Zeolites oxygen

Adsorption Equilibrium Numerous purification and recovery processes for gases and liquids Activated carbon-based applications Desiccation using silica gels, aluminas, and zeolites Oxygen from air by PSA using LiX and 5A zeolites... 

ZSM-5, 228, 229, 246, 247-248 Zeolites, oxygenations in Wagnerova Class I intrazeolite photooxygenation, 233-253... 

Xenon has been considered as the diffusing species in simulations of microporous frameworks other than faujasite (10-12, 21). Pickett et al. (10) considered the silicalite framework, the all-silica polymorph of ZSM-5. Once again, the framework was assumed to be rigid and a 6-12 Lennard-Jones potential was used to describe the interactions between Xe and zeolite oxygen atoms and interactions between Xe atoms. The potential parameters were slightly different from those used by Yashonath for migration of Xe in NaY zeolite (13). In total, 32 Xe atoms were distributed randomly over 8 unit cells of silicalite at the beginning of the simulations and calculations were made for a run time of 300 ps at temperatures from 77 to 450 K. At 298 K, the diffusion coefficient was calculated to be 1.86 X 10 9 m2/s. This... 

June et al. (12) used TST as an alternative method to investigate Xe diffusion in silicalite. Interactions between the zeolite oxygen atoms and the Xe atoms were modeled with a 6-12 Lennard-Jones function, with potential parameters similar to those used in previous MD simulations (11). Simulations were performed with both a rigid and a flexible zeolite lattice, and those that included flexibility of the zeolite framework employed a harmonic term to describe the motion of the zeolite atoms, with a force constant and bond length data taken from previous simulations (26). 

Hope et al. (116) presented a combined volumetric sorption and theoretical study of the sorption of Kr in silicalite. The theoretical calculation was based on a potential model related to that of Sanders et al. (117), which includes electrostatic terms and a simple bond-bending formalism for the portion of the framework (120 atoms) that is allowed to relax during the simulations. In contrast to the potential developed by Sanders et al., these calculations employed hard, unpolarizable oxygen ions. Polarizability was, however, included in the description of the Kr atoms. Intermolecular potential terms accounting for the interaction of Kr atoms with the zeolite oxygen atoms were derived from fitting experimental results characterizing the interatomic potentials of rare gas mixtures. In contrast to the situation for hydrocarbons, there are few direct empirical data to aid parameterization, but the use of Ne-Kr potentials is reasonable, because Ne is isoelectronic with O2-. 

Figure 16 shows that the reaction coordinate for exchange involves two zeolite oxygen atoms, one which acts as a proton donor (acid) and the other as a proton acceptor (base). Both exchangeable hydrogen atoms lie approximately halfway between the carbon atom and their respective oxygens. Comparison of literature geometries indicates that the precise... 

We may conclude that the divalent cobalt ions move out into the large cavities upon adsorption of NH3 to form a hexacoordinate cobalt(II)-ammonia complex. Following adsorption of 02 in the ammoniated Co(II)Y zeolites, oxygen enters the coordination sphere of the Co2+ ions. This is accompanied by a charge-transfer process to form a [Co(III) (NH3)502 ]2+ complex. The general intermolecular redox process can be approximated by the reactions... 

Researchers attempted to find correlations between the composition of the unit cell and the selectivity for paraxylene. In this respect, D. Barthomeuf (9) proposed an approach based on Sanderson s intermediate electronegativity which allows us to estimate the basicity of zeolite oxygens, and hence the strength of the acid-base interaction between xylene molecules and zeolites. It should be noted that these calculations provide an insight into the interactions between the zeolite structure and the molecules at low loading only, i.e. when the interactions between adsorbed molecules are negligible. 

In zeolites, this barrier is even higher. As discussed in Section II.B, the lower acid strength and the interaction between the zeolitic oxygen atoms and the hydrocarbon fragments lead to the formation of alkoxides rather than carbenium ions. Thus, extra energy is needed to transform these esters into carbonium ionlike transition states. Quantum-chemical calculations of hydride transfer between C2-C4 adsorbed alkenes and free alkanes on clusters representing zeolitic acid sites led to activation energies of approximately 200 kJ/mol for isobutane/tert-butoxide (29), 230-305 kJ/mol for propane/sec-propoxide, and 240 kJ/mol for isobutane/tert-butoxide (32), 130-150 kJ/mol for ethane/ethene (63), 95-105 kJ/mol for propane/propene, 88-109 kJ/mol for isobutane/isobutylene, and... 

Figure 3. Hexane occlude in a mordenite zeolite micropore. The hydrocarbon molecule can only interact with zeolitic oxygen atoms.

For the isomerization of toluene via methoxy and benzene intermediate, the similar proton activation, which results in the formation of a phenoxy intermediate, is achieved (see Figure 9). As previously, the bond between the zeolitic oxygen atom and the aromatic carbon atom stretches out. A "free" Wheland complex is eventually reached which can reorient to favor the position of the toluene methyl group with the demethylation transition state. 

The geometry of this transition state can be described as a methenium carbocation sandwiched in between a benzene molecule and a deprotonated Bn0nsted site. The methenium ion is planar, and its carbon atom is located along the line defined by the zeolitic oxygen atom to which it will be bonded and the aromatic carbon atom to which it was bonded. The activation energy which is required to achieve this reaction is act = + 279 kJ/mol. 

Figure I. Location of a sodium cation at a model zeolite six-ring. Zeolite oxygen centers directed toward inside and outside the cluster are also shown.

Some of the most thoroughly characterized supported metal complexes are zeolite-supported metal carbonyls. These have been prepared, for example, by the adsorption of Rh(CO)2(acac) on zeolites (e.g., the faujasite zeolite NaY [26] or dealuminated zeolite Y [27]) followed by CO treatment of the resultant material (Fig. 19.3). The IR spectra (not shown, but found in [26, 27]) of the rhodium dicarbonyl represented in Fig. 19.3 are consistent with a square-planar complex (formally Rh(I)) with the Rh atom bonded to two zeolite oxygen atoms. 

In a hydrated sample, Cu(Il) is octahedrally coordinated to three zeolitic oxygen and three water molecules. This species most likely exists in site I in Figure... 

In summary, if the framework structure is analogous, the ion-exchange sites in SAPO-n are generally similar to those of aluminosilicate zeolites, i.e., the sites are octahedrally coordinated to three zeolitic oxygens in the framework of SAPO. However, the number of studies on the ion-exchanged sites in SAPO-n or MeAPO-n is limited but... 

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