Highly efficient metal/solvent-free chemical fixation of CO2 at atmospheric pressure conditions using functionalized porous covalent organic frameworks

Rational Design of Porous Organic Framework (POF) for Efficient Conversion of CO2 to Cyclic Carbonates and 2-Oxazolidinones at Atmospheric Pressure Conditions

Carbon dioxide (CO2) capture and its subsequent catalytic fixation into usable compounds represent a potential approach for addressing the energy problem and the implications of global warming. Hence, it is necessary to develop effective catalytic systems required for the transformation of CO2 into valuable chemicals/fuels. Herein, we rationally designed a hydroxyl-functionalized porous organic framework (OH-POF) consisting of both acidic (OH) as well as basic N sites for the transformation of CO2 using epoxides for the production of cyclic carbonates (CCs), a useful commodity chemical under environmental-friendly, metal/solvent/co-catalyst-free conditions. Moreover, OH-POF was post-synthetically modified to anchor non-noble metal, Zn(II) to generate Zn-POF and further explored it for the efficient functionalization of CO2 with propargylic amines to generate valuable bioactive 2-oxazolidinones. Significantly, both OH-POF and Zn-POF demonstrated exceptional reusability with catalytic efficacy retained across numerous cycles of use. Notably, this study showcases a green and sustainable process for utilization of CO2 under environmentally favorable ambient conditions into two highly valuable compounds, viz cyclic carbonates and 2-oxazolidinones.

Catechol-Functionalized Covalent Organic Framework: Synthesis, Characterization and Metal-Free Organocatalyst for CO2 Fixation Under Mild Conditions

Covalent organic frameworks (COFs) incorporating hydrogen bond donor (HBD) moieties show great promise for heterogeneous catalyst for CO2 cycloaddition. In this work, a catechol-functionalized COF (BL-TF-COF) was constructed via Schiff-base condensation under solvothermal conditions, which was characterized using powder X-ray diffraction (PXRD), Fourier transform infrared spectroscopy (FT-IR), solid-state 13C nuclear magnetic resonance (NMR), scanning electron microscope (SEM), high resolution transmission electron microscope (HR-TEM), diffuse reflection spectroscopy (DRS) and cyclic voltammetry (CV). BL-TF-COF presents high crystallinity, large surface area (523 m2 g-1) and remarkable chemical stability, along with abundant hydroxyl functional groups distributed on the pore wall. This framework shows a CO2 adsorption capacity up to 88 mg g-1 at 273 K and 1 bar. The binding locations of the adsorption of CO2 over BL-TF-COF was studied by grand canonical Monte Carlo (GCMC) simulations. BL-TF-COF displays remarkable catalytic performance for the cycloaddition of CO2 with epoxides under mild conditions. Under the condition of the epichlorohydrin/tetrabutylammonium bromide/BL-TF-COF molar ratio of 10210:630:4, the system achieved 62.5% substrate conversion at 25 °C under atmospheric CO2 pressure (1 bar) within 48 h. Mechanistic insights into epoxide adsorption and activation processes were investigated through density functional theory (DFT) calculations.

Robust Tertiary Amine Suspended HCIPs for Catalytic Conversion of CO2 into Cyclic Carbonates under Mild Conditions

A series of tertiary amine suspended hyper-cross-linked ionic polymers (HCIPs), characterized by a rich mesoporous structure, high ionic liquid (IL) density, and good CO2 adsorption capability, were readily prepared via a postsynthetic method. The self-polymerization of 1,3,5-tris(bromomethyl) benzene (TBB) or its copolymerization with 4,4'-bis(bromomethyl) biphenyl (BBP) in varying ratios, followed by grafting with N,N,N',N'-tetramethyl-1,3-propanediamine (TMPDA), yielded the target TMPDA-HCIPs. These HCIPs constitute one of the limited categories of heterogeneous water-tolerant catalyst types ever developed for the cycloaddition reaction between CO2 and epoxides. Specifically, chloropropylene carbonate (CPC) was produced in 99.9% yield with 99% selectivity at 80 °C and 1 bar of CO2 pressure in the presence of 22 mol % water relative to the epoxide substrate. Furthermore, when simulated flue gas served as the CO2 source, the same ratio of water enhanced the CPC yield from 81.9% to 91.5% under 1 MPa pressure, with the selectivity only slightly decreasing from 99% to 94.1%. Additionally, the catalyst could be easily recovered and maintained a high catalytic performance after six cycles. In conclusion, this study presents a robust water-tolerant heterogeneous catalyst for the efficient synthesis of cyclic carbonates from CO2 under mild conditions, potentially reducing the high costs of purifying real flue gas that contains water vapor.

