|Elucidating CO2 Chemisorption in Diamine-Appended Metal–Organic Frameworks (April 2019)||Cooperative Adsorption of Carbon Disulfide in Diamine-Appended Metal-Organic Frameworks (February 2019)|
|Cooperative Adsorption of Gas in a MOF Can Happen Even in the Absence of an Underlying Phase Transition (December 2018)||Zeolitic Imidazolate Framework Membranes Made by Ligand-Induced Permselectivation (LIPS) (October 2018)|
|Redox-Switchable Breathing Behavior in Metal–Organic Frameworks (June 2018)||Separation of Xylene Isomers through Multiple Metal Site Interactions in Metal-Organic Frameworks (May 2018)|
|Unexpected Diffusion Anisotropy of Carbon Dioxide in the Metal-Organic Framework Zn2(dobpdc) (March 2018)||On the Direct Synthesis of Cu(BDC) MOF Nanosheets and their Performance in Mixed Matrix Membranes (February 2018)|
|Relaxometry and Diffusometry of Small Molecules in MOFs (December 2017)||A Spin Transition Mechanism for Cooperative Adsorption in Metal–Organic Frameworks (November 2017)|
|The Chemistry of CO2 Capture in an Amine-Functionalized Metal–Organic Framework under Dry and Humid Conditions (October 2017)||Controlling Cooperative CO2 Adsorption in Diamine-Appended Metal–Organic Frameworks (August 2017)|
|Structural Characterization of Framework–Gas Interactions by in Situ Single-Crystal X-ray Diffraction (June 2017)||The Influence of Intrinsic Framework Flexibility on Adsorption in Nanoporous Materials (May 2017)|
|Ultra-Selective High-Flux Membranes from Directly Synthesized Zeolite Nanosheets (April 2017)||Continuous Flow Processing of ZIF-8 Membranes on Polymeric Porous Hollow Fiber Supports for CO2 Capture (March 2017)|
|Chemical Conversion of Linkages in Covalent Organic Framework (February 2017)||New Adsorbate-Induced Deformation of a Series of Metal-Organic Frameworks (January 2017)|
|New Method to Incorporate Alkylamine into Metal-Organic Frameworks for CO2 Capture (December 2016)||In silico Design and Screening of Hypothetical MOF-74 Analogs and Their Experimental Synthesis (October 2016)|
|Reducing Plasticization Effects in Polymer Membranes using Metal-Organic Framework Nanocrystals (September 2016)||Selective Gas Capture via Kinetic Trapping (August 2016)|
|An In Situ One-Pot Synthetic Approach towards Multivariate Zirconium MOFs (July 2016)||Reversible CO Scavenging via Adsorbate-Dependent Spin Transitions in an Fe(II)-Triazolate Metal-Organic Framework (June 2016)|
|Enhanced Separation and Mitigated Plasticization in Membranes using Metal-Organic Framework Nanoparticles (May 2016)||Systematic Tuning and Multi-Functionalization of Covalent Organic Polymers for Enhanced Carbon Capture (April 2016)|
|Layered ZIF-Polymer Membranes Through On-Polymer Chemical Transformations of Colloidal Nanocrystal Films (January 2016)||Covalent Organic Frameworks Comprising Cobalt Porphyrins for Catalytic CO2 Reduction in Water (December 2015)|
|MOFs Can Tune the Critical Point of Gases (November 2015)||Novel CO2 Binding Mechanism Determined Via in-situ X-ray Absorption Spectroscopy & Theory (August 2015)|
|Ultrastable Polymolybdate-Based Metal-Organic Frameworks as Highly Active Electrocatalysts for Hydrogen Generation (July 2015)||Understanding Small Molecule Interactions in Metal-Organic Frameworks: Coupling Experiment with Theory (June 2015)|
|Photochromic MOFs for Control of Singlet Oxygen Generation (May 2015)||Introduction of Functionality, Selection of Topology, and Enhancement of Gas Adsorption in Multivariate Metal−Organic Framework-177 (April 2015)|
|Low-Energy CO2 Capture through Cooperative Adsorption (March 2015)||Ab Initio Rational Design of New MOFs for Separations and Flue Gas Capture (February 2015)|
|Topology Guided Design and Syntheses of Stable Mesoporous MOFs with High Surface Area (January 2015)||Evaluating Different Classes of Porous Materials for Carbon Capture (December 2014)|
|Hybrid Absorption–Adsorption Carbon Capture (November 2014)||Reversible CO Binding in Metal-Organic Frameworks (October 2014)|
|Metal-Organic Frameworks with Precisely Designed Interior for Carbon Dioxide Capture in the Presence of Water (September 2014)||Stepwise Synthesis of Robust Metal-Organic Frameworks via Post-Synthetic Metathesis and Oxidation of Metal Nodes (September 2014)|
|Capture and Release of Guest Molecules by Optical Responsive Metal–Organic Polyhedra (MOP) (August 2014)||Rigidifying Fluorescent Linkers by MOF Formation for Quantum Yield Enhancement (July 2014)|
|Redox Chemistry And Metal-Insulator Transitions Intertwined (June 2014)||High Methane Storage Capacity in Aluminum Metal−Organic Frameworks (MOFs) (May 2014)|
|Water Adsorption in Metal-Organic Frameworks (April 2014)||Cooperative Interactions Boost Adsorption Performance (April 2014)|
|Symmetry-Guided Design of Highly Porous MOFs (March 2014)||Highly Stable Porphyrinic Zr-MOFs for CO2 Fixation (February 2014)|
|Understanding Porosity in Amorphous Porous Molecular Solids (January 2014)||Probing CO2 Adsorption in Metal-Organic Frameworks with Open Metal Sites (December 2013)|
|A Stable Zr-Porphyrinic MOF Exhibiting pH-Dependent Fluorescence (November 2013)||The Chemistry and Applications of Metal-Organic Frameworks (October 2013)|
|Mapping of Functional Groups in Metal-Organic Frameworks (September 2013)||Solid, Porous Material for Improved Efficiency of Gasoline Production and Low-Cost and Non-Toxic Enhancement of Gasoline Quality (August 2013)|
|CO2 adsorption mechanism in alkylamine-functionalized Mg2(dobpdc) (July 2013)||Mail-Order Metal-Organic Frameworks: Designing Isoreticular MOF-5 Analogues Comprising Commercially Available Organic Molecules (June 2013)|
|New Materials for Methane Capture from Dilute and Medium-Concentration Sources (May 2013)||Understanding CO2 Dynamics Inside MOF Open Metal Sites (April 2013)|
|New Structure found at Ionic Liquid Surfaces (March 2013)||Pre-Designed Single-Molecule Traps for CO2 Capture (February 2013)|
|CO2 Capture from Air Using Porous Polymer Networks (January 2013)||Predicting Large CO2 Adsorption in Aluminosilicate Zeolites for Postcombustion Carbon Dioxide Capture (December 2012)|
|High Performance Composite Membranes for Separation of Carbon Dioxide from Methane (December 2012)||Efficient Discovery of Zeolite Materials for Adsorption-based Separations (November 2012)|
|Ultrastable Metal-Organic Frameworks with Large Channels (November 2012)||Confinement of Metal−Organic Polyhedra in Silica Nanopores (October 2012)|
|CO2 Dynamics in a Metal-Organic Framework with Open Metal Sites (September 2012)||Pore Surface Engineering with Controlled Loadings of Functional Groups via Click Chemistry in Highly Stable Metal–Organic Frameworks (August 2012)|
|Ab initio Carbon Capture in Open-Site Metal Organic Frameworks (August 2012)||Systematic Expansion of Porous Crystals to Include Large Molecules (July 2012)|
|In Silico Screening of Carbon Capture Materials (June 2012)||Diffusion In Confinement: Kinetic Simulations of Self- and Collective-Diffusion Behavior of Adsorbed Gases (May 2012)|
|Reduced Regeneration Energy CO2 Adsorbent (April 2012)||Novel Material for Efficient and Low-Cost Separation of Gases for Fuels and Plastics (March 2012)|
|Enhanced CO2 Capture in Metal-Organic Frameworks (February 2012)||Stimulus CO2 adsorption in Metal-Organic Frameworks (January 2012)|
|New X-ray Technique to Study Molecular Orientation through Non-crystalline Thin Films (April 2011)||Metal-Organic Frameworks Capture CO2 From Coal Gasification Flue Gas (March 2011)|
|Subnanometer Porous Thin Films by the Co-assembly of Nanotube Subunits and Block Copolymers (January 2011)||Metal Binding in an Aluminum Based Metal-Organic Framework for Carbon Dioxide Capture (September 2010)|
|Nitrogen/Oxygen Separations in Metal-Organic Frameworks for Clean Fossil Fuel Combustion (May 2010)||New Synthetic Strategy for Porous Molecular Materials towards Gas Separation (February 2010)|
The widespread deployment of carbon capture and sequestration as a climate change mitigation strategy could be facilitated by the development of more energy-efficient adsorbents. Diamine-appended metal–organic frameworks of the type diamine–M2(dobpdc) (M = Mg, Mn, Fe, Co, Ni, Zn; dobpdc4- = 4,4′-dioxidobiphenyl-3,3′-dicarboxylate) have shown promise for carbon capture applications, although questions remain regarding the molecular mechanisms of CO2 uptake in these materials. Here, we leverage the crystallinity and tunability of this class of frameworks to perform a comprehensive study of CO2 chemisorption. Using multinuclear nuclear magnetic resonance (NMR) spectroscopy experiments and calculations for thirteen diamine–M2(dobpdc) variants, we demonstrate that the canonical CO2 chemisorption products—ammonium carbamate chains and carbamic acid pairs—can be readily distinguished, and that ammonium carbamate chain formation dominates for diamine–Mg2(dobpdc) materials. In addition, we elucidate a new chemisorption mechanism in the material dmpn–Mg2(dobpdc) (dmpn = 2,2-dimethyl-1,3-diaminopropane), which involves formation of a 1:1 mixture of ammonium carbamate and carbamic acid and accounts for the unusual adsorption properties of this material. Finally, we show that the presence of water plays an important role in directing the mechanisms for CO2 uptake in diamine–M2(dobpdc) materials. Overall, our combined NMR and calculations approach gives a thorough understanding of CO2 adsorption within diamine–M2(dobpdc) compounds, which may aid similar studies in other amine-functionalized adsorbents in the future.
Over one million tons of CS2 are produced annually, and emissions of this volatile and toxic liquid, known to generate acid rain, remain poorly controlled. As such, materials capable of reversibly capturing this commodity chemical in an energy-efficient manner are of interest. Recently, we reported diamine-appended metal-organic frameworks capable of selectively capturing CO2 via a cooperative insertion mechanism that promotes efficient adsorption–desorption cycling. We therefore sought to explore the ability of these materials to capture CS2 through a similar mechanism. Employing crystallography, spectroscopy, and gas adsorption analysis, we demonstrate that CS2 is indeed cooperatively adsorbed in N,N-dimethylethylenediamine-appended framework M2(dobpdc) (M = Mg, Mn, Zn; dobpdc4− = 4,4′-dioxidobiphenyl-3,3′-dicarboxylate), via the formation of electrostatically paired ammonium dithiocarbamate chains. In the weakly thiophilic Mg congener, chemisorption is cleanly reversible with mild thermal input. This work demonstrates that the cooperative insertion mechanism can be generalized to other high-impact target molecules.
Cooperative (sharp) adsorption of gases by porous frameworks, which permits more efficient uptake and removal than the more usual non-cooperative (Langmuir-type) adsorption, usually results from a phase transition of the framework. We show how cooperativity emerges in the class of metal-organic frameworks mmen-M2(dobpdc) (mmen = N,N′-dimethylethylenediamine; dobpdc4− = 4,4′-dioxido-3,3′-biphenyldicarboxylate; M = Mg, Mn, Fe, Co, Zn, and Ni) in the absence of a phase transition. Our study provides a microscopic understanding of the emergent features of cooperative binding, including the position, slope, and height of the isotherm step, and indicates how to optimize gas storage and separation in these materials.
