Removal of Trace Benzene from Cyclohexane Using a MOF Molecular Sieve

Cyclohexane
(Cy), commonly produced by the catalytic
hydrogenation
of benzene (Bz), is used in large quantities as a solvent or feedstock
for nylon polymers. Removing trace unreacted Bz from the Cy product
is technically difficult due to their similar molecular structures
and physical properties. Herein, we report that a metal–organic
framework (MOF) adsorbent shows a molecular sieving effect for Bz
and Cy with record-high Bz/Cy adsorption selectivities (216, 723,
and 1027) in their liquid mixtures (v/v = 1:1, 1:10, and 1:20), and
traps Bz molecules effectively even at low partial pressure in the
vapor phase (e.g., 2.49 mmol/g at 8.2 Pa) or at low content in liquid-phase
Cy (e.g., 128 mg/g at 20 ppm). Over 99% removal of trace Bz (1000
ppm) from liquid Cy could be achieved in one simple stripping step
at room temperature using this sorbent, producing a Cy with >99.999%
purity. Single-crystal structure analyses for guest-free and Bz-loaded
phases of the MOF disclosed that a narrow slit-like pore aperture
and the strong uniting of multiple weak host–guest and guest–guest
interactions are the co-origin of its distinct adsorption property
for Bz and Cy.