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).