Pillared clay studies

Pillared clays (MELS) are aLso covered in this review. MELS have three-dimensional network structure like zeolites, and, unlike clays, which have two-dimensional layered structures, pillared cationic and anionic clays have been studied. Phthalocyanins intercalated in anionic clays have given interesting results for wastewater purification (Vaccari, 1998). 

Auroux, A. (2002) Microcalorimetry methods to study the acidity and reactivity of zeolites. Pillared clays and mesoporous materials. Top. Catal., 19, 205-213. 

The acidic properties of alumina pillared clays have been extensively studied from the interest in using the pillared clays as cracking catalysts [21-24]. Sakurai et al. [25] studied the acidic properties of the alumina pillared clays with different kinds of silicate layers and concluded that the alumina pillars between the silicate layers did not have any acidity and that the role played by the pillars was only to make the original acidity of the silicate interlayers more easily accessible through opening the interlayer spaces. 

The acidity of the sol oxide pillared clays was also studied by a titration method with Hammett indicators [16], The acidity distributions of the three kinds of sol oxide pillared clays are shown in Fig. 4. The Ti02 pillared clay... 

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. 

Al3+-exchanged synthetic hectorite is a good catalyst for these conversions, and the 13C NMR spectrum obtained in the interlamellar, proton-catalyzed addition of water to 2-methylpropene is indistinguishable (Fig. 79) from that of f-butanol. Doubtless studies of this kind, where natural-abundance, 3C NMR signals are used to probe the chemical identity and motional freedom of reactant and product species situated in the interlamellar spaces of clays or pillared clays (see below), will become increasingly popular. Using l3C NMR linewidths and spin-lattice relaxation studies, Matsumoto et al. (466) have succeeded in discriminating between the internal and external surfaces of pillared montmorillonites. 

The gas phase isomerisation of o-dichlorobenzene (odCB) was studied over protonic zeolites HZSM5, HMOR, HMAZ, HOFF, HBETA and a pillared clay HPILC. All the catalysts deactivate. The deactivation rate is the highest when dry air is used as carrier gas, and the lowest with nitrogen... 

The similarities in catalytic reactivity between Cr3 53-montmorillonite and chromia supported on alumina suggest that the structure of the intercalated chromia particles may resemble the structure of the bulk oxide. In order to obtain structural information for the chromia aggregates in pillared clays, we have initiated structural studies of these materials. Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy is being recognized as an effective tool for determining the local structure of a variety of materials. The basic principles and utility of this technique have been discussed elsewhere (18.). ... 

Future EXAFS studies of the local structure of reactive sites in pillared clay derivatives and related hydroxy-interlayered clay minerals should help elucidate the reactivity of these materials as catalysts for synthetic organic chemical conversions, as well as for reactions occurring in the natural soil environment. 

Nanoporous materials like zeolites and related materials, mesoporous molecular sieves, clays, pillared clays, the majority of silica, alumina, active carbons, titanium dioxides, magnesium oxides, carbon nanotubes and metal-organic frameworks are the most widely studied and applied adsorbents. In the case of crystalline and ordered nanoporous materials such as zeolites and related materials, and mesoporous molecular sieves, their categorization as nanoporous materials are not debated. However, in the case of amorphous porous materials, they possess bigger pores together with pores sized less than 100 nm. Nevertheless, in the majority of cases, the nanoporous component is the most important part of the porosity. 

In the case of pillared clays (PILCs), notwithstanding the fact that these materials were initially developed as catalysts, there have also been other investigations, where PILCs have been studied as potential sorbents, especially for gas separation applications. These materials have a developed micropore structure with a relatively large BET surface area, normally between 300 and 400m2/g and a micropore volume around 0.15 cm3/g [154,155],... 

Many new adsorbents have been developed over the past 20 years including carbon molecular sieves, new zeolites and aluminophosphates, pillared clays and model mesoporous solids. In addition, various spectroscopic, microscopic and scattering techniques can now be employed for studying the state of the adsorbate and microstructure of the adsorbent. Major advances have been made in the experimental measurement of isotherms and heats of adsorption and in the computer simulation of physisorption. 

Various procedures have been proposed for the evaluation of the true intracrystalline capacity and the external surface area (see Sayari et al., 1991). The as-method is one way of analysing composite isotherms, which has been applied to nitrogen isotherms on different samples of ZSM-5 (Sing, 1989). This approach was used by Gil et al. (1995) in their recent study of the microporosity of pillared clays and zeolites. By this means, mesopores were estimated to have contributed about 25% to the total pore volume of a commercial sample of HZSM-5. 

At this point, the most promising types of pillared clays for use as cracking catalysts are the pillared rectorite developed by Jie et al. (3) and the large pore Ce/Al-pillared smectites developed by McCauley W. Further studies will have to be performed in order to determine the feasibility of these to types of materials for this application. 

The data of Figure 1 clearly show that irradiation of the titania pillared clay systems leads to an enhancement in the moles of Cl produced during photodegradation of dichloromethane. Prolonged pre-irradiation leads to lower conversion than shorter periods of pre-Irradlatlon. This is a general result and was found for all supported systems studied here. 

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