Synthetic clays, pillaring

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). 

By intercalating the clay with aluminium oligomers pillared clays of enhanced surface area have been synthesized. The increase in catalytic activity with enhanced basal surface area indicates that the acidic sites are located in the basal plane of the synthetic clay particles. 

Pillaring selected synthetic clay minerals with suitable pillaring agents may lead to a breakthrough in the theory and application of pillared clays. This chapter reports on the preparation and characterization of a novel pillared synthetic heteroatom-containing clay, that is, pillared synthetic boron-containing saponite. 

Prepciration of Molecular Sieves by Pillaring of Synthetic Clays... 

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. 

Al3+-exchanged synthetic hectorite is a good catalyst for these conversions, and the 13C NMR spectrum obtained in the interlamellar, proton-catalyzed addition of water to 2-methylpropene is indistinguishable (Fig. 79) from that of f-butanol. Doubtless studies of this kind, where natural-abundance, 3C NMR signals are used to probe the chemical identity and motional freedom of reactant and product species situated in the interlamellar spaces of clays or pillared clays (see below), will become increasingly popular. Using l3C NMR linewidths and spin-lattice relaxation studies, Matsumoto et al. (466) have succeeded in discriminating between the internal and external surfaces of pillared montmorillonites. 

It is a matter of some significance catalytically to be able to ascertain whether the aluminum present in a natural clay or its synthetic analogue is in a state of octahedral substitution (as in montmorillonite) or whether there is some tetrahedral substitution (as in beidellite). 27A1 MAS NMR readily provides the necessary answers. For example, Diddams et at. (462) in a study of the synthesis, characterization, and catalytic performance of synthetic beidellites and their pillared analogues, monitored the fate of AI from the gel precursor to the sheet silicate and to its pillared state by 27A1 MAS NMR (see... 

Future EXAFS studies of the local structure of reactive sites in pillared clay derivatives and related hydroxy-interlayered clay minerals should help elucidate the reactivity of these materials as catalysts for synthetic organic chemical conversions, as well as for reactions occurring in the natural soil environment. 

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. 

Adams, J. M. 1987. Synthetic organic chemistry using pillared, cation-exchanged and acid-treated montmorillonite catalysts a review. Appl. Clay Sci, 2 309-342. 

The main type of inorganic-LDH hybrid system comes from intercalation of oxometalates into the interlayer space of LDHs because of the high catalytic activity of both oxometalates and LDHs. Typical synthetic routes are similar to those of pillared clays, as mentioned before. After pristine nitrate/chloride-LDHs are anion exchanged with polyoxometalates, usually at elevated temperatures, the pillared polyoxometalates are stabilized within the interlayer space by calcination. 

Montmorillonites (natural and synthetic) Natural clays can have swelling structure giving microporosity pillared clays have larger pores add-treated clays are partially mesoporous (and may change with agdng)... 

Pinnavaia [31] showed similarly that fully occupied silicate layers do not eliminate the possibility of reaction of the AI13 cation with the clay layers. A synthetic fluorohectorite is found to be pillared by AI13, and, more importantly, it is apparent from the Si and Al spectra that structural rearrangements do occur for a clay that has Si fully occupying the silicate layer. Thus Si-O-Al linkages are formed between the cation and the silicate sheet (Fig. 6). 

The uptake of hydrogen cyanide into protonic forms of synthetic zeolites Y, Beta, mordenite, as well as into naturally occurring clinoptilolite, ferrierite, stil-bite and aliunina-pillared clays were studied by Jamis et al. [954] using FTIR measurements of the C=N stretching vibration to characterize the binding of HCN to the various Br0nsted-acid sites of the molecular sieve materials. 

---