Hyperfine adjustment of flexible pore-surface pockets enables smart recognition of gas size and quadrupole moment

Abstract

The pore size and framework flexibility of hosts are of vital importance for molecular recognition and related applications, but accurate control of these parameters is very challenging. We use the slight difference of metal ion size to achieve continuous hundredth-nanometer pore-size adjustments and drastic flexibility modulations in an ultramicroporous metal-organic framework, giving controllable N₂ adsorption isotherm steps, unprecedented/reversed loading-dependence of H₂ adsorption enthalpy, quadrupole-moment sieving of C₂H₂/CO₂, and an exceptionally high working capacity for C₂H₂ storage under practical conditions (98 times that of an empty cylinder). In situ single-crystal X-ray diffraction measurements and multilevel computational simulations revealed the importance of pore-surface pockets, which utilize their size and electrostatic potential to smartly recognize the molecular sizes and quadruple moments of gas molecules to control their accessibility to the strongest adsorption sites.

Fig. 1. (a) The framework and pore structure of [M₃(vtz)₆] (black dashed lines: linkers of the dia topology, yellow sphere: cavity of the pore system, green cylinders: channels connecting adjacent cavities, orange spheres: pore-surface pockets, and light green cylinders: pocket entrances). (b)–(d) Structures of the pore-surface pockets of Zn, Mn and Cd, respectively, in a static point of view (entrances are highlighted by light-green spheres with aperture diameters in the unit of Å).

Fig. 2. (a) Stoichiometric/non-stoichiometric N₂ adsorption isotherms of [M₃(vtz)₆] measured at 77 K. (b) PES of a N₂ molecule inserting into the pocket calculated using DFT based on rigid structures. D is the distance between the pocket entrance and the molecular centroid of N₂. The insets are the three typical host–N₂ structures for Mn. (c) Top and (d) side views of host–guest configurations of four kinds of pocket in the single-crystal structure of Cd·2N₂. Thermal ellipsoids are drawn at 50% probability. The N₂ molecule at Site-IIIa exhibits symmetry-induced 3-fold disorder.

Fig. 3. (a) The coverage-dependent H₂ adsorption enthalpies (Q _st) calculated using the Clausius–Clapeyron equation using original data without fitting (points) and using the virial equation (lines). (b) PES of a H₂ molecule inserting into the pore-surface pockets calculated using DFT based on rigid structures. D is the distance between the pocket entrance and the molecular centroid of H₂. The insets are four typical host–H₂ structures for Mn placed at their corresponding PES positions.

Fig. 4. PES of a CO₂/C₂H₂ molecule moving linearly between the bottoms of two pockets (connected by a channel) in [M₃(vtz)₆] supposing a rigid host. D is the distance between the centers of the channel and the gas molecule. Inset: a portion of Cd (scaled to fit the abscissa) with three typical guest positions.

Fig. 5. (a) C₂H₂ adsorption isotherms of Cd at 273, 283 and 298 K. The predicted isotherm was obtained based on the Clausius–Clapeyron equation and isotherms measured at 273, 283, and 298 K. The two dashed lines represent the practical working limits of the charging and discharging pressures. (b) Comparison of the USCs and utilization ratios for C₂H₂ storage parameters of representative MOFs.