Smectite clays pillaring

An equivalent surface area of 460 m g was determined from the monolayer volume, Vj. The value obtained for the dimensionless energetic constant, C=260, was characteristic of a microporous material. Although the BET surface area may not be a physically precise quantity due to the fact that the nitrogen molecule does not exhibit the same cross-sectional area in a microporous environment as on a flat surface, the BET value is useful for comparisons of relative porosities among a related class of adsorbents. For instance, smectite clays pillared by metal oxide aggregates typically exhibit BET surface areas in the range 150 - 400 m /g. Thus, the TSLS complex is among the more porous intercalated nanocomposites derived from smectite clays. 

Matsuda, T., Nagashima, H., and Kikuchi, E. 1988. Physical and catalytic properties of smectite clays pillared by alumina in disproportionation of 1,2,4-trimethylben-zene. Appl, Catal, 45 171-182. 

FIGURE 7.27 Representation of the formation of pillars in a smectite clay where is the pillaring cation. 

Here we report the synthesis and catalytic application of a new porous clay heterostructure material derived from synthetic saponite as the layered host. Saponite is a tetrahedrally charged smectite clay wherein the aluminum substitutes for silicon in the tetrahedral sheet of the 2 1 layer lattice structure. In alumina - pillared form saponite is an effective solid acid catalyst [8-10], but its catalytic utility is limited in part by a pore structure in the micropore domain. The PCH form of saponite should be much more accessible for large molecule catalysis. Accordingly, Friedel-Crafts alkylation of bulky 2, 4-di-tert-butylphenol (DBP) (molecular size (A) 9.5x6.1x4.4) with cinnamyl alcohol to produce 6,8-di-tert-butyl-2, 3-dihydro[4H] benzopyran (molecular size (A) 13.5x7.9x 4.9) was used as a probe reaction for SAP-PCH. This large substrate reaction also was selected in part because only mesoporous molecular sieves are known to provide the accessible acid sites for catalysis [11]. Conventional zeolites and pillared clays are poor catalysts for this reaction because the reagents cannot readily access the small micropores. 

Layered silicate clays intercalated by pillaring poly-oxocations are precursors to an important class of mi-croporous catalysts. Smectite clay was the only host structure known to be pillarable by purely inorganic oxo ions. Recently, layered double hydroxides (LDH) pillaring oxo ions were reported by Pinnavaia and coworkers [79, 80]. 

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). 

We have been investigating the use of imogolite as a pillaring agent for smectite clays with layer lattice structures ". The regular intercalation of the tubes within the layered host results in the formation of a tubular silicate-layered silicate (TSLS) complex. These new nanocomposite materials may be viewed as pillared clays in which the pillars themselves are microporous. Significantly, the TSLS structure is thermally stable up to 450 C when montmorillonite is selected as the layered host . 

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. 

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. 

The smectite clays exhibit catalytic activity related to the presence of add sites contained at the surface of the clay layer. The role the pillar plays in the catalytic activity of these materials has not been successfiiUy elucidated due to the overriding activity found in the clay itself. The aluminum pillar, for example, produces both Bronsted and Lewis acidity upon thermal treatment ... 

It has been a decade and a half ce the first report of pillaring of the smectite clays. Since that time a better understanding of the formation of these phases and their characterization has occurred. Like the zeolite molecular sieves, the interest in these phases has mainly been confined to the petroleum industry and related applications. The direction of catalyst testing of the PLS is expected to parallel those areas now being pursued in the zeolite molecular sieves. 

Smectite clay catalysts are potential alternative adsorbents, although some modifications of the natural mineral are necessary. Interlayer sites in smectite dehydrate at temperatures above 200°C, collapsing to an illitic structure. Since the ion-exchange capacity of smectite centres on the interlayer site, collapse must be prevented if clay catalysts are to be used in thermal treatments of chemical organic toxins. The intercalation of thermally stable cations, which act as molecular props or pillars, is one... 

Fig. 1 Schematic diagram showing props or pillars in the interlayer position in smectite clay. T, tetrahedral sheet 0, octahedral sheet.

Figure 4. Most pillaring has been done with smectite clays. This figure shows a variety of new sheet structures recently pillared with various species shown in Figure 3, or related polymers.

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. ... 

Zielk, R. C., and T. J. Pinnavaia. 1988. Modified clays for the adsorption of environmental toxicants binding of chlorophenols to pillared, delaminated, and hydroxy-interlayered smectites. Clays Clay Miner. 36(5) 403-8. 

MOLECULAR ENGINEERING OF LAMELLAR SOLIDS. I. PRINCIPLES DERIVED FROM THE PILLARING OF SMECTITE CLAYS... 

The present work summarizes our efforts to synthesize supergallery pillared derivatives of three general families of layered compounds. The host structures of interest include smectite clays, layered double hydroxides, and layered silicic acids. 

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