Zeolitic imidazolate frameworks (ZIFs), a very important subfamily of metal-organic frameworks (MOFs), are constructed from tetrahedrally coordinated M2+ (Zn2+ or Co2+) ions, which are interlinked by imidazolate linkers. Recently, we discovered that the prototypical ZIF-4 (chemical composition M(im)2, im– = imidazolate) undergoes a phase transition from an open pore (op) to a closed pore (cp) phase under hydrostatic pressure. Here we further present the high-pressure (HP) behaviour of a series of isostructural ZIF-4 derivatives, named ZIF-62, with the general chemical composition M(im)2-x(bim)x (bim– = benzimidazolate, 0.02 < x < 0.35). The ZIF-62 materials crystallize in the space group Pbca and feature the same network topology (cag) as the original ZIF-4, however, some of the im– linkers are substituted by the bulkier bim– linkers. HP-PXRD experiments were performed at beamline I15 of Diamond Light Source in the pressure range from ambient up to 4000 bar using silicon oil as a non-penetrating pressure transmitting medium and a hydraulic pressure cell. Under hydrostatic pressure, all ZIF-62 derivatives reversibly contract from the op to the cp phase only by rotations about the imidazolate-metal bonds (Fig. 1). Crystal symmetry and network topology are preserved. With increasing bim–-concentration, the threshold pressure for the op-cp phase transition increases from 700 bar to 1300 bar and the overall volume contraction across the transition decreases from 25% to 17% of the initial volume. Most importantly, the 1st order (discontinuous) transition transfers to a 2nd order (continuous) transition for x > 0.30. Thus, the void space and pore openings of ZIF-62 can be tuned continuously by the application of mechanical pressure – a unique feature which might be useful for adjusting and enhancing the gas separation performance of these flexible MOFs.
Fig. 1. Structural models of ZIF-62 with the composition Zn(im)1.65(bim)0.35 at ambient pressure (left) and 4000 bar (right). The void space available for CO2 molecules (kinetic diameter = 3.30 Å) is shown in gold.
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