Cation siting, zeolites

In most of the reactions discussed the active entity of the zeolite catalysts is introduced via ion exchange. Thus a knowledge of the possible siting of cations is a prerequisite for an understanding of the location and nature of the active sites in zeolites. In this respect the periodicity of the internal surface of the zeolites provides an almost unique opportunity to study the surface composition in considerable detail using powerful analytical methods such as X-ray diffraction. 

In recent years Seff and co-workers (9) have extensively studied cation siting in zeolite A using single-crystal X-ray diffraction techniques. In favorable cases these workers have also been able to obtain detailed information on the interactions between cations and absorbate molecules. Two examples are shown in Fig. 4, where the adsorption complexes formed when acetylene and NO are adsorbed in Co(II)A have been resolved. In the former case it is proposed that a weak complex is formed via an induced dipole interaction with the polarizable 7i-orbitals of the acetylene molecule. For the NO complex there is good evidence for electron transfer resulting in a complex between CO(III) and NO. In both cases the organic molecules 

The siting of cations in mordenite is generally less well understood than that in the zeolites described above. Smith and co-workers (12) have, however, in recent years carried out a number of single-crystal X-ray analyses on various cation-exchanged forms of mordenite. These workers correctly emphasize, however, that the cation population densities are subject to unknown errors due to pseudosymmetry. The alkali metal ions are distributed over four major sites, namely  

Only Na+ ions were found in site I due to space restrictions. 

Alternative methods to ion exchange may be employed to introduce active metal components into zeolites such as pore volume impregnation and vapor-phase adsorption of volatile compounds. In these cases the siting of such species in the zeolite pore structure is generally less well defined. 

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A notable exception are chemisorbed complexes in zeolites, which have been characterized both structurally and spectroscopically, and for which the interpretation of electronic spectra has met with a considerable success. The reason for the former is the well-defined, although complex, structure of the zeolite framework in which the cations are distributed among a few types of available sites the fortunate circumstance of the latter is that the interaction between the cations, which act as selective chemisorption centers, and the zeolite framework is primarily only electrostatic. The theory that applies for this case is the ligand field theory of the ion-molecule complexes usually placed in trigonal fields of the zeolite cation sites (29). Quantum mechanical exchange interactions with the zeolite framework are justifiably neglected except for very small effects in resonance energy transfer (J30). ... 

The influence of the preparation method of In-modified zeolite catalysts for the SCR of NOx by methane on the structure of In species formed and the catalytic activity has been studied. The structure of the catalysts has been investigated by XPS, ISS, XAFS, FTIR, electron microscopy and TPR. Dependent on the preparation, indium may occupy zeolite cation sites or form intra- and extra-zeolite oxide aggregates. It was found that indium ions at cation sites provide a low-temperature SCR activity while clustered species are active at high temperatures. 

Figure 2.10. The active site and its local environment for different catalytic systems. (A) The single monoatomic zeolitic cation site (Fe in ferrierite) (B) the single site enzyme and its local cavity which provide multiple points of additional contact (methanol oxidase) the ensemble of metal atoms, (C) site for Fe-catalyzed ammonia synthesis.

The important feature is the formation of a coordinatively unsaturated site (cus), permitting the reaction to occur in the coordinative sphere of the metal cation. The cus is a metal cationic site that is able to present at least three vacancies permitting, in the DeNOx process, to insert ligands such as NO, CO, H20, and any olefin or CxHyOz species that is able to behave like ligands in its coordinative environment. A cus can be located on kinks, ledges or corners of crystals [16] in such a location, they are unsaturated. This situation is quite comparable to an exchanged cation in a zeolite, as studied by Iizuka and Lundsford [17] or to a transition metal complex in solution, as studied by Hendriksen et al. [18] for NO reduction in the presence of CO. 

The spectrum of Mn2+ in zeolites has been used to study the bonding and cation sites in these crystalline materials. This is a 3d5 ion hence, one would expect a zero-field splitting effect. A detailed analysis of this system was carried out by Nicula et al. (170). When the symmetry of the environment is less than cubic, the resonance field for transitions other than those between the + and — electron spin states varies rapidly with orientation, and that portion of the spectrum is spread over several hundred gauss. The energies of the levels are given by the equation... 

Vibrational dynamics of small molecules adsorbed on cation sites in zeolite channel systems IR and DFT investigation... 

The interaction of CO and acetonitrile with extra-framework metal-cation sites in zeolites was investigated at the periodic DFT level and using IR spectroscopy. The stability and IR spectra of adsorption complexes formed in M+-zcolitcs can be understood in detail only when both, (i) the interaction of the adsorbed molecule with the metal cation and (ii) the interaction of the opposite end of the molecule (the hydrocarbon part of acetonitrile or the oxygen atom of CO) with the zeolite are considered. These effects, which can be classified as the effect from the bottom and the effect from the top, respectively, are critically analyzed and discussed. 

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. 

The effect from the top is behind the differences in IR spectra of CO adsorbed on various Na-zeolites (Fig. 1). The IR spectrum of CO adsorbed on the high-silica Na-FER shows only one band (centred at 2175 cm 1) that is due to the carbonyl complexes formed on isolated Na+ sites. When the content of Na+ in the sample increases (Na-FER with Si/Al=8), in addition to the band at 2175 cm 1 a new band at 2158 cm"1 appears due to the formation of linearly bridged carbonyl complexes on dual cation sites. The IR spectrum of CO adsorbed on Na-A,which has a large concentration of Na+ cations, shows bands centred at 2163, 2145, and 2129 cm 1 the band at 2163 cm"1 is due to the carbonyl species formed on dual cation sites, while bands at 2145 and 2129 cm"1 are due to carbonyls formed on multiple cation sites (Table 1), i.e., on adsorption sites involving more than two cations. 

