Matrimid®/polysulfone blend mixed matrix membranes containing ZIF-8 nanoparticles for high pressure stability in natural gas separation

Plasticization is of important concern in high pressure natural gas separation. Majority of the pure polymers and MOF-MMM systems suffer from plasticization at low pressures. Combination of polymer blending and MMM approach could lead to plasticization resistant membranes with improved membrane performance. In this work, the gas separation performance of Matrimid® and PSF blend membranes containing ZIF-8 nanoparticles, were studied. The effect of ZIF-8 loading on the gas transport and plasticization behavior of the blend-MOF MMMs was investigated. Gas permeability measurements of PI/PSF-ZIF-8 MMMs showed significant improvements in permeability of both CO₂ and CH₄, compared to pure reference polymer membrane. At a high loading of 30 wt% ZIF-8, MMM showed an enhancement of 136% in CO₂ permeability compared to the pure PI/PSF membrane, while the selectivity stayed constant. This increase in the permeability could be related to the moderate increase in sorption capacity and faster diffusion of gases through the ZIF-8 nanoparticles. More detailed analysis of the gas transport performance of the MMMs showed that the CO₂ permeability and the CO₂/CH₄ selectivity were mainly governed by the increase in CO₂ diffusion coefficient. Blend MMMs were considerably more resistant to plasticization phenomenon compared to the pure PI membrane. For pure gases, native PI/PSF blend membrane showed a plasticization pressure of ∼18 bar, which increased to ∼25 bar up to 30 wt% ZIF-8 loading. Short and long CO₂ exposure time experiments, with different upstream pressures, showed that the permeability stayed constant in time at pressures below the plasticization pressure while a noticeable increase in permeability was recorded in time at higher pressures. Additionally, these experiments showed that the extent of dilation in the pure PI/PSF membrane was much higher compared to the PI/PSF-ZIF-8 MMMs. In mixed gas (50/50 mol% CO₂/CH₄) experiments, the pure PI/PSF membrane and the MMMs did not show plasticization over the pressure range investigated, as confirmed by a constant mixed gas CH₄ permeability and nearly constant selectivity with pressure. While the pure PI and PI-ZIF-8 MMMs showed the plasticization behavior. Comparing the mixed gas performance of PI/PSF-ZIF-8 membranes with that of pure PI/PSF membranes at high pressures an increase of 197% in CO₂ permeability and 30% in CO₂/CH₄ selectivity was obtained. These results showed that the combination of polymer blending and the mixed matrix membrane approach offers a simple and economically versatile tool to approach higher CO₂ permeabilities and CO₂/CH₄ selectivities while maintaining a stable separation performance in processes involving higher CO₂ partial pressures.