ReviewFunctionalization of covalent organic frameworks by metal modification: Construction, properties and applications
Graphical abstract
Introduction
Covalent organic frameworks (COFs) and metal organic frameworks (MOFs) have often been compared owing to their unique properties and similar crystal configurations. MOFs have porous structures with functional diversity of organic ligands and metal ions, but the unstable skeletons linked by coordination bonds limit their further development [1], [2]. While COFs, first synthesized by Yaghi’s group in 2005 [3], exhibit highly ordered structures through reversible reactions, which allow accurate integration of atoms in organic building units to generate pre-designed nano-channels and frameworks. They not only contain the main merits of MOFs (large surface areas, regular pore channels and rigid structures), but also have relatively low densities owing to the construction of light-organic elements (B, C, O, N and Si) [4]. Particularly, the COF skeletons linked by strong covalent bonds have higher stability [5], [6].
Based on the different geometries of the building units, COFs can be classified into two-dimensional (2D) types and three-dimensional (3D) types. The ordered π systems with columnar stacking structures of 2D COFs can facilitate substances transport and charges transfer for catalysis [7]. Moreover, the exposure of potential catalytic positions on COFs is helpful to understand the mechanism of catalyzing chemical reactions [8]. On the other hand, through sp3 hybridization of C or Si atoms in building units, COFs expand their skeletons to 3D types with large number of open sites available for modification. The high surface areas (up to 4210 m2 g−1) and low densities (low to 0.17 g cm−3) of 3D COFs are promising for gas adsorption [9].
However, the combination of light gases with COFs is mainly controlled by the van der Waals (vdW) force and dispersion interaction, which means that the interaction energies between gas molecules with COF skeletons are weak, limiting their adsorption performance at ambient temperature [10]. In addition, the catalytic activity and selectivity of COFs are often disappointing under harsh conditions due to the shortage of active sites [11]. Hence, in order to strengthen the practical applications of COFs, it is necessary to overcome such disadvantages. As one of the most striking features of COFs, modifiability can alter their structural properties, consolidate the stability, and broaden their applications [12], [13], [14]. Up to now, plenty of COFs with various functionalities have been explored through different modification strategies. The channel-wall modification strategy anchors functional groups (alkyl chains, carboxylic acid, ester group, etc.) into COF channels by addition reactions or substitution reactions [15], [16]. The skeleton modification strategy, including the conversion of linkages and the exchange of building units, is often applied to improve the crystallinity and stability of COFs [17], [18]. In addition, some COFs-based composites have also been developed to endow COFs with distinctive functions through integrating COFs with MOFs [19], enzymes [20], quantum dots [21] or other functional materials [22], [23]. Among them, the metal modification strategy which utilizes strong coordination reactions of organic building units containing vast heteroatoms with metal species (atoms, ions and nanoparticles) to incorporate metal active sites into COFs, is considered to be an efficient strategy for COF functionalization [24], [25], [26]. It balances the properties between COFs and MOFs with strong covalent bonds and open metal sites (Fig. 1), resulting in new specific applications for the metal-modified COFs.
The presence of metals will cause the gas molecules to be polarized and then to be adsorbed through electrostatic binding force, which is stronger than the vdW force and thus improving the adsorption capacity [27], [28]. Due to the existence of empty d-electron orbitals and small energy level spacing, transition metals can form the coordination bonds with reactants to participate in the catalytic reactions [29], [30], [31]. However, bare metals with high surface energy often tend to agglomerate and resulting in the decrease or disappearance of metal activity, the COFs can be the potential support materials for dispersing and immobilizing metal species without loss of adsorption and catalytic performance. Furthermore, in view of the diversity of COF structures, a variety of metal sites can be integrated into COFs and maintain stability in different chemical systems.
Due to the flexibility of design and the diversity of modification strategies, increasing instructive reviews of COFs have been conducted. Chen and co-workers reported the recent advances in synthetic strategies and procedures toward high-quality COF crystallites and films [32]. Jiang and co-workers summarized the mechanisms of COF functional design by emphasizing structure–function correlations [33]. Li and co-workers published a review on the design and construction of functionalized COFs and their applications to sensing [34]. Other reviews mainly focused on the structural pre-design [35], morphologic control [36] and promising performance of COFs [37]. As one of the most accessible modification approaches of COFs, however, a relatively comprehensive discussion on metal modification strategy and the applications of metallized COFs are rarely mentioned. The purpose of this review article is not to reorganize the work of previous reviews but to provide a base and rapid update for researchers in the field of COF metallization. Herein, we review these progresses and discuss the pristine properties and characterization techniques of COFs to fully recognize these charming materials. Particular attentions are given to the construction and properties of metal-modified COFs in view of verifying the feasibility of modification. Moreover, the applications of hybrids in gas adsorption and catalysis are presented, from theoretical simulations to experimental verifications.
