Bottom-Up Synthesis of Covalent Organic Frameworks with Quasi-Three-Dimensional Integrated Architecture via Interlayer Cross-Linking.

Developing strategies to enhance the structural robustness of covalent organic frameworks (COFs) is of great importance. Here, we rationally design and synthesize a class of cross-linked COFs (CCOFs), in which the two-dimensional (2D) COF layers are anchored and connected by polyethylene glycol (PEG) or alkyl chains through covalent bonds. The bottom-up fabrication of these CCOFs is achieved by the condensation of cross-linked aldehyde monomers and tritopic amino monomers. All the synthesized CCOFs possess high crystallinity and porosity, and enhanced structural robustness surpassing the typical 2D COFs, which means that they cannot be exfoliated under ultrasonication and grinding due to the cross-linking effect. Furthermore, the cross-linked patterns of PEG units are uncovered by experimental results and Monte Carlo molecular dynamics simulations. It is found that all CCOFs are dominated by vertical cross-layer (interlayer) connections (clearly observed in high-resolution transmission electron microscopy images), allowing them to form quasi-three-dimensional (quasi-3D) structures. This work bridges the gap between 2D COFs and 3D COFs and provides an efficient way to improve the interlayered stability of COFs.

Highly Robust Microporous Metal-Organic Frameworks for Efficient Ethylene Purification under Dry and Humid Conditions.

Two C2 H6 -selective metal-organic framework (MOF) adsorbents with ultrahigh stability, high surface areas, and suitable pore size have been designed and synthesized for one-step separation of ethane/ethylene (C2 H6 /C2 H4 ) under humid conditions to produce polymer-grade pure C2 H4 . Experimental results reveal that these two MOFs not only adsorb a high amount of C2 H6 but also display good C2 H6 /C2 H4 selectivity verified by fixed bed column breakthrough experiments. Most importantly, the good water stability and hydrophobic pore environments make these two MOFs capable of efficiently separating C2 H6 /C2 H4 under humid conditions, exhibiting the benchmark performance among all reported adsorbents for separation of C2 H6 /C2 H4 under humid conditions. Moreover, the affinity sites and their static adsorption energies were successfully revealed by single crystal data and computation studies. Adsorbents described in this work can be used to address major chemical industrial challenges.

Rationally Fabricating Three-Dimensional Covalent Organic Frameworks for Propyne/Propylene Separation.

Efficient propyne/propylene separation to obtain polymer-grade propylene is a crucial and challenging process in industrial production, but it has not yet been realized in the covalent organic framework (COF) field. Addressing this challenge, we synthesize two three-dimensional COF adsorbents via a [8 + 4] construction approach based on an octatopic aldehyde monomer. Upon using the continuous rotation electron diffraction technique and structural simulation, both COFs are successfully determined as rare flu topology. Various characterization techniques prove that both COFs exhibit high crystallinity, high porosity, and good stability. Attributed to their interconnected micropores and nonpolar pore environment, these COFs can efficiently remove trace amounts of propyne from the propyne/propylene (1/99, and 0.1/99.9, v/v) mixture to obtain high-purity propylene (>99.99%), validated by dynamic breakthrough experiments. This work paves a new avenue for propyne/propylene separation using COFs as highly efficient adsorbents.

Bottom-Up Synthesis of 8-Connected Three-Dimensional Covalent Organic Frameworks for Highly Efficient Ethylene/Ethane Separation.

Developing cost-/energy-efficient separation techniques for purifying ethylene from an ethylene/ethane mixture is highly important but very challenging in the industrial process. Herein, using a bottom-up [8 + 2] construction approach, we rationally designed and synthesized three three-dimensional covalent organic frameworks (COFs) with 8-connected bcu networks, which can selectively remove ethane from an ethylene/ethane mixture with high efficiency. These COF materials, which are fabricated by the condensation reaction of a customer-designed octatopic aldehyde monomer with linear diamino linkers, possess high crystallinity, good structural robustness, and high porosity. Attributed to the well-organized micro-sized pores with a nonpolar/inert pore environment, these COFs display high ethane adsorption capacity and good selectivity over ethylene, making them among the best ethane-selective adsorbents for ethylene purification. Their excellent ethylene/ethane separation performance is validated by dynamic breakthrough experiments with high-purity ethylene (>99.99%) produced through a single adsorption process. The separation performance surpasses all reported C2H6-selective COFs and even some benchmark metal-organic frameworks. This work provides important guidance for the design of new adsorbents for value-added gas purification.

A Class of Rigid-Flexible Coupling Crystalline Crosslinked Polymers as Vapomechanical Actuators.

Fabricating mechanically responsive actuators that can efficiently convert external stimuli into mechanical work is of great significance for real-world applications. Herein, we rationally design a class of rigid-flexible coupling crystalline crosslinked polymers (CCPs) to fabricate vapomechanically responsive actuators. Interfacial condensation reactions of flexible macromers with rigid monomers afford a series of freestanding CCP membranes. Notably, it is the first example where crosslinked polymers show high crystallinity and porosity. Moreover, the CCP membranes exhibit good mechanical properties and interesting vapor-triggered actuation performance, which is reversible and repeatable. We find that the unusual polymer structures, high vapor sorption, and anisotropic membranes contribute to the directional deformation performance of the CCP actuators. The synthesis approach in this work provides new insights into the design and fabrication of smart materials for advanced applications.

