Water, zeolites

Figure C2.12.8. Schematics of tlie dealumination of zeolites. Water adsorbed on a Br( msted site hydrolyses tire Al-O bond and fonns tire first silanol group. The remaining Al-0 bonds are successively hydrolysed leaving a silanol nest and extra-framework aluminium. Aluminium is cationic at low pH.

Meunier, F., Second law analysis of a solid adsorption heat pump operating on reversible cascade cycles application to the zeolite-water pair. Heat Recovery Systems, 1985, 5, 133 141. 

The gases used were purchased premixed in aluminum cylinders to avoid carbonyl formation. The high purity gas mixture was further purified by a zeolite water trap and a copper carbonyl trap. The gas pressure in the reactor was measured with a capci-tance manometer and the fTow monitored with a mass fTow controT-ler. The typical gas flow rates were 15 cc/min (STP) and the maximum conversion was 1% based on integration of hydrocarbon products. The hydrocarbon products were analyzed by gas chromatography (temperature programmed chromosorb 102, FID). 

Chemically bound water, or water of crystallization, is divided into two main categories, bound water and zeolite water. They are distinguished by the effect upon... 

Alefeld [19] worked with closed Zeolite/water systems. They reached storage capacities of about 250 kWh/i related to the dry mass of Zeolites in their experiments. The change in the water uptake was about 25% of the Zeolites dry mass. The load levelling within a district heating net was identified... 

Gopa et al. [23] reported about open Zeolite/water systems for the storage of solar heat. In their work they were presenting stability tests, methods for the definition of the adsorption enthalpy in dependence on the adsorbed amount of water and the possible heating power during the discharging process. 

Gopal, R., B.R. Hollebone, C.H. Langford and R.A. Shigeishi, 1982. The rates of solar energy storage and retrieval in a zeolite water system, Solar Energy, 28, 421-424. 

Table 2. Variation in cooling COP of a zeolite - water regenerative cycle with evaporating, condensing, adsorption rejection and maximum desorption temperatures 0°C, 40°C, 50°C and 350°C respectively.

In general, the 2 1 clays are not very simple systems in which to study the interaction of water and surfaces. They have complex and variable compositions and their structures are poorly understood. Water occurs in several different environments zeolitic water in the interlayer regions, water adsorbed on the external surfaces of the crystallites, water coordinating the exchangeable cations, and, often, as pore water filling voids between the crystallites. Thus, there are many variables and the effects of each on the properties of water are difficult to separate. 

Isomaltose from isomaltose, glucose Dealuminated-beta zeolite Water/ethanol [228]... 

E., Hu, W., and Ni, L. (2005) Selechve adsorphon of p-chloronihobenzene from aqueous mixture of p-chloroni-trobenzene and o-chloronitrobenzene using HZSM-5 zeolite. Water Res., 39, 1174. 

R. Gans Discovered that zeolites could be used to soften hard water. He also invented processes for synthesizing zeolites and designed the equipment—the zeolite water softener— used for the recovery of gold from sea water. 1905... 

Zeolites are tectosilicates and their structure is based upon the feldspar model. The major chemical and structural substitution is M+A1 = Si, i.e., silicon replaces aluminum plus one (or in the case of calcium) ion. Usually the exchange of one ion for two is accompanied by an increase of free, zeolite water in the structure. As this substitution increases in importance, the structure becomes more "open" and more variable substitutions can take place, for example NII for K+, and so forth. 

Tihe presence of exchangeable cations, framework oxygens, cavities of A different sizes, and previously adsorbed water molecules makes the interaction of water with zeolites complicated (1). As a result, in zeolites, water molecules with different physical properties exist, from tightly bound to liquid-like water. This was shown by NMR measurements (2,3 4) ... 

The properties of zeolitic water and the behavior of the exchangeable cations can be studied simultaneously by dielectric measurements (5, 6). In X-type zeolites Schirmer et al. (7) interpreted the dielectric relaxation as a jump of cations from sites II to III or from sites II to II. Jansen and Schoonheydt found only relaxations of cations on sites III in the dehydrated zeolites (8) as well as in the hydrated samples (9). Matron et al. (10) found three relaxations, a, (, and 7, in partially hydrated and hydrated NaX. They ascribed them respectively to cations on sites I and II, on sites III, and to water molecules. 

