Highly porous and stable metal-organic frameworks: Structure design and sorption properties

Abstract

Gas sorption isotherm measurements performed on the evacuated derivatives of four porous metal-organic frameworks (MOF-n), Zn(BDC)·(DMF)(H2O) (DMF = N,N'-dimethylformamide, BDC = 1,4- benzenedicarboxylate) (MOF-2), Zn3(BDC)3·6CH3OH (MOF-3), Zn2(BTC)NO3·(C2H5OH)5H2O (BTC = 1,3,5-benzenetricarboxylate) (MOF-4), and Zn4O(BDC)3·(DMF)8C6H5Cl (MOF-5), reveal type I isotherms for n = 2, 3, and 5, which is evidence of microporous and accessible channels having high structural integrity and organization. Although gas sorption into MOF-4 was not observed, careful examination of its ethanol sorption isotherms at 22 and 32 °C point to the presence of coordinatively unsaturated zinc centers within its pores, which upon ethanol sorption undergo coordination transitions from 3- to 4-, 4- to 5-, and 5- to 6-coordination. MOF-n materials were produced by building the extended analogues of molecular metal carboxylate clusters - a strategy that has allowed the realization of the most porous and thermally stable framework yet reported: the evacuated form of MOF-5 is especially stable in air at 300 °C and has a free pore volume representing 55-60% of its crystal as determined by gas sorption and single- crystal diffraction studies.