Question

Which one of the following statements about the zeolite is true? (1) They have open structure which enables them to take up small molecules. (2) They are used as cation exchangers (3) Zeolites are aluminosilicates having three dimensional network. (4) None of the \(\mathrm{SiO}_{4}^{4-}\) units are replaced by \(\mathrm{AlO}_{4}^{5-}\) and \(\mathrm{AlO}_{6}^{9-}\) ions in zeolites (a) \(1,2,3\) (b) \(1,2,4\) (c) \(2,3,4\) (d) 3,4

Short answer

The correct statements are 1, 2, and 3, so the answer is (a).

Step-by-step solution

Understanding Zeolite Structure

Zeolites are microporous, aluminosilicate minerals commonly used as commercial adsorbents and catalysts. They possess a three-dimensional framework that contains channels and cavities, capable of hosting cations, water, or other small molecules.

Analyzing Option 1

Statement 1 states that zeolites have open structures allowing small molecules to be taken up. This is true for zeolites, as their porous nature contributes to adsorbing molecules into their cavities.

Analyzing Option 2

Statement 2 claims zeolites are used as cation exchangers. This is correct, as their ability to exchange cations, like Calcium or Sodium, within their structures is a well-known property used in water softening processes.

Analyzing Option 3

Statement 3 mentions that zeolites are aluminosilicates with a three-dimensional network. This is accurate since zeolites' composition is of aluminosilicate origin which contributes to their robust framework and structural features.

Analyzing Option 4

Statement 4 suggests none of the \({SiO_4}^{4-}\) units are replaced by \(AlO_4^{5-}\) and \(AlO_6^{9-}\) ions in zeolites, which is incorrect. In zeolites, \({SiO_4}^{4-}\) can be replaced by \(AlO_4^{5-}\) to maintain their charge balance and structure.

Compiling Correct Options

The true statements about zeolites are 1, 2, and 3. Therefore, the correct choice is option (a) which corresponds to statements 1, 2, and 3 being true.

Key concepts

Aluminosilicates

Aluminosilicates form the backbone of zeolite structures. These materials are made of a combination of aluminum, silicon, and oxygen. In zeolites, aluminosilicates create a continuous framework of \({SiO_4}^{4-}\) and \({AlO_4}^{5-}\) tetrahedra linked together. This intricate lattice is what gives zeolites their remarkable characteristics.

When aluminum replaces some of the silicon in the framework, it introduces a charge difference. Silicon's natural state is neutral, while aluminum results in a slight negative charge when it enters the framework. Because of this, zeolites need cations, such as sodium or calcium, to balance this charge. These cations are not tightly bound and can move freely within the structure, which contributes to the zeolite’s functionality in ion exchange processes.

One key property of aluminosilicates in zeolites is their stability and robustness. Their robust three-dimensional network enables them to withstand high temperatures and pressures. This structure offers a unique advantage, making them ideal for various industrial and environmental applications, such as catalysis and separation processes.

Cation Exchange

Cation exchange is one of the signature properties of zeolites. It refers to the ability of zeolites to swap cations inside their structure with those in a surrounding solution. Imagine zeolites as a big net with negatively charged points; these points are offset by positively charged cations.

In practical terms, cation exchange involves exchanging these cations without altering the framework of the zeolite.

• This process is widely exploited in water softening. Here, ions like calcium or magnesium – which cause water hardness – are replaced by more benign sodium ions.
• Additionally, this property makes zeolites essential in catalysis where replacing specific cations can fine-tune the properties of a zeolite to optimize reactions.

The exchange process is reversible and selective, meaning the type of exchanged cation can be controlled depending on the application. Zeolites’ efficiency in cation exchange makes them highly valuable in ecological conservation, such as trapping radioactive ions in waste management.

Microporous Materials

Zeolites are quintessential examples of microporous materials. They have very small pores, typically less than 2 nanometers wide. These tiny spaces aren't just random but are precisely arranged in a highly ordered framework.

This molecular sieving capability allows them to selectively sort molecules based on size. Zeolites can capture smaller molecules while excluding larger ones, making them ideal for applications in filtration and purification.

• Aside from filtering, their microporous nature permits high surface reactivity, enhancing their role in chemical catalysis.
• Industrially, this property is leveraged in petroleum refining and environmental cleanup where pollutants are absorbed and immobilized within the micropores.

Microporosity also plays a crucial role in the thermal properties of zeolites, as the retained smaller molecules can act as thermal buffers. Their resilience to temperature changes and pressure stability greatly enhances their durability and functionality across various domains.