Experimental Study of CO₂, CH₄, and Water Vapor Adsorption on a Dimethyl-Functionalized UiO-66 Framework

A water resistant, highly robust dimethyl-functionalized UiO-66 analogue (UiO-66-DM) has been synthesized and characterized by using N₂ adsorption, powder X-ray diffraction, ¹H nuclear magnetic resonance, Fourier transform infrared spectroscopy, and thermogravimetric analysis followed by mass spectroscopy. High-pressure (up to 20 bar) single-component adsorption isotherm measurements of CO₂ and CH₄ have been performed on UiO-66-DM at different temperatures (293–308 K). Adsorption isotherm data were modeled by using the Toth equation, and isosteric heats of adsorption were calculated by using the Clausius–Clapeyron equation. The Ideal Adsorbed Solution Theory (IAST) was used to calculate mixture selectivities. Water adsorption experiments show that water adsorption loadings in UiO-66-DM are reduced by almost 50% compared to the parent material. In the low-pressure region, functionalization of the benzenedicarboxylic ligand by 2,5-dimethyl leads to higher interactions with both CO₂ and CH₄ compared to the parent UiO-66. CO₂/CH₄ selectivity calculated from IAST is higher for UiO-66 in the low-pressure region (<5 bar) but for pressures >5 bar, UiO-66-DM is more CO₂ selective than UiO-66

Computational Screening of Functionalized UiO-66 Materials for Selective Contaminant Removal from Air

Metal–organic frameworks (MOFs) have potential applications for efficient filtration of toxic gases from ambient air. We have used computational methods to examine the efficacy of functionalized UiO-66 with a wide range of functional groups to identify materials suitable for selective adsorption of NH₃, H₂S, or CO₂ under humid conditions. To this end, adsorption energies at various favorable positions in the structures are obtained from both cluster-based and periodic models. Our cluster calculations show that DFT calculations using the PBE-D2 functional can reliably predict the ranking of materials obtained at the MP2 level. Performing PBE-D2 calculations using periodic models gives rankings of materials that are significantly different from those of cluster calculations, showing that confinement effects are important in these materials. On the basis of these calculations, recommendations for high performing materials are made using PBE-D2 calculations from periodic models that use the full structure of each MOF

Synthesis, Characterization, and Adsorption Studies of Nickel(II), Zinc(II), and Magnesium(II) Coordination Frameworks of BTTB

Three porous metal–organic frameworks {[Ni­(H₂BTTB)·(H₂O)₂]·(DIOX)₂}<i>_n</i> (<b>1</b>), {[Zn­(H₂BTTB)]·(DEF)₃·(H₂O)₂}_<i>n</i> (<b>2</b>), and {[Mg­(H₂BTTB)·(C₂H₅OH)₂]·(DEF)₄}<i>_n</i> (<b>3</b>) based on the 4,4′,4″,4‴-benzene-1,2,4,5-tetrayltetrabenzoic acid (H₄BTTB) ligand have been synthesized under solvothermal conditions (DIOX = dioxane). These three MOFs show structural diversities: compound <b>1</b> is a two-dimensional (2D) grid layer, compound <b>2</b> is a 2-fold interpenetrated 3D framework with a pillared-layer structure, and compound <b>3</b> is a noninterpenetrated 3D framework with a (4, 4)-connected binodal net. Compound <b>1</b> and compound <b>2</b> have BET surface areas of 391 and 447 m²/g, respectively; however, the surface area of compound <b>3</b> cannot be experimentally determined. All three MOFs have a higher adsorption preference for CO₂ over N₂ and CH₄. Ideal adsorbed solution theory was used to estimate binary adsorption selectivities. Compound <b>2</b> shows the highest capacity for all three gases, whereas compound <b>1</b> shows the highest selectivity for CO₂ over CH₄ and N₂. Compound <b>1</b> exhibits a selectivity of ∼30 for CO₂ over N₂ in equimolar mixtures

Does Mixed Linker-Induced Surface Heterogeneity Impact the Accuracy of IAST Predictions in UiO-66-NH₂?

To move toward more energy-efficient adsorption-based processes, there is a need for accurate multicomponent data under realistic conditions. While the Ideal Adsorbed Solution Theory (IAST) has been established as the preferred prediction method due to its simplicity, limitations and inaccuracies for less ideal adsorption systems have been reported. Here, we use amine-functionalized derivatives of the UiO-66 structure to change the extent of homogeneity of the internal surface toward the adsorption of the two probe molecules carbon dioxide and ethylene. Although it might seem plausible that more functional groups lead to more heterogeneity and, thus, less accurate predictions by IAST, we find a mixed-linker system with increased heterogeneity in terms of added adsorption sites where IAST predictions and experimental loadings agree exceptionally well. We show that incorporating uncertainty analysis into predictions with IAST is important for assessing the accuracy of these predictions. Energetic investigations combined with Grand Canonical Monte Carlo simulations reveal almost homogeneous carbon dioxide but heterogeneous ethylene adsorption in the mixed-linker material, resulting in local, almost pure phases of the individual components

Does Mixed Linker-Induced Surface Heterogeneity Impact the Accuracy of IAST Predictions in UiO-66-NH₂?

