A post-synthetic modification strategy for enhancing Pt adsorption efficiency in MOF/polymer composites.

Growing polymers inside porous metal-organic frameworks (MOFs) can allow incoming guests to access the backbone of otherwise non-porous polymers, boosting the number and/or strength of available adsorption sites inside the porous support. In the present work, we have devised a novel post-synthetic modification (PSM) strategy that allows one to graft metal-chelating functionality onto a polymer backbone while inside MOF pores, enhancing the material's ability to recover Pt(iv) from complex liquids. For this, polydopamine (PDA) was first grown inside of a MOF, known as Fe-BTC (or MIL-100 Fe). Next, a small thiol-containing molecule, 2,3-dimercapto-1-propanol (DIP), was grafted to the PDA via a Michael addition. After the modification of the PDA, the Pt adsorption capacity and selectivity were greatly enhanced, particularly in the low concentration regime, due to the high affinity of the thiols towards Pt. Moreover, the modified composite was found to be highly selective for precious metals (Pt, Pd, and Au) over common base metals found in electronic waste (i.e., Pb, Cu, Ni, and Zn). X-ray photoelectron spectroscopy (XPS) and in situ X-ray absorption spectroscopy (XAS) provided insight into the Pt adsorption/reduction process. Last, the PSM was extended to various thiols to demonstrate the versatility of the chemistry. It is hoped that this work will open pathways for the future design of novel adsorbents that are fine-tuned for the rapid, selective retrieval of high-value and/or critical metals from complex liquids.

How Reproducible is the Synthesis of Zr-Porphyrin Metal-Organic Frameworks? An Interlaboratory Study.

Metal-organic frameworks (MOFs) are a rapidly growing class of materials that offer great promise in various applications. However, the synthesis remains challenging: for example, a range of crystal structures can often be accessed from the same building blocks, which complicates the phase selectivity. Likewise, the high sensitivity to slight changes in synthesis conditions may cause reproducibility issues. This is crucial, as it hampers the research and commercialization of affected MOFs. Here, it presents the first-ever interlaboratory study of the synthetic reproducibility of two Zr-porphyrin MOFs, PCN-222 and PCN-224, to investigate the scope of this problem. For PCN-222, only one sample out of ten was phase pure and of the correct symmetry, while for PCN-224, three are phase pure, although none of these show the spatial linker order characteristic of PCN-224. Instead, these samples resemble dPCN-224 (disordered PCN-224), which has recently been reported. The variability in thermal behavior, defect content, and surface area of the synthesised samples are also studied. The results have important ramifications for field of metal-organic frameworks and their crystallization, by highlighting the synthetic challenges associated with a multi-variable synthesis space and flat energy landscapes characteristic of MOFs.

Rapid, Selective Extraction of Silver from Complex Water Matrices with a Metal-Organic Framework/Oligomer Composite Constructed via Supercritical CO2.

Every year vast quantities of silver are lost in various waste streams; this, combined with its limited, diminishing supply and rising demand, makes silver recovery of increasing importance. Thus, herein, we report a controllable, green process to produce a host of highly porous metal-organic framework (MOF)/oligomer composites using supercritical carbon dioxide (ScCO2 ) as a medium. One resulting composite, referred to as MIL-127/Poly-o-phenylenediamine (PoPD), has an excellent Ag+ adsorption capacity, removal efficiency (>99 %) and provides rapid Ag+ extraction in as little as 5 min from complex liquid matrices. Notably, the composite can also reduce sliver concentrations below the levels (<0.1 ppm) established by the United States Environmental Protection Agency. Using theoretical simulations, we find that there are spatially ordered polymeric units inside the MOF that promote the complexation of Ag+ over other common competing ions. Moreover, the oligomer is able to reduce silver to its metallic state, also providing antibacterial properties.

Uranium(IV) and Thorium(IV) Coordination Polymers Based on Tritopic Carboxylic Acids.

