Abstract
Effective and economical carbon dioxide-methane separation (CO 2 /CH 4 ) is highly desirable in several industries such as sweetening natural gases and renewable natural gas (RNG) from biogas and landfills. Among the different separation technologies, membrane separation has been shown to have lower cost of production and lower CH 4 losses. In this study, Zeolitic Imidazole Frameworks (ZIF-8) crystals with sizes varying from 45 nm to 450 nm were synthesized and incorporated in the polymer of intrinsic microporosity, PIM-1, to form mixed matrix membranes (MMMs). The structure, morphology, and physicochemical properties of the MMMs were characterized by 1 H NMR, FTIR, XRD, TGA, and SEM. ZIF-8 crystal size was controlled using the concentration of sodium formate. The influence of the ZIF-8 crystal size on MMMs was studied by sorption, gas permeability, and aging of the membranes. The MMMs with ZIF-8 crystals of 120 nm particle diameter yielded the greatest improvement in gas transport properties; the CO 2 /CH 4 selectivity-CO 2 permeability was 11.4 and 9700 Barrer compared to PIM-1 with 6.4 and 9300 Barrer respectively. The former is near the Robeson 2008 upper bound, while PIM-1 is on the 1991 upper bound. After 40 days of aging, selectivity increased and permeability decreased; the changes were parallel to the Robeson upper bounds indicating increased polymer packing and diffusivity selectivity. • The incorporation of different ZIF-8 crystal sizes in PIM-1/ZIF-8 MMMs showed a significant impact on CO 2 /CH 4 separation performance. • PIM-1/ZIF-8 MMMs with ZIF-8 a crystal diameter of 120 nm improved CO 2 /CH 4 compared to PIM-1, increasing both selectivity and permeability. • The 120 nm diameter ZIF-8 crystal sizes were best dispersed in the MMMs, smaller and larger crystals (i.e., 45, 250, and 450 nm) aggregated and agglomerated respectively, with inferior permeation properties. • PIM-1/ZIF-8B MMMs achieved a good performance, which is close to the Robeson 2008 upper bound. • Physical aging after 40 days decreased the permeability, the selectivity increased as predicted by the Robseon upper bound slope.
Highlights
Carbon dioxide (CO2) is usually found in natural gas streams, fossil fuel combustion, biogas, and as a by-product of coal gasification [1]
The interactions between Zeolitic Imidazole Frameworks (ZIFs)-8 and Polymers of intrinsic microporosity (PIMs)-1 in the PIM-1/ZIF-8 membranes were investigated by FTIR spectroscopy (Fig. 1c)
In the PIM-1 membrane spectrum, the absorption bands at 2239 cm− 1 and 1265 cm− 1 are associated with nitrile groups (C≡N stretching)
Summary
Carbon dioxide (CO2) is usually found in natural gas streams, fossil fuel combustion, biogas, and as a by-product of coal gasification [1]. Membrane materials with higher selectivity, for low methane losses, and high permeability, for low capital cost and small footprints, are highly desirable [9,10,11,12,13,14,15]. In both glassy and rubbery polymers, there is a trade-off relationship between membrane permeability and selec tivity [6,7]. Mixed matrix membranes (MMMs) combine the advantages of both classes of mate rials; high selectivity of inorganic nanoparticles and the processability of polymers. MMMs are a synergistic approach yielding selectivity-permeability beyond the single materials and surpassing the 2008 upper-bound [18,19,20,21]
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