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69) > C6H6, Me (ZIF-68, -79) > H (ZIF-70) > BPL carbon (Figure 16 ). Most notably, t he u ptake capacity f or t he nitro-functionalized s tructure ( ZIF-78) i s ne arly t hree timesthatofcommercial BPLcarbon,withmorethan twice the selectivity over N2. While selective ad sorption ser ves as o ne m echanism for potential g as s eparation, t he un ique s tructure of Z IFs, containing l arge p ores w ith sm all ap ertures, p rovides an additional pa thway for gas s eparation: kinetic s eparation [81, 82]. The controllable aperture diameter of ZIFs means that it may be possible to design a Z IF with an aperturesized to allow thepassageofonegas,while excluding (partially) the flux of a second gas. The performance of ZIFs as selective adsorbents for carbon dioxide ultimately depends on their specific interactions with t he CO2 molecules. A lthough l ittle direct experiment da ta ( such a s ne utron or x -ray diffraction) exists to date, molecular simulations yielded some insight about the predominant interaction mechanisms [82, 84]. These simulations employ empirical interaction potentials, and the results depend somewhat sen sitively o n t he ex act f orce field employed. I n su ch si mulations, t he m ost f avorable adsorption site for the CO2 is found to be in the small cages of the ZIF [84]. At low pressures the predominant in teraction i s b etween CO2 and the p olar su bstituents o f I M, l ocalizing i n t he sm all p ores made by the polar groups [83]. The electrostatic interactions of the CO2 with the framework atoms is quite significant, and cannot be neglected at or below ambient pressures; at much higher pressure (above those likely ap plicable for f lue gas sep aration) the e ffect o f el ectrostatic interaction seem s t o b ecome o f secondary importance [85]. ZIFs e xhibit impressive t hermal s tability [ 75], up to 500 ° C, w hich is m oderately hi gher than f or prototypical MOFs [77, 78]. Yet it is the chemical and solvent stability of ZIFs that strongly differentiates them f rom m ost ot her M OFs, di splaying e xcellent r esistance t o m any c ommon s olvents [ 75]. F or example, a group of ZIFs were shown to be resistant to 7 days of refluxing in both benzene and methanol [77, 80]. Another ZIF, ZIF-8, was resistant to 7 days of exposure to 50 °C water, and even to 24 hours in 0.8M N aOH a t 100 ° C [75]. The en hanced ch emical stability o f Z IFs f acilitates t heir post-synthetic functionalization, allowing for modulation of the CO2 capacity and selectivity [86]. The unusual solvent resistance of ZIFs has been attributed to either unusually strong metal-ligand bonds, or the hydrophobic nature of the ZIF ligands protecting the sensitive metal-ligand bonds to hydrolysis [75]. Some support for the latter hypothesis is given by subsequent experimental work on water adsorption, where it was shown that ZIF-8 is extremely hydrophobic [87]. In fact, the ZIF displays almost no water adsorption until the condensation point is reached. Note that in this work, in contrast to that of Park et al., this later work found a few additional peaks in the powder XRD spectrum after exposure to water at 50 °C for 24 hours, suggesting that although their solvent resistance is good, the ZIFs are far from inert [87]. 3.2.3 Current Limitations in CO2 Capture by MOFs and ZIFS It seems clear that it will be feasible to develop MOFs with truly outstanding levels of performance in the separation of CO2 from mixtures with N2. T heir eventual use on the enormous scale required for CO2 capture from power plant flue gases, however, still presents a number of serious challenges. Some of the issues to consider are economical, with uncertainties arising, for example, in the availability of materials and the ultimate costs of manufacture of MOFs if they are to be produced on such an enormous scale. Other challenges involve fundamental science. For example, little is currently known about the necessary physical and chemical properties of the sorbent that would be required for utilization in a practical flue Carbon Capture Factual Document 31 Figure 16. S electivity for CO binding over CH , 24 N2,andO2 inselected ZIFsadoptingthe gme structure t ype an d h aving a v ariety o f s urface functional groups.PDF Image | 2020 Carbon Capture
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