Improved photocatalytic performance of metal-organic frameworks for CO2 conversion by ligand modification

Porphyrin-Based Metal-Organic Framework Photocatalysts: Structure, Mechanism and Applications

In recent years, porphyrins have been frequently reported as photocatalysts due to their fascinating photochemical properties. However, porphyrins have the same shortcomings as other homogeneous photocatalysts, such as poor stability and difficulty in recovering. To solve this problem, it is a good strategy to form a porphyrin-based metal-organic framework (PMOF) by modifying porphyrin functional groups and adding metals as nodes to connect and control the arrangement of porphyrins. The metal nodes control the rigidity and connectivity of the porphyrin modules to order them in the MOF, which improves the stability of the porphyrins, avoids porphyrin aggregation and folding, and increases the active sites for photocatalytic reactions. This review summarized the research progress of PMOF photocatalysts in the last ten years and analyzed the effects of the spatial structure, porphyrin ligands, porphyrin central metals, and metal nodes of PMOF on the photocatalytic performance. The applications of PMOF-based photocatalysts in H2 production, CO2 reduction, pollutant degradation, and sterilization are reviewed. In addition, the mechanism of these processes is described in detail. Finally, some suggestions on the development of PMOF photocatalysts are put forward.

Metal-Organic Frameworks for Enhanced Hydrogen Generation from Syngas: A Density Functional Theory Approach

Although methane poses environmental concerns, it is employed in hydrogen production processes such as steam-methane reforming (SMR), which has an issue of by-products released. Initiatives are being pursued to address CO and CO2 emissions using carbon capture methods, with the goal of minimizing environmental harm while improving pure hydrogen generation from syngas. In this study, porous coordination network (PCN-250) has been studied for its selective adsorption property towards CO, CO2 and H2O as syngas mixture to obtain pure hydrogen. For this purpose, the trimetallic cluster node Fe2M (where Fe2 represents the 3+ oxidation state and M is Cr(II), Mn(II), Fe(II), Co(II), Ni(II), and Zn(II)) has been considered. Fe(III) in combination with metal atoms like Cr(II), Co(II), or Ni(II) has been found to exhibit enhanced adsorption properties towards CO, CO2 and H2O. The molecule with the strongest interaction was found to be H2O over Fe(III)2Zn(II) cluster and weakest interaction was found between H2 and Fe(III)2Ni(II). Charge transfer, natural bond orbital (NBO) and spin density analyses reveal the electronic structural properties of this combination, leading to enhanced adsorption of CO and CO2.

Z-scheme heterojunction enhanced photocatalytic performance for CO2 reduction to CH4

Due to the high charge separation efficiency leading to high photocatalytic activity, there has been significant interest in enhancing the charge separation ability of photocatalysts by controlling the heterojunction structure. To investigate the effect of the heterojunction structure on the photocatalytic performance of composite catalysts and understand its corresponding mechanism, a Z-scheme ZnFe2O4/ZnO/CdS heterojunction was constructed using the ultrasound method and used for CO2 photoreduction. The Z-scheme heterojunction catalyst demonstrates elevated photocatalytic and charge separation efficiencies. Specifically, the conversion rate for the photocatalytic conversion of CO2 to CH4 reaches 105.9 μmol g-1 h-1, surpassing that of the majority of previously reported semiconductor photocatalysts like ZnFe2O4/CdS. This research offers a fresh perspective on the development of innovative heterojunction photocatalysts and broadens the utilization of ternary composite materials in CO2 photoreduction.

High-Efficiency Photocatalytic Oxidation of Benzyl Alcohol by NH2-UiO-66-(Indole-2,3-Dione)-Fe

The photocatalytic oxidation of biomass-derived benzyl alcohol provides a promising way for the synthesis of benzoic acid, which is an important intermediate with wide applications. To improve the efficiency of photocatalytic benzyl alcohol oxidation to benzoic acid is of great interest. In this work, we propose the utilization of NH2-UiO-66-ID-Fe catalyst for photocatalytic oxidation of benzyl alcohol to benzoic acid, where NH2-UiO-66 is a typically used metal-organic framework, ID is indole-2,3-dione (ID) that has biocompatibility, light absorption property and can be covalently combined with amino-functionalized substances. The NH2-UiO-66-ID-Fe catalyst exhibits improved light absorption and photo-generated electron-hole separation ability compared with NH2-UiO-66. The photocatalytic performance of NH2-UiO-66-ID-Fe was examined for the oxidation of bio-based benzyl alcohol under mild conditions of air atmosphere, room temperature and no additive or additional oxidant involved. The results show that the conversion of benzyl alcohol and the selectivity to benzoic acid could both reach over 99 % in 6 h, and the generation rate of benzoic acid per gram of catalyst is 3.36 mmol g-1 h-1. The reaction mechanism was detected by radical trapping method and in situ electron paramagnetic resonance. This study presents an efficient and environmentally benign avenue for the synthesis of carboxylic acid compounds.

