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gas sep aration sy stem. While t hese properties would depend on the d etailed structure o f the separation system, even order of magnitude estimates would be extremely useful. In particular, estimates for the for the following are required: capacities (translated into laboratory measurable units, such as mmol CO2 / g or mmol CO2 / L of sorbent at a given pressure or temperature), adsorption selectivity for CO2 over other relevant flue gases, transport diffusion constants for CO2 through the sorbent, and heat of CO2 adsorption, are essential for guiding future synthetic work. Our fundamental understanding of the interaction of MOFs and ZIFs with CO2 and other flue gases, in particular N2 is still limited. Experimental efforts to probe these interactions via either diffraction studies or sp ectroscopic (IR, p erhaps ev en N MR) m ethods may p rovide v aluable i nsight t hat is n ot cu rrently available. Additional insights can also be gained through computational techniques, including molecular simulations and electronic structure methods. These computational efforts are somewhat hindered by the fact that d ifferent em pirical force f ields h ave y ielded d ifferent r esults, a nd the w eak intermolecular interactions between the framework and adsorbate are difficult to probe using common density functional theory techniques. On the other hand, it remains particularly difficult to predict the CO2 uptake in MOFs that contain open metal sites or other strongly interacting functional groups. The models typically used in current simulations are based on a c lassical “force field” model and do not take into account orbital interactions and thus are generally not expected to perform well for strong binding sites. In addition to the above-mentioned fundamental problems, there are some practical issues that need to be resolved before MOFs and ZIFs can be used effectively for CO2 capture: Performance in the Presence of Water. The management of water will be an important factor for the industrial a pplication of MOFs i n C O2 capture. Unless r igorously d ried, m ost i ndustrial g as st reams contain some amount of moisture, and untreated flue gas contains 5-7% water vapor by weight. Ideally, a MOF suitable for CO2 capture should be stable to the sustained presence of water vapor at this level. Although many MOFs are unstable to water, a growing number are held together by very strong metal- ligand bond s a nd c an s urvive e ven e xtreme hy drothermal c onditions [87]. Importantly, t he M OFs displaying a high water stability include Mg2(dhtp), HCu[(Cu4Cl)3(BTTri)8], and most ZIFs [71, 80, 89]. In addition, since water has a large dipole moment, it will tend to adsorb to charged sites on a MOF surface preferentially over CO2, potentially interfering with CO2 capture. In general, many more measurements on MOFs using mixed gas streams that include water vapor are needed to probe what effects w ater w ill h ave. Interestingly, h owever, t here ar e i ndications that cer tain MO Fs m ay act ually perform better in the presence of water [89]. Stability Towards Impurities. In natural, synthesis, and especially flue gas streams, there are impurities that can be acidic, such as SO2, H2S, HCl, and NOx, and could potentially be corrosive to MOFs. Ideally, a MOF would be stable to exposure to any potential flue gas impurities; although certain of these gases may already be removed in power plants due to environmental legislations. Further measurements on the impact of such trace gases on MOFs are needed. Gas Diffusion Rates. Most of the work on MOFs to date has focused on thermodynamic aspects of their performance. Equally important, however, is the kinetics of how a flue gas will permeate a MOF. Very few measurements of this type have yet been carried out, and many more are needed. Similarly, very little experimental or theoretical data exists for diffusion of CO2 in ZIFs. Wang et al. noted that slow diffusion seems present during gas absorption (although this is not quantified), which they attribute to the effect of constricted por e a pertures [ 79]. A ha ndful of s imulation studies ha ve l ooked a t di ffusion in Z IFs a nd found that it could be up to an order of magnitude slower than in typical MOF systems [83, 84]. The decreased diffusion was attributed to the the smaller pores in the ZIF system, as well as steric hindrance due to substituents on the IM linkers. It would seem that experimental measurements of gas diffusion in ZIF sy stems ar e cr ucially important in or der to e valuate w hether t he obs erved di ffusion rates a re Carbon Capture Factual Document 32PDF Image | 2020 Carbon Capture
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