Solution

The catalytic reaction that depends upon the pore structure of the catalyst and the size of the reactant and product molecules is called shape-selective catalysis. Zeolites are good shape-selective catalysts because of their honeycomb-like structures. An important zeolite catalyst used in the petroleum industry is ZSM-$$5$$. It converts alcohol directly into gasoline (petrol). This is because of the following: (A) Zeolites are natural ion exchangers. They take up calcium ions from hard water and release sodium ions in water. Hence, they act as water softeners. Hence, option A is correct. (B) Zeolites trap small molecules (such as water, ammonia and carbon dioxide) in their interstices reversibly. Hence, they act as molecular sieves. Thus, option B is not correct. (C) Zeolites are complex oxide catalysts commonly used in petrochemical industries. Hence, option C is correct. (D) The activity of zeolites is increased by heating them in a vacuum. Hence, option D is correct.