Zeolitic imidazolate framework (ZIF) membranes are a promising energy-efficient separation technology. However, the existing fabrication methods of ZIF membranes rely on solvothermal synthesis that is difficult to scale up. The CGS team developed a novel all-vapor-phase processing method based on atomic layer deposition (ALD) and ligand-vapor treatment that holds promise for reliable and scalable manufacturing of high-performance molecular sieving membranes. This method starts with ALD, a well-established materials processing technology, to deposit ZnO inside a porous alumina support using diethyl zinc + water. After ALD, an impermeable and nonselective ZnO-alumina composite is obtained. The ALD treated membrane is then subjected to a treatment of vapors of ligands that can react with the oxide and transform it to ZIF. After ligand-vapor treatment (2-methylimidazole at 125 °C for 24 h), the ALD-deposited ZnO is partially converted to ZIF, resulting in a ZIF nanocomposite membrane that shows stable performance with high propylene flux and high propylene/propane selectivity. Because of the transformation from an impermeable to selective membrane, this process is named ligand-induced permselectivation (LIPS). The LIPS approach is based on solvent-free, seed-free, all-vapor processing with ALD, and is shown to be simple and highly reproducible. It establishes a robust approach for the fabrication of ZIF and possibly many other MOF membranes and nanocomposites.
Metal-organic frameworks (MOFs) that respond to external stimuli such as guest molecules, temperature, or redox conditions are highly desirable for their potential applications as smart absorbents for gas separation and storage. A representative example of framework flexibility is the breathing effect in which the framework experiences a reversible unit-cell dimensional change resulting from guest adsorption or desorption. We demonstrated that the flexibility and the associated breathing behavior of MOFs can be controlled by redox chemistry. Guided by topology, two flexible isomeric MOFs with a formula of In(Me2NH2)(TTFTB) were constructed via a combination of [In(COO)4]- metal nodes and tetratopic tetrathiafulvalene-based linkers (TTFTB). The breathing behaviors of two compounds upon N2 sorption were studied by single-crystal X-ray diffractions and molecular simulations. More importantly, the rigidity of TTF-based linkers can be switched by reversible redox reaction, which in turn controls the flexibility of MOFs. The redox-controlled dynamic behavior of MOFs is reminiscent of sophisticated biological behavior such as redox regulation of enzymes. We believe that the redox-switchable flexible MOFs can potentially be applied to the design of smart absorbents with higher storage capacities and efficient gas release.
The separation of alkylaromatics o-xylene, m-xylene, p-xylene, and ethylbenzene is of enormous importance industrially, but very technically challenging due to the different isomers' similar physical properties. Metal–organic frameworks are promising as potential selective adsorbents, but separation of larger hydrocarbon mixtures remains a challenge. Here, we demonstrate the ability of Co2(dobdc) (dobdc4– = 2,5-dioxide-1,4-benzenedicarboxylate) and Co2(m-dobdc) (m-dobdc4– = 4,6-dioxido-1,3-benzenedicarboxylate) to differentiate these different isomers, with Co2(dobdc) preferentially binding o-xylene > ethylebenzene > m-xylene > p-xylene. This was shown through single component vapor adsorption isotherms, multi-component vapor breakthrough measurements, and multi-component solution phase adsorption measurements. Structural analysis through single crystal and powder X-ray diffraction showed that these materials perform these separations through multiple metal site interactions, in which adjacent Co(II) centers bind to the same xylene molecule. Due to the different shapes of these isomers, the degree to which each xylene molecule can effectively bind to both Co(II) sites dictates the strength of interaction, providing an effective basis for separation. Finally, previously unobserved pore flexing in the hexagonal channels of Co2(dobdc) allows for greater xylene uptake capacities.
Metal–organic frameworks are promising materials for energy-efficient gas separations, but little is known about the diffusion of adsorbates in materials featuring one-dimensional porosity at the nanoscale. An understanding of the interplay between framework structure and gas diffusion is important for the practical application of these materials as adsorbents or in mixed-matrix membranes. Here, we investigated the diffusion of CO2 within the pores of Zn2(dobpdc) (dobpdc4– = 4,4′-dioxidobiphenyl-3,3′-dicarboxylate) using pulsed field gradient (PFG) nuclear magnetic resonance (NMR) spectroscopy and molecular dynamics (MD) simulations. Importantly, our PFG NMR method allowed measurement of self-diffusion in different crystallographic directions. In addition to observing CO2 diffusion through the channels parallel to the crystallographic c axis we unexpectedly found that CO2 is also able to diffuse between the hexagonal channels in the crystallographic ab plane, despite the walls of these channels appearing impermeable by single-crystal X-ray crystallography and flexible lattice MD simulations. Observation of such unexpected diffusion in the ab plane suggests the presence of defects that enable effective multidimensional CO2 transport in a metal–organic framework with nominally one-dimensional porosity.
Metal-organic framework (MOF)-based membranes have gained increasing attention for their potential utility in the separation of CO2 from natural gas and flue gas streams. Mixed matrix membranes (MMMs), which consist of MOF particles imbedded in a polymer matrix, are particularly appealing from a manufacturing standpoint. Compared to isotropic MOF crystals, the use of high aspect-ratio MOF nanosheets may enhance the separation performance of MMMs, although it has been a challenge to develop simple and scalable methods for nanosheet synthesis. In this work, we describe the facile preparation of Cu(BDC) (BDC2– = 1,4-benzenedicarboxylate) nanosheets with aspect ratios as high as 100 via direct synthesis from a well-mixed solution of metal and ligand precursors. Notably, incorporation of the Cu(BDC) nanosheets into a Matrimid polymer matrix results in a MMM with a 70% increase in CO2/CH4 selectivity compared to neat Matrimid. Our approach is suitable for large-scale synthesis and can also be extended to the preparation of other varieties of MOF nanosheets. Analysis of gas permeation data for Cu(BDC) MMMs using a mathematical model also indicates that additional membrane performance improvements may be achieved by varying the choice of polymer used in the continuous phase.
How do molecules move when confined within small spaces? We placed three molecules (ortho-, meta-, and para-xylene) inside of a special material (metal-organic framework IRMOF-1) that has small pores, about the twice the size of these molecules. NMR spectroscopy revealed differences in how fast the molecules diffuse, as well as how the molecules rotate when confined to the MOF. Diffusion measurements come from a method known as NMR pulsed-field gradient diffusion and compare quite well with computer simulations of diffusion using sophisticated force fields created by quantum chemistry. The most cylindrically-shaped molecule, para-xylene, diffuses the fastest of the three because it has the weakest interactions with the MOF walls, as revealed by computer simulations. The rotation of these molecules was discerned via NMR relaxation times and, when combined with computer simulated probability maps, show that these molecules rotate like a Frisbee, and not like a flipping pancake, while in the MOF. Interestingly, the para-xylene molecules diffuse the fastest, yet have the highest energy barrier to rotate because these molecules don’t fit well in the MOF pores.
Designing new adsorbents for industrial gas separations requires maximizing both selectivity for the gas of interest and the recyclability of the material, where the selective adsorbent can be easily regenerated under mild conditions. We have made significant progress in designing selective adsorbents known as metal–organic frameworks (MOFs) that exhibit cooperative carbon monoxide adsorption for highly energy-efficient separations. Specifically, these materials contain interacting iron(II) sites where binding carbon monoxide at one iron site assists the binding of CO at neighboring metal sites, much like biological systems such as hemoglobin. This occurs through a spin transition mechanism where during this binding process neighboring iron sites undergo a simultaneous transition from high-spin to low-spin. Due to this adsorption mechanism, these materials can exhibit large working capacities utilizing small temperature swings, making them highly energy efficient in terms of regeneration while also remaining selective for CO adsorption. Importantly, this spin transition was shown to be highly tunable through variation of the organic linkers of the framework. As a similar response can be achieved through adsorption of a variety of different industrially relevant gases, further modification of this system can result in next generation materials for several different separation processes.
New products resulting from CO2 capture in porous materials can lead to enhanced efficiency of the solid sorbents. The use of two primary alkylamine functionalities covalently tethered to the linkers of IRMOF-74-III results in a material that can uptake CO2 at low pressures through a chemisorption mechanism. In contrast to other primary amine-functionalized solid adsorbents that uptake CO2 primarily as ammonium carbamates, we observe using solid state NMR that the major chemisorption product for this material is carbamic acid. The equilibrium of reaction products also shifts to ammonium carbamate when water vapor is present; a new finding that has impact on control of the chemistry of CO2 capture in MOF materials. This finding also highlights the importance of geometric constraints within the pores of MOFs as the amines in IRMOF-74-III are positioned such that formation of carbamic acid is favored under dry conditions. The understanding of this chemistry gleaned here can be applied to the synthesis of next-generation solid CO2 sorbents.
In the transition to a clean-energy future, CO2 separations will play a critical role in mitigating current greenhouse gas emissions and facilitating conversion to cleaner-burning and renewable fuels. Diamine-appended variants of the metal–organic framework Mg2(dobpdc) (dobpdc4- = 4,4′-dioxidobiphenyl-3,3′-dicarboxylate) are a particularly promising materials for CO2 separations owing to their high selectivities for CO2 adsorption, large CO2 removal capacities, and low regeneration energies. These frameworks feature step-shaped CO2 adsorption isotherms resulting from cooperative and reversible insertion of CO2 into metal–amine bonds to form ammonium carbamate chains. A detailed structure–activity study revealed that small modifications to the diamine structure can shift the threshold pressure for cooperative CO2 adsorption by over 4 orders of magnitude at a given temperature, and the observed trends were rationalized on the basis of crystal structures of the isostructural zinc frameworks. These results can be leveraged to precisely tailor adsorbents to the conditions of a given CO2 separation process, highlighting the potential of diamine-appended frameworks as next-generation adsorbents for a wide array of CO2 separations.
The crystallographic characterization of framework-guest interactions provides unparalleled insight on the nature and location of guest binding sites within metal-organic frameworks. In collaboration with Beamline 11.3.1 at the Advanced Light Source, experimental apparatus and techniques have been developed for in situ single-crystal X-ray diffraction experiments on porous crystals. These methods have enabled the direct observation of the adsorption of small molecules (CO, N2, O2, CH4, Ar, and P4) in Co2(dobdc) (dobdc4- = 2,5-dioxido-1,4-benzene-dicarboxylate), a metal-organic framework with coordinatively unsaturated cobalt(II) sites. The resulting structures reveal the location of the primary, secondary (for N2, O2, and Ar) and tertiary (for O2) adsorption sites for these gases within the framework. Remarkably, these gases interact with the framework cobalt(II) centers through distinctly weak interactions compared to those found in molecular complexes. As a consequence, this work represents the first report of the characterization of such species by single-crystal X-ray diffraction. Furthermore, these results are correlated with low- and high-pressure gas adsorption isotherms to establish the relationship between structure and adsorption behavior.
For applications of metal-organic frameworks (MOFs) such as gas storage and separation, flexibility is often seen as a parameter that can tune material performance. In this work we aim to determine the optimal flexibility for the shape selective separation of similarly sized molecules (e.g., Xe/Kr mixtures). To obtain systematic insight into how the flexibility impacts this type of separation, we develop a simple analytical model that predicts a material’s Henry regime adsorption and selectivity as a function of flexibility. Selectivity performance is either improved or reduced with increasing flexibility, depending on the material’s pore size characteristics. However, the selectivity of a material with the pore size and chemistry that already maximizes selectivity in the rigid approximation is continuously diminished with increasing flexibility, demonstrating that the globally optimal separation exists within an entirely rigid pore. Molecular simulations show that our simple model predicts performance trends that are observed when screening the adsorption behavior of flexible MOFs. Thus, for shape selective adsorption applications, the globally optimal material will have the optimal pore size/chemistry and minimal intrinsic flexibility even though other nonoptimal materials' selectivity can actually be improved by flexibility. Equally important, we find that flexible simulations can be critical for correctly modeling adsorption in these types of systems.
