High-temperature in situ crystallographic observation of reversible gas sorption in impermeable organic cages

CO2 Adsorption in a Robust Iron(III) Pyrazolate-Based MOF: Molecular-Level Details and Frameworks Dynamics From Powder X-ray Diffraction Adsorption Isotherms

Understanding adsorption processes at the molecular level, with multi-technique approaches, is nowadays at the frontier of porous materials research. In this work it is shown that with a proper data treatment, in situ high-resolution powder X-ray diffraction (HR-PXRD) at variable temperature and gas pressure can reveal atomic details of the accommodation sites, the framework dynamics as well as thermodynamic information (isosteric heat of adsorption) of the CO2 adsorption process in the robust iron(III) pyrazolate-based MOF Fe2(BDP)3 [H2BDP = 1,4-bis(1H-pyrazol-4-yl)benzene]. Highly reliable "HR-PXRD adsorption isotherms" can be constructed from occupancy values of CO2 molecules. The "HR-PXRD adsorption isotherms" accurately match the results of conventional static and dynamic gas sorption experiments and Monte Carlo simulations. These results are indicative of the impact of the molecular-level behavior on the bulk properties of the system under study and of the potential of the presented multi-technique approach to understand adsorption processes in metal-organic frameworks.

Endogenous metal-ion dynamic nuclear polarization for NMR signal enhancement in metal organic frameworks

Rational design of metal-organic framework (MOF)-based materials for catalysis, gas capture and storage, requires deep understanding of the host-guest interactions between the MOF and the adsorbed molecules. Solid-State NMR spectroscopy is an established tool for obtaining such structural information, however its low sensitivity limits its application. This limitation can be overcome with dynamic nuclear polarization (DNP) which is based on polarization transfer from unpaired electrons to the nuclei of interest and, as a result, enhancement of the NMR signal. Typically, DNP is achieved by impregnating or wetting the MOF material with a solution of nitroxide biradicals, which prevents or interferes with the study of host-guest interactions. Here we demonstrate how Gd(iii) ions doped into the MOF structure, LaBTB (BTB = 4,4',4''-benzene-1,3,5-triyl-trisbenzoate), can be employed as an efficient polarization agent, yielding up to 30-fold 13C signal enhancement for the MOF linkers, while leaving the pores empty for potential guests. Furthermore, we demonstrate that ethylene glycol, loaded into the MOF as a guest, can also be polarized using our approach. We identify specific challenges in DNP studies of MOFs, associated with residual oxygen trapped within the MOF pores and the dynamics of the framework and its guests, even at cryogenic temperatures. To address these, we describe optimal conditions for carrying out and maximizing the enhancement achieved in DNP-NMR experiments. The approach presented here can be expanded to other porous materials which are currently the state-of-the-art in energy and sustainability research.

Engineering Single-Atom Sites into Pore-Confined Nanospaces of Porphyrinic Metal-Organic Frameworks for the Highly Efficient Photocatalytic Hydrogen Evolution Reaction

As a type of heterogeneous catalyst expected for the maximum atom efficiency, a series of single-atom catalysts (SACs) containing spatially isolated metal single atoms (M-SAs) have been successfully prepared by confining M-SAs in the pore-nanospaces of porphyrinic metal-organic frameworks (MOFs). The prepared MOF composites of M-SAs@Pd-PCN-222-NH₂ (M = Pt, Ir, Au, and Ru) display exceptionally high and persistent efficiency in the photocatalytic hydrogen evolution reaction with a turnover number (TON) of up to 21713 in 32 h and a beginning/lasting turnover frequency (TOF) larger than 1200/600 h^-1 based on M-SAs under visible light irradiation (λ ≥ 420 nm). The photo-/electrochemical property studies and density functional theory calculations disclose that the close proximity of the catalytically active Pt-SAs to the Pd-porphyrin photosensitizers with the confinement and stabilization effect by chemical binding could accelerate electron-hole separation and charge transfer in pore-nanospaces, thus promoting the catalytic H₂ evolution reaction with lasting effectiveness.

