Textures pillared clays

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. 

The activated carbons used in this work present a wide distribution of acid groups and some of which exceed in strength those existing on acidic zeolites and pillared clays, although they are not always accessible due to the presence of constrictions in the pore network. As a consequence of this, kinetic re-arrangements are produced. Thus, not only a large number of chemical surface groups is desirable, but also a proper textural structure that facilitates the access to them. 

In order to improve the textural properties of particle-clay nanohybrids, bulky organic cations are intercalated as a kind of template into particle-intercalated clays before stabilization procedures. Intercalation of the organic cations results in the removal of some of the intercalated nanoparticles and/or in their rearrangement. Subsequent calcination leads to formation of additional pore space that is highly correlated to the geometry and size of the templates. This technique allows fine tuning of textural properties in the preparation of particle-clay nanohybrids. The clay nanohybrids intercalated with metals, oxides, and complexes have a broad range of applications. In particular, metal oxide particle-pillared clays have excellent potentials as catalysts, catalyst supports, selective adsorbents, etc. " ... 

The XRD patterns of zirconium sulfate pillared clays obtained after 90 hours of intercalation with different zirconium acetate concentrations using 0.5 as sulfate to Zr ratio and the same clay concentration as used earlier are presented in Fig. 5. The diffraction data show the appearance of two first order reflections. The first one is at 23.4 A for the lowest zirconium concentration and appears as a shoulder at the same distance for 0.05 mol/L concentration. The second reflection is observed at approximately 12.3 A for the lowest concentration and at 13.7 A for 0.1 mol/L zirconium acetate. The first one results from the intercalation of sulfated zirconium species. Those species are more voluminous than the non sulfated one which gives a distance spacing at only 19.6 A. The better intercalation of sulfated zirconium species at low Zr concentration is probably due to the slow progress of polycondensation reactions. This process reduces the number of different zirconium species and gives a better cristallinity of the solid. Table 2 summarizes the textural properties of samples prepared with different zirconium concentrations. The decrease of the surface area with the decrease of the Zr concentration is probably due to the increase of the sodium clay layers by comparison with the intercalated layers. The microporous volume increases when the Zr concentration decreases. The higher microporosity is due to the important basal distance of this sample. 

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). 

Because of the house-of-cards -structure, and the limited size and stacking of the platelets of all saponites prepared in this work, the specific surface areas and pore volumes are extremely high, which is favourable for application in organic reactions. Except for Zn-saponites, all surface areas and pore volumes are comparable (Co-saponite) or even higher (Ni- and Mg-saponite) than those reported thus far for pillared clays [10]. Calcination of the synthetic saponites at 450°C for 4 hours do not affect strongly the textural properties the BET surface area and pore volume decrease by 1 to 10 percent. 

Timofeeva, M., Khankhasaeva, S., Chesalov, Y., et al. (2009). Synthesis of Fe,Al-pillared Clays Starting from the Al,Fe-polymeric Precursor Effect of Synthesis Parameters on Textural and Catalytic Properties, AjPjpZ. Catal. B-Environ., 88, pp. 127-134. 

A comparative study of the structure developed by two alumina-pillared clays using the results of adsorption from nitrogen, hydrogen and carbon dioxide is reported. The samples considered come from a montmorillonite and a saponite that have been treated with solutions of hydrolysed aluminium. The solids resulting of the intercalation have been calcined at 473, 623 and 773 K for 4 h. The textural properties of the samples derived from the results of nitrogen adsorption at 77 K, show a decrease of the specific surface area and the micropore volume as the temperature of calcination increases. Results from hydrogen adsorption indicate the presence of micropores of widths smaller than the diameter of the molecule of nitrogen, while carbon dioxide adsorption reveals the presence of specific sites of chemisorption. 

The complementary use of nitrogen and carbon dioxide adsorption to charactaj microporous materials as activated carbons has been recommended by various auttmrs [12,13], In the case of pillared clays, little work has been done using carbon dioxide as adsorbate. In a previous work [14], we found that the adsorption of carbon dioxide and that of nitrogen are not sensible to the same type of microporosity. It is also necessary to take into account the possibility of chemisorption when carbon dioxide is used as adsorbate. In this way and from the results presented in this work, there is an effect between the temperature of calcination of the samples (GAmont-Al) and the adsorption of carbon dioxide that cannot be explained from a modification of the texture of the pillarai clays the nature of the surface sites created at 473 K would merit further research. 

The microporous structure of the PILCs is characterised by the distance between the clay layers and the distance between the intercalated units (pillars), the interlayer spacing and the interpillar spacing respectively [6]. Several phenomena can influence these distances such as the elimination of organic moieties from the polycations and the dehydration or sintering of the pillars. Regarding the MPSDs presented in Figures 2-4, more information about the distribution of the aluminium fixed and the evolution of the textural properties with the calcination temperature is required in order to achieve a better knowledge of the microporous structure of these solids. 

Chromia pillared and pillared-delaminated clays have been synthesized from different montmori I Ionites and characterized by a variety of methods. Chromia-sulfide pillared materials show a high activity and selectivity in thiophene HDS and the consecutive hydrogenation of butene. The use of different clays as starting materials for the preparation of Cr-PILC enables control of their textural properties and chromium loading and thus to tailor the activity of these catalysts. 

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. 

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