Porous crystalline materials with uniform pore sizes and high pore densities are promising for membrane separation applications, which are energy-efficient alternative to the conventional separation processes. However, the fabrication cost of such material is relatively high, and cost reduction or performance improvement is required for industrial applications. One promising approach is to fabricate a thin and intergrown membrane based on 2-dimensional crystals (nanosheets). In this work, we have, for the first time, developed a direct synthesis method of single MFI nanosheets and demonstrated their high separation performances. Previously, zeolite MFI nanosheets were prepared by exfoliation of multi-lamella MFI materials. This exfoliation process is costly and time-consuming and suffers from fracture of the nanosheets (<300 nm). In comparison, our direct synthesis method can yield larger (~2 µm) nanosheets with an improved production yield. This was enabled by seeded growth based on twining, which triggers the emergence of nanosheet from the seed crystal. These nanosheets with high aspect ratios can form high-density coating on porous supports, which are further intergrown into continuous membranes. These thin membranes exhibit superior separation performances, as established for xylene isomer mixtures, alcohol/water mixture, and linear/branched hydrocarbon mixtures.
Inorganic membranes can have very high gas permeability and selectivity compared to conventional polymeric membranes. But the fabrication of inorganic membranes is costly, time consuming and needs aggressive synthesis conditions. Also, scale-up and reproducibility are big problems, in part due to the use of expensive ceramic supports. In this work, we have successfully fabricated defect-free ZIF-8 membranes on polymeric hollow fiber supports using a continuous flow processing method that is simple, scalable, reduces manufacturing costs, and is environmentally friendly. The formation of a continuous ZIF-8 membrane that is ~8 µm thick was controlled by flowing an aqueous metal solution on the shell side of a polymer hollow fiber while flowing the 2-methylimidazole linker through the bore. The formation of ZIF-8 was confirmed using XRD and EDX. The ZIF-8 membrane was grown and anchored to the microporous region of the outer surface of supports for better mechanical properties and to avoid the delamination of membrane. These membranes demonstrated CO2 permeance of 22 GPU and CO2/N2 selectivity of 52. This method is very useful to scale up the fabrication of inorganic membranes for industrial applications, as the membrane can be formed in situ in a pre-assembled hollow fiber module.
Covalent organic frameworks (COFs) are ordered, porous polymers formed from the assembly of small molecular building blocks. The typical linking reactions employed are reversible so that any defects arising during COF synthesis can be dissolved and subsequently reformed. The inherent linkage reversibility leaves the frameworks subject to chemical degradation through this reverse reaction. This report describes a chemical modification of the framework from easily hydrolyzed imine linkages to more chemically stable amide functionalities. This reaction can be performed under mild oxidative conditions at room temperature, and leaves the underlying structure of the starting materials unchanged. The conversion of these materials can be confirmed can both infrared and solid state nuclear magnetic resonance (NMR) spectroscopy. In addition, the products maintain the crystallinity of the parent compounds, as measured by powder X-ray diffraction. While the amide-linked materials are permanently porous, their surface area is lowered by this transformation, probably due to included oligomeric species, whose existence is supported by preliminary NMR experiments. The improved chemical stability of these new materials was assessed by measurement of diffraction before and after treatment with strong aqueous acid and base.
Flexible metal-organic frameworks (MOFs) have unique adsorption properties. However, most simulation studies of adsorption in MOFs assume that the crystal lattice is rigid. The development of new flexible framework models is required to compare to and interpret experimental data. Recently, a new deformation pattern has been observed by the adsorption of argon in the IRMOF-74 series. To describe this behavior, we used a combination of Monte Carlo and molecular dynamics simulation techniques, which demonstrates that adsorbate molecules can induce a change in the crystal lattice associated with a lowering of the crystal symmetry. The simulations show that the crystal lattice changes from regular hexagons to a complex pattern of some regular hexagons surrounded by a spiral of irregular hexagons. Adsorption simulations show that irregular hexagons in the deformed lattice have a slightly smaller pore volume, which enhances adsorbate interactions. We compared the X-ray scattering data associated with the deformed lattice to results published in the journal Nature, and found that deformed lattice X-ray peaks corresponded with experimental observations. The conclusions indicate that lattice deformation is an alternative explanation for in situ small angle X-ray scattering data of this series of materials.
Inexpensive and effective CO2 adsorbents are highly desired to stabilize atmospheric CO2 levels. Alkylamine modified metal-organic frameworks (MOFs) are promising candidate sorbent materials, since they bind CO2 with strong affinity even in the presence of water and can be easily regenerated by moderate heating under reduced pressure. However, it is challenging to synthesize adsorbent materials replete with amine functionalities. In this work, we developed a simple method to synthesize alkylamine modified MOFs based on a Brønsted acid-base reaction through which alkylamines are tethered to sulfonic acid sites in the framework. By systematically optimizing the amine tethering process, we generated an adsorbent that demonstrates strong CO2 binding under conditions relevant for capture from both flue gas and air. Importantly, the CO2 uptake capacity was unchanged after 15 cycles of CO2 capture and regeneration, indicating good long-term stability for multiple cycles of use. The low-cost starting materials and simple synthetic procedure of this adsorbent make it a promising candidate for large-scale production as a carbon capture material.
Our in silico reticular chemistry study combines an application that has only been studied experimentally, the creation of MOF-74 analogs, with a computational method for the automated generation of hypothetical analogs of MOFs exhibiting a 1-D rod topology. With MOF-74 as the target system for our in silico structure generation, only 61 ligands (0.0001%) were identified to assemble valid MOF-74 analogs from the chemical space spanned by the PubChem compounds database. We necessarily developed a novel in silico building algorithm since MOF-74 exhibits 1-D secondary building unit (SBU) rods and complex connectivity between ligands and SBUs. Additionally, we utilized Density Functional Theory (DFT) and Grand Canonical Monte Carlo (GCMC) to simulate and understand the CO2 adsorption trends in this library of materials. One ligand in the library that was also identified as commercially available, known by its pharmaceutical name olsalazine or 3,3′-azobis(6-hydroxybenzoate)salicylic acid, was used to successfully synthesize a MOF-74 analog. We significantly increase the impact of our in silico screening by demonstrating that novel, predicted structures are indeed synthesizable.
The efficient separation of CO2 from various gas streams, in processes such as natural-gas purification and post-combustion carbon capture, presents major opportunities for advancing clean energy technologies. Membrane-based gas separations are less energy intense compared to conventional CO2 separation methodologies, but new membrane materials with improved separation performance under realistic process conditions are needed. Here, we utilize strong metal-organic framework nanoparticle/polymer interactions to improve membrane performance under realistic feed environments, which tend to diminish the separation properties of neat polymer membranes. We demonstrate that the incorporation of Ni2(dobdc) (dobdc4- = dioxidobenzenedicarboxylate) metal-organic framework nanocrystals into various polyimides can improve the performance of membranes for separating CO2 from CH4 under mixed-gas conditions. Four upper-bound 2,2′-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA)-based polyimides, as well as the commercial polymer Matrimid®, show improved selectivity under mixed-gas feeds when loaded with 15-25 wt% Ni2(dobdc), while the neat polyimides show diminishing selectivity upon increasing feed pressure. This approach presents an alternative to chemical crosslinking for achieving plasticization resistance, with the added benefit of retaining or increasing permeability while simultaneously reducing chain mobility.
The burning of carbon based fossil-fuels and the consequent release of carbon dioxide in the atmosphere poses a threat to the environment. Capture and sequestration of CO2 from a flue-gas mixture is therefore a pressing need. Adsorbents like metal-organic frameworks (MOFs) are used for this purpose, but gas uptake processes rely on equilibrium conditions, severely restricting the parameter space of conditions and materials available to industry. Furthermore, nonequilibrium aspects of gas capture in MOFs remains unexplored. Here, using a simple statistical mechanical model of gas diffusion and binding, parameterized by quantum mechanical data, we show that selective gas capture in MOFs can be effected under nonequilibrium conditions. Employing ideas of statistical mechanics, we uncover the emergent gas separation mechanism that arises due to different mobilities of different gas types within a crowded framework. We predict optimum nonequilibrium strategies for effective gas uptake. Our simulation study provides a new perspective on the problem of gas capture, and identifies a path toward using previously discounted or new materials and conditions to achieve it.
Integrating multi-functionality into stable metal-organic frameworks (MOFs) has attracted growing attention as it plays a critical role in realizing the potential of MOFs for a wide range of applications. This work shows that tetratopic porphyrin ligands can be incorporated, rather than simply encapsulated, into UiO-66 through a one-pot, thermodynamically controlled synthesis from mixed ligands. The MOFs displayed PXRD patterns consistent with that of phase-pure UiO-66. SEM showed that the powders are octahedral microcrystals with an even distribution of sizes. Chemical stability tests showed that the MOFs are stable in both acidic solutions (6 M HCl) and basic solutions (10 mM NaOH). Control experiments demonstrated that the number of carboxylic groups plays a vital role in integrating TCPP into UiO-66, very likely through coordination to Zr6 clusters during the synthesis, generating defects and larger pores to allow the residence of TCPP in the framework. Through modifying BDC moieties and porphyrin moieties, 49 MOFs with multi-functionalities were obtained. This strategy goes beyond the limit of conventional mixed-ligand strategies and post-synthetic modifications and expands the diversity of functionality for stable MOFs modification, holding great potential for exploring the applications of MOFs extensively.
Gas separations involving carbon monoxide are performed on large industrial scales in many processes. Some examples include hydrogen production, where the CO produced as a byproduct needs to be eliminated entirely, typically through energetically costly cryogenic distillation. Other processes, like steel manufacturing, underutilize this valuable carbon starting material due to contamination with N2 and CO2. A material that is able to bind CO selectively and reversibly at low pressures and reasonable temperatures is desirable for many of these applications. Porous materials have been explored as possible adsorbents, but thus far face selectivity issues, have low capacities at low pressures, or adsorb CO so strongly that it is irreversible. We have synthesized a new microporous material that binds CO at extremely low pressures, allowing for potential scavenging of CO at these low pressures. This material is also able to release the CO, allowing for a fully regenerable material. These features arise from a new mechanism in which the Fe(II) sites of the material interconvert between a high-spin form, which binds CO weakly, and a low-spin form, which binds CO strongly, allowing for strong CO binding at low pressures but release of CO upon conversion back to high-spin Fe(II). This material shows unprecedented selectivity for CO over H2, N2, and even CO2, which typically competes with CO adsorption in porous materials. We envision that this spin transition mechanism can be applied to other gases separations for further improvements in selective adsorbents.
The implementation of membranes for ethylene/ethane separations is challenging due to low membrane selectivities under both pure and mixed-gas conditions. Traditional approaches of compositing membranes do not yield improved ethylene/ethane performance, because they rely on a size-sieving based mechanism. Here, the improved adsorption selectivity in M2(dobdc) nanoparticles is instead leveraged to improve membrane permselectivity. The open-metal sites in the metal-organic framework pores selectively adsorb ethylene, increasing the total concentration of ethylene in the film relative to ethane (Fig. a). This leads to an increase in permselectivity as well as permeability in the case of Ni2(dobdc) and Co2(dobdc), while the permeability greatly increases in the case of Mg2(dobdc) and Mn2(dobdc) but a slight decrease in selectivity is observed (Fig. b). This can be explained primarily by variations in the particle size, where the larger agglomerates of Mg2(dobdc) and Mn2(dobdc) lead to non-selective transport pathways (Fig c), while Ni2(dobdc) and Co2(dobdc) effectively crosslink the polymer (Fig. d), reducing chain mobility and improving membrane performance under mixed-gas conditions.