Nine new coordination polymers based on U(IV) and Th(IV) were synthesized solvothermally utilizing four different trianionic carboxylates (H3BHTC = biphenyl-3,4',5-tricarboxylic acid, H3NTB = 4,4',4″-nitrilotribenzoic acid, H3BTB = 4,4',4″-benzene-1,3,5-triyl-tris(benzoic acid), H3BTE = 4,4',4″-(1,3,5-benzenetriyltri-2,1-ethynediyl)trisbenzoic acid). The influence of the ligand architecture, the pH, the stoichiometry, the nature of the metal, and the concentration on the structure and dimensionality of the final actinide assembly is discussed. The H3BHTC ligand allowed the synthesis of a cationic three-dimensional (3D) framework [U(BHTC)(DMF)3]I (1), which is the first example of a cationic U(IV) polymer. The H3NTB ligand yielded the 3D neutral polymer [U3(NTB)4] (2) or the two-dimensional (2D) cationic polymer [U(NTB)(NMP)3]I (3), depending on the solvent. When conditions leading to (2) were used with a Th(IV) precursor, the 2D neutral polymer [Th(NTB)(DMF)3Cl] (4) was obtained. The ligand H3BTB allowed the synthesis of two 3D cationic networks [U(BTB)(DMF)2]I (5) and [U(BTB)(DMF)3]I (7) or the neutral 3D analogue [U3(BTB)4] (6), depending on the precursor's oxidation state and the acidity of the reaction mixture. The ligand H3BTE allowed the synthesis of the anionic 3D [(CH3)2NH2][U2(BTE)3] (8) framework featuring large accessible pores, and under the same conditions, an isostructural Th(IV) was also obtained [(CH3)2NH2][Th2(BTE)3] (8-Th). All isolated coordination polymers were characterized by single-crystal X-ray diffraction (SCXRD). The Langmuir surface areas of the U(IV) polymers (2), (7), and (8) increased from 140 to 310 m2/g owing to the increasing size of the linker, with polymer (8) showing a value that is comparable to the highest surface area reported to date. The effect of the postsynthetic solvent substitution was also studied, revealing a crystal-to-crystal transformation of the cationic framework (7) to the neutral framework [U(BTB)(THF)I] (7c).

A customized MOF-polymer composite for rapid gold extraction from water matrices.

With the fast-growing accumulation of electronic waste and rising demand for rare metals, it is compelling to develop technologies that can promotionally recover targeted metals, like gold, from waste, a process referred to as urban mining. Thus, there is increasing interest in the design of materials to achieve rapid, selective gold capture while maintaining high adsorption capacity, especially in complex aqueous-based matrices. Here, a highly porous metal-organic framework (MOF)-polymer composite, BUT-33-poly(para-phenylenediamine) (PpPD), is assessed for gold extraction from several matrices including river water, seawater, and leaching solutions from CPUs. BUT-33-PpPD exhibits a record-breaking extraction rate, with high Au3+ removal efficiency (>99%) within seconds (less than 45 s), a competitive capacity (1600 mg/g), high selectivity, long-term stability, and recycling ability. Furthermore, the high porosity and redox adsorption mechanism were shown to be underlying reasons for the material's excellent performance. Given the accumulation of recovered metallic gold nanoparticles inside, the material was also efficiently applied as a catalyst.

A Simple, Transition Metal Catalyst-Free Method for the Design of Complex Organic Building Blocks Used to Construct Porous Metal-Organic Frameworks.

The design of metal-organic frameworks (MOFs) having large pore sizes and volumes often requires the use of complex organic ligands, currently synthesized using costly and time-consuming palladium-catalyzed coupling chemistry. Thus, in the present work, a new strategy for ligand design is reported, where piperazine and dihydrophenazine units are used as substitutes for benzene rings, which are the basic building block of most MOF ligands. This chemistry, which is based on simple, nucleophilic aromatic substitution (SN Ar) reactions, is used for the transition metal catalyst-free construction of 21 new, carboxylate-based ligands with varying sizes, shapes, and denticity and 15 linear di- and tetra-nitriles. Moreover, to demonstrate the utility of the ligands as building blocks, 16 new structurally diverse MOFs having surface areas up to 3100 m2 g-1 were also synthesized.

Solid-state synthesis of a MOF/polymer composite for hydrodeoxygenation of vanillin.

