Question

How does the Al:Si ratio affect the chemistry of the zeolite?

What are nitrogen isotherms, and what information does one obtain from them?

Answer 1

Answer.

A zeolite's ability to adsorb the cations is measured by its negative charge which occurs due to isomorphous substitution of Si by Al. The Zeolites with lower Si/Al ratio or higher H⁺ adsorption selectivity. Consequently, this adsorption of H⁺ leads to decrease in cation adsorption ability of the zeolite and may lead to dissolution of zeolites in aqueous media.

Nitrogen isotherms are graphs or plots between pressure on u axis and volume adsorbed on the x axis. The adsorbed volume is determined by measuring the amount of N₂ gas adsorbed on the surface and the subsequent volume that desorbs at a constant temperature.

There are two main points of information that can be obtained from nitrogen isotherms. They are surface area of catalyst and pore size determination.

The isothermal adsorption of nitrogen is used for determining surface properties of catalysts which in turn are responsible for their activity and stability of a catalyst.

Nitrogen isotherms are also used for pore size analysis. Classic methods required formation of multilayer over pores to determine pore size. However due to sufficient adsorptive specificity of nitrogen, the pore size can be determined by monolayer formation over pores, from nitrogen isotherms.