A systematic strategy is proposed for preparing multi-functionalized covalent organic polymers (COPs) using the efficient Ullmann cross-coupling reaction. Using this strategy, 17 novel multiblock COPs were synthesized with finely-tuned porosities from a core tetrahedral linker, tetrakis(4-bromophenyl) methane (TBM), and a variety of aromatic linear, trigonal, and even tetragonal linkers. The COPs synthesized in this work have remarkably high porosities and hydrothermal stabilities, which are critical for the adoption of these materials for industrial applications. By tailoring the length and geometry of building blocks, it is possible to tune the BET specific surface areas (SSAs) and pore volumes of these COPs. As a result, the material COP-20 (composed of TBM + DB-OH) has been synthesized with the largest measured pore volume in the field of porous organic materials (3.5 cm3∙g−1). The synthetic approach also allows for incorporation of different functional groups into COPs; 5 distinct functional groups have been successfully incorporated, i.e., –NO2, –NH2, –CH3, –SO2Cl and –OH, in groups of two and three into individual COPs. Notably, functionalizing COPs with multiple groups in one phase can lead to improved properties that are not simply linear combinations of those of the pure components. By incorporating two and three distinct functional groups, i.e., –NH2, –SO2Cl and –OH, into one phase, the multi-functionalized COP-21 and COP-22 (COP-21 = TBM + DB-OH + DB-NH2; COP-22 = TBM + DB-OH + DB-NH2 + DB-SO2Cl) exhibit an enhanced selectivity, roughly twice as large as any of the singly-functionalized COPs. Therefore, incorporating multiple functional groups within a single COP may very well be useful for improving the carbon capture properties of a given material. At typical flue gas conditions, both COP-21 and COP-22 performed the best and displayed the lowest parasitic energies (1608 kJ/kg and 1632 kJ/kg) among all the frameworks studied, and, of the singly-functionalized frameworks, the amine functionalized COP-17 performed best. In a similar PPN study, it was observed that functionalizing the frameworks with longer amine chains improved their carbon capture performance. For these reasons, it is anticipated that functionalizing the polymers presented here with longer amine chains that extend into the pore space will lead to further reduction in the parasitic energies of these frameworks. (Linker abbreviations: TBM = tetrakis(4-bromophenyl)methane, DB = dibromobenzene, TBB = tribromobenze, TBA = tribromoamine, and derivatives of these.)
Here it is shown that sub-micron coatings of zeolitic imidazolate frameworks (ZIFs) and even ZIF–ZIF bilayers can be grown directly on polymers of intrinsic microporosity from zinc oxide (ZnO) nanocrystal precursor films, yielding a new class of all-microporous layered hybrids. The ZnO-to-ZIF chemical transformation proceeded in less than 30 min under microwave conditions using a solution of the imidazole ligand in N,N-dimethylformamide (DMF), water, or mixtures thereof. By varying the ratio of DMF to water, it was possible to control the morphology of the ZIF-on-polymer from isolated crystallites to continuous films. Grazing incidence X-ray diffraction was used to confirm the presence of crystalline ZIF in the thin films, and X-ray absorption spectroscopy was used to quantify film purity, revealing films with little to no residual ZnO. The role solvent plays in the transformation mechanism is discussed in light of these findings, which suggest the ZnO nanocrystals may be necessary to localize heterogeneous nucleation of the ZIF to the polymer surface.
In this work, the concepts of molecular homogenous and heterogeneous catalysis were merged as illustrated by incorporation of molecular CO2 reduction catalysts into the backbone of a spatially well-defined covalent organic framework. This prototype system gives exceptionally high activity (turnover number 290,000, and turnover frequency 9,400 h−1) as well as selectivity over competing proton reduction even in pH 7 water. Covalent organic frameworks (COFs) were chosen instead of the more intensively studied metal organic frameworks (MOFs) owing to the high degree of conjugation and π-π stacking between layers in the former, which result in higher charge-carrier mobilities than found in MOFs. In addition to the conductivity and high surface area, a key feature of COFs is that their active sites can be modified at will with molecular-level control by tuning the building blocks constituting the frameworks, thus offering a significant advantage over other solid-state catalysts where tuning the catalytic properties with that level of rational design remains a major challenge. In comparison with homogeneous molecular catalysts, COFs are stable to water and can operate in aqueous electrolyte with high selectivity over competing water reduction. Moreover, they offer the unique possibility to introduce heterogeneity in the number and ratio of functionalities into the otherwise well-defined pore environment. Using this approach it was possible to show that frameworks bearing catalytic cobalt and structural copper porphyrin units give rise to materials with emergent properties that are greater than the sum of the individual parts. This notion of heterogeneity within order is a concept that is difficult to realize in other substance classes. To improve this prototype system toward commercialization, further efforts are seeking to increase the number of electroactive cobalt centers and achieve lower overpotentials while maintaining high activity and selectivity for CO2 reduction over proton reduction. In this work, initial results are also reported on the fabrication of COF thin films on various substrates to achieve this goal. In addition work is being carried out to expand the types of value-added carbon products that can be made using COFs and related frameworks.
Molecular simulations and NMR relaxometry experiments demonstrate that pure benzene or xylene confined in isoreticular metal–organic frameworks (IRMOFs) exhibit true vapor-liquid phase equilibria where the effective critical point may be reduced by tuning the structure of the MOF. Our results are consistent with vapor and liquid phases extending over many MOF unit cells. These results are counterintuitive since the MOF pore diameters are approximately the same length scale as the adsorbate molecules. As applications of these materials in catalysis, separations, and gas storage rely on the ability to tune the properties of adsorbed molecules, we anticipate that the ability to systematically control the critical point, thereby preparing spatially inhomogeneous local adsorbate densities, could add a new design tool for MOF applications.
X-ray absorption spectroscopy (XAS) probes the local partial density of molecular orbital states around distinct absorbing atoms, and thus provides unique information about the location and chemical nature of the CO2 adsorption process in metal-organic frameworks (MOFs). The figure shows absorption spectra of the nitrogen and oxygen in diamine-appended Mg2(dobpdc); in each case the blue and red spectra represent the MOF without and with adsorbed CO2 present, respectively. Arrows indicate significant spectral changes. First principles theoretical spectra for distinct CO2 adsorption models were also calculated. Three different models had been proposed for the adsorption of CO2 in this MOF, and by comparison of measured and computed spectra for these models we learned that only the insertion model accounts for the observed spectral changes. In this model, the two nitrogen atoms in the diamines participate in chemically distinct ammonium and carbamate interactions with the CO2, and XAS changes reflect these distinct interactions. The oxygen spectral changes result from adsorbed CO2 and are consistent with its reaction within the MOF to form carbamate. Such direct experimental sensitivity to different chemical motifs is rare, and this capability highlights the advantage of using XAS as a technique for characterizing gas adsorption in MOFs.
Metal-organic frameworks (MOFs) exhibit permanent porosity and high surface area, which may provide advantages toward catalytic reactions. Polyoxometalate (POM) ions with redox activity show great promise as redox catalysts, while POM-based MOFs combine the redox nature of the POM moiety and the porosity of a MOF structure, which may favor hydrogen generation. Herein are reported two novel POM-based MOFs, NENU-500 and NENU-501. These materials exhibit not only good air-stability but also tolerance to acid and base media (from pH = 1-12). Furthermore, NENU-500 shows the highest activity for electrochemical hydrogen generation from acidic water among all MOF materials. As shown in the bottom figure, the onset potential of NENU-500 is closer to that of a platinum catalyst than other MOFs. Using a Zn-ε-Keggin-Cl fragment as a model, DFT computations were also carried out and the resulting calculated free energy change for hydrogen adsorption helps to rationalize the good catalytic capacity of NENU-500 and NENU-501. The present study demonstrates successful construction of stable POM-based MOFs with porosity and novel hydrogen-evolving electrocatalysts with excellent activity.
With separation processes consuming an estimated 10-15% of global energy and the expectation that this consumption will greatly increase with population growth and the implementation of large-scale carbon capture and sequestration technologies, there are intensive scientific efforts focused on the development of new physical adsorbents that might enable more energetically favorable gas separations relative to traditional distillation or absorption processes. This feat is not easy, as the differences in the molecules of interest, such as CO2 and N2—the main components in a postcombustion flue gas—are minimal. As such, these separations require tailor-made adsorbent materials with molecule-specific chemical interactions on their internal surface. Metal-organic frameworks (MOFs) have gained much attention as next generation porous media for gas separations and storage, and new MOFs are regularly reported. However, to develop better materials in a timely manner for specific applications, the interactions between guest molecules and the internal surface of the framework must first be understood. In this review, a combined experimental and theoretical approach is presented that proves essential for the elucidation of small-molecule interactions in a model MOF system known as M2(dobdc) (dobdc4− = 2,5-dioxido-1,4-benzenedicarboxylate; M = Mg, Mn, Fe, Co, Ni, Cu, or Zn), a material whose adsorption properties can be readily tuned via chemical substitution. It is additionally shown that the study of extensive families like this one can provide a platform to test the efficacy and accuracy of developing computational methodologies in slightly varying chemical environments, a task that is necessary for their evolution into viable, robust tools for screening large numbers of materials.
The controlled generation of singlet oxygen is of great interest owing to its potential applications, including industrial wastewater treatment, photochemistry, and photodynamic therapy. Two photochromic metal–organic frameworks (or porous coordination networks), PC-PCN and SO-PCN, were developed. A photochromic reaction was successfully realized in PC-PCN while maintaining its single crystallinity. In particular, as a solid-state material that inherently integrates the photochromic switch and photosensitizer, SO-PCN demonstrated reversible control of 1O2 generation. Additionally, SO-PCN showed catalytic activity towards photooxidation of 1,5-dihydroxynaphthalene.
Generally, in the chemistry of metal-organic frameworks (MOFs), frameworks having the most symmetric nets are expected to form (termed default nets). However, one of the exceptions to this principle is MOF-177, which is constructed from the octahedral Zn4O(COO) 6 clusters and triangular 1,3,5-benzenetribenzoate (BTB) organic linkers to form the qom net, which has a lower symmetry compared to pyrite (pyr) or rutile (rtl) nets. This qom-based system was investigated with the aim of (i) elaborating an original thesis of high-symmetry and default nets, (ii) providing insight into the design principles of MOFs, and (iii) showing how these principles could be used to further improve properties of MOF-177 and other related systems. In this work, two approaches were studied to assess the tolerance of the qom net to functionalization of the organic linker and the mixing of linkers bearing different functionalities. It was found that the position of functional groups on the linker is critical in selecting for qom net versus pyr and rtl nets, and that mixing of linkers exclusively affords the qom net. Furthermore, by mixing two or three different type of linkers, 14 multivariate (MTV) MOF-177 derivatives were prepared with the same underlying qom net but with varied ratios of incorporated functionalities. Among them, five fully functionalized MOF-177 analogues and seven MTV-MOF-177 derivatives were selected for gas adsorption studies, and gas adsorption data revealed that some MOF-177 derivatives exhibit higher volumetric gas uptake capacity than the original MOF-177.
A new adsorption mechanism was elucidated for CO2 in porous metal-organic frameworks. Through detailed spectroscopic, diffraction, and computational experiments, diamine-appended metal-organic frameworks were shown to adsorb CO2 via an unprecedented cooperative insertion mechanism. The CO2 adsorption isotherms of materials utilizing this mechanism are characterized by prominent steps that shift dramatically with temperature. The presence of sharp steps allow for large working capacities for CO2 capture to be realized using only moderate temperature swings. Significantly, changing the strength of the metal-diamine bond allows for the position of the steps to be rationally tuned. Beyond the low regeneration energy, the new class of adsorbents described can potentially allow for drastic reductions in the capital cost of carbon capture owing to new processes that now become feasible.
Metal-organic frameworks (MOFs) are highly promising for efficient and cost-effective selective capture and separation of specific gases relevant for a host of clean energy applications. However, given the large number of possible MOFs, theory and computation are required to develop new understanding of binding mechanisms – and associated design rules – that can guide synthesis efforts. Using the most rigorous density functional theory calculations performed to date on 140 unique systems, the origin the binding enthalpies of 14 different small molecules across 10 members of the isostructural series M-MOF-74 were systematically computed and elucidated. The calculations employed nonlocal van der Waals density functionals, Hubbard U corrections, and include zero-point motion and finite temperature effects; the impact of these corrections was assessed quantitatively. For systems with reported measured binding enthalpies, the calculations lead to excellent agreement with experiment. For systems as of yet unrealized or uncharacterized, a new intuition was developed for the role of different transition-metal cations in small-molecule binding at open-metal sites, leading to new predictions. For example, it was predicted that Cu-MOF-74 will select CO2 over H2O and Mn-MOF-74 can be used to separate trace flue-gas impurities and toxic gases.
