Investigation of ZIF-78 Morphology and Feed Composition on the Mixed Gas CO₂/N₂ Separation Performance in Mixed Matrix Membranes

In this work, the influence of the zeolitic imidazolate framework 78 (ZIF‑78) morphology, with 1D pores, on the mixed matrix membrane (MMM) CO₂/N₂ mixed gas separation performance is investigated as well as the influence of the feed composition and pressure. Low aspect ratio and a high aspect ratio ZIF‑78 particles are synthesized and incorporated in Matrimid with 10 and 20 wt% additive content. High pressure CO₂ and N₂ sorption measurements show that both the low and high aspect ratio ZIF‑78 metal–organic frameworks (MOFs) exhibit similar sorption behavior. The incorporation of ZIF‑78 into Matrimid results in improved CO₂ permeabilities up to 39%, without compromising the selectivity, relative to native Matrimid membranes. Both at low and high CO₂ partial pressures, the MMMs containing the different ZIF‑78 morphologies show equal CO₂ permeabilities, indicating that the ZIF‑78 morphology and consequently the aspect ratio are insignificant for these particular MMMs, contradicting previous observations in literature. Thus, depending on the MOF/polymer system, the MOF morphology and aspect ratio can be considered as a design aspect. Finally, a clear influence of feed composition and pressure on the MMM solubility coefficient and diffusivity is observed, emphasizing the importance of the mixed gas composition and measurement conditions.