This paper reviews the synthesis of smectites and porous pillared clay catalysts. Synthetic as well as natural smectites serve as precursors for the synthesis of Al, Zr, Ti, Fe, Cr, Ga, V, Si, and other pillared clays as well as mixed Fe/Al, Ga/Al, Si/Al, Zr/Al and other mixed metal/Al pillared clays. The use of these pillared clays in some catalytic reactions is also briefly reviewed.

Synthesis of Smectites and Porous Pillared Clay Catalysts: A Review

Titanium pillared clays using montmorillonite, saponite and rectorite
hosts have been prepared from TiCl
 4
–ethanol solutions.
The products have been characterised structurally by PXRD and IR studies
and for acidity by cyclohexylamine and pyridine
adsorption–desorption studies. Catalytic activity data for pentanol
dehydration and cumene cracking are also presented. The amount of Ti
incorporated is related to the cation-exchange capacity of the host. The
thermal stability of the pillared material decreases from rectorite
through montmorillonite to saponite. Cyclohexylamine desorption suggests
that the saponite sample has the highest acidity, and this is reflected in
the highest conversion for cumene cracking. In the case of pentanol
dehydration, however, the highest activity is seen for the rectorite
sample, despite its apparent lower acidity. Similar effects have been
reported previously for this reaction with alumina pillared
clays.

Dependence of the properties of titanium-pillared clays on the host matrix : A comparison of montmorillonite, saponite and rectorite pillared materials

Monohydroxy aluminium oxide (Boehmite, AlOOH) has been used as an extrusion aid along with α-alumina and γ-alumina particulates. Extended ball milling of the mixture using alumina grinding media imparts nucleating seeds to boehmite for an early conversion to α-alumina. The boehmite-γ-alumina mixture appears to result in a denser alumina ceramic accompanied by an enhanced grain growth indicating the higher reactivity at higher temperatures compared to the boehmite-α-alumina system. The reason for this novel behaviour is probably the coating of nanosize boehmite containing nucleating sites on the porous γ-alumina powders resulting in enhanced diffusion reactions. The present study therefore provides scope for low temperature alumina ceramics using boehmite binder along with partly dehydroxylated aluminium hydroxide.

Monohydroxy aluminium oxide (Boehmite, AlOOH) as a reactive binder for extrusion of alumina ceramics

Delamination through sonication for hydroxy metal oxide sol intercalation of montmorillonite

Factors which affect the microstructure of pillared materials derived from acid activated clays have been investigated by varying the conditions used in their preparation and the level of acid treatment of the host clay prior to pillaring. The pillared acid-activated clays prepared are found to exhibit both micro and mesoporosity with a higher proportion of mesoporosity compared to conventional pillared clays. For materials calcined at 500°C, typical values obtained are in the range 0.02–0.1 cc/g and 50–200 m2/g for micropore volume and surface area respectively with corresponding mesopore values ranging between 0.2–0.4 cc/g and 150–250 m2/g. In all cases, the development of microporosity is at the expense of mesoporosity and therefore, high micropore values are accompanied with low mesopore values and vice versa. The microstructure developed by these materials is critically dependent on the amount (and nature) of pillaring species incorporated by the host clay. Consequently, the preparation conditions (i.e., Al/clay ratio and pillaring (exchange) temperature) affect the microstructure in as much as they influence the amount of alumina incorporated as pillar. The development of microporosity is highest for air-dried materials prepared at 80°C at an Al/clay ratio of 6 mmole/g.

The microstructure of alumina pillared acid-activated clays

Synthesis of pillared smectites from hydrous metal oxide sol-smectite mixtures under reflux conditions

A novel method for the synthesis of pillared smectites (cross linked intercalated smectites) by refluxing a mixture of bentonite (2 wt%) and nano hydroxy aluminium oxide (boehmite, AlOOH) sol in aqueous medium is reported. Mixed sol containing boehmite and hydrous cerium oxide has resulted in mixed oxide pillared smectites. The various conditions of the intercalation are reported and the characteristics of the intercalated smectites have been evaluated using XRD, surface area analysis and thermogravimetric techniques. The presnet method is shown to be effective in reducing the time of pillaring. High order of intercalation 20 A and specific surface area of 260 m2 g−1 are achieved by this method. Further, the possibility of handling a higher concentration of the smectite, which has been a limiting factor in the usual methods, is highlighted. The dielectric constants of the dried smectites after intercalation by reflux are compared with those prepared by usual methods. The present technique of refluxing a mixture of mono ion exchanged smectite and hydrous metal oxide sols is novel due to its capability to intercalate smectites having high surface area, increased interlayer spacing and also enhanced thermal stability.

A. D. Damodaran

Papers

Synthesis of pillared smectites from hydrous metal oxide sol-smectite mixtures under reflux conditions

Synthesis of Pillared Smectites from Hydrous Metal Oxide Sol-Smectite Mixtures Under Reflux Conditions.