Zr-pillaring

The Li ions were introduced in two different ways either before or after Zr intercalation. The montmorillonite (Weston L-Eccagun) was first exchanged with NaCl (IN) and washed. Two montmorillonites with reduced charge were prepared following the Brindley and Ertem method (13). Part of the Na+ montmorillonite was first saturated with LiCl (IN) and washed. The Li+ clay thus obtained and Na+ clay suspension were stirred for 24 hours at 25°C and dried on glass plate. The films were then heated at 220°C for 24 h in order to allow Li diffusion in the clay structure. Two different Li concentrations (F=0.4 and F=0.6) were used. The Na Li+ modified montmorillonite were dispersed in water acetone solution (1/1). The ZrOCla, 8H2O solution was added to the Na+Li+ montmorillonite (0.02g.l l Zr/Clay=5.CEC). The suspension was stirred with NaOH solution (0.1 N) up to a OH/Zr ratio of 0.5. The final pH of the suspension was 1.85. After two hours of reaction at 40°C the Zr pillared clay was washed up to constant conductivity of the solution, freeze-dried and calcined at different temperatures up to 700°C (Eni-02 and EIII-03). 

The DRX spectra of the solids calcined up to 500°C are illustrated on fig. 3. The position of the d 001 diffraction line versus the calcination temperature for the Na+ montmorillonite (EIII-01) and the Zr pillared modified clays (EIII-02, EIII-03) is reported on fig. 4. 

A small increase of the (d 001) basal spacing is observed for the Li containing Zr pillared clays. However, the thermal stability of these solids drastically decrease. At high temperature, the collapse of the strucutre is also supported by the decrease of the surface area which is, at 700°C, almost identical to those measured for the montmorillonite. Different hypothesis may be proposed to explain the increase of the interlayer distance at low temperature (i) a better polymerization of the intercalated complex (ii) a modification of the distribution of the pillars (iii) a lower interaction between the pillar and the silica layer. The first hypothesis may easily be eliminated since the small variation of the height of the pillars (less than 1 A) cannot be explained by structural changes of the... 

This last assumption is strongly supported by the variation of interlayer distance of the Zr pillared after ethylene glycol saturation. 

The Li diffusion in the clay structure slightly enhances the acidity of the Zr pillared montomorillonite as shown by the variation of the amount of desorbed NH3 We also observed a parallel decrease of the Lewis and increase of the Brdnsted sites. 

The diffusion of Li+ in the octahedral cavities of the Na+montmorillonite allows to control the density of the pillars of the Zr pillared montmorillonite. The solids, stable up to 300°C, have larger surface area basal distancy than the pure Zir montmorillonite. The distance between the pillars increases while the interaction strength between the pillars and the clay layer decreases. 

However, the thermal stability of the Li-Zr pillared clays is drastically influenced after calcination at temperatures higher than 400°C. This is mainly due to Li acting as flux. 

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. 

Cool et al. (80,81) performed a templated synthesis of Zr-pillared laponite using ethylenediamine in an amount exceeding the CEC of laponite. Here, the amines not only have a positive influence on the pillar distribution, but also favor the parallel orientation of the clay sheets, resulting in a more homogeneously pillared structure with increased microporosity. Thus, on laponite, the main function of the template is not to block exchange sites, but to influence the stacking of clay layers. The porosity and adsorption characteristics of the different pillared clays are summarized in Table 6. 

Compared to the Al-pillaring, the Zr-pillaring of clays is less investigated and documented in the literature. Despite the many experiments, the nature of the... 

When dissolved in water, a rapid polymerization of the tetramers occurs. Clearfield suggested the following sequence of events during the aging or hydrolysis process of Zr-pillaring solutions [49] ... 

For the Zr-pillaring solution 0.1 M ZrOCl2" 8H2O is used. The aging process is also performed under reflux. The pH of the final solution is close to 1, i. e. very acidic due to polymerization. 

Heylen et al. reported a surface area and a micropore volume which was 2.5 times higher for an Fe-PILC synthesized with butylammonium as template, in comparison to the unmodified Fe-PlLC [27]. An important increase in the adsorption capacity towards Nj, Oj and CO at 194 K (Pgq=4.5x 10 Pa) has been observed after the modification. Adsorption capacities of 0.23 mmol/g, 0.17 mmol/g and 0.30 mmol/g for, respectively, Nj, O2 and CO were reached for the modified BuA-Fe-PILC. When compared to the adsorption on the unmodified Fe-PILC, the following capacities were obtained 0.00 mmol/g N2, 0.03 mmol/g O2 and 0.27 mmol/g CO. Also, it is proven that ammonium templates can be used to optimize the Zr-pillaring of laponite leading to enhanced pore volumes and surface areas. 

In order to adsorb organics selectively from aqueous solution, it is very important that the adsorbents are hydrophobic. Therefore, Shu et al. (68) investigated the adsorption of the same organic toxicants on a hydrophobic surfactant (Tergitol)-modified Zr-pillared montmorillonite. The surfactant-modified external surfaces have a high affinity for the organics, and this affinity is related to the surfactant loading. An adsorption capacity of phenol equal to 0.8 mmol g has been reached at an equilibrium concentration of 7 mg mL ... 

Recently, Quevedo et al. evaluated different solids as catalysts— a natural bentonite from Colombia, homogenized calcium bentonite, Al-pillared bentonite, a mixed oxide (Al203-Mg0, M3+/(M2++M +)=0.5), and an Al-Zr pillared vermiculite— in the Pictet-Spengler reaction between dopamine hydrobromide and aromatic aldehydes to synthe-tize the 1,2,3,4-tetrahydroisoquinoline [141]. The best yields, up to total conversion, were obtained when the mixed oxide was used as catalyst. It seems that this catalyst exhibited acid-base pairs due to the presence of sites... 

Increasing the pH of the pillaring solution by addition of NaOH increases the thermal stability of Zr pillared clays. The improvement may be diiwtly coirelated to the amount of Z1O2 and to the density of the pill. The strong interaction between the pillars and the clays promotes high acidity of this microporous solid. 

In this work we present the influence of the concentration of NaOH, introduced in two different ways, on the thermal stability, porosity and acidity of Zr-pillared montmorillonite. The basic solution was added either in the ZrOCl2 solution before pillaring (ex-situ) or after the Zr solution was contacted with the montmorillonite suspension (in-situ). 

Table 1. Zr pillared clays pH during the pillaring process, cationic exchange capacity and concentration of Z1O2 after pillaring.

The chemical composition of the Zr-pillared clays as well as the Na montmorillonite are reported in table 2. Silica, alumina and zirconia have been analyzed by X-Ray fluorescence the other elements by atomic absorption spectroscopy after sulfofluorhydric dissolution of the clay (6). 

All the Zr pillared montmorillonite present much higher total acidity than the Na noontmotillonite except for the solids EII-04. 

Fig. 4. Schematic representation of the architecture of the Zr pillared clays. CONCLUSIONS...

When the pH of the pillaring solution is adjusted at 3.9, Zr(OH)4 precipitates. The addition of NaOH to the Zi-clay suspension led to a mixed system in which bulk Z1O2 and Zr pillars are present. When the adjustment of the pH to 3.9 is done on the Z1OCI2 solution, no pillars are formed and most of the zirconium could be deposited as bulk Z1O2. 

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