Isomerisation catalysts: Among the options below, which catalyst has been classically used in refinery isomerisation processes?

Introduction:
Isomerisation rearranges the carbon skeleton of paraffins (e.g., normal butane to isobutane, or C5–C6 normal paraffins to isoparaffins) to boost octane or provide feed for alkylation. Catalysts can be homogeneous (Lewis acids) or heterogeneous (noble metal on halogenated supports). The question asks for a classic catalyst associated with refinery isomerisation history.

Given Data / Assumptions:

• We compare well-known catalysts from different process families.
• Focus is on established textbook associations.

Concept / Approach:
Historically, aluminium chloride (AlCl3) served as a Lewis acid catalyst for isomerisation (and related carbocation chemistry) under specific conditions. Modern units more commonly employ platinum on chlorided alumina for C5–C6 isomerisation or solid acids, but AlCl3 remains the classical answer. Alumina by itself is largely an inert support, nickel is chiefly a hydrogenation catalyst, and phosphoric acid is associated with polymerisation of olefins (not isomerisation). Copper chromite is used in other hydrogenation/dehydrogenation services, not as a primary isomerisation catalyst.

Step-by-Step Solution:

Verification / Alternative check:
Older refinery literature and exam syllabi reference AlCl3 for isomerisation, while noting modern Pt/chlorided alumina systems for contemporary units.

Why Other Options Are Wrong:

• Alumina only: Support, lacks necessary acidity.
• Nickel: Hydrogenation/dehydrogenation rather than skeletal isomerisation.
• Phosphoric acid: Polymerisation catalyst for olefins.
• Copper chromite: Not the standard isomerisation catalyst for paraffins.

Common Pitfalls:
Conflating polymerisation catalysts with isomerisation catalysts due to both involving carbocation-type chemistry in some systems.

Final Answer:
Aluminium chloride