Cations in zeolites

In the vast majority of cases where NMR is used to localise cations in zeolites, one or both of the following techniques is used Magic Angle Spinning (MAS) or Multiple Quantum Magic Angle Spinning (MQMAS). 

The resultant cycloalkenyl carbenium ions, especially the cyclopentenyl cations, are very stable (103,104) and can even be observed as free cations in zeolites 105,106). These ions can oligomerize further and, within zeolites, irreversibly block the acidic hydroxyl groups. With liquid acids, the oligomers will dilute the acid and thus lower its acid strength. 

Feuerstein, M. and Lobo, R.F. (1998) Characterization of Li cations in zeolite LiX by solid-state NMR spectroscopy and neutron diffraction. Chem. Mater., 10, 2197-2204. 

This approach allowed more precise identification of the nature of the bonds between many chemisorbed molecules and different kind of surface metallic sites (cations in zeolites or on the surface of metal oxides, metal atoms on the surface... 

Table 3. Distributions of cations in zeolite L, (H,K)9,5Al9.5Si26.s072.

Cation in zeolite Concentration of the most strong pro-tic centres VoH=36I0on I [imol/g Concentration of aprotic ... 

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 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... 

Fig. 63. 13C MAS NMR spectrum with cross-polarization of tetrapropylammonium (TPA+) cation in zeolite ZSM-S at 50.29 MHz (330).

They exhibit strong acidity, which is usually of the Bronsted type, partly because of the influence of the strong internal electrostatic fields (ca. 106 Vcm 1) exerted on the interlamellar water (which generates protons by dissociation) or, because of the additional influence of certain hydrated interlamellar cations, notably Al3+. Cation hydrolysis, just as with strongly polarizing cations in zeolites, yields free protons, thus ... 

Theoretical and Experimental NMR techniques provide powerful tools for the investigation of heterogeneous catalysis. Recent advances in in situ NMR techniques are summarized, as are advances in theoretical methods. The utility of our combined theoretical/experimental approach is illustrated by studies of the pentamethylbenzenium cation and the 1,3-dimethylcyclopentenyl cation in zeolite HZSM-5, acetylene adsorption on MgO, and the isopropyl cation on frozen SbF5. We also discuss the role of the basicity of adsorbates in the formation of stable carbenium ions on zeolites. 

The reported values of the diffusion coefficients of cations in zeolites are around 10 13-10-15 [m2/s] [5,37,48], and the factors which affect the velocity of cationic exchange are... 

Differential interactions between cations in zeolites and the products of a photoreaction may result in selectivity. One such example is the selective photoisomerization of nms-l,2-diphenylcyclopropane to the cis isomer [136]. Triplet sensitization of 1,2-diphenylcyclopropane in solution results in a photostationary state mixture consisting of 55% cis and 45% trans isomers. When the same process is carried out within NaY zeolite the cis isomer is formed in excess of 95%. The preference for the cis isomer within NaY is attributed to the preferential binding of the cation to the cis... 

The complementarity and interplay of the results of EXAFS and Mossbauer spectroscopies provide means by which crystal chemistry of certain cations in zeolites may be successfully studied. While neither technique is completely adequate as a "stand-alone tool for such studies, the combination of the two can be used to map the chemical nature, environment and location of cations where this information would otherwise be inaccessible. 

At this time, the locations of cations in zeolites have been determined primarily by X-ray diffraction (XRD) techniques. Unfortunately, this method has the drawback of being able to locate only the most stationary cations in zeolites. In some studies of hydrated zeolites, less than 50% of the total cation population can be accounted for. A higher percentage of the cations can be located in dehydrated samples, but the effect of the dehydration step on the location of the cations is generally not well known. NMR measurements, on the other hand, are most sensitive to mobile cations and cations in high symmetry sites. 

Quantum Chemical Modeling the Location of Extraframework Metal Cations in Zeolites... 

The movement from relatively loosely bound sites within the larger cavities to smaller cages with little associated water is common behaviour for cations in zeolites. Their presence in the large cavities, in what has been described as a brine is, of course, the source of facile cation-exchange properties. It... 

The use of CO and nitriles as probes also reveal the possibility of interactions of adsorbed molecules with other cations in zeolite channels [162, 163],... 

Different charge-compensating cations in zeolite L have been tested for their promotional effect in n-hexane aromatization. Apparently, high basicity of the alkaline and alkaline earth promoter favors n-hexane aromatization. Basicity and selectivity both increase from Li and Cs 331) and from Mg to Ba (22,25). Bezouhanova et al. studied the FTIR bands of linearly adsorbed CO in the range of 2060-2075 cm . One band at 2075 cm", which is also found on unsupported Pt, is attributed to extrazeolite Pt particles, a second band shifts from 2060 cm" for Li to lower wavenumbers with K and Rb 331). Another criterion, used by Larsen and Haller, is the measured rate of competitive hydrogenation of benzene and toluene, which has been found to correlate with the zeolite basicity (25). As described in a previous section, this method had previously been used by Tri el al. to probe for the electron deficiency of Pt particles in acidic zeolites 332). The rate data are analyzed in terms of a Langmuir-Hinshelwood model and the ratio of the adsorption coefficients of toluene and benzene, A, /b, is determined. It was found to decrease from 8.6 for Pt/Si02, and 5.4 for Pt/MgL, to 4.4 for Pt/BaL. As direct electron transfer from the cations to neutral Pt particles is unlikely, an interaction of Pt with the zeolite framework or with... 

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