Dehydroxylation montmorillonite

Abbreviations used cristobalite (crist.) dehydroxylated montmorillonite (dehy. mont.), mata-kaolinite (metakaol.), montmorillonite (mont.), and trace (tr.). 

Montmorillonite-Calcite (me) Mixture. Heated mixtures of montmorillonite and calcite yielded the phases given in Table I. Although the montmorillonite structure persisted through 400 °C, it underwent dehydroxy lation between 400 and 500 °C. Grim and Bradley (16) have shown that the general layered structure is able to survive the elimination of the (OH) water with moderate readjustments. This structure produces an X-ray diffraction pattern like that given in Table III. Table III represents data close to those observed in this study. This phase is called dehydroxylated montmorillonite in Table I. This phase disappeared between 700 and 800 °C as a result of the complete destruction of the montmorillonite crystal structure. Calcite decomposed between 500 and 600 °C to form lime that was present through 900 °C. 

Anorthite began to crystallize at 800 °C and was observed at all temperatures through 1100 °C. Anorthite apparently formed by chemical reactions between lime and the dehydroxylated montmorillonite. 

Fig. 14. Compensation plot for dehydroxylation of kaolinite ( ) and other layer-type silicates (X = montmorillonite, illite and muscovite) data and sources given in Table 11. (Redrawn, with permission, from Advances in Catalysis, ref. 36).

Kodama and Brydon [631] identify the dehydroxylation of microcrystalline mica as a diffusion-controlled reaction. It is suggested that the large difference between the value of E (222 kJ mole-1) and the enthalpy of reaction (43 kJ mole-1) could arise from the production of an amorphous transition layer during reaction (though none was detected) or an energy barrier to the interaction of hydroxyl groups. Water vapour reduced the rate of water release from montmorillonite and from illite and... 

Montmorillonite-Dolomite (md) Mixture The crystalline phases formed by heating the md mixture at various temperatures (Table I) were similar to those found in the me mixture, with some variations. Montmorillonite dehydroxylated between 400 and 500 °C to form the phase shown in Table III and finally broke down completely between 700 and 800 °C. The dolomite decomposed to yield lime and periclase between 500 and 600 °C. These phases diminished by 900 °C and were not detected at higher temperatures, although theoretically they are stable at these temperatures. Anorthite formed at 900 °C and above. 

During the ablation experiment, temperature within the char layer exceeds 1000°C and approach 2000-2500°C at the surface. At these temperatures, any carbonaceous residue from the pol3oner will contain graphite. Additionally, mica-type layered silicates, such as montmorillonite, irreversibly transform into other aluminosilicate phases. Between 600 and 1000 C, montmorillonite dehydroxylates and has been observed to initially transform into spinel, cristobolite, mullite and/or pyroxenes (enstatite) (24). At temperatures greater than 1300 C, mullite, cristobolite and cordierite form and subsequently melt at temperatures in excess of 1500 C (mullite 1850 C, pure cristobolite 1728°C and cordierite --ISSO C) (25). The presence of an inorganic that transforms into a high viscosity melt on the surface of the char will improve ablation resistance by flowing to self-heal surface flaws. This is known to occur in silica-filled ablatives (26). 

Unmodified sodium montmorillonite (Na+MMT) displays good thermal stability in the temperamre range 20-500 °C, evolving only moderate quantities of physically adsorbed water at temperatures up to 120 °C [11, 12] and the water from hydrated ions in the temperature range from 85 to 183 °C [13]. The dehydroxylation of the crystal lattice of MMT was observed at temperatures above 500 °C, at which most commercially available polymers have already degraded. 

The results of TG analysis of montmorillonite and its hexylammonium derivative support this concept the weight loss in the latter case in the temperature range 550-710 C, (where the former undergoes dehydroxylation), reaches only 52% of that of the former, however, the expelled H2O was not detected in GC analysis. As a consequence, the Bronsted acid sites which might catalyze the interlayer reaction of -hexylamine diffusing out from the silicate lattice cannot be formed on the surface. 

In Fig. 3.58 is shown the integrated intensities of the band at 1540 cm" for pillared beidellite and montmorillonite against the outgassing temperature. Increasing the calcination temperature prior to pyridine adsorption results in a steep drop in the proton content in the case of pillared montmorillonite, while pillared beidellite keeps its acidity. The steep drop of the Bronsted acid sites observed for pillared montmorillonite was attributed to the fact that, upon thermal activation, the protons migrate into the octahedral layer of the clay, where they induce a premature dehydroxylation. Thus, the acidity is mainly of the Lewis type for samples treated at higher temperatures. A similar result was reported also for bentonites pillared with alumina clusters. ... 

Soils and clays, in general, when calcined give off adsorbed, interlayer, and hydrated types of water. These effects produce endothermal peaks or loss of weight in DTA and TG, respectively. The endothermal peaks are followed by exothermal peaks that are caused by re-crystalliza-tion. Although many types of clay minerals such as montmorillonite, illite, and some shales show these effects, they are not suitable as pozzolans in concrete. Metakaolin, formed by heating kaolinite, seems to be the most suitable additive material for cement. Heating of kaolinite involves removal of adsorbed water at about 100°C and dehydroxylation at above 600°C, followed by the formation of metakaolinite, an almost amorphous product. The sequence of reactions is as follows ... 

It shows pronounced thermal effects on heating and generally has a more ordered structure than other clay minerals. Figures I and 2 illustrate typical DTA data for kaolinite, halloysite, and montmorillonite. Kaolinite and halloysite lose their hydroxyls between 450 to 600°C. Variations within this range are attributed to differences in entrapped water vapor that is dependent on sample size and shape factors. The loss of hydroxyls from montmo-rillonites in the range of 450 to 650°C is t5q)ical for dioctahedral forms of these minerals. Dehydroxylation is more gradual for trioctahedral forms and can continue to temperatures up to 850°C. 

Multi-component clay-based chemistries involving reactions between clays and lime and pozzolans are of interest in the area of soil stabilization. Thermogramst for kaolinite and montmorillonite treated with lime are presented in Fig. 18. Addition of lime results in the gradual diminution of the primary kaolinite dehydroxylation peak (500-600°C) to a greater extent than can be accounted for by dilution alone. All samples have a small peak at about 130°C and a broad endothermic peak at about 210°C. The decomposition of carbonated lime is associated with endothermic reactions at 700-800°C. 

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