Prototypical Metal-organic Frameworks Research Articles

Defect strategies in Mn-doped metal–organic frameworks to enhance adsorption-based NOx gas sensing performance

Post-synthetic metalation (PSM) represents a powerful toolkit for modifying the chemical composition of pre-formed metal-organic frameworks (MOFs). However, accessing defective MOFs with exchanged metal nodes via this approach remains challenging. Herein, we report a transmetalation strategy to induce desired metal exchange frustrated flexibility defects in a prototypical MOF platform. Specifically, the pristine Zn-based MOF undergoes selective Zn-to-Mn exchange through a carefully controlled solvothermal transmetalation process. This protocol generates a series of Mn-doped MOFs with increasing Mn integration and the concomitant formation of defect sites within the framework. Detailed structural characterization confirms the successful Mn doping and illuminates the generation of vacancies and distortions around the exchanged Mn nodes. The transmetallated Mn-doped MOF materials exhibit altered chemical environments, as probed by spectroscopic techniques, and display promising sensing activity for NOx through the MOF matrix membrane. The proposed application of the MOF samples as an adsorbent-based chemical sensor includes sensing mechanisms for NO2 gas.

The effect of adsorbent shaping on the equilibrium and kinetic CO2 adsorption properties of ZIF-8

To be used at scale, adsorbents must be shaped into macroscopic objects, e.g. pellets, granules, and monoliths. Shaping may involve various processing steps and additives that collectively contribute to the final equilibrium and kinetic sorption properties of the adsorbent. Understanding the fundamental links between these processing steps and the resulting sorption performance is needed to rationalise the design of shaped adsorbents. Our study aims to advance the state of knowledge in this area. By focusing on ZIF-8, a prototypical metal organic framework (MOF), and CO2, a common small gas adsorbate, we compared a commercial binderless ZIF-8 extrudate and two purpose-made ZIF-8 pellets shaped with a blend of binders. We used analytical, spectroscopic, and imaging techniques to characterise the samples’ chemical, textural, and morphological properties. In addition, we collected equilibrium and kinetic CO2 adsorption data at 283, 293, and 303 K and up to 1 bar. The binderless ZIF-8 extrudate exhibited a homogeneous structure with BET area, micro- and meso-porosity only slightly reduced compared to ZIF-8 powder. The ZIF-8 pellets also maintained the overall BET area of the ZIF-8 powder, but displayed enhanced macroporosity and skeletal density as well as reduced microporosity – features that likely result from the pressure-induced strains of pelletisation. The extrudates are much less mechanically robust than the pellets. All samples displayed CO2 adsorption capacity in line with the CO2 uptake of their respective components. The CO2 kinetics measurements reveal clear distinctions between binderless ZIF-8 extrudate and ZIF-8 pellets, the latter indicating a behaviour associated with the additional presence of macropores. Our study presents quantifiable evidence for the effect of adsorbent shaping on gas adsorption equilibria and kinetics, providing a valuable resource for preliminary process design.

Molecular driving forces for water adsorption in MOF-808: A comparative analysis with UiO-66.

Metal-organic frameworks (MOFs), with their unique porous structures and versatile functionality, have emerged as promising materials for the adsorption, separation, and storage of diverse molecular species. In this study, we investigate water adsorption in MOF-808, a prototypical MOF that shares the same secondary building unit (SBU) as UiO-66, and elucidate how differences in topology and connectivity between the two MOFs influence the adsorption mechanism. To this end, molecular dynamics simulations were performed to calculate several thermodynamic and dynamical properties of water in MOF-808 as a function of relative humidity (RH), from the initial adsorption step to full pore filling. At low RH, the μ3-OH groups of the SBUs form hydrogen bonds with the initial water molecules entering the pores, which triggers the filling of these pores before the μ3-OH groups in other pores become engaged in hydrogen bonding with water molecules. Our analyses indicate that the pores of MOF-808 become filled by water sequentially as the RH increases. A similar mechanism has been reported for water adsorption in UiO-66. Despite this similarity, our study highlights distinct thermodynamic properties and framework characteristics that influence the adsorption process differently in MOF-808 and UiO-66.

Crystal size-dependent framework flexibility of a prototypical metal organic framework is related to metal content: zeolitic imidazolate framework-7

Reducing crystal size in ZIF-7 MOFs leads to significantly increased framework flexibility with smaller crystals exhibiting more point defects and lower metal content. Such size-dependent behaviour may enable rational design of various materials.

Evidence of Organic Polymeric Behavior in the Glass Transition of Metal-Organic Frameworks.

