NMR Spectroscopy Reveals Adsorbate Binding Sites in the Metal–Organic Framework UiO-66(Zr)

We assign ¹H and ¹³C NMR resonances emanating from acetone, methanol, and cyclohexane adsorbed inside the pores of UiO-66­(Zr). These results are informed by density functional theory (DFT) calculations, which probe the role of two competing effects inside of the pore environment: (i) nucleus independent chemical shifts (NICSs) generated by ring currents in conjugated linkers and (ii) small molecule coordination to the metal-oxyhydroxy cluster. These interactions are found to perturb the chemical shift of in-pore adsorbate relative to ex-pore adsorbate (which resides in spaces between the MOF particles). Changes in self-solvation upon adsorption may also perturb the chemical shift. Our results indicate that cyclohexane preferentially adsorbs in the tetrahedral pores of UiO-66­(Zr), while acetone and methanol adsorb at the Zr–OH moieties on the metal-oxyhydroxy clusters in a more complex fashion. This method may be used to probe molecular adsorption sites and material void saturation with selected adsorbates, and with further development may eventually be used to trace in-pore chemistry of MOF materials

Understanding the Impact of Multi-Chain Ion Coordination in Poly(ether-Acetal) Electrolytes

Performant solid polymer electrolytes for battery applications usually have a low glass transition temperature and good ion solvation. Recently, to understand the success of PEO for solid-sate battery applications and explore alternatives, we have studied a series of polyacetals along with PEO, both from an experimental and a computational standpoint. We observed that even though the mechanism of transport may be more optimal in polyacetals, the lower glass transition temperature of the PEO-salt electrolyte system still makes it the best option, in this class of polymers, for battery applications. In this work, we explored the free-energy landscape of PEO and P(EO-MO) at various compositions and temperatures using metadynamics simulations to gain deeper insights into the various factors that affect the glass transition temperatures in these systems. In particular, we study the competition between intra- and inter-chain coordination of the cation in these systems that we had hypothesized in our previous work was responsible for the differences in the glass transition temperature. We observe that in PEO, the single-chain binding motif is thermodynamically more stable than the multi-chain binding motif, unlike P(EO-MO), where the opposite is true. We also show that multi-chain coordination, and the associated higher glass transition temperature, in P(EO-MO) is due to a larger strain energy for single-chain coordination that originates in the introduced OCO linkages (relative to PEO’s consistent OCCO linkages). Furthermore, the type of pathways to move from one transition state to another in the various systems do not change at higher concentrations though the relative probability of cation–anion coordinated states increases. Calculations at different temperatures to understand the entropic effect on the stability of these coordination environments reveal that as we increase the temperature, single-chain coordination becomes relatively more stable due to the entropic cost of multi-chain coordination, reducing the number of accessible states for the polymer. The various insights into the factors that affect glass transition temperature in these systems suggest design principles for polymer electrolyte systems with lower glass transition temperatures that need further research to compete with PEO at the same absolute battery working temperatures

CO₂ Dynamics in a Metal–Organic Framework with Open Metal Sites

Metal–organic frameworks (MOFs) with open metal sites are promising candidates for CO₂ capture from dry flue gas. We applied <i>in situ</i> ¹³C NMR spectroscopy to investigate CO₂ adsorbed in Mg₂(dobdc) (H₄dobdc = 2,5-dihydroxyterephthalic acid; Mg-MOF-74, CPO-27-Mg), a key MOF in which exposed Mg²⁺ cation sites give rise to exceptional CO₂ capture properties. Analysis of the resulting spectra reveals details of the binding and CO₂ rotational motion within the material. The dynamics of the motional processes are evaluated via analysis of the NMR line shapes and relaxation times observed between 12 and 400 K. These results form stringent and quantifiable metrics for computer simulations that seek to screen and improve the design of new MOFs for CO₂ capture

Revisiting Anisotropic Diffusion of Carbon Dioxide in the Metal–Organic Framework Zn₂(dobpdc)

