Sustainable Synthesis of Zeolitic Imidazolate Frameworks at Room Temperature in Water with Exact Zn/Linker Stoichiometry

Abstract

Zeolitic imidazolate frameworks (ZIFs) are widely used MOFs because of certain characteristics, but also because they can be prepared at room temperature using water as the unique solvent. However, these a priori sustainable conditions inevitably entail a huge and somehow unusable excess of linker. Here, we present the formation of ZIFs at room temperature in water, starting from mixtures with a linker/metal ratio of two, that is, coinciding with the stoichiometry found in the final MOFs, in the presence of amines. ZIF-8 can be prepared with triethylamine (TEA), giving a yield of Zn of 96.6%. Other bases, like NaOH, tetraethylammonium hydroxide or ammonium hydroxide, do not lead to ZIF-8 under the same conditions. The so-obtained ZIF-8 contains TEA inside its cavities, making it less porous than its conventionally prepared counterparts. Amine can be removed by mild thermal treatments (200–250 °C). Such thermal treatments induce the generation of g-C3N4-like species which could give added value to these materials as potential photocatalysts, increasing their affinity to CO2, as proved in this work. This methodology can be successfully extended to other amines, like N,N-dicyclohexylmethylamine, as well as to other prepared ZIFs, like Co-based ZIF-67, isostructural to ZIF-8.