Conclusions
In this study, ZIF-8 and Ni-ZIF-8 nanoparticles were incorporated with PDMS for the synthesis of MMMs, and the separation mechanisms of butadiene and nitrogen, molecular solution-diffusion properties through the MMMs, were investigated comprehensively.
Ni-ZIF-8 nanoparticles have been used for the first time with PDMS to synthesize MMMs. The resulting Ni-ZIF-8/PDMS MMMs significantly improved thermal stability, microstructure, permeance, and selectivity relative to pure PDMS, and ZIF-8/PDMS MMMs. Interestingly, Ni-ZIF-8 MMM significantly improved BD permeance and selectivity at the same time, breaking down the trade-off effect. Ni-ZIF-8/PDMS MMMs showed greater BD affinity relative to ZIF-8/PDMS MMMs under the same experimental conditions. BD permeance in 15% Ni-ZIF-8 was improved by 67% from PDMS, and 32% from 15% ZIF-8 MMM. The ideal selectivity of BD/N2 of the corresponding 15% Ni-ZIF-8 MMM was increased by 81% and 37% from pure PDMS and 15% ZIF-8/MMM, respectively. The higher BD/N2 ideal selectivity of Ni-ZIF-8/PDMS over that of ZIF-8/PDMS was attributed to the effective aperture size and high affinity of Ni-ZIF-8 with butadiene.
The solution-diffusion results revealed in detail for these mixed-matrix membranes. The solubility, diffusivity, solubility selectivity, and diffusion selectivity were improved in Ni-ZIF-8@PDMS MMMs. Permeation activation energies revealed that the 15% Ni-ZIF-8 MMM showed less activation energy of diffusion, the activation energy of permeation, activation entropy change, and sorption enthalpy relative to pure PDMS, and 15% ZIF-8 MMM. All of these findings strongly support the use of 15% Ni-ZIF-8 MMM for the efficient separation of BD and nitrogen, which is available in the tail gas of the synthetic rubber industry. This study will provide in-depth knowledge of gas transport properties in ZIFs-based MMMs for the separation of VOCs from permanent gases.