A new solid-state method was used to introduce a furan-thiourea polymer into the pores of a MOF, Cr-BDC. Next, the activity of the new MOF-polymer composite containing Pd was assessed in the catalytic hydrodeoxygenation of vanillin, a biomass derived chemical.

3D vs. turbostratic: controlling metal-organic framework dimensionality via N-heterocyclic carbene chemistry.

Using azolium-based ligands for the construction of metal-organic frameworks (MOFs) is a viable strategy to immobilize catalytically active N-heterocyclic carbenes (NHC) or NHC-derived species inside MOF pores. Thus, in the present work, a novel copper MOF referred to as Cu-Sp5-BF4, is constructed using an imidazolinium ligand, H2Sp5-BF4, 1,3-bis(4-carboxyphenyl)-4,5-dihydro-1H-imidazole-3-ium tetrafluoroborate. The resulting framework, which offers large pore apertures, enables the post-synthetic modification of the C2 carbon on the ligand backbone with methoxide units. A combination of X-ray diffraction (XRD), solid-state nuclear magnetic resonance (ssNMR) and electron microscopy (EM), are used to show that the post-synthetic methoxide modification alters the dimensionality of the material, forming a turbostratic phase, an event that further improves the accessibility of the NHC sites promoting a second modification step that is carried out via grafting iridium to the NHC. A combination of X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) methods are used to shed light on the iridium speciation, and the catalytic activity of the Ir-NHC containing MOF is demonstrated using a model reaction, stilbene hydrogenation.

Hybridization of Synthetic Humins with a Metal-Organic Framework for Precious Metal Recovery and Reuse.

The number of synthetic strategies used to functionalize MOFs with polymers is rapidly growing; this stems from the knowledge that non-native polymeric guests can significantly boost MOF performance in a number of desirable applications. The current work presents a scalable and solid-state method for MOF/polymer composite production. This simple method constitutes mixing a MOF powder, namely, Fe-BTC (BTC = 1,3,5-benzenetricarboxylate), with a biomass-derived solid monomer, 5-hydroxymethylfurfural (HMF), and subsequently heating the solids; the latter promotes both solid-state diffusion of HMF into the MOF and the formation of polymeric humin species with a high density of accessible hydroxyl functionality within the MOF pore. The resulting composite, Fe-BTC/humin, was found to selectively extract Ag+ ions from laundry wastewater. Subsequent reduction of the Ag+ species yields a novel catalyst, Fe-BTC/humin/Ag, that is able to drive the organic transformation of cinnamaldehyde in a highly selective manner. Moreover, the catalyst exhibited recyclability up to five cycles, which is in contrast to the Fe-BTC/Ag catalyst without the humin-based polymer. It is envisioned that MOF/polymer composites that are able to selectively extract precious metals from liquid waste streams can be used for the future production of sustainable catalysts; this work was aimed at demonstrating a proof of concept in this regard. Moreover, this study brings more understanding of the impact that MOFs can have on polymer functionalities. Understanding the polymer structure and how it can be manipulated will help us realize the high degree of future potential of this distinct class of composite materials.

A Two Step Postsynthetic Modification Strategy: Appending Short Chain Polyamines to Zn-NH2-BDC MOF for Enhanced CO2 Adsorption.

