Effect of Building Block Transformation in Covalent Triazine-Based Frameworks for Enhanced CO2 Uptake and Metal-Free Heterogeneous Catalysis

In situ growth preparation of a new chiral covalent triazine framework core-shell microspheres used for HPLC enantioseparation

The manufacturing of chiral covalent triazine framework core-shell microspheres CC-MP CCTF@SiO₂ composite is reported as stationary phase for HPLC enantioseparation. The CC-MP CCTF@SiO₂ core-shell microspheres were prepared by immobilizing chiral COF CC-MP CCTF constructed using cyanuric chloride and (S)-2-methylpiperazine on the surface of activated SiO₂ through an in-situ growth approach. Various racemates as analytes were separated on the CC-MP CCTF@SiO₂-packed column. The experimental results indicate that 19 pairs of enantiomers were well separated on the CC-MP CCTF@SiO₂-packed column, including alcohols, phenols, amines, ketones, and organic acids. Among them, there are 17 pairs of enantiomers that can achieve baseline separation with good peak shapes. Their resolution values on this chiral column are between 0.4 and 5.61. The influences of analyte mass, column temperature, and composition of the mobile phase on the resolution of enantiomers were studied. In addition, the chiral resolution ability of CC-MP CCTF@SiO₂-packed column was compared with the commercial chiral chromatographic columns (Chiralpak AD-H and Chiralcel OD-H columns) and some CCOF@SiO₂ chiral columns (β-CD-COF@SiO₂, CTpBD@SiO₂, and MDI-β-CD-modified COF@SiO₂). The CC-MP CCTF@SiO₂-packed column exhibited some unique advantages and can complement these chiral columns in chiral separations. The research results show that the CC-MP CCTF@SiO₂ chiral column offered high column efficiency (e.g., 17680 plates m^-1 for ethyl mandelate), low column backpressure (5-9 bar), high enantioselectivity, and excellent chiral resolution ability for HPLC enantioseparation with good stability and reproducibility. The relative standard deviations (RSD) (n = 5) of the retention time, and peak areas by repeated separation of ethyl mandelate are 0.23% and 0.67%, respectively. It demonstrates that the CC-MP CCTF@SiO₂ core-shell microsphere composite has great potential in enantiomeric separation by HPLC.

Porous Polymer Materials for CO2 Capture and Electrocatalytic Reduction

Efficient capture of CO₂ and its conversion into other high value-added compounds by electrochemical methods is an effective way to reduce excess CO₂ in the atmosphere. Porous polymeric materials hold great promise for selective adsorption and electrocatalytic reduction of CO₂ due to their high specific surface area, tunable porosity, structural diversity, and chemical stability. Here, we review recent research advances in this field, including design of porous organic polymers (POPs), porous coordination polymers (PCPs), covalent organic frameworks (COFs), and functional nitrogen-containing polymers for capture and electrocatalytic reduction of CO₂. In addition, key issues and prospects for the optimal design of porous polymers for future development are elucidated. This review is expected to shed new light on the development of advanced porous polymer electrocatalysts for efficient CO₂ reduction.

Porous organic polymers for CO2 capture, separation and conversion

Porous organic polymers (POPs) have long been considered as prime candidates for carbon dioxide (CO2) capture, separation, and conversion. Especially their permanent porosity, structural tunability, stability and relatively low cost are key factors in such considerations. Whereas heteratom-rich microporous networks as well as their amine impregnation/functionalization have been actively exploited to boost the CO2 affinity of POPs, recently, the focus has shifted to engineering the pore environment, resulting in a new generation of highly microporous POPs rich in heteroatoms and featuring abundant catalytic sites for the capture and conversion of CO2 into value-added products. In this review, we aim to provide key insights into structure-property relationships governing the separation, capture and conversion of CO2 using POPs and highlight recent advances in the field.

CO2 Adsorption on Modified Mesoporous Silicas: The Role of the Adsorption Sites

The post-synthesis procedure for cyclic amine (morpholine and 1-methylpiperazine) modified mesoporous MCM-48 and SBA-15 silicas was developed. The procedure for preparation of the modified mesoporous materials does not affect the structural characteristics of the initial mesoporous silicas strongly. The initial and modified materials were characterized by XRD, N₂ physisorption, thermal analysis, and solid-state NMR. The CO₂ adsorption of the obtained materials was tested under dynamic and equilibrium conditions. The NMR data revealed the formation of different CO₂ adsorbed forms. The materials exhibited high CO₂ absorption capacity lying above the benchmark value of 2 mmol/g and stretching out to the outstanding 4.4 mmol/g in the case of 1-methylpiperazin modified MCM-48. The materials are reusable, and their CO₂ adsorption capacities are slightly lower in three adsorption/desorption cycles.

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.

