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. 

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). 

The possibility of pillaring smectites with various other large polyhydroxides (e.g. of titanium and various transition metals) is under active investigation. For example, the preparation of thermally stable TiPILCs (e.g. with BET areas > 300 m2 g at 600°C) has been described by Sterte (1991) and characterized by Bernier et al. (1991). The use of different pillaring agents offers the prospect of the development of... 

Aceman, S., N. Lahav, and S. Yariv. 2000. A thermo-XRD study of Al-pillared smectites differing in source of charge, obtained in dialyzed, non-dialyzed and washed systems. Appl. Clay Sci. 17 99-126. 

The potential of pillared smectites as catalysts for catalytic cracking... 

Si02/Al203-pillared smectites were prepared according to two alternative procedures treatment of a mixture of orthosilicic acid and AICI3 with aqueous NaOH, followed by aging of the solution (method A) and preparation and aging of hydroxy-Al oligocations followed by reaction of the latter with orthosilicic acid (method B). The preparation of these solutions is further described in reference (6). ... 

Si02/Al203-pillared smectites were prepared using a continuous mixing procedure and a clay dispersion having a concentration of 2. 5 g/1 (see further ref. (6) ). 

Aluminum oxide pillared smectites. The effect of steam deactivation (18 hr, 100% steam) of conventional Al203-pillared montmorillonite on surface area and cracking performance is shown in Table I. 

Table I. MAT-Cracking results and surface areas of pillared smectites...

Vaughan et al. (2) found that, due to further polymerization of the pillaring cations, hydrothermal treatment of the aluminum chlorohydrate solution (reflux conditions) used in the preparation of the pillared smectites resulted in a material with improved hydrothermal stability. Tokarz and Shabtai (H) conducted a similar study using base hydrolyzed AlCl3-solutions. They found that refluxing of the solutions for 6- 8 hr. was sufficient to produce pillared smectites with improved thermal stability and higher porosity as compared with those prepared from solutions aged at room temperature for two weeks. 

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. 

Several attempts were made to prepare pillared smectites with sufficient hydrothermal stability for use as active components in catalysts for catalytic cracking of heavy oil fractions. Although improvements were made, none of the attempts resulted in pillared materials stable enough to withstand the hydrothermal conditions found in the regenerator of a commercial FCC. One type of materials studied, i.e. alumina-montmorillonites, may be attractive alternatives to the active matrices, often alumina, currently used in FCC-catalysts designed for cracking of heavy oils. The alumina-montmorillonites can, perhaps, not be considered to be bona fide pillared smectites as they have considerably larger pores and a wider pore-size distribution than what is characteristic for pillared smectites. 

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. 

Advancements in the preparation of new PLS s nearly parallels that of the zeolite and zeolite-like phases. Initially the pillared smectite clays were investigated but the quest for new materials with new properties led to e qiloring the pillaring of other layered phases. These include, most notably, the layered zirconium phosphates, double hydroxides (hydrotalcites), sihcas and metal oxides. The parallel paths of discovery in new material compositions for the layered phases and the microporous (zeoUte) phases are summarized in Table 1. A conq>arison between the pore architectures of the zeohtes and the two dimensional PLS is shown in Table 2. 

Recent Catalytic Studies Involving the Pillared Smectites... 

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