Zeolites Li-A

Figure 2. Representations of the three distinct topologies derived by simulated annealing based on data for a lithium gallosilicate (orthorhombic, Pna2i a = 18.5A, c = 7.5 A, 2 unique T-sites, 8 T-atoms in the unit cell) [66], The correct model (which proves isotopological with the parent zeolite Li-A(BW)) is (c).

The 30 successive packing calculations consumed, respectively for zeolite Li-A(BW) and zeolite 4A, some 3s and 12s on a workstation, with each structure optimization then reqmring an additional 2 or 8 min. respectively. In the latter case, in which the local structure provided by the simulations and the sample-averaged structure yielded by difiraction differ, the modeling results also allow the local effects of the site disorder to be explored. These encouraging results set the stage for extensions to still more complex systems and to structures for which less direct experimental data are available. 

P. Norby, A.N. Christensen, and I.G.K. Andersen, Hydrothermal Preparation of Zeolite Li-A(BW), LiAlSi04 H20, and Structure Determination from Powder Diffraction Data by Direct Methods. Acta Chem. Scand. Sen, A, 1986, 40, 500-506. 

Figure 15.6 Three dimensional plot of powder diffraction profiles as a function of time during the hydrothermal conversion of zeolite Li/Na LTA to zeolite Li A(BW). The temperature was ramped to 200 °C in 5 min and kept at that temperature. ...

Table 5.1 Potential parameters used in placement of Li+ and Na+ extra-framework cations in zeolites Li-A(BW) and 4A.

Figure 5.8 Lithium ion binding sites identified in the zeolite Li-A(BW) using energy minimization the initial step illustrated here introduces Li+ cations at non-clashing...

Li, Y. and Yang, W. (2008) Microwave synthesis of zeolite membranes a review. Journal ofMemhrance Science, 316, 3-17. 

Adsorption enthalpies and vibrational frequencies of small molecules adsorbed on cation sites in zeolites are often related to acidity (either Bronsted or Lewis acidity of H+ and alkali metal cations, respectively) of particular sites. It is now well accepted that the local environment of the cation (the way it is coordinated with the framework oxygen atoms) affects both, vibrational dynamics and adsorption enthalpies of adsorbed molecules. Only recently it has been demonstrated that in addition to the interaction of one end of the molecule with the cation (effect from the bottom) also the interaction of the other end of the molecule with a second cation or with the zeolite framework (effect from the top) has a substantial effect on vibrational frequencies of the adsorbed molecule [1,2]. The effect from bottom mainly reflects the coordination of the metal cation with the framework - the tighter is the cation-framework coordination the lower is the ability of that cation to bind molecules and the smaller is the effect on the vibrational frequencies of adsorbed molecules. This effect is most prominent for Li+ cations [3-6], In this contribution we focus on the discussion of the effect from top. The interaction of acetonitrile (AN) and carbon monoxide with sodium exchanged zeolites Na-A (Si/AM) andNa-FER (Si/Al= 8.5 and 27) is investigated. 

Li and Armor reported that Co-exchanged zeolites present a very high catalytic performance for the CH4-SCR, even in oxygen excess conditions [1, 3], Bimetallic Pt-and Pd-Co zeolites have revealed an increase of activity, selectivity towards N2 and stability, when compared with monometallic Co catalysts [4-8] even in the presence of water in the feed. Recent works show that these catalytic improvements are due to the presence of specific metal species as isolated metal ions, clusters and oxides and their location inside the cavities or in the external surface of zeolite crystallites [9, 10],... 

The catalytic isomerization of 1-methylnaphthalene and all lation of 2-methylnaphtha-lene with methanol were studied at ambient pressure in a flow-type fixed bed reactor. Acid zeolites with a Spaciousness Index between ca. 2 and 16 were found to be excellent isomerization catalysts which completely suppress the undesired disproportionation into nwhthalene and dimethylnaphthalenes due to transition state shape selectivity. Examples are HZSM-12, H-EU-1 and H-Beta. Optimum catalysts for the shape selective methylation of 2-methylnaphthalene are HZSM-5 and HZSM-li. All experimental finding concerning this reaction can be readily accounted for by conventional product shape selectivity combined with coke selectivation, so there is no need for invoking shape selectivity effects at the external surface or "nest effects", at variance with recent pubhcations from other groups. 

The 7Li resonance in zeolites is also difficult to interpret, even though the quadrupole moment is much lower. Lechert et al. (227) believe that the 7Li linewidth is controlled by the dipole-dipole interaction with 27A1 nuclei in the aluminosilicate framework. According to Herden et al. (232) the increase of 7Li frequency from 9 to 21 MHz does not affect the second moment of the spectra in zeolites Li-X and Li-Y, which means that the quadrupolar interaction is small. The second moment was also independent of the Si/Al ratio. The mean Li-Al distance calculated from the van Vleck formula was 2.35 A. Small amounts of divalent cations reduce the movement of Li + considerably, with the activation energy for this process increasing from 30 to 60 kJ/mol. 

