In petroleum refining isomerisation of light paraffins (to improve octane by increasing branching), which classic Lewis acid catalyst is historically associated with the process?

Introduction:
Isomerisation rearranges straight-chain paraffins into branched isomers, improving gasoline octane or alkylation feed quality. Historically, several acid catalysts have been used, with different safety and selectivity profiles, and modern units often use chlorinated alumina with platinum under hydrogen.

Given Data / Assumptions:

• Target chemistry: skeletal isomerisation of C4–C6 paraffins.
• We focus on classic, well-known isomerisation catalysts.
• We distinguish isomerisation from alkylation and polymerisation.

Concept / Approach:
Friedel–Crafts type Lewis acids, notably aluminium chloride (AlCl3), have long-standing association with isomerisation in earlier refinery practice. While today’s mainstream technologies frequently apply Pt on chlorinated alumina or zeolitic systems, AlCl3 remains the historically emblematic catalyst among the listed choices.

Step-by-Step Solution:
1) Separate isomerisation from alkylation: HF/H2SO4 are typical for alkylation, not isomerisation.2) Identify the classic Lewis acid used for isomerisation: AlCl3.3) Select AlCl3 as the correct answer.

Verification / Alternative check:
Refining texts catalogue AlCl3 among early isomerisation catalysts; shift to chlorinated alumina/Pt reflects improved stability and handling but does not negate AlCl3’s historical role.

Why Other Options Are Wrong:
H2SO4/HF: Hallmark alkylation acids.H3PO4: Used in polymerization, not the classic choice for light paraffin isomerisation.Zeolite Y only: Zeolites are important but the question asks for the classic Lewis acid association.

Common Pitfalls:
Conflating isomerisation with alkylation due to both using strong acids; the processes serve different objectives and use different conventional catalysts.

Final Answer:
AlCl3