Reaction Environment Modification in Covalent Organic Frameworks for Catalytic Performance Enhancement

Anomalous thermo-osmotic conversion performance of ionic covalent-organic-framework membranes in response to charge variations

Increasing the charge density of ionic membranes is believed to be beneficial for generating high output osmotic energy. Herein, we systematically investigated how the membrane charge populations affect permselectivity by decoupling their effects from the impact of the pore structure using a multivariate strategy for constructing covalent-organic-framework membranes. The thermo-osmotic energy conversion efficiency is improved by increasing the membrane charge density, affording 210 W m⁻² with a temperature gradient of 40 K. However, this enhancement occurs only within a narrow window, and subsequently, the efficiency plateaued beyond a threshold density (0.04 C m⁻²). The complex interplay between pore-pore interactions in response to charge variations for ion transport across the upscaled nanoporous membranes helps explain the obtained results. This study has far-reaching implications for the rational design of ionic membranes to augment energy extraction rather than intuitively focusing on achieving high densities.

The development of ionic membranes with a high charge population is critical for realizing efficient thermo-osmotic energy conversion. Here, the authors demonstrated that the thermo-osmotic energy conversion efficiency can be improved by increasing the membrane charge density but this enhancement only occurs within a narrow window.

Nanopores of a Covalent Organic Framework: A Customizable Vessel for Organocatalysis

Covalent organic frameworks (COFs) as crystalline polymers possess ordered nanochannels. When their channels are adorned with catalytically active functional groups, their highly insoluble and fluffy powder texture makes them apt heterogeneous catalysts that can be dispersed in a range of solvents and heated to high temperatures (80-180 °C). This would mean very high catalyst density, facile active-site access, and easy separation leading to high isolated yields. Different approaches have been devised to anchor or disperse the catalytic sites into the nanospaces offered by the COF pores. Such engineered COFs have been investigated as catalysts for many organic transformation reactions. These range from Suzuki-Miyaura coupling, Heck coupling, Knoevenagel condensation, Michael addition, alkene epoxidation, CO2 utilization, and more complex biomimetic catalysis. Such catalysts employ COF as a "passive" support that merely docks catalytically active inorganic clusters, or in other cases, the COF itself participates as an "active" support by altering the electronics of the inorganic catalytic sites through the redox activity of its framework. Even more, catalytic organic pockets or metal complexes have been directly tethered to COF walls to make them behave like single-site organocatalysts. Here, we have listed most COF-based organic transformations by categorizing them as metal-free non-noble-metal@COF and noble-metal@COF. The initial part of this review highlights the advantages of COFs as a component of a heterogeneous catalyst, while the latter part discusses all of the current literature on this topic.

Thermo-Osmotic Energy Conversion Enabled by Covalent-Organic-Framework Membranes with Record Output Power Density

A vast amount of energy can be extracted from the untapped low-grade heat from sources below 100 °C and the Gibbs free energy from salinity gradients. Therefore, a process for simultaneous and direct conversion of these energies into electricity using permselective membranes was developed in this study. These membranes screen charges of ion flux driven by the combined salinity and temperature gradients to achieve thermo-osmotic energy conversion. Increasing the charge density in the pore channels enhanced the permselectivity and ion conductance, leading to a larger osmotic voltage and current. A 14-fold increase in power density was achieved by adjusting the ionic site population of covalent organic framework (COF) membranes. The optimal COF membrane was operated under simulated estuary conditions at a temperature difference of 60 K, which yielded a power density of ≈231 W m^-2 , placing it among the best performing upscaled membranes. The developed system can pave the way to the utilization of the enormous supply of untapped osmotic power and low-grade heat energy, indicating the tremendous potential of using COF membranes for energy conversion applications.

