Zeolites sorption capacity

During the last decade large progresses have been performed in the so much difficult art of zeolites synthesis. As a consequence, the amounts of structural defects and chemical impurities have been reduced in zeolite samples (crystallites of larger sizes and well-defined morphology have been synthesized ). At the same time, the zeolite sorption capacities increase. Such an observation is well illustrated by the sorption... 

We have observed large variations in the sorption capacities of zeolite samples characterized by (ID) channel systems, as for instance AFI (AIPO4-5 zeolite) and MTW (ZSM-12 zeolite) architectural framework types. Indeed, for such unconnected micropore networks, point defects or chemisorbed impurities can annihilate a huge number of sorption sites. Detailed analysis, by neutron diffraction of the structural properties of the sorbed phase / host zeolite system, has pointed out clear evidence of closed porosity existence. Percentage of such an enclosed porosity has been determined. 

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. 

Though not a general adsorption equilibrium model the Kelvin equation does provide the relationship between the depression of the vapor pressure of a condensable sorbate and the radius (r) of the pores into which it is condensing. This equation is useful for characterization of pore size distribution by N2 adsorption at or near its dew point. The same equation can also describe the onset of capillary condensation the enhancement of sorption capacity in meso- and macro-pores of formed zeolite adsorbents. 

In a zeolite catalyst sample, which was coked via dealkylation of ethylbenzene at reaction temperatures somewhat higher than those of the sorption experiments, the diffusion coefficient of ethylbenzene remained essentially unchanged even though the sorption capacity significantly decreased due to deposition of carbonaceous material. 

The windows to the channels thus form a three-dimensional sieve with mesh widths between about 300 and 1000 pm, thus the well-known name molecular sieve for these crystalline aluminosilicates. Zeolites thus have large internal surface areas and high sorption capacities for molecules small enough to pass through the window into the cavities. They can be used to separate mixtures such as straight-chain and branched-chain hydrocarbons. 

Sorption capacity is one of the major properties used for industrial applications of zeolites. H. Lee reviews the aspects of zeolites used as adsorbents. The other papers in the section deal with the theory of sorption and diffusion in porous systems, the variation of sorption behavior upon modification, and the variation of crystal parameters upon adsorption. NMR and ESR studies of sorption complexes are reported. H. Resing reviews the mobility of adsorbed species in zeolites studied by NMR. 

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. 

The observed sorption capacity Q for nitrogen is shown as a function of pressure p, for the dehydrated zeolites in Figure 1. Here, and throughout this paper, sorption capacities are expressed as one-hundred times the mass sorbate x per unit mass of dehydrated sorbent m,... 

The observed variation of nitrogen sorption capacity at atmospheric pressure with content of presorbed water is shown in Figure 2. In all cases, the nitrogen capacity is seen to fall off regularly with increasing amounts of presorbed water, the rate of fall being highest in zeolite NaA and lowest in zeolite NaY. 

Table VII. Reported Sorption Capacities for Nitrogen on Zeolite NaA at 25°C and Atmospheric Pressure...

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... 

Our main focus for this review is to briefly and critically describe some of the defluoridation techniques as a means of getting a basis to support the adsorption technique, to evaluate the defluoridation adsorbents now being utilized and those novel defluoridation adsorbents reported in literature over the last two decades, with special reference to drinking water. Emphasis is laid toward the adsorbents availability, fluoride sorption capacity and where applicable their kinetic adsorption characteristics and column performances are reported. Detailed characteristics of fluoride adsorption onto surface-tailored zeolite are provided. In addition, various adsorber configurations are reexamined and challenges to and prospects for their application to less developed countries (LDCs) are discussed. 

The intracrystalline pore volume of the catalysts was evaluated by n-hexane sorption as shown in Fig. 6. Sorption capacities for samples SI to S3 are comparable to that of the zeolite before Ga impregnation and correspond to the value expected for an unaltered ZSM-5 type material (S10). Sorption capacity decreases for samples S3, S4, S5, and S6, because of intracrystalline volume blockage by coke deposits and possibly also (silica)-alumina debris [6] in the aged catalyt S6. In addition, the sorption rate for S6 is about twice the rate observed for the other samples, suggesting that adsorption occurs mostly at the external surface of the S6 catalyst crystallites. Thus, it appears that coke deposited on S6, probably as polyaromatic species, has almost blocked the channel pore mouths and/or practically occupied the whole intracrystalline pore volume. It explains the poor catalytic performance of S6. 

Zeolite-based materials are also promising for the removal of organic compounds from industrial waste water (13). This is particularly true for chlorinated pollutants and the preferred process is based on adsorption/separation using hydrophobic molecular sieves (HMS). Compared to carbon adsorbents, HMS presents a good compromise between sorption capacities, selectivity to organics compared to water, and regenerability (vide infra, section 16.3.1.). 

A pressure swing adsorption process (PSA) has been described with high efficiency for separation and capture of C02 in N2 at content from 16 to 25% (22). High purity C02 (> 99%) was recovered with efficiency ranging from 53% to 70% depending on C02 concentration. The selectivity and sorption capacity of zeolite 13X (FAU type) was much better than those of activated carbon. However, the influence of H20 on process efficiency was not reported. It is clear that H20, always present in flue gases from combustion, should first be separated to prevent inhibition of the zeolite. 

Clay minerals, natural and synthetic zeolites, silica and aluminum oxide forms generally are a mineral phase in mineral-carbon adsorbents. Natural aluminosilicates, particularly zeolites, due to the existence in their structure of ultramicropores and micropores (with pore diameter below 2 nm) with hydrophilic properties, exhibit high sorption capacity for particles of water vapor as well as sieve properties. They also demonstrate very good ion exchange properties. For instance, the ion exchange capacity of zeolite NaA is about 700 mval/100 g. 

After the standard activation procedure (described below), the NaY and SK-500 remained highly crystalline. The degree of crystallinity as observed by J. F. Charnell at the Mobil Oil Co. equalled the maximum found there for other samples of each zeolite. Neither sample contained appreciable amorphous matter (26). The benzene sorption capacity after ion exchange and activation of each of the type Y zeolites was essentially the same and equal to the value reported for NaX (8), further indicating that these procedures did not result in any marked changes in the crystal structure. However, use of a slightly larger molecule such as triethyl-amine would have been a more critical test. 

Generally speaking, the single-gas flux through supported zeolite membranes, for a given temperature, depends on the sorption capacity of the gas on the zeolite pores and its equilibrium adsorption constant (Langmuir isotherm is often used to describe the relationship between the amount adsorbed and the gas-phase pressure), the gas diffusion coefficient, the thickness of the zeolite layer, the porosity of the support, and the pressure at the feed and permeate sides. 

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