The Effect of Pressure on ZIF-8: Increasing Pore Size with Pressure and the Formation of a High-Pressure Phase at 1.47 GPa.

Recent interest in gas storage materials has led to a plethora of papers on the synthesis of novel metal-org. framework materials (MOFs). Since sorption measurements were performed at pressures up to 0.01 GPa, the effect of higher pressures on the framework is relatively unknown, though the mech. stability of some MOFs at high temps. has been discussed. Herein the authors present the 1st high-pressure study on a porous ZIF, ZIF-8 (Zn(MeIM)2, MelM = 2-methylimidazolate) with a sodalite zeolite-type structure and a large accessible pore vol. (greater than 2000 Å3 per unit cell). In summary, by applying pressure to ZIF-8 the hydrostatic medium can be forced to enter the pore, initially increasing the vol. of both the nanopore and the unit cell. On increasing pressure further, more solvent can be forced into the nanopore, even though the nanopore vol. decreases, until the sample undergoes a phase transition. This transition not only allows more solvent to enter the original nanopore, but it increases the size of the narrow channels that connect these pores, resulting in an overall increase in porous vol. and amt. of sequestered solvent. Modification of the pore vol., size, shape, and therefore selectivity has thus been achieved on application of pressure. Pressure could therefore be used as a means of inserting larger mols. into the pores which would otherwise be too small. [on SciFinder(R)]