organic linkers. The obtained ordered organic-inorganic networks exhibit high surface areas and their pore size, aperture and functionality is highly tunable, due to the variability in available organic linkers. [1] For example, a series of iron(III) carboxylate MOFs shows varying framework flexibility, pore size, pore aperture, and 3D structure by varying the carboxylate linker type. [2-6] These properties make MOFs very attractive materials in the field of gas storage and separation, since gas sorption and diffusion are tunable by linker variation. [7,8] Despite these promising characteristics, MOFs often lack chemical and thermal stability, which limits their practical application. Nonetheless, the number of stable MOFs has been increasing over the years. [9] One subclass of thermally and chemically stable MOFs are zeolitic imidazolate frameworks (ZIFs), which consist out of zinc or cobalt cations and a variety of imidazolate linkers. Since the first reported ZIFs, many new ZIF structures and topologies have been unraveled. [10] Due to their versatility, stability, and microporous nature (i.e., high gas uptake), multiple ZIF containing membrane structures have been investigated for gas separation purposes. [11,12] By the use of high-throughput synthesis, ZIF structures analogous to the zeolite gmelinite (GME) topology were found, which exhibited extraordinary high CO 2 uptake. [13] These hexagonal anisotropic GME ZIF crystals have two 1D channel types (one pore exists out of KNO cages, with pore apertures ranging from 4.5 to 8 Å, the other out of alternating GME and HPR cages, with pore apertures ranging from 3.6 to 4.3 Å) that are oriented parallel along the c-axis of the crystal structure (Figure 1). [14,15] GME ZIFs are constructed by linking Zn 2+ with 2-nitroimidazolate (nIm) and another imidazolate linker, where the other imidazolate linker determines the pore size, aperture, and polarity. [13,14] Controlling size and shape of the mixed linker ZIFs is rather difficult, since the difference in pK a values between the variable imidazoles and 2-nitroimidazole (nIm) causes different degrees of deprotonation in the reaction mixture. [16] Nonetheless, for ZIF-69 and 78 (both with GME topology) it was shown that their size and aspect ratio could be controlled by using specific zinc salts and deprotonating agents and adapting the ratio between zinc and the imidazoles. [17-19] In this work, the influence of the zeolitic imidazolate framework 78 (ZIF-78) morphology, with 1D pores, on the mixed matrix membrane (MMM) CO 2 /N 2 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 CO 2 and N 2 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 CO 2 permeabilit...

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