Zeolite cation positions

Figure 2 (a) Zeolite L framework with the different cation positions A to E and the -cage (dashed) viewed perpendicularly to the c axis, (b) Side view of the 12-ring channel along the c axis, (c) SEM image of zeolite L. 

The isomorphous replacement of aluminum by gallium in the framework structure of zeolites (beta, MFI, offretite, faujasite) offers new opportunities for modified acidity and subsequently modified catalytic activity such as enhanced selectivity toward aromatic hydrocarbons [249,250]. The Ga + ions in zeolites can occupy tetrahedral framework sites (T) and nonframework cationic positions. 

The presence and position of the cations in zeolites is important for several reasons. The cross section of the rings and channels in the structures can be altered by changing the size or the charge (and thus the number) of the cations, and this... 

The replacement of Si4+ by Al3+ ions in the tetrahedra generates a deficit of one positive charge per aluminum ion, which must be compensated by the incorporation of extrinsic cations in the zeolite structure. The sodium or calcium ions which are most commonly found in natural or synthetic zeolites can be exchanged with other alkali, alkaline-earth, rare-earth, or transition metal ions. The zeolite open structure can accommodate not only the extraframework cations, but also various molecules provided that their size is smaller than the zeolite apertures. A key feature of cation-exchanged zeolites is the local electrostatic field associated with the cations. This has led to the view of zeolites as solid solvents (258 and references therein). 

The principal components of the trityl cation in zeolite HY are <5 = 282 ppm and <5j = 55 ppm. It is instructive to tabulate all of the 13C principal component data measured for free carbenium ions in zeolites as well as for a few carbenium ions characterized in other solid acid media (Table III). The zeolitic species, in addition to the trityl cation (119), are the substituted cyclopentenyl cation 8 (102), the phenylindanyl cation 13, and the methylindanyl cation 12 (113). Values for the rert-butyl cation 2 and methylcyclopentyl cation 17 (prepared on metal halides) (43, 45) are included for comparison. Note that the ordering of isotropic chemical shifts is reasonably consistent with one s intuition from resonance structures i.e., the more delocalized the positive charge, the smaller the isotropic shift. This effect is even more apparent in the magnitudes of the CSA. Since... 

The crystal structure analysis of palladium-exchanged zeolite allows the determination of initial cation positions in the dehydrated porous framework. Similar studies after reduction by hydrogen at various temperatures should permit the observation of palladium removal from the cation sites and thus the estimation of the reduction level. Moreover, the presence of metal on the external surface is easily detected. Hence, x-ray diffraction techniques should give a good picture of hydrogen reduction of palladium in Y zeolites. 

Clinoptilolite is an abundant, naturally occurring zeolite whose structure has recently been determined (2, 3). The clino structure is nearly identical to that of heulandTte. A combination of lower Al+++ in the framework and an extra stable cation position appears to render clino thermally stable relative to heulandite. 

Sufficient ammonium iron fluoride salt was added to the NH4Y to replace 55% of the framework A1 in the Y zeolite. The LZ-224 product contained 16.9 wt.% Fe203. The product was 48% dealuminated with an apparent value of 95% substitution of Fe into the dealuminated sites. This value is inflated due to the fact that iron Is also exchanged into the normal cation positions as well. The color of the LZ-224 product was brown, indicating the presence of the Fe cations. The normal cation equivalent,... 

Nearly all of the iron was incorporated Into the zeolite. The product color was brown, again indicative of the presence of part of the Fe in cation positions. The cation equivalent was low, even the M+/A1 being only 0.72. The adsorption capacity was fully maintained, suggesting that all of the iron is either substituted in the framework or exchanged in the zeolite as an extra framework cation. 

In this experiment sufficient iron salt as (NH4)3FeF6 was added to replace 75% of the framework A1 of the NH4L. The LZ-228 product contained 13.5 wt.% Fe203. The LZ-228 product was 57% depleted in A1, with nearly 50% of the added iron incorporated into the zeolite, presumably in both framework and cation positions. The sample was colored beige and the cation equivalent, M+/A1, was only 0.55. The oxygen and water capacities were fully maintained, despite the presence of more than 13 wt.% Fe203 in the product. 

In these calculations averaged charges on the intra-tetrahedral lattice cation positions were used. The difference between the two heats of formation due to ionic bonding is added to the heat of formation due to covalent bonding resulting from the simple Extended Huckel Method for zeolitic silicas in order to arrive at the total heat of formation of the zeolite structure as a function of the amount of aluminum. 

For clarity, the positions of the cations and zeolitic water in these structures have been omitted the vertices represent T sites and the straight lines joining them bridging oxygens. Viewed in this way, we see that the structures of these seemingly unrelated zeolites do have a familial pattern.10 11... 

A corresponding Fm3c refinement of the silver zeolite-A structure, using a sample with a Si/Al ratio of 1.02, has also been achieved. The final profile R-factor is 12.3 percent and a bimodal distribution of Si-0 and Al-0 bond lengths is again obtained, confirming once more the alternation of Si and Al. Our analysis of the cation positions is not yet complete and we are examining the results for evidence of silver clusters, as reported by Uytterhoeven and co-workers (17). 

Ab initio calculations of [(H03T0T(0H)3]" clusters were also performed by Sauer and Engelhardt (119), whose formulation of the problem was quite similar to that of the previous works. The only distinction in the scheme of computations was that the charge compensation was achieved by means of a crystal field simulated by six point charges q [q = e or e for (Si, Al)- and (Al, Al)2-, respectively]. In addition, the alternative structures with point charges q = e, 2e located at the cation positions in a zeolite were also considered. 

