Noncovalent interaction guided selectivity of haloaromatic isomers in a flexible porous coordination polymer.

Porous, supramolecular structures exhibit preferential encapsulation of guest molecules, primarily by means of differences in the order of (noncovalent) interactions. The encapsulation preferences can be for geometry (dimension and shape) and the chemical nature of the guest. While geometry-based sorting is relatively straightforward using advanced porous materials, designing a "chemical nature" specific host is not. To introduce "chemical specificity", the host must retain an accessible and complementary recognition site. In the case of a supramolecular, porous coordination polymer (PCP) [Zn(o-phen)(ndc)] (o-phen: 1,10-phenanthroline, ndc: 2,6-naphthalenedicarboxylate) host, equipped with an adaptable recognition pocket, we have discovered that the preferential encapsulation of a haloaromatic isomer is not only for dimension and shape, but also for the "chemical nature" of the guest. This selectivity, i.e., preference for the dimension, shape and chemical nature, is not guided by any complementary recognition site, which is commonly required for "chemical specificity". Insights from crystal structures and computational studies unveil that the differences in the different types of noncovalent host-guest interaction strengths, acting in a concerted fashion, yield the unique selectivity.

Tailoring a robust Al-MOF for trapping C2H6 and C2H2 towards efficient C2H4 purification from quaternary mixtures.

Light hydrocarbon separation is considered one of the most industrially challenging and desired chemical separation processes and is highly essential in polymer and chemical industries. Among them, separating ethylene (C2H4) from C2 hydrocarbon mixtures such as ethane (C2H6), acetylene (C2H2), and other natural gas elements (CO2, CH4) is of paramount importance and poses significant difficulty. We demonstrate such separations using an Al-MOF synthesised earlier as a non-porous material, but herein endowed with hierarchical porosity created under microwave conditions in an equimolar water/ethanol solution. The material possessing a large surface area (793 m2 g-1) exhibits an excellent uptake capacity for major industrial hydrocarbons in the order of C2H2 > C2H6 > CO2 > C2H4 > CH4 under ambient conditions. It shows an outstanding dynamic breakthrough separation of ethylene (C2H4) not only for a binary mixture (C2H6/C2H4) but also for a quaternary combination (C2H4/C2H6/C2H2/CO2 and C2H4/C2H6/C2H2/CH4) of varying concentrations. The detailed separation/purification mechanism was unveiled by gas adsorption isotherms, mixed-gas adsorption calculations, selectivity estimations, advanced computer simulations such as density functional theory (DFT), grand canonical Monte Carlo (GCMC) and ab initio molecular dynamics (AIMD), and stepwise multicomponent dynamic breakthrough experiments.

Gate Opening without Volume Change Triggers Cooperative Gas Interactions, Underpins an Isotherm Step in Metal-Organic Frameworks.

Three halogenated metal-organic frameworks (MOFs) reported recently exhibited a second step in their CO2 gas adsorption isotherms. The emergence of halogen-bonding interactions beyond a threshold gas pressure between the framework halogen and the CO2 guest was conjectured to be the underlying reason for the additional step in the isotherm. Our investigation employing periodic density functional theory calculations did not show significant interactions between the halogen and CO2 molecules. Further, using a combination of DFT-based ab initio molecular dynamics and grand canonical Monte Carlo simulations, we find that the increased separation of framework nitrate pairs facing each other across the pore channel enables the accommodation of an additional CO2 molecule which is further stabilized by cooperative interactions─an observation that facilely explains the second isotherm step. The increased separation between the nitrate groups can occur without any lattice expansion, consistent with experiments. The results point to a structural feature to achieve this isotherm step in MOFs that neither possess large pores nor exhibit large-scale structural changes such as breathing.

A Dynamic Chemical Clip in Supramolecular Framework for Sorting Alkylaromatic Isomers using Thermodynamic and Kinetic Preferences.

Adsorptive chemical separation is at the forefront of future technologies, for use in chemical and petrochemical industries. In this process, a porous adsorbent selectively allows a single component from a mixture of three or more chemical components to be adsorbed or permeate. To separate the unsorted chemicals, a different adsorbent is needed. A unique adsorbent which can recognize and separate each of the chemicals from a mixture of three or more components is the necessity for the next generation porous materials. In this regard, we demonstrate a "dynamic chemical clip" in a supramolecular framework capable of thermodynamic and kinetics-based chemical separation. The dynamic space, featuring a strong preference for aromatic guests through π-π and C-H⋅⋅⋅π interactions and adaptability, can recognize the individual chemical isomers from mixtures and separate those based on thermodynamic and kinetic factors. The liquid-phase selectivity and separation of the aromatic isomers are possible by the adaptability of the "chemical clip" and here we elucidate the prime factors in a combinatorial approach involving crystallographic evidence and detailed computational studies.

Unraveling the Sorption Mechanism of CO2 in a Molecular Crystal without Intrinsic Porosity.

The facile uptake of CO2 gas in a nonporous molecular crystal constituted by long molecules with carbazole and ethynylphenyl moieties was reported in experiments recently. Herein, the mechanism of gas uptake by this crystal is elucidated using atomistic molecular simulations. The uptake of CO2 is shown to be facilitated by (i) the capacity of the crystal to expand in volume because of weak intermolecular interactions, (ii) the parallel orientation of the long molecules in the crystal, and (iii) the ability of the molecule to marginally bend, yet not lose crystallinity because of the anchoring of the terminal carbazole groups. The retention of crystallinity upon sorption and desorption cycles is also demonstrated. At high enough pressures, near-neighbor CO2 molecules sorbed in the crystal are found to be oriented parallel to each other.