Smectite clays, pillaring with cations

More recently, various attempts have been made to develop cracking catalysts from pillared smectite clays, in which the layers are separated and held apart by the intercalation of large cations. Pillared clays (PILCs) have large surface areas within fairly well-ordered micropore structures (pore widths in die approximate range 0.6-1.2 nm). It is not surprising that these materials have attracted considerable interest with the prospect of an alternative type of catalytic shape selectivity (Thomas, 1994 Thomas etal., 1997 Fripiat, 1997). 

Over the past 15-20 years, there has been a renewed and growing interest in the use of clay minerals as catalysts or catalyst supports. Most of this interest has focused on the pillaring of smectite clays, such as montmorillonite, with various types of cations, such as hydrated metal cations, alkylammonium cations and polycations, and polynuclear hydroxy metal cations (1-17). By changing the size of the cation used to separate the anionic sheets in the clay structure, molecular sieve-like materials can be made with pore sizes much larger than those of conventional zeolites. 

A third conclusion, which follows directly from the results presented here, is that if one purposefully wants to design intercalated systems for which different spatial distributions of pillaring cations influence in a signiflcant way the overall reaction efficiency, the way to achieve this is to utilize small crystallites of smectite clays rather than extended, ordered arrays. Decreasing the size of the crystallite will also influence the relative efficiency of two- versus three-dimensional flows of the reactant to the target molecule. This is consistent with, and is another illustration of, the Adam-Delbruck concept of reduction of dimensionality. ... 

In the first method, commercial, partially neutralized AP solutions are modified by addition of La + or Ce +, and the pillaring process of a smectite clay mineral is performed under reflux or under hydrothermal conditions. A material is obtained that is, at least partially, expanded to a t/ooi spacing of 2.6 nm. The structure and composition of the pillaring oligomer is unknown, nor is it known to what extent the La or Ce cations are incorporated into the pillar. In view of the pH conditions of the pillaring process, only traces are expected to be incorporated in the pillar some may be ion exchanged or adsorbed on the pillar. Anyhow, the data of Table 2 clearly show the increase in surface area and pore volume with respect to the regularly pillared materials. 

The original pillared clays were made by (/) mixing smectite with a polymeric cationic hydroxy metal complex such as aluminum chlorhydrol (2) allowing a minimal amount of time for the cationic hydroxy metal complex to exchange with the interlayer cations and (7) calcining the resulting material to decompose the hydroxy metal complex (110). A number of newer methods have been developed to make pillared clays (111—117). 

In order to determine the structural factors, concerning host clays, which improve on the catalytic efficiency of pillared clay by the fixation of cations, the following were chosen for comparison with TSM montmorillonite (as smectite, having less of a layer charge than TSM, but an octahedral vacancy like TSM) and taeniolite (as mica, having the same layer charge as TSM, but no octahedral vacancy, unlike TSM). Table 14-2 shows the catalytic activities for cumene... 

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. 

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