Chemistry - A Molecular Science

Chapter 8 Solid Materials

Chemistry can also take place inside the pores. Indeed, some reactions that occur with

difficulty on their own occur with relative

ease within the caviti

es of a zeolite. The

enhanced reactivity results because interacti

ons with the aluminosilicate framework can

make substances more reactive,

i.e.

, zeolites can function as

catalysts

.* Indeed, the largest

use of zeolites is to catalyze reactions that c

onvert some of the less useful molecules found

in petroleum into more useful ones. The por

e size dictates the size of both the reactants

and the products, so chemists can select to

react only certain molecules in a mixture by

selecting a zeolite with the appropriate pore si

ze to exclude larger molecules. Chemists

also create zeolites with chemical groups attach

ed to them that enhance certain reactions;

i.e

., they

functionalize

zeolites.

Clays

are the most abundant minerals found in

soils, rocks and waters. Like zeolites,

they are usually aluminosilicates. However, Mg

is substituted for Al in many clays, and Fe

fills certain of the Al sites in some clays to

give them a red color. All clays exhibit a two-

dimensional layered lattice structure rather than the porous network structure of zeolites. The two major types of clays, kaolinite and smectic clays pictured in Figures 8.23a and 8.23b, respectively, differ in the organization of tetrahedral (SiO

) and octahedral (AlO 4

or 6

MgO

) building blocks. The differences in their 6

structures result in dramatically different

properties for these two types of clays.

Kaolinite clays

, Figure 8.23a, consist of aluminosilicate sheets composed of a silicate

layer (SiO

tetrahedra shown in blue) and an aluminate layer (AlO 4

octahedra shown in 6

green). In their sedimentary formation,

kaolinite clays may have water between the

aluminosilicate layers. However, upon heatin

g (firing) the water is driven out from

between the layers, leaving only OH bonds on th

e surface formed by the aluminate layers.

These terminal OH groups form strong hydrogen

bonds to the oxygen atoms of the silicate

portion of the neighboring layer providing a rigi

d material, which is why kaolinite clay is

the main component of china clay.

Smectic

(or swelling) clays, Figure 8.23b, consist of sheets composed of a layer of

aluminate octahedra sandwiched between two la

yers of silicate tetrahedra. Other cations

surrounded by water (hydrated metal ions), represented by the larger gray spheres in the figure, often fill the space between these layers. There are fewer OH groups in this type of structure, and those that are present are ‘bur

ied’ in the aluminate layer between the two

silicate layers, which effectively shields them from forming inter-layer hydrogen bonds. As a result, the layers in a smectic clay are

not tightly held together, and other ions and

molecules can penetrate in between these layers and force the sheets apart. This property

* Catalysts are materials that speed the rate of reaction. They will be

discussed in Chapter 9. (a) (b)

O-H bond

O-H bondmetalcation

Al Si

Figure 8.23 (a) Kaolinite and (b) Smectic clays Si atoms and Si-O bonds are blue, Al atoms and Al-O bonds are green, O atoms are red, and H atoms and O-H bonds are white.

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