Chabazite, adsorption

Czepirski, L. Komorowska-Czepirska, E., and Cacciola, G., Adsorption equilibria and kinetics of water vapour on modified chabazite, Adsorpt. Sci. Technol., 14(2), 83-88 (1996). 

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. 

The low silica zeolites represented by zeolites A and X are aluminum-saturated, have the highest cation concentration and give optimum adsorption properties in terms of capacity, pore size and three-dimensional channel systems. They represent highly heterogeneous surfaces with a strongly hydrophilic surface selectivity. The intermediate Si/Al zeolites (Si/Al of 2-5) consist of the natural zeohtes eri-onite, chabazite, clinoptilolite and mordenite, and the synthetic zeolites Y, mordenite, omega and L. These materials are still hydrophilic in this Si/Al range. 

Weigel, O. and Steinhoff, E. (1925) Adsorption of organic liquid vapors by chabazite. Z. Kristallogr., 61,125-154. 

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. 

Figure 4. Isosteric heats of adsorption of argon on halozeolite and on Ca-chabazite (11)...

First molecular sieve effect (Adsorption on Chabazite, Weigel, Steinhoff) 1932 Molecular sieve porous material that acts as sieve on a molecular scale (Mac Bain)... 

It has already been mentioned that zeolites are shape selective with respect to molecular adsorption. This property relates to their micropores stmcture. The zeolite framework shows a limited flexibility, which is essential. For instance, Yashonath et al. have shown in their classical dynamic simulations study of molecular diffusion within zeolite micropore that the zeolite framework flexibility affects significantly diffusion when the molecules have a size comparable with the micropore size. To get an idea of the order of magnitude of this flexibility, one can consider the hybrid semi-empirical DFT periodic study of chabazite zeolite of Ugliengo et al. V They introduced in the unit cell of chabazite Br0nsted acidic sites which are known to induce an increase of the volume of around 10 This increase of the volume relates with the difference of volume between a Si04 tetraheron and a... 

Shah and coworkers also studied methanol adsorption in sodalite and, in contrast to the case of chabazite, the HB complex with two hydrogen bonds between the methanol and the zeolite lattice was found to represent the mini-... 

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. 

Fig. 13. Changes in energy of adsorption of a protonated molecule with respect to the gas phase for protonation of propylene in chabazite. Fads is the adsorption energy of propene in chabazite.

From the previous discussion, it follows that the intracrystalline volume in zeolites is accessible only to those molecules whose size and shape permits sorption through the entry pores thus, a highly selective form of catalysis, based on sieving effects, is possible. Weisz and coworkers 7) have conclusively established that the locus of catalytic activity is within the intracrystalline pores when Linde 5A sieve ( 5 A pore diameter) was used, selective cracking of linear paraffins, but not branched paraffins, was observed. Furthermore, isoparaffin products were essentially absent. With the same catalyst, -butanol, but not isobutanol, was smoothly dehydrated at 230-260°. At very high temperatures, slight conversion of the excluded branched alcohol was observed, suggesting catalysis by a small number of active sites located at the exterior surface. Similar selectivity between adsorption of n-paraffins and branched-chain or aromatic hydrocarbons is shown by chabazite and erionite (18). 

Comprehensive review papers on the sorption properties of natural zeolites can be found in literature [72,73]. Referring in particular to the main sedimentary zeolites, the last two columns in Table 2 show some structural features of interest for sorption applications. Chabazite, clinoptilolite, faujasite and mordenite, which couple reasonably large to large window sizes with wide inner volumes (except mordenite), appear the most suitable materials for adsorption processes. 

Fig. 6 (a, b), showing some adsorption isotherms of gas and vapours of environmental interest, on chabazite-, clinoptilolite- and faujasitc-rich tuffs [74], confirms the expectations. In fact, the best results are presented by the material containing the large-pore faujasite, which displays a normalised adsorption capacity roughly twice that of chabazitc-rich tuff capacity and even better, compared to the clinoptilolite-rich tuff. Adsorption rates appear rather high for all the three samples. As regards sulfur dioxide, the performances of the faujasite sample are still the most favourable, whereas those of the other two zeolites are comparable with each other. The results of ammonia adsorption on faujasite arc of interest, too. Adsorption capacity is, however, only about half of that displayed for sulfur dioxide and also kinetics appears slower. 

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. 

Differential heats of adsorption for several gases on a sample of a polar adsorbent (natural zeolite chabazite) are shown as a function of the 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 adsorption selectivity ... 

Zecchina et al. (2005a) studied the adsorption of hydrogen at 77 K and pressures up to 1 bar on a number of different zeolites with the chabazite structure (CHA). Based on previous work and their own observations they identified three characteristics deemed important in hydrogen uptake (i)... 

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