Adsorbents chabazite

Adsorbents Table 16-3 classifies common adsorbents by structure type and water adsorption characteristics. Structured adsorbents take advantage of their crystalline structure (zeolites and sllicalite) and/or their molecular sieving properties. The hydrophobic (nonpolar surface) or hydrophihc (polar surface) character may vary depending on the competing adsorbate. A large number of zeolites have been identified, and these include both synthetic and naturally occurring (e.g., mordenite and chabazite) varieties. 

Haase, F., Sauer, J., Hutter, J., 1997, Ab Initio Molecular Dynamics Simulation of Methanol Adsorbed in Chabazite , Chem. Phys. Lett., 266, 397. 

A new transition-state-searching algorithm was used to determine the mechanism for methanol condensation to form dimethyl ether within the microporous environment of the zeolite, chabazite, using periodic boundary conditions and density functional theory. An acid site in the zeolite produces MeOH2+ for nucleophilic attack by a second adsorbed MeOH molecule. 

The experimental entropies of adsorption were calculated after obtaining the free energies of adsorption at 0 = /% from the gas pressure in equilibrium with half the amount of adsorbate required to form the monolayer. The same principles were used to obtain the figure for the entropy of adsorption of O2 on unreduced steel. The values for carbon tetrachloride were taken directly from Foster s paper (4). The results for adsorption in chabazite were obtained from the work of Barrer and Ibbitson (15) with the slight modification needed to allow for the different standard states in the two phases used by them. The figures in the last column... 

The question of methanol protonation was revisited by Shah et al. (237, 238), who used first-principles calculations to study the adsorption of methanol in chabazite and sodalite. The computational demands of this technique are such that only the most symmetrical zeolite lattices are accessible at present, but this limitation is sure to change in the future. Pseudopotentials were used to model the core electrons, verified by reproduction of the lattice parameter of a-quartz and the gas-phase geometry of methanol. In chabazite, methanol was found to be adsorbed in the 8-ring channel of the structure. The optimized structure corresponds to the ion-paired complex, previously designated as a saddle point on the basis of cluster calculations. No stable minimum was found corresponding to the neutral complex. Shah et al. (237) concluded that any barrier to protonation is more than compensated for by the electrostatic potential within the 8-ring. 

In the fall of 1948, I was measuring the adsorption characteristics of numerous commercial adsorbents and of the natural zeolite, chabazite. Several uses for silica gel in air separation plants were identified. But the more we learned about chabazite, the more intrigued I became by its potential as a commercial adsorbent as well as its possible use in air purification and separation. I envisioned, as others had before me [1-5], major new separation processes based on a series of different pore size zeolites. The stumbling blocks were that (1) chabazite was the only known zeolite with seemingly practical adsorption... 

The small pore (8-ring) structures all adsorb oxygen but only the chabazite-types and levynite adsorb n-butane or n-hexane and the rate of adsorption is very strongly dependent on particle size. Larger adsorbates are completely excluded. The very small pore structures (6-ring) adsorb only water and exclude oxygen. 

Differential heats of adsoiption for several gases on a sample of a polar adsorbent (natural zeolite chabazite) are shown as a function of die quantities adsorbed in Figure 5 (4). Consideration of the electrical properties of the adsorbates, included in Table 2, allows the correct prediction of the relative order of adsoiption selectivity7 ... 

The first natural microporous aluminosilicate, i.e., natural zeolite, was discovered more than 200 years ago, and after long-term practical applications, the intrinsic properties of natural zeolites such as reversible water-adsorption capacity were fully recognized.13 41 By the end of the 19th century, during exploitation of ion-exchange capacity of some soils, it was found that natural zeolites exhibited similar properties some cations in natural zeolites could be ion-exchanged by other metal cations. Meanwhile, natural chabazite could adsorb water, methanol, ethanol, and formic acid vapor, but could hardly adsorb acetone, diethyl ether, or benzene. Soon afterwards, scientists began to realize the importance of such features, and use these materials as adsorbents and desiccants. Later, natural zeolites were also used widely in the field of separation and purification of air. 

In 1953, Barrel and Brook 19) observed that butyl chloride adsorbed upon the external surface of chabazite crystals reacted smoothly at room temperature to give HCl and oily polymers derived from isobutylene. Decomposition of isopropyl chloride over chabazite did not occur until 150 , however. Milton and Breck (20) demonstrated smooth dehydrochlorination of butyl chloride over clay-bonded CaA at 100-150°. These j8-eliminations may be assumed to have occurred on the external surface of the small pore zeolites. 

Fig. 6. (a) Adsorption isotherms of water vapour on a -90% faujasite-rich tuff (enriched sample) from Aritayn (north-east Jordan) (squares) a chabazite-rich tuff (47% chabazite, 16% phillipsite) from Vulsini (central Italy) (triangles) and a 44% clinoplilolitc-rich tuff from Palestra (north-east Greece) (circles), at 25°C [74], Non-adsorbent phases are not reported, (b) Adsorption isotherms of sulfur dioxide on the same materials (same symbols) at the same temperature. Filled symbols ammonia adsorption on Aritayn faujasite. 

Fig. 5. Differential heats of sorption in nature chabazite (4). = N2 = Ar Q = 02 <0> = CO = C02. See Table 2 for polarizability, dipole moment, and quadrupole moment values for the gases. Volume adsorbed is expressed as cm3 of adsorbate as liquid. To convert kj to kcal, divide by 4.184.

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