Aluminum-oxygen zeolite

X-ray crystallography a diffraction technique employed to study the distances between atoms or ions in crystalline materials zeolite a mineral typically composed of silicon, aluminum, oxygen, and metals that is characterized by the presence of uniform pores of molecular scale... 

Acid-treated clays were the first catalysts used in catalytic cracking processes, but have been replaced by synthetic amorphous silica-alumina, which is more active and stable.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 ... 

Zeolite is sometimes called molecular sieve. It has a well defined lattice structure. Its basic building blocks are silica and alumina tetrahedra (pyramids). Each tetrahedron (Figure 3-1) consists of a silicon or aluminum atom at the center of the tetrahedron, with oxygen atoms at the four comers. 

As stated above, a typical zeolite consists of silicon and aluminum atoms that are tetrahedrally joined by four oxygen atoms. Silicon is in a +4 oxidation state therefore, a tetrahedron containing silicon is neutral in charge. In contrast, aluminum is in a +3 oxidation state. This indicates that each tetrahedron containing aluminum has a net charge of -1, which must be balanced by a positive ion. 

The isomorphic substituted aluminum atom within the zeolite framework has a negative charge that is compensated by a counterion. When the counterion is a proton, a Bronsted acid site is created. Moreover, framework oxygen atoms can give rise to weak Lewis base activity. Noble metal ions can be introduced by ion exchanging the cations after synthesis. Incorporation of metals like Ti, V, Fe, and Cr in the framework can provide the zeolite with activity for redox reactions. 

Hyperfine interaction has also been used to study adsorption sites on several catalysts. One paramagnetic probe is the same superoxide ion formed from oxygen-16, which has no nuclear magnetic moment. Examination of the spectrum shown in Fig. 5 shows that the adsorbed molecule ion reacts rather strongly with one aluminum atom in a decationated zeolite (S3). The spectrum can be resolved into three sets of six hyperfine lines. Each set of lines represents the hyperfine interaction with WA1 (I = f) along one of the three principal axes. The fairly uniform splitting in the three directions indicates that the impaired electron is mixing with an... 

According to the results of Ben Taarit and co-workers (76) and Neikam (77) Ce(III) Y zeolites will not form anthracene cation radicals but upon oxidation to Ce(IV) the radicals are readily formed. This experiment suggests that one role of oxygen during calcination may be to oxidize certain cations. The surface may be oxidized by molecules other than oxygen since the chlorination of 7-alumina by carbon tetrachloride considerably increases the sites responsible for the acceptor character. These sites, which oxidize perylene into the paramagnetic radical ion, have been attributed to biocoordinated positive aluminum atoms (78). 

The effect of probe molecules on the 27A1 NMR has attracted some attention recently. In particular, the determination of the quadrupole coupling constant, Cq, is a sensitive means to learn more about the bonding situation at the aluminum in acid sites, and how it reflects the interaction with basic probe molecules. If one of the four oxygen atoms in an AIO4 tetrahedral coordination is protonated, as in a zeolitic acid site, the coordination is somewhat in between a trigonal and a tetrahedral A1 environment [232]. The protonated oxygen decreases its bond order to A1 to approximately half of its size compared to an unprotonated zeolite. 

X-ray studies carried out by Gallezot et al. (46) on a 53 percent EDTA-dealuminated Y zeolite, have shown that the aluminum extraction does not leave any vacancies in the framework after calcination at 400°C in flowing, dry oxygen and nitrogen (46). It was suggested that a local re-crystall-ization of the framework occurs even in the absence of steam. The silica necessary for the process presumably originates in the destroyed surface layers of the crystallite and diffuses into its interior. 

Zeolite structures typically consist of silicon and aluminum finked by tetrahedrally coordinating oxygen atoms. However, similar structures as found for these aluminosilicates can be formed by substitution of the aluminum by other elements (e.g., Ga in gallosilicates or Ti in titanosilicates). Even the substitution of both Si and A1 is possible, as for example in aluminophosphates or... 

Martra and coworkers studied low-temperature CO and room-temperature CO2 adsorphon on Na- and Ba-exchanged X and Y zeolites [138]. They were able to determine from the CO adsorphon IR data that Ba ions located in the Sn sites in both X and Y zeolites exhibit higher Lewis acid strength compared to Na ions in the same sites. Furthermore, the Lewis acid shength of both Ba and Na ions was dependent on the framework aluminum content of the zeolite. Using CO2 as a probe, they found that subshtuhng Na with Ba caused a reduchon in the basicity of the framework oxygens in proximity to the cahons. 

Different results are obtained when NH4Y is activated at higher temperatures (600°C). Vedrine et al. (266) showed that y irradiation in vacuo of such an activated zeolite leads to two types of signals. The signal with g = 2.0125, g = 2.0030, and a 12-line hyperfine structure with A aiso = 10.0 G was attributed to a positive hole (V center) trapped on an oxygen bridging two aluminum atoms ... 

It is interesting to note than an 11-line hyperfine structure with aiso = 10 G was observed at gjm = 2.007 when the Al-exchanged HY zeolite was irradiated in vacuo. This hyperfine structure was reported to be reversibly broadened by oxygen (103). Although the hyperfine structure appears to be similar to that observed by Vedrine et al. (266) for the V center associated with two aluminum atoms, it does not lead to the formation of OJ on adsorption of oxygen. 

Zeolite molecular sieves are composed of silicon and aluminum and can be natural or manmade minerals. Molecular sieves are crystalline, hydrated aluminosilicates of (most commonly) sodium, calcium, potassium, and magnesium. The alumininosilicate portion of the structure is a three-dimensional open framework consisting of a network of A104 and Si04 tetrahedra linked to each other by sharing all of the oxygens (Sherman, 1978). Zeolites may be represented by the empirical formula... 

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