Rational Design Of Metal-organic Frameworks Research Articles

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

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

Ru-modulated morphology and electronic structure of nickel organic framework bifunctional electrocatalyst for efficient overall water splitting

Metal-organic frameworks (MOFs) are considered as prospective electrocatalysts for water splitting due to their adjustable pore structure and abundant active sites. However, the rational design of MOFs with excellent oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) performances is still an enormous challenge. Herein, Ru-doped nickel thiophene dicarboxylic acid (Ni-TDA)/nickel foam (NF) were prepared via a one-pot solvothermal method, and their morphologies and electronic structures can be facilely modulated by Ru doping contents. The as-prepared self-supporting Ni5Ru-TDA/NF with Ni/Ru molar ratio of 5:1 show the excellent OER and HER activities with low overpotentials of 235 and 35 mV at a current density of 10 mA cm−2 in 1 M KOH, respectively. Furthermore, Ni5Ru-TDA/NF indicate the remarkable stability during the process of OER and HER with the slight decrease after the stability test of 60 h. The excellent electrocatalytic activities can be attributed to high intrinsic activity of multiple Ru species, the synergistic effect of Ru and Ni sites and abundance active sites in Ru-doped Ni-TDA. The water electrolysis system using Ni5Ru-TDA/NF as the anode and cathode, respectively, can achieve a current density of 10 mA cm−2 with a cell voltage of only 1.53 V, which is promising for practical applications.

DigiMOF: A Database of Metal-Organic Framework Synthesis Information Generated via Text Mining.

The vastness of materials space, particularly that which is concerned with metal-organic frameworks (MOFs), creates the critical problem of performing efficient identification of promising materials for specific applications. Although high-throughput computational approaches, including the use of machine learning, have been useful in rapid screening and rational design of MOFs, they tend to neglect descriptors related to their synthesis. One way to improve the efficiency of MOF discovery is to data-mine published MOF papers to extract the materials informatics knowledge contained within journal articles. Here, by adapting the chemistry-aware natural language processing tool, ChemDataExtractor (CDE), we generated an open-source database of MOFs focused on their synthetic properties: the DigiMOF database. Using the CDE web scraping package alongside the Cambridge Structural Database (CSD) MOF subset, we automatically downloaded 43,281 unique MOF journal articles, extracted 15,501 unique MOF materials, and text-mined over 52,680 associated properties including the synthesis method, solvent, organic linker, metal precursor, and topology. Additionally, we developed an alternative data extraction technique to obtain and transform the chemical names assigned to each CSD entry in order to determine linker types for each structure in the CSD MOF subset. This data enabled us to match MOFs to a list of known linkers provided by Tokyo Chemical Industry UK Ltd. (TCI) and analyze the cost of these important chemicals. This centralized, structured database reveals the MOF synthetic data embedded within thousands of MOF publications and contains further topology, metal type, accessible surface area, largest cavity diameter, pore limiting diameter, open metal sites, and density calculations for all 3D MOFs in the CSD MOF subset. The DigiMOF database and associated software are publicly available for other researchers to rapidly search for MOFs with specific properties, conduct further analysis of alternative MOF production pathways, and create additional parsers to search for additional desirable properties.

Exploring the Isoreticular Continuum between Phosphonate- and Phosphinate-Based Metal-Organic Frameworks.

The rational design of metal-organic frameworks (MOFs) is one of the driving forces behind the great success that this class of materials is experiencing. The so-called isoreticular approach is a key design tool, very often used to tune the size, steric properties, and additional functional groups of the linker used. In this work, we go one step further and show that even linkers with two different coordinating groups, namely, phosphonate and phosphinate, can form isoreticular MOFs. This effectively bridges the gap between MOFs utilizing phosphinate and phosphonate coordinating groups. Using a novel bifunctional ligand, 4-[hydroxy(methyl)phosphoryl]phenylphosphonic acid [H3PPP(Me)], we were able to prepare ICR-12, a MOF isoreticular to already published MOFs containing bisphosphinate linkers (e.g., ICR-4). An isostructural MOF ICR-13 was also successfully prepared using 1,4-benzenediphosphonic acid. We envisage that this strategy can be used to further enlarge the pool of MOFs.

