Lower Activation Energy for Catalytic Reactions through Host–Guest Cooperation within Metal–Organic Frameworks

Porous Organic Polymers for Catalytic Conversion of Carbon Dioxide

To overcome the challenges of global warming and environmental pollution, it is necessary to reduce the concentration of carbon dioxide (CO₂ ) in the atmosphere, which is mainly accumulated in the air through the burning of fossil fuels. Therefore, the development of environmentally friendly strategies to capture carbon dioxide and convert it into value-added products offers a promising way forward for reducing carbon dioxide concentration in the atmosphere. In this context, POPs (porous organic polymers) have shown great potential as CO₂ selective adsorbents due to their high specific surface area, chemical stability, nanoscale porosity and structural diversity, as well as POPs based heterogeneous catalysts for CO₂ conversion. This review provides a concise account of preparation methods of various POPs, challenges and current development trends of POPs in photocatalytic CO₂ reduction, electrocatalytic CO₂ reduction and chemical CO₂ conversion.

Selective Implantation of Diamines for Cooperative Catalysis in Isoreticular Heterometallic Titanium-Organic Frameworks

We introduce the first example of isoreticular titanium-organic frameworks, MUV-10 and MUV-12, to show how the different affinity of hard Ti(IV) and soft Ca(II) metal sites can be used to direct selective grafting of amines. This enables the combination of Lewis acid titanium centers and available -NH₂ sites in two sizeable pores for cooperative cycloaddition of CO₂ to epoxides at room temperature and atmospheric pressure. The selective grafting of molecules to heterometallic clusters adds up to the pool of methodologies available for controlling the positioning and distribution of chemical functions in precise positions of the framework required for definitive control of pore chemistry.

Rational Design of a ZnII MOF with Multiple Functional Sites for Highly Efficient Fixation of CO2 under Mild Conditions: Combined Experimental and Theoretical Investigation

The development of efficient heterogeneous catalysts suitable for carbon capture and utilization (CCU) under mild conditions is a promising step towards mitigating the growing concentration of CO₂ in the atmosphere. Herein, we report the construction of a hydrogen-bonded 3D framework, {[Zn(hfipbba)(MA)]⋅3 DMF}_n (hfipbba=4,4'-(hexaflouroisopropylene)bis(benzoic acid)) (HbMOF1) utilizing Zn^II center, a partially fluorinated, long-chain dicarboxylate ligand (hfipbba), and an amine-rich melamine (MA) co-ligand. Interestingly, the framework possesses two types of 1D channels decorated with CO₂ -philic (-NH₂ and -CF₃ ) groups that promote the highly selective CO₂ adsorption by the framework, which was supported by computational simulations. Further, the synergistic involvement of both Lewis acidic and basic sites exposed in the confined 1D channels along with high thermal and chemical stability rendered HbMOF1 a good heterogeneous catalyst for the highly efficient fixation of CO₂ in a reaction with terminal/internal epoxides at mild conditions (RT and 1 bar CO₂ ). Moreover, in-depth theoretical studies were carried out using periodic DFT to obtain the relative energies for each stage involved in the catalytic reaction and an insight mechanistic details of the reaction is presented. Overall, this work represents a rare demonstration of rational design of a porous Zn^II MOF incorporating multiple functional sites suitable for highly efficient fixation of CO₂ with terminal/internal epoxides at mild conditions supported by comprehensive theoretical studies.

Carbon Dioxide Conversion Upgraded by Host-guest Cooperation between Nitrogen-Rich Covalent Organic Framework and Imidazolium-Based Ionic Polymer

The chemical conversion of CO₂ into value-added chemicals is one promising approach for CO₂ utilization. It is crucial to explore highly efficient catalysts containing task-specific components for CO₂ fixation. Here, a host-guest catalytic system was developed by integrating nitrogen-rich covalent organic framework (TT-COF) and imidazolium-based ionic polymer (ImIP), which serve as hydrogen-bonding donor and nucleophilic agent, respectively, for cooperatively facilitating the activation of the epoxides and subsequent CO₂ cycloaddition. The catalytic activity of the host-guest system was remarkably superior to those of ImIP, TT-COF, and their physical mixture. Furthermore, selective adsorption for CO₂ over N₂ rendered this catalytic system effective for the cycloaddition reaction of the simulated flue gas. The protocols for the unification of two catalytically active components provide new opportunities for the development of composite systems in multiple applications.

Crosslinked Resin-Supported Bifunctional Organocatalyst for Conversion of CO2 into Cyclic Carbonates

The development of solvent-free, metal-free, recyclable organic catalysts is required for the current chemical fixation of carbon dioxide converted into cyclic carbonates. With the goal of reducing the cost, time, and energy consumption for the coupling reaction of CO₂ and epoxides, a series of highly active heterogeneous catalysts, based on a thiourea and quaternary ammonium salt system, are synthesized by using a thiol-ene click reaction under ultraviolet light. Benefitting from synergistic interactions of the electrophilic center (thiourea) and the nucleophilic site (ammonium bromide), the catalysts exhibit excellent catalytic selectivity (99 %) for the cycloaddition of carbon dioxide with a diverse range of epoxides under mild conditions (1.2 MPa, 100 °C). Moreover, the catalyst can be easily recycled by facile filtration and reused for 5 times without noticeable loss of activity and selectivity. This work provides a potential heterogeneous catalyst for the conversion of carbon dioxide into high value-added chemicals with the combined advantages of low cost, easy recovery, and satisfactory catalytic properties.

