Production, pillared smectites

When pillared smectites without tetrahedral substitution are calcined, there is no reaction between the pillars and the smectite layers. By contrast, a considerable structural transformation occurs when pillared beidellite is calcined, which has been interpreted as the growth of a three-dimensional quasi-zeolitic framework between the two-dimensional clay layers. The acidic properties of the product are comparable with those of zeolite Y and much more pronounced than those of calcined pillared smectites without tetrahedral substitution. 

Silicon oxide / Aluminum oxide pillared smectites. Si02/Al203-pillared smectites were prepared using two different methods (6). The surface areas of the pillared products decreased with increasing Si/Al-ratio in the pillaring solution while the basal spacings were essentially independent of this ratio. 

One way to prepare pillared smectites (and probably the simplest method for scaling up to industrial production) is introducing polyhydroxoaluminum cations by cation exchange. These cations are formed when NaOH is added to aluminum salt solutions in amounts NaOH/AP < 3 (typical 1—2.5) [100—105]. Usually, the polyhydroxoaluminum smectites are heated to 300—400°C to dehydrate them into polyoxoaluminum smectites. As adsorbents they may also be used in the uncalcinated form. 

Another way to modify pillared smectites is proposed by Michot and Pinnavaia [109]. Sodium montmoril-lonite is reaeted with a solution containing the [A1i304(0H)24+x(H20,2J cation [110] and a technical alkyl pentaethylene oxide (Tergitol 15s-5). The reaction product is a pillared smectite loaded with the nonionic surfactant. The surfactant occupied micropores between the pillars are the adsorption sites for 3-chlorophenol, 3,5-dichlorophenol, 3,4,5-trichlorophenol and pentachlorophenol. The uptake of the pollutants increases with the number of chlorine atoms. The toxicant loaded clay can be recycled by calcination at 500 °C and re-adsorption of the surfactant. 

No systematic investigation on the dependence of properties of the Ce/Al- pillared products upon different synthesis parameters was, however, reported. Although the work of McCauley is primarily concerned with the preparation of Ce/Al-pillared smectites, he claims that, in addition to Ce(III), other rare earth (RE) cations in admixture with ACH can be used for the preparation of large-pore pillared products. 

Pr/Al- and Nd/Al-solutions treated at 120-160°C for 12-96 h did not yield large-pore pillared products. All samples prepared from these solutions showed basal spacings in the range 19-20 A which is within the range characteristic for conventional Al-pillared smectites. Autoclave treatment of RE-free ACH-solutions at 120-160°C for 12-96 h resulted in the formation of colloidal dispersions or geis of boehmite or pseudoboehmite. 

The intercalation of imogolite monolayers in smectite cogently demonstrates the concept of forming regular, supergallery pillared products by direct intercalation on metal oxide sol particles of regular size and shape. 

For industrial production of pillared clays, the pillaring process has to be optimized and scaled up. Sonication of the clay suspension, acid and base treatments, and competitive exchange are some of the techniques found in the literature (9). In laboratory preparations, large amounts of water are needed. Thus pillaring of concentrated aqueous smectite suspensions has been tried in order to find better conditions for scale-up (10). 

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