Rational Design of Metal-Organic Frameworks for Electroreduction of CO2 to Hydrocarbons and Carbon Oxygenates.

Since CO2 can be reutilized by using renewable electricity in form of product diversity, electrochemical CO2 reduction (ECR) is expected to be a burgeoning strategy to tackle environmental problems and the energy crisis. Nevertheless, owing to the limited selectivity and reaction efficiency for a single component product, ECR is still far from a large-scale application. Therefore, designing high performance electrocatalysts is the key objective in CO2 conversion and utilization. Unlike most other types of electrocatalysts, metal-organic frameworks (MOFs) have clear, designable, and tunable catalytic active sites and chemical microenvironments, which are highly conducive to establish a clear structure-performance relationship and guide the further design of high-performance electrocatalysts. This Outlook concisely and critically discusses the rational design strategies of MOF catalysts for ECR in terms of reaction selectivity, current density, and catalyst stability, and outlines the prospects for the development of MOF electrocatalysts and industrial applications. In the future, more efforts should be devoted to designing MOF structures with high stability and electronic conductivity besides high activity and selectivity, as well as to develop efficient electrolytic devices suitable for MOF catalysts.

Ligand-Assisted Controllable Growth of Self-Supporting Ultrathin Two-Dimensional Metal-Organic Framework Nanosheet Electrodes for an Efficient Oxygen Evolution Reaction.

Rational design of metal-organic frameworks (MOFs) into ultrathin two-dimensional (2D) nanosheets with controllable thickness is significantly attractive but is also a significant challenge. Herein, the authors report, for the first time, the synthesis of ultrathin 2D nickel-based MOF nanosheets with a thickness of only about 2 nm via a ligand-assisted controllable growth strategy, which cannot be acquired from the exfoliation of their bulky counterparts or the conventional hydrothermal method. The correlation between 2D nanosheets and crystal growth preference was demonstrated through a judicious choice of a specific [Ni(BIP)(p-BDC)(H2O)2]n framework (BIP = (3,5-bis(1-imidazoly)pyridine), p-H2BDC = terephthalic acid) to underlie the geometry of the resultant morphology. Under the modulation by the dosage of terephthalic acid through a corrosion-dissolution-coordination process, the nanosheets of Ni-MOFs with a controllable thickness can be tuned to 50 and 100 nm. Ultrathin 2D Ni-MOF nanosheet-derived N-doped Ni@carbon exhibits a satisfactory electrocatalytic performance with a small overpotential of 170 mV to achieve a current density of 10 mA cm-2, much outperforming the bulk Ni-MOF and the most reported non-noble-metal oxygen evolution reaction electrocatalysts to date. It is believed that this ligand-assisted controllable growth strategy represents a novel and simple path to prepare high-performance MOF-based electrocatalysts for wide applications.

Significantly enhanced electrochemical performance of 2D Ni-MOF by carbon quantum dot for high-performance supercapacitors

Rational design of metal-organic framework (MOF) materials with rich active sites and high conductivity is very important for their applications in the energy storage devices, but it is still a great challenge. Herein, carbon quantum dot (CQD)/NiMOF composites are synthesized via a one-pot hydrothermal method. The experimental results show that CQDs can effectively enhance the electrical conductivity of NiMOF and regulate its morphology to expose abundant active sites, which are beneficial for electrochemical performance. The as-prepared CQD@MOF composite delivers a specific capacitance of 2457.9 F g−1 at 1 A g−1, and the outstanding cycling stability with the capacitance retention of 84.5% after 5000 charge/discharge cycles. Moreover, the hybrid supercapacitors with activated carbon (AC) as the negative electrode and CQD@MOF as the positive electrode exhibits a wide voltage window of 1.7 V and a high energy density of 67.1 Wh kg−1 at a power density of 5.1 kW kg−1. The results can effectively boost the electrochemical performance of MOF and promote its application in supercapacitor electrode materials.

