Confinement Effects in Catalysis Using Well-Defined Materials and Cages

This review focuses on the effects that confinement of molecular and heterogeneous catalysts with well-defined structure has on the selectivity and activity of these systems. A general introduction about catalysis and how the working principles of enzymes can be used as a source of inspiration for the preparation of catalysts with enhanced performance is provided. Subsequently, relevant studies demonstrate the importance of second coordination sphere effects in a broad sense (in homogeneous and heterogeneous catalysis). Firstly, we discuss examples involving zeolites, MOFs and COFs as heterogeneous catalysts with well-defined structures where confinement influences catalytic performance. Then, specific cases of homogeneous catalysts where non-covalent interactions determine the selectivity and activity are treated in detail. This includes examples based on cyclodextrins, calix[n]arenes, cucurbit[n]urils, and self-assembled container molecules. Throughout the review, the impact of confined spaces is emphasized and put into context, in order to get a better understanding of the effects of confinement on catalyst performance. In addition, this analysis intends to showcase the similarities between homogeneous and heterogeneous catalysts, which may aid the development of novel strategies

Iron oxide-promoted photochemical oxygen reduction to hydrogen peroxide (H₂O₂)

Hydrogen peroxide (H2O2) is a valuable green oxidant with a wide range of applications. Furthermore, it is recognized as a possible future energy carrier achieving safe operation, storage and transportation. The photochemical production of H2O2 serves as a promising alternative to the waste- and energy-intensive anthraquinone process. Following the 12 principles of Green Chemistry, we demonstrate a facile and general approach to sustainable catalyst development utilizing earth-abundant iron and biobased sources only. We developed several iron oxide (FeOx) nanoparticles (NPs) for successful photochemical oxygen reduction to H2O2 under visible light illumination (445 nm). Achieving a selectivity for H2O2 of >99%, the catalyst material could be recycled for up to four consecutive rounds. An apparent quantum yield (AQY) of 0.11% was achieved for the photochemical oxygen reduction to H2O2 with visible light (445 nm) at ambient temperatures and pressures (9.4–14.8 mmol g−1 L−1). Reaching productivities of H2O2 of at least 1.7 ± 0.3 mmol g−1 L−1 h−1, production of H2O2 was further possible via sunlight irradiation and in seawater. Finally, a detailed mechanism has been proposed on the basis of experimental investigation of the catalyst's properties and computational results

Frontiers, Opportunities, and Challenges in Biochemical and Chemical Catalysis of CO_2 Fixation

Two major energy-related problems confront the world in the next 50 years. First, increased worldwide competition for gradually depleting fossil fuel reserves (derived from past photosynthesis) will lead to higher costs, both monetarily and politically. Second, atmospheric CO_2 levels are at their highest recorded level since records began. Further increases are predicted to produce large and uncontrollable impacts on the world climate. These projected impacts extend beyond climate to ocean acidification, because the ocean is a major sink for atmospheric CO2.1 Providing a future energy supply that is secure and CO_2-neutral will require switching to nonfossil energy sources such as wind, solar, nuclear, and geothermal energy and developing methods for transforming the energy produced by these new sources into forms that can be stored, transported, and used upon demand

Synthesis of functional ‘polyolefins’: state of the art and remaining challenges

Functional polyolefins (i.e., polyethene or polypropene bearing functional groups) are highly desired materials, due to their beneficial surface properties. Many different pathways exist for the synthesis of these materials, each with its own advantages and drawbacks. This review focuses on those synthetic pathways that build up a polymer chain from ethene/propene and functionalised polar vinyl monomers. Despite many recent advances in the various fields of olefin polymerisation, it still remains a challenge to synthesise high molecular- weight copolymers with tuneable amounts of functional groups, preferably with consecutive insertions of polar monomers occurring in a stereoselective way. To overcome some of these challenges, polymerisation of alternative functionalised monomers is explored as well

New Endeavors in Gold Catalysis-Size Matters

International audienceGolden times: Recent breakthroughs in gold-catalyzed transformations using nanosized homogeneous gold catalysts are highlighted. These catalysts have activities and stabilities comparable to (or even surpassing) heterogeneous catalysts. Well-defined, ligand-supported gold clusters turned out to be active in homogeneous catalysis, a catalyst concept which holds potential for future studies

Transition-Metal Encapsulation within Supramolecular Diphosphine Capsules

International audienceThis review provides a detailed overview on the research carried out in self-assembled supramolecular capsules enabling en-capsulation of transition metals within their inner space. The supramolecular assemblies discussed are based on cationic/anionic interactions between cone-shaped building blocks, derived from quaternary ammonium and sulfonated salts, which upon assembly give rise to closed cavitands. Furthermore, one or both of the ionic building blocks contain diphosphine groups located at the cavity core. Thus, upon chelation of a transition-metal ion to the phosphorus(III) donor atoms, the metal fragment lies positioned in the molecular container. In addition, metal coordination does not affect the overall structure of the supramolecular capsule