The comparison between the results listed in Table 5 with pure gas permeance (see Table 3) reveals that SLV 319 in all fabricated membranes, the CO2 permeance as well as the CO2/CH4 selectivity is lower than those of pure gases. The CO2 permeance for Matrimid membrane decreased from 5.1 to 4.8 GPU, when the mixed gas was fed to the membrane and also the CO2/CH4 selectivity was reduced from 14.8 to 13.5. The high concentration of CH4 in gas mixture which resulted in the CO2/CH4 competitive sorption caused the decline in performance of Matrimid membrane under mixed gas condition (Ebadi Amooghin et al., 2015). In other words, co-presence of CH4 with CO2 in the mixed gas feed prevents CO2 to pass through membrane easily.
A similar behavior in CO2 permeance and CO2/CH4 selectivity in MMMs was perceived. Again the permeance values of CO2 as well as the selectivity of CO2/CH4 are lower than pure gases permeation test. The decrease of CO2 permeance and the following reduction of CO2/CH4 selectivity values in MMMs are mostly attributed to the three subsequent reasons:i)In MMMs those containing MIL-53, the presence of CH4 with CO2 in gas mixture, prevents further adsorption of CO2 by MIL-53 particlesii)Existence of CH4 inside the membrane prevents the extra condensation of CO2. Consequently, the CO2 solubility and its permeance are decreased.iii)CO2 molecules have plasticization effect on polymer matrix which increases the polymer chain movement and consequently increases the membrane free volume; therefore a larger CH4 molecule in gas mixture can diffuse through membrane.