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Researchers h ave r ecently b egun t o co mpare C O2 uptake acr oss f amilies o f d ifferent MO Fs. F or example, Y ang et al . [74] i nvestigated t he e ffects of or ganic linker, p ore s ize, por e t opology, a nd electrostatic f ields on t he a dsorption of C O2 in ni ne different MOFs. T hey f ound t hat the e nthalpy of adsorption plays an important role at low pressures, so that there is a correlation between CO2 uptake and the adsorption enthalpy. At higher pressures, CO2 uptake correlates with the material surface area and free volume. A similar trend was observed by Yazaydin et al. [70], who examined a more diverse group of 14 MOFs and reported experimental uptake at conditions relevant for flue gas capture in addition to results from molecular simulation. The simulation results were in good agreement with experiment, especially in ranking the M OFs f or CO2 uptake a t 0.1 ba r ( representative o f flue g as conditions). For example, the simulations correctly predicted the top 5 MOFs for uptake of CO2 at 0.1 bar in agreement with experiment [70]. It should be emphasized that good agreement between simulation and experiment relies on careful characterization of the materials used in the experiments. Simulations model adsorption in the “perfect” MOF cr ystal st ructures r eported f rom si ngle-crystal x -ray di ffraction. I f pow der s amples us ed f or adsorption m easurements differ from t hese s tructures, one s hould not e xpect g ood a greement be tween simulation and experiment. Deviations could include partial collapse of the MOF upon solvent removal, residual solvent molecules in the MOF pores, or unreacted molecules from the MOF synthesis. 3.2.2 Zeolitic Imidazolate Frameworks (ZIFs) (a) (b) Zeolitic imidazolate frameworks (ZIFs) are a subclass of m etal-organic f rameworks t hat consist o f a tetrahedral c ation c oordinated by a n or ganic imidazolate (IM) b identate l igand, o r s ubstituted derivative thereof [75]. In the large majority of ZIFs that have been synthesized to date the central cation is Zn2+, al though sev eral ex amples o f Z IFs co ntaining Co2+ can be found in the literature [75-77]. ZIFs carry their “ zeolitic” m oniker not from a ny chemical similarity t o s ilicon-based z eolites, but r ather d ue to their structural analogy: the Zn-IM-Zn bond angle in a ZIF is nearly identical to the Si-O-Si in conventional zeolites (Figure 14) [75]. This correspondence leads to th e f ormation o f s imilar th ree-dimensional s tructures f or bo th c ompounds, c ontaining l arge por e regions, interconnected by (typically) more constricted entrances (apertures) (Figure 15). Yet in contrast to traditional zeolites, ZIFs offer tremendous potential for design and modification, allowing for control of the physical (pore size, aperture diameter) and chemical properties by functionalization of the organic IM l inker [ 78]. T his flexibility o pens en tirely n ew p ossibilities t hat a re i naccessible u sing t raditional zeolite chemistry. Like t he MO Fs c ounterparts, Z IFs f orm st able, t hree-dimensional, crystalline microporous solids. Due to their porous nature and large interior pores, ZIFs possess very low density, and extraordinarily high s urface a reas, o ften i n e xcess of 1000 -2000 m 2/g [ 77, 78] . 7.5 Å Furthermore, r ecent s tudies i ndicated t hat Z IFs may di splay v ery good selectivity for CO2 [77-80]. Both the capacity and selectivity of t he Z IFs c an be i nfluenced a nd t ailored by pr oper functionalization of the I M l inker [ 77, 78 ]. This w as r ecently demonstrated in a n i sostructural s eries o f Z IFs of f ormulae Zn(nIm)(Lbim) (n Im– = 2-nitroimidazolate; L bim– = 5-L- benzimidazolate w here L is a v ariety of functional g roups) [[80]. Thus, at 1 ba r and 298 K, the CO2 uptake selectivity was found to vary along the series: L = NO2 (ZIF-78) > CN, Br, Cl (ZIF-82, -81, - N IM Zn2+ ~145¡ ZIF O ~145¡ Zn2+ Si N NN N N Si Figure 14. (a) Structure of an imidazolate anion [top], and a benzyl-substituted imidazolate [bottom]. (b) Comparison of ZIF and zeolite structures, showing a similar bond angle; adapted from Ref. 59. Carbon Capture Factual Document 30 bIM Zeolite 10.2 Å Figure 15. ZIF-68, with a large cavity and smaller aperture. Aperture sizes range from 0.7–13Å in various ZIFs.PDF Image | 2020 Carbon Capture
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