Shape-Selective Zeolite Catalysis: The Molecular Sieve (2D)

Pore Geometry

Pore diameter 0.55 nm

Feed Composition (relative weight)

● n-Hexane (linear, 0.43 nm) 1

● Isooctane (branched, 0.62 nm) 1

● Mesitylene (bulky aromatic, 0.74 nm) 1

Kinetics

Active-site reaction rate 1.20 s⁻¹

Feed rate 6 /s

Live Readout

Molecules fed

Overall yield

0.0%

Admitted / Blocked

0 / 0

Selectivity (linear)

—

Species	Fed	Adm.	Prod.

How it works

A zeolite's crystalline framework is riddled with rigid, uniform channels. Shape-selective catalysis relies on a purely geometric filter: a molecule can only enter a pore — and reach the internal active site — if its kinetic diameter is smaller than the pore's free diameter. Everything else collides with the crystal exterior and never reacts.

Species	Kinetic Ø
n-Hexane (linear)	0.43 nm
Isooctane (branched)	0.62 nm
Mesitylene (bulky aromatic)	0.74 nm

These three values and the 0.3–0.8 nm slider range mirror real zeolites: ZSM-5's ~0.55 nm channels pass linear paraffins but reject branched and bulky aromatics (the basis of catalytic dewaxing), while wider-pore faujasite (~0.74 nm) admits nearly everything.

Diffusion — each molecule performs a size-biased random walk (larger molecules diffuse more slowly, Stokes–Einstein-style) toward the membrane.
Pass/block test — at each pore gate, diameter(molecule) ≤ diameter(pore) is the only admission rule; misses bounce straight back into the feed.
Reaction — an admitted molecule reacts at the internal active site with probability (reaction rate × Δt) per frame, then exits right as product, feeding the yield counter.

n-Hexane (linear) Isooctane (branched) Mesitylene (bulky) Product