Controlling Guest Diffusion by Local Dynamic Motion in Soft Porous Crystals to Separate Water Isotopologues and Similar Gases.

Abstract

ConspectusThe precise and effective separation of similar mixtures is one of the fundamental issues and essential tasks in chemical research. In the field of gas/vapor separation, the size difference among the molecular pairs/isomers of light hydrocarbons and aromatic compounds is generally 0.3-0.5 Å, and the boiling-point difference is generally 6-15 K. These are necessary industrial raw materials and have great separation demands. Still, their separation mainly relies on energy-intensive distillation technology. On the other hand, remarkably similar substances such as oxygen/argon and isotopologues usually exhibit size differences of only 0-0.07 Å and boiling-point differences of only 1-3 K. Although their industrial separation can be realized, their efficiency is considerably low. Therefore, effectively separating remarkably similar mixtures is crucial in fundamental chemistry and industry, but it remains a significant challenge. Porous coordination polymers (PCPs) or metal-organic frameworks (MOFs) are emerging materials platforms for designing adsorbents for separating similar mixtures. However, the reported PCPs did not work well for separating remarkably similar substances. The framework structures of the mainstream PCPs remain unchanged (rigid) or significantly change (globally flexible) upon adsorption. However, rigid and globally flexible PCPs find controlling the pore aperture in subangstrom precision challenging, a prerequisite for distinguishing remarkably similar substances. Thus, novel mechanisms and materials design principles are urgently needed to realize PCPs-based adsorptive separation of remarkably similar mixtures.To confront the obstacles in separating remarkably similar mixtures, our group started contributing to this field in 2017. We employed locally flexible PCPs as the materials designing platform, whose local motions of the side substituent groups potentially regulate the pore apertures to design and control the gas/vapor diffusion in PCPs. Specifically, we encoded dynamic flipping molecular motions into the diffusion-regulatory gate functionality. The ligands were designed by integrating carboxylic coordination groups with nonplanar fused-ring moieties, with the latter moieties exhibiting flipping motion around their equilibrium positions with small energy increases. Such local motions of ligands lead to the dynamic opening and blocking of PCP channels, thus termed flipping dynamic crystals (FDCs). FDCs feature distinctive temperature-responsive adsorption behaviors due to the competition of thermodynamics and kinetics under diffusion regulation, enabling differentiation of remarkably similar mixtures by each gate-admission temperature much higher than the boiling-point temperature of each component. Even when the molecular sizes are the same in the water isotopologue mixtures, FDCs can separate each isotopologue by amplifying their diffusion-rate differences. Finally, by combining the thermodynamic and kinetic factors, FDCs achieve temperature-switched recognition of CO₂/C₂H₂ and diffusion-rate sieving of C₃H₆/C₃H₈. Therefore, our work provides a platform for designing locally flexible PCPs by introducing subangstrom precision in flexibility. This opens up the feasibility of separating remarkably similar mixtures on scientific principles. In this Account, we summarize our above ongoing research contributions, including (i) the design of flipping ligands and FDCs, (ii) the characterization of flipping motions, (iii) the gas/isotopologue sorption behaviors, and (iv) the separation of gases and isotopologues. Overall, our studies offer a new aspect of soft porous crystals and provide future opportunities for relevant researchers in this field.