. Zeolite Catalysis: The details of heterogeneous catalysis will be incomplete, if zeolites are not discussed. Zeolites are microporous, crystalline, hydrated, alumino silicates, made of silicon and aluminium tetrahedron. There are about natural zeolites and synthetic zeolites.
As Temperature ºC Rate of reaction Optimum temperature is usually around ºC X Y At high temperatures enzymes denature, rate falls rapidly Figure . Rate vs Temperature pH pH opt Rate of reaction Figure . Rate vs pH XII U10-Surface XII U10-Surface - - - - silicon is tetravalent and aluminium is trivalent, the zeolite matrix carries extra negative charge. To balance the negative charge, there are extra framework cations for example, H + or Na + ions.
Zeolites carrying protons are used as solid acid catalysts and they are extensively used in the petrochemical industry for cracking heavy hydrocarbon fractions into gasoline, diesel,etc., Zeolites carrying Na + ions are used as basic catalysts. One of the most important applications of zeolites is their shape selectivity. In zeolites, the active sites namely protons are lying inside their pores. So, reactions occur only inside the pores of zeolites.
Reactant selectivity: When bulkier molecules in a reactant mixture are prevented from reaching the active sites within the zeolite crystal, this selectivity is called reactant shape selectivity. Transition state selectivity: If the transition state of a reaction is large compared to the pore size of the zeolite, then no product will be formed. Product selectivity: It is encountered when certain product molecules are too big to diffuse out of the zeolite pores. Phase Transfer catalysis: Suppose the reactant of a reaction is present in one solvent and the other reactant is present in an another solvent.
The reaction between them is very slow, if the solvents are immiscible. As the solvents form separate phases, the reactants have to migrate across the boundary to react. But migration of reactants across the boundary is not easy. For such situations a third solvent is added which is