Smectite, pillared clay

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). 

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). 

ESR has also been used in the characterization of species adsorbed on pillared clays, i.e. smectites with hydroxy-aluminium interlayers. Adsorption of Cu(II) on hydroxy-aluminium hectorite produced mobile hexaaquacopper(II) and Cu(II) chemisorbed to... 

Fast deactivation rates due to coking and the limited hydrothermal stability of pillared clays have probably retarded the commercial development of these new type of catalysts and prevented (to date) their acceptance by chemical producers and refiners. However, there is a large economic incentive justifying efforts to convert inexpensive (i.e. 40-100/ton) smectites into commercially viable (pillared clay) catalysts (56). Therefore, it is believed that work on the chemical modification of natural (and synthetic) clays, and work on the preparation and characterization of new pillared clays with improved hydrothermal stability are, and will remain, areas of interest to the academic community, as well as to researchers in industrial laboratories (56). 

Iron impurities in clays have been thought responsible for these type of catalysts low carbon selectivity (4,5). The purpose of this paper is to investigate and report the influence that the location, chemical state and environment of iron impurities have on the cracking properties of pillared clays prepared by reacting several smectites with aluminum chlorhydroxide solutions. 

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. 

The values of Dt of (1.89 0.09) and (1.94 0.10) reported by Van Damme and Fripiat (1985) for pillared clays were derived from the multilayer capacities of nitrogen and various organic adsorptives. The fact that Dt 2 appeared to confirm that the basal smectite surface was smooth and that the pillars were regularly distributed. It was argued by Van Damme and Fripiat that a random distribution of the pillars would necessarily lead to some localized molecular sieving and that this in turn would result in D% > 2. 

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). 

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. 

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 . 

Pillared interlayered clays (PILC) can be regarded as nanocomposites, in which oxide particles of nano- and subnanometer scales are incorporated into the interlayer space of two-dimensional aluminosilicates [1]. In recent years, much attention has been focused on this new type of materials with large heights of pillars, because they provide larger pores in comparison to conventional zeolites. Smectites pillared with transition metal oxides (e.g. Cr, Fe, Ti) are of particular interest because the incorporated phases have themselves catalytic properties. Such solids are claimed to possess a remarkable activity in a notable number of reactions [2,3]. 

Hitherto the most successful pillaring has been carried out on smectite type clays. The original research on pillaring included several series of unsuccessful experiments on mica, vermiculite and the sheet silicic acid minerals magadiite, silhydrite and kenyaite. Recently renewed attention has focused on these and other alternative sheet structures. They include rectorite, zirconium phosphates, tetrasilicic micas, hydrotalcites and silicic acids, the general structural features of which are shown in Figure 4. 

Finally, between the TOT layers of a smectite, large cationic species that are polymeric or oligomeric hydroxyl metal cations formed by the hydrolysis of metal salts of aluminum, gallium, chromium(III), silicon, titanium(IV), iron(III), and mixtures of them can be inserted by cation exchange, giving the so-called pillared clays... 

At microactivity test (MAT) conditions, the calcined (500°C/2h) expanded clay exhibits high cracking activity and selectivities typical of pillared clays. In contrast to pillared smectites, these materials have some hydrothermd stability and, can be... 

Recently, Takahama et al. (18) have reported that a montmorillonite expanded with SiOj TiOj sol particles (4), when dried with a supercritical fluid, can generate an expanded clay mineral with a surface area and pore volume more typical of silicas than of pillared clays (18,12). It is the purpose of this chapter to examine the physicochemical properties of two smectite (montmorillonite and saponite) samples expanded with Si02 Ti02 clusters and dried using a CO2 fluid at supercritical conditions. 

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