ZSM-5 for Alkylation

ZSM-5 is the shape-selective zeolite for alkylation. Its 10-MR channels (5.1-5.6 Å) discriminate between alkylation products by molecular shape: the linear para-isomer diffuses out of the pore system faster than bulkier ortho- and meta-isomers, producing a product enriched in the para-isomer beyond thermodynamic equilibrium. This is commercially important for p-xylene and p-ethyltoluene production.

How Shape Selectivity Works in Alkylation

Alkylation of toluene (or ethylbenzene) over ZSM-5 generates all three ring isomers at the acid sites inside the pores. The para-isomer — the most linear — diffuses through the 10-MR channels far faster than the bulkier ortho- and meta-isomers. Para-isomer selectivity of 80-97% is achievable, versus the ~24% para fraction at thermodynamic equilibrium.

Applications

• Toluene methylation to p-xylene — the para-selective route to purified terephthalic acid feedstock
• Ethylbenzene/ethyltoluene production with para enrichment
• Benzene alkylation where shape-selective product distribution is valued

Recommended ZSM-5 Grades

Smaller ZSM-5 crystals (100-500 nm) reduce diffusion path length and further enhance para-selectivity. Phosphorus, magnesium, or silicon modification is commonly applied to poison external surface sites that would isomerise the para-product after it exits the pores.

When requesting a sample for alkylation evaluation, specify your reactant pair, target para-selectivity, and preferred crystal size. See the ZSM-5 product page for the full grade range.