Ionic Liquid-Modified Porous Organometallic Polymers as Efficient and Selective Photocatalysts for Visible-Light-Driven CO2 Reduction

Ionic Liquids in Metal, Photo-, Electro-, and (Bio) Catalysis

Ionic liquids (ILs) have unique physicochemical properties that make them advantageous for catalysis, such as low vapor pressure, non-flammability, high thermal and chemical stabilities, and the ability to enhance the activity and stability of (bio)catalysts. ILs can improve the efficiency, selectivity, and sustainability of bio(transformations) by acting as activators of enzymes, selectively dissolving substrates and products, and reducing toxicity. They can also be recycled and reused multiple times without losing their effectiveness. ILs based on imidazolium cation are preferred for structural organization aspects, with a semiorganized layer surrounding the catalyst. ILs act as a container, providing a confined space that allows modulation of electronic and geometric effects, miscibility of reactants and products, and residence time of species. ILs can stabilize ionic and radical species and control the catalytic activity of dynamic processes. Supported IL phase (SILP) derivatives and polymeric ILs (PILs) are good options for molecular engineering of greener catalytic processes. The major factors governing metal, photo-, electro-, and biocatalysts in ILs are discussed in detail based on the vast literature available over the past two and a half decades. Catalytic reactions, ranging from hydrogenation and cross-coupling to oxidations, promoted by homogeneous and heterogeneous catalysts in both single and multiphase conditions, are extensively reviewed and discussed considering the knowledge accumulated until now.

Incorporating Catalytic Units into Nanomaterials: Rational Design of Multipurpose Catalysts for CO2 Valorization

ConspectusCO2 conversion to valuable chemicals is effective at reducing CO2 emissions. We previously proposed valorization strategies and developed efficient catalysts to address thermodynamic stability and kinetic inertness issues related to CO2 conversion. Earlier, we developed molecular capture reagents and catalysts to integrate CO2 capture and conversion, i.e., in situ transformation. Based on the mechanistic understanding of CO2 capture, activation, and transformation at a molecular level, we set out to develop heterogeneous catalysts by incorporating catalytic units into nanomaterials via the immobilization of active molecular catalysts onto nanomaterials and designing nanomaterials with intrinsic catalytic sites.In thermocatalytic CO2 conversion, carbonaceous and metal-organic framework (MOF)-based catalysts were developed for nonreductive and reductive CO2 conversion. Novel Cu- and Zn-based MOFs and carbon-supported Cu catalysts were prepared and successfully applied to the cycloaddition, carboxylation, and carboxylative cyclization reactions with CO2, generating cyclic carbonates, carboxyl acids, and oxazolidinones as respective target products. Reductive conversion of CO2, especially reductive functionalization with CO2, is a promising transformation strategy to produce valuable chemicals, alleviating chemical production that relies on petrochemistry. We explored the hierarchical reductive functionalization of CO2 using organocatalysts and proposed strategies to regulate the CO2 reduction level, triggering heterogeneous catalyst investigation. Introducing multiple active sites into nanomaterials opens possibilities to develop novel CO2 transformation strategies. CO2 capture and in situ conversion were realized with an N-doped carbon-supported Zn complex and MOF materials as CO2 adsorbents and catalysts. These nanomaterial-based catalysts feature high stability and excellent efficiency and act as shape-selective catalysts in some cases due to their unique pore structure.Nanomaterial-based catalysts are also appealing candidates for photocatalytic CO2 reduction (PCO2RR) and electrocatalytic CO2 reduction (ECO2RR), so we developed a series of hybrid photo-/electrocatalysts by incorporating active metal complexes into different matrixes such as porous organic polymers (POPs), metal-organic layers (MOLs), micelles, and conducting polymers. By introducing Re-bipyridine and Fe-porphyrin complexes into POPs and regulating the structure of the polymer chain, catalyst stability and efficiency increased in PCO2RR. PCO2RR in aqueous solution was realized by designing the Re-bipyridine-containing amphiphilic polymer to form micelles in aqueous solution and act as nanoreactors. We prepared MOLs with two different metallic centers, i.e., the Ni-bipyridine site and Ni-O node, to improve the efficiency for PCO2RR due to the synergistic effect of these metal centers. Sulfylphenoxy-decorated cobalt phthalocyanine (CoPc) cross-linked polypyrrole was prepared and used as a cathode, achieving the electrocatalytic transformation of diluted CO2 benefiting from the CO2 adsorption capability of polypyrrole. We fabricated immobilized 4-(t-butyl)-phenoxy cobalt phthalocyanine and Bi-MOF as cathodes to promote the paired electrolysis of CO2 and 5-hydroxymethylfurfural (HMF) and obtained CO2 reductive products and 2,5-furandicarboxylic acid (FDCA) efficiently.

Dynamic Active Sites in Bi5O7I Promoted by Surface Tensile Strain Enable Selective Visible Light CO2 Photoreduction

Surface defects with abundant localized electrons on bismuth oxyhalide catalysts are proved to have the capability to capture and activate CO₂. However, bismuth oxyhalide materials are susceptible to photocorrosion, making the surface defects easily deactivated and therefore losing their function as active sites. Construction of deactivation-resistant surface defects on catalyst is essential for stable CO₂ photoreduction, but is a universal challenge. In this work, the Bi₅O₇I nanotubes with surface tensile strain are synthesized, which are favorable for the visible light-induced dynamic I defects generation. The CO₂ molecules absorbed on I defects are constantly reduced by the incoming photogenerated electrons from I-deficient Bi₅O₇I nanotubes and the successive protonation of CO₂ molecules is thus highly promoted, realizing the selective CO₂ conversion process via the route of CO₂-COOH^--CO. The efficient and stable photoreduction of CO₂ into CO with 100% selectivity can be achieved even under visible light (λ >420 nm) irradiation benefited from the dynamic I defects as active sites. The results presented herein demonstrate the unique action mechanism of light-induced dynamic defects during CO₂ photoreduction process and provide a new strategy into rational design of deactivation-resistant catalysts for selective CO₂ photoreduction.

Boosting the photocatalytic hydrogen production performance of graphitic carbon nitride nanosheets by tailoring the cyano groups

Graphitic carbon nitride (g-C₃N₄) is a promising visible light responsive photocatalyst for solar hydrogen production. However, pristine g-C₃N₄ suffers from severe charge recombination, resulting in a poor photocatalytic activity. Herein, a facile KOH-assisted sealed heating process is designed to tailor the electronic structure of g-C₃N₄, leading to a significantly enhanced and stable photocatalytic hydrogen production rate of 225.1 µmol h^-1 using only 50 mg of the photocatalyst. An excellent apparent quantum efficiency of 16.82% is achieved at 420 nm. Systematic studies reveal that KOH-assisted sealed heating can generate more cyano groups onto the framework of g-C₃N₄, which can increase the charge carrier density and reduce the surface charge transfer resistance, promoting charge separation and transfer. The new findings demonstrated in this work provide a facile strategy for the design of low-cost and efficient photocatalyst for solar fuel production.