Covalent organic framework-based porous materials for harmful gas purification

Covalent organic frameworks (COFs) with 2D or 3D networks are a class of novel porous crystalline materials, and have attracted more and more attention in the field of gas purification owing to their attractive physicochemical properties, such as high surface area, adjustable functionality and structure, low density, and high stability. However, few systematic reviews about the application statuses of COFs in gas purification are available, especially about non-CO₂ harmful gases. In this review, the recent progress of COFs about the capture, catalysis, and detection of common harmful gases (such as CO₂, NO_x, SO₂, H₂S, NH₃ and volatile pollutants) were comprehensively discussed. The design strategies of COF functional materials from porosity adjustment to surface functionalization (including bottom-up approach, post-synthetic approach, and blending with other materials) for certain application were summarized in detail. Furthermore, the faced challenges and future research directions of COFs in the harmful gas treatment were clearly proposed to inspire the development of COFs.

A Nanographene-Based Two-Dimensional Covalent Organic Framework as a Stable and Efficient Photocatalyst

Synthesis of covalent organic frameworks (COFs) with desirable organic units furnishes advanced materials with unique functionalities. As an emerging class of two-dimensional (2D) COFs, sp2 -carbon-conjugated COFs provide a facile platform to build highly stable and crystalline porous polymers. Herein, a 2D olefin-linked COF was prepared by employing nanographene, namely, dibenzo[hi,st]ovalene (DBOV), as a building block. The DBOV-COF exhibits unique ABC-stacked lattices, enhanced stability, and charge-carrier mobility of ≈0.6 cm2  V-1  s-1 inferred from ultrafast terahertz photoconductivity measurements. The ABC-stacking structure was revealed by the high-resolution transmission electron microscopy and powder X-ray diffraction. DBOV-COF demonstrated remarkable photocatalytic activity in hydroxylation, which was attributed to the exposure of narrow-energy-gap DBOV cores in the COF pores, in conjunction with efficient charge transport following light absorption.

Olefin Metathesis in Confinement: Towards Covalent Organic Framework Scaffolds for Increased Macrocyclization Selectivity

Covalent organic frameworks (COFs) offer vast structural and chemical diversity enabling a wide and growing range of applications. While COFs are well‐established as heterogeneous catalysts, so far, their high and ordered porosity has scarcely been utilized to its full potential when it comes to spatially confined reactions in COF pores to alter the outcome of reactions. Here, we present a highly porous and crystalline, large‐pore COF as catalytic support in α,ω‐diene ring‐closing metathesis reactions, leading to increased macrocyclization selectivity. COF pore‐wall modification by immobilization of a Grubbs‐Hoveyda‐type catalyst via a mild silylation reaction provides a molecularly precise heterogeneous olefin metathesis catalyst. An increased macro(mono)cyclization (MMC) selectivity over oligomerization (O) for the heterogeneous COF‐catalyst (MMC:O=1.35) of up to 51 % compared to the homogeneous catalyst (MMC:O=0.90) was observed along with a substrate‐size dependency in selectivity, pointing to diffusion limitations induced by the pore confinement.

A Heteromeric Carboxylic-acid-based Single Crystalline Crosslinked Organic Framework

The development of large pore single-crystalline covalently linked organic frameworks is critical in revealing the detailed structure-property relationship with substrates. One emergent approach is to photo-crosslink hydrogen-bonded molecular crystals. Introducing complementary hydrogen-bonded carboxylic acid building blocks is promising to construct large pore networks, but these molecules often form interpenetrated networks or non-porous solids. Herein, we introduced heteromeric carboxylic acid dimers to construct a non-interpenetrated molecular crystal. Crosslinking this crystal precursor with dithiols afforded a large pore single-crystalline hydrogen-bonded crosslinked organic framework HCOF-101. X-ray diffraction analysis revealed HCOF-101 as an interlayer connected hexagonal network, which possesses flexible linkages and large porous channels to host a hydrazone photoswitch. Multicycle Z/E-isomerization of the hydrazone took place reversibly within HCOF-101, showcasing the potential use of HCOF-101 for optical information storage.