Salen-decorated Periodic Mesoporous Organosilica: From Metal-assisted Epoxidation to Metal-free CO2 Insertion

We clicked a salen ligand onto a thiol-ethane bridged periodic mesoporous organosilica (Salen-PMO) using a photo-initiated thiol-ene click reaction. This process resulted in a covalently bonded salen ligand on the PMO material. The final BET surface area amounts 511 m² /g and the pore size diameter is approximately 7 nm. The functionalized PMO material showed an excellent carbon dioxide uptake capacity of 1.29 mmol/g at 273 K and 1 bar. More importantly, by coordinating a MoO₂ ²⁺ complex onto the Salen-PMO material, we obtained a heterogeneous catalyst with a good catalytic performance for the epoxidation of cyclohexene. The catalyst was highly reusable, as no decrease in its activity was observed for at least four runs (99% conversion). Finally, the metal-free Salen-PMO showed an exceptional catalytic performance in the cycloaddition of CO₂ to epoxides. The obtained results clearly demonstrate the versatility of the Salen-PMO material not only as metal-free catalyst but also as a support material to anchor metal complexes for specific catalytic applications. With the same catalytic platform, we were able to firstly create epoxides out of alkenes, and subsequently turn these epoxides into cyclic carbonates, consuming CO₂ .

Rigid Nanoporous Urea-Based Covalent Triazine Frameworks for C2/C1 and CO2/CH4 Gas Separation

C2/C1 hydrocarbon separation is an important industrial process that relies on energy-intensive cryogenic distillation methods. The use of porous adsorbents to selectively separate these gases is a viable alternative. Highly stable covalent triazine frameworks (urea-CTFs) have been synthesized using 1,3-bis(4-cyanophenyl)urea. Urea-CTFs exhibited gas uptakes of C₂H₂ (3.86 mmol/g) and C₂H₄ (2.92 mmol/g) at 273 K and 1 bar and is selective over CH₄. Breakthrough simulations show the potential of urea-CTFs for C2/C1 separation.

Fluorocarbon-Functionalized Superhydrophobic Metal-Organic Framework: Enhanced CO2 Uptake via Photoinduced Postsynthetic Modification

The design and synthesis of porous materials for selective capture of CO₂ in the presence of water vapor is of paramount importance in the context of practical separation of CO₂ from the flue gas stream. Here, we report the synthesis and structural characterization of a photoresponsive fluorinated MOF {[Cd(bpee)(hfbba)]·EtOH}_n (1) constructed by using 4,4'-(hexafluoroisopropylidene)bis(benzoic acid) (hfbba), Cd(NO₃)₂, and 1,2-bis(4-pyridyl)ethylene (bpee) as building units. Due to the presence of the fluoroalkyl -CF₃ functionality, compound 1 exhibits superhydrophobicity, which is validated by both water vapor adsorption and contact angle measurements (152°). The parallel arrangement of the bpee linkers makes compound 1 a photoresponsive material that transforms to {[Cd₂(rctt-tpcb)(hfbba)₂]·2EtOH}_n (rctt-tpcb = regio cis,trans,trans-tetrakis(4-pyridyl)cyclobutane; 1IR) after a [2 + 2] cycloaddition reaction. The photomodified framework 1IR exhibits increased uptake of CO₂ in comparison to 1 under ambient conditions due to alteration of the pore surface that leads to additional weak electron donor-acceptor interactions with the -CF₃ groups, as examined through periodic density functional theory calculations. The enhanced uptake is also aided by an expansion of the pore window, which contributes to increasing the rotational entropy of CO₂, as demonstrated through force field based free energy calculations.

Illustrating the Role of Quaternary-N of BINOL Covalent Triazine-Based Frameworks in Oxygen Reduction and Hydrogen Evolution Reactions

Defective nitrogen-doped carbon materials have shown a promising application as metal-free electrocatalysts in the oxygen reduction reaction (ORR) and the hydrogen evolution reaction (HER). However, there are still some challenges in the tuning of metal-free electrocatalysts and in understanding the roles of various nitrogen species in their electrocatalytic performance. Herein, we design a covalent triazine framework (CTF)-based material as an effective metal-free bifunctional electrocatalyst. We chose BINOL-CN (2,2'-dihydroxy-[1,1'-binaphthalene]-6,6'-dicarbonitrile) as both a carbon and a nitrogen source for the fabrication of N-containing CTF-based materials. Four BINOL-CTFs with varying N-functionalities (pyridinic-N/triazine-N, pyrrolic-N, quaternary-N, and pyridine-N-oxide) were successfully obtained. These materials were evaluated in the ORR and the HER in basic and acidic conditions, respectively. The best material has an onset potential of 0.793 V and a half-wave potential of 0.737 V, and it follows first-order kinetics in a 4e^- pathway in the ORR reaction. The same material shows an impressive HER activity with an overpotential of 0.31 V to achieve 10 mA/cm² and a small Tafel slope of 41 mV/dec, which is comparable to 31 mV/dec for Pt/C, making it a potential bifunctional electrocatalyst. We showed that the ORR and HER reactivity of CTF-based materials depends exclusively on the amount of quaternary-N species and on the available surface area and pore volume. This work highlights the engineering of CTF materials with varying amounts of N species as high-performance bifunctional electrocatalysts.

Coinage metal metallacycles involving a fluorinated 3,5-diarylpyrazolate

Photoluminescent, trinuclear, coinage metal pyrazolates have been isolated using a fluorinated diaryl-pyrazolate.