The origin of the glass transition is still an open debate, especially for the new class of glasses, formed from metal-organic compounds. High-temperature in situ 2H Nuclear Magnetic Resonance (NMR) experiments are performed on deuterated samples of ZIF-62 (Zn(C3H4N2)2-x(C7H6N2)x, with x=0.25 and x=0.05), the prototypical metal-organic framework glass former. Using lineshape analysis, frequencies and angular amplitudes of oscillations of the imidazolate ring during heating up to the melt progressively increasing from ≈10 to 150MHz, and from ≈5° to 25° are found. This behavior is compositionally dependent and points to the origin of the glass transition lying in organic linker movement, in a similar vein to that witnessed in some organics and contrary to the purely inorganic-based view of Metal-Organic Framework (MOF) glasses taken to date. This experimental approach shows the potential to elucidate the melting and/or decomposition process for a wide range of MOFs.

Uncertainty in Composite Membranes: From Defect Engineering to Film Processing.

Composite membranes featuring metal-organic framework (MOF)-dispersed polymers have attracted tremendous attention in recent years. However, evaluating commercial viability is oftentimes obscured by the irreproducibility in both MOF synthesis and film manufacturing protocols. Variability in MOF property sets are typically ascribed to crystal defects resulting from subtle variations in synthesis, but quantitative studies investigating the role of defects on transport properties are exceedingly rare. Likewise, controlled film formation protocols are rarely reported in the open literature, making it difficult to provide substantial and informative structure-property correlations. This study aims to address these uncertainties. To this end, two samples of a prototypical MOF, UiO-66-NH2, were synthesized to feature similar particle size, morphology, and colloidal stability. However, defect engineering protocols coupled with careful screening experiments were developed to synthesize the two MOFs with maximally different porosities. Composite membranes were prepared for each MOF and a high-performance polymer, 6FDA-Durene, and then tested for light gas permeation measurements, revealing a small and unexpected enhancement in CO2/CH4 performance for samples containing low-porosity UiO-66-NH2. Mechanistic studies on sorption revealed a surprising 50% decrease in sorption capacity for high-porosity UiO-66-NH2, completely offsetting enhancements from increased gas diffusion. By using multiple replicate experiments, the sample-to-sample variation was large enough to obscure any differences in permeability and selectivity between the two types of MOF composites at low volume fractions. Application of the Maxwell model to extrapolate pure-MOF performance led to significant variations in predicted values, demonstrating the importance of collecting and reporting replicate experiments for membrane preparation and testing.

Quantification of gas-accessible microporosity in metal-organic framework glasses

Metal-organic framework (MOF) glasses are a new class of glass materials with immense potential for applications ranging from gas separation to optics and solid electrolytes. Due to the inherent difficulty to determine the atomistic structure of amorphous glasses, the intrinsic structural porosity of MOF glasses is only poorly understood. Here, we investigate the porosity features (pore size and pore limiting diameter) of a series of prototypical MOF glass formers from the family of zeolitic imidazolate frameworks (ZIFs) and their corresponding glasses. CO2 sorption at 195 K allows quantifying the microporosity of these materials in their crystalline and glassy states, also providing excess to the micropore volume and the apparent density of the ZIF glasses. Additional hydrocarbon sorption data together with X-ray total scattering experiments prove that the porosity features of the ZIF glasses depend on the types of organic linkers. This allows formulating design principles for a targeted tuning of the intrinsic microporosity of MOF glasses. These principles are counterintuitive and contrary to those established for crystalline MOFs but show similarities to strategies previously developed for porous polymers.

Evaluation of ZIF-8 flexible force fields for structural and mechanical properties

Metal–Organic Frameworks (MOFs) offer considerable potential for applications in adsorption due to their large pore volumes and surface areas. Studies on mechanical stability of MOFs are scarce. Seminal experimental work has shed a new light on the role that elastic constants play in establishing the structural stability of the prototypical ZIF-8 MOF, with its elastic deformation mechanism being linked to the pliant ZnN4 tetrahedra (Tan et al., 2012). Over the past decade several classical flexible force fields have been proposed to study the physical properties of the system using simulations (Zheng et al., 2012; Hu et al., 2012; Zhang et al., 2013, Wu et al., 2014; Krokidas et al., 2015; Weng and Schmidt, 2019; Dürholt et al. 2019). In this work, we evaluated the majority of them for reproducing structural and mechanical properties (unit cell sizes as a function of temperature and pressure, and elastic constants as a function of pressure), compared them to existing DFT calculations (Tan et al., 2012; Maul et al., 2019) and found that they provide different results under the same testing conditions. The obtained results provide insight into the relationship between fundamental elastic properties and the chosen force field parametrization, allowing us to characterize the applicability of each of the force fields. Furthermore, the employed two-code approach allowed us to find significant discrepancies in elastic constant values for the same force field between methodologies that employ different energy minimization algorithms, suggesting that eigenmode-following approaches might be needed to guarantee true minimum energy configurations for ZIFs.