Mesoporous metal-organic frameworks (MOFs) have attracted great interest as heterogeneous platforms to immobilize or encapsulate functional moieties. However, as isoreticular chemistry is applied to augment the internal cavity of known MOF structures, the pore size and stability are usually inversely correlated. One strategy to compensate for the low stability of MOFs constructed with large linkers is to increase the connectivity of both organic linkers and inorganic clusters. In order to achieve the high connectivity, extended tetratopic carboxylate porphyrin ligand and the twelve connected Zr6 cluster were used to construct mesoporous MOFs. Topological connection of these two nodes could ideally give rise to a high connected ftw-a network. However, not only the connectivity and symmetry of nodes, but also the direction and relative position between each node are crucial for topological design and the latter is often neglected. In this study, guided by topology and symmetry, porphyrinic linkers were extended with desired conformation by arranging the vicinal phenyl ring and carboxylate group. Through combination of the organic linkers and twelve connected Zr6 cluster, a series of mesoporous MOFs with ftw-a topology were synthesized. Moreover, these MOFs show excellent stability in aqueous solutions with pH ranging from 0 to 12.
The steady increase of the CO2 concentration in the atmosphere reveals the imperative need for performing Carbon Capture and Sequestration (CCS) in power plants. One separation technology that has been considered to accomplish this task is CO2 adsorption with solid nanoporous materials. The question addressed in this work was how to meaningfully screen and rank the enormous amount of possible materials to identify the most suitable ones for CO2 capture. It is argued that parasitic energy is a useful metric that predicts the total energy penalty imposed on a power plant by conducting CCS on a material-by-material basis as it combines several thermodynamic properties. This work aimed to relate the concept of parasitic energy and its advantages to other criteria that have been used as a metric for carbon capture. The main source for the electricity loss was also revealed, providing important insights into the energetic costs for power companies and further it was demonstrated how the CO2 concentration influences the selection of the optimal material. A large set of experimental and hypothetical structures were ranked to identify the best material for CCS, which could potentially reduce the energy penalty on a power plant compared to the state-of-art technology, amine scrubbing. It is anticipated that this metric will be well-suited to conduct high-throughput screenings of large material databases.
Scientists at the China University of Petroleum, Beijing University of Chemical Technology, EPFL, and the University of California, Berkeley have developed a slurry-based process that can revolutionize carbon capture. The slurry, consisting of a zeolitic imidazolate framework-8 (ZIF-8) powder suspended in glycol-2-methylimidazole, offers the efficient large-scale implementation of a liquid while maintaining the lower costs and energy efficiency of solid carbon-capturing materials. The current technology to separate CO2 from flue gasses uses amine scrubbing. For amine solutions, regeneration is the most energy-consuming part because CO2 is strongly bound to the amine molecules. An alternative to liquids is to use metal-organic frameworks (MOFs) and indeed within the CGS researchers have been able to tune the interactions with CO2 to minimize the regeneration energy. From a practical point of view, however, a solid-adsorption based process is a challenge because, unlike liquids, solids cannot be transported easily. Lin et al. have combined carbon-capturing solids and liquids to develop a slurry-based process that offers the best of both worlds: as a liquid it is relatively simple to implement on a large scale, while it maintains the lower costs and energy efficiency of a solid carbon-capturing material. The solid part of the slurry is ZIF-8, which is suspended in a 2-methylimidazole glycol liquid mixture. The innovation is that ZIF-8 has pores that are large enough to capture CO2 but too small for the solvent. With this slurry it is possible to capture CO2 from flue gases without the typical associated transportation difficulties. This breakthrough therefore removes the main bottlenecks of a solid adsorption process. Similar to amine scrubbing, a MOF-slurry can be used in a continuous process with full heat integration. The slurry represents a new template for developing similar combinations in the future.
Scientists at the University of California, Berkeley and Lawrence Berkeley National Laboratory, completed a detailed study of CO adsorption in six metal−organic frameworks with exposed divalent metal cations. Six metal−organic frameworks of the M2(dobdc) (M = Mg, Mn, Fe, Co, Ni, Zn; dobdc4− = 2,5-dioxido-1,4-benzenedicarboxylate) structure type were demonstrated to bind carbon monoxide reversibly and at high capacity. Infrared spectra indicated that, upon coordination of CO to the divalent metal cations lining the pores within these frameworks, the C−O stretching frequency is blue-shifted, consistent with nonclassical metal-CO interactions. Structure determinations revealed M−CO distances ranging from 2.09(2) Å for M = Ni to 2.49(1) Å for M = Zn and M−C−O angles ranging from 161.2(7)° for M = Mg to 176.9(6)° for M = Fe. Electronic structure calculations employing density functional theory (DFT) resulted in good agreement with the trends apparent in the infrared spectra and crystal structures. These results represent the first crystallographically characterized magnesium and zinc carbonyl compounds and the first high-spin manganese(II), iron(II), cobalt(II), and nickel(II) carbonyl species. Adsorption isotherms also indicated reversible adsorption, with capacities for the Fe, Co, and Ni frameworks approaching one CO per metal cation site at 1 bar, corresponding to loadings as high as 6.0 mmol/g and 157 cm3/cm3. The six frameworks display (negative) isosteric heats of CO adsorption ranging from 52.7 to 27.2 kJ/mol along the series Ni > Co > Fe > Mg > Mn > Zn, following the Irving−Williams stability order. The reversible CO binding suggests that these frameworks may be of utility for the separation of CO from various industrial gas mixtures, including CO/H2 and CO/N2. Selectivities determined from gas adsorption isotherm data using ideal adsorbed solution theory (IAST) over a range of gas compositions at 1 bar and 298 K indicated that all six M2(dobdc) frameworks could potentially be used as solid adsorbents to replace current cryogenic distillation technologies, with the choice of M dictating adsorbent regeneration energy and the level of purity of the resulting gases. (Structure images: red spheres = oxygen; gray spheres = carbon).
In this work, a methodology was developed to introduce amino-functionalities in IRMOF-74-III structures by using protecting groups (Boc = tert-butyloxycarbonyl) and a post-synthetic deprotection method. CO2 isotherms were measured at 25 °C for IRMOF-74-III functionalized with –CH3, –NH2, –CH2NHBoc, –CH2NMeBoc, –CH2NH2, and –CH2NHMe), and it was found that all compounds showed significant uptake. In particular, materials with –CH2NH2 and –CH2NHMe showed the highest uptake capacities for a low CO2 pressure range (<1 Torr). It was confirmed by cross-polarization magic angle spinning 13C NMR spectra that these two compounds bind CO2 chemically. Among them, IRMOF-74-III-CH2NH2 can be recovered under milder conditions after CO2 adsoption and therefore its capability for CO2 capture in the presence of water was analyzed and compared to IRMOF-74-CH3. Breakthrough experiments showed that while IRMOF-74-CH3 exhibits an 80% decrease in its CO2 capacity in the presence of moisture, the IRMOF-74-III-CH2NH2 uptake and selectivity remains nearly identical and, more significantly, with full preservation of its structure.
Synthesis of stable MOFs with high valence metal ions has become the focus of some recent research efforts. Utilizing the material PCN-426-Mg (PCN = porous coordination network) as a template, two robust MOFs, PCN-426-Fe(III) and PCN-426-Cr(III), were synthesized through a strategy of post-synthetic metathesis and oxidation (PSMO) of the metal nodes step by step. The frameworks remained in their single crystal form throughout this process. Furthermore, the stability and porosity of the frameworks were significantly improved after PSMO. By taking advantage of the kinetically labile metal-ligand exchange reactions prior to oxidation (Fe(II)-O or Cr(II)-O), and the kinetically inert metal-ligand bonds after oxidation (Fe(III)-O or Cr(III)-O), robust MOFs, which would otherwise be difficult to synthesize, can be readily prepared. This method has the following advantages: (1) A Mg-MOF was chosen as a template so that the labile metal−oxygen bonds would drive the metal exchange to completion; and (2) the MOF template was first exchanged with low-oxidation-state but kinetically labile metal ions, which were subsequently oxidized to a high oxidation state to accelerate the metal-exchange and at the same time preserve the integrity of the framework. MOFs based on high-valence metal ions are usually produced in amorphous or powder forms, and in general chromium MOFs have obtained in powder form almost exclusively in the literature, until now with PCN-426-Cr(III), which was made through the PSMO synthetic route. Therefore, this route is therefore of critical importance for the synthesis and characterization of robust MOFs, which are otherwise difficult or unfeasible through traditional synthetic routes.
It is demonstrated that the stimuli responsive (sr) metal-organic polyhedron (MOP)-1 functionalized with azobenzene groups can undergo reversible trans/cis isomerization. trans-srMOP-1 exhibits low solubility and strong interaction among cages. Guest methylene blue (MB) molecules can be captured in the pockets between individual trans-srMOP-1. Upon UV irradiation, trans-srMOP-1 becomes cis-srMOP-1, decreasing the interaction among srMOP-1 cages while increasing its solubility. As a result, trans to cis isomerization of srMOP-1 facilitates the release of MB. Remarkably, highly reversible capture and release of MB was achieved by alternate irradiation of blue and UV lights. Before any practical applications of such optically responsive MOPs may be considered, there are still many aspects that must be explored. For instance, biocompatibility, optimization of isomerization conditions to enhance energy efficiency, and modification of srMOPs for visible or near-IR induced isomerization and for versatile solvent systems need to be studied. Nonetheless, the reversible capture/release of guest molecules presented here will provide a new direction in the ever diversifying field of MOPs, whilst laying the groundwork for new optically responsive materials.
Fluorescent solid materials have attracted significant attention because of their wide applications especially as inorganic and organic light-emitting diodes and solid state sensors. A highly fluorescent MOF, PCN-94, composed of a tetraphenylethylene (TPE)-core linker and a Zr6 cluster, was synthesized. It has been demonstrated that the band gap and quantum yield of fluorophores can be tuned by rigidifying them as linkers of metal-organic framework. The twisted linker conformation in PCN-94 induced bright-blue fluorescence emission at 470 nm, which is hypsochromically-shifted from the 545 nm yellow emission of linker precursor H4ETTC. The quantum yield of PCN-94 is as high as unity (99.9 ± 0.5%). The high quantum yield of PCN-94 is primarily attributed to the immobilization of the ETTC linker as it is strongly coordinated to Zr(IV), and to the diminution of concentration quenching that is very common for luminescent organic molecules. Rigidifying the linker and the repulsion between H atoms on adjacent phenyl rings of ETTC (H4ETTC = 4′,4′′′,4′′′′′,4′′′′′′′-(ethene-1,1,2,2-tetrayl)tetrakis(([1,1′-biphenyl]-4-carboxylic acid) both effectively eliminate thermal motions of ETTC, making the excited linker release the extra energy through fluorescence, and substantially increase the quantum yield. This work demonstrates a rare example of how linker conformation can produce significant effects on the photophysical properties of MOFs, and represents a new strategy to attain light-emitting MOFs that may be suitable for potential applications in molecular electronics.
In a pre-combustion Carbon Capture process fossil fuels are burned with pure oxygen. As the flue gas contains only carbon dioxide and water, the carbon dioxide can easily be captured by condensing the water. The bottleneck in this process is that one needs to separate oxygen from the air, which is very energy intensive. Researchers in the CGS previously developed a new metal-organic framework (MOF) Fe-MOF74, which features parallel, quasi-one-dimensional chains of exposed Fe(II) sites and preferentially adsorbs oxygen from the air. The material adsorbs O2 reversibly at lower temperatures, which are suitable for oxygen separation from air, but the adsorption is irreversible at higher temperatures. Now, based on a theoretical study a mechanism is proposed for oxygen chemisorption in this quasi-one-dimensional adsorbent. Namely, oxygen adsorption-desorption is an oxidation-reduction chemical process that involves metal-insulator transitions rather than charge transfer within individual adsorption sites. Upon oxygen adsorption at the exposed Fe(II) sites, oxygen molecules partially reduce, and the material, which is an insulator, partially oxidizes as it undergoes an insulator-to-metal transition that delocalizes electrons along the quasi-one-dimensional chains. The adsorption sites in the metallic state are identical Fe sites of the valence intermediate between (II) and (III). The material returns to the original Fe(II) insulating state upon desorption of O2 from the metallic phase. However, if O2 further reduces at higher temperatures, the metallic state undergoes a metal to insulator transition so that the valence of the Fe sites in the insulating state oscillates between (II) and (III) along the chains. The insulating state becomes metallic again if O2 reduces even further. The activation barrier for this metal-insulator-metal transition depends on the gap in the insulator. As the activation barrier is overcome at a high enough temperature, O2 desorbs from Fe(II)-like sites but remains on Fe(III)-like sites and is further reduced. The resulting Fe(III) material cannot be easily reversed. Apart from providing theoretical insight for improving upon oxygen separation characteristics of the material, this mechanism also points to a class of nano-porous adsorbents where redox chemistry and low-dimensional metal-insulator transitions intertwine. These materials must exhibit unusual chemical profiles, which can be exploited in developing novel redox-active nanoporous adsorbents and catalysts.
