Pillared layered clays

The inherent limitations of the use of zeolites as catalysts, i.e. their small pore sizes and long diffusion paths, have been addressed extensively. Corma reviewed the area of mesopore-containing microporous oxides,[67] with emphasis on extra-large pore zeolites and pillared-layered clay-type structures. Here we present a brief overview of different approaches to overcoming the limitations regarding the accessibility of catalytic sites in microporous oxide catalysts. In the first part, structures with hierarchical pore architectures, i.e. containing both microporous and mesoporous domains, are discussed. This is followed by a section on the modification of mesoporous host materials with nanometre-sized catalytically active metal oxide particles. 

Pillared layered clays (PILC) containing different hydrated cations in the interlayer space are also selective catalysts. Pinacol and 2,3-diphenyl-2,3-butanediol have been studied in the presence of Al-PILC, Zr-PILC, and Cr-PILC [38], Zr-PILC, with the lowest acidity, gave rise to the largest amount of diene from pinacol (32 % and 22 %, depending on the method of preparation). The rearrangement was selective in the presence of a mixed-metal pillared layered clay (Fe Aln-PILC, sealed tube, 398 K, 1 h, catalyst/diol = 5) [40],... 

Acidic clays are widely applied in the dehydration of alcohols [38]. Although similar to zeolites in their capacity to induce the formation of both alkenes and ethers, selective alkene synthesis is possible. Various layered materials (clays, ion-exchanged montmorillonite, pillared layered clays) are very active and, in general, selective in transforming primary, secondary, and tertiary aliphatic alcohols to 1-alkenes [39-43]. Al -exchanged montmorillonite, however, induces ether formation from primary alcohols and 2-propanol [41]. Substituted 1-phenyl-1-ethanols yield the corresponding styrene derivatives at high temperature (653-673 K) [44]. 

M. Campanati, M. Casagrande, 1. Fagiolino, M. Lenarda, L. Storaro, M. Battagliarin, A. Vaccari. Mild Hydrogenation of Quinoline. 2. A Novel Rh-containing Pillared Layered Clay Catalyst. J. Mol. Catal. A Chemical in press. 

The aged pillaring reagents were heated to 65 C and 325 bentonite, American Colloid Company, whose major constituent is the layered clay mineral, montmorilIonite, was added. There was always a 5-fold excess of aluminum in solution and the volume of solution per gram of clay was always 45 cc/g or more. The reaction was carried out for 2 hours. The slurry was filtered and the solids washed two times with water, dried, sized, and calcined at 500 C for 2 hours in air. 

In all previous cases, V was incorporated in a monomeric form. There are also methods to introduce oligomeric V into inorganic structures. Choudary et al. (48) advocated the use of a montmorillonite, pillared with V oligomers (V-PILC). V-PILC catalyzes the epoxidation of allylic alcohols with i-BuOOH. Oligomeric V is also used to pillar anionic clays such as layered double hydroxides (LDHs) with decavanadate anions (VioOjg) (49). 

Wang, J. D., Serrette, Y., Tian, Y. and Clearfield, A. (1995). Synthetic and catalytic studies of inorganically pillared and organically pillared layered double hydroxides. Appl. Clay Sci. 10, 103. 

Adsorption by Clays, Pillared Layer Structures and Zeolites... 

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). 

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. 

Pillared zirconium clay modified by sulfate constitutes a new class of materials. Nevertheless, the classical procedure consisting of impregnating these solids with sulfate solutions presents some disadvantages such as low surface area and a poor sulfur thermal stability. In a previous work, we have developed a new in situ sulfation preparation method using zirconyl chloride as zirconium precursor. However, the use of this salt gives an intercalation solution with a very low pH which can affect the clay layers. Furthermore, the addition of sulfate ions to the ZrOz solution is limited by Hauser salt or polymeric phase precipitation. 

Exploration of the pillar-clay sheet reactivity and connectivity also indicate the important role of the specific clay type. 27 1 and 29si-MASNMR experiments have shown distinctive differences between pillaring mechanisms in trioctahedral hectorite and dioctahedral montmorillonite. Whereas Plee et al. (22) concluded that chemical crosslinking may occur between the pillar and tetrahedral layer in a beidellite montmorillonite, Pinnavaia et al. (23) showed that it did not occur in a hectorite. These are the first observations of a complex process that may depend upon several structural and chemical factors, such as substitution of Al in the tetrahedral layer, or the need for vacancies in the octahedral layer to allow rotation of structural units or migration of reactant species to facilitate crosslinking. Ongoing research should further elucidate refinements on these mechanisms, and direct the technology towards more optimized catalysts - presumably those which form chemical bonds between the pillar and clay layer. 

The pillaring of clays has become an established technique of preparing a new class of porous materials. While their promise as cracking catalysts has not been fulfilled, never-the-less they exhibit interesting catalytic behavior. We have extended the technique to the pillaring of many other types of non-clay layered compoimds which include phosphates, oxides and layered double hydroxides. This extension broadens the field to the point where one can choose the degree of acidity or basicity and framework metals required for specific catalytic processes. 

PILLARED LAYERED COMPOUNDS (PLC) OTHER THAN CLAYS... 

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