Functionalizing metal-organic frameworks (MOFs) with amines is a commonly used strategy to enhance their performance in CO2 capture applications. As such, in this work, a two-step strategy to covalently functionalize NH2-containing MOFs with short chain polyamines was developed. In the first step, the parent MOF, Zn4O(NH2-BDC)3, was exposed to bromoacetyl bromide (BrAcBr), which readily reacts with pendant -NH2 groups on the 2-amino-1,4-benzenedicarboxylate (NH2-BDC2-) ligand. 1H NMR of the digested MOF sample revealed that as much as 90% of the MOF ligands could be functionalized in the first step. Next, the MOF samples 60% of the ligands functionalized with acetyl bromide, Zn4O(NH2-BDC)1.2(BrAcNH-BDC)1.8, was exposed to several short chain amines including ethylenediamine (ED), diethylenetriamine (DETA), and tris(2-aminoethyl)amine (TAEA). Subsequent digested 1H NMR analysis indicated that a total of 30%, 28%, and 19% of the MOF ligands were successfully grafted to ED, DETA, and TAEA, respectively. Next, the CO2 adsorption properties of the amine grafted MOFs were studied. The best performing material, TAEA-appended-Zn4O(NH2-BDC)1.2(BrAcNH-BDC)1.8, exhibits a zero-coverage isosteric heat of CO2 adsorption of -62.5 kJ/mol, a value that is considerably higher than the one observed for the parent framework, -21 kJ/mol. Although the boosted CO2 affinity only leads to a slight increase in the CO2 adsorption capacity in the low-pressure regime (0.15 bar), which is of interest in postcombustion carbon dioxide capture, the CO2/N2 (15/85) selectivity at 313 K is 143, a value that is ∼35 times higher than the one observed for Zn4O(NH2-BDC)3, 4.1. Such enhancements are attributed to accessible primary amines, which were grafted to the MOF ligand. This hypothesis was further supported via in situ DRIFTS measurements of TAEA-Ac-Zn4O(NH2-BDC)1.2(BrAcNH-BDC)1.8 after exposure to CO2, which revealed the chemisorption of CO2 via the formation of hydrogen bonded carbamates/carbamic acid and CO2δ- species; the latter are adducts formed between CO2 and [amineH]+Br- salts that are produced during the amine grafting step.

Mechanistic Study on Thermally Induced Lattice Stiffening of ZIF-8.

The flexibility of the ZIF-8 aperture, which inhibits a molecular cutoff of 3.4 Å, can be reduced by rapid heat treatment to obtain CO2-selective membranes. However, the early stages of the structural, morphological, and chemical changes responsible for the lattice rigidification remain elusive. Herein, using ex situ and in situ experiments, we determine that a small shrinkage of the unit-cell parameter, ∼0.2%, is mainly responsible for this transformation. Systematic gas permeation studies show that one needs to achieve this shrinkage without a disproportionately large shrinkage in the grain size of the polycrystalline film to avoid the formation of cracks. We show that this condition is uniquely achieved in a short time by exposure of ZIF-8 to a mildly humid environment where lattice parameter shrinkage is accelerated by the incorporation of linker vacancy defects, while the shrinkage in grain size is limited. The water-vapor-led incorporation of linker vacancy defects takes place with an energy barrier of 123 kJ mol-1, much higher than that for the thermal degradation of ZIF-8, <80 kJ mol-1. The latter is promoted by heat treatment in a dry environment at a relatively higher temperature; however, this condition does not shrink the lattice parameters at short exposure time.

Preparation of Highly Porous Metal-Organic Framework Beads for Metal Extraction from Liquid Streams.

Metal-organic frameworks (MOFs) offer great promise in a variety of gas- and liquid-phase separations. However, the excellent performance on the lab scale hardly translates into pilot- or industrial-scale applications due to the microcrystalline nature of MOFs. Therefore, the structuring of MOFs into pellets or beads is a highly solicited and timely requirement. In this work, a general structuring method is developed for preparing MOF-polymer composite beads based on an easy polymerization strategy. This method adopts biocompatible, biodegradable poly(acrylic acid) (PAA) and sodium alginate monomers, which are cross-linked using Ca2+ ions. Also, the preparation procedure employs water and hence is nontoxic. Moreover, the universal method has been applied to 12 different structurally diverse MOFs and three MOF-based composites. To validate the applicability of the structuring method, beads consisting of a MOF composite, namely Fe-BTC/PDA, were subsequently employed for the extraction of Pb and Pd ions from real-world water samples. For example, we find that just 1 g of Fe-BTC/PDA beads is able to decontaminate >10 L of freshwater containing highly toxic lead (Pb) concentrations of 600 ppb while under continuous flow. Moreover, the beads offer one of the highest Pd capacities to date, 498 mg of Pd per gram of composite bead. Furthermore, large quantities of Pd, 7.8 wt %, can be readily concentrated inside the bead while under continuous flow, and this value can be readily increased with regenerative cycling.

Evaluation of materials for iodine and technetium immobilization through sorption and redox-driven processes.