Emission-Tunable Soft Porous Organic Crystal Based on Squaraine for Single-Crystal Analysis of Guest-Induced Gate-Opening Transformation

Soft porous crystals (SPCs) with both crystallinity and flexibility have evolved as emerging materials for lots of applications. However, the development of purely organic SPCs (SPOCs) with advanced functionalities significantly lags behind. Herein, we report the construction of an emission-tunable SPOC with a rationally designed squaraine derivative (named as SPOC-SQ). SPOC-SQ is featured with a squaraine core and four peripheries with electron donor-π-acceptor (D-π-A) characteristics, which facilitates the formation of porous crystal framework stabilized by π-π interactions and H bonds and at the same time provides structural flexibility through phenyl rotations. This SPOC can be easily obtained from its dichloromethane (DCM) solution and exhibits reversible stimuli-responsive single-crystal-to-single-crystal (SCSC) structural transformation, accompanied by bright and tunable emission. In addition, this activated SPOC (SPOC-SQ-a) selectively recognizes and absorbs acetylene (C₂H₂) over other gases without destroying the single crystallinity, enabling the single-crystal XRD analysis of the structural transformation. Close inspection of single-crystal XRD results of SPOC-SQ-C₂H₂ facilitates the understanding of the host-guest interactions. More interestingly, upon interacting with C₂H₂, a one-dimensional (1D) channel is formed in the crystal to adopt C₂H₂, which proves the SCSC process and provides molecular-level insights into the gate-opening process. Furthermore, C₂H₂ adsorption dynamics can be monitored in real time by tracking the fluorescence wavelength changes of SPOC-SQ framework. Thus, the unique gate-opening sorption attribute of SPOC-SQ-a crystals toward C₂H₂ enables its potential applications for gas separation.

Porous Metal-Organic Polyhedra: Morphology, Porosity, and Guest Binding

Designing porous materials which can selectively adsorb CO₂ or CH₄ is an important environmental and industrial goal which requires an understanding of the host–guest interactions involved at the atomic scale. Metal–organic polyhedra (MOPs) showing permanent porosity upon desolvation are rarely observed. We report a family of MOPs (Cu-1a, Cu-1b, Cu-2), which derive their permanent porosity from cavities between packed cages rather than from within the polyhedra. Thus, for Cu-1a, the void fraction outside the cages totals 56% with only 2% within. The relative stabilities of these MOP structures are rationalized by considering their weak nondirectional packing interactions using Hirshfeld surface analyses. The exceptional stability of Cu-1a enables a detailed structural investigation into the adsorption of CO₂ and CH₄ using in situ X-ray and neutron diffraction, coupled with DFT calculations. The primary binding sites for adsorbed CO₂ and CH₄ in Cu-1a are found to be the open metal sites and pockets defined by the faces of phenyl rings. More importantly, the structural analysis of a hydrated sample of Cu-1a reveals a strong hydrogen bond between the adsorbed CO₂ molecule and the Cu(II)-bound water molecule, shedding light on previous empirical and theoretical observations that partial hydration of metal−organic framework (MOF) materials containing open metal sites increases their uptake of CO₂. The results of the crystallographic study on MOP–gas binding have been rationalized using DFT calculations, yielding individual binding energies for the various pore environments of Cu-1a.

We report a family of metal−organic polyhedra (MOP), which derive their permanent porosity from cavities between packed cages rather than from within the polyhedra. The relative stabilities of these MOP structures are rationalized by considering their weak nondirectional packing interactions using Hirshfeld surface analysis. A detailed structural investigation into the adsorption of CO₂ and CH₄ is reported using in situ X-ray and neutron diffraction, coupled with DFT calculations.

Adsorption under nanoconfinement: a theoretical-computational study revealing significant enhancement beyond the Langmuirian levels

The principal goal of this work is to predict characteristics unique to equilibrated adsorption of a small number of molecules on atomic sites located inside a closed nanoscale space. Compared to the thermodynamic limit of macroscopic systems, significantly enlarged adsorbate coverage under nanoconfinement constitutes a major finding of the modeling. Concomitantly, nanoconfined adsorbates are expected to exhibit extra thermal stability against desorption. These effects on adsorption are explored using canonical partition-functions as well as an original relationship between coverage variations and the Langmuir constant, both in the frameworks of the ideal gas and lattice-gas models. With reported DFT adsorption-energies as input, adsorption isotherms are derived numerically for H2 on Ti-doped graphene-like nanostructures. Remarkable deviations from the classical Langmuir isotherm are predicted for the first time, namely, system-size dependent enhanced H2 adsorbate coverage. The effects are computed also for CO2 inside MOF single-molecule traps, including their relationships to adsorption-energy, specific-heat and to coverage fluctuations. According to preliminary modeling, nanoconfinement effects are anticipated also for adsorption in nanopores undergoing molecular exchange with the external environment, and for impurity segregation in nanoparticle and nanocrystalline solids. The entropic origin of the nanoconfinement effect on equilibrium adsorption (NCEEA) is demonstrated analogously to the nanoconfinement effect on equilibrated chemical reactions studied by us previously. Besides unraveling some basic theoretical issues in physical nanochemistry, this study is expected to be pertinent to nanotechnological applications, such as gas storage and separation in nanoporous materials and other solid adsorbents.