There is a great interest in expanding the use of methane for fueling automobiles because of its wide availability and its lower carbon emission compared to petroleum. A current challenge for the implementation of this technology is to find materials that are able to store and deliver large amounts of methane near room temperature and at low pressures. The U.S. DOE has initiated a research program aimed at operating methane storage fueling systems at room temperature and moderate pressure that is relevant to commercially and widely available equipment. Researchers at the Lawrence Berkeley National Laboratory synthesized two new aluminum metal−organic frameworks [termed MOF-519: Al8(OH)8(BTB)4(H2BTB) 4, and MOF-520: Al8(OH)8(BTB)4(HCOO)4, BTB = 4,4′,4′′-benzene-1,3,5-tryil-tribenzoate], and their crystal structures and methane adsorption properties were examined. Both materials exhibit permanent porosity and high methane volumetric storage capacity: a volumetric capacity of MOF-519 and 520 at 80 bar is 279 and 231 cm3/cm3 at 298 K, respectively, where MOF-519 outperforms any other reported MOF. Furthermore, MOF-519 exhibits an exceptional working capacity, being able to deliver a large amount of methane at pressures between 5 and 35 bar, 151 cm3/cm3, and between 5 and 80 bar, 230 cm3/cm3, which is the largest working capacity obtained for any of the top performing MOFs and porous carbon AX-21. At 80 bar, a tank filled with MOF-519 would deliver almost three times more methane than an empty tank.
Water adsorption in porous materials is important for many applications such as dehumidification, thermal batteries, and delivery of drinking water in remote areas. Researchers at Lawrence Berkeley National Laboratory identified three criteria for achieving high performing porous materials for water adsorption, which are condensation pressure of water in the pores, uptake capacity, and recyclability and water stability of the material. They studied and compared the water adsorption properties of twenty-three materials, twenty of which are metal-organic frameworks (MOFs), including six new MOFs. These new MOFs are made from the Zr6O4 (OH) 4 (–CO2)n secondary building units (n = 6, 8, 10, or 12) and variously shaped carboxyl organic linkers to make extended porous frameworks. The water sorption properties revealed that two Zr-MOFs, MOF-801-P and MOF-841, are the highest performers. In particular, a thermal battery system built with 15 kg of MOF-801-P operating at 65% efficiency would have a power capability equivalent to 1.8 kWh. An X-ray single-crystal and a powder neutron diffraction study revealed the position of the water adsorption sites in MOF-801 and highlighted the importance of the intermolecular interaction between adsorbed water molecules within the pores. On the other hand, MOF-841 has potential to be used for the capture and release of water in remote areas. This MOF is able to take up an amount of water equivalent to 40% of its weight and release it with just a change in temperature. Thus, if 6 kg of MOF-841 were deployed in a desert area with climate conditions similar to the ones in the city of Tabuk (Saudi Arabia), they would be able to deliver the amount of drinking water (2.4 L) that a person needs per day, without the need of any electric power supply.
Researchers at the University of California, Berkeley have discovered a new approach to creating microporous materials that feature a very high density of acidic functional groups on the interior of the material. Furthermore, these functional groups are spatially aligned in such a way that multiple groups can act cooperatively on a single gas molecule, which resulted in the exceptional adsorption of ammonia. First, as a control experiment, a family of structured organic polymers was developed that contained isolated functional groups that ranged from very weakly acidic (–NH2) to strongly acidic (–SO3H). These materials had acceptable performance for ammonia adsorption, but were below current best-in-class porous frameworks. Using a newly developed catalytic polymerization protocol, a second family of materials was reported. Although the functional groups in these materials were only moderately acidic (–CO2H), ammonia adsorption (at 1 bar pressure and room temperature) was achieved that was the highest yet observed in a stable porous adsorbent—a result of their particular arrangement. Importantly, the use of modern polymerization catalysts will allow for this approach to be extended to other functional groups, and therefore, other gas separations.
CGS researchers at Texas A&M University have reported successful implementation of the rational design of MOFs with desired properties. This work is based on MOF topochemistry, where previous work typically relies on the analysis of framework topology after the MOF structure is resolved. However, the symmetry-guided design implies it is possible to achieve a bottom-up design with expected topology and porosity, and a careful inspection of the net topology of simple mineral structures may provide novel insights into the rational design of MOFs. The fluorite topology is especially intriguing for the construction of highly porous materials. Its structure can be conceived of as a cubic close packing (ccp) of the calcium cations (Ca2+) in which the fluoride anions (F−) fill all its tetrahedral interstitial cavities, leaving all the octahedral interstitial cavities unoccupied. Materials with larger octahedral cavities can be generated by augmenting the 4-connected nodes with a rigid tetrahedron. Additionally, frameworks with fluorite topology cannot exhibit self-interpenetration. Single crystals of the material PCN-521 were obtained by solvothermal reaction of the MTBC ligand (MTBC4− = 4′,4′′,4′′′,4′′′′-methanetetrayltetrabiphenyl-4-carboxylate) and ZrCl4 in DMF in the presence of benzoic acid. Its octahedral cavity is significantly augmented with a size of 20.5 x 20.5 x 37.4 Å, the largest pore size of any MOF with tetrahedral linkers. This material also possesses the largest solvent accessible volume (78.50%) and Brunauer−Emmett−Teller (BET) surface area (3411 m2·g−1) among all the MOFs with tetrahedral ligands. It is the first example of a Zr MOF made from a tetrahedral linker, and the first example of a non-interpenetrated MOF constructed from the MTBC ligand.
Metalloporphyrins exhibit excellent catalytic activities, and immobilization of porphyrin catalysts in polymers and zeolites has been extensively explored. With uniform but tunable pore sizes, MOFs provide a special platform for the effective utilization of the porphyrinic catalytic centers. Recently, porphyrin derivatives have been introduced into MOFs by either linker extension or encapsulation. Catalytic activities and optical properties have been investigated in such porphyrinic MOFs. However, most of these MOFs are constructed by the combination of soft acids (low-oxidative transition metal ions) and hard bases (carboxylates), and consequently have relatively weak chemical stabilities. Hence, the applications of porphyrinic MOFs in catalysis have been restricted to mild reaction conditions. Combing the versatility of metalloporphyrins and the stability of Zr-carboxylate MOFs, a series of porhyrinic Zr-MOFs with excellent chemical stability have been obtained through ligand elimination strategy. Interestingly, PCN-224(Co) shows very high efficiency as a heterogeneous catalyst for the CO2 and epoxide coupling reaction with good recyclability.
A computational methodology was developed for generating structural models of amorphous porous organic cages that can be used in gas separations. These organic cage molecules have structures with protected internal pore volume that cannot be in-filled, irrespective of the solid-state packing mode: that is, they are intrinsically porous. Amorphous packings can give higher pore volumes than crystalline packings for these materials, but the precise nature of this additional porosity is hard to understand for disordered solids that cannot be characterized by X-ray diffraction. The developed methodology provides computational models of such amorphous materials, and enables molecular dynamics simulations of these species. These simulations provide information about time evolution of the topology and geometry of pores in these materials as well as help to rationalize the experimentally observed gas selectivities. The analysis of the generated structural models was facilitated by the LBNL-developed porous structure analysis software – Zeo++.
It is important to understand carbon dioxide adsorption interactions inside metal-organic frameworks (MOFs) in order to optimize materials design for carbon capture applications. Scientists at Lawrence Berkeley National Lab and UC Berkeley have made the first measurements of how the local electronic structure at MOF internal open metal sites changes on adsorption of CO2 and compared these results with those predicted using first principles theoretical calculations of the underlying MOF electronic structure. The X-ray absorption spectroscopy measurements are sensitive to the active electronic structure at the Mg2+ sites and reveal a strong spectral signature that is reversibly suppressed upon gas adsorption. This behavior is reproduced in calculated spectra, revealing a distinct, distorted electronic state arising from the unusual molecular symmetry at the Mg2+ sites in the MOF. This state shows unique interactions for weakly-binding CO2 gas versus strongly-binding dimethylformamide, resulting in a stronger spectral signature for stronger adsorption. The qualitative agreement between experiment and theory largely validates the theoretical design approach for this system.
A novel Zr-based MOF has been developed based on a porphyrinic tetracarboxylic acid ligand. Strikingly, the chemical bonds between Zr4+ cations and carboxylates are very strong and resistant to chemical attack due to a hard-acid and hard-base combination. Thus the Zr-MOF exhibits exceptionally high chemical stability in aqueous solutions with pH ranging from 1 to 11, which represents one of the most extensive pH ranges that a porphyrinic MOF can survive. Remarkably, given the stability of the MOF over a wide-pH range and the shift of the protonation−deprotonation equilibrium of the porphyrin center upon pH variation, the fluorescence intensity of the MOF is in close correlation with the pH value, and in particular from pH 7 to 10 there exists an almost linear relationship between the pH value and the fluorescence intensity. The results indicate the Zr-porphyrinic MOF is promising for pH sensing. In addition, the easy installation of exposed metal sites in the porphyrin center in the Zr-MOF, combined with the high stability, should increase its potential for a variety of applications in clean-energy related areas such as gas separation, catalysis, and light harvesting.
Metal-organic frameworks (MOFs) are made by linking inorganic and organic units by strong bonds (reticular chemistry). The flexibility with which the constituents' geometry, size, and functionality can be varied has led to more than 20,000 different MOFs, and these materials are ideal candidates for applications such as the storage of fuels (hydrogen and methane), capture of carbon dioxide, and catalysis applications, to mention a few. In this review, four key developments were introduced that were particularly important in advancing this field. These developments were (1) the realization of the geometrical principles for linking inorganic units with organic linkers, (2) the exploitation of the isoreticular principle in the design of MOFs with ultrahigh porosity, (3) the use of a powerful tool for covalent functionalization of pores (post-synthetic modification), and (4) the discovery of multivariate MOFs. The most promising applications for MOFs are also highlighted. The precise control over the assembly of MOFs is expected to propel this field further into new realms of synthetic chemistry, in which far more sophisticated materials may be accessed. Several future directions of MOF chemistry are proposed in the final section of the review.
It has previously been shown by Yaghi et al. that one can make metal-organic frameworks (MOFs) from a mixture of two different organic linkers, each having the same backbone but different functional groups. These so-called multivariate (MTV)-MOFs with different functionalities exhibit an increased CO2 selectivity over corresponding single-component MOFs, by as much as two orders of magnitude. Interestingly, these MTV-MOFs are crystalline, however, the spatial arrangement of the functional groups introduces some disorder. To resolve the structure of such a material, the conventional methods fail as these rely on diffraction techniques, which require (nearly) perfectly ordered crystals. The fundamental question underlying these MOFs and many other materials, in which there is heterogeneity within order: what is the special arrangement of the heterogeneity? In case of the MTV-MOFs knowing this spatial arrangement, or apportionment, of the functional groups is an essential step to understand and control the performance of these materials. To address the issue of heterogeneity within order, a combined solid-state NMR and computational characterization method was developed. This method shows that, depending on functionality composition and ratio, MTV-MOFs exist in alternating, random, or clustered apportionments. This study also revealed that the resulting molecular model of the MTV-MOF can explain the enhancement of the CO2 separation selectivity as observed previously. Notably, this new research opens up the possibility of controlling the apportionment in MTV-MOFs, where one can now think about synthesizing structures in which the inner or outer surface of a given solid can be "tiled" with regions of different functionalities apportioned and juxtaposed to ultimately carryout complex functions. Additionally, there are many other systems in which disorder prohibits a molecular understanding of material structure, and for these a combined NMR-computational approach might provide a solution.
