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)(H 2 O) (DMF = N,N ‘ -dimethylformamide, BDC = 1,4-benzenedicarboxylate) (MOF-2), Zn 3 (BDC) 3 ·6CH 3 OH (MOF-3), Zn 2 (BTC)NO 3 ·(C 2 H 5 OH) 5 H 2 O (BTC = 1,3,5-benzenetricarboxylate) (MOF-4), and Zn 4 O(BDC) 3 ·(DMF) 8 C 6 H 5 Cl (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.