Anhydrous aluminium chloride is used as a catalyst.

Answer

### Step-by-Step Solution:

1. **Identify the Compound**:
- The compound in question is anhydrous aluminium chloride (AlCl3).

2. **Understand the Role of AlCl3**:
- Anhydrous aluminium chloride is used as a catalyst in various chemical reactions, particularly in organic chemistry.

3. **Lewis Acid Concept**:
- Aluminium chloride acts as a Lewis acid. A Lewis acid is defined as a substance that can accept an electron pair from a donor (Lewis base) to form a covalent bond.

4. **Electron Configuration of Aluminium**:
- Aluminium (Al) has three electrons in its outer shell (valence shell). In AlCl3, aluminium effectively has six electrons in its outer shell because it forms bonds with three chloride ions (Cl-). This means that aluminium does not have a complete octet (8 electrons).

5. **Need for Electrons**:
- Since aluminium has only six electrons in its outer shell, it requires two more electrons to complete its octet. This deficiency makes it a good candidate for accepting electron pairs.

6. **Formation of Intermediates**:
- When AlCl3 acts as a Lewis acid, it accepts an electron pair from a Lewis base, forming a complex or intermediate. This intermediate is often more reactive than the original reactants.

7. **Catalytic Activity**:
- By forming these intermediates, AlCl3 speeds up reactions, particularly in electrophilic aromatic substitution reactions such as Friedel-Crafts alkylation and acylation.

8. **Application in Reactions**:
- In these reactions, AlCl3 facilitates the formation of carbocations, which are essential for the electrophilic attack on aromatic compounds like benzene.

9. **Conclusion**:
- Therefore, anhydrous aluminium chloride is used as a catalyst because it acts as a Lewis acid, accepting electron pairs to form intermediates that enhance the rate of chemical reactions.