Zeolite water loss

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. 

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. 

Earlier discussion introduced the concept of using the characteristic truncated octahedral elements of the sodalite framework to explain the molecular architecture of the synthetic zeolites X and Y (see Section. 2.4.3). There are other structural correlations that can be drawn between felspathoids and zeolites, for example, that the cancrinite cage (11-hedron) is a face-sharing element seen in the LTL, ERI, OFF, and EOS frameworks. Furthermore, in nature, salt ion pairs are contained in felspathoid minerals and when these are removed the residual framework exhibits the zeolitic properties of ion exchange and reversible water loss. Other similarities arise in that zeolites can imbibe salt ion pairs, and isotypic structures... 

The thermal behaviour of zeolites has thoroughly been investigated. When heated, a zeolite powder undergoes a series of physical and chemical changes, which include water loss, decomposition and gas evolution, phase transition, structure breakdown, re-crystallisation, melting, and others [75J. The thermal characterisation of natural zeolites has been carried out by various techniques and the relevant data may be found in several publications [44,76-78]. 

The residual zeolite does not manifest the same thermal properties as the initial zeolite. Its x-ray diffraction pattern is nevertheless the same. It loses its adsorption properties when an attempt is made to desorb it at 450°C for 2 hours. We limited ourselves to determining its water loss after saturation at 25 °C under 15 torr. Since grinding of the untreated zeolite does not change its adsorption properties and its structure as determined by x-ray diffraction, we shall attribute the lack of thermal stability of the residual zeolite to a change in composition owing to the formation of solid B possess the exact... 

This facile loss of water was shown to be reversible by Damour (1840) and, in 1858, Eichorn showed that the zeolite chabazite contained alkali and alkaline earth metals, which were capable of being reversibly replaced, that is, the zeolite exhibited cation-exchange properties. Analysis of zeolite minerals showed them to be aluminosilicates and their easy loss of water and cation exchange was evidence for the open nature of their structures, often likened to a sponge. The description zeolitic water has been widely used to describe loosely held water in any solid. 

Water molecules of different nature are normally present on needle-like clays. These can be well distinguished via thermo-gravimetric (TGA) test (Figure 12.6). By increasing the temperature up to 1000°C, four main weight losses are observed which can be attributed respectively—for ascending temperature—to the release of adsorbed and zeolitic water, the release of the first structural water, the release of the second structural water, and the dehydroxylation of the Mg-OH groups [19]. 

First peak appears at 51.38 8C, due to a weight loss of 1.81% of the initial weight. It corresponds to the departure of water (moisture or adsorption) due to attraction on the surface of the sample and zeolitic water inserted between the layers or in the cavities of the crystalline structure. Second peak which maximum appears toward 749.87 8C corresponds to the dehydroxylation. 

The thermogravimetric curve below 180° shows a weight loss of 9%, which may be linked to the loss of the hygroscopic or zeolitic water. A second loss of weight occurs between 200° and 350°. Lastly, a more progressive 6 % loss of weight takes place between 350° and 650°. 

If the structure of the mineral is now considered, the first loss of weight includes hygroscopic and zeolitic water. The second should affect the OH—H, and the third, the hydroxyls of the network. It is, therefore, possible that certain details of structure should be revised. 

If this behavior is related to structure, the first loss of water corresponds to hygroscopic water and to zeolitic water. It should be remembered that the zeolitic water is the one lodged in the channels of the structure. 

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