Figure 9: 13C solid-state NMR on ZIF-62 and silver-loaded ZIF-62 a)crystals and b)glasses.
Solid State NMR has also been performed to help probe the interaction of the carbon atoms within the ZIF-62 structure in the presence of silver. The chemical shift at 125 ppm and 137 ppm could be attributed to the carbon atoms available in the imidazole ligands, while the additional peaks at 115 ppm, 120 ppm, 149 ppm and 160 ppm can be attributed to the carbon atoms present in the benzimidazole ligands. This can be clearly seen in both crystal and melted 13C solid-state NMR in Figure 9, at which the peaks responsible for the benzimidazole ligands are much more pronounced in higher benzimidazole ratio ZIF-62 variants. The 13C solid-state NMR do not show a clear difference between the detected C bonds, except for the case of mAgZIF-62-a, at which an additional peak appears around 123 ppm. This peak can be attributed to the presence of the ZIF-zni structure in the system of mAgZIF-62-a. This is as, according to literature, when ZIF-4 was melted into ZIF-zni, a new set of peaks appeared around the region between 127.8 ppm – 128.3 ppm, which is slightly higher than the other imidazole-related peak in ZIF-4 around 123.9 ppm – 125.8 ppm.[33] As this peak exists very close to the peak responsible for carbon atoms in the imidazolate ligand, it indicates that the presence of Ag may have strongly interacted with the imidazolate ligand during melting/heating for the lower benzimidazole ratio ZIF-62 (AgZIF-62-a).
A quick probe into the porosity behaviour of the synthesised ZIF-62 silver composites was also performed. CO2 adsorption behaviour was studied as previous reports for ZIF-62 have stated that limited N2 adsorption can be seen for ZIF-62 glasses due to their relatively small pore windows and cavities.[21] From the CO2adsorption behaviour seen in Figure S 4, it can be seen that only a slight adsorption behaviour can be seen when silver was doped into the ZIF-62 glass, indicating that minimal pore size change occurred after silver adsorption. The slight increase of CO2 adsorption for ZIF-62-a variation when silver was added may be due to the disruption of ultra-dense ZIF-zni structure by the silver composite, which formed additional pores allowing for some extra pore formation allowing for extra CO2 adsorption. The slight decrease of CO2 adsorption capacity in the ZIF-62-c variation upon the addition of silver composites may be due to the filling up of some pores in the amorphous ZIF-62 glassy structure by silver composite, leading to lower CO2 adsorption.