How to Directly Observe and Understand Adsorption: Gas Adsorption Crystallography

Abstract

Conspectus Adsorption has been explorednot only to improve gas storage capacitybut also to understand the pre-stage of reactions. Porous crystals,such as metal-organic frameworks, zeolites, and mesoporoussilica crystals, have attracted a lot of interest as adsorbents becauseof their well-defined structures and large porosities and the possibilityto design local environments by chemical transformation. Measurementof gas adsorption isotherms is a general approach for the characterizationof porous crystals and in the development of their applications. Unfortunately,such measurements do not directly give crucial information concerningthe adsorption behavior of adsorbates in porous materials, even thoughthey provide knowledge of the overall gas uptake within the material.To overcome this limitation, X-ray diffraction (XRD) combined withgas adsorption (in situ gas adsorption XRD) has been developed inrecent decades. Refinement of in situ XRD data can provide directstructural information on the substrate during adsorption, and Fourieranalysis of structure factors obtained from reflection intensitiesin the in situ XRD data provides information on the total electroncharge distribution and structural information on both the adsorbatemolecules and the porous crystalline material. In this Account, wehighlight our previous studies of the collective adsorption behaviorof porous crystals through measurement and analysis of in situ gasadsorption XRD data, termed "gas adsorption crystallography".Using this technique, it has been possible to determine the pore structuresof mesoporous crystals which were not straightforward to establishdue to their structural nonuniformity from the amorphous walls andfluctuations in liquid crystal phases. Moreover, detailed quantitativemonitoring of adsorption processes in porous crystals with differentpore geometries and chemistries has shown the effect of the pore environmentsof substrates and the nature of the adsorbate species on the collectiveadsorption behavior. As a consequence of these factors, some uniqueevents have been observed and unveiled by gas adsorption crystallographythat could not be detected directly from conventional isotherms, includingthe formation of an adsorbate superlattice structure in IRMOF-74-V-hex,sequential pore filling in PCN-224 and ZIF-412, reverse sequentialpore filling of MOF-205, selective coverage of adsorbates on the porewalls of periodic mesoporous organosilica, and a structural transitionof zeolite MFI caused by adsorption. In addition, molecular simulationcoupled with gas adsorption crystallography has been developed toprovide theoretical knowledge of how the interactions between adsorbatesand the substrate, controlled by the pore environments of the substrateand the nature of the adsorbate species, influence the collectiveadsorption behavior. The aim of this Account is to show that gas adsorptioncrystallography can provide a rigorous physicochemical understandingof adsorption behavior, which can help in the design of adsorbentswith good guest selectivity and high uptake capacity.