps to understand the role of particular minerals in sorption properties of complex mineral assemblies. Iron and managanese oxide phases were detected in polished thin sections of natural zeolitic tuff by SXRF. This section was contacted with synthetic ground water containing Pu(V), and Pu was found to be predominantly associated with manganese oxide, but not with hematite [41]. Micro SXRF can be combined with micro-XANES, and micro-EXAFS to determine the oxidation state and coordination environment of Pu adsorbed at different regions of the tuff [42]. 

Comprehensive review papers on the sorption properties of natural zeolites can be found in literature [72,73]. Referring in particular to the main sedimentary zeolites, the last two columns in Table 2 show some structural features of interest for sorption applications. Chabazite, clinoptilolite, faujasite and mordenite, which couple reasonably large to large window sizes with wide inner volumes (except mordenite), appear the most suitable materials for adsorption processes. 

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. 

It seems that the zeolites have been well screened in a qualitative sense, for their catalytic properties. This paper is concerned with the quantitative aspects of catalytic reaction rates in zeolites. The question whether the model of coupled surface adsorption and reaction is still meaningful in the case of zeolite catalysis was already raised by Weisz and Frilette (4) when they wrote In conventional surface catalysis the termination of a three-dimensional solid structure is considered to be the locus of activity. For these zeolites the concept of surface loses its conventional meaning.. . It is the purpose of the present article to examine critically some possibile models representing equilibrium and rate phenomena in gas-zeolite systems, in order to obtain an understanding of the kinetics of chemical reactions in zeolites. Sorption equilibria, on the one hand, and rates of sorption/desorption, exchange, and catalytic reaction on the other hand are closely related and therefore have to be represented in terms of the same model. 

Some zeolites possess sorption properties close to those of small-pore zeolites, even when they have pore openings exceeding 0.45 nm. This is due to blockage of pores by large cations or structural dislocations. Offretite, zeolite T, ZSM-34, and clinoptilolite belong to this category. 

It is known that the number and size of cations in the channels and cavities as well as the polarizability of the sorbate molecules influence the sorption properties of zeolites. This effect must also be considered in the case of R-exchanged zeolites. 

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