It is known that the 29Si NMR chemical shift in zeolites is sensitive to the type of the exchangeable cation (56), which indicates the presence of interactions between cations and the framework. In particular, the substitution of Na+ by Li+ in zeolite A and in synthetic faujasite moves the 29Si resonances ca. 4 ppm downfield in both cases. Melchior et al. (235) have used this effect to study the location of cations in a series of partially exchanged zeolites (Li,Na)-A. They found that the average 29Si chemical shift is not a... 

Sherry, in the last few years, has been generalizing all knowledge about selectivity during ion exchange in zeolites this author has summarized the selectivity rules as follows [21] every zeolite has preference for Na+ instead of Li+ and NH4 instead of Na+ zeolites with a low Si/Al ratio have preference for Ca2+ and zeolites with a high Si/Al ratio have preference for alkaline cations zeolites are selective for polarizable cations and the electroselectivity, molecular sieving, and space limitations rules are valid. 

Silver species have been studied in a variety of A-zeolites including Nai2-A, K -A, Li -A, CsyNas-A and Cag-A (11). Complete exchange of cesium ion for sodium ion cannot be achieved in the A-zeolites. Typically the major cation was exchanged by silver to an extent of about 0.7 ion per unit cell which is 6% of the exchangeable cations. After irradiation about 0.003 silver ions per unit cell were converted to atomic silver species. 

In 1989, Chao [2] reported that LiLSX (lithium ion-exchanged low silica X zeolite, having a Si/Al ratio close to I.O) showed an unexpected high capacitiy and selectivity for nitrogen over oxygen. He found a Li exchange threshold value in LiNaLSX at about 2/3. Below... 

Plevert J., Di Renzo F., Fajula F. and Chiari G., Structure of dehydrated zeolite Li-LSX by neutron diffraction Evidence for a low-temperature orthorhombic faujasite. J. Phys. Chem. B 101 (1997) pp. 10340-10346... 

Ni(C0)4 in dealuminated NaY (Si/Al > 400) shows one band at 2046 cm , similar to that of tetrahedrally coordinated Ni(CO)4 in THF solution. No change of the Ni oxidation state and no loss of CO ligands after adsorption of Ni(CO)4 in alkali zeolite Y are detected with XANES and EXAFS spectroscopies. However, the appearance of four IR bands, which shift when the Ni(CO>4 loading or the alkali cations are varied, indicates an interaction of the type -OC—Ni, where = Na or Li. A reactive Ni(CO)3 in-... 

Li per unit cell, isosteric heats were determined by R. M. Barrer and R. M. Gibbons for 68 Li" 6 Na", calorimetric measurements were made by N. N. Avgul, E. S. Dobrova, and A. V. Kiselev for 40 LL + 25 Na", calorimetric (points) and isosteric (filled curve) heats were obtained by N. N. Avgul, B. G. Aristov, A. V. Kiselev, L. Ya. Kurdyukova, and N. V. Frolova. In the case of GO2 adsorption, the calorimetric heat values coincide with the isosteric. These examples clearly show that the physicochemical constants calculated from experiments (Henry constant, second virial coeflBcient, corresponding heat of adsorption, etc.) are influenced by the zeolite structure and chemical composition. Therefore, it is quite necessary to indicate this composition in the representation and discussion of the thermodynamic results. Uncertain results were often obtained for zeolites having a binding material. 

Reactions of aqueous alkaline media (containing TT, Ba2+ + Tl+, Baz+ + Li+, or Ba2+ + Na+) with metakaolinite and kaolinite (for Ba2++Li+ only) yield a wide variety of products.457 Non-zeolites included a barium silicate hydrate, barium aluminate, and a number of unidentified Tl-containing species. Zeolites of the following types were grown ... 

Fig. 17. The system used to study vibrational properties of the CO molecule trapped inside the metal exchanged ZSM5 zeolite. Subsystem A comprises the alkaline cation (Li+, Na+, or K+) and the CO molecule, whereas the (AlSii5C>2iH32)- cluster represents the zeolite framework (subsystem B). Hydrogens were used to saturate the cut Si-O bonds. Details of calculations are given in [Wesolowski et al, J. Chem. Phys., 115 (2001) 4791].

The microporous alumino-silicate zeolites (Types A, X, and mordenite are frequently used) provide a variety of pore openings (3-10 A), cavity and channel sizes, and framework Si/Al ratios. They are also available in various cationic exchanged forms (Na, K, Li, Ag, Ca, Ba, Mg), which govern their pore openings and cationic adsorption site polarities. They are highly hydrophilic materials and must be dehydrated before use. The amorphous adsorbents contain an intricate network of micropores and mesopores of various shapes and sizes. The pore size distribution may vary over a wide range. The activated carbons and the polymeric sorbents are relatively hydrophobic in nature. The silica and alumina gels are more hydrophilic (less than zeolites) and they must also be dehydrated before use. 

Values of free energy can be used to generate a selectivity series. This is done by taking a zeolite in a pure homoionic form (say Na) and using it to construct isotherms by contacting it with isonormal solutions of in-going ions (say Li,K,Rb,Cs). The AG° values obtained provide an assessment of the affinity that the Na zeolite has for the other alkali metals. 

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