Control of Pore Size and Functionality in Isoreticular Zeolitic Imidazolate Frameworks and their Carbon Dioxide Selective Capture Properties

Abstract

Five new crystalline zeolitic imidazolate frameworks (ZIFs), ZIF-78 to -82, were prepared from zinc(II) nitrate and mixtures of 2-nitroimidazole and five different functionalized imidazoles and were found to have the GME topology. These structures, along with three previously reported GME ZIFs, constitute a series of highly porous materials with Brunauer−Emmet−Teller surface areas ranging from 620 to 1730 m²/g. The pore diameters and apertures vary incrementally from 7.1 to 15.9 Å and 3.8 to 13.1 Å, respectively, and the functionalities decorating the pores vary from polar cyano- and nitro- groups to nonpolar alkyl groups. The variability expressed in these materials makes them highly attractive for study as gas-separation media. Selectivity values calculated for separation of CO₂ and CH₄ predict that the ZIFs with polar functionality, ZIF-78 (10.6:1) and -82 (9.6:1), retain CO₂ gas to a greater degree than the other members of the GME series and BPL-activated carbon. These predictions are borne out in dynamic breakthrough studies, which confirm the increased capacity of ZIF-78 and -82 and demonstrate the promise of this class of materials.