Democratizing the Assessment of Thermal Robustness of Metal-Organic Frameworks.

With the pressing need of having reliable materials for carbon dioxide capture, metal-organic frameworks (MOFs) have shown promising performance due to their flexible sign and tunable functionality by applying reticular chemistry principles. One of the main characteristics of practical MOFs is to design thermally robust candidates for sustainable functionality. Here, we introduce a comprehensive methodology for examining the thermal stability of MOFs by combining theoretical calculations and affordable experimental methods to fully describe their performance under thermal variations. We chose the prototypical MOF, HKUST-1, to assess the methodology by performing density functional theory and classical molecular dynamics simulations and validating with experiments such as in situ powder X-ray diffraction, differential scanning calorimetry, and thermogravimetric analysis. HKUST-1 shows thermal robustness until a temperature of 240 °C at different atmospheric gases with a reversible breathing trend with temperature. This methodology is affordable as it uses minimal experimental testing and can be applied to any MOF materials to explore its suitability for practical applications.

Dispersive x-ray absorption spectroscopy for time-resolved in situ monitoring of mechanochemical reactions.

X-ray absorption spectroscopy (XAS) provides a unique, atom-specific tool to probe the electronic structure of solids. By surmounting long-held limitations of powder-based XAS using a dynamically averaged powder in a Resonant Acoustic Mixer (RAM), we demonstrate how time-resolved in situ (TRIS) XAS provides unprecedented detail of mechanochemical synthesis. The use of a custom-designed dispersive XAS (DXAS) setup allows us to increase the time resolution over existing fluorescence measurements from ∼15minto 2s for a complete absorption spectrum. Hence, we here establish TRIS-XAS as a viable method for studying mechanochemical reactions and sampling reaction kinetics. The generality of our approach is demonstrated through RAM-induced (i) bottom-up Au nanoparticle mechanosynthesis and (ii) the synthesis of a prototypical metal organic framework, ZIF-8. Moreover, we demonstrate that our approach also works with the addition of a stainless steel milling ball, opening the door to using TRIS-DXAS for following conventional ball milling reactions. We expect that our TRIS-DXAS approach will become an essential part of the mechanochemical tool box.

Low-Temperature and Additive-Free Synthesis of Spherical MIL-101(Cr) with Enhanced Dye Adsorption Performance

The chromium-benzenedicarboxylate metal–organic framework (MOF), MIL-101(Cr), is one of the most well-investigated and widely used prototypical MOFs. Regarding its synthesis, the use of a toxic modulator (usually HF) and high reaction temperature (220 °C) are the main factors hindering its further expansion of production and utilization. In fact, high quality MIL-101(Cr) crystals can be prepared at a much lower temperature (160 °C) with spherical morphology via an additive-free approach. Compared to traditional octahedral MIL-101(Cr), the spherical MIL-101(Cr) possesses higher adsorption performance toward dye molecules, including methyl orange (MO) and rhodamine B (RB). The results suggest that toxic additives and high reaction temperatures are not essential in the synthesis of MIL-101(Cr), and the fabrication of spherical MIL-101(Cr) may offer a facile and effective pathway for the large-scale industrial application of MIL-101(Cr).

Catching the Reversible Formation and Reactivity of Surface Defective Sites in Metal-Organic Frameworks: An Operando Ambient Pressure-NEXAFS Investigation.

In this work, we apply for the first time ambient pressure operando soft X-ray absorption spectroscopy (XAS) to investigate the location, structural properties, and reactivity of the defective sites present in the prototypical metal–organic framework HKUST-1. We obtained direct evidence that Cu+ defective sites form upon temperature treatment of the powdered form of HKUST-1 at 160 °C and that they are largely distributed on the material surface. Further, a thorough structural characterization of the Cu+/Cu2+ dimeric complexes arising from the temperature-induced dehydration/decarboxylation of the pristine Cu2+/Cu2+ paddlewheel units is reported. In addition to characterizing the surface defects, we demonstrate that CO2 may be reversibly adsorbed and desorbed from the surface defective Cu+/Cu2+ sites. These findings show that ambient pressure soft-XAS, combined with state-of-the-art theoretical calculations, allowed us to shed light on the mechanism involving the decarboxylation of the paddlewheel units on the surface to yield Cu+/Cu2+ complexes and their reversible restoration upon exposure to gaseous CO2.

