Molecular separations that enable selective transport of target molecules from gas and liquid molecular mixtures, such as CO2 capture, olefin/paraffin separations, and organic solvent nanofiltration, represent the most energy sensitive and significant demands. Membranes are favored for molecular separations owing to the advantages of energy efficiency, simplicity, scalability, and small environmental footprint. A number of emerging microporous organic materials have displayed great potential as building blocks of molecular separation membranes, which not only integrate the rigid, engineered pore structures and desirable stability of inorganic molecular sieve membranes, but also exhibit a high degree of freedom to create chemically rich combinations/sequences. To gain a deep insight into the intrinsic connections and characteristics of these microporous organic material-based membranes, in this review, for the first time, we propose the concept of organic molecular sieve membranes (OMSMs) with a focus on the precise construction of membrane structures and efficient intensification of membrane processes. The platform chemistries, designing principles, and assembly methods for the precise construction of OMSMs are elaborated. Conventional mass transport mechanisms are analyzed based on the interactions between OMSMs and penetrate(s). Particularly, the 'STEM' guidelines of OMSMs are highlighted to guide the precise construction of OMSM structures and efficient intensification of OMSM processes. Emerging mass transport mechanisms are elucidated inspired by the phenomena and principles of the mass transport processes in the biological realm. The representative applications of OMSMs in gas and liquid molecular mixture separations are highlighted. The major challenges and brief perspectives for the fundamental science and practical applications of OMSMs are tentatively identified.

Organic molecular sieve membranes for chemical separations.

Single clusters have attracted extensive research interest in the field of catalysis. However, achieving a highly uniform dispersion of a single-cluster catalyst is challenging. In this work, for the first time, we present a versatile strategy for uniformly dispersed polyoxometalates (POMs) in covalent organic frameworks (COFs) through confining POM cluster into the regular nanopores of COF by a covalent linkage. These COF-POM composites combine the properties of light absorption, electron transfer, and suitable catalytic active sites; as a result, they exhibit outstanding catalytic activity in artificial photosynthesis: that is, CO2 photoreduction with H2O as the electron donor. Among them, TCOF-MnMo6 achieved the highest CO yield (37.25 μmol g-1 h-1 with ca. 100% selectivity) in a gas-solid reaction system. Furthermore, a mechanism study based on density functional theory (DFT) calculations demonstrated that the photoinduced electron transfer (PET) process occurs from the COF to the POM, and then CO2 reduction and H2O oxidation occur on the POM and COF, respectively. This work developed a method for a uniform dispersion of POM single clusters into a COF, which also shows the potential of using COF-POM functional materials in the field of photocatalysis.

Confining and Highly Dispersing Single Polyoxometalate Clusters in Covalent Organic Frameworks by Covalent Linkages for CO2 Photoreduction.

As a new generation of porous materials, porous organic polymers (POPs), have recently emerged as a powerful platform of heterogeneous photocatalysis. POPs are constructed using extensive organic synthesis methodologies, with various functional organic units being connected via high-energy covalent bonds. This review systematically presents the recent advances in POPs for visible-light driven organic transformations. Herein, we firstly summarize the common construction strategies for POP-based photocatalysts based on two major approaches: pre-design and post-modification; secondly, we categorize and summarize the synthesis methods and organic reaction types for constructing various types of POPs. We then classify and introduce the specific reactions of current light-driven POP-mediated organic transformations. Finally, we outline the current state of development and the problems faced in light-driven organic transformations by POPs, and we present some perspectives to motivate the reader to explore solutions to these problems and confront the present challenges in the development process.

Porous organic polymers for light-driven organic transformations.

Mimicking natural photosynthesis to convert CO2 with H2O into value-added fuels achieving overall reaction is a promising way to reduce the atmospheric CO2 level. Casting the catalyst of two or more catalytic sites with rapid electron transfer and interaction may be an effective strategy for coupling photocatalytic CO2 reduction and H2O oxidation. Herein, based on the MOF ∪ COF collaboration, we have carefully designed and synthesized a crystalline hetero-metallic cluster catalyst denoted MCOF-Ti6Cu3 with spatial separation and functional cooperation between oxidative and reductive clusters. It utilizes dynamic covalent bonds between clusters to promote photo-induced charge separation and transfer efficiency, to drive both the photocatalytic oxidative and reductive reactions. MCOF-Ti6Cu3 exhibits fine activity in the conversion of CO2 with water into HCOOH (169.8 μmol g-1h-1). Remarkably, experiments and theoretical calculations reveal that photo-excited electrons are transferred from Ti to Cu, indicating that the Cu cluster is the catalytic reduction center. 

