Pillaring of smectite clays

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. 

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

The mechanism of pillaring of smectite clays is beginning to be elucidated. Such understanding could lead to the synthesis of derivatives with improved thermal stability. 

Figure 16. A model for the pillaring of smectite clays by hydrolyzed polyoxyalumimam cations, abbreviated kl " Alj ". The average...

Drame, H. 2005. Cation exchange and pillaring of smectites by aqueous Fe nitrate solutions. Clays Clay Miner. 53 335-347. 

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. ... 

Another very important property of smectite clays for pillaring is their ability to be swollen by water. Swelling provides access to the exchange ions on the basal surfaces, and allows facile replacement by pillaring cations. The charge... 

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. 

Six of the eleven chapters are devoted primarily to the intercalation chemistry of smectite clays, the most versatile among all lamellar compounds. Two of these chapters are devoted to the experimental and theoretical aspects of the clay structures and surface chemistry, including chemical catalysis. Organo clays and polymer-clay nanocomposites, the adsorption of nitroaromatic compounds of environmental significance onto clay surfaces, photochemical processes, and pillared clays and porous clay heterostructures are the subjects of the remaining four chapters. These six chapters provide detailed discussions of the factors that influence access to the intragallery surfaces of the clay host and the materials properties of the resulting intercalates. 

When the lateral dimensions of the clay layers are small (< 0.05 pm) and the layer morphology is lath-like, the flocculation of smectite clays by polyoxycations can lead to delaminated aggregates [45, 61]. Under these conditions, the previously discussed card-house structure has been proposed for delaminated clays. It differs dramatically from the well-ordered F-to-F lamellar structures formed by pillared clays when the layer size is large (< 2 pm) and pancake-like in morphology. Schematic drawings of the pillared and delaminated clay... 

Intercalation of smectite clays with polyoxycations provides a new class of porous materials. Intercalated clays are called pillared clays and they have high thermal stability and large surface area. Vaughan and Lussier(ref. 1) were the first to point out the shape selective sorption property of pillared montmorillonite using various probe hydrocarbons. The shape selective catalysis in cracking of alkylbenzenes was demonstrated by Shabtai and co-workers(ref. 2). 

Pillared clays are smectite minerals or iUite-smectite minerals that have been stmcturaHy modified to contain pillars of stable inorganic oxide. The pillars prop open the smectite stmcture so they have a basal space of approximately 3.0 nm. Typical metals in the pillars include Al, Zr, Ti, Ce, and Fe, and these materials are used in catalytic processes to crack heavy cmde oils (110—112). 

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). 

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