What is Alkylation?
Alkylation is the reaction that attaches an alkyl group to an aromatic ring or other substrate — the industrial route to ethylbenzene, cumene, and linear alkylbenzene, the intermediates for styrene, phenol/acetone, and detergents. Zeolite catalysts have largely replaced liquid mineral acids (HF, H₂SO₄, AlCl₃) in these processes, because solid-acid catalysis eliminates the corrosion, separation, and disposal problems of liquid acids.
The Alkylation Reaction
In aromatic alkylation, an olefin (ethylene, propylene, or longer) reacts with an aromatic (benzene, toluene) over a solid acid catalyst:
- Benzene + ethylene → ethylbenzene
- Benzene + propylene → cumene (isopropylbenzene)
- Benzene + C₁₀-C₁₃ olefin → linear alkylbenzene (LAB)
The reaction proceeds through carbocation intermediates on Brønsted acid sites. Zeolites provide these sites in a shape-selective, regenerable, environmentally benign form.
Zeolite vs Liquid Acid Catalysts
| Property | Zeolite (solid acid) | HF / AlCl₃ (liquid acid) |
|---|---|---|
| Corrosion | None | Severe |
| Waste/neutralisation | None | Significant (CaF₂, Al sludge) |
| Separation | Catalyst stays in reactor | Acid must be separated and recycled |
| Regeneration | Thermal, multiple cycles | Fresh acid makeup |
| Shape selectivity | Available (ZSM-5 para-selectivity) | None |
Which Zeolites Are Used
| Zeolite | Role | Examples |
|---|---|---|
| Beta | General aromatic alkylation, large-pore access | Ethylbenzene, cumene, LAB |
| Y zeolite | High-throughput, high-capacity alkylation | Bulk alkylation duty |
| ZSM-5 | Para-selective alkylation | p-xylene, p-ethyltoluene |
Beta zeolite (SiO₂/Al₂O₃ 20-50) is the most common general alkylation catalyst, combining 12-MR 3D pore access for aromatics and olefins with strong Brønsted acidity. ZSM-5 is chosen specifically when the product must be enriched in the para-isomer beyond thermodynamic equilibrium.
See best zeolite for alkylation for the selection comparison. When requesting a sample, specify your reactants, target product, and whether you need H-form powder or extrudates.
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