Figure 12: Demonstration of
reverse hydrocarbon selectivity for olefin species with the presence of
silver nanoparticles.
Another observation was that
with similar carbon content, alkene/olefin always has a much higher
permeability than its alkane/paraffin counterpart. This can be explained
by the π–Ag complexation allowing the alkene bond (C=C) to overlap with
the 5s and 4d orbitals present in Ag.[10,42,43]However, as can be seen in Figure 12, the permeability trend with
increasing carbon amount differs with the presence of silver
nanoparticles. An expectation of the permeability trend of molecules
with very similar physical and chemical properties is that it should
follow the trend of smaller particles having higher permeability than
their bigger counterpart. This trend can be seen for paraffin molecules
despite loading type and amount (CH4 >
C2H6 >
C3H8). However, in the presence of
silver, the permeability of alkene increases with an increase in
molecular size (C3H6 >
C2H4), while the reverse of that
behaviour was seen in membranes and mixed matrix membranes with no
silver composites (C3H6 <
C2H4). Although this behaviour has been
shown in terms of adsorption with the presence of silver ions, to our
knowledge, it has never been demonstrated for silver nanoparticles and
in terms of membrane permeability.[43] The
adsorption capacity data of olefins for Ag+ decorated
particles from literature also follows the order of C3> C2 > C1,
which was explained to be due to the decreasing sequence of the light
hydrocarbon’s critical temperatures (Tc). However, this
behaviour was also reported to be similar for paraffins, which is not
the case for our silver loaded mixed matrix membranes. Thus, this may
demonstrate a facilitated diffusion-based transport in our silver loaded
membrane system, at which the silver complexation helps improve the
diffusion of olefin over paraffin without strongly bonding to the olefin
molecules.
A possible explanation for the observation of this behaviour may come
from the transport mechanism of the olefin itself. As each of the
propylene and ethylene has only one alkene bond (C=C), and the system
has a constant amount of π bond site available on the Ag surface;
theoretically, the molar amount of π–Ag complexation that is able to be
achieved by both propylene and ethylene should be relatively similar.
The π–Ag complexation also occurs in the same electron orbital for both
propylene and ethylene, and thus, the complexation energy (for
adsorption and desorption) should be relatively close. Propylene has a
higher molecular weight and consequentially higher gas density when
compared to ethylene. Thus, when measured on a weight or volume basis, a
larger amount of propylene will be adsorbed than ethylene (when
considering that a similar molar amount was adsorbed for both species).
This higher weight/volume amount of propylene-Ag complex formed can then
be translated to a higher facilitated transport rate by weight/volume
when compared to ethylene, which is consistent with the result seen in
Figure 12. However, more experiments will have to be performed to
confirm this theory as many other factors also come into play, such as
the effect of propylene vs ethylene sizes which may reduce the molar
adsorption density on the Ag nanoparticle surface, and possible
differences of (C=C) π–Ag complexation energies due to the presence of
extra alkyl chain in propylene when compared to ethylene.