Understanding Diffusion in Hierarchical Zeolites with House-of-Cards Nanosheets

Introducing mesoporosity to conventional microporous sorbents or catalysts is often proposed as a solution to enhance their mass transport rates. Here, we show that diffusion in these hierarchical materials is more complex and exhibits non-monotonic dependence on sorbate loading. Our atomistic simulations of <i>n</i>-hexane in a model system containing microporous nanosheets and mesopore channels indicate that diffusivity can be smaller than in a conventional zeolite with the same micropore structure, and this observation holds true even if we confine the analysis to molecules completely inside the microporous nanosheets. Only at high sorbate loadings or elevated temperatures, when the mesopores begin to be sufficiently populated, does the overall diffusion in the hierarchical material exceed that in conventional microporous zeolites. Our model system is free of structural defects, such as pore blocking or surface disorder, that are typically invoked to explain slower-than-expected diffusion phenomena in experimental measurements. Examination of free energy profiles and visualization of molecular diffusion pathways demonstrates that the large free energy cost (mostly enthalpic in origin) for escaping from the microporous region into the mesopores leads to more tortuous diffusion paths and causes this unusual transport behavior in hierarchical nanoporous materials. This knowledge allows us to re-examine zero-length-column chromatography data and show that these experimental measurements are consistent with the simulation data when the crystallite size instead of the nanosheet thickness is used for the nominal diffusional length

CO₂ Adsorption in M‑IRMOF-10 (M = Mg, Ca, Fe, Cu, Zn, Ge, Sr, Cd, Sn, Ba)

Metal–organic frameworks (MOFs) have been studied extensively for application in flue gas separation because of their tunability, structural stability, and large surface area. M-IRMOF-10 (M = transition metal or main-group atom) is a well-studied series of structures and is composed of saturated tetrahedral Zn₄O nodes and dicarboxylate linkers that form a cubic unit cell. We report the results of a computational study on the effects that changing the metal atoms within IRMOF-10 has on the affinity of the material towards CO₂. Force fields were parametrized using quantum mechanical calculations to systematically compare the effects of different metal centers on CO₂ adsorption at high and low pressure. Two different methods for the determination of partial charges (DDEC and CM5) and force field parameter sets (TraPPE and UFF) were employed. TraPPE parameters with fitted metal–CO₂ interactions and CM5 charges resulted in isotherms that were closer to experiment than pure UFF. The results indicate that exchanging the Zn²⁺ ions in the IRMOF-10 series with metals that have larger ionic radii (Sn²⁺ and Ba²⁺) can lead to an increase in CO₂ affinity due to the increased exposure of the positive metal charge to the oxygen atoms of CO₂ and the increased interaction from the more diffuse electrons

Selective, Tunable O₂ Binding in Cobalt(II)–Triazolate/Pyrazolate Metal–Organic Frameworks

The air-free reaction of CoCl₂ with 1,3,5-tri­(1<i>H</i>-1,2,3-triazol-5-yl)­benzene (H₃BTTri) in <i>N</i>,<i>N</i>-dimethylformamide (DMF) and methanol leads to the formation of Co-BTTri (Co₃[(Co₄Cl)₃(BTTri)₈]₂·DMF), a sodalite-type metal–organic framework. Desolvation of this material generates coordinatively unsaturated low-spin cobalt­(II) centers that exhibit a strong preference for binding O₂ over N₂, with isosteric heats of adsorption (<i>Q</i>_st) of −34(1) and −12(1) kJ/mol, respectively. The low-spin (<i>S</i> = 1/2) electronic configuration of the metal centers in the desolvated framework is supported by structural, magnetic susceptibility, and computational studies. A single-crystal X-ray structure determination reveals that O₂ binds end-on to each framework cobalt center in a 1:1 ratio with a Co–O₂ bond distance of 1.973(6) Å. Replacement of one of the triazolate linkers with a more electron-donating pyrazolate group leads to the isostructural framework Co-BDTriP (Co₃[(Co₄Cl)₃(BDTriP)₈]₂·DMF; H₃BDTriP = 5,5′-(5-(1<i>H</i>-pyrazol-4-yl)-1,3-phenylene)­bis­(1<i>H</i>-1,2,3-triazole)), which demonstrates markedly higher yet still fully reversible O₂ affinities (<i>Q</i>_st = −47(1) kJ/mol at low loadings). Electronic structure calculations suggest that the O₂ adducts in Co-BTTri are best described as cobalt­(II)–dioxygen species with partial electron transfer, while the stronger binding sites in Co-BDTriP form cobalt­(III)–superoxo moieties. The stability, selectivity, and high O₂ adsorption capacity of these materials render them promising new adsorbents for air separation processes

La Lanterne : journal politique quotidien

06 juillet 18901890/07/06 (N4824,A14)