In industry, cyclohexane is obtained exclusively through the hydrogenation of benzene, but the subsequent complete separation of these liquids is very difficult, since they have similar boiling points and form an azeotrope. Their selective adsorptive separation using metal–organic frameworks (MOFs) is one of the promising approaches. Two- and three-dimensional MOFs contain cavities, the size, geometry and chemical nature of which enable the isolation of the desired component from a difficult-to-separate mixture. Combining various organic ligands and metal ions provides almost unlimited design possibilities for MOFs.
The present study considers the sorption of benzene and cyclohexane for five previously reported MOFs based on trans-1,4-cyclohexanedicarboxylic acid acting as an example of a ligand with a saturated carbon skeleton: [Ga(OH)(C8H10O4)] (1), [Ca(H2O)2(C8H10O4)]·H2O (2), [Zr6O4(OH)4(C8H10O4)6] (3) [1], [Cu2(C8H10O4)2] (4) [2], [Al(OH)(C8H10O4)]·H2O (5). The adsorption of individual compounds from the liquid and gas phase and competitive adsorption from mixtures were investigated.
For MOFs 3 and 4, the high selectivity of benzene sorption over cyclohexane was determined, with volume-by-volume selectivity up to ~13. Further, benzene/cyclohexane adsorption selectivities were calculated from the adsorption data using two methods. According to the Henry method, the C6H6/C6H12 Henry constant ratios at low pressures were 7105 for [Zr6O4(OH)4(chdc)6] and 3.63 for [Cu2(chdc)2]. According to the ideal adsorbed solution theory (IAST) for near-atmosphere pressures, the corresponding selectivity coefficients were 23.6 for [Zr6O4(OH)4(chdc)6] and 1.23 for [Cu2(chdc)2].
These results indicate the promising potential of using metal–organic frameworks 3 and 4 with an alicyclic ligand as highly effective sorbents for the separation of the industrial mixtures of benzene and cyclohexane.
[1] Bueken B. et al., Chemistry–A European Journal, 2016, 22, 3264.
[2] Lannoeye J et al., Microporous and Mesoporous Materials, 2016, 226, 292.
