The rapidly rising level of carbon dioxide in the atmosphere resulting from human activity is one of the greatest environmental problems facing our civilization today. Most technologies are not yet sufficiently developed to move existing infrastructure to cleaner alternatives. Therefore, techniques for capturing carbon dioxide from emission sources may play a key role at the moment. The structure of the UiO-66 material not only meets the requirement of high stability in contact with water vapor but through the water pre-adsorbed in the pores, the selectivity of carbon dioxide adsorption is increased. We successfully applied the recently developed methodology for water adsorption modelling. It allowed to elucidate the influence of water on CO 2 adsorption and study the mechanism of this effect. We showed that water is adsorbed in octahedral cage and stands for promotor for CO 2 adsorption in less favorable space than tetrahedral cages. Water plays a role of a mediator of adsorption, what is a general idea of improving affinity of adsorbate. On the basis of pre-adsorption of methanol as another polar solvent, we have shown that the adsorption sites play a key role here, and not, as previously thought, only the interaction between the solvent and quadrupole carbon dioxide. Overall, we explained the mechanism of increased CO 2 adsorption in the presence of water and methanol, as polar solvents, in the UiO-66 pores for a potential post-combustion carbon dioxide capture application.
Proton-conducting
metal–organic frameworks (MOFs) have been
gaining attention for their role as solid-state electrolytes in various
devices for energy conversion and storage. Here, we present a convenient
strategy for inducing and tuning of superprotonic conductivity in
MOFs with open metal sites via postsynthetic incorporation of charge
carriers enabled by solvent-free mechanochemistry and anion coordination.
This scalable approach is demonstrated using a series of
CPO-27/MOF-74
[M
2
(dobdc); M = Mg
2+
, Zn
2+
, Ni
2+
; dobdc = 2,5-dioxido-1,4-benzenedicarboxylate] materials
loaded with various stoichiometric amounts of NH
4
SCN. The
modified materials are not achievable by conventional immersion in
solutions. Periodic density functional theory (DFT) calculations,
supported by infrared (IR) spectroscopy and powder X-ray diffraction,
provide structures of the modified MOFs including positions of inserted
ions inside the [001] channels. Despite the same type and concentration
of proton carriers, the MOFs can be arranged in the increasing order
of conductivity (Ni < Zn < Mg), which strongly correlates with
amounts of water vapor adsorbed. We conclude that the proton conductivity
of
CPO-27
materials can be controlled over a few orders
of magnitude by metal selection and mechanochemical dosing of ammonium
thiocyanate. The dosing of a solid is shown for the first time as
a useful, simple, and ecological method for the control of material
conductivity.
Structural defects in metal–organic frameworks can be exploited to tune material properties. In the case of UiO-66 material, they may change its nature from hydrophobic to hydrophilic and therefore affect the mechanism of adsorption of polar and non-polar molecules. In this work, we focused on understanding this mechanism during adsorption of molecules with different dipole moments, using the standard volumetric adsorption measurements, IR spectroscopy, DFT + D calculations, and Monte Carlo calculations. Average occupation profiles showed that polar and nonpolar molecules change their preferences for adsorption sites. Hence, defects in the structure can be used to tune the adsorption properties of the MOF as well as to control the position of the adsorbates within the micropores of UiO-66.
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