Nanoporous materials: Clays
Nanomaterials
Materials are substances that are used for various purposes ranging from large scale to as small as nano scale. Materials used at nano scale are called nanomaterials; specifically, these are materials whose particle sizes are controlled within the nano-range by the concept of quantum confinement. Nanomaterials are classified into many types based on chemical classification and physical structure, one such type is nanoporous materials.
Nanoporous materials:
Materials which have a porous structure with pores of sizes in nano-range are called nanoporous materials. It is desirable to develop porous materials with a wide range of pore-width for application to various kinds of catalytic reactions. These materials are classified as:
· Microporous materials: 0.2–2 nm
· Mesoporous materials: 2–50 nm
· Macroporous materials: 50–1000 nm
Further, there are largely three kinds of nanoporous materials, namely, ion exchange resins, zeolites and clays.
Ion exchange resins:
An organic ion exchange resin is composed of high-molecular-weight polyelectrolyte that can exchange their mobile ions for ions of similar charge from the surrounding medium. Ion exchange resins are very versatile catalysts which play an important role in the large scale manufacture of petrochemicals and solvents.
Zeolites:
Zeolite is the broad term used to describe a family of minerals called tectosilicates. Zeolites are constructed of tetrahedral AlO4-5 and SiO4-4 molecules bounded by oxygen atoms in a three dimensional arrangement.
However, resins and zeolites are artificially synthesised but an important class of naturally occurring nanoporous materials are clays.
Clays:
Clays are silicate minerals which are rock-forming minerals and they belong to the specific group of silicate minerals called the phyllosilicates. The evidence that clays are nanoporous materials is in their basal spacing. (Basal spacing: The spacing between two layers of any clay mineral). The basal spacing of one of the clays called montmorillonite is 12.1 Å or ~1nm.
Clays are hydrous aluminium phyllosilicates i.e. water containing minerals of aluminium silicates. They form parallel flat hexagonal sheets of silicate tetrahedra with SiO4. However, there are two different types of sheets that are formed, namely, tetrahedral sheets composing of (Al, Si)3O4 and octahedral sheets containing metal cations like aluminium, sodium, magnesium, etc., coordinated to six oxygen atoms.
The two important structural classifications of clays:
1:1 Structure:
Every tetrahedral layer is bonded to one octahedral layer through electrostatic attraction.
2:1 structure:
Two tetrahedral layers sandwich one octahedral layer through electrostatic attractions.
Uses of clays:
Clays are used for many purposes like fillers in cosmetics, medicines, building materials, etc. however, major use of clays is as solid acid catalysts.
Clay as a solid acid catalyst:
The well-known use of mineral acids for catalysis has been now replaced by the use of solid acid catalysts like clays which are equally and sometimes much more environmentally benign than the other catalysts due to some of their properties like the following:
· Clays are heterogeneous in nature.
· They are easily removable from the reaction mixture and reusable.
· They have greater selectivity, better yields and shorter reaction times (compared to mineral acids).
· They are economical and environmentally benign.
· They require only milder conditions for organic reactions.
· They can be modified to suit our needs.
One among the clays used for used for such purposes is montmorillonite, one of the most common members of the smectite group of clays having a general formula of A0.3D2-3[T4O10]Z2.nH2O.
The physical and chemical specifications of montmorillonite are as follows,
Physical specifications:
• Colour: White, Pale pink, Blue, Yellow, Red, Green
• Lustre: Dull, Earthy
Chemical specifications:
Ø Chemical formula: (Na,Ca)0.33(Al,Mg)2(Si4O10)(OH)2.nH2O
• Crystal system: Monoclinic
• Structure of montmorillonite: 2:1