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Linking oxidative and reductive clusters to prepare crystalline porous catalysts for photocatalytic CO2 reduction with H2O

Conjugated microporous polymers for energy storage: Recent progress and challenges

Covalent organic frameworks offer a molecular platform for integrating organic units into periodically ordered yet extended two- and three-dimensional polymers to create topologically well-defined polygonal lattices and built-in discrete micropores and/or mesopores. This polymer architecture is unique as it enables predesigning both primary- and high-order structures, greatly enhancing our capabilities of designing organic materials to produce predictable structures and to achieve unique properties and functions. Progress over the past 15 years in the design, synthesis and functional exploration of COFs has successively established the basis of the COF field and COFs have shown the great potential of chemistry in developing a class of amazing organic materials. In this review, we focus on analysing the historic developments of COFs to uncover a full materials and application picture by providing comprehensive yet clear guidance for molecular design, synthetic control and functional exploration. We scrutinise the structural components of COFs including building blocks, reactive sites and functional groups with the aim of finding the origins of structural designability and diversity, as well as multiple functionalities. We disclose strategies for designing and synthesising frameworks to construct various tailor-made interfaces, and for exploring skeletons and pores to design properties and functions. With well-defined skeletons, pores and interfaces that offer a chemical basis to trigger and control interactions with photons, excitons, phonons, polarons, electrons, holes, spins, ions and molecules, we illustrate the current status of our understandings of structure-property correlations, and unveil the principles for establishing a regime to design unique functions that originate from and are inherent to structures. We predict the key central issues in design and synthesis, the challenges in functional design and the future directions from the perspectives of chemistry, physics and materials science.

Covalent organic frameworks: an ideal platform for designing ordered materials and advanced applications

Nature evolves fascinating molecular pores to achieve unique biological functions based on a single pore or channel as observed for aquaporins and ion channels. An artificial system, on the other hand, explores porous structures to construct dense pores in materials. Progress in chemistry over the past century has greatly improved our capability to synthesize porous materials. This is evident by the advancement from inorganic to organic units, from trial-and-error tests to module fabrication and further to fully predesignable pores, and from harsh preparation protocols to ambient synthetic methods. Over the past 15 years, a molecular platform based on organic and polymer chemistry has been explored to enable the design of artificial pores to achieve different pore size, shape, wall, and interface. This becomes possible with a class of emerging polymer-covalent organic frameworks (COFs). COFs are a class of crystalline porous polymers that integrate organic units into extended molecular frameworks with periodically ordered skeletons and well-defined pores. We have focused on exploring COFs over the past 15 years to design and synthesize porous structures with the aim of developing chemistry that leads to the creation of tailor-made pore interfaces (Nagai, A. et al. Nat. Commun., 2011, 2, 536). In this Account, we summarize the general concept of our approaches to various pore interfaces by emphasizing design principle, synthetic strategy, and distinct porous features and their impacts. We illustrate pore interface design by highlighting general strategies based on direct polymerization and pore surface engineering to construct different pore walls with a diversity of functional units. One distinct feature is that these functional groups are predesigned and synthetically controlled to achieve a predetermined component, position, and density, leading to a general way to install various specific pore wall interfaces to each pore. We showcase hierarchical pore interface architectures by elucidating the nature of interplays between interfaces and molecules and ions, ranging broadly from hydrogen bond to dipole-dipole/quadrupole interactions, electrostatic interaction, acid-base interaction, coordination, and electronic interactions. We scrutinize the unique properties and functions of adsorption and separation, catalysis, energy transformation and storage, and proton and metal ion transport by disclosing functional design schemes and interface-function correlations. We predict the fundamental key issues to be addressed and show future directions in designing artificial pores to target at ultimate functions. This chemistry on pore interface engineering opens a way to porous materials that have remained challenging in the predesign of both structure and function.

Covalent Organic Frameworks: Pore Design and Interface Engineering

Attempts to develop photocatalysts for hydrogen production from water usually result in low efficiency. Here we report the finding of photocatalysts by integrated interfacial design of stable covalent organic frameworks. We predesigned and constructed different molecular interfaces by fabricating ordered or amorphous π skeletons, installing ligating or non-ligating walls and engineering hydrophobic or hydrophilic pores. This systematic interfacial control over electron transfer, active site immobilisation and water transport enables to identify their distinct roles in the photocatalytic process. The frameworks, combined ordered π skeletons, ligating walls and hydrophilic channels, work under 300-1000 nm with non-noble metal co-catalyst and achieve a hydrogen evolution rate over 11 mmol g-1 h-1, a quantum yield of 3.6% at 600 nm and a three-order-of-magnitude-increased turnover frequency of 18.8 h-1 compared to those obtained with hydrophobic networks. This integrated interfacial design approach is a step towards designing solar-to-chemical energy conversion systems. 

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Zhuoer Li

Papers

Covalent organic frameworks: an ideal platform for designing ordered materials and advanced applications

Covalent Organic Frameworks: Pore Design and Interface Engineering

Integrated interfacial design of covalent organic framework photocatalysts to promote hydrogen evolution from water