Computational and experimental collaborative efforts have facilitated a quantum chemical computational study to understand the mechanism of CO2 adsorption in alkyl-amine appended Metal Organic Frameworks (MOF). The conventional Amine−CO2 reactions in aqueous solution are zwitter-ionic and charged species are stabilized by the polar medium; one would expect a CO2:amine stoichiometry of 1:2. Surprisingly, experimentally 1:1 chemistry was found in these alkyl-amine appended MOFs, which doubles the capacity of these materials. An explanation why the chemistry in these materials is different from solution is lacking. Quantum chemical calculations were performed on a cluster mimicking these alkyl-amine-appended MOFs to investigate the CO2 chemisorption mechanism. These calculations revealed that in the absence of solvent, the formation of charged species is no longer possible because neither the neighboring amines nor the framework can accept a proton. These calculations revealed a novel mechanism, allowing recruiting an additional CO2 molecule, resulting in 2:2 stoichiometry. For the proposed mechanism an adsorption energy of −138.25 kJ per 2 moles of CO2 was calculated, corresponding to an average adsorption energy of −69.13 kJ/mol. This predicted adsorption energy is in good agreement with the experimental Qst value of −71 kJ/mol. This study also predicted that the length of the amine chains and the distances between the amines imposed by the framework dictate the reaction path.
A synthetically realistic database of hypothetical metal-organic framework (MOF) materials has been assembled. By focusing only on commercially-available organic linker molecules and utilizing the synthetically common topology of MOF-5, this database of over 100 computationally-designed structures represents a set of materials close to experimental realization. All structures were relaxed using semi-empirical electronic structure calculations, and evaluated for vehicular methane storage performance using Grand Canonical Monte Carlo simulation. A number of revealing relationships between material structure and methane adsorption performance were identified that can be exploited in the design of high-performance sorbents. High volumetric methane uptake can be achieved by utilizing short, wide organic molecules in the MOF structure, with the highest gravimetric uptake occurring for the longest molecules. A strong correlation was identified between gravimetric surface area and gravimetric methane adsorption, enabling high uptake materials to be designed using a simple geometric criterion.
Using high-throughput in silico screening, scientists at UC Berkeley, Lawrence Berkeley National Laboratories, and Lawrence Livermore National Laboratories have discovered candidate zeolite structures suitable for methane capture from dilute and medium-concentration sources. These materials possess large methane uptake capabilities and can selectively adsorb CH4 over N2 and CO2, a rare quality, given methane's negligible dipole and quadrupole moments. The work involved density functional theory calculations, implicit solvation schemes, and molecular mechanics using well-tested force fields. The researchers employed a very efficient molecular simulation code based on graphics processing units (GPUs), and managed to screen close to one hundred thousand materials within a reasonable compute time.
It is important to understand carbon dioxide dynamics inside metal-organic frameworks (MOFs) in order to optimize materials design for carbon capture applications. In this study, techniques in molecular simulations as well as experimental NMR measurements were used reveal in detail various molecular mechanisms behind the CO2 dynamics inside MOFs with open metal sites. Two different types of CO2 motions, hopping between adsorption sites and localized fluctuation, have been identified as the underlying mechanisms to quantitatively interpret the experimental NMR data. This study identified CO2 hops between different metal sites as the signature motion of the experimentally measured NMR 13C chemical shift anisotropy (CSA) patterns, which have a uniaxial-rotational-like pattern. In addition, together with the localized fluctuation motions, the apparent changes in the behavior of the "rotational angle" from the observed NMR line-shapes can also be accurately explained from molecular simulations. This study demonstrates the utility of integrating simulation and experimental tools.
The molecular-scale structure of the ionic liquid [C18mim]+[FAP]− near its free surface was studied by complementary methods. X-ray absorption spectroscopy and resonant soft X-ray reflectivity revealed a depth-decaying near-surface layering. Element specific interfacial profiles were extracted with sub-molecular resolution from energy-dependent soft X-ray reflectivity data. Temperature-dependent hard X-ray reflectivity, small- and wide-angle X-ray scattering, and infrared spectroscopy uncovered an intriguing melting mechanism for the layered region, where alkyl chain melting drove a negative thermal expansion of the surface layer spacing.
Novel porous metal-organic polyhedra (MOPs) and metal-organic frameworks (MOFs) with pre-designed "single-molecule traps (SMTs)" have been developed. These materials can selectively adsorb CO2 over N2 and CH4. It is proposed and validated, both experimentally and computationally, that these materials contain a precisely designed cavity, termed a "single-molecule trap (SMT)", with the desired size and properties suitable for trapping target CO2 molecules. Such a SMT can strengthen CO2–host interactions without involving chemical bonding, thus showing potential for CO2 capture. In principle, by judiciously designing a SMT, this method can be applied in the synthesis of other new metal-organic materials and composite materials for other speciﬁc applications, such as H2 storage and purification, and even CO2 activation.
Novel amine-appended porous polymer networks (PPNs) have been discovered that exhibit immense selectivity and capture capacity for CO2 from ultradilute sources such as air. The amine-grafted porous polymer networks were investigated for CO2 capture directly from air (400 ppm CO2, 78.96% N2, and 21% O2). Under these ultra-dilute conditions, the material PPN-6-CH2DETA was found to exhibit an extraordinarily high CO2 selectivity (3.6 x 1010) and loading capacity (1.04 mol/kg) as calculated using Ideal Adsorption Solution Theory. In addition, this material outperforms others based on simulated breakthrough calculations, and thus may have great potential to be used in direct air capture applications.
A computational approach has been developed to understand the effects of changing the cation concentration on the adsorption of CO2 in zeolites. Zeolites can be synthesized with different Al:Si ratios. Given the +3 charge of Al and +4 for Si, cations are required to compensate for the net charge on the zeolite lattice if the Al to Si ratio is increased. Large-scale simulations of aluminosilicate zeolites were conducted to identify structures exhibiting large CO2 uptake for postcombustion carbon dioxide capture. These structures include the experimentally know zeolite structures as well as a large database of predicted structures. The aim of the study was to find those structures that have the highest CO2 uptake. Subsequent analysis of the structures with the maximum uptake was carried out to identify the factors that make a particular structure ideal. These calculations showed that the optimal aluminosilicate zeolite structures have an underlying all-silica lattice with a large free volume and a framework topology that maximizes the regions with nearest-neighbor framework atom distances from 3 to 4.5 Å. These predictors extend well to different Si:Al ratios and for both Na+ and Ca2+ cations, demonstrating their universal applicability in identifying the best-performing aluminosilicate zeolite structures. In particular, this study identified the structures SAO and RWY as the most promising known zeolite structures.
Natural gas can contain large quantities of carbon dioxide, and removal of this carbon dioxide is an energy-consuming step in gas processing. The three most common technologies used for the removal of carbon dioxide from sour gas are "scrubbing", adsorption, and cryogenic separation. The lack of suitable membranes hinders the wide application of this technology in the natural gas industry. Researchers at the Molecular Foundry at Lawrence Berkeley National Laboratory have developed polypropylene-supported polyaniline membranes whose surfaces are functionalized with diamines to separate carbon dioxide from methane. After solvation with water, these membranes exhibit both a permeability and a selectivity for carbon dioxide orders of magnitude larger than values observed for other polymer membranes. This result is remarkable since the general observation is that increasing either of these parameters leads to a corresponding decrease of the other one. The presence of water in these membranes promotes the permeation of carbon dioxide via the facilitated transport mechanism, in which both the fixed amine moieties and water play a major role.
A novel, computationally efficient method for discovering promising candidate zeolite materials for adsorption-based separations has been developed. Traditionally, computationally expensive molecular simulations are used to accurately model the adsorption performance of a material for gas separations. However, the recent development of very large material databases, coupled with the rarity of outstanding adsorption materials, means that exhaustive simulation is not an inefficient method of discovery. By examining the best known CO2 separation materials, it was determined that strong local binding sites for CO2 dominate adsorption behavior, and that these sites consist of framework atoms arranged so as to envelope a guest CO2 molecule. Hence, an alternative approach for discovering zeolites was developed, based upon screening for this signature arrangement of atoms in zeolite frameworks. Utilizing this approach, a database of over one million zeolites, an order of magnitude larger than has been examined using exhaustive molecular simulation, was efficiently screening for outstanding CO2 adsorbing materials. Accordingly, promising candidates for adsorption-based separations can be discovered, bypassing the need for exhaustive molecular simulation, and ensuring that computationally expensive methods can be performed sparingly on only the most promising materials.
A new series of new zirconium-based metal-organic frameworks has been synthesized, named PCN-222 (Fe, Mn, Co, Ni, Cu, Zn). These materials possess outstanding stability relative to all known MOFs. Even after the treatment in boiling water and concentrated HCl, the framework remains intact, a property which has rarely been seen in all reported MOFs. Moreover, the PCN-222 MOFs have one dimensional (1D) channels with diameters as large as 3.7 nm, which is the second largest 1D channel diameter among all the reported MOFs. More importantly, PCN-222(Fe) was found to provide a high density of catalytic sites. One active site per 1286 Da is obtained in PCN-222, and in sharp contrast 44174 Da is necessary for each active site in natural enzyme horseradish peroxidase. Therefore, PCN-222(Fe) is an extraordinary example that combines exceptionally high chemical stability, ultra-large pore size, and highly dense catalytically active centers, with excellent biomimetic catalytic activity.
In this work by researchers in the group of Joe Zhou at Texas A&M University, metal−organic polyhedra (MOPs) were successfully incorporated into silica nanopores. Three MOPs with identical geometries but different ligand functionality (namely tert-butyl, hydroxyl, and sulfonic groups) were employed. A typical mesoporous silica, SBA-15, with a two-dimensional hexagonal pore regularity was used as the host. In comparison with bulk MOPs, which prefer to aggregate, MOPs confined in silica nanopores can be well dispersed, making the active sites and pores in the MOPs accessible. These dispersed MOPs showed apparently superior H2 adsorption capacity in comparison with aggregated bulk MOPs. Moreover, the thermal stability of the MOPs was enhanced upon their confinement in silica nanopores. The present strategy should enable various porous MOPs as well as other supramolecular architectures to be introduced into mesoporous materials with a range of pore symmetries and pore sizes, resulting in the fabrication of new types of porous composites with potential in various applications.
The rotational motion of CO2 in a MOF with open metal sites (here, Mg2(dobdc) or Mg-MOF-74) was discovered using solid-state NMR spectroscopy. This discovery not only confirmed previous study that CO2 molecules are strongly bond to the open Mg2+ sites, but also revealed detailed CO2 dynamics in a wide temperature range from 12 K to 400 K. The findings are based on the C-13 chemical shift anisotropy (CSA) patterns of CO2 adsorbed in Mg2(dobdc). In contrast to the density function theory (DFT) calculations and neutron scattering studies that are confined to the rigid limit, the CSA patterns are sensitive to the motional dynamics at practical conditions. We have shown that at temperatures in between 200 to 400 K, the CO2 is rotating at a single fixed angle of around 56 to 69 degrees. The motion persists even below 100 K and comes to a stop at around 10 K. We have also calculated the activation energy and correlation times of the rotational motion from NMR spin-lattice relaxation times. These results provide very stringent constraints for DFT and molecular dynamics simulations of CO2 adsorption and motion that seek to improve the design of materials possessing open metal sites for carbon capture.
Pore surface modification with desired functional groups in well-defined MOFs remains a significant challenge. Currently, the modification of MOF pore surfaces is mostly based on the use of predesigned ligands with specific functional groups. This approach is somewhat limited because of the cumbersome multistep process of ligand synthesis and the often unpredictable coordination between reactive functional groups (e.g., −OH, −COOH, N-donating groups, etc.) and metal centers during the MOF assembly process. Moreover, it is generally difficult to obtain MOFs with long and/or large groups appended on the pore walls by direct solvothermal reactions. In this work, it was possible to engineer MOF pore surfaces with controlled loadings of different functional groups via postsynthetic click reactions. Reactions of ZrCl4 and single or mixed linear ligands bearing methyl or azide groups led to highly stable isoreticular Zr-based MOFs with accessible, reactive azide groups in large pores that enable the MOFs to undergo a quantitative click reaction with alkynes to form triazole-linked groups on the pore wall surfaces. Significantly, such a synthetic route allows accurate control of the loading of azide groups on the internal surface of a MOF for the first time. The Zr-based MOFs offer an ideal platform for pore surface engineering. A variety of functional groups can be anchored onto the pore walls of the MOFs with precise control over the loading, density, and functionality. More importantly, given the high stability of Zr-MOFs, the resultant click products have been demonstrated to possess well-retained frameworks and accessible functionalized pore surfaces. This work provides a general approach for introducing diverse functional groups, especially large and/or reactive groups, into framework pores to produce MOFs that otherwise could not be directly synthesized by conventional solvothermal reactions. The tailoring of pore surfaces in MOFs with control over both the type and density of functional groups will extend their functionalities further toward broader applications.
