Metal-organic frameworks (MOFs) are porous crystals built from inorganic and organic building blocks, which can be varied nearly at will.  Many MOFs have porosity exceeding the traditional porous materials, porous carbon and the all-inorganic zeolites. Although interactions of adsorbates with the internal MOF surface also among themselves within individual pores have been extensively studied, adsorbate-adsorbate interaction across pore wall have not been explored (Fig. 1).

     Gas adsorption isotherm of IRMOF-74 showed very interesting behavior especially two distinct stages after capillary condensation (Fig. 2/ Fig 4 in the previous Science paper [1]).  In order to track and map the distribution of adsorbates through electron charge distribution, we have set up a new “in-situ” small angle X-ray scattering (SAXS) instrument at KAIST.  We observed extra adsorbtion domains, that is, domains spanning several neighbouring pores which have higher adsorbate density than non-domain pores.  Furthermore in the case of IRMOF-74-V-hex, we could clearly observe transformation to an ordered arrangement of excess adsorbate pores onto a basic hexagonal lattice called “superlattice structure of excess adsorbate pores” on the order of 40 nm in size.

     The whole adsorption process can be schematically drawn showing layer-by-layer, formations of extra adsorption domains followed by superlattice formation, and uniform expansion of the framework (Fig. 3). Through the cooperative behavior of the adsorbates,  these organized gases represent 22 % of the uptake usually not deciphered in the routine gas adsorption (Fig. 2) [2].  

[1] Large-Pore Apertures in a Series of Metal-Organic Frameworks, Hexiang Deng et al., Science 336, 2012, 1018-1023.

[2] Extra adsorption and adsorbate superlattice formation in metal-organic frameworks, Hae Sung Cho et al., Nature 527, 2015, 503-507.

Fig. 1 Three adsorbate-interaction regimes in mesoporous MOFs.  In regime A, adsorbed molecules interact (green arrows) with pore framework walls.  In regime B, adsorbates interact amongst each other (blue arrows) within a pore. These two types of interaction and the corresponding regimes have been well studied. Regime C, however, has not been explored; here, adsorbates interact with each other (red arrows) across pore walls, in a way that is mediated by the framework.

Fig. 2  Five stages (1 to 5) in the gas adsorption isotherm with distinct slopes   Three points (dark-red, dark green and light blue) are highlighted for the start, end/start and end of two events, unobserved in type IV isotherms.  The appearance and disappearance of the broad peak indicates the formation of extra adsorption domains over pores (aggregation, dark-red) and the even distribution of adsorbates (homogenization, blue). Superlattice structure appears as stage 3 turns to stage 4 (organization, green) and disappears at the end of stage 4 (homogenization, blue).

Fig. 3    Schematic illustration of gas adsorption process; Gas molecules adsorb on the pore walls in random and uniform fashion under 27 kPa,  the extra adsorption domains (aggregation point, 30 kPa) and superlattice (organization point, 33 kPa) formed as a result of argon being distributed unevenly among adjacent mesopores. The size of the argon superlattice domain at 33 kPa is about 40 nm.