Pillared clay studies thermal stabilities

Brindley and Sempels (1), Vaughan et al. (2) and Shabtai (3) have shown that the experimental conditions of Al intercalation influences the physicochemical properties of the clay. The nature, amount and spacial distribution of the pillars change the thermal stability, texture and acidity of the pillared clays. For example, Rausch and Bale (4) have reported that the OH/Al ratio modifies the structure of the Al complex and that monomeric [Al(0H)x(H20)6-x] " or polymeric [A1i304(0H)24(H20)i2] species can be obtained. Clearfield (5) demonstrated that the polymerisation state of Zr species depends on the temperature, concentration and pH of the solutions. In any case, the height of pillars is largely controlled by the polymerisation state of the intercalated complexes. However, in order to maintain the accessibility of the inner surface, the density or spacial distribution of the pillars has to be controlled. This parameter has been studied by Flee et al (5), and Shabtai et al (7) for Al pillared clays and Farfan-Torres et al (8) for zirconium. 

Some attempts eonsisting of adding ammonium sulfate to the pillared montmorillonite suspension have been reported [4,5], These preparation methods show that adding sulfate ions decreases considerably the basal spacing without developing high acidity. Other studies attempted to prepare zirconium sulfate pillared clays by adding in situ sulfate. However, the solids obtained have a low surface area, low thermal stability [6] and low sulfate to zirconium ratios [7], Earlier, we have prepared a zirconium sulfate pillared clay by in situ... 

When the Zr concentration and the S04 Zr ratio increase, the formation of a polymeric phase is accentuated. For low S04 Zr ratio, the pillars in the intercalated clay fraction consist of non sulfated polycations. But when the zirconium concentration is low (0.025 mol/L) and the S04 Zr ratio is high, the intercalation of sulfated polycations occurs. These preparation conditions give a solid with a high textural thermal stability. Sulfate ions linked to Zr-pillars seem to be more stable than those incorporated in the polymeric phase. Those sulfate ions dispersed in the solid could generate an interesting catalytic behaviour that merits further study. 

The last decade has seen a growing interest for the study of pillared clays and several papers on the preparation and characterization of these microporous materials appear in the literature. The main emphasis was oriented towards the preparation of new systems or catalysts presenting advanced structural-textural properties and thermal stability (1). 

The thermal stabilities of their corresponding pillar intmlayered montmorillonite clay minerals (PILCs) were characterized through the use of powder X-ray diffraction, differential thermal analyses and surface area measurements. These relative stabilities were also found to increase in the order Gaia-PILC < Alia-PILC < GaAl -PlLC. The relative Brdnsted and Lewis acidities of the montmorillonite itself, and of the clay mineral pillared with the different polyoxocations have been examined through infrared pyridine sorption studies. 

Typically, the pillared structure of montmorilIonites can be destroyed either by heating at 675°C for lOh or at 730°C for 2h in presence of steam (12). In addition to a high thermal stability in air, the ACH-rectorite under study retain most of their surface and cracking properties even after steamaging at 760°C/5h with 100 steam in a fluidized bed. Table 3. Steaming may have affected the iron distribution (migration) in the clay silicate layers. 

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