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

Periodic Reporting for period 1 - PHOTOCATALYSIS (Recyclable Metal-free Photocatalysts for Synthetic Chemistry based on Covalent Organic Frameworks)

Okres sprawozdawczy: 2019-09-01 do 2021-08-31

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.
[1]. Y. Zhao and M. Antonietti, Angew. Chem. Int. Ed., 2017, 56, 9336-9340.
The objectives and goals have been addressed through four specific work packages (WPs); (1) WP1 - Designed synthesis of photoactive monomers, (2) WP2 - Synthesis and structure of the photoactive COFs/CTFs, (3) WP3 – Application of COFs/CTFs for photocatalytic Diels-Alder (D–A) reaction, (4) WP4 – Implementation of these catalysts in a continuous flow reactor. However, the work performed during this project running period is ascribed to three specific work packages (WP1, WP2, WP3), and the results are identified through achieved milestones (M1, M2, M3, M4). At first, the fellow has been focused on the COF-based metal-free photocatalytic D-A reactions (Figure 1). The photoactive amine/aldehyde functionalized aromatic building blocks/monomers, including pyrene, tetraphenylethylene, triphenylbenzene, triazine, and porphyrin were synthesized (M1). By adopting appropriate topology (Scheme 1), monomer symmetry, and reactive functional group, imine-linked (C=N) COFs were synthesized (M2, M3) by using solvothermal synthetic methods.[2] The synthesized monomers and COFs were characterized by using various analytical characterization methods, such as nuclear magnetic resonance (NMR) spectroscopy, X-ray diffraction (XRD), and Brunauer-Emmett-Teller (BET) surface area measurement. These photoactive COFs exhibited high crystallinity and porosity, thus suitable for photocatalysis.[3-8] These COFs were used for visible light (excited with blue LED, 450 nm) photocatalytic [4+2] D-A cycloaddition reaction of trans-anethole (TA) and isoprene in nitromethane solvent at room temperature (M4). The characterization of the D-A reaction product(s) and catalytic performance is yet to be optimized. However, during this project tenure fellow has been collaboratively worked on other project related research topics, such as ‘CTF as a physiological molecular thermometer’ (published in the Journal of Materials Chemistry C, 2021, DOI: 10.1039/D1TC00999K RSC, IF: 7.059) ‘selective metal-free catalysis by CTFs’ (revised manuscript submitted in the Applied Catalysis B: Environmental, Elsevier, IF: 16.683) and metal-free photocatalytic D-A reactions (manuscript in preparation).
[2]. S. Dalapati, M. Addicoat, S. Jin, T. Sakurai, J. Gao, H. Xu, S. Irle, S. Seki and D. Jiang, Nat. Commun., 2015, 6, 7786.
[3]. M. Li, H. Zhang, Y. Zhang, B. Hou, C. Li, X. Wang, J. Zhang, L. Xiao, Z. Cui and Y. Ao, J. Mat. Chem. C, 2016, 4, 9094-9102.
[4]. V. S. Vyas, F. Haase, L. Stegbauer, G. Savasci, F. Podjaski, C. Ochsenfeld and B. V. Lotsch, Nat. Commun., 2015, 6, 8508.
[5]. M. Yuasa, K. Oyaizu, A. Yamaguchi and M. Kuwakado, J. Am. Chem. Soc., 2004, 126, 11128-11129.
[6]. W. Chen, Z. Yang, Z. Xie, Y. Li, X. Yu, F. Lu and L. Chen, J. Mat. Chem. A, 2019, 7, 998-1004.
[7]. V. S. Vyas, F. Haase, L. Stegbauer, G. Savasci, F. Podjaski, C. Ochsenfeld and B. V. Lotsch, Nat. Commun., 2015, 6, 8508.
[8]. L. Ascherl, E. W. Evans, M. Hennemann, D. Di Nuzzo, A. G. Hufnagel, M. Beetz, R. H. Friend, T. Clark, T. Bein and F. Auras, Nat. Commun., 2018, 9, 3802.
The heterogeneous photocatalysis design, synthesis and, the industrial application remains a challenging issue. Herein, the fellow has been envisaged a proof-of-concept for designing heterogeneous photocatalytic systems for important photocatalytic Diels-Alder (D–A) cycloaddition reactions. The characterization of the D-A reaction product(s) and further investigation on catalytic performance are yet to be optimized. However, a parallel investigation was performed to reproduce the original research results on photocatalytic [4+2] D-A cycloaddition reaction by using commercially available homogeneous Ru-photocatalyst and found that it is working perfectly under a similar experimental condition as for COFs.[11] These investigations suggest that the development of COFs based metal-free heterogeneous photocatalytic system would be possible by following the described strategy herein. Moreover, once the COFs system is optimized for exclusive photocatalytic D-A reactions, it will be great for developing heterogeneous COFs based metal-free and recyclable catalysts towards more environmentally benign processes, safer and cheaper products for a sustainable society. Notably, these COFs can also be applied in a continuous flow reactor for rapid and scalable product synthesis in the industry.
[9]. S. Lin, M. A. Ischay, C. G. Fry and T. P. Yoon, J. Am. Chem. Soc., 2011, 133, 19350-19353.
Scheme 1. Topology for COFs synthesis
Figure 1. Proposed mechanism of photocatalytic Diels-Alder (D-A) reaction