Doping light emitters into metal-organic frameworks

Space division with red cubes: Doping metal-organic frameworks with another metal component gives a further opportunity to tune their properties. Recent work successfully introduced europium into the inorganic nodes of frameworks. Although the doping element does not affect the framework topology, highly improved emissive performance was measured thanks to the intrinsic red emission of europium.     

Copper-based metal-organic frameworks for electrochemical reduction of CO2

The electrochemical CO₂ reduction reaction (CO₂ER) is an emerging process that involves utilizing CO₂ to produce valuable chemicals and fuels by consuming excess electricity from renewable sources. Recently, Cu and Cu-based nanoparticles, as earth-abundant and economical metal sources, have been attracting significant interest. The chemical and physical properties of Cu-based nanoparticles are modified by different strategies, and CO₂ can be converted into multicarbon products. Among various Cu-based nanoparticles, Cu-based metal-organic frameworks (MOFs) are gaining increasing interest in the field of catalysis because of their textural, topological, and electrocatalytic properties. In this minireview, we summarized and highlighted the main achievements in the research on Cu-based MOFs and their advantages in the CO₂ER as electrocatalysts, supports, or precursors.     

Preparation,structure and photocatalysis of metal-organic frameworks derived from aromatic carboxylate and imidazole-based ligands

Three 3-D metal-organic frameworks (MOFs), [Cd(NDC)(biim-4)]·0.5H₂O (1), [Cd₂(TDC)₂(biim-4)₂(H₂O)₂] (2) and [Zn₂(biim-4)₂(TDC)₂]·2.5H₂O (3) (biim-4 = 1,4-bis(1-imidazolyl) butyric alkyl; H₂NDC = 1,4-naphthalene dicarboxylic acid; H₂TDC = thiophene-2,5-dicarboxylic acid), have been synthesized under hydrothermal conditions and structurally characterized by single X-ray diffraction. The three MOFs have high photocatalytic degradation effects on methyl orange under UV irradiation. Through electronic structure analysis combined with time-dependent density functional theory calculations, catalytic performances of these materials are correlated with the molecular composition and the optoelectronic properties of the samples.     

Molecular simulation of carbon dioxide/methane/hydrogen mixture adsorption in metal-organic frameworks

This work performs a systematic computational study toward a molecular understanding of the separation characteristics of metal-organic frameworks (MOFs), for which the purification of synthetic gas by two representative MOFs, MOF-5 and Cu-BTC, is adopted as an example. The simulations show that both geometry and pore size affect largely the separation efficiency, complex selectivity behaviors with different steps can occur in MOFs, and the electrostatic interactions that exist can enhance greatly the separation efficiency of gas mixtures composed of components with different chemistries. Furthermore, the macroscopic separation behaviors of the MOF materials are elucidated at a molecular level to give insight into the underlying mechanisms. The findings as well as the molecular-level elucidations provide useful microscopic information toward a complete understanding of the separation characteristics of MOFs that may lead to general design strategies for synthesizing new MOFs with tailored properties, as well as guiding their practical applications.     

Carbon dioxide capture-related gas adsorption and separation in metal-organic frameworks

Reducing anthropogenic CO₂ emission and lowering the concentration of greenhouse gases in the atmosphere has quickly become one of the most urgent environmental issues of our age. Carbon capture and storage (CCS) is one option for reducing these harmful CO₂ emissions. While a variety of technologies and methods have been developed, the separation of CO₂ from gas streams is still a critical issue. Apart from establishing new techniques, the exploration of capture materials with high separation performance and low capital cost are of paramount importance. Metal-organic frameworks (MOFs), a new class of crystalline porous materials constructed by metal-containing nodes bonded to organic bridging ligands hold great potential as adsorbents or membrane materials in gas separation. In this paper, we review the research progress (from experimental results to molecular simulations) in MOFs for CO₂ adsorption, storage, and separations (adsorptive separation and membrane-based separation) that are directly related to CO₂ capture.     