The diffusion of gases confined in nanoporous materials underpins membrane and adsorption-based gas separations, yet relatively few measurements of diffusion coefficients in the promising class of materials, metal–organic frameworks (MOFs), have been reported to date. Recently we reported self-diffusion coefficients for ¹³CO₂ in the MOF Zn₂(dobpdc) (dobpdc^4– = 4,4′-dioxidobiphenyl-3,3′-dicarboxylate) which has one-dimensional channels with a diameter of approximately 2 nm [Forse, A. C.; J. Am. Chem. Soc. 2018, 140, 1663−1673]. By analyzing the evolution of the residual ¹³C chemical shift anisotropy line shape at different gradient strengths, we obtained self-diffusion coefficients both along (<i>D</i>_∥) and between (<i>D</i>_⊥) the one-dimensional MOF channels. The observation of nonzero <i>D</i>_⊥ was unexpected based on the single crystal X-ray diffraction structure and flexible lattice molecular dynamics simulations, and we proposed that structural defects may be responsible for self-diffusion between the MOF channels. Here we revisit this analysis and show that homogeneous line broadening must be taken into account to obtain accurate values for <i>D</i>_⊥. In the presence of homogeneous line broadening, intensity at a particular NMR frequency represents signal from crystals <i>with a range of orientations</i> relative to the applied magnetic field and magnetic gradient field. To quantify these effects, we perform spectral simulations that take into account homogeneous broadening and allow improved <i>D</i>_⊥ values to be obtained. Our new analysis best supports nonzero <i>D</i>_⊥ at all studied dosing pressures and shows that our previous analysis overestimated <i>D</i>_⊥

Revisiting Anisotropic Diffusion of Carbon Dioxide in the Metal-Organic Framework Zn2(dobpdc)

The diffusion of gases confined in nanoporous materials underpins membrane and adsorption-based gas separations, yet relatively few measurements of diffusion coefficients in the promising class of materials, metal-organic frameworks (MOFs), have been reported to date. Recently we reported self-diffusion coefficients for ¹³CO₂ in the MOF, Zn₂(dobpdc), (dobpdc^4– = 4,4′-dioxidobiphenyl-3,3′-dicarboxylate) that has one-dimensional channels with a diameter of approximately 2 nm. By analyzing the evolution of the residual ¹³C chemical shift anisotropy lineshape at different gradient strengths, we obtained self-diffusion coefficients both along (D_||) and between (D_⊥) the one-dimensional MOF channels. The observation of non-zero D⊥ was unexpected based on the single crystal X-ray diffraction structure and flexible lattice molecular dynamics simulations, and we proposed that structural defects may be responsible for self-diffusion between the MOF channels. Here we revisit this analysis and show that homogeneous line broadening must be taken into account to obtain accurate values for D⊥. In the presence of homogeneous line broadening, intensity at a particular NMR frequency represents signal from crystals with a range of orientations relative to the applied magnetic field and magnetic gradient field. To quantify these effects, we perform spectral simulations that take into account homogeneous broadening and allow improved D⊥ values to be obtained. Our new analysis best supports non-zero D⊥ at all studied dosing pressures and shows that our previous analysis overestimated D⊥

Assessment of Adsorbate π‑Backbonding in Copper(I) Metal–Organic Frameworks via Multinuclear NMR Spectroscopy and Density Functional Theory Calculations

We assess the binding of C2H4 to the coordinately unsaturated copper­(I) sites of the metal–organic frameworks Cu­(I)-ZrTpmC* and Cu­(I)-MFU-4l via 13C solid-state nuclear magnetic resonance spectroscopy, density functional theory (DFT), and natural localized molecular orbital analysis. Using these methods, forward-donation and back-donation contributions between C2H4 and the exposed Cu­(I) are delineated, and high binding enthalpies are contextualized as a function of electronic changes upon site modification and adsorption. With the infrastructure for DFT and solid-state 13C NMR becoming more routine for scientists, we envision that these results will support the study of exposed electron-rich metal sites in a variety of chemical applications

In Situ Formation of Wilkinson-Type Hydroformylation Catalysts: Insights into the Structure, Stability, and Kinetics of Triphenylphosphine- and Xantphos-Modified Rh/SiO₂

