Dual-ligand zeolitic imidazolate framework crystals and oriented films derived from metastable mono-ligand ZIF-108

Abstract

Post-synthetic ligand exchange of metal-organic frameworks (MOFs) is a feasible target-oriented method of preparing dual-ligand MOF crystals with fine-tuneable compositions and functions. Herein, a series of dual-ligand zeolitic imidazolate framework (ZIF) crystals with SOD, GME and GIS topologies were constructed by ligand exchange of the mono-ligand parent materials ZIF-108, Zn(2-nitroimidazolate)2. In-depth investigations on the formation processes of the daughter ZIFs indicated that ligand exchange is a heterogeneous nucleation process using ZIF-108 as seeds. The metastability of ZIF-108 makes the ligand substitution of ZIF-108 require lower activation energy than homogeneous nucleation. This ligand exchange process paves the way for facile synthesis of dual-ligand and dual-metal-dual-ligand ZIF crystals with diverse porous networks. Based on these results, c-out-of-plane oriented dual-ligand ZIF-78 films were synthesized through the evolutionary selection in a van der Drift's type growth originated from randomly oriented ZIF-108 seed layer.