Effect of stepwise protonation of an N-containing ligand on the formation of metal–organic salts and coordination complexes in the solid state

Controlled synthesis of a series of metal–organic salts and coordination complexes is tuned by the protonation of ligand, and their transformations are induced by the solid–gas and concentration of [H⁺].

Unraveling the Sorption Mechanism of CO2 in a Molecular Crystal without Intrinsic Porosity

The facile uptake of CO₂ gas in a nonporous molecular crystal constituted by long molecules with carbazole and ethynylphenyl moieties was reported in experiments recently. Herein, the mechanism of gas uptake by this crystal is elucidated using atomistic molecular simulations. The uptake of CO₂ is shown to be facilitated by (i) the capacity of the crystal to expand in volume because of weak intermolecular interactions, (ii) the parallel orientation of the long molecules in the crystal, and (iii) the ability of the molecule to marginally bend, yet not lose crystallinity because of the anchoring of the terminal carbazole groups. The retention of crystallinity upon sorption and desorption cycles is also demonstrated. At high enough pressures, near-neighbor CO₂ molecules sorbed in the crystal are found to be oriented parallel to each other.

Efficient separation of C2 hydrocarbons in a permanently porous hydrogen-bonded organic framework

A highly robust porous hydrogen-bonded organic framework (HOF) constructed by 4,4',4''-benzene-1,3,5-triyl-tris(benzoic acid) not only achieves the highest uptakes of ethylene and ethane among the HOF materials, but also exhibits unusual adsorption selectivity of C2H6 over other C2 gases. Besides, it exhibits the second highest acetylene uptake among all the reported HOF materials.

Dynamic behaviour in nonporous hybrid metal–organic materials via mechanochemical and gas–solid reactions

Nonporous hybrid metal organic materials have been studied in the solid-state by mechanochemical reactions, chemisorption and release of HCl by heating. The dynamic behavior of the hybrid materials allows the breaking and formation of chemical bonds.

Self-assembly of 1,3,5-benzenetribenzoic acid on Ag and Cu at the liquid/solid interface

Big difference: molecules of benzenetribenzoic acid are disordered on Cu but highly organised on Ag, forming a monopodal row structure.

Gas adsorption in Mg-porphyrin-based porous organic frameworks: A computational simulation by first-principles derived force field

A novel type of porous organic frameworks, based on Mg-porphyrin, with diamond-like topology, named POF-Mgs is computationally designed, and the gas uptakes of CO₂ , H₂ , N₂ , and H₂ O in POF-Mgs are investigated by Grand canonical Monte Carlo simulations based on first-principles derived force fields (FF). The FF, which describes the interactions between POF-Mgs and gases, are fitted by dispersion corrected double-hybrid density functional theory, B2PLYP-D3. The good agreement between the obtained FF and the first-principle energies data confirms the reliability of the FF. Furthermore our simulation shows the presence of a small amount of H₂ O (≤ 0.01 kPa) does not much affect the adsorption quantity of CO₂ , but the presence of higher partial pressure of H₂ O (≥ 0.1 kPa) results in the CO₂ adsorption decrease significantly. The good performance of POF-Mgs in the simulation inspires us to design novel porous materials experimentally for gas adsorption and purification. © 2017 Wiley Periodicals, Inc.

High and Selective Carbon Dioxide Capture in Nitrogen-Containing Aerogels via Synergistic Effects of Electrostatic In-Plane and Dispersive π-π-Stacking Interactions

A new strategy for CO₂ capture is reported based on the synergistic effect of electrostatic in-plane and dispersive π-π-stacking interactions of amide and indole with CO₂. Density functional theory illustrated that the amide group can have an increased ability to capture CO₂ molecules that were just desorbed from an adjacent indole unit. We used this strategy to fabricate a microporous aerogel that exhibited a superior CO₂ capture performance in both dry and wet conditions. The proposed synergistic effect is expected to be a new rationale for the design of CO₂ capture materials.

Hierarchical Structure and Molecular Dynamics of Metal-Organic Framework as Characterized by Solid State NMR

Metal-organic framework (MOF) stands out as a promising material with great potential in application areas, such as gas separation and catalysis, due to its extraordinary properties. In order to fully characterize the structure of MOFs, especially those without single crystal, Solid State NMR (SSNMR) is an indispensable tool. As a complimentary analytical technique to X-ray diffraction, SSNMR could provide detailed atomic level structure information. Meanwhile, SSNMR can characterize molecular dynamics over a wide dynamics range. In this review, selected applications of SSNMR on various MOFs are summarized and discussed.