Section snippets
Properties of pristine COFs
Utilizing the principle of dynamic organic chemistry which is controlled by thermodynamics and thus forms reversible reactions with “error checking” and “self-correction”, the construction of COFs is accompanied with the crystallization, and the checking-correction process suppresses the occurrence of structural defects and facilitates the formation of ordered COF structures [38], [39]. A COF framework consists of two modules: linkers (building units) and linkages (bonds formed between building
Characterization techniques
The characterization of COFs relies on various analytical and spectroscopic techniques, which provide pivotal information on structure, morphology and porosity. Since the building units of COFs maintain geometric regularity during assembly, their crystalline quality and structural parameters are widely performed by powder X-ray diffraction (PXRD). Through comparing the intensity of experimental and simulated diffraction peaks, the crystallinity of COFs can be simply judged. In addition, under
Construction of metal-modified COFs
Generally, the metallization of COFs can be performed by the bottom-up method and the post-synthetic method. In the bottom-up method, the building units are pre-modified with metal precursor during the synthesis of COFs. In the post-synthetic method, the metal precursor is incorporated or attached to the pore surface of the established frameworks. However, detailed experimental screenings for the design of optimal modification options are not simple. Theoretical simulations provide feasible
Properties of metal-modified COFs
In order to verify the feasibility of the metal modification strategy, three problems ought to be considered. First, metals prefer to occupy which positions in COF skeletons and whether they will form clusters. Second, whether the binding energies between metals and COFs are large enough for stable combination. Third, whether metals incorporation will influence the structural properties of COFs. Answers to these questions will help to understand and design multifunctional metal-modified COF
Gas adsorption
A lot of experiments and theories showed that the interaction between light gases and COFs is mainly controlled by the vdW force and dispersion interaction, hindering the application in gas adsorption of these materials at ambient conditions [129]. Metal-modified COFs are outstanding platforms for gas adsorption owing to the tight bonding of metals and gas molecules. Most studies are based on multiscale theoretical approaches, for example, the first-principles calculations are used to evaluate
Conclusions and perspectives
The unique pristine properties of COFs make them become new era of organic porous materials. In this context, improving structural complexity and versatility is an important direction in the exploration of COF functionalization, which can be achieved by metal modification strategy. In this review, we analyze the development of COFs and metal/COF hybrids from different aspects.
In light of dynamic organic chemistry, the pore sizes and shapes of frameworks can be tailored by the length and
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The study was financially supported by the Program for Changjiang Scholars and Innovative Research Team in University (IRT-13R17), the National Natural Science Foundation of China (51979103, 51679085, 51378192, 51039001, 51378190, 51521006, 51508177), the Fundamental Research Funds for the Central Universities of China (531118010055), the Funds of Hunan Science and Technology Innovation Project (2018RS3115).
References (203)
- et al.
Metal or metal-containing nanoparticle@MOF nanocomposites as a promising type of photocatalyst
Coord. Chem. Rev.
(2019) - et al.
Multivariate crystalline porous materials: synthesis, property and potential application
Coord. Chem. Rev.
(2019) - et al.
A new electrochemically responsive 2D π-conjugated covalent organic framework as a high performance supercapacitor
Microporous Mesoporous Mat.
(2018) - et al.
Grafting of quantum dots on covalent organic frameworks via a reverse microemulsion for highly selective and sensitive protein optosensing
Sens. Actuator B-Chem.
(2018) - et al.
Covalent organic frameworks as heterogeneous catalysts
Chin. J. Catal.
(2018) - et al.
Lithium doping on covalent organic framework-320 for enhancing hydrogen storage at ambient temperature
J. Solid State Chem.
(2016) - et al.
Prussian blue analogue derived magnetic Cu-Fe oxide as a recyclable photo-Fenton catalyst for the efficient removal of sulfamethazine at near neutral pH values
Chem. Eng. J.
(2019) - et al.
Iron-mediated activation of persulfate and peroxymonosulfate in both homogeneous and heterogeneous ways: a review
Chem. Eng. J.
(2020) - et al.
