Molecule-Based Crystalline Adsorbents: Advancing Adsorption Theory and Storage/Separation Applications

As a simple and common physicochemical process, adsorption is the basis of storage, separation, and many other applications. Compared to conventional adsorbents, molecule-based crystalline materials show advantages of extremely rich and easily designable/synthesized/characterized structures as well as remarkable flexibility. The emergence of new adsorbent materials has brought forth both opportunities and challenges for adsorption theory and its applications.

Focusing on the simplest applications of adsorption, i.e., storage and separation, this Account aims to analyze the representative adsorbent engineering strategies. First, we provide a brief introduction to conventional adsorption theory and the fundamental principles of adsorptive storage and separation applications. Following that, we discuss how the special structural characteristics of molecule-based crystalline adsorbents, especially their flexibility, provide new insights and directions.

According to the well-established adsorption theory, molecule-based crystalline porous materials offer not only exceptionally large pore volumes and specific surface areas but also a wide variety of adsorption sites with a tunable guest binding affinity. More importantly, the concentration and position of the adsorption sites can be engineered and straightforwardly visualized. By rationally tuning host–guest and guest–guest interactions, the adsorption isotherm shape can be regulated to increase working capacity and selectivity and even inverse selectivity to meet practical separation demands.

Inspired by the structural flexibility of molecule-based crystalline materials, we need to consider the structural transformations of host–guest systems in the adsorption processes, in not only the thermodynamic but also the kinetic aspects. Rational classification of the various types of flexibility or structural transformations is crucial for elucidating the structure–property relationships in these host–guest systems. As the most well-known type of flexibility, guest-induced crystal-to-crystal structural transformations are thermodynamically controlled and occur periodically at the equilibrium state, which can be conveniently and straightforwardly visualized by diffraction techniques. Particularly, the pore-opening action (nonporous-to-porous transformation) can offer exceptionally high working capacity and remarkable advantages in terms of thermal effects. In the matter of crystalline adsorbents, this flexibility can also be aperiodic, which can effectively address the coadsorption and leakage issues to give high adsorption selectivity and purification productivity. The structural flexibility of host–guest systems can also be kinetically controlled and occur at the nonequilibrium state (and aperiodically). Gating flexibility describes the transient structural transformations for diffusion of oversized guest molecules, which not only provides more comprehensive mechanisms and criteria for kinetic separation and molecular sieving but also can be used to achieve unique thermodynamic behaviors, such as guest flexibility. The combination of advanced adsorption theory and the unique characteristics of molecule-based crystalline adsorbents will open up new avenues for related applications.