An investigation has been carried out to identify the effects of catalyst preparation on the activity, selectivity, and stability of phosphine-modified rhodium/silica catalysts (Rh/SiO₂) for propene hydroformylation. High selectivity to aldehydes was achieved, without the formation of propane or butanol. Catalyst activity and selectivity was found to depend strongly on the nature and concentration of the phosphine ligands and the amount of rhodium dispersed on the silica support. Screening of different ligands showed that a bidentate xantphos (X) ligand was ∼2-fold more active than the monodentate phosphine ligand (PPh₃) screened at a ligand-to-rhodium ratio of 15:1. Investigation of the effects of reaction temperature, reactant partial pressures, and phosphine-to-rhodium ratio indicates that the kinetics of propene hydroformylation over X-promoted Rh/SiO₂ is nearly identical to those for sulfoxantphos-modified rhodium-containing supported ionic liquid phase (SX-Rh SILP) catalysts. In-situ FTIR and solid-state ³¹P MAS NMR characterization provide evidence for the formation of HRh­(CO)_<i>n</i>(PPh₃)_4–<i>n</i> species on PPh₃-modified Rh/SiO₂, and HRh­(CO)₂(X) species on xantphos-modified Rh/SiO₂. The high catalytic activity observed over rhodium-containing silica catalysts is attributed to formation of Rh^(I)(CO)₂ species by the process of corrosive chemisorption of Rh nanoparticles by CO and the subsequent ligation of phosphine ligands to the dicarbonyl species. Evidence is also presented suggesting that the active form of the catalyst resides on the surface of the Rh nanoparticles

Water Enables Efficient CO₂ Capture from Natural Gas Flue Emissions in an Oxidation-Resistant Diamine-Appended Metal–Organic Framework

Supported by increasingly available reserves, natural gas is achieving greater adoption as a cleaner-burning alternative to coal in the power sector. As a result, carbon capture and sequestration from natural gas-fired power plants is an attractive strategy to mitigate global anthropogenic CO2 emissions. However, the separation of CO2 from other components in the flue streams of gas-fired power plants is particularly challenging due to the low CO2 partial pressure (∼40 mbar), which necessitates that candidate separation materials bind CO2 strongly at low partial pressures (≤4 mbar) to capture ≥90% of the emitted CO2. High partial pressures of O2 (120 mbar) and water (80 mbar) in these flue streams have also presented significant barriers to the deployment of new technologies for CO2 capture from gas-fired power plants. Here, we demonstrate that functionalization of the metal–organic framework Mg2(dobpdc) (dobpdc4– = 4,4′-dioxidobiphenyl-3,3′-dicarboxylate) with the cyclic diamine 2-(aminomethyl)­piperidine (2-ampd) produces an adsorbent that is capable of ≥90% CO2 capture from a humid natural gas flue emission stream, as confirmed by breakthrough measurements. This material captures CO2 by a cooperative mechanism that enables access to a large CO2 cycling capacity with a small temperature swing (2.4 mmol CO2/g with ΔT = 100 °C). Significantly, multicomponent adsorption experiments, infrared spectroscopy, magic angle spinning solid-state NMR spectroscopy, and van der Waals-corrected density functional theory studies suggest that water enhances CO2 capture in 2-ampd–Mg2(dobpdc) through hydrogen-bonding interactions with the carbamate groups of the ammonium carbamate chains formed upon CO2 adsorption, thereby increasing the thermodynamic driving force for CO2 binding. In light of the exceptional thermal and oxidative stability of 2-ampd–Mg2(dobpdc), its high CO2 adsorption capacity, and its high CO2 capture rate from a simulated natural gas flue emission stream, this material is one of the most promising adsorbents to date for this important separation

Influence of Pore Size on Carbon Dioxide Diffusion in Two Isoreticular Metal–Organic Frameworks

The rapid diffusion of molecules in porous materials is critical for numerous applications including separations, energy storage, sensing, and catalysis. A common strategy for tuning guest diffusion rates is to vary the material pore size, although detailed studies that isolate the effect of changing this particular variable are lacking. Here, we begin to address this challenge by measuring the diffusion of carbon dioxide in two isoreticular metal–organic frameworks featuring channels with different diameters, Zn2(dobdc) (dobdc4– = 2,5-dioxido­benzene-1,4-dicarboxylate) and Zn2(dobpdc) (dobpdc4– = 4,4′-dioxido­biphenyl-3,3′-dicarboxylate), using pulsed field gradient NMR spectroscopy. An increase in the pore diameter from 15 Å in Zn2(dobdc) to 22 Å in Zn2(dobpdc) is accompanied by an increase in the self-diffusion of CO2 by a factor of 4 to 6, depending on the gas pressure. Analysis of the diffusion anisotropy in Zn2(dobdc) reveals that the self-diffusion coefficient for motion of CO2 along the framework channels is at least 10000 times greater than for motion between the framework channels. Our findings should aid the design of improved porous materials for a range of applications where diffusion plays a critical role in determining performance