3.4. Utilisation of Ag Zn-ZIF-62 for gas separation processes
In order to study the practical applicability of the synthesised Ag-doped glassy materials, its capability to selectively separate gases was tested. Silver in its different states, such as Ag ionic liquid, Ag salts, Ag nanoparticles and even AgO, have been shown to have the capability of selectively transporting different gas pairs such as CO2/N2 and olefin/paraffin.[9,34–36] In this section, we attempt to demonstrate the accessibility of the silver nanoparticles embedded within the glassy matrix by studying the gas separation performance in a mixed matrix membrane set. 6FDA-DAM was chosen as the polymer matrix due to its good capabilities in separating CO2/N2, as well as its relatively good selectivity for olefin/paraffin-based separation.[37–39] The melted AgZIF-62 and ZIF-62 particles were dispersed in a 6FDA-DAM polymer solution before being cast and activated.
The membranes were characterised to understand their intrinsic properties. SEM and SEM-EDS were used to study the contact between the dispersed particles and the polymer matrix as can be seen in Figure S 5. As can be seen in Figure S 5a and d, the contact between the filler particles and the polymer matrix is considerably good. No clear void was observed between the AgZIF-62 fillers with the 6FDA-DAM polymer matrix. SEM-EDS showed the presence of the fillers embedded within the membrane, with reasonable particle dispersion and particle size of around 5-10µm. The silver atoms were also seen to be well dispersed within the regions at which Zn is present, indicating that the Ag is well embedded in the ZIF-62 glass/zni and does not leach into the main polymeric section of the membrane. This would allow any improvement to be mainly attributed to the filler rather than any leached silver ions in the 6FDA-DAM matrix.
An analysis using XRD has also shown the successful incorporation of different fillers at different loadings into the 6FDA-DAM matrix as can be seen in Figure S 6a, as the filler retains its characteristic peaks within the 6FDA-DAM matrix, with its intensity increasing with respect to its loading in the membrane. As can be seen in Figure S 6b, all the FTIR peaks present in pure 6FDA-DAM polymers over the range of 600 cm-1 to 4000 cm-1 were present in all of the membranes, indicating that there were no significant changes in the chemical bonds and structure of the polymer matrix were seen. However, in the range between 600 cm-1 to 850 cm-1, a few new peaks were seen in the MMMs. A peak between 610 cm-1 to 615 cm-1 was present only in AgZIF-62 variants of MMMs, which may indicate the interaction of Ag with the polymer matrix. This is as the lower region peaks are usually attributed to the interaction of metal nanoparticles/ions with an organic ligand (typically seen in MOFs). A peak at 665 cm-1 was seen in all AgZIF-62 and ZIF-62 loaded MMMs, which comes from the ZIF-62 particles themselves. Two peaks at about 775cm-1 and 835 cm-1 were seen only in AgZIF-62-a loaded MMMs, which is also present in the AgZIF-62-a particles due to the presence of dense ZIF-zni phase.[32]
A TGA analysis, as can be found in Figure S 6c, demonstrates the membrane decomposition in air at around 500 oC, which shows that the 6FDA-DAM polymer has a thermal decomposition behaviour similar to the previous TGA analysis of the filler particles. The 6FDA-DAM neat polymer has a remaining weight of about 2%, while the 10%, 20% and 30% loaded mAgZIF-62-a membranes all have a residual weight of 5%, 12% and 21%, respectively. The 20% mZIF-62-a membrane has a slightly lower remaining weight of 9.5% when compared to the Ag-loaded variation of similar loading, which is also consistent with the previous particle TGA analysis due to the absence of Ag nanoparticles in the resulting weight. This demonstrates that these membranes are relatively stable up to 500 oC and can be considered for higher temperature-based gas separation in the future.