Zeolites interactions with adsorbed species

The acidic properties of zeolitic materials are of considerable importance with respect to catalyzed reactions in heterogeneous catalysis. It is vital to know the concentration, strength and accessibility of the Brpnsted and Lewis acid sites and the details of their interaction with adsorbed species (12). For zeolites, for example, 29si MAS NMR plays a crucial role in the determination of the amount of aluminium which is part of the zeolite lattice as well as the... 

The results of the present study are summarized in Table 2. For comparison reasons, the well defined carbonyls and nitrosyls of Rh on US-Ex are also included. As in the case of Rh surface complexes [1-4] US-Ex act as a kind of unique matrix for the formation of well defined surface carbonyls. The following properties of US-Ex might be responsible for these effects the remaining amount of Al atoms as centres for the localization of cationic carbonyl species is small (at a Si Al ratio of ca.lOO only about 2 per unit cell). At the same time, also the amount of other cations (as Na in the case of NaX or NaY) and adsorbed molecular water is rather small in US-Ex. The carbonyls formed in US-Ex are, therefore, isolated from each other and free of interaction with other species in the supercages of the zeolite framework. 

Examples of recent in situ studies using conventional XAS scarming techniques have focused on the characterization of the local environment of nonframework metal cations (e.g.,Pd in Na-zeolite Y [78],Rh/Pt clusters in Na-zeo-lite Y [79], Cu(l) in Cu-ZSM-5 [80]) or on the interaction of framework metal atoms with adsorbed species (e.g.,the interaction of acetonitrile with cobalt alu-minophosphates [81]). 

We have recently shown that metal-exchanged zeolites give rise to carbocationic reactions, through the interactions with alkylhalides (metal cation acts as Lewis acid sites, coordinating with the alkylhalide to form a metal-halide species and an alkyl-aluminumsilyl oxonium ion bonded to the zeolite structure, which acts as an adsorbed carbocation (scheme 2). We were able to show that they can catalyze Friedel-Crafts reactions (9) and isobutane/2-butene alkylation (70), with a superior performance than a protic zeolite catalyst. 

ESR and ESEM studies of Cu(II) in a series of alkali metal ion-exchanged Tl-X zeolites were able to demonstrate the influence of mixed co-cations on the coordination and location of Cu(II) (60). The presence of Tl(l) forces of Cu(II) into the -cage to form a hexaaqua species, whereas Na and K result in the formation of triaqua or monoaqua species. In NaTl-X zeolite, both species are present with the same intensity, indicating that both cations can influence the location and coordination geometry of Cu(II). The Cu(II) species observed after dehydration of Tl-rich NaTl-X and KT1-X zeolites was able to interact with ethanol and DMSO adsorbates but no such interaction was observed with CsTl-X zeolites. This interaction with polar adsorbates was interpreted in terms of migrations of the copper from the -cages. 

Concentrations of proton and non-proton sites in zeolites were changed by thermal treatment of Na, NH zeolites at different temperatures (.100°, 250°, 350°, 450°, 550°, 650°, and 750°C). Molecules of N,N-dimethylaniline interact at 20°C with both the proton-donating and electron-deficient zeolite sites. Effects of these interactions are evident in the spectra of the adsorbed species. 

Typically, the UV Raman spectra of various hydrocarbons adsorbed in zeolites have been found to be similar to their spectra in solution, as a pure liquid, or as a pure solid (25). This is an important finding because the UV Raman spectra of free molecules (which are relatively quick and easy to measure) can be used for fingerprint identification of adsorbed species. One minor exception to this rule is the Raman spectrum of naphthalene, which shows some changes in the pattern of peak intensities between solid naphthalene and naphthalene adsorbed in ultrastable Y-zeolite. In this case, the adsorbed naphthalene spectrum more closely resembles that of the molecule in solution with benzene or CCI4, which suggests that interaction with the pore walls of the zeolite was similar to solvent interactions. The smaller pore diameters and pore intersections in zeolite MFI compared to Y-zeolite might be expected to produce more pronounced changes in molecular vibrational spectra as a consequence of steric interactions of the molecules with the pore walls. 

The second contribution, i.e., permeation through the microporous zeolitic-channel network, can be due to activated gaseous diffusion or surface diffusion of adsorbed species. As a general rule, the smaller the pores size the greater the interaction of the adsorbed molecule with the pore walls. 

Upon adsorption of increasing amount of cumene on H(3, the OH bands shifted to lower wavenumbers, and bands characteristic for adsorbed cumene appeared (Fig. 4). IR spectra clearly show that cumene first interacts with the bridging OH groups and then with the Broensted acid sites due to extraframework aluminium species. It is interesting to remark that even the terminal silanols interact with cumene. Cumene is not easily removed from the zeolite, the background spectrum of zeolite could not be restored. 

According to this interpretation a small proportion of the non-framework Si-OH is able to interact with pyridine. Does this mean that these Si-OH groups behave as Bronsted sites Such a surprisingly acidic behaviour is unlikely. Adsorption experiments performed on silica-rich amorphous silica-alumina indicate that the Si-OH band located at 3745 cm 1 is partly affected by pyridine adsorption, even after evacuation at 723 K. Taking into account that non-protonated pyridine species are detected under such conditions, the perturbation of the Si-OH would result from an indirect interaction with pyridine adsorbed on near-by Lewis acid sites. The same phenomenon is likely to occur for non-framework Si-OH present in modified zeolites since the solids are known to contain a large amount of extra-framework Lewis sites (4). Moreover this interpretation would support the presence of a silica rich extra-framework phase in HT as well as in HTA solids. 

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