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 optimized sodium cation positions in a six-ring of FAU zeolite structure containing two A1 atoms in para-position (denoted as Na-Al-2p) ° is shown in Figure 1. This position of the cation is representative for Sn cation position in Y and X zeolites," as well as the position of Na in Na-EMT zeolite. As expected, for this and the other zeolite model structures, Na" prefers positions near to oxygen centers bonded to A1 atoms rather than those of Si-O-Si bridges. Also, tbe cation is far from oxygen centers which are connected to compensating cations, an additional proton in this case. 

The comparison should best be made against the spectra of Cu Y zeolites. Such spectra have been reported by Nicula, Stamires and Turkevich J. Chem. Physics, 1965, 42, 3684), who, however, made no attempt to interpret the e.s.r. spectrum of dehydrated Cu Y in terms of specific cation positions. Our independent study with a thoroughly dehydrated Cu Y sample is still in progress. Although a complete assignment of the spectrum has not yet been achieved, because of the complexity of the hyperfine structures, it is clear that the Cu i ions are distributed between at least two different kinds of sites. 

The Effect of Adsorbed Molecules on the Spectrum of the Cu Cations in Zeolites. Figure 5 shows the change in the spectrum corresponding to the transitions between d-electron levels of Cu during dehydration of the zeolite. The spectrum of completely hydrated zeolite revealed a broad absorption band with a maximum at 12,100 cm" Thermal treatment of the zeolite at 100 °C resulted in the appearance of a new absorption band at approximately 15,500 cm" After vacuum treatment at high temperatures, there appeared in the spectrum an absorption band at 11,200 cm" The position of the absorption band due to Cu " in the spectrum of completely hydrated zeolite is close to that of the [Cu(H20)e] complex (12,600 cm" ) (2). This indicates that Cu " enters the hydrated zeolite structure as an octahedral hexaquo-complex. The same conclusion has been reached by other investigators 4, 18, 20) on the basis of e.s.r. spectroscopic measurements of the Cu " cations in completely hydrated zeolites. 

Octahedral and cubic cation positions may be occupied by univalent and/or divalent cations, or they may be unoccupied Le., they may remain as holes. In our opinion, the number and distribution of unoccupied cation positions are responsible for deceleration or acceleration of cation diffusion. Designating as Iq and Ic, respectively, the number of unoccupied octahedral and cubic cation positions per cavity, as rrio and nic the number of univalent cations, as Po and Pc the number of divalent cations situated in the octahedral and cubic cation positions, and as Ni, Nmy and Np the total number of unoccupied cation positions, of monovalent and of divalent cations, then for uptake t/ on a type A zeolite we have ... 

Table I shows the corresponding distribution of ions, Na" and Ca or Mn as well as unoccupied cation positions as a function of zeolite uptake.

Table I. Cation Distribution and Unoccupied Cation Positions at the Octahedral and Cubic Oxygen Rings in Na,Ca and Na,Mn Type A Zeolites...

For a Na,Mg zeolite, the distribution is different at t/ = 0.50 and y = 0.83 because of the higher polarizing activity of Mg as compared with Ca or Mn. For these values of y there should be, as compared vsdth the corresponding Na,Ca or Na,Mn zeolites, one additional Na ion at the cubic cation positions, so that there is one more unoccupied position remaining at the octahedral cation positions. 

Thus, it is necessary for unhampered diffusion of cations in a zeolite that at least 1/3 of the octahedral and 1/4 of the cubic cation positions shall be unoccupied simultaneously, since every cubic cation position is adjacent to 3 octahedral positions, and every octahedral position is adjacent to 4 cubic cation positions—i.e., lo = 6/3 = 2 and Ic = 8/4 = 2. If lo a 2 and/or Ic < 2, cation diffusion by this mechanism is impossible. In that case, it proceeds by simultaneous position interchange between 2 cations. For Ca zeolites and Mn zeolites, the resulting maxi-mums are at Np = 0, 2, 3, and 5, or for Mg zeolites at Np = 0 and 2, while the minimums for Ca zeolites and Mn zeolites are at Np = 1, 3, and 6, or for Mg zeolites at Np = 1, 3, 4, 5, and 6, confirming these assumptions... 

The positions of non-framework cations in aluminosilicate zeolites can control or fine-tune their sorptive and catalytic properties. Measurement, however, requires careful and usually protracted analyses of accurate single crystal or powder dif action data. In cases for which extensive experimental data are available, statistical mechanics analyses can yield insight into relative site energies [53-55] etirlier analyses have also attempted to quantify the relative importance of short and long-range interactions in controlling site occupancy patterns [56]. Earlier atomistic simulations in this area [57-62] had mixed results. Recent developments in methods and interatomic potentials have allowed non-framework cation positions to be simulated based solely on a knowledge of the framework structure in zeolite systems for which validatory experimental data are available [113]. 

It is clear that in the case of MFI, the zeolite pore entrances should not be considered as rigid apertures. Instead, zeolite framework topologies can show flexibility. While the O-Si-0 angle in the tetrahedral unit is rigid (109 + 1 °), the Si-O-Si angle between the units can vary between 145 and 180°. Based on isomorphous substitution of Si by other T-atoms in the framework [18], framework defects [19], cation positions, changes in the water content [16], external forces on the crystalline material [20] and upon adsorption of guest molecules [21] phase transitions can occur that have a dramatic influence in particular cases on the framework atom positions. 

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