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Microporous selective adsorbents

Adsorption. Although several types of microporous soHds are used as adsorbents for the separation of vapor or Hquid mixtures, the distribution of pore diameters does not enable separations based on the molecular-sieve effect. The most important molecular-sieve effects are shown by crystalline zeoHtes, which selectively adsorb or reject molecules based on differences in molecular size, shape, and other properties such as polarity. The sieve effect may be total or partial. [Pg.447]

Gas-adsorption processes Involve the selective concentration (adsorption) of one or more components (adsorbates) of a gas (or vapor) at the surface of a microporous solid (adsorbent) The attractive forces causing the adsorption are generally weaker than those of chemical bonds and are such that, by Increasing the temperature of the adsorbent or reducing an adsorbate s partial pressure, the adsorbate can be desorbed The desorption step Is quite Important in the overall process First, desorption allows recovery of adsorbates In those separations where they are valuable, and second, It permits reuse of the adsorbent for further cycles ... [Pg.149]

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). [Pg.273]

The aluminosilicate zeolites may be regarded as the most important and well-established members of a special class of microporous adsorbents in which the porosity is intra-crystalline. Although zeolites have been known for over 200 years, their potential value as highly selective adsorbents was first realized about 50 years ago (Barrer, 1945, 1978). Interest was further stimulated by die announcement by Breck et al. (1956) of the synthesis of the hitherto unknown zeolite A (i.e. Linde sieve A). Since then several hundred new porous zeolites have been synthesized. [Pg.356]

Zeosils are microporous solids with tetrahedral frameworks, which are similar to those of aluminosilicate zeolites, but which are built from pure SiOz [1, 2]. With their neutral frameworks, zeosils do not show the typical properties of zeolites such as ion exchange, hydrophilicity, and catalytic activity instead, these materials are hydrophobic and non-reactive. Zeosils find their main qrplications as highly selective adsorbents for sorbing nonpolar molecules from wet gas streams or aqueous solutions. [Pg.930]

The molecular-sieve zeolites are distiact from other three major npore size. Although other microporous solids are used as adsorbents for the separation of vapor or liquid mixtures, the distribution of pore diameters does not enable separations based on the ssolecular-sieve effect, that is. sepurations caused by difference in the molecular size of the materials to be separated. The most impurtanr molecular-sieve effects are shown by dehydrated crystalline zsoliles. Zeolites selectively adsorb or reject molecules based on differences in molecular size, shepe. and other properties such as polarity. Daring the ndsorption of various molecules, the micropores fill and empty reversibly. Adsorption in zeolites is a matter of pore filling, and the usual surface-area concepts are not applicable. [Pg.646]

ZMlites were first recognized as a new type of mineral in 1756. Studies of the gas-adsorpdon properties of dehydrated natural zeolite crystals more than 60 years ago led to the discovery of their molecular-sieve behavior. As microporous solids with uniform pore sizes that range from 0.3 to 0.8 nm, these materials can selectively adsorb or reject molecules based on their molecular size. This effect, with obvious commercial overtones leading to novel processes for separadon of materials, inspired attempts to duplicate the natural materials by synthesis. Many new crystalline zeolites have been synthesized, and several fulfill important functions in the cherrtical and petroleum industries. Mote than 150 synthetic zeolite types and 40 zeolite minerals ate known. The most irnportam molecular sieve zeolite adsorbents ate the synthetic Type A, Type X, synthetic mordenite, and their ion-exchanged variations, and the mineral zeolites, cha-buite and mordenite. [Pg.646]

The adsorption of alkenes with different chain length (C6-C12) from alkane solvents (C5-C14) on NaY (Si Al 2.79) was studied using a batch experimental technique. Under these conditions the zeolite micropores are close to saturation, since the solvent (alkane) will show a tendency to fill up the remaining free space. Already at low alkene concentrations, the alkenes are selectively adsorbed from their mixture with an alkane as a result of the specific interactions between 7i-electrons of the double bond and zeolite cations. The amount alkene adsorbed depends on the chain length of both the alkene and the alkane solvent in an unexpected way. Two remarkable effects are observed (1) shorter alkenes are preferentially adsorbed compared to longer alkenes and (2) with longer alkane solvents, the hexene/dodecene selectivity decreases (Figure 2). [Pg.142]