In this contribution the analysis of the TPD data obtained for Cu-K-FER zeolite is presented, considering the formation of carbonyl complexes on dual cation sites. 

Due to the presence of low-temperature desorption peak a new desorption site was included to phenomenological model of TPD experiments previously used for the description of the Cu-Na-FER samples [5], The fit of experimental TPD curves was performed in order to obtain adsorption energies and populations for individual site types sites denoted A (A1 pair), B (sites in P channel (A1 at T1 or T2)), C (sites in the M channel and intersection (A1 at T3 or T4)) [3] and D (newly introduced site). The new four-site model was able to reproduce experimental TPD curves (Figure 1). The desorption energy of site D is cu. 82 kJ.mol"1. This value is rather close to desorption energy of 84 kJ.mol"1 found for the site B , however, the desorption entropy obtained for sites B and D are rather different -70 J.K. mol 1 and -130 J.K. mol"1 for sites B and D , respectively. We propose that the desorption site D can be attributed to so-called heterogeneous dual-cation site, where the CO molecule is bonded between monovalent copper ion and potassium cation. The sum of the calculated populations of sites B and D (Figure 2) fits well previously published population of B site for the Cu-Na-FER zeolite [3], Because the population of C type sites was... 

Interaction of the CO molecule with CuX-FER zeolites (X is an alkali-metal or proton as a co-cation) was investigated by IR spectroscopy and DFT calculations. An absorption band at 2138 cm 1 observed in IR spectra of CO on CuK- and CuCs-FER zeolites was assigned to a new type of CO adsorption complex on heterogeneous dual cation sites. CO molecule interacts simultaneously with Cu+ and alkali metal cations (via C- and O-end, respectively) in this type of complex. Interaction of CO with the secondary (alkali metal) cation led to a slight destabilization of the carbonyl complex. 

The IR spectra of CO adsorbed on CuK-FER with the same Cu/Al ratio (Cu/Al = 0.09) but different Si/Al ratio (nominal value of Si/Al ratio are 8.6 and 27.5) were taken at the same experimental conditions (Figure IB). Since dual cation sites capable of bridging CO should be more abundant in the FER matrix with a lower Si/Al ratio (i.e., with a higher concentration of Al in the framework and, thus, higher concentration of extraframework cations in the zeolite), the band at 2138 cm"1 should be more prominent for the sample with lower Si/Al ratio. Indeed, it is clearly seen that the band at 2138 cm"1 is more pronounced in the case of the CuK-FER sample with lower Si/Al ratio (Figure IB). 

Evidence of heterogeneous dual cation sites in zeolites by combined IR andDFT investigation... 

Combination of IR spectroscopy and DFT calculations provides evidence that heterogeneous dual cation sites can be formed in zeolites. Bridged carbonyl complexes can be formed whenever two metal cations are at the right distance apart from each other and give rise to a low energy CO stretching band in IR spectra. 

Fe-zeolites were prepared using the NH4 form of BEA Si/Al = 13.5. Parent BEA zeolite (average particle size of 300 nm or 1pm) was dried at 150 °C for 4 h and then mixed with a solution of FeCl3 in acetyl acetone. After 12 h of mixing, excess of the solution was removed, the solid was dried at room temperature and heated under vacuum at 350 °C for 4 h. A sample was washed with distilled water and dried in an air at room temperature. Then, the remaining organic species in the Fe-zeolites was removed by calcination at 450 °C in air for 10 h. The produced catalysts contain 0.6 wt% of Fe. This preparation procedure predominantly provides iron introduction into cationic sites [3], Two types of catalysts were prepared, Fe-BEA with a particle size of 1 pm (Fe/m-BEA) and Fe-BEA with particle size of 300 nm (Fe/n-BEA). 

The ammonia is released and the protons remain in the zeolite, which then can be used as acidic catalysts. Applying this method, all extra-framework cations can be replaced by protons. Protonated zeolites with a low Si/Al ratio are not very stable. Their framework structure decomposes even upon moderate thermal treatment [8-10], A framework stabilization of Zeolite X or Y can be achieved by introducing rare earth (RE) cations in the Sodalite cages of these zeolites. Acidic sites are obtained by exchanging the zeolites with RE cations and subsequent heat treatment. During the heating, protons are formed due to the autoprotolysis of water molecules in the presence of the RE cations as follows ... 

Martra, G., Ocule, R., Marchses, L, Centi, G., and Coluccia, S. (2002) Alkali and alkaline-earth exchanged faujasites strength of lewis base and add centres and cation site occupancy in Na- and BaY and Na- and BaX zeolites. Catal. Today, 73, 83-93. 

The typical unit cell content of zeolite L is (K,Na)gAi9Si27072.nH20 and its Si/AI ratio varies in the range of 2.6 - 3.5 [1-4]. Takaishi recently determined the distribution of Al atoms in the framework of zeolite L ly analyzing Si-MAS-NMR spectra. He thereby deduced five kinds of extra-framework cation sites as shown in Fig. 1., and estimated the relative strengths of their cation affinities [5]. 

Fig. 1. Cation sites in zeolite L. A AI atom, A AI atom located on a hidden site,0 site A,. site 6, (f site B", 0 site C, site D, 0 site D , site E...

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