Catalyst system for low-pressure Ziegler process producing high-density polyethylene (HDPE): identify the correct catalyst combination.

Introduction / Context:
Ziegler–Natta catalysts revolutionised polyethylene production by enabling polymerisation at low pressure with controlled stereochemistry and higher density. Recognising the classical catalyst pairs is essential in polymer process engineering.

Given Data / Assumptions:

• HDPE via low-pressure routes uses transition metal halides with organoaluminium co-catalysts.
• Typical systems: TiCl3/TiCl4 with AlEt3 (aluminium triethyl).

Concept / Approach:
The organoaluminium compound alkylates and activates the titanium species, producing active sites for ethylene insertion. Precious metals like platinum are not used for HDPE Ziegler processes; nickel and molybdenum are not the classical choices here.

Step-by-Step Solution:
Select the titanium + organoaluminium pair: AlEt3 + TiCl4.Eliminate single-metal options lacking Ziegler pairing.

Verification / Alternative check:
Polymerisation texts list TiClx/AlEt3 variants as industry standards for HDPE and PP.

Why Other Options Are Wrong:
Pt, Ni, Mo are not the conventional catalysts for low-pressure HDPE Ziegler systems.

Common Pitfalls:
Confusing Ziegler–Natta with Phillips (Cr/SiO2) catalysts; both produce HDPE but are different families.

Final Answer:
Aluminium triethyl activated with TiCl4