Layered Silicates\nExhibiting MOF-Like Gate-Opening\nBehaviors in Liquid-Phase Adsorptions: Experimental and Theoretical\nInvestigations

Layered silicates, including clay minerals, can be used\nas liquid-phase\nadsorbents in many important applications. However, because their\ntwo-dimensional interlayer space is narrow and not entirely opened\ndue to the presence of interlayer species, guest species are forced\nto penetrate while expanding the interlayer space, which limits their\nadsorption performances compared with microporous materials such as\nMOFs and zeolites. Herein, as reported for the adsorption of gaseous\nspecies on flexible MOFs, we report a layered silicate that exhibits\ngate-opening adsorption in liquid phases. This layered silicate, synthesized\nvia dilute acid treatment of the parent sodium-type, exhibits an abrupt\nincrease in the basal spacing (layer thickness + interlayer space)\nto reach a plateau even at an earlier stage of benzoic acid adsorption\nfrom acetonitrile, whereas a typical layered silicate, magadiite,\nexhibits a gradual increase in the basal spacing as adsorption progress\nunder identical conditions. The layered silicate shows an excellent\nadsorption capacity and rate for benzoic acid uptake from acetonitrile,\nwhich is considerably higher than that of magadiite. With comprehensive\nadsorption tests using different adsorbates and solvents, we propose\nthat the layered silicate has zeolite-like but distorted, flexible\nopen microchannels within each layer, and the <i>intra</i>layer microchannels can effectively and rapidly accommodate the solvent\n(acetonitrile) molecules, which are capable of expanding the framework\nto initiate the adsorption of aromatic compounds. The density function\ntheory calculation revealed the adsorption mechanism, where the layered\nsilicate accommodates acetonitrile in the <i>intra</i>layer\nmicrochannel followed by the interlayer space, and the former selectively\nplays a role as the adsorption site of aromatic compounds via exchange\nwith acetonitrile.