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.