Exploiting Reaction-Diffusion Conditions to Trigger Pathway Complexity in the Growth of a MOF.

Coordination polymers (CPs), including metal-organic frameworks (MOFs), are crystalline materials with promising applications in electronics, magnetism, catalysis, and gas storage/separation. However, the mechanisms and pathways underlying their formation remain largely undisclosed. Herein, we demonstrate that diffusion-controlled mixing of reagents at the very early stages of the crystallization process (i.e., within ≈40 ms), achieved by using continuous-flow microfluidic devices, can be used to enable novel crystallization pathways of a prototypical spin-crossover MOF towards its thermodynamic product. In particular, two distinct and unprecedented nucleation-growth pathways were experimentally observed when crystallization was triggered under microfluidic mixing. Full-atom molecular dynamics simulations also confirm the occurrence of these two distinct pathways during crystal growth. In sharp contrast, a crystallization by particle attachment was observed under bulk (turbulent) mixing. These unprecedented results provide a sound basis for understanding the growth of CPs and open up new avenues for the engineering of porous materials by using out-of-equilibrium conditions.

Defect Termination in the UiO-66 Family of Metal-Organic Frameworks: The Role of Water and Modulator.

The defect concentration in the prototypical metal-organic framework UiO-66 can be well controlled during synthesis, leading to precisely tunable physicochemical properties for this structure. However, there has been a long-standing debate regarding the nature of the compensating species present at the defective sites. Here, we present unambiguous spectroscopic evidence that the missing-linker defect sites in an ambient environment are compensated with both carboxylate and water (bound through intermolecular hydrogen bonding), which is further supported by ab initio calculations. In contrast to the prevailing assumption that the monocarboxylate groups (COO-) of the modulators form bidentate bonding with two Zr4+ sites, COO- is found to coordinate to an open Zr4+ site in an unidentate mode. The neighboring Zr4+ site is terminated by a coordinating H2O molecule, which helps to stabilize the COO- group. This finding not only provides a new understanding of defect termination in UiO-66, but also sheds light on the origin of its catalytic activity.

Comparative Evaluation of Different MOF and Non‐MOF Porous Materials for SO2 Adsorption and Separation Showing the Importance of Small Pore Diameters for Low‐Pressure Uptake

AbstractThe search for adsorbents for flue gas desulfurization processes is a current interest. For the first time, a comparative experimental study of SO2 adsorption by porous materials including the prototypical metal–organic frameworks NH2‐MIL‐101(Cr), Basolite F300 (Fe‐1,3,5‐BTC), HKUST‐1 (Cu‐BTC), the zeolitic imidazolate frameworks (ZIF)‐8, ZIF‐67, the alumosilicate Zeolite Y, the silicoaluminumphosphate (SAPO)‐34, Silica gel 60, the covalent triazine framework (CTF)‐1, and the active carbon Ketjenblack is carried out. Microporous materials with pore sizes in the range of 4–8 Å or with nitrogen heterocycles are found to be optimal for SO2 uptake in the low‐pressure range. The SO2 uptake capacity at 1 bar correlates with the Brunauer‐Emmett‐Teller‐surface area and pore volume rather independently of the surface microstructure. Zeolite Y and SAPO‐34 are stable toward humid SO2. The materials Zeolite Y and CTF‐1(600) show the most promising SO2/CO2 selectivity results with an ideal adsorbed solution theory selectivity in the range of 265–149 and 63–43 with a mole fraction of 0.01–0.5 SO2, respectively, at 293 K and 1 bar.

H2S Stability of Metal–Organic Frameworks: A Computational Assessment

The H2S stability of a range of metal-organic frameworks (MOFs) was systematically assessed by first-principles calculations. The most likely degradation mechanism was first determined and we identified the rate constant of the degradation reaction as a reliable descriptor for characterizing the H2S stability of MOFs. A qualitative H2S stability ranking was thus established for the list of investigated materials. Structure-stability relationships were further envisaged considering several variables including the nature of the linkers and their grafted functional groups, the pore size, the nature of metal sites, and the presence/nature of coordinatively unsaturated sites. This knowledge enabled the anticipation of the H2S stability of one prototypical MOF, e.g., MIL-91(Ti), which has been previously proposed as a good candidate for CO2 capture. This computational strategy enables an accurate and easy handling assessment of the H2S stability of MOFs and offers a solid alternative to experimental characterizations that require the manipulation of a highly toxic and corrosive molecule.

