Pillaring process

Physicochemical and morphological characterisations demonstated that the pillaring process of the original montmollorinite was effective. Table 1 shows the most salient characterisation results. 

The pillaring process also affected the concentration and the strength of acid sites, as confirmed by NH3-TPD (Table 1). Also, the ion exchange with Ni2+ cations modified the acid properties of surfaces new Lewis (nickel cations) and Bronsted (H+) acid sites have been created during the ion exchange and thermal activation (eq. 1), respectively. 

The basal spacings (d(OOl)) of the intercalated and pillared samples determined from the (001) reflection in the XRD powder patterns ranged from 18.4 to 23.1 and from 15.0 to 17.6 A, respectively. These values fit well into the ranges generally reported in the literature for alumina-pillared clays and indicate that the intercalating and pillaring processes have been succesfully accomplished in every case. 

Table 3 presents the micropore volume of the samples, calculated by the t-method, after thermal treatment at 400, 500, 600 and 700"C. A high micropore volume is generated with the pillared process in all the pillared samples, because the raw material has no microporosity. Again, this increase is greater in the Al/La- pillared samples than in the Al-Wy sample. 

Table 3 shows that the volume of micropores remains high in the Al/La-pillared samples after the successive thermal treatments. On the other hand, the thermal stability of the micropore volume developed after the pillaring process is lower in the Al-Wy sample, being reduced practically to zero after thermal treatment at 700 C, whereas in the Al/La-pillared samples it is 0.031-0.057 cmVg at 700°C. 

FIGURE 13.6 Schematic illustration of pillaring process in bidimensional clay (a) ion exchange with precursor cations and (b) conversion to oxide by calcination. (From Yamanaka, S., Design and synthesis of functional layered nanocomposites. Am. Ceram. Soc. Bull., 70, 1056, 1991. With permission.). 

Fe a-montmorillonites (Fe,Na-Mont) were prepared from natural Na-montmorillonite (Na-Mont) (Bentolite H, SCP Laport) by ion-exchange in iron chloride solution containing the same amount of iron as in e synthesis of Keggin ion applied in the pillaring process. Ion-exchange was performed at 353 K for 8 hours followed by washing the sample free of chloride. Finally, the solid material was separated by centrifogation and dried at room temperature. 

The features described above have been used for identification of the MCM-22 layers. Images containing the quality of detail shown in features A and B depend on well-ordered materials in near perfect orientation. The disruption caused to the material s structure by the swelling and/or pillaring process reduces the probability of observing such images in exfoliated materials. The layers in heavily exfoliated materials viewed edge-on usually resemble those shown in the features C. 

As will be clear from the following explanation, the pillar process has the same advantages, in terms of Si real estate, as has the tungsten plug process (see chapter I). The process steps have been described by Welch et al.184(see also Yeh et al.185 for pillar variations). In figure 8.10 we see a sketch of the complicated process flow. The starting metal layer consists of two AlCu films sandwiched between three TiW layers. On the stack a photoresist is spin coated in which the inverse via mask is printed (figure... 

The specific surface area (Table 1) shows an increase in BENa (homogenized montmorillonite) and in the pillared sample. The increase up to 247 m /g is related to the creation of micropores in the pillaring process. Of the zeolitic samples, those treated more severely (ZE--P) show values similar to that of the starting material, whereas the ZE—X samples present a slight increase. These results can be attributed to the formation in ZE--P of a structure with a pore size smaller than the N2 used in the isotherm analysis, whereas the resulting ZE—products have a more open structure. 

The zeolitic materials obtained by the alkaline treatment and pillaring process, produced new materials with a specific surface area higher than the raw materid and also with a higher acidity. 

For the most part, structural questions that arise in clays and pillared clays are essentially the same as those that arise in solid-state NMR of silicates and zeolites. The pillaring process, especially, has been of great interest, and structural issues related to pillaring (at least for (Ali V) as pillars) are nicely outlined by Fripiat 16],... 

Figure 4 Schematic flowsheet of clay pillaring process. (From Ref. 1.)...

Most studies of water in clays are not necessarily related directly to heterogeneous catalysis, but the state of the water in the interlayer is clearly important in the pillaring process. Woessner at Mobil pioneered this area 20 years ago. It was discovered that interlayer water in clays was undergoing anisotropic motion, the result being a partially motionally averaged Pake doublet spectrum (53-551. Under typical hydration conditions, many clays appear to maintain the equivalent of one or two monolayers of water between the clay sheets, so it is not surprising that the motion is restricted. 

M. Before pillaring, the solution is stirred for 3 h at 25 °C. The pillaring process consists in slowly adding the Na-montmorillonite (4 g.l i) to the hydrolyzed or partially hydrolyzed titanium solution (10 mmoles Ti/g day). After 12 h under permanent stirring at 25 °C, the pillared clay is washed and centrifuged (5000 rpm for 5 mn). The pillared montmorillonite is then dried and caldned at 500 °C. 

Clays are layered minerals with space between the layers where they can adsorb positive and negative ions as well as water molecules. Clays undergo exchange interactions of adsorbed ions with the outside too. Although clays are very useful for many applications, they have one main disadvantage i.e. lack of permanent porosity. To overcome this problem, researchers are looking for a way to prop and support the clay layers with molecular pillars. Most of the clays can swell and thus increase the space between their layers to accommodate the adsorbed water and ionic species. These clays are employed in the pillaring process. 

As described by these authors, the pillaring process consists of two... 

The pillaring process is an attempt to obtain thermally stable catalysts with a large interlayer space and a large porosity. 

Chlorhydrol is commonly used for pillaring processes, and in spite of the presence of other more or less condensed hydroxylated species it r( resents a convenient source of the polycation [AI13 O4 (OH)24 (H20)i2l (noted [AI13]). It is prepared by an acidic attack of A1 metal by HCl and does not contain anions such as N03 or C03 which may influence the cationic exchange. 

Plee, D., Gatineau, L., and Fripiat, J. J. 1987. Pillaring processes of smectites with and without tetrahedral substitution. Clays Clay Minerals 35 81-88. 

The description that we have given of most of the anionic clays corresponds to PLS. The intercalates of other lamellar compounds (graphites, clays, phosphates, phosphonates, oxides, oxy-halides, and chalcogenides) have been intensively studied for their pillaring properties. The primary reason for this interest is the possibility of engineering the pore sizes and distribution during the pillaring process. 

XRD patterns and surface area measurements provide the basic test of the efficiency of the pillaring process (Table 1). The changes observed in the dppi basal spacing and the increase in the surface area confirm that pillaring procedures were effective. 

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