Tethering Flexible Polymers to Crystalline Porous Materials: A Win-Win Hybridization Approach.

Crystalline porous materials (CPMs) have been widely studied over the past few decades due to their well-defined, customizable, and porous structures. However, the limited stability and machinability restrict their particle applications. Recently, a new trend to use hybrid strategies to bridge the gap between CPMs and flexible polymers has emerged as an effective solution to tackle the above issues. Polymers can be either cross-linked within the framework or anchored to the outer surface of the materials, that endow CPMs with polymer features, such as flexibility, ductility, and machinability. Such resulting polymer-tethered CPMs (named as polyCPMs) have demonstrated improved performances in many application scopes such as membrane separation, heterogeneous catalysis and smart response. This minireview highlights state of the art for the hybridization strategies of CPMs with flexible polymers, and provide a perspective on their potential applications and future directions.

Homochiral BINAPDA-Zr-MOF for Heterogeneous Asymmetric Cyanosilylation of Aldehydes.

A new homochiral BINAPDA-Zr-MOF was prepared by a new chiral organic linker of (R)-4,4'-(6,6'-dichloro-2,2'-diethoxyl-[1,1'-binaphthalene]-4,4'-diyl)dibenzoic acid (R-L) and ZrCl4 under solvothermal conditions. Its structure was determined by Pawley refinement on the basis of the measured PXRD pattern determined for BINAPDA-Zr-MOF, and it showed that the obtained chiral MOF crystallized in the F23 space group with the same topological structure as that of UiO-66. The obtained BINAPDA-Zr-MOF can be a very active catalyst to catalyze aldehyde cyanosilylation. In addition, the chiral BINAPDA-Zr-MOF was a typical solid catalyst, which was proved by a hot leaching test; moreover, it could be reused at least five times without loss of its catalytic activity and enantioselectivity.

Dual Heterogeneous Catalyst Pd-Au@Mn(II)-MOF for One-Pot Tandem Synthesis of Imines from Alcohols and Amines.

A new Mn(II) metal-organic framework (MOF) 1 was synthesized by the combination of 4,4,4-trifluoro-1-(4-(pyridin-4-yl)phenyl)butane-1,3-dione (L) and Mn(OAc)2 in solution. 1 features a threefold-interpenetrating NbO net containing honeycomb-like channels, in which the opposite Mn(II)···Mn(II) distance is 23.5075(10) Å. Furthermore, 1 can be an ideal platform to support Pd-Au bimetallic alloy nanoparticles to generate a composite catalytic system of Pd-Au@Mn(II)-MOF (2). 2 can be a highly active bifunctional heterogeneous catalyst for the one-pot tandem synthesis of imines from benzyl alcohols and anilines and from benzyl alcohols and benzylamines.

Pd@Cu(II)-MOF-Catalyzed Aerobic Oxidation of Benzylic Alcohols in Air with High Conversion and Selectivity.

A new 3D porous Cu(II)-MOF (1) was synthesized based on a ditopic pyridyl substituted diketonate ligand and Cu(OAc)2 in solution, and it features a 3D NbO motif which is determined by the X-ray crystallography. Furthermore, the Pd NPs-loaded hybrid material Pd@Cu(II)-MOF (2) was prepared based on 1 via solution impregnation, and its structure was confirmed by HRTEM, SEM, XRPD, gas adsorption-desorption, and ICP measurement. 2 exhibits excellent catalytic activity (conversion, 93% to >99%) and selectivity (>99% to benzaldehydes) for various benzyl alcohol substrates (benzyl alcohol and its derivatives with electron-withdrawing and electron-donating groups) oxidation reactions in air. In addition, 2 is a typical heterogeneous catalyst, which was confirmed by hot solution leaching experiment, and it can be recycled at least six times without significant loss of its catalytic activity and selectivity.

Au@Cu(II)-MOF: Highly Efficient Bifunctional Heterogeneous Catalyst for Successive Oxidation-Condensation Reactions.

A new composite Au@Cu(II)-MOF catalyst has been synthesized via solution impregnation and full characterized by HRTEM, SEM-EDS, XRD, gas adsorption-desorption, XPS, and ICP analysis. It has been shown here that the Cu(II)-framework can be a useful platform to stabilize and support gold nanoparticles (Au NPs). The obtained Au@Cu(II)-MOF exhibits a bifunctional catalytic behavior and is able to promote selective aerobic benzyl alcohol oxidation-Knoevenagel condensation in a stepwise way.

Fabrication of Biomolecule-Covalent-Organic-Framework Composites as Responsive Platforms for Smart Regulation of Fermentation Application.

Exploration of novel material platforms to protect biological preservatives and realize intelligent regulation during fermentation is of great significance in industry. Herein, we established an intelligent responsive platform by introducing antimicrobial biomolecules (nisin) into rationally designed covalent organic frameworks (COFs), resulting in a new type of "smart formulation", which could responsively inhibit microbial contamination and ensure the orderly progression of the fermentation process. The encapsulated biomolecules retained their activity while exhibiting enhanced stability and pH-responsive releasing process (100% bacteriostatic efficiency at a pH of 3), which can ingeniously adapt to the environmental variation during the fermentation process and smartly fulfill the regulation needs. Moreover, the nisin@COF composites would not affect the fermentation strains. This study will pave a new avenue for the preparation of highly efficient and intelligent antimicrobial agents for the regulation of the fermentation process and play valuable roles in the drive toward green and sustainable biomanufacturing.