Zeolite anionic oxygens

Zeolites exhibit many unique adsorption properties, mainly because of their unique surface chemistry. The surface of the framework is essentially oxygen atoms, because Si and A1 are buried or recessed in the tetrahedra of oxygen atoms, so they are not exposed and cannot be accessed by adsorbate molecules. Also, the anionic oxygen atoms are much more polarizable... 

On the other hand, in highly siliceous zeolites like ZSM-5, the A102 units of the framework are so far apart that charge cannot be considered to be shared equally among anionic oxygens. Sites on a given kind of cation sub-lattice do not new have equal probabilities of occupation. Indeed only those near an AIO2 could have a cation occupant. 

Acid-treated clays were the first catalysts used in catalytic cracking processes, but have been replaced by synthetic amorphous silica-alumina, which is more active and stable. Incorporating zeolites (crystalline alumina-silica) with the silica/alumina catalyst improves selectivity towards aromatics. These catalysts have both Fewis and Bronsted acid sites that promote carbonium ion formation. An important structural feature of zeolites is the presence of holes in the crystal lattice, which are formed by the silica-alumina tetrahedra. Each tetrahedron is made of four oxygen anions with either an aluminum or a silicon cation in the center. Each oxygen anion with a -2 oxidation state is shared between either two silicon, two aluminum, or an aluminum and a silicon cation. 

The four oxygen anions in the tetrahedron are balanced by the -i-4 oxidation state of the silicon cation, while the four oxygen anions connecting the aluminum cation are not balanced. This results in -1 net charge, which should be balanced. Metal cations such as Na", Mg ", or protons (H" ) balance the charge of the alumina tetrahedra. A two-dimensional representation of an H-zeolite tetrahedra is shown ... 

Chapter 6, by Clennan, on oxygenation in zeolites critically summarizes the results on oxidation of organic molecules within zeolites by singlet oxygen and superoxide anion. This chapter complements Chapter 6, Volume 5 by Va-senko and Frei. In addition to Chapter 6, Chapter 7 in this volume and Chapters 3, 4, and 5 in Volume 5 are exceptional resources in the area of excited-state behavior of organic molecules within zeolites. 

In IR experiments it was confirmed that NO could adsorb as NO, NO and (NO)2- species on the Cu-zeolite, and the anionic species decreased with adsorption time to yield N2 and N2O in the gas phase whereas NO" " increased. After adsorption of NO for about 1 h, anionic species had almost disappeared and the intensity of NO species became approximately constant. These results indicate that all the Cu ions generated through pretieatment at elevated temperature were oxidized to Cu2 ions by oxygen produced in the NO decomposition at ambient temperature and the resulting CU2+ ions acted as adsorption sites for NO" " (Cu2+ + NO = Cu -NO ). This NO species could not be desorbed by evacuation at room temp ature. The IR spectra indicated the presoice of a large amount of NO and small amounts of NO2 and NO3 after the evacuation, i.e., weakly adsorbed or physisorbed NO molecules were absent from the zeolite under these condititHis. These phenomena were further confirmed by ESR experiments the adsorption-desorption cycles of NO resulted in a decrease-increase in the intensity of Cu2+ ESR signals. 

The effective area of the anionic aluminosilicate framework in the pores of a zeolite is at least 100 times the external surface area, and it can be as high as 1000 m2 g-1. Consequently zeolites are unusually effective as catalysts for reactions that are favored by aluminosilicate surfaces. Substitution of Sj4+ Al3+ in a silica framework makes it acidic and, potentially, coordinatively unsaturated. Suppose, for example, that we heat the NH4+ form of a zeolite. Ammonia is driven off, and one H+ remains to counterbalance each Al3+ that has substituted for a silicon. The protons are attached to oxygens of the aluminosilicate framework ... 

In this work, 1 1 oxygenated low-spin cobalt(II)-ammonia complexes were synthesized within the zeolite framework by the adsorption of NH3 and 02 in Co(II)Y zeolites with differing cobalt(II) content. Spin densities and the nature of the superoxide anion (02 ) were estimated by introducing oxygen-17 in the ammoniated Co(II) zeolites. Questions concerning the equivalence of the two oxygen atoms have arisen in studies on oxygen adducts of Co (II) Schiff base compounds (7), and it was of interest to study this problem in cobalt(II) ammonia complexes. 

NH3 is similar to H2O in that they both possess large dipole moments and are both small molecules. The presence of NH3 in a zeolite is chemically similar to the presence of H2O in a zeolite. Therefore, the hydrated cation distribution in zeolites is probably more typical of NH3 adsorption in zeolites than the dehydrated cation distribution. According to Breck (18), for hydrated zeolite X, cations are found in sites SI, SI, SII, and SIV. Of these sites, SI, SII, and SIV would all be adsorption lattice solution sites. The cationic and anionic lattice solution sites (in the supercavity of NaX) are illustrated in Figure 8. For NH3, the subscript J1 will refer to SII sites, the subscript J2 will refer to SI sites, and J3 will refer to SIV sites. The anionic sites are two and are (l) in the center U-membered ring of the connecting frame and (2) near the center of the 0(2)—0(1)—0(l) triad of oxygen atoms. For NH3, the subscript il will refer to the first anionic site the subscript i2 will refer to the second anionic site. 

In his monograph (9), Rabo reported the first studies of solid-state reactions between zeolites, mainly Y zeolite, and some salts. These studies revealed either ion exchange or more or less reversible occlusion of the salt. In cases of occlusion, the salt anion (halide, nitrate, or oxygenated chlorine anions) was usually located in sodalite cavities. 

Treatments with hydrogen (at 480°C) resulted in complete and partial losses in activity (139) for impregnated and ion-exchanged catalysts, respectively. This could be correlated with the degree of reduction which was incomplete under these conditions for the ion-exhanged catalysts. It was therefore concluded that the active site involved Ni2+ ions bonded to the oxygen anions of the zeolite framework and that nickel metal was inactive in the temperature range 150-250°C. 

In the zeolites each lattice cation is tetrahedrally coordinated to four oxygen anions (see Fig. 4.57). Each oxygen anion shares two lattice cations. In smectite clays an octahedral layer, usually containing Al3+ or Mg2+, connects two tetrahedral layers. The tetrahedral layer can be considered neutral when the tetrahedral site contains a four valent cation. This is usually a Si4+ ion. A top view and side view are shown in Fig. 4.59. 

We also illustrate this for water protonation. The zeolite proton will attach to the water oxygen atom, The proton of water will bind to the basic lattice oxygen anion ... 

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