Review
Functionalization of covalent organic frameworks by metal modification: Construction, properties and applications

https://doi.org/10.1016/j.cej.2020.127136Get rights and content

Highlights

  • This review focuses on the metal modification strategy of COFs.

  • The pristine properties and characterization techniques of COFs are discussed.

  • The construction and properties toward metal-modified COFs are highlighted.

  • The applications and development prospects are summarized.

Abstract

As emerging crystalline porous materials, covalent organic frameworks (COFs) have drawn great interest due to their tunable porosity and modifiable skeletons. Multiple applications of COFs rely on the previous chemical functionality of organic building units or posterior modification of frameworks. The metal modification strategy utilizes metal–ligand coordination reactions to incorporate metal species into COF skeletons, providing the possibility for functionalization. In this review, we discuss the pristine properties (pore design, chemical stability and scalability) and characterization techniques of COFs to fully recognize these charming materials. Particular attentions are given to the construction (theoretical simulations and actual operation synthesis methods) and metallized properties (adsorption site, binding energy and structural effect) of metal-modified COFs in view of verifying the feasibility of modification. Moreover, the applications of hybrids in gas adsorption and catalysis are presented. Finally, we summarize the challenges and give outlook in the future perspectives of this modification strategy.

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).

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