Effect of temperature on solubility and diffusivity
Effect of temperature was also tested for both penetrant solubilities
and diffusivities in pure PDMS, ZIF-8, and Ni-ZIF-8 based MMMs. Figure
6(a, b), and Figure S15 (a, b) show the solution coefficients and
solution selectivities of BD with respect to the feed temperatures over
Ni-ZIF-8 MMMs, and ZIF-8 MMMs, respectively. It was noticed that the
solution coefficient of both the penetrants was decreased with respect
to the feed temperatures. The BD solution coefficient values decreased
from 35.13 to 26.5 in 15% in Ni-ZIF-8 MMM, from 31.9 to 24.12 in 15%
ZIF-8 MMM, and from 24.67 to 19
(cm3(STP).cm-3 membrane.
bar-1) in PDMS from 40 to 50°C, respectively. These
findings indicate the sorption domination in BD permeation across the
membrane, as the BD permeance and ideal selectivity also decreased at
elevated temperatures.
The diffusion coefficient values of BD and diffusion selectivity
increase with respect to feed temperatures shown in Figure 6 (c, d), and
Figure S15 (c, d) for Ni-ZIF-8 MMMs, and ZIF-8 MMMs, respectively. In
rubbery polymers such as PDMS, the diffusion coefficient normally
increased by the increase in temperature. BD diffusion coefficient
increased 19% in pure PDMS, 16% in 15% ZIF-8 MMM, and 17% in 15%
Ni-ZIF-8 MMM, respectively. On the other side, the diffusion selectivity
also increased with respect to the increase in temperature. It was noted
that the selectivity enhancement was slightly more (11%) in pure PDMS
as compared with 15% ZIF-8 MMM (9%), and 15% Ni-ZIF-8 MMM (8.5%),
which can be attributed to decreased BD sorption at elevated
temperatures in ZIF-8 and Ni-ZIF-8.