Iron-Complex-Based Supramolecular Framework Catalyst for Visible-Light-Driven CO2 Reduction

Molecule-based heterogeneous photocatalysts without noble metals are one of the most attractive systems for visible-light-driven CO₂ reduction. However, reports on this class of photocatalysts are still limited, and their activities are quite low compared to those containing noble metals. Herein, we report an iron-complex-based heterogeneous photocatalyst for CO₂ reduction with high activity. The key to our success is the use of a supramolecular framework composed of iron porphyrin complexes bearing pyrene moieties at meso positions. The catalyst exhibited high activity for CO₂ reduction under visible-light irradiation (29100 μmol g^-1 h^-1 for CO production, selectivity 99.9%), which is the highest among relevant systems. The performance of this catalyst is also excellent in terms of apparent quantum yield for CO production (0.298% at 400 nm) and stability (up to 96 h). This study provides a facile strategy to create a highly active, selective, and stable photocatalyst for CO₂ reduction without utilizing noble metals.

Development of γ-Al2O3 with oxygen vacancies induced by amorphous structures for photocatalytic reduction of CO2

Inducing amorphous components into Al₂O₃ leads to elongation of the Al-O bond and the formation of oxygen vacancies, which makes Al₂O₃ an independent photocatalyst for CO₂ adsorption and reduction. The generation rate of CO can reach 36.5 μmol g^-1 h^-1, which is 6.5 times that of P25 TiO₂.

Periodically nanoporous hydrogen-bonded organic frameworks for high performance photocatalysis

The development of highly catalytic hydrogen-bonded organic frameworks (HOFs) is of great importance, but remains challenging. Herein, we demonstrate the fabrication of a periodically nanoporous HOF for high performance photocatalysis. Compared with the conventional microporous HOFs, the nanoporous HOF architecture has a larger number of free carboxyl groups on the surface and presents greatly improved photoelectrochemical properties. It exhibits high catalytic activity for the photo-oxidative coupling of amines under mild conditions such as air atmosphere and room temperature and without any co-catalysts, sacrificial reagents or photosensitizers. The relationship between the structure, properties and catalytic performance of the nanoporous HOF was studied by experimental and theoretical investigations. It shows that such a HOF structure facilitates reactant adsorption and O₂ dissociation, thus promoting the oxidative coupling reaction. This work provides a new way for improving the catalytic performance of a single HOF.

Controlled synthesis of Bi- and tri-nuclear Cu-oxo nanoclusters on metal-organic frameworks and the structure-reactivity correlations

Precisely tuning the nuclearity of supported metal nanoclusters is pivotal for designing more superior catalytic systems, but it remains practically challenging. By utilising the chemical and molecular specificity of UiO-66-NH2 (a Zr-based metal-organic framework), we report the controlled synthesis of supported bi- and trinuclear Cu-oxo nanoclusters on the Zr6O4 nodal centres of UiO-66-NH2. We revealed the interplay between the surface structures of the active sites, adsorption configurations, catalytic reactivities and associated reaction energetics of structurally related Cu-based 'single atoms' and bi- and trinuclear species over our model photocatalytic formic acid reforming reaction. This work will offer practical insight that fills the critical knowledge gap in the design and engineering of new-generation atomic and nanocluster catalysts. The precise control of the structure and surface sensitivities is important as it can effectively lead to more reactive and selective catalytic systems. The supported bi- and trinuclear Cu-oxo nanoclusters exhibit notably different catalytic properties compared with the mononuclear 'Cu1' analogue, which provides critical insight for the engineering of more superior catalytic systems.

Application of Porous Materials for CO2 Reutilization: A Review

CO2 reutilization processes contribute to the mitigation of CO2 as a potent greenhouse gas (GHG) through reusing and converting it into economically valuable chemical products including methanol, dimethyl ether, and methane. Solar thermochemical conversion and photochemical and electrochemical CO2 reduction processes are emerging technologies in which solar energy is utilized to provide the energy required for the endothermic dissociation of CO2. Owing to the surface-dependent nature of these technologies, their performance is significantly reliant on the solid reactant/catalyst accessible surface area. Solid porous structures either entirely made from the catalyst or used as a support for coating the catalyst/solid reactants can increase the number of active reaction sites and, thus, the kinetics of CO2 reutilization reactions. This paper reviews the principles and application of porous materials for CO2 reutilization pathways in solar thermochemical, photochemical, and electrochemical reduction technologies. Then, the state of the development of each technology is critically reviewed and evaluated with the focus on the use of porous materials. Finally, the research needs and challenges are presented to further advance the implementation of porous materials in the CO2 reutilization processes and the commercialization of the aforementioned technologies.