Construction of Flexible Amine-linked Covalent Organic Frameworks by Catalysis and Reduction of Formic Acid via the Eschweiler-Clarke Reaction

Compared to the current mainstream rigid covalent organic frameworks (COFs) linked by imine bonds, flexible COFs have certain advantages of elasticity and self-adaptability, but their construction and application are greatly limited by the complexity in synthesis and difficulty in obtaining regular structure. Herein, we reported for the first time a series of flexible amine-linked COFs with high crystallinity synthesized by formic acid with unique catalytic and reductive bifunctional properties, rather than acetic acid, the most common catalyst for COF synthesis. The reaction mechanism was demonstrated to be a synchronous in situ reduction during the formation of imine bond. The flexibilities of the products endow them with accommodative adaptability to guest molecules, thus increasing the adsorption capacities for nitrogen and iodine by 27 % and 22 %, respectively. Impressively, a novel concept of flexibilization degree was proposed firstly, which provides an effective approach to rationally measure the flexibility of COFs.

Construction of Covalent Organic Frameworks with Crown Ether Struts

Crown ethers are a class of macrocyclic molecules with unique flexible structures but they are rarely integrated in covalent organic frameworks (COFs). To date, employing flexible organic units such as crown ethers to construct COFs with high crystallinity and surface area are still a challenge. In this work, two new COFs with different flexible crown ethers as backbone rather than side chains are synthesized and further employed for alkali metal ions separation. Both of COFs possess high surface areas, good crystallinity, and excellent chemical stability. Interestingly, these two new COFs with 18-crown-6 or 24-crown-8 units showed remarkable binding ability of K⁺ or Cs⁺ owing to the size-fit effect. This work demonstrated that the unique structural features of crown ethers will lead to increase interest in fabricating COFs with crown ethers.

N-Heterocyclic Carbene Functionalized Covalent Organic Framework for Transesterification of Glycerol with Dialkyl Carbonates

The development of a heterogeneous catalyst through the combination of novel carrier and powerful catalytic active sites is of particular interest. Herein, the successful integration of an N-Heterocyclic carbene (NHC) moiety into a covalent organic framework (COF) was achieved by coupling 4,4′,4′′,4′′′-(pyrene-1,3,6,8-tetrayl) tetraaniline (PyTTA) and equimolar 4,7-bis(4-formylphenyl)-1-methyl-1H-benzimidazole (IM) and 2′3′5′6′-tetrafluoro-[1,1′:4′,1′′-terphenyl]-4,4′-dicarbaldehyde (4F) followed by ionization with 1-bromobutane (C4H9Br) and then deprotonation upon addition of a base. The resulting material exhibited promising heterogeneous catalytic activity towards transesterification reaction of glycerol with dialkyl carbonate. Moreover, good recyclability granted no substantial loss of activity upon five cycles. Combination of COFs and NHCs might synergize their characteristics, thus providing more possibilities for creating new patterns of catalytic reactivity.

A novel fluorescent covalent organic framework containing boric acid groups for selective capture and sensing of cis-diol molecules

Owing to specific formation of five-membered or six-membered cyclic esters between boric acid groups and cis-diol molecules, boric acid bearing fluorescent materials can not only selectively capture but also specifically identify cis-diol substances. In this work, a novel covalent organic framework containing boric acid groups (COF-BA) was prepared through post-modification via the aza-Diels-Alder cycloaddition reaction. COF-BA with good stability, a permanent pore structure, a high specific surface area (606 m2 g-1) and a uniform pore size (2.59 nm) exhibited unique selectivity toward the cis-diol guest molecule 1,2-dihydroxyanthracene-9,10-dione (1,2-Doa) with a high adsorption capacity of 177.95 mg g-1. However, as for the isomers of 1,2-Doa (1,4-dihydroxyanthracene-9,10-dione and 2,6-dihydroxyanthracene-9,10-dione), the corresponding uptake capacities are distinctively decreased to 40.86 mg g-1 and 3.05 mg g-1, respectively. It is worth noting that the COF-BA can be recovered and recycled. Moreover, because the formation of the quinoline enhanced the conjugation effect of the COF skeleton, it was unexpectedly found that COF-BA possessed an intrinsic fluorescence property and could be used as an optical sensor for 1,2-Doa.