Development of soluble UiO-66 to improve photocatalytic CO2 reduction

The conversion of CO2 to valuable chemicals by photocatalysis has attracted widespread interesting. Owing to the linear structure of CO2 molecule with strong CO bond, the surface reaction is considered as one of the main issues to restrict the conversion efficiency. Herein, soluble UiO-66 has been developed as co-catalyst to enhance the photocatalytic performance for CO2 reduction. The soluble UiO-66 has presented the performance ca. 1.5 higher than the insoluble version, yielding 30.6 µmol/h for CO2 to CO conversion by using 5.0 mg sample. This improvement can be attributed to the preserved framework by soluble UiO-66. Therefore, the chemisorption of CO2 molecules can be improved, boosting the reduction reaction for CO production. This research provides a new path for a rational design of metal-organic framework (MOF) for photoredox catalysis, meanwhile inspiring advanced application of MOF catalysis with the soluble manner.

Metal-Organic Frameworks and Metal-Organic Gels for Oxygen Electrocatalysis: Structural and Compositional Considerations.

Increasing demand for sustainable and clean energy is calling for the next-generation energy conversion and storage technologies such as fuel cells, water electrolyzers, CO2 /N2 reduction electrolyzers, metal-air batteries, etc. All these electrochemical processes involve oxygen electrocatalysis. Boosting the intrinsic activity and the active-site density through rational design of metal-organic frameworks (MOFs) and metal-organic gels (MOGs) as precursors represents a new approach toward improving oxygen electrocatalysis efficiency. MOFs/MOGs afford a broad selection of combinations between metal nodes and organic linkers and are known to produce electrocatalysts with high surface areas, variable porosity, and excellent activity after pyrolysis. Some recent studies on MOFs/MOGs for oxygen electrocatalysis and their new perspectives in synthesis, characterization, and performance are discussed. New insights on the structural and compositional design in MOF/MOG-derived oxygen electrocatalysts are summarized. Critical challenges and future research directions are also outlined.

Proton conductivity as a function of the metal center in porphyrinylphosphonate-based MOFs.

The rational design of metal-organic frameworks (MOFs) is highly important for the development of new proton conductors. Porphyrinylphosphonate-based MOFs, providing the directed tuning of physical and chemical properties of materials through the modification of a macrocycle, are potentially high-conducting systems. In this work the synthesis and characterization of novel anionic Zn-containing MOF based on palladium(ii) meso-tetrakis(3-(phosphonatophenyl))porphyrinate, IPCE-2Pd, are reported. Moreover, the proton-conductive properties and structures of two anionic Zn-containing MOFs based on previously described nickel(ii) and novel palladium(ii) porphyrinylphosphonates, IPCE-2M (M = Ni(ii) or Pd(ii)), are compared in details. The high proton conductivity of 1.0 × 10-2 S cm-1 at 75 °C and 95% relative humidity (RH) is revealed for IPCE-2Ni, while IPCE-2Pd exhibits higher hydrolytic and thermal stability of the material (up to 420 °C) simultaneously maintaining a comparable value of conductivity (8.11 × 10-3 S cm-1 at 95 °C and 95% RH). The nature of the porphyrin metal center is responsible for the features of crystal structure of materials, obtained under identical reaction conditions. The structures of IPCE-2Pd and its dehydrated derivative IPCE-2Pd-HT are determined from the synchrotron powder diffraction data. The presence of phosphonic groups in compared materials IPCE-2M affords a high concentration of proton carriers that together with the sorption of water molecules leads to a high proton conductivity.

Rational design of metal-organic frameworks to deliver methotrexate for targeted rheumatoid arthritis therapy