To move toward more energy-efficient adsorption-based processes, there is a need for accurate multicomponent data under realistic conditions. While the Ideal Adsorbed Solution Theory (IAST) has been established as the preferred prediction method due to its simplicity, limitations and inaccuracies for less ideal adsorption systems have been reported. Here, we use amine-functionalized derivatives of the UiO-66 structure to change the extent of homogeneity of the internal surface toward the adsorption of the two probe molecules carbon dioxide and ethylene. Although it might seem plausible that more functional groups lead to more heterogeneity and, thus, less accurate predictions by IAST, we find a mixed-linker system with increased heterogeneity in terms of added adsorption sites where IAST predictions and experimental loadings agree exceptionally well. We show that incorporating uncertainty analysis into predictions with IAST is important for assessing the accuracy of these predictions. Energetic investigations combined with Grand Canonical Monte Carlo simulations reveal almost homogeneous carbon dioxide but heterogeneous ethylene adsorption in the mixed-linker material, resulting in local, almost pure phases of the individual components

Molecular-level Insight into Unusual Low Pressure CO₂ Affinity in Pillared Metal–Organic Frameworks

Fundamental insight into how low pressure adsorption properties are affected by chemical functionalization is critical to the development of next-generation porous materials for postcombustion CO₂ capture. In this work, we present a systematic approach to understanding low pressure CO₂ affinity in isostructural metal–organic frameworks (MOFs) using molecular simulations and apply it to obtain quantitative, molecular-level insight into interesting experimental low pressure adsorption trends in a series of pillared MOFs. Our experimental results show that increasing the number of nonpolar functional groups on the benzene dicarboxylate (BDC) linker in the pillared DMOF-1 [Zn₂(BDC)₂(DABCO)] structure is an effective way to tune the CO₂ Henry’s coefficient in this isostructural series. These findings are contrary to the common scenario where polar functional groups induce the greatest increase in low pressure affinity through polarization of the CO₂ molecule. Instead, MOFs in this isostructural series containing nitro, hydroxyl, fluorine, chlorine, and bromine functional groups result in little increase to the low pressure CO₂ affinity. Strong agreement between simulated and experimental Henry’s coefficient values is obtained from simulations on representative structures, and a powerful yet simple approach involving the analysis of the simulated heats of adsorption, adsorbate density distributions, and minimum energy 0 K binding sites is presented to elucidate the intermolecular interactions governing these interesting trends. Through a combined experimental and simulation approach, we demonstrate how subtle, structure-specific differences in CO₂ affinity induced by functionalization can be understood at the molecular-level through classical simulations. This work also illustrates how structure–property relationships resulting from chemical functionalization can be very specific to the topology and electrostatic environment in the structure of interest. Given the excellent agreement between experiments and simulation, predicted CO₂ selectivities over N₂, CH₄, and CO are also investigated to demonstrate that methyl groups also provide the greatest increase in CO₂ selectivity relative to the other functional groups. These results indicate that methyl ligand functionalization may be a promising approach for creating both water stable and CO₂ selective variations of other MOFs for various industrial applications

Does Mixed Linker-Induced Surface Heterogeneity Impact the Accuracy of IAST Predictions in UiO-66-NH₂?

To move toward more energy-efficient adsorption-based processes, there is a need for accurate multicomponent data under realistic conditions. While the Ideal Adsorbed Solution Theory (IAST) has been established as the preferred prediction method due to its simplicity, limitations and inaccuracies for less ideal adsorption systems have been reported. Here, we use amine-functionalized derivatives of the UiO-66 structure to change the extent of homogeneity of the internal surface toward the adsorption of the two probe molecules carbon dioxide and ethylene. Although it might seem plausible that more functional groups lead to more heterogeneity and, thus, less accurate predictions by IAST, we find a mixed-linker system with increased heterogeneity in terms of added adsorption sites where IAST predictions and experimental loadings agree exceptionally well. We show that incorporating uncertainty analysis into predictions with IAST is important for assessing the accuracy of these predictions. Energetic investigations combined with Grand Canonical Monte Carlo simulations reveal almost homogeneous carbon dioxide but heterogeneous ethylene adsorption in the mixed-linker material, resulting in local, almost pure phases of the individual components