Recent advances in the construction of functionalized covalent organic frameworks and their applications to sensing
Biosens. Bioelectron.
(2019) - et al.
Graphene-synergized 2D covalent organic framework for adsorption: a mutual promotion strategy to achieve stabilization and functionalization simultaneously
J. Hazard. Mater.
(2018)
Effects of carbon nanotubes on biodegradation of pollutants: positive or negative?
Ecotox. Environ. Safe.
Effect of surfactants on the interaction of phenol with laccase: molecular docking and molecular dynamics simulation studies
J. Hazard. Mater.
Application of molecular docking for the degradation of organic pollutants in the environmental remediation: a review
Chemosphere
DFT calculations of molecular magnetic properties of coordination compounds
Coord. Chem. Rev.
Ab initio molecular dynamics for liquid metals
J. Non-Cryst. Solids
Metal-organic framework nanosheets (MONs): a new dimension in materials chemistry
J. Mater. Chem. A
Porous, crystalline, covalent organic frameworks
Science
Covalent organic frameworks: a materials platform for structural and functional designs
Nat. Rev. Mater.
Covalent chemistry beyond molecules
J. Am. Chem. Soc.
Tuning the stacking behaviour of a 2D covalent organic framework through non-covalent interactions
Mater. Chem. Front.
Designed synthesis of 3D covalent organic frameworks
Science
Computer simulation of the adsorption of light gases in covalent organic frameworks
Langmuir
Triazine-based covalent organic polymers: design, synthesis and applications in heterogeneous catalysis
J. Mater. Chem. A
Sequential pore wall modification in a covalent organic framework for application in lactic acid adsorption
Chem. Mat.
Experimental and computational studies of pyridine-assisted post-synthesis modified air stable covalent-organic frameworks
Chem. Commun.
Metal-functionalized covalent organic frameworks as precursors of supercapacitive porous N-doped graphene
J. Mater. Chem. A
Pore surface engineering of covalent organic frameworks: structural diversity and applications
Nanoscale
Tailor-made pore surface engineering in covalent organic frameworks: systematic functionalization for performance screening
J. Am. Chem. Soc.
Chemical conversion of linkages in covalent organic frameworks
J. Am. Chem. Soc.
Toward covalent organic frameworks bearing three different kinds of pores: the strategy for construction and COF-to-COF transformation via heterogeneous linker exchange
J. Am. Chem. Soc.
Synthesis of robust MOFs@COFs porous hybrid materials via aza-Diels-Alder reaction: towards high performance supercapacitor materials
Angew. Chem.-Int. Edit.
Covalent organic frameworks with chirality enriched by biomolecules for efficient chiral separation
Angew. Chem.-Int. Edit.
Covalent organic framework photocatalysts: structures and applications
Chem. Soc. Rev.
Integrating suitable linkage of covalent organic frameworks into covalently bridged inorganic/organic hybrids toward efficient photocatalysis
J. Am. Chem. Soc.
Solvothermal in situ metal/ligand reactions: a new bridge between coordination chemistry and organic synthetic chemistry
Accounts Chem. Res.
Highly dispersed gold nanoparticles anchoring on post-modified covalent organic framework for catalytic application
Chem. Eng. J.
First-principles study of hydrogen adsorption in metal-doped COF-10
J. Chem. Phys.
Charge transfer spectra of transition metal complexes
J. Chem. Educ.
New synthetic strategies toward covalent organic frameworks
Chem. Soc. Rev.
Covalent organic frameworks: chemical approaches to designer structures and built-in functions
Angew. Chem.-Int. Edit.
Covalent organic framework electrocatalysts for clean energy conversion
Adv. Mater.
Covalent organic frameworks as a platform for multidimensional polymerization
ACS Central Sci.
Covalent organic frameworks (COFs): perspectives of industrialization
CrystEngComm
Covalent organic frameworks
Chem. Soc. Rev.
Weaving of organic threads into a crystalline covalent organic framework
Science
Reticular synthesis of microporous and mesoporous 2D covalent organic frameworks
J. Am. Chem. Soc.
Enhanced hydrolytic stability of self-assembling alkylated two-dimensional covalent organic frameworks
J. Am. Chem. Soc.
Covalent triazine frameworks: synthesis and applications
J. Mater. Chem. A
A crystalline imine-linked 3-D porous covalent organic framework
J. Am. Chem. Soc.
Crystalline covalent organic frameworks with hydrazone linkages
J. Am. Chem. Soc.
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The authors contribute equally to this paper.