Methotrexate (MTX) has been used as an anchor drug for the treatment of rheumatoid arthritis (RA), while the patients with chronic MTX administration suffer from severe side-effects. To this end, targeted delivery of MTX by nanomedicine has attracted great interest. In this work, we aimed to employ metal-organic frameworks (MOFs) as nanocarrier to deliver MTX by virtue of its facile and green preparation and exceptionally high drug loading. While MTX could be easily and effectively loaded via different MOF construction strategies, such as direct coordination, physical encapsulation, and covalent conjugation, we found that most of the MTX loading MOFs showed premature and burst drug release, attributable to the unstable coordination between MTX and metals. To address this issue, we rationally designed the MOFs by conjugating MTX with tannic acid (TA) at 2:1 M ratio and then coordinating with ferric ion (Fe3+), followed by surface modification of hyaluronic acid (HA). The resulting MOFs achieved ultra-high drug loading (45%) and sustained drug release, and could selectively recognize the diseased cells for anti-inflammatory effect. The in vivo therapeutic evaluation suggested that the MOFs could enhance the anti-rheumatic activity of MTX while minimizing its toxic effects by targeted drug delivery, resulting in improved therapeutic index. This work provides a biocompatible nano-platform to deliver MTX for RA treatment, and importantly, calls for special attention to the gap between MOFs design and their biological applications, and the gap needs to be filled by careful evaluation of in vivo stability and burst drug release.

Geometry Mismatch and Reticular Chemistry: Strategies To Assemble Metal-Organic Frameworks with Non-default Topologies.

The past 20 years have witnessed tremendous advances in the field of porous materials, including the development of novel metal-organic frameworks (MOFs) that show great potential for practical applications aimed at addressing global environmental and industrial challenges. A critical tool enabling this progress has been reticular chemistry, through which researchers can design materials that exhibit highly regular (i.e., edge-transitive) topologies, based on the assembly of geometrically matched building blocks into specific nets. However, innovation sometimes demands that researchers steer away from default topologies to instead pursue unusual geometries. In this Perspective, we cover this aspect and introduce the concept of geometry mismatch, in which seemingly incompatible building blocks are combined to generate non-default structures. We describe diverse MOF assemblies built through geometry mismatch generated by use of ligand bend angles, twisted functional groups, zigzag ligands and other elements, focusing on carboxylate-based MOFs combined with common inorganic clusters. We aim to provide a fresh perspective on rational design of MOFs and to help readers understand the countless options now available to achieve greater structural complexity in MOFs.

Rational Design of Catalytic Centers in Crystalline Frameworks.

Crystalline frameworks including primarily metal organic frameworks (MOF) and covalent organic frameworks (COF) have received much attention in the field of heterogeneous catalysts recently. Beyond providing large surface area and spatial confinement, these crystalline frameworks can be designed to either directly act as or influence the catalytic sites at molecular level. This approach offers a unique advantage to gain deeper insights of structure-activity correlations in solid materials, leading to new guiding principles for rational design of advanced solid catalysts for potential important applications related to energy and fine chemical synthesis. In this review, recent key progress achieved in designing MOF- and COF-based molecular solid catalysts and the mechanistic understanding of the catalytic centers and associated reaction pathways are summarized. The state-of-the-art rational design of MOF- and COF-based solid catalysts in this review is grouped into seven different areas: (i) metalated linkers, (ii) metalated moieties anchored on linkers, (iii) organic moieties anchored on linkers, (iv) encapsulated single sites in pores, and (v) metal-mode-based active sites in MOFs. Along with this, some attention is paid to theoretical studies about the reaction mechanisms. Finally, technical challenges and possible solutions in applying these catalysts for practical applications are also presented.

Hollow Zn/Co ZIF Particles Derived from Core-Shell ZIF-67@ZIF-8 as Selective Catalyst for the Semi-Hydrogenation of Acetylene.

The rational design of metal-organic frameworks (MOFs) with hollow features and tunable porosity at the nanoscale can enhance their intrinsic properties and stimulates increasing attentions. In this Communication, we demonstrate that methanol can affect the coordination mode of ZIF-67 in the presence of Co(2+) and induces a mild phase transformation under solvothermal conditions. By applying this transformation process to the ZIF-67@ZIF-8 core-shell structures, a well-defined hollow Zn/Co ZIF rhombic dodecahedron can be obtained. The manufacturing of hollow MOFs enables us to prepare a noble metal@MOF yolk-shell composite with controlled spatial distribution and morphology. The enhanced gas storage and porous confinement that originate from the hollow interior and coating of ZIF-8 confers this unique catalyst with superior activity and selectivity toward the semi-hydrogenation of acetylene.