Figure 10: Gas separation performance for AgZIF-62-a at different composite loading for a) CO2, N2, and CH4 based separation, b )ethylene/ethane based separation, and c) propylene/propane-based separation
The activated ZIF-62@6FDA-DAM membrane was then tested for different gases, including CO2, N2, CH4, ethane, ethylene, propane, and propylene. As mentioned in earlier sections, silver, both in the elemental and ionic state, has been shown in the literature to improve gas separation performance. In the case of CO2/N2 and CO2/CH4 separation, silver nanoparticles embedded in Pebax mixed matrix membrane have been shown to consistently help towards CO2 permeance improvements and selectivity.[40] Similar behaviour can be seen in this experiment, too, as per Figure 10 and Figure 11, whereby the addition of mAgZIF-62 continually increases the permeability of CO2 and increases CO2/N2selectivity until a maximum loading of 20%. At 20% loading, a maximum CO2/N2 selectivity of 27.1±1.7 and CO2 permeability of 864.3±4.4 barrer were achieved, which is significantly higher than pure 6FDA-DAM with a selectivity of 17±0.9 and CO2 permeability of 642.4±4.8 barrer. This corresponds to a selectivity increase of 60% and permeability of 34.5% as compared to a pure 6FDA-DAM membrane. However, an increase to 30% loading starts to lower the selectivity significantly despite a continual increase in permeability, which may be caused by aggregation of mAgZIF-62 particles in the membrane at higher loading and the possible formation of interfacial voids. However, no clear increase in CO2/CH4 selectivity can be seen, which may be caused by proportionally similar permeance improvements for both CH4 and CO2 with increased loading.
Other than CO2-targeted separation, silver nanoparticles have also been demonstrated to help with light hydrocarbon separation. Although silver ions have been shown to result in good selectivity for olefin/paraffin-based separation, they are unstable over an extended period of time.[41] The utilisation of silver nanoparticles has been shown in the literature to be more stable over longer periods, with good selectivity of olefin over paraffin.[10,42] This behaviour is also consistent with our data, as can be seen in Figure 10 and Figure 11. Like previous CO2 – targeted separation, the selectivity for ethylene/ethane and propylene/propane both increases until mAgZIF-62 loading of 20% before dropping, with a maximum selectivity of 3.57 and 10.21, respectively. This corresponds to ethylene/ethane and propylene/propane selectivity increases of 32.8% and 17.2% when compared to the pure 6FDA-DAM membrane. The permeability of both ethylene and propylene also increases by 29.3% and 18.4% to a value of 44.4 and 31.1 barrer respectively at 20% loading.