Conductive phthalocyanine-based metal-organic framework as a highly efficient electrocatalyst for carbon dioxide reduction reaction

Porous crystalline metal-organic frameworks(MOFs) are one class of promising electrode materials for CO_2 electroreduction reaction(CO_2 RR) by virtue of their large CO_2 adsorption capacities and abundant tunable active sites, but their insulating nature usually leads to low current density. Herein, a two-dimensional(2 D) Ni-phthalocyanine-based MOF(Ni Pc-Ni(NH)_4) constructed by 2,3,9,10,16,17,23,24-octaaminophthalocyaninato nickel(II)(Ni Pc-(NH_2)_8) and nickel(II) ions attained high electrical conductivity due to the high overlap of d-π conjugation orbitals between the nickel node and the Ni-phthalocyanine-substituted o-phenylenediamine. During CO_2 RR, the Ni Pc-Ni(NH)_4 nanosheets achieved a high CO Faradaic efficiency of 96.4% at -0.7 V and a large CO partial current density of 24.8 m A cm~(-2) at-1.1 V, which surpassed all the reported two-dimensional MOF electrocatalysts evaluated in an H-cell. The control experiments and density functional theory(DFT) calculations suggested that the Ni-N_4 units of the phthalocyanine ring are the catalytic active sites. This work provides a new route to the design of highly efficient porous framework materials for the enhanced electrocatalysis via improving electrical conductivity.     

Phenothiazine-based covalent organic frameworks with low exciton binding energies for photocatalysis

Designing delocalized excitons with low binding energy (E_b) in organic semiconductors is urgently required for efficient photochemistry because the excitons in most organic materials are localized with a high E_b of >300 meV. In this work, we report the achievement of a low E_b of ∼50 meV by constructing phenothiazine-based covalent organic frameworks (COFs) with inherent crystallinity, porosity, chemical robustness, and feasibility of bandgap engineering. The low E_b facilitates effective exciton dissociation and thus promotes photocatalysis by using these COFs. As a demonstration, we subject these COFs to photocatalytic polymerization to synthesize polymers with remarkably high molecular weight without any requirement of the metal catalyst. Our results can facilitate the rational design of porous materials with low E_b for efficient photocatalysis.

We report the construction of phenothiazine-based covalent organic frameworks, which exhibited diverse structures, the feasibility of bandgap engineering, and unprecedented ultralow exciton binding energy of ∼50 meV for photocatalytic polymerization.     

Linker engineering in metal–organic frameworks for dark photocatalysis

Dark reactions featuring continuous activity under light off conditions play a critical role in natural photosynthesis. However, most artificial photocatalysts are inactive upon the removal of the light source, and the artificial photocatalysts with dark photocatalysis abilities have been rarely explored. Herein, we report a Ti-based metal–organic framework (MOF), MIL-125, exhibiting the capability of dark photocatalytic hydrogen production. Remarkably, the introduction of different functional groups onto the linkers enables distinctly different activities of the resulting MOFs (MIL-125-X, X = NH₂, NO₂, Br). Dynamic and thermodynamic investigations indicate that the production and lifetime of the Ti³⁺ intermediate are the key factors, due to the electron-donating/-withdrawing effect of the functional groups. As far as we know, this is the first report on dark photocatalysis over MOFs, providing new insights into the storage of irradiation energy and demonstrating their great potential in dark photocatalysis due to the great MOF diversity.

A Ti-based MOF with long-lived Ti³⁺ can achieve dark photocatalysis. The different groups on the organic linker modulate electron storage ability and the lifetime of Ti³⁺, significantly regulating dark photocatalytic activity in H₂ production.     

Metal-organic frameworks for electrochemical reduction of carbon dioxide: The role of metal centers

Direct electrochemical reduction of CO_2 into valuable chemicals and fuel is one of the most promising approaches to address the current energy crisis and lower CO_2 emission. Recently, numerous metal-organic framework(MOF) and their derived materials have extensively been developed as electrocatalysts for CO_2 reduction owing to their unique structure including porosity, large specific surface area, and tunable chemical structures. In this review, the recent progress of MOF-based electrocatalysts for CO_2 reduction was summarized and discussed. Detailed discussions mainly focus on the synthesis and mechanism of pristine MOFs and MOF-derived materials for electrocatalytic CO_2 reduction. These examples are expected to provide clues to rational design and synthesis of stable and high-performance MOFs-based electrocatalysts for CO_2 reduction.     

Atmospheric-pressure, liquid-phase, selective aerobic oxidation of alkanes catalysed by metal-organic frameworks

Aerobic oxidation of cyclooctane to its corresponding ol/one mixture at atmospheric pressure and in the liquid phase is efficiently promoted by an Fe(BTC) (BTC=1,3,5-benzenetricarboxylate) metal-organic framework, incorporating N-hydroxyphthalimide and in several cases reaches a selectivity over 90% at 28% conversion. This catalytic system is further extended to other hydrocarbons, such as ethylbenzene and 1,2,3,4-tetrahydronaphthalene (tetralin), with high selectivity (>85%). This high selectivity in the product distribution arises from a radical reaction mechanism that occurs inside a hydrophobic cavity that preferentially adsorbs hydrocarbons over their corresponding alcohols. The system can be reused although there is a gradual decrease in turnover frequency, caused by minor changes in the crystal structure due to the formation of iron oxide nanoparticles. Given the sustainable nature of the oxidant and the mild conditions used, this discovery could serve to develop a new catalyst generation for the oxyfunctionalisation of hydrocarbon feedstocks with the real possibility of finding industrial application.     

