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.