Which of the followign statements about zerolites is not correct ?

Options

A. Zeolites are open structures of silica in which trivalent aluminium is substituted by a fraction of silicon atoms.
B. Shape selectivity of zeolites depends upon porous structure of the catalyst
C. Zeolites are synthetic microporous aluminosilicates which do not exist naturally.
D. Zeolites are aluminosilicates having three dimensional network.

Explanation

To determine which statement about zeolites is not correct, let's analyze each option step by step:

1. **Option 1: Zeolites are open structures of silica in which trivalent aluminum is substituted by a fraction of silicon atoms.**
- **Analysis:** This statement is correct. Zeolites are indeed aluminosilicates where aluminum can replace silicon in the structure, leading to the formation of an open framework that allows for the presence of cations and water molecules.

2. **Option 2: Shape selectivity of zeolites depends upon the porous structure of the catalyst.**
- **Analysis:** This statement is also correct. The unique porous structure of zeolites allows them to selectively adsorb certain molecules based on their size and shape, which is a key feature in catalysis.

3. **Option 3: Zeolites are synthetic microporous aluminosilicates which do not exist naturally.**
- **Analysis:** This statement is incorrect. Zeolites can be both natural and synthetic. Many zeolites occur naturally in volcanic rocks and sedimentary deposits, while others are synthesized for specific applications.

4. **Option 4: Zeolites are the aluminosilicates having a three-dimensional network.**
- **Analysis:** This statement is correct. Zeolites have a three-dimensional framework structure that contributes to their properties and functions.

**Conclusion:** The statement that is not correct is **Option 3**: "Zeolites are synthetic microporous aluminosilicates which do not exist naturally."