Radioactive iodine-129 (129I) and technetium-99 (99Tc) pose a risk to groundwater due to their long half-lives, toxicity, and high environmental mobility. Based on literature reviewed in Moore et al. (2019) and Pearce et al. (2019), natural and engineered materials, including iron oxides, low-solubility sulfides, tin-based materials, bismuth-based materials, organoclays, and metal organic frameworks, were tested for potential use as a deployed technology for the treatment of 129I and 99Tc to reduce environmental mobility. Materials were evaluated with metrics including capacity for IO3- and TcO4- uptake, selectivity and long-term immobilization potential. Batch testing was used to determine IO3- and TcO4- sorption under aerobic conditions for each material in synthetic groundwater at different solution to solid ratios. Material association with IO3- and TcO4- was spatially resolved using scanning electron microscopy and X-ray microprobe mapping. The potential for redox reactions was assessed using X-ray absorption near edge structure spectroscopy. Of the materials tested, bismuth oxy(hydroxide) and ferrihydrite performed the best for IO3-. The commercial Purolite A530E anion-exchange resin outperformed all materials in its sorption capacity for TcO4-. Tin-based materials had high capacity for TcO4-, but immobilized TcO4- via reductive precipitation. Bismuth-based materials had high capacity for TcO4-, though slightly lower than the tin-based materials, but did not immobilize TcO4- by a redox-drive process, mitigating potential negative re-oxidation effects over longer time periods under oxic conditions. Cationic metal organic frameworks and polymer networks had high Tc removal capacity, with TcO4- trapped within the framework of the sorbent material. Although organoclays did not have the highest capacity for IO3- and TcO4- removal in batch experiments, they are available commercially in large quantities, are relatively low cost and have low environmental impact, so were investigated in column experiments, demonstrating scale-up and removal of IO3- and TcO4- via sorption, and reductive immobilization with iron- and sulfur-based species.

A metal-organic framework/polymer derived catalyst containing single-atom nickel species for electrocatalysis.

While metal-organic frameworks (MOF) alone offer a wide range of structural tunability, the formation of composites, through the introduction of other non-native species, like polymers, can further broaden their structure/property spectrum. Here we demonstrate that a polymer, placed inside the MOF pores, can support the collapsible MOF and help inhibit the aggregation of nickel during pyrolysis; this leads to the formation of single atom nickel species in the resulting nitrogen doped carbons, and dramatically improves the activity, CO selectivity and stability in electrochemical CO2 reduction reaction. Considering the vast number of multifarious MOFs and polymers to choose from, we believe this strategy can open up more possibilities in the field of catalyst design, and further contribute to the already expansive set of MOF applications.

Understanding How Ligand Functionalization Influences CO2 and N2 Adsorption in a Sodalite Metal-Organic Framework.

In this work, a detailed study is conducted to understand how ligand substitution influences the CO2 and N2 adsorption properties of two highly crystalline sodalite metal-organic frameworks (MOFs) known as Cu-BTT (BTT-3 = 1,3,5-benzenetristetrazolate) and Cu-BTTri (BTTri-3 = 1,3,5-benzenetristriazolate). The enthalpy of adsorption and observed adsorption capacities at a given pressure are significantly lower for Cu-BTTri compared to its tetrazole counterpart, Cu-BTT. In situ X-ray and neutron diffraction, which allow visualization of the CO2 and N2 binding sites on the internal surface of Cu-BTTri, provide insights into understanding the subtle differences. As expected, slightly elongated distances between the open Cu2+ sites and surface-bound CO2 in Cu-BTTri can be explained by the fact that the triazolate ligand is a better electron donor than the tetrazolate. The more pronounced Jahn-Teller effect in Cu-BTTri leads to weaker guest binding. The results of the aforementioned structural analysis were complemented by the prediction of the binding energies at each CO2 and N2 adsorption site by density functional theory calculations. In addition, variable temperature in situ diffraction measurements shed light on the fine structural changes of the framework and CO2 occupancies at different adsorption sites as a function of temperature. Finally, simulated breakthrough curves obtained for both sodalite MOFs demonstrate the materials' potential performance in dry postcombustion CO2 capture. The simulation, which considers both framework uptake capacity and selectivity, predicts better separation performance for Cu-BTT. The information obtained in this work highlights how ligand substitution can influence adsorption properties and hence provides further insights into the material optimization for important separations.