Non-Noble Metal Anchored 2D Covalent Organic Framework for Ambient CO2 Fixation to High-Value Compounds

The catalytic functionalization of CO2 into high-value compounds comprises a promising approach to mitigate its atmospheric content and sustainable generation of fine chemicals. In this respect, covalent organic frameworks (COFs) offer great potential in carbon dioxide capture and utilization. Herein, we report application of a crystalline, nanoporous 2D COF (ET-BP-COF) obtained by condensation of 4,4',4'',4'''-(ethene-1,1,2,2-tetrayl) tetraaniline (ET-NH2) and 2,2'-bipyridyl-5,5'-dialdehyde (BP-CHO) building blocks for strategic utilization of CO2. The ET-BP-COF features a unique 2D kagome (kgm) topology composed of hexagonal and triangular 1D channels decorated with bipyridine sites, which were exploited for covalent anchoring of eco-friendly, alkynophilic Cu(I) by the post-synthetic method. The Cu(I) engrafted COF was applied as a recyclable catalyst for coupling CO2 with alkynes to generate two high-value compounds, α-alkylidene cyclic carbonates (α-ACCs) and 2-oxazolidinones. Notably, Cu(I)@ET-BP-COF demonstrated excellent catalytic performance for transforming propargylic amine and CO2 to 2-oxazolidinone, an essential building block for antibiotics. Besides, an efficient transformation of propargylic alcohols to generate α-ACCs, valuable commodity chemicals, has been achieved by utilizing carbon dioxide. Further, detailed theoretical simulations disclosed the insight mechanistic path of Cu(I) catalyzed coupling of CO2 and alkynes to produce 2-oxazolidinones and α-ACCs. Significantly, the Cu(I)@COF was reusable for multiple cycles without losing framework rigidity and catalytic performance. This study showcases the potential application of ET-BP-COF for stable anchoring of eco-friendly metals as catalytic sites for effective utilization of CO2 to produce two high-value products, 2-oxazolidinones and α-ACCs, under mild atmospheric conditions.

Pyrene-Based Nanoporous Covalent Organic Framework for Carboxylation of C-H Bonds with CO2 and Value-Added 2-Oxazolidinones Synthesis under Ambient Conditions

The selective carbon capture and utilization (CCU) as a one-carbon (C1) feedstock offers dual advantages for mitigating the rising atmospheric CO2 content and producing fine chemicals/fuels. In this context, herein, we report the application of a porous bipyridine-functionalized, pyrene-based covalent organic framework (Pybpy-COF) for the stable anchoring of catalytic Ag(0) nanoparticles (NPs) and its catalytic investigation for fixation of CO2 to commodity chemicals at ambient conditions. Notably, Ag@Pybpy-COF showed excellent catalytic activity for the carboxylation of various terminal alkynes to corresponding alkynyl carboxylic acids/phenylpropiolic acids via C-H bond activation under atmospheric pressure conditions. Besides, carboxylative cyclization of various propargylic amines with CO2 to generate 2-oxazolidinones, an important class of antibiotics, has also been achieved under mild conditions. This significant catalytic activity of Ag@Pybpy-COF with wide functional group tolerance is rendered by the presence of highly exposed, alkynophilic Ag(0) catalytic sites decorated on the pore walls of high surface area (787 m2 g-1) Pybpy-COF. Further, density functional theory calculations unveiled the detailed mechanistic path of the Ag@Pybpy-COF-catalyzed transformation of CO2 to alkynyl carboxylic acids and 2-oxazolidinones. Moreover, the catalyst showed high recyclability for several cycles of reuse without significant loss in its catalytic activity and structural rigidity. This work demonstrates the promising application of Pybpy-COF for stable anchoring of Ag NPs for successful transformation of CO2 to valuable commodity chemicals at ambient conditions.

Hierarchical Amino-Functionalized Ionic Liquids@MOF Composite Gel for Catalytic Conversion of CO2

Hybridization of metal-organic frameworks (MOFs) and homogeneous ionic liquids (ILs) endows the heterogeneous composite with high porosity and accessible multiple active sites (e.g., acidic or basic sites), which exhibits great potential in CO2 capture and conversion. Nevertheless, the majority of MOF composites are synthesized as powders, significantly restricting their practical applications due to inherent problems such as poor handling properties, high pressure drops, and mechanical instability. Thus, it is crucial to shape MOF composites into various monoliths that allow efficient processing, especially for industrial purposes. In this work, a hierarchical ILs@nanoMOF composite gel (H-IL@UiO-66-gel) featuring both intraparticle micropores and interparticle mesopores and multiple active sites was successfully fabricated by a two-step approach. Benefiting from the integrated advantages of the hierarchically porous MOF for enhanced mass transfer and affinity of ILs for activating CO2 molecules, the resultant H-IL@UiO-66-gel exhibits excellent uptake of macromolecules and catalytic activity toward CO2 cycloaddition with epoxides under moderate conditions, far beyond the traditional microporous IL@UiO-66-gel and unfunctionalized H-UiO-66-gel. Furthermore, the H-IL@UiO-66 composite monolith can be effortlessly separated and reused at least three times without depletion of catalytic activity. It is believed that this fabrication method for the shaping of MOF composites is highly versatile and can be extended to other types of MOFs for various application fields.

