From Trash to Treasure: Probing Cycloaddition and Photocatalytic Reduction of CO₂ over Cerium-Based Metal–Organic Frameworks

Abstract

Fixing the greenhouse gas CO2 via cycloaddition reactions to value-added cyclic carbonates or photocatalytic reduction of CO2 to produce desirable fuels is the most coveted, although challenging, energy-efficient valorization technique owing to the chemical and thermodynamic stability of CO2. Therefore, catalytic materials with an optimum amount of surface porosity and Lewis acidity and basicity play an instrumental and crucial role in CO2 activation in the CO2 fixation reaction. On the other hand, the efficacy of the photocatalytic reduction of CO2 depends on the appropriate band alignment of the catalytic materials and the competitive H2 evolution reaction. Here, we have synthesized Ce-MOFs with and without amine functionalization and compared both MOFs for CO2 fixation with epoxides and photocatalytic reduction of CO2. The amine-functionalized MOF, Ce-BDC-NH2, not only exhibited efficient CO2 fixation to produce cyclic carbonate at room temperature and pressure but also demonstrated effective photoreduction of CO2 with high selectivity toward CH3OH and HCO2H. The probing of the physical properties of the MOFs unveiled that due to the optimum amount of Lewis acidic site Ce3+ and Ce4+, Ce-BDC-NH2 could effectively adsorb the epoxide for the CO2 cycloaddition reaction, whereas the amino functionalization highly influenced the band maxima and minima to facilitate the photoreduction of CO2 with minimized H2 evolution reaction.

Ligand Defect Density Regulation in Metal–Organic Frameworks by Functional Group Engineering on Linkers

“…Intuitively, the Ce-MOFs with a high density of ligand defects possess abundant unsaturated coordination sites (i.e., ligand defect sites), which could serve as active sites for various catalytic reactions, including C–H oxidation, cyanosilylation, and CO 2 cycloaddition reaction. − Although the MOFs with unsaturated Ce sites have been demonstrated to be useful to CO 2 cycloaddition reaction, ,, the limitation of adjustment in the concentration of unsaturated Ce sites may hinder the identification of accurate active site. In this case, the generated four isomorphic Ce-UiO-66-X samples with flexible tunability of ligand defect densities (i.e., unsaturated Ce sites) is thus conducive to the establishment of the structure–activity relationship.…”

S-Scheme Bi-oxide/Ti-oxide Molecular Hybrid for Photocatalytic Cycloaddition of Carbon Dioxide to Epoxides

“…While this conversion could be effectively catalyzed with Lewis acidic and basic reagents at high temperatures, there have been unremitting endeavors to develop new catalytic systems that use sunlight as the energy source and operate under mild conditions (e.g., ambient temperature and 1 atm CO 2 ). In this regard, photocatalysis, which uses photogenerated electrons and holes to trigger CO 2 cycloadditions via new reaction pathways distinct from traditional Lewis acid/base thermocatalysis, represents a promising new direction. − Due to their high charge-separation efficiency, semiconductor heterojunctions composed of two transition metal-oxide components are a class of highly efficient photocatalysts. Among them, the S-scheme systems (also known as the direct Z-scheme) are known for their high redox power due to their unique charge carrier-transfer mode, − which has been shown to benefit challenging reactions like CO 2 reduction. − Given the cycloaddition reactions of CO 2 and epoxides, it is assumed that S-scheme systems may benefit from a synergistic effect of the very strong driving force and the surface coordination-unsaturated transition metal ions that could serve as Lewis acidic sites.…”

Rational Design of Metal–Organic Framework‐Based Materials for Photocatalytic CO2 Reduction

“…Intrigued by the merits of amine groups, some studies on the design of new MOF structures composed of linkers with amine groups as efficient photocatalysts for the reduction of CO 2 have also been reported. [ 31 ]…”