Rational Localization of Metal Nanoparticles in Yolk-Shell MOFs for Enhancing Catalytic Performance in Selective Hydrogenation of Cinnamaldehyde

The development of sustainable catalysts to simultaneously improve activity and selectivity remains a challenge. Herein, it is demonstrated that metal nanoparticles (MNPs) can be encapsulated into a yolk-shell metal-organic framework (MOF) with controllable spatial localization to optimize catalytic performance. When the MNPs are located in the void space between the shell and the core of the MOF, the resulting MNPs@MOF composites show both high catalytic activity and selectivity toward the hydrogenation of α,β-unsaturated aldehydes. In particular, the easily recoverable and stable Pt_void @MOF(Y) shows an exceptionally high selectivity of 98.2 % for cinnamyl alcohol at a high conversion of 97 %. The excellent performance can be attributed to easy diffusion of the reactants to access highly exposed MNPs in the MOF support, as well as the improved adsorption of the reactant and desorption of the product due to the appropriate metal-support interaction and rich void space between core and shell.

Formation of CX Bonds in CO2 Chemical Fixation Catalyzed by Metal-Organic Frameworks

Transformation of CO₂ based on metal-organic framework (MOF) catalysts is becoming a hot research topic, not only because it will help to reduce greenhouse gas emission, but also because it will allow for the production of valuable chemicals. In addition, a large number of impressive products have been synthesized by utilizing CO₂ . In fact, it is the formation of new covalent bonds between CO₂ and substrate molecules that successfully result in CO₂ solidly inserting into the products, and only four types of new CX bonds, including CH, CC, CN, and CO bonds, are observed in this exploration. An overview of recent progress in constructing CX bonds for CO₂ conversion catalyzed by various MOF catalysts is provided. The catalytic mechanism of generating different CX bonds is further discussed according to both structural features of MOFs and the interactions among CO₂ , substrates, as well as MOFs. The future opportunities and challenges in this field are also tentatively covered.

Construction of Hierarchical Metal-Organic Frameworks by Competitive Coordination Strategy for Highly Efficient CO2 Conversion

Hierarchical porosity and functionalization help to fully make use of metal-organic frameworks (MOFs) for their diverse applications. Herein, a simple strategy is reported to construct hierarchically porous MOFs through a competitive coordination method using tetrafluoroborate (M(BF₄ )_x , where M is metal site) as both functional sites and etching agents. The resulting MOFs have in situ formed defect-mesopores and functional sites without sacrificing their structure stability. The formation mechanism of the defect-mesopores is elucidated by a combination of experimental and first-principles calculation method, indicating the general feasibility of this new approach. Compared with the original microporous counterparts, the new hierarchical MOFs exhibit superior adsorption for the bulky dye molecules and catalytic performance for the CO₂ conversion attributed to their specific hierarchical pore structures.

Reaction Environment Modification in Covalent Organic Frameworks for Catalytic Performance Enhancement

Herein, we show how the spatial environment in the functional pores of covalent organic frameworks (COFs) can be manipulated in order to exert control in catalysis. The underlying mechanism of this strategy relies on the placement of linear polymers in the pore channels that are anchored with catalytic species, analogous to outer-sphere residue cooperativity within the active sites of enzymes. This approach benefits from the flexibility and enriched concentration of the functional moieties on the linear polymers, enabling the desired reaction environment in close proximity to the active sites, thereby impacting the reaction outcomes. Specifically, in the representative dehydration of fructose to produce 5-hydroxymethylfurfural, dramatic activity and selectivity improvements have been achieved for the active center of sulfonic acid groups in COFs after encapsulation of polymeric solvent analogues 1-methyl-2-pyrrolidinone and ionic liquid.

Sustainable Heterogeneous Catalysts for CO2 Utilization by Using Dual Ligand ZnII /CdII Metal-Organic Frameworks

Two-dimensional Zn^II /Cd^II -based dual ligand metal-organic frameworks (MOFs) {[M(CHDC)(L)]⋅H₂ O}_n involving 4-pyridyl carboxaldehyde isonicotinoylhydrazone (L) in combination with flexible 1,4-cyclohexanedicarboxylic acid (H₂ CHDC) as linkers have been synthesized by adaptable synthetic protocols including a green mechanochemical (grinding) method. Characterization, chemical/thermal stability, phase purity, and solid-state luminescent properties of both MOFs have been established by various analytical methods. Structural analysis revealed dimeric metal clusters composed of [M₂ (CHDC)₂ ]_n loops doubly pillared with L, generating a 2D framework. Both MOFs can be used as highly active solvent-free binary catalysts for CO₂ cycloaddition with epoxides in the presence of the co-catalyst tetrabutylammonium bromide (TBAB) with good catalytic conversion in up to six catalytic cycles without significant loss of activity. The present investigation demonstrates the application of MOFs as efficient heterogeneous catalysts for CO₂ utilization under moderate reaction conditions. Based on the single-crystal X-ray data, a probable mechanism for the cycloaddition reaction has also been proposed.