The application of metal-organic frameworks in electrocatalytic nitrogen reduction

In recent years,the research of nitrogen reduction reaction(NRR) under ambient conditions has attracted wide attention for their relatively low energy consumption,in which rational design of electrocatalysts is the key to achieve high-performance NRR.Metal-organic frameworks(MOFs),as a new kind of porous material,have been intensively studied in the past few decades owing to not only their structural versatility and tunability but also intrinsic porosity.Due to their structural features,MOFs als...     

Highly ordered macroporous dual-element-doped carbon from metal–organic frameworks for catalyzing oxygen reduction

Multiple heteroatom-doped carbons with 3D ordered macro/meso-microporous structures have not been realized by simple carbonization of metal–organic frameworks (MOFs). Herein, ordered macroporous phosphorus- and nitrogen-doped carbon (M-PNC) is prepared successfully by carbonization of double-solvent-induced MOF/polystyrene sphere (PS) precursors accompanied with spontaneous removal of the PS template, followed by post-doping. M-PNC shows a high specific surface area of 837 m² g⁻¹, nitrogen doping of 3.17 at%, and phosphorus doping of 1.12 at%. Thanks to the hierarchical structure, high specific surface area, and multiple heteroatom-doping, M-PNC exhibits unusual catalytic activity as an electrocatalyst for the oxygen reduction reaction. Computational calculation reveals that the P O group helps stabilize the adsorption of intermediates, and the position of P O relative to graphitic N significantly improves the activity of the adjacent carbons for electrocatalysis.

Multiple heteroatoms-doped carbon with 3D ordered macroporous structures, which showing outstanding catalytic activity for oxygen reduction, was prepared by carbonization of double-solvent-induced MOF/polystyrene sphere accompanied with post-doping.     

金属有机骨架及其衍生材料在电解水和锌-空气电池中的应用

电解水和锌-空气电池(ZABs)技术为解决能源危机、实现碳中和目标开辟了一条新的途径。然而,这些技术的实际应用在很大程度上受到析氢反应(HER)、析氧反应(OER)以及氧还原反应(ORR)缓慢动力学的限制。因此,迫切需要开发高效、稳定的电催化剂有效降低反应过电位,加快电催化反应进程。金属有机骨架(MOFs)由于其灵活可调的组成和精确可控的结构,已成为催化领域研究最广泛的材料之一。本文聚焦于MOFs基电催化剂的制备策略和结构特性,主要介绍它们在电解水和ZABs方面近期的研究进展,并对该领域存在的问题和发展趋势进行了总结和展望。     

A cofacial metal–organic framework based photocathode for carbon dioxide reduction

Innovative and robust photosensitisation materials play a cardinal role in advancing the combined effort towards efficient solar energy harvesting. Here, we demonstrate the photocathode functionality of a Metal–Organic Framework (MOF) featuring cofacial pairs of photo- and electro-active 1,4,5,8-naphthalenediimide (NDI) ligands, which was successfully applied to markedly reduce the overpotential required for CO₂ reduction to CO by a well-known rhenium molecular electrocatalyst. Reduction of [Cd(DPNDI)(TDC)]_n (DPNDI = N,N′-di(4-pyridyl)-1,4,5,8-naphthalenediimide, H₂TDC = thiophene-2,5-dicarboxylic acid) to its mixed-valence state induces through-space Intervalence Charge Transfer (IVCT) within cofacial DPNDI units. Irradiation of the mixed-valence MOF in the visible region generates a DPNDI photoexcited radical monoanion state, which is stabilised as a persistent species by the inherent IVCT interactions and has been rationalised using Density Functional Theory (DFT). This photoexcited radical monoanion state was able to undergo charge transfer (CT) reduction of the rhenium molecular electrocatalyst to effect CO generation at a lower overpotential than that required by the discrete electrocatalyst itself. The exploitation of cofacial MOFs opens new directions for the design philosophy behind light harvesting materials.

The photocathode functionality of a Metal–Organic Framework (MOF) featuring cofacial photo- and electro-active ligands provides a new approach to CO₂ reduction via charge transfer with a rhenium electrocatalyst.

