Zeolite isolated cations

The first two examples both involved the creation of cationic species on an acidic zeolite. In both cases we did not need to model the interaction of the cation with the zeolite framework good agreement was obtained with just calculation of the isolated cation. Apparently, the cation is not strongly perturbed by the presence of the zeolite. Such fortunate circumstances are rare. Here we show an example of how theoretical NMR calculations can help elucidate chemistry on a basic metal oxide surface, in particular, the adsorption of acetylene on MgO (26). For this study we needed to model the active sites of the catalyst, for which there are many possibilities. It is assumed the reactive sites are those in which Mg and O are substantially less coordinated than in the bulk. Comer sites are those in which Mg or O are three-coordinate, whereas Edge sites have four-fold coordination. These sites are where the strongest binding of the adsorbates are obtained. 

Once loaded with a-oxygen, the a-sites can oxidize benzene to phenol and methane to methanol at room temperature. The catalysts have to be loaded at a higher temperature with N2O and can then perform the selective oxidation. The structural properties responsible for this chemistry are under intense debate. A number of authors suggest, as already mentioned, that dimeric species are responsible for the formation of the a-oxygen, while others suggest isolated cations. Fe(lV) species have been suggested by a few authors, but disputed by others [56]. Until 2006, iron in zeolites was the only catalyst for this rather unusual reaction. Only since then have other systems been found that can perform this selective oxidation [57, 58]. 

In present work the influence of cobalt additive on catalytic properties of CuH-ZSM-5 zeolite, in both ethane oxidation and N2O decomposition, was studied. ESR spectroscopy was used for monitoring the change in either the valence or coordinative state of Cu isolated cations, located inside zeolitic channels, upon different treatments of the samples. 

The major difficulty in assessing the catalytic activity of oxides is in discriminating their action from that of the isolated cation and particularly fi om that of cations pairs known to form in zeolite matrices (67). This difficulty mainly arises from the poor means to characterize small oxide particles in zeolites or over other supports in contrast to metal particles. 

The polarizabilities of N2, O2, and Ar are nearly the same (1.74, 1.58, and 1.63 in units of 10 " cm, respectively), and are all nonpolar. Consequently, they adsorb nearly the same on all sorbents except zeolites. The fact that zeolites can distinguish between N2 and O2 was observed as early as 1938 (Barrer, 1937 1938). Barrer reported values for heats of adsorption of N2 on chabazite as high as 8 kcal/mol. The high heats of adsorption were subsequently explained quantitatively in terms of the quadrupole-electric field gradient interactions (Drain, 1953 Kington and Macleod, 1959). The unique adsorption properties of zeolites derive from the fact that their surfaces are composed of negatively charged oxides with isolated cations that are located above the surface planes. Despite... 

For Fe- and Cu-modified zeolites, there is general agreement that NH3-SCR can proceed on isolated cation sites, but their intrinsic activities appear to depend in an unknown way on the cation position in the zeolite. There are indications for the involvement of ohgomeric oxide stmctures in catalysts as well although this is not generally accepted. Fast SCR over Fe zeolites uses exclusively isolated sites, probably a type stabilized as Fe " by two nearby framework A1 ions. 

The introduction of monovalent and bivalent transition metal cations into zeolites is also possible and introduces in zeolites sites with redox activity. Several of these systems have wide application in catalysis. In particular, Co-zeolites, such as Co-MFI and Co-FER, have been deeply investigated for their activity in the CH4-SCR reaction [246]. In this case the adsorption of bases such as nitriles and ammonia, followed by IR and by TPD technique, show that they act as medium-strong Lewis acid sites. The current opinion is that these sites are catalytically active for the DeNO c reaction just when they are isolated in the zeolite cavities. A recent investigation provided evidence for the deposition of part of Co ions also at the external surface of the zeolite upon cation exchanging [85] and to their likely nonnegligible catalytic activity [247]. The deposition of Co species at the external cavities can be a reason for only apparent over-exchanging (i.e., production of zeolites with Co +/AP+ atomic ratios >0.5). 

In order to introduce vanadium into hydrogen forms of zeolites (H,Na-MOR,H-MOR with nsi/nAi = 5 and H,Na-ZSM-5, H-ZSM-5 with nsi/nAi = 35), mixtures of V2O5 and the zeolites were subjected to heat-treatment at 1073 K in air [92,190, 191 ]. Electron spin resonance spectroscopy (ESR) yielded a spectrum (as shown for the example of H-ZSM-5 in Fig. 52) exhibiting a well-resolved hyperfine (HF) signal ofvanadyl cations with g = 193,gj = 2.02,A = 19.8 mT,and Aj = 8.3 mT. These parameters are typical of isolated cations in an almost square-planar coordination. 

