A charge-assisted hydrogen-bonded organic framework with polar porosities for selective separation of o-xylene from C₈H₁₀ aromatic isomers

Porous materials have been extensively explored to facilitate the development of energy-efficient separation processes towards handling industrial challenges. Herein, a microporous charge-assisted hydrogen-bonded organic framework, HOF-35, was assembled from 2,3,5,6-tetrakis(4-carboxyphenyl)pyrazine with dimethylamine formed from the decomposition of N,N-dimethylformamide under solvothermal conditions. Single crystal X-ray diffraction (SCXRD) analysis reveals that HOF-35 was assembled from divalent H₂L²⁻ anions and dimethylammonium cations via hydrogen bonding interactions. Three single-crystal to single-crystal transformations were achieved through two-step solvent exchanges followed by a desolvent process, resulting in the activated sample HOF-35a with a Brunauer-Emmett-Teller surface area of 464 m² g⁻¹ according to CO₂ sorption experiment at 196 K. In particular, HOF-35a can selectively adsorb o-xylene over m-xylene, p-xylene, and ethylbenzene on the basis of multicomponent liquid-phase adsorption results. SCXRD studies disclose the exact binding sites and multiple host-guest interactions of the four C8 molecules accommodated in this HOF. Thermogravimetric-differential scanning calorimetry analyses further estimate the highest adsorption enthalpy for o-xylene among these four C8 molecules, accounting for its preferential adsorption in the activated HOF from a thermodynamic perspective. Time-dependent liquid-phase adsorption experiments unveil the kinetic diffusion of C8 mixture. Obviously, the thermodynamic effect in combination with the kinetic diffusion process synergistically promotes the above-mentioned selective separation.