Fe(III)-Anchored Porphyrin-Based Nanoporous Covalent Organic Frameworks for Green Synthesis of Cyclic Carbonates from Olefins and CO2 under Atmospheric Pressure Conditions

The utilization of carbon dioxide (CO2) as a C1 source coupled with olefins, readily accessible feedstocks, offers dual advantages of mitigating atmospheric carbon dioxide and green synthesis of valuable chemicals. In this regard, herein we demonstrate the application of Fe(III)-anchored porphyrin-based covalent organic framework (P-COF) as a promising recyclable catalyst for one-step generation of cyclic carbonates (CCs), value-added commodity chemicals from olefins and CO2, under mild atmospheric pressure conditions. Moreover, this one-pot synthesis was applied to transform various olefins (aliphatic and aromatic) into the corresponding CCs in good yield and selectivity. In addition, the Fe(III)@P-COF showed good recyclability and durability for multiple reuse cycles without losing its catalytic activity. Notably, this one-step synthesis strategy presents an eco-friendly, atom-economic alternative to the conventional two-step process requiring epoxides. This work represents a rare demonstration of porphyrin COF-catalyzed one-pot CC synthesis by utilizing readily available olefins at atmospheric pressure of carbon dioxide.

Two Stable Sodalite-Cage-Based MOFs for Highly Gas Selective Capture and Conversion in Cycloaddition Reaction

Stable metal-organic frameworks, containing periodically arranged nanosized cages or pores and active Lewis acid-base sites, are considered ideal candidates for efficient heterogeneous catalysis. Herein, based on the light of reticular chemistry design principles, the ingenious assembly of two pyridine N-rich multifunctional triangular linkers, H₃TBA [3,5-di (1h-tetrazol-5-yl) benzoic acid] and H₂TZI [5-(1H-tetrazol-5-yl)isophthalic acid], with Mn^II formed PCP-33(Mn) and PCP-34(Mn), respectively. PCP-33(Mn) and PCP-34(Mn) are typical sod topology zeolitic metal-organic frameworks (ZMOFs) with hierarchical tetragonal micropores and metal organic polyhedral sodalite-like cages. The inner walls of these cages are modified by open metal sites Mn^II and Lewis acid-base sites of halide ions and N pyridine atoms. The characteristics of the cages' structures make two MOFs exhibit high surface area and a small window, which promote their outstanding gas capture ability (C₂H₂, 131.8 cm³ g^-1; CO₂, 77.9 cm³ g^-1 at 273 K) and selective separation performance (C₂H₂/CH₄, 226.2, CO₂/CH₄, 50.3 at 298 K), and are also suitable as catalytic reactors for metal/solvent-free chemical fixation of CO₂ with epoxides to achieve high-efficiency CO₂ conversion. Furthermore, they are greatly recyclable for several cycles while retaining their structural rigidity and catalytic activity.

Amino-Functionalized Ionic-Liquid-Grafted Covalent Organic Frameworks for High-Efficiency CO2 Capture and Conversion

Rationally integrating desired functional components into a composite material can endow the tailored function to achieve the corresponding purpose. This is the first case where a series of [AeImBr]_X%-TAPT-COFs (X = 0, 17, 33, 50, 67, 83, 100) were fabricated by chemically integrating the amino-functionalized imidazole ionic liquid (NH₂-IL) onto channel walls of mesoporous covalent organic framework materials ([HO]_X%-TAPT-COFs). By virtue of the polar groups (amino groups) and abundant imidazole cations of NH₂-IL and its microporous nature, the obtained [AeImBr]_X%-TAPT-COFs exhibit higher CO₂ capture activity than [HO]_X%-TAPT-COFs. Correspondingly, the CO₂ equilibrium capture capacity increases from 62.6 to 117.4 mg/g, which is crucial to the storage of enough CO₂ around the catalytic active sites. Additionally, the synergistic effect of -NH₂ and Br^- in NH₂-IL can also improve the cycloaddition reaction rate. The characteristics of [AeImBr]_X%-TAPT-COFs contribute to the efficient generation of cyclic carbonate through heterogeneously catalyzing CO₂-epoxide cycloaddition without any solvents and cocatalysts. Specifically, [AeImBr]_83%-TAPT-COF has a CO₂ equilibrium capture capacity of 117.4 mg/g and cyclochloroallyl carbonate yield of 99.1%. As a result of the use of the chemical grafting method, [AeImBr]_X%-TAPT-COFs possess excellent stability and cycle life. The equilibrium capture capacity and cyclochloroallyl carbonate yield reach 112.7 mg CO₂/g adsorbent and 95.0% at the eighth cycle.