During the formation of some metal organic frameworks, the metal centers are coordinated by both linker and solvent molecules. Subsequent activation removes the solvent molecules and creates open metal sites, which have strong affinity for CO2. At present, the interactions of CO2 with these open metal sites are so poorly understood that commonly used force fields underestimate the adsorption of CO2 in Mg-MOF-74 at carbon capture conditions by as much as two orders of magnitude. Here we present a systematic procedure to develop a force field for Mg-MOF-74 and similar systems using high-level quantum chemical calculations. Monte Carlo simulations based on an ab initio force field generated for CO2 in Mg-MOF-74 shed some light on the interpretation of thermodynamic data of flue gas in this material. The force field accurately describes the chemistry of the open metal sites, and this approach may serve in molecular simulations in general and in the study of fluid–solid interactions.
Until recently, pores larger than 32 Angstroms have been difficult to achieve in metal-organic frameworks (MOFs). Three major obstacles to expanding pore size are: (a) limited solubility of large organic links; (b) structure interpenetration; and (c) collapse of pores after guest removal. In this study, strategies were designed for the first time to overcome all three limitations. In particular, a well-studied framework, MOF-74, which contains linkers that have one phenylene ring, was expanded to two, three, four, five, six, seven, nine, and eleven phenylene rings, to generate a series of structures with sequentially larger pores while maintaining the same topology of MOF-74. The obtained pores are large enough to accommodate passage of biomolecules such as myoglobin and green fluorescent proteins (GFPs). The development of this series of MOFs significantly expands the scope of guest molecules that can be included into porous crystals from small gases to large biomaterials and opens the way for potential applications in molecular recognition and drug delivery.
One of the main bottlenecks to deploying large-scale carbon dioxide capture and storage (CCS) in power plants is the energy required to separate the CO2 from flue gas. For example, near-term CCS technology applied to coal-fired power plants is projected to reduce the net output of the plant by ~30% and to increase the cost of electricity by 60-80%. Developing capture materials and processes that reduce the parasitic energy imposed by CCS is therefore an important area of research. Parasitic energy refers to the minimized electric load, at optimal conditions, that is imposed upon a power plant using a carbon capture material in a temperature-pressure swing capture process, followed by compression. Exploiting the parallel processing power of clusters of graphics processing units (GPUs), as well as novel material informatics methods, a computational procedure was developed to rapidly calculate the thermodynamic and geometric properties of a material that are critical in determining its parasitic energy penalty. These computational advances enabled the parasitic energy to be determined for hundreds of thousands of known and computationally predicted zeolites and zeolitic imidazolate frameworks (ZIFs) and allowed a direct comparison with existing carbon capture technology. Notably, many of these materials have the potential to reduce the parasitic energy of CCS by 30-40% compared to near-term technologies. Important observations were also made about optimal structures which will guide the future design and synthesis of outstanding carbon capture materials.
A set of simulation tools has been developed to more quickly and accurately determine the diffusion behavior of adsorbed gases. Adapting transition state theory for use in a kinetic Monte Carlo simulation allows for a much faster characterization of the diffusion behavior than the previous benchmark, molecular dynamics, without compromising accuracy. These new tools were used to explore how the relationship between self- and collective-diffusion behaviors is affected by adsorbate loading in different topologies. These new tools and insights will aid in the characterization and identification of materials for the efficient capture of CO2, or any other gas separation.
Metal-organic frameworks have shown exceptional promise as materials for CO2 capture from coal flue gas. The synthesis of a new metal-organic framework containing a high density of secondary amines was accomplished by the post-synthetic modification of the metal-organic framework Mg2(dobpdc) (dobpdc2− = 4,4′-dioxidio-3,3′-biphenyldicarboxylate) with the N,N′-dimethylethylenediamine (top figure). The CO2 isotherms saturated at very low pressures, with a capacity for CO2 of over 2 mmol/g (~9 wt %) at 390 ppm, which corresponds to the pressure of CO2 in air (bottom figure). At 150 mbar, the pressure relevant for coal flue gas, the material reversibly adsorbs over 3.3 mmol/g (~15 wt %). The material can be regenerated at temperatures between 120 and 150 °C, which is comparable to how current aqueous amines used for CO2 removal are regenerated. However, the large working capacity of the adsorbent and reduced heat capacity of the solid may reduce the required regeneration energy by at least 10% and potentially up to 25%. The material showed little degradation over 50 adsorption/desorption cycles, has rapid adsorption kinetics, and is stable to water and temperatures below 170 °C. Lastly, the low saturation capacity may make this an extremely attractive adsorbent for removing CO2 from the flue gas of natural gas power plants, which typically have CO2 partial pressures near 50 mbar.
A novel metal-organic framework (MOF) material has been discovered that shows excellent performance for the purification of gases such as methane, ethylene, and propylene from gas mixtures, importantly at more ambient temperatures than those currently used for their separation. This discovery could help oil and chemical industries save tremendous amounts of capital and energy and also lower environmental impacts by replacing existing large-scale energy-intensive gas separation processes. To meet demand for these materials, petrochemical companies continue to increase production that involves cracking longer chain hydrocarbons at high temperatures (up to 500-600°C) and separating the resulting products at high pressures and cryogenic temperatures (−100°C). The new solid iron-based adsorbent features a large surface area and exposed iron cation sites, which can preferentially adsorb gases such as ethylene and propylene with high selectivity, over their saturated counterparts, ethane and propane. This material may also be capable of purifying natural gas streams containing a number of impurity gases.
A new mechanism has been identified by which CO2 binds to metal-organic frameworks (MOFs) with exceptional strength. Discovering this novel CO2 binding mechanism may help researchers design materials to capture and store anthropogenic carbon emissions. While it is well-established that metal atoms in MOFs can selectively bind CO2 over other species common in exhaust gas, such as N2, the organic linker molecules, on the other hand, have been largely considered spectators in this process. In this work, researchers used ab initio methods to discover that the affinity of certain MOFs for CO2 can be greatly enhanced via a new adsorption mechanism involving both the metal atoms and the organic molecules. Indeed, with the appropriate linking molecules, CO2 binding increased by 50%. This discovery may therefore lead to new routes to design MOFs with strong CO2 absorption by manipulating both the metal and the organic components of the framework.
A novel metal-organic framework (MOF) has been developed that shows reversible alteration of CO2 adsorption upon photochemical or thermal treatment. Current CO2 capture technology has a high parasitic energy cost associated with CO2 adsorption and sorbent regeneration. In order to be competitive adsorbents for carbon capture, MOFs need to exhibit high selectivity for CO2 over N2 and be regenerated in an energy-efficient manner. The MOF reported by Zhou and coworkers contains azobenzene functional groups which can switch their conformation upon light irradiation or heat treatment. As a result, the freshly made trans-MOF adsorbs a significant amount of CO2 but negligible N2. Upon light irradiation, the total uptake of CO2 decreases readily due to trans to cis isomerization of the azobenzene groups inside the pores of the MOF. The adsorbent returns to its original state with gentle heating. This discovery shows that a potential route to release bound CO2 from MOFs is to use light irradiation. This optically-induced CO2 release can combine with current regeneration methods such as temperature swing adsorption (TSA) or pressure swing desorption (PSA), leading to cost-effective CO2 capture.
A new technique was developed to study depth profiles of molecular orientation in thin films made of soft matter, such as polymer membranes for gas separation or conductive polymers for organic electronics. The method is based on the reflectivity of "soft" x-rays, which are less energetic than those generally used in conventional x-ray scattering. Their interaction with matter is very sensitive to the chemical structure of the studied material and the orientation of specific molecular bonds with respect to the polarization of the soft x-ray beam. This provides a precise tool to detect depth profiles of this orientation through thin films. The capabilities of this novel technique were demonstrated with a study of well-aligned liquid crystalline polymer films. All the experimental data, obtained using synchrotron sources at the Advanced Light Source and at ELETTRA, can be modeled quantitatively. For films up to 50 nm in thickness, the degree of orientation was found to be independent of depth and film thickness. This new technique paves the way for new kinds of studies on a wide class of interesting systems ranging from polymers to biomaterials.
Researchers within the CGS have discovered a new alternative for CO2 capture: separation of the products of coal gasification (CO2 and H2) using metal-organic frameworks. Both coal gasification and the separation of CO2 and H2 have been studied for decades as an excellent alternative to traditional coal combustion and its highly problematic CO2 capture options. In this study, researchers explored the use of metal-organic frameworks for this technology and discovered a novel class of frameworks with vast improvements over traditionally used materials. Specifically, metal-organic frameworks that have Lewis acidic open metal sites decorating their pores are highly selective for CO2 in the presence of H2 and still maintain the enormous capacity for holding CO2 once it is bound, making them strong frontrunners in this exciting CO2 capture option.
Researchers at the CGS have demonstrated for the first time that membranes with pores of sub-nanometer size can be fabricated by the growth of directed cyclic peptide nanotubes within the nanoscopic framework set by block copolymers (i.e. polymer chains made of two chemically different chains connected together). Membranes containing high density arrays of vertically aligned nanotubes spanning the film exhibit enhanced mass transport and selectivity based on the sizes of gas molecules passing through the nanotubes. This new strategy for fabricating highly efficient separation membranes circumvents impediments associated with aligning and organizing high aspect ratio nano-objects normal to the surface.
Researchers at the DOE Energy Frontier Research Center (EFRC) led by the University of California-Berkeley have synthesized a new aluminum based metal-organic framework, i.e. a three dimensional network build from metal atoms linked by organic units, with the ability to bind an extremely large variety of metal ions. This framework exhibits a high surface area, thermal stability, and is water stable, a necessity for capturing CO2 from power plant flue gas streams. Metals can be inserted into the binding units of the framework by simply soaking the material in various salt solutions. Researchers found that the material's ability to selectively adsorb CO2 over N2 was increased upon binding of specific copper salts into the porous framework. Not only does the material become more selective upon addition of metal salts, CO2 capacity is also greatly increased. Precious metal ions can be added to the framework, possibly making it a useful platform for heterogeneous catalysis.
Highly selective and reversible oxygen storage has been achieved in a chromium-based metal-organic framework, i.e. a three dimensional network build from metal atoms linked by organic units. This framework shows tremendous selectivity for the adsorption of O2 over N2 at room temperature and an uptake capacity of 11 weight percent O2 at very low pressures. The unprecedented O2/N2 selectivity of this compound arises as a result of the chromium ions in the framework being able to donate electrons to adsorbing oxygen molecules while being inactive towards the other components of air, most importantly nitrogen. Heating the sample to just 50°C in a vacuum releases most of the adsorbed oxygen, allowing the material to be used multiple times as an adsorbent. Although it showed modest decreases in oxygen uptake capacity over multiple cycles, this new metal-organic framework was successfully cycled 15 times, maintaining a high adsorption capacity and O2/N2 selectivity.
Researchers at the DOE Energy Frontier Research Center (EFRC) led by the University of California-Berkeley have synthesized a new aluminum based metal-organic framework, i.e. a three dimensional network build from metal atoms linked by organic units, with the ability to bind an extremely large variety of metal ions. This framework exhibits a high surface area, thermal stability, and is water stable, a necessity for capturing CO2 from power plant flue gas streams. Metals can be inserted into the binding units of the framework by simply soaking the material in various salt solutions. Researchers found that the material's ability to selectively adsorb CO2 over N2 was increased upon binding of specific copper salts into the porous framework. Not only does the material become more selective upon addition of metal salts, CO2 capacity is also greatly increased. Precious metal ions can be added to the framework, possibly making it a useful platform for heterogeneous catalysis.