Pillared Clays

Pillared clays are usually smectite clay minerals in which the interlayer cations are three-dimensional species which in some cases, after appropriate treatment, are fixed to the layers of the host clay. The shape and size of these cations allows them to function as molecular pillars which keep the layers apart at a fixed distance. The pillaring phenomenon therefore exposes much of the intercrystal basal surfaces for adsorption and molecular sieving purposes. Permanent porosity may be introduced in montmorillon-ite by replacing the interlayer alkali or alkaline earth cation with a variety of species such as tetraalkylam-monium ions, tris-metal chelates, bicyclic amine cations and polymeric oxymetal cations. Clays pillared by oxycations or metal oxides are of greatest interest because they exhibit thermal stability in excess of 500°C and, depending on preparation methods, materials with large pore diameters and surface area (Table 1).

The most extensively studied pillared clays are those containing polymeric hydroxy-aluminium species as the pillaring cation. In this paper such Al pillared clays are used to illustrate the nature and properties generally possessed by metal oxide pillared clays. In the non-calcined so-called precursor pillared clay, layer charge is balanced by the pillaring polyca-tions which in the case of Al pillared clays is the Keggin-like [Al13O4(OH)24(H2O)12]7 + ion. On calcination this ion is converted into an oxide with the layer charge balanced by the release of an equivalent number of protons, i.e.

Figure 4 Three-dimensional illustration of the structure of silicate clays.

2. Trioctahedral smectites (all octahedral sites are occupied by divalent cations)

(b) Hectorite

[M,(Si8)(Mg6-,Li,)O2o(OH)4 • «H2O] Octahedral substitution.

Smectite clays can intercalate other compounds in a three component system:

1. Host layer with an overall negative layer charge.

2. Exchangeable intercalates (ions) which compensate for the overall negative charge.

3. Neutral molecules (e.g. water) which occur between the layers and are associated with the inter-layer cations and the layers.

2[Al13O4(OH)24(H2O)12]7

p 13Al2O3 # 14H-pillar

# 4IH2O

The formation of pillars fixed to the layers of the host clay is dependent on the calcination temperature. In general the basal (0 0 1) spacing of the precursor-Al pillared clay decreases to a fixed value upon

Table 1 Pillar type and corresponding basal spacing and surface area for montmorillonite pillared clays

Pillar type

Basal spacing (As)

Surface area (m2g 1)

Alumina

18-19

250-400

Iron oxide

17-18

& 280

Chromia

19-21

350-400

Zirconia

18-22

250-300

Titania

18-20; 25-29

300-350

Silica

12-13; 16-20

40-200; 150-400

Silica/alumina

16-19

350-500

Titania/silica

38-40

250-400

heating at 500°C. Heating to temperatures up to 400°C causes some contraction but does not prevent re-expansion of the clay upon exposure to moisture. However, the pillared clay obtained by calcination at 500°C usually shows no tendency to expand. This is because in the temperature range 400-500°C an irreversible contraction in the layer spacing occurs, during which the pillars are held within the host aluminosilicate sheets resulting in cross-linked materials. Therefore the precursor pillaring species dehydroxylates progressively on heating to 400°C, releasing protons which migrate into the clay structure and at 500°C condensation takes place of terminal hydroxy groups present on the polymeric ions with the lattice hydroxy groups on the clay. The oxide pillars formed become linked directly via oxygen to the aluminium and magnesium atoms in the octahedral layer resulting in a rigid cross-linked structure resistant to expansion. These changes are illustrated in Figure 5.

The microstructure of pillared clays is controlled by the wet chemistry of synthesis and, to a large

Figure 5 Schematic description of pillaring. In the case of an alumina pillared clay prepared from Ca-montmorillonite, d, = 14.4 A, d2 = 20.5 A, d3 = 19.0 A.

extent, the method used to dry the precursor pillared clay. The basal spacing of the pillared clays depends on the age of the pillaring reagent, the degree of hydrolysis (polymerization) of the pillaring reagent, the amount of reactants (i.e. Al/clay ratio) and the temperature of pillaring. Pillaring of clays increases their surface area from as low as 30m2g_1 to 500m2g_1 (Table 1) and generates a microporous structure similar but less constrained than that of zeolites. The volume created can be used for adsorption purposes; the adsorption characteristics are known to vary with the method employed in drying the pillared clay. Air-dried pillared clays are zeolitelike products which cannot adsorb molecules of kinetic diameter 9.2 A (e.g. 1,2,5-triethylbenzene) but can adsorb molecules of kinetic diameter 6.0 A. Freeze-dried pillared clays can, however, adsorb appreciable amounts of molecules with kinetic diameter of 10.0 A. Freeze-dried pillared clays therefore contain a significant fraction of pore openings > 10.0 A whereas all the pore openings of air-dried pillared clays are < 9.0 A. This is related to the mechanism of layer aggregation during drying. The aggregation may be face to face (for air-dried materials) or edge to face and edge to edge layer contact for freeze-dried materials. Air-dried pillared clays therefore exhibit long range lamellar order with a regular and relatively narrow pore size distribution while freeze-dried pillared clays, on the other hand, exhibit less lamellar order and a broad pore size range.

Metal oxide pillared clays in general tend to possess pores in both the micropore and mesopore size range. The ratio of micropore to mesopore volume largely depends on the interlayer spacing (pillar height) and the interpillar distance. The interpillar distance may be controlled by varying the ion exchange capacity of the host clay; this in turn determines the number of pillaring polycations required to balance the host layer charge. A low exchange capacity favours a low pillar density and vice versa. The interlayer spacing, on the other hand, may be controlled by varying the pillar type. Figure 6 gives a diagrammatic representation of two common pillar types and Table 1 gives some examples of pillar type and basal spacing for montmorillonite clay.

The porosity of pillared clays may also be varied by combining the pillaring process with other treatments such as competitive ion exchange with monocations or acid activation. Indeed acid activation of clays (see below) prior to pillaring yields a different class of materials, generally referred to as pillared acid-activated clays, with quite distinct properties.

An important characteristic of pillared clays (and clays in general) which is in some cases crucial to their

expressed as:

Figure 6 Diagrammatic illustration of polymeric hydroxy-Al (A) and -Ti (B) pillaring cations.

expressed as:

Figure 6 Diagrammatic illustration of polymeric hydroxy-Al (A) and -Ti (B) pillaring cations.

use in separation processes is that they possess considerable acidity and may be classified as solid acids. For example non-calcined precursor-alumina pillared clay possesses Br+nsted acidity which arises through the following mechanisms:

1. Polarization of interlamellar water by initial exchangeable cations not replaced by the hydroxy-Al polycations. This is especially the case if the initial exchangeable cation is acidic.

2. The pillaring polymer may hydrolyse to release protons, i.e.

3. The OH groups of the clay lattice and the pillar may also act as Br+nsted acid sites.

However, for a pillared clay calcined at temperatures above 400°C, Br+nsted acidity is weaker than Lewis acidity. This is due to the migration of protons from the interlayer region into the layer structure where they neutralize the negative layer charge thus removing some Br+nsted acid sites.

Acid activated clays When 'activatable' clay minerals are treated in acid, their chemical composition and physical properties are altered. The activation process enhances properties already present in the clay minerals and gives them certain desirable properties with respect to their applicability as adsorbents and catalysts. The clays of choice for acid activation are non-swelling bentonites containing montmorillonite as the major component. In general terms the acid activation of montmorillonites proceeds via the removal of octahedral ions and any isomorphously substituted tetrahedral ions. The changes that take place in an idealized montmoril-lonite with no isomorphous substitution may be

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