Separation of Benzene-Cyclohexane at the Liquid-Solid and Vapor-Solid Interfaces Using Adaptive Molecular Crystals of...

Separation of mixtures containing low-molecular-weight hydrocarbons using sustainable and less energy-intensive processes is one of the grand challenges facing the chemical sciences. In this work, we report the peculiar ability of molecular crystals containing boron-based receptors A1 assembled through dative B←N bonds between 2-(naphthalen-1-yl)naphtho[2,3-d][1,2,3]dioxaborolane and 3,6-di(pyridin-4-yl)-1,2,4,5-tetrazine (BiPyTz), to readily capture and separate small molecules of great industrial interest like benzene (Bz) and cyclohexane (Cy) via the formation of stoichiometric solvates A1·2Cy and A1·2Bz with high conversions. Sorption in mixtures Bz/Cy 1:1 (v/v) showed high selectivity (≥99%) for Bz over Cy in liquid-solid experiments and up to 96% in vapor-solid tests. The dynamics of the vapor-solid sorption process revealed a transient formation of A1·2Cy that gradually converts to the more stable A1·2Bz. Recyclability of the sorbent materials was accomplished without heating through the reversible transformation of A1·2Bz toward the unsolvated form of A1 or A1·MeCN. Receptor A1 is prone to crystallize with MeCN in various stoichiometric ratios with noncentrosymmetric conformations displaying syn orientation of the naphthylboronic fragments, whereas crystals of A1 containing Bz or Cy exhibited a more stable conformation with antiparallel disposition of the aromatic substituents and crystallographic inversion symmetry. Computational studies of adduct-adduct and solvent-adduct interaction energies in the crystals of A1·MeCN, A1·2MeCN, and A1·2Bz confirmed a favorable solid phase transformation toward the Bz solvate due to two factors: (i) the conformational adaptability of receptor A1 to optimize adduct-adduct aromatic interactions, accounting for the major contribution to the crystal stability, and (ii) a larger attachment energy of Bz over MeCN. Overall, our studies demonstrate the potential of receptor-based molecular crystalline materials containing flexible and adaptive acyclic B←N adducts to effectively separate a very challenging hydrocarbon solvent system under gentle conditions and with recycling capabilities.