Preserving Porosity of Mesoporous Metal-Organic Frameworks through the Introduction of Polymer Guests.

High internal surface areas, an asset that is highly sought after in material design, has brought metal-organic frameworks (MOFs) to the forefront of materials research. In fact, a major focus in the field is on creating innovative ways to maximize MOF surface areas. Despite this, large-pore MOFs, particularly those with mesopores, continue to face problems with pore collapse upon activation. Herein, we demonstrate an easy method to inhibit this problem via the introduction of small quantities of polymer. For several mesoporous, isostructural MOFs, known as M2(NDISA) (where M = Ni2+, Co2+, Mg2+, or Zn2+), the accessible surface areas are increased dramatically, from 5 to 50 times, as the polymer effectively pins the MOFs open. Postpolymerization, the high surface areas and crystallinity are now readily maintained after heating the materials to 150 °C under vacuum. These activation conditions, which could not previously be attained due to pore collapse, also provide accessibility to high densities of open metal coordination sites. Molecular simulations are used to provide insight into the origin of instability of the M2(NDISA) series and to propose a potential mechanism for how the polymers immobilize the linkers, improving framework stability. Last, we demonstrate that the resulting MOF-polymer composites, referred to as M2(NDISA)-PDA, offer a perfect platform for the appendage/immobilization of small nanocrystals inside rendering high-performance catalysts. After decorating one of the composites with Pd (average size: 2 nm) nanocrystals, the material shows outstanding catalytic activity for Suzuki-Miyaura cross-coupling reactions.

A new post-synthetic polymerization strategy makes metal-organic frameworks more stable.

Metal-organic frameworks are of interest in a number of host-guest applications. However, their weak coordination bonding often leads to instability in aqueous environments, particularly at extreme pH, and hence, is a challenging topic in the field. In this work, a two-step, post-synthetic polymerization method is used to create a series of highly hydrophobic, stable MOF composites. The MOFs are first coated with thin layers of polydopamine from free-base dopamine under a mild oxygen atmosphere, which then undergoes a Michael addition to covalently graft hydrophobic molecules to the external MOF surface. This easy, mild post-synthetic modification is shown to significantly improve the stability of a number of structurally diverse MOFs including HKUST-1 (Cu), ZIF-67 (Co), ZIF-8 (Zn), UiO-66 (Zr), Cu-TDPAT (Cu), Mg-MOF-74 (Mg) and MIL-100 (Fe) in wet, caustic (acidic and basic) environments as determined by powder X-ray diffraction and surface area measurements.

Recent Advances of MOFs and MOF-Derived Materials in Thermally Driven Organic Transformations.

Owing to the almost boundless structural tunability, MOF and MOF-derived catalysts have recently exhibited structures of higher complexity, and hence, have demonstrated activity in a wide array of organic transformations. These reactions have a broad range of important applications ranging from pharmaceuticals to agriculture. Given the increasing number of publications in the area, this Minireview is focused on the most recent advancements in thermally driven organic transformations using both MOFs, nanoparticle@MOF (NP@MOF) composites, and several classes of MOF-derived materials. The most recent advancements made in materials design and the utility of these materials in a broad range of reactions are discussed.

An In-Situ Neutron Diffraction and DFT Study of Hydrogen Adsorption in a Sodalite-Type Metal-Organic Framework, Cu-BTTri.

Herein we present a detailed study of the hydrogen adsorption properties of Cu-BTTri, a robust crystalline metal-organic framework containing open metal-coordination sites. Diffraction techniques, carried out on the activated framework, reveal a structure that is different from what was previously reported. Further, combining standard hydrogen adsorption measurements with in-situ neutron diffraction techniques provides molecular level insight into the hydrogen adsorption process. The diffraction experiments unveil the location of four D2 adsorption sites in Cu-BTTri and shed light on the structural features that promote hydrogen adsorption in this material. Density functional theory (DFT), used to predict the location and strength of binding sites, corroborate the experimental findings. By decomposing binding energies in different sites in various energetic contributions, we show that van der Waals interactions play a crucial role, suggesting a possible route to enhancing the binding energy around open metal coordination sites.