The description of the separation of multicomponent mixtures requires a more complex approach, for example by using Maxwell-Stefan methodology. However, the real membrane often assumes a more complex structure, in which, beside the microporous zeolite layer, the mesoporosity of the intra-crystalline-defects and of the underlying support can play an important role, especially when the capillary condensation phenomenon can occur, as in the case of the permeation of vapour. Kondo and Kita (Kondo and Kita, 2010) attempted an interpretation of the dehydration process by including narrow non-zeolitic pores into the support. The water molecules in the feed selectively adsorbed in zeolite pores are then transported to the non-zeolitic pore, where they are released in the permeate side of the membrane. [Pg.253]

If a Type I isotherm exhibits a nearly constant adsorption at high relative pressure, the micropore volume is given by the amount adsorbed (converted to a liquid volume) in the plateau region, since the mesopore volume and the external surface are both relatively small. In the more usual case where the Type I isotherm has a finite slope at high relative pressures, both the external area and the micropore volume can be evaluated by the a,-method provided that a standard isotherm on a suitable non-porous reference solid is available. Alternatively, the nonane pre-adsorption method may be used in appropriate cases to separate the processes of micropore filling and surface coverage. At present, however, there is no reliable procedure for the computation of micropore size distribution from a single isotherm but if the size extends down to micropores of molecular dimensions, adsorptive molecules of selected size can be employed as molecular probes. [Pg.286]

Principal Adsorbent Types. Commercially useful adsorbents can be classified by the nature of their stmcture (amorphous or crystalline), by the sizes of their pores (micropores, mesopores, and macropores), by the nature of their surfaces (polar, nonpolar, or intermediate), or by their chemical composition. AH of these characteristics are important in the selection of the best adsorbent for any particular appHcation. [Pg.275]

The search for a suitable adsorbent is generally the first step in the development of an adsorption process. A practical adsorbent has four primary requirements selectivity, capacity, mass transfer rate, and long-term stabiUty. The requirement for adequate adsorptive capacity restricts the choice of adsorbents to microporous soUds with pore diameters ranging from a few tenths to a few tens of nanometers. [Pg.292]

Effectiveness of selective adsorption of phenanthrene in Triton X-100 solution depends on surface area, pore size distribution, and surface chemical properties of adsorbents. Since the micellar structure is not rigid, the monomer enters the pores and is adsorbed on the internal surfaces. The size of a monomer of Triton X-100 (27 A) is larger than phenanthrene (11.8 A) [4]. Therefore, only phenanthrene enters micropores with width between 11.8 A and 27 A. Table 1 shows that the area only for phenanthrene adsorption is the highest for 20 40 mesh. From XPS results, the carbon content on the surfaces was increased with decreasing particle size. Thus, 20 40 mesh activated carbon is more beneficial for selective adsorption of phenanthrene compared to Triton X-100. [Pg.462]

Recent reports describe the use of various porous carbon materials for protein adsorption. For example, Hyeon and coworkers summarized the recent development of porous carbon materials in their review [163], where the successful use of mesoporous carbons as adsorbents for bulky pollutants, as electrodes for supercapacitors and fuel cells, and as hosts for protein immobilization are described. Gogotsi and coworkers synthesized novel mesoporous carbon materials using ternary MAX-phase carbides that can be optimized for efficient adsorption of large inflammatory proteins [164]. The synthesized carbons possess tunable pore size with a large volume of slit-shaped mesopores. They demonstrated that not only micropores (0.4—2 nm) but also mesopores (2-50 nm) can be tuned in a controlled way by extraction of metals from carbides, providing a mechanism for the optimization of adsorption systems for selective adsorption of a large variety of biomolecules. Furthermore, Vinu and coworkers have successfully developed the synthesis of... [Pg.132]


See other pages where Microporous selective adsorbents is mentioned: [Pg.446]    [Pg.13]    [Pg.425]    [Pg.132]    [Pg.133]    [Pg.30]    [Pg.109]    [Pg.246]    [Pg.158]    [Pg.171]    [Pg.13]    [Pg.47]    [Pg.112]    [Pg.22]    [Pg.114]    [Pg.277]    [Pg.420]    [Pg.374]    [Pg.107]    [Pg.207]    [Pg.23]    [Pg.348]    [Pg.646]    [Pg.458]    [Pg.251]    [Pg.282]    [Pg.300]    [Pg.192]    [Pg.48]    [Pg.185]   
See also in sourсe #XX -- [ Pg.47 ]




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