How to Directly Observe and Understand Adsorption: Gas Adsorption Crystallography

ConspectusAdsorption has been explored not only to improve gas storage capacity but also to understand the pre-stage of reactions. Porous crystals, such as metal–organic frameworks, zeolites, and mesoporous silica crystals, have attracted a lot of interest as adsorbents because of their well-defined structures and large porosities and the possibility to design local environments by chemical transformation. Measurement of gas adsorption isotherms is a general approach for the characterization of porous crystals and in the development of their applications. Unfortunately, such measurements do not directly give crucial information concerning the adsorption behavior of adsorbates in porous materials, even though they provide knowledge of the overall gas uptake within the material. To overcome this limitation, X-ray diffraction (XRD) combined with gas adsorption (in situ gas adsorption XRD) has been developed in recent decades. Refinement of in situ XRD data can provide direct structural information on the substrate during adsorption, and Fourier analysis of structure factors obtained from reflection intensities in the in situ XRD data provides information on the total electron charge distribution and structural information on both the adsorbate molecules and the porous crystalline material. In this Account, we highlight our previous studies of the collective adsorption behavior of porous crystals through measurement and analysis of in situ gas adsorption XRD data, termed "gas adsorption crystallography". Using this technique, it has been possible to determine the pore structures of mesoporous crystals which were not straightforward to establish due to their structural nonuniformity from the amorphous walls and fluctuations in liquid crystal phases. Moreover, detailed quantitative monitoring of adsorption processes in porous crystals with different pore geometries and chemistries has shown the effect of the pore environments of substrates and the nature of the adsorbate species on the collective adsorption behavior. As a consequence of these factors, some unique events have been observed and unveiled by gas adsorption crystallography that could not be detected directly from conventional isotherms, including the formation of an adsorbate superlattice structure in IRMOF-74-V-hex, sequential pore filling in PCN-224 and ZIF-412, reverse sequential pore filling of MOF-205, selective coverage of adsorbates on the pore walls of periodic mesoporous organosilica, and a structural transition of zeolite MFI caused by adsorption. In addition, molecular simulation coupled with gas adsorption crystallography has been developed to provide theoretical knowledge of how the interactions between adsorbates and the substrate, controlled by the pore environments of the substrate and the nature of the adsorbate species, influence the collective adsorption behavior. The aim of this Account is to show that gas adsorption crystallography can provide a rigorous physicochemical understanding of adsorption behavior, which can help in the design of adsorbents with good guest selectivity and high uptake capacity.