Parameters affecting the impregnation process
The pH of the solution is an important parameter in the impregnation process. On liquid side pH determines the most abundant species in solution to be deposited on to the support. HNO3,
carboxylic acids, ammonia are usually used for adjusting the pH because these can decompose during thermal treatment. On solid side, pH controls the nature of surface charge and the number of charged sites, in other words, the zeta potential. During impregnation, the extent of interaction between the metal complex and the support is controlled by parameters such as isoelectric point of oxide support, temperature and nature of support and dopants.
Ion exchange
Ion exchange consists of replacing an ionic species by another ionic species in electrostatic interaction of the precursors with the surface of a support. The support containing the ion A is dipped into an excess volume (compared to the pore volume) of a solution containing ion B that is to be introduced. Ion B gradually penetrates into the pore space of the support and takes the place of ion A, which passes into the solution. This continues until an equilibrium is established corresponding to a given distribution of the two ions between the solid and solutions.
Natural ion exchangers: Natural exchangers are composed of a frame work having charges that is neutralized by ions of opposite charges. For example, zeolite has negatively charged framework due to the particular environment of aluminium. Aluminium just like silicone is effectively situated in the centre of a tetrahedron of four oxygen atoms which have four negative charges, where as aluminium itself has only three positive charges. The tetrahedron (AlO4) thus have an overall one negative charge distributed over the oxygen atoms and this charge is neutralized by the presence of various cations such as Na+, K+ etc. These cations are not definitively bonded to the framework but may be replaced by other cations during an ion exchange operation. Zeolite has a constant number of these exchange sites which are equal to the number of aluminium atoms in their framework. Clays, silicates are also cation exchangers.