PHOTOCATALYSIS · Recyclable Metal-free Photocatalysts for Synthetic Chemistry based on Covalent Organic Frameworks

Recyclable Metal-free Photocatalysts for Synthetic Chemistry based on Covalent Organic Frameworks

The motivation of the photocatalysis project is to identify stable, recyclable, and metal-free heterogeneous photocatalysts for the Diels-Alder (D–A) reaction.[1] The D–A cycloaddition reaction is one of the most powerful synthetic protocols for the synthesis of unsaturated six-membered rings, yet to be established for the industry. Designed synthesis of photoactive porous organic polymers, namely, crystalline Covalent Organic Frameworks (COFs)/Covalent Triazine Frameworks (CTFs) has been considered here as a metal-free photocatalytic platform for D-A reaction because they allow synergistic utilization of their skeleton and pores. High porosity and uniform pores of the COFs/CTFs act as a nanoreactor for selective catalysis, which can be controlled by elementary pore design based on a topological diagram. These metal-free insoluble polymer-catalysts exhibited high thermal and chemical stability. Therefore, these heterogeneous catalysts can be easily separated from the reaction mixture and re-activated for cycle use. The discovery of smart and highly efficient catalysts for solar energy conversion and green fuel production is a global scientific challenge due to increasing energy demand and related environmental consequences. The synthetic photocatalyst is highly promising but employs to date is expensive and/or toxic metals (Pt, Au, Ru). This hampers the development of large-scale synthesis and introduces detrimental effects to the environment. Along this line, one of the biggest challenges in the case of homogeneous catalysts is the recyclability of the catalyst itself, thus waste chemical management is another challenge after catalytic application by the homogeneous catalyst to avoid environmental contamination. Therefore, the search for inexpensive, efficient, and environment-friendly catalysts is to deliver safer and cheaper products to society. Furthermore, heterogeneous catalysts are ideal candidates for synthesizing materials in a continuous-flow reactor, as they stay in a place during the continuous flow process and, thus easily be recycled. Objectives of this Marie Skłodowska Curie Action (MSCA) have been to (a) design and synthesize stable, highly crystalline and, highly porous COFs/CTFs, (b) introduce desired functionality via elementary molecular design of appropriate monomers; suitable for efficient visible light absorption, (c) control pore size and porosity by employing framework construction topology for selective mass transport during the catalytic application, (d) investigate COFs/CTFs catalysts for important photocatalytic organic syntheses, such as D-A reactions, (e) implement photocatalysts in flow-chemistry, (f) interpret the structure-function correlation and feedback to the catalyst design and effectiveness of metal-free photocatalytic D-A reactions. References: [1]. Y. Zhao and M. Antonietti, Angew. Chem. Int. Ed., 2017, 56, 9336-9340.

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