Non-conjugated flexible network for the functional design of silica-based stationary phase for mixed-mode liquid chromatography

The functionalized non-conjugated organic network modified silica microspheres are proposed as the stationary phase of liquid chromatography, which can effectively avoid some defects of organic framework materials in liquid chromatography. Due to the co-existing of pyridine ring, carbonyl group, amide group and triazine ring in the non-conjugated flexible organic network on the silica surface, the developed mixed-mode stationary phase exhibits multiple interactions between the stationary phase and the analytes during the separation process. A variety of nucleoside bases, organic acids, antibiotics, pesticides, alkylbenzenes, polycyclic aromatic hydrocarbons and sulfonamides achieved ideal resolution and flexible selectivity in separation. Compared with the commercial chromatographic columns under their optimized chromatographic conditions, it shows better performance for the separation of complex analytes. The influence of chromatographic conditions on retention behavior indicates that the column's multiple retention mechanisms make it suitable for mixed-mode liquid chromatography. The stationary phase prepared by the new design strategy also has excellent chromatographic reproducibility, repeatability and stability with the intraday RSD of 0.09%-0.12% (n = 10) and the interday RSD of 0.37%-1.64% (n = 5) for the retention time. The separation results of actual samples also prove its potential in the analysis of complex samples. In short, we designed and prepared the non-conjugated flexible network modified silica stationary phase material for liquid chromatography that is different from organic framework materials. Its excellent separation ability shows that we have successfully reported a new kind of liquid chromatography packing with functional design and facile preparation method.

Water-Assisted Highly Efficient Photocatalytic Reduction of CO2 to CO with Noble Metal-Free Bis(terpyridine)iron(II) Complexes and an Organic Photosensitizer

Photocatalytic CO₂ reduction reaction is believed to be a promising approach for CO₂ utilization. In this work, a noble metal-free photocatalytic system, composed of bis(terpyridine)iron(II) complexes and an organic thermally activated delayed fluorescence compound, has been developed for selective reduction of CO₂ to CO with a maximum turnover number up to 6320, 99.4% selectivity, and turnover frequency of 127 min^-1 under visible-light irradiation in dimethylformamide/H₂O solution. More than 0.3 mmol CO was generated using 0.05 μmol catalyst after 2 h of light irradiation. The apparent quantum yield was found to be 9.5% at 440 nm (180 mW cm^-2). Control experiments and UV-vis-NIR spectroscopy studies further demonstrated that water strongly promoted the photocatalytic cycle and terpyridine ligands rather than Fe(II) were initially reduced during the photocatalytic process.

High selective photocatalytic CO2 conversion into liquid solar fuel over a cobalt porphyrin-based metal-organic framework

In the present study, porphyrin-based metal-organic framework (Co/PMOF) was synthesized and characterized by different spectra analyses. The photoluminescence properties of porphyrin and Co/PMOF revealed that the photoluminescence of Co/PMOF was quenched compared to the porphyrin, indicating that the lifetime of photo-generated charge carriers in Co/PMOF is longer than porphyrin. The prepared Co/PMOF was applied as an efficient photocatalyst for CO₂ photoconversion to formate in the presence of triethanolamine (TEOA) as a sacrificial agent under visible-light irradiation. The photoreaction results showed that 23.21 µmol HCOO^- was produced over Co/PMOF during the 6 h photocatalytic reaction under visible illumination, showing much better activity than the porphyrin, 4.56 µmol HCOO^-. No other products were detected, suggesting that this reaction over Co/PMOF has high selectivity. Co/PMOF reusability and stability were examined through recycling tests and there were no remarkable losses of photoactivity even after three cycles of photoreaction. Moreover, FTIR measurement and UV-Vis spectra demonstrated no notable changes in Co/PMOF structure. As a result, superior photocatalytic behavior of Co/PMOF was implied for CO₂ photoreduction which highlights the great potential of assembly porphyrin and cobalt into MOFs for CO₂ photoreduction.

Porous framework-based hybrid materials for solar-to-chemical energy conversion: from powder photocatalysts to photoelectrodes

Porous framework materials such as metal organic frameworks (MOFs) and covalent organic frameworks (COFs) can be considered promising materials for solar-to-chemical energy conversion.