5. Conclusion
The presence of silver as guest molecules has been shown to provide a significant impact on the melting behaviour of ZIF-62. The benzimidazole ratio within the ZIF-62 structure was shown to directly impact the resulting dynamic thermal behaviour. At a lower benzimidazole ratio, the ZIF-62 tends to form ZIF-zni at a much lower temperature, while at a higher benzimidazole ratio, ZIF-62 was able to melt fully into an amorphous glass without any resulting ZIF-zni. An additional look into the Ag interaction using THz-IR shows that the Ag may have significant interaction with the ligands, especially the benzimidazole ligand, which may explain the difference in behaviour at a lower benzimidazole ratio. This chemical interaction analysis, coupled with solid-state NMR, also indicates that the Ag may have interacted at different sites within the ZIF-62 structure based on the initial benzimidazole ratio amount in the structure. The resulting Ag-doped thermally treated ZIF-62, both in ZIF-zni and amorphous glass form, forms accessible Ag nanoparticles within the thermally treated ZIF-62 framework. This was shown by good gas separation performance when the materials were applied as a gas separation membrane in a mixed-matrix membrane setting. The gas separation analysis also shows Ag nanoparticles’ capability to reverse the alkene selectivity as its size increase (C3H6 > C2H4), which has never been demonstrated before in a membrane.
The success in the formation of functional thermally treated ZIF-62 in both ZIF-zni and amorphous ZIF-62 when doped with silver demonstrates the suitability of this material to be composited with other functional metallic molecules. The ease of processibility of these meltable ZIFs also opens its usage in the form of a thin film for a variety of real-life applications such as for gas separation or film-based catalysis and functional films (eg. antimicrobial glass).