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.