It is well known also that higher alkanes suffer radical gas phase oxidation above 723 K. Therefore, their use requires catalysts active and selective for deNOx at lower temperatures. The mechanism of NOx elimination is still debated a redox mechanism involving Cu ions is probable, and isolated Cu cations exchanged into MFI [4,5] or mordenite [6] have been found to be more active than CuO clusters. It must be emphasized, however, that acid zeolites exhibit good activity at high temperature, and acid mechanisms have been proposed [7-10]. In presence of Cu this acid mechanism disappears probably due to the decrease of the acidity of mordenite upon Cu exchange [6]. According to... 

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 order to rationalize the complex reaction mixtures in these slurry reactions the authors suggested that irradiations of the oxygen CT complexes resulted in simultaneous formation of an epoxide and dioxetane36 (Fig. 34). The epoxide products were isolated only when pyridine was co-included in the zeolite during the reaction. Collapse of the 1,1-diarylethylene radical cation superoxide ion pair provides a reasonable explanation for the formation of the dioxetane, however, epoxide formation is more difficult to rationalize. However, we do point out that photochemical formation of oxygen atoms has previously been observed in other systems.141 All the other products were formed either thermally or photochemically from these two primary photoproducts (Fig. 34). The thermal (acid catalyzed) formation of 1,1-diphenylacetaldehyde from the epoxide during photooxygenation of 30 (Fig. 34) was independently verified by addition of an authentic sample of the epoxide to NaY. The formation of diphenylmethane in the reaction of 30 but not 31 is also consistent with the well-established facile (at 254 nm but not 366 or 420 nm) Norrish Type I... 

Let us compare M-ZSM-5 zeolites with M = H+, Li+, Na, K+, Rb, Cs, AF+, on one hand, and organic electron donors of variable ionization potentials, on the other. Zeolite H-ZSM-5 generates cation-radicals from substrates with an oxidation potential of up to 1.65 V (Ramamurthy et al. 1991). The naphthalene sorption by Al-ZSM-5 zeolites calcified in an atmosphere of oxygen or argon leads to the appearance of two occluded particles—the naphthalene cation-radical and isolated electron. Both particles were fixed by ESR method. Back reaction between the oppositely charged particles proceeds in an extremely slow manner and both the signals persist over several weeks at room temperature (Moissette et al. 2003). 

Other restricted media in which radical cations of organic substrates are formed are zeolites. Typical anisotropic features of the naphthalene radical cation were observed after exposure of naphthalene to aluminated H ZSM-5 (n = 3, 3.4) calcined under oxygen at 773 K. These spectra contain an overlapping feature, which was assigned to isolated electrons. These signals persist over several weeks at room temperature29. 

In 1995, Maciel and co-workers (118) synthesized the trityl cation in the supercages of zeolite HY by a clever application of Friedel-Crafts chemistry—13CC14 was reacted with an excess of benzene (Fig. 15). Maciel and co-workers carried out a number of spectroscopic and chemical manipulations that unambiguously demonstrated that the product was the trityl cation and that the cation was in the zeolite. Ab initio calculations at various levels of theory predict that the point group of isolated 16 is D3 rather than Dih. It is interesting to speculate about the extent to which the zeolite environment might force the degree of twist away from the gas-phase equilibrium value. 

As a numerical example, consider a partially dealuminated H-Y zeolite that contains 32 Alf/u.c., all of which are isolated, and 8 extraframework Al cations/u.c. This example is similar to the case of the partially dealuminated H-ZSM-20 zeolite in Figure 6 if one assumes that most of the extraframework Al is present in the cationic form. If the extraframework Al is complexed such that each Al has an equivalent charge of +2, 16 protons would be required to balance the framework charge. Here it is assumed that the cationic extraframework... 

The section above described how alkali metal species such as solvated M+ ions and M ions can be formed and isolated within the lattices ofcertainhquid solvents. It is also possible to obtain single alkah metal atoms (M) and clusters (usually cationic ones, (M) +) in various solid hosts such as zeolites and graphite. There is considerable interest in clustered species since they are especially relevant to the question of what has been called the "metal/nonmetal transition, that is, how many atoms need to be gathered together before metallic (rather than molecular, nonmetalhc) properties become apparent The few electron alkah elements lend themselves to state-of-the-art... 

Kucherov et al.[9] have shown that at low Cu levels, all of the Cu exchanged into the zeolite is detected as isolated Cu2+ cations existing in two different coordination states four coordinate square planar and five coordinate square pyramidal. Figure 2 shows a typical spectrum of Cu2 + exchanged MFI, showing the two different Cu2+ species.[10]. Both species are located in the main channels of the MFI structure [9]. 

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