Zeolite Type water isotherm

The N2 adsorption-desorption isotherms of dried chitosan gel and chitosan-zeolite composites are reported in Figure 4 (a). Dried chitosan gels present a surface area lower than 5 m2 g"1 and virtually no porosity, the evaporation of water having brought about the coalescence of the polymer fibrils. The composites with a small amount of zeolites (less than 8 % for the zeolite X composite) present a type 4 isotherm leaning towards... 

This work prompted a flurry of activity in the mid- to late 1980s to find the type IM isotherm. A number of inventions can be found in which alumina, or silica gel are blended with zeolites type X or Y to mimic the shape of the isotherm that Collier defined. Mol Sieve type DDZ-70(g) is in fact one of only a few true type IM isotherms. This product and Engelhard s type ETS-10 both have the required isotherm shape for water and deliver the benefits expected, to wit excellent capacity for water, self-sharpening mass transfer zone and low energy investment required to regenerate. Mol Sieve type DDZ-70(g) is used commercially in rotors... 

Fig. 5 (a) shows the nitrogen adsorption isotherms of aluminum hydroxy pillared clays after heat-treatment at 300-500°C. These are of the typical Langmuir type isotherm for microporous crystals. Fig, 5 (b) shows the water adsorption isotherms on the same Al-hydroxy pillared clays [27]. Unlike the water adsorption isotherms for hydrophilic zeolites, such as zeolites X and A, apparently these isotherms cannot be explained by Langmuir nor BET adsorption equations the water adsorption in the early stages is greatly suppressed, and shows hydrophobicity. Water adsorption isotherms for several microporous crystals [20] are compared with that of the alumina pillared clay in Fig. 6. Zeolites NaX and 4A have very steep Langmuir type adsorption isotherms, while new microporous crystals such as silicalite and AlPO -S having no cations in the...

Isotherm I is typical of adsorption in micropores, which occurs, for instance, in zeolites and in activated carbons. Isotherm II represents multilayer physisorption on a flat surface (valid for many nonporous substances). Type HI and V isotherms are characteristics of weak gas-solid interactions (water adsorption on gold and bromine adsorption on silica are good examples). The type IV isotherm is frequently found in the study of heterogeneous catalysts its shape is characteristics... 

Type I isotherms [3] are char8u terized by an as)onptotic approach to a saturation cap8u ity with increasing pressure. This class of isotherms is most commonly observed for gases or vapors (water is an exception) adsorbed in zeolites or activated carbon. A t3rpical set of Type 1 adsorption isotherms is shown in Fig. 1. Several questions may be asked about sets of isotherms like these. How is the saturation capacity measured Is the saturation capau i1y a constant or does it decrease with temperature as suggested by Fig. 1 ... 

The water and n-hexane adsorption isotherms of the zeolitic mesoporous materials obtained are compared to that of a 4S0 nm colloidal silicalite-1 in Figure 5. The water adsorption isotherms are distinctively type HI, whereas the n-hexane isotherms are type 1. The lowest water isotherm was for the colloidal silicalite-1, where the first point measured for the n-hexane isotherm was already at 80 mg g. The amount of n-hexane adsorbed reached 2S0 mg g at high pressure, which roughly corresponds to the filling of silicalite-1 micropores. 

Figure 1 also shows the pure water vapour adsorption isotherms on UOP 13X zeolite and a sample of Davison silica gel at 30°C. These data were measured at Air Products laboratories. They are, respectively, type I and IV in shape. The zeolite adsorbs water very strongly (very high capacity at low x). The water adsorption capacities of the silica gel is comparable to those of the aluminas at low x but it exhibits higher water adsorption... 

Zeolite could selectively adsorb A -nitrosamines in the solution of methylene chloride or water, and the equilibrium data were fitted to Freundlich-type isotherms. Textural and acid-basic properties of zeolite determined their adsorption capacity. The extraordinary adsorption properties of NaA zeolite for iV-nitrosamines is inferred that the adsorbates inert the channel with the group -N=N-0. Larger amount of A-nitrosamines was adsorbed on ZSM-5 zeolite in water instead in methylene chloride, due to the hydrophobicity of the zeolite. Application of zeolite to remove A-nitrosamines from beer seems successful. Up to 100% of the worst carcinogenic compounds could thus be removed with 1.4 g/L of zeolite which was proven to be better adsorbent than silica or alumina. On Na 3 zeolite and MCM-41 mesoporous material A-nitrosodimethylamine decomposed above 573 K and the liberated NOx could be detected even at 773 K during the TPSR process, indicating the strong adsorption of A-nitrosamines on molecular sieves that makes zeolite become the functional materials for environmental protection. 

Compound beds of alumina and zeolite X have been employed successfully in industrial dehydration by PSA. In both types of applications, the more favorable shape of the zeolite isotherms shorten the mass transfer zone and simultaneously allow for achievement of lower mole fractions of water or lower dew points for the product gas. 

All zeolites have a highly hydrophilic surface and are very efficient desiccants. Contrary to other nonzeolitic desiccants such as silica gel and activated alumina, zeolite adsorbents have type I adsorption isotherms for water—i.e., a high water adsorption capacity at a low concentration of water. To obtain extremely dry gases and liquids, therefore, zeolite adsorbents are strongly preferred over amorphous desiccants. The 3A mo-... 

In a joint work with A. A. Isirikyan with the participation of G. U. Rakhmat-Kariyev, we carried out direct measurements of differential heats of adsorption of water vapors on crystalline and molded zeolite NaA at 22 °C using a Tian-Calvet-type calorimeter. The calorimetric installation enabled us to measure thermal effects for each point of the adsorption isotherm for a period of 300 hours and more (Figure 1). The squares and circles in the upper part of the graph denote our data for... 

The main goal of the present paper is to outline how could be different the structural and dynamic properties of water phases confined in two model hydrophobic porous materials, the mesoporous MCM-41 material and the microporous AIPO4-5 zeolite, althou their sorption behaviours are characterized by the same type V of adsorption isotherm. 

The adsorption and desorption isotherms of water vapor are drawn at 25°C for dealuminated HY zeolites upon framework Si/Al ratio. The isotherms are compared to that of the parent NaY zeolite. The isotherm changes in shape from the type I to the type IV with an hysteresis loop changing from the type H4 to the type H2, as increases the Si/Al ratio. The POLANYI-DUBININ theory is used to determine the micropore volume accessible to water. It decreases with increasing Si/Al ratios, down to zero at a Si/Al ratio of 35. Such a result is accounted by the adsorption on the hydrophilic centers which are the cations (H ) associated with the structural aluminium ions, each cation being coordinated by 8H2O. 

Adsorption and desorption isotherms of water vapor on HY zeolites change in shape with increasing dealumination. Thus, for only steamed samples the isotherm is close to the type I whereas for zeolites submitted to an additional acid leaching, it is close to the type IV with a progressive lowering of the point B towards the pressure axis, what emphasizes a more and more hydrophobic character of adsorption. The micropore volume accessible to water and the unit cell parameter are simply correlated with the framework Si/Al ratio. The water molecule does not allow to determine quantitatively structural and secondary pore volumes, but appears to be a selective molecular probe of the structural aluminium ions. For HY zeolites and without any restraint, 8H2O are associated with such a framework aluminic site. 

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