Enzyme-Inspired Assembly: Incorporating Multivariate Interactions to Optimize the Host-Guest Configuration for High-Speed Enantioselective Catalysis

To achieve a rapid asymmetry conversion, the substrate objects suffer from accelerated kinetic velocity and random rotation at the cost of selectivity. Inspired by natural enzymes, optimizing the host-guest configuration will realize the high-performance enantioselective conversion of chemical reactions. Herein, multivariate binding interactions were introduced into the 1D channel of a chiral catalyst to simulate the enzymatic action. An imidazolium group was used to electrophilically activate the C═O unit of a ketone substrate, and the counterion binds the hydrogen donor isopropanol. This binding effect around the catalytic center produces strong stereo-induction, resulting in high conversion (99.5% yield) and enantioselectivity (99.5% ee) for the asymmetric hydrogenation of biomass-derived acetophenone. In addition, the turnover frequency of the resulting catalyst (5160 h^-1 TOF) is more than 58 times that of a homogeneous Ru-TsDPEN catalyst (88 h^-1 TOF) under the same condition, which corresponds to the best performance reported till date among all existing catalysts for the considered reaction.

Triphenylphosphine-Based Covalent Organic Frameworks and Heterogeneous Rh-P-COFs Catalysts

The synthesis of phosphine-based functional covalent organic frameworks (COFs) has attracted great attention recently. Herein, we present two examples of triphenylphosphine-based COFs (termed P-COFs) with well-defined crystalline structures, high specific surface areas, and good thermal stability. Furthermore, rhodium catalysts with these P-COFs as support material show high turnover frequency for the hydroformylation of olefins, as well as excellent recycling performance. This work not only extends the phosphine-based COF family, but also demonstrates their application in immobilizing homogeneous metal-based (e.g., Rh-phosphine) catalysts for application in heterogeneous catalysis.

Integrating single Ni sites into biomimetic networks of covalent organic frameworks for selective photoreduction of CO2

Selective photoreduction of CO2 into a given product is a great challenge but desirable. Inspired by natural photosynthesis occurring in hierarchical networks over non-precious molecular metal catalysts, we demonstrate an integration of single Ni sites into the hexagonal pores of polyimide covalent organic frameworks (PI-COFs) for selective photoreduction of CO2 to CO. The single Ni sites in the hexagonal pores of the COFs serve as active sites for CO2 activation and conversion, while the PI-COFs not only act as a photosensitizer to generate charge carriers but also exert a promoting effect on the selectivity. The optimized PI-COF with a triazine ring exhibits excellent activity and selectivity. A possible intra- and inter-molecular charge-transfer mechanism was proposed, in which the photogenerated electrons in PI-COFs are efficiently separated from the central ring to the diimide linkage, and then transferred to the single Ni active sites, as evidenced by theoretical calculations.

Recent Progress in Metal-Free Covalent Organic Frameworks as Heterogeneous Catalysts

Covalent organic frameworks (COFs), connecting different organic units into one system through covalent bonds, are crystalline organic porous materials with 2D or 3D networks. Compared with conventional porous materials such as inorganic zeolite, active carbon, and metal-organic frameworks, COFs are a new type of porous materials with well-designed pore structure, high surface area, outstanding stability, and easy functionalization at the molecular level, which have attracted extensive attention in various fields, such as energy storage, gas separation, sensing, photoluminescence, proton conduction, magnetic properties, drug delivery, and heterogeneous catalysis. Herein, the recent advances in metal-free COFs as a versatile platform for heterogeneous catalysis in a wide range of chemical reactions are presented and the synthetic strategy and promising catalytic applications of COF-based catalysts (including photocatalysis) are summarized. According to the types of catalytic reactions, this review is divided into the following five parts for discussion: achiral organic catalysis, chiral organic conversion, photocatalytic organic reactions, photocatalytic energy conversion (including water splitting and the reduction of carbon dioxide), and photocatalytic pollutant degradation. Furthermore, the remaining challenges and prospects of COFs as heterogeneous catalysts are also presented.