The development of photocathode materials for CO₂ reduction and hydrogen evolution catalyses has traditionally focussed on photosensitising transition metal complexes or nanostructured solid state semiconductors.^1,2 At the nascent frontier between robust solid state semiconductors and synthetically protean metal complexes are photo-/electro-active Metal–Organic Frameworks (MOFs) that consolidate the flexibility of homogeneous systems into the robust heterogeneous phase.³ Contrasting with reported MOF examples, natural photosynthesis remains one of the most efficient light harvesting systems.⁴ One common reaction centre adopted in photosynthesis features a redox-active cofacial dimer of chlorophyll pigment molecules.⁵ This cofacial moiety stabilises the photoexcited charge separated state through intra-dimer Intervalence Charge Transfer (IVCT) interactions, enabling the trapping and conversion of light to chemical energy. Recently, we characterised IVCT interactions upon reduction to the mixed-valence state in the MOF [Zn₂(TDC)₂(DPPTzTz)₂]_n (DPPTzTz = 2,5-bis(4-(4-pyridyl)phenyl)thiazolo[5,4-d]thiazole and H₂TDC = thiophene-2,5-dicarboxylic acid) featuring cofacial dimers of the thiazolothiazole redox-active core, and probed its structure–activity dependence computationally and experimentally.^6–9 Subsequently, we sought design a new MOF featuring cofacial pairs of the photo- and redox-active N,N′-di(4-pyridyl)-1,4,5,8-naphthalenediimide (DPNDI) ligand, as a conceptually neoteric photosensitiser for incorporation into systems relevant towards artificial photosynthesis.The naphthalene diimide (NDI) core was selected for its photoactive radical monoanion state.¹⁰ For a number of discrete systems, Wasielewski and coworkers have computationally and experimentally demonstrated the ability to photoexcite the easily accessible NDI radical monoanion using visible light, facilitating its transient photoelectrochemical reduction of Re based catalytic CO₂ reduction sites.^2,11–14 Recently, Goswami et al. synthesised a Zr NDI-based MOF, applying this as a radical state heterogeneous photosensitiser to decompose dichloromethane.¹⁵Here, we describe the synthesis of a new photo- and redox-active MOF [Cd(DPNDI)(TDC)]_n, denoted csiMOF-6 (cofacial stacked IVCT), featuring cofacial dimers of the DPNDI ligand. Cofacial DPNDI MOFs have been reported previously by Takashima et al.¹⁶ and Sikdar et al.,¹⁷ where guest dependent charge transfer (CT) and neutral state photoexcitation behaviours were examined. Dinolfo et al. also incorporated DPNDI into a rhenium based cofacial complex, where its mixed-valence IVCT behaviour was probed using electrochemical and spectroelectrochemical (SEC) techniques.¹⁸ We envisaged that the cofacial NDI units in csiMOF-6 would stabilise its photoexcited radical monoanion state by IVCT interactions, akin to cofacial moieties in natural photosynthsesis processes. This strengthens the persistence of the NDI photoexcited radical monoanion state, thereby improving its efficacy at photoelectrochemical reduction of catalytically active sites. Effectiveness of the cofacial design principle behind csiMOF-6 photocathodes was verified using a combined experimental and computational approach. The successful photocathode performance of csiMOF-6 under broad band visible light irradiation encompassed its photoelectrochemical reduction of the [Re(bipy-tBu)(CO)₃Cl] (bipy-tBu = 4,4′-di-tert-butyl-2,2′-bipyridine, developed by Smieja et al.¹⁹) CO₂ reduction electrocatalyst, resulting in CO generation at reduced overpotential requirements.     

MoS_3 loaded TiO_2 nanoplates for photocatalytic water and carbon dioxide reduction

Photocatalytic water splitting and carbon dioxide reduction provide us clean and sustainable energy resources. The carbon dioxide reduction is also the redemption of the greenhouse effect. MoS_3/TiO_2 photocatalysts based on TiO_2 nanoplates have been synthesized via a hydrothermal acidification route for water and carbon dioxide reduction reactions. This facile approach generates well dispersed Mo S3 with low crystallinity on the surface of TiO_2 nanoplates. The as-synthesized MoS_3/TiO_2 photocatalyst showed considerable activity for both water reduction and carbon dioxide reduction. The thermal treatment effects of TiO_2 , the loading percentage of MoS_3 and the crystalline phase of TiO_2 have been investigated towards the photocatalytic performance. TiO_2 nanoplate synthesized through hydrothermal reaction with the presence of HF acid is an ideal semiconductor material for the loading of MoS_3 for photocatalytic water and carbon dioxide reduction simultaneously in EDTA sacrificial solution.