Includes hydroxyl water. b Molecular zeolitic water only. 

In the Lenn -Guinier heating camera the total heating time was 30 hr. Changes in unit cell dimensions with temperature are shown for zeolite L in Figure 1. An increase in a was often accompanied by a decrease in c and at 500° C, the temperature at which zeolitic water was lost, in all forms save those bearing Li and H the a and c dimensions had regained their... 

For H- and Li-enriched forms of zeolite L and for H-offretite the weight loss continued up to 1000° C but for other forms well defined limits of weight loss had occurred before 500° C. In the H-zeolites this indicates dehydrox-ylation subsequent to the loss of zeolitic water. In H-offretite the weight loss also includes that caused by removal of N(CH3)4 cations. In Li-L some hydrolysis yielding LiOH seems possible, giving Li20 at elevated temperature. 

Since the presentation of this model new data have appeared which allow various tests and new conclusions. The diffusion coefficients of Karger (14), together with Equation 1 and the median jump time from the relaxation data at room temperature yield a jump distance of 2.7 A for the zeolitic water as compared with 2.2 A in bulk water (see Table III for a data summary). One might be tempted to explain the jump distance in terms of some geometrical constant of the zeolite structure such as the distance between Sn and Sm ionic sites (40), but with the cages full of... 

Table III. Mobility of Bulk and Zeolitic Water Compared at 25°C...

Finally, the 29Si CP/MAS-NMR spectrum of a partially dehydrated sepiolite that was subsequently exposed to acetone vapor is presented in Fig. 2c, and is strikingly similar to the spectrum of the original, untreated sepiolite (Fig. 2a). Since zeolitic water molecules are not present in this sample, and in light of the discussion of the partially dehydrated sepiolite sample, it appears that the acetone molecules have penetrated inside the microporous channels and reversed the structural changes that were caused by partial dehydration. Thus Fig. 2c confirms that acetone molecules enter the microporous channels of sepiolite, and are not simply adsorbed on the crystallite exterior surfaces. 

Therefore, it seems that tridireotional zeolites HY and HB are the most promi sing ones for liquid phase reactions. In the case of HB zeolites, two items deserve special comments. Firstly, the yield (82%) of ethyl phenylacetate for the equimolar esterification of phenylacetic acid and ethanol in the presence of the fl-10 sample is substantially higher than that of the equilibrium (69%) at the same temperature and solvent (Table 3). Analogous results have been already observed with dealuminated acidic Y faujasites and can be due to zeolite water adsorption and/or to the hydrophobicity of the in surfaces (ref. 2). The hydrophobic character of high silica... 

The size-exclusion and ion-exchange properties of zeolites have been exploited to cause electroactive species to align at a zeolite—water interface (233—235). The zeolite thus acts as a template for the self-organization of electron transfer (ET) chains that may find function as biomimetic photosynthetic systems, current rectifiers, and photodiodes. An example is the three subunit ET chain comprising Fe(CN )g anion (which is charge-excluded from the anionic zeolite pore structure), Os( bipyridine (which is an interfacial cation due to size exclusion of the bipyridine ligand), and an intrazeolite cation (trimethylamino)methylferrocene+ (F+ ). A cationic polymer bound to the (CN)6 anion holds the self-assembled structure at an... 

Prior to sorption measurements, zeolite samples were activated by evacuation at elevated temperatures. There is frequently some question as to how precisely one can establish the mass of a zeolite sample from which all zeolitic water, but no water arising from collapse of structural hydroxyl groups, has been removed (l f ). In order to establish that the (zeolitic-water-free) masses of the activated zeolite samples used here are well defined, the following stepwise activation procedure was used. Each sample was first heated in vacuo at 300°C. When the pressure had dropped to below about 10 torr, the balance was isolated from the pumps, the rate of pressure increase measured, and evacuation resumed. This process was repeated until the rate of pressure increase fell to below 5 X 10 torr min l, a duration of time which was from 15 to 30 minutes. This is a rate such that were the increase due to water vapor alone, and were the rate to remain constant, the weight loss would still be undetectable after 2h hrs., a duration seldom exceeded in activating zeolites. 

Groffman, A., Peterson, S., and Brookins, D. (1992). Removing lead from wastewater using zeolite. Water Environ. Tech., 5, 54. 

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