MOFs industrialization: a complete assessment of production costs.

The potential of safe and low-cost batch production processes for Metal-Organic Frameworks (MOFs) at an industrial scale has been evaluated based on the prototypical MOF MIL-160(Al), a bio-derived material of high practical interest that can be made with a high space-time yield using green ambient pressure conditions. A simple method to calculate the production cost of this material has been determined based on a simulated process constructed with the data collected from laboratory pilot large-scale tests taking into account for the first time in MOF cost evaluation all the process parameters such as the scale, the cost of the raw materials, recirculation, and washing. The investment for a production plant established the ground for the estimation of the complete cost. The expected cost ranged from ca. 55 $ per kg at 100 tons per year down to 29.5 $ per kg for 1 kton per year production with longer term perspectives of reaching costs below 10 $ per kg once the bio-derived ligand is considered for the large-scale production of bioplastics.

Isoreticular Microporous Metal–Organic Frameworks for Carbon Dioxide Capture

The isoreticular principle has been applied to construct two copper metal-organic framework (MOF) analogues with different porosities for the adsorptive capture of CO2 from N2 and CH4 at 1 atm and 298 K. By using a 4-substituted isophthalate linker with a bulky nitro group, the microporous MOF [Cu(BDC-NO2)(DMF)] (UTSA-93 or CuBDC-NO2; H2BDC-NO2 = 4-nitroisophthalic acid and DMF = N,N'-dimethylformamide) has been synthesized with mot topology, showing a compact pore structure with a size of 6.0 × 7.0 Å2 in contrast to that of 6.9 × 8.5 Å2 in the prototypical MOF with a bromo group. The optimized pore structure allows the nitro-functionalized MOF to capture CO2 with a higher capacity of about 2.40 mmol g-1 under ambient conditions, in contrast to 1.08 mmol g-1 in the bromo-functionalized analogue. The adsorption selectivity of CuBDC-NO2-a for a CO2/N2 (15:85) mixture (28) under ambient conditions is also higher than that of the bromo-substituted prototype (25) and comparable with those of several MOF materials. Moreover, dynamic breakthrough experiments of the nitro-functionalized MOF have been performed to illustrate its separation potential toward a CO2/N2 mixture.

Tunable Metal-Organic Frameworks Based on 8-Connected Metal Trimers for High Ethane Uptake.

Metal trimers [M3 (O/OH)](OOCR)6 are among the most important structural building blocks. From these trimers, a great success has been achieved in the design of 6- or 9-connected framework materials with various topological features and outstanding gas-sorption properties. In comparison, 8-connected trimer-based metal-organic frameworks (MOFs) are rare. Given multiple competitive pathways for the formation of 6- or 9-connected frameworks, it remains challenging to identify synthetic or structural parameters that can be used to direct the self-assembly process toward trimer-based 8-connected materials. Here, a viable strategy called angle bending modulation is revealed for creating a prototypical MOF type based on 8-connected M3 (OH)(OOCR)5 (Py-R)3 trimers (M = Zn, Co, Fe). As a proof of concept, six members in this family are synthesized using three types of ligands (CPM-80, -81, and -82). These materials do not possess open-metal sites and show excellent uptake capacity for various hydrocarbon gas molecules and inverse C2 H6 /C2 H4 selectivity. CPM-81-Co, made from 2,5-furandicarboxylate and isonicotinate, features selectivity of 1.80 with high uptake capacity for ethane (123 cm3 g-1 ) and ethylene (113 cm3 g-1 ) at 298 K and 1bar.

Surfactant-controlled crystal growth of metal–organic frameworks and their nanoparticle pyrolysis products

The effect of a common surfactant, cocamidopropyl betaine (CAPB), on the growth and meso- to micro-structural morphology of a prototypical metal–organic framework, HKUST-1 (Hong Kong University of Science and Technology-1, copper 1,3,5-benzenetricarboxylate) has been investigated. Surfactant molecules direct growth of HKUST-1 into wheat sheaf-shaped structures, comprising flat, ribbon-like individual ‘stems’ of HKUST-1. Comparison of wheat sheaf-like structures to bulk HKUST-1, as well as their respective decomposition products, was made. Upon pyrolysis, both bulk and wheat sheaf structures thermally decomposed to amorphous carbon decorated with metallic copper nanoparticles, that in each case retained the base morphology of the starting MOF.