Single-layer metal-organic framework materials enable efficient artificial photosynthesis

On November 10, 2022, Professor Wenbin Lin’s team at the University of Chicago published a research paper entitled “Biomimetic active sites on monolayered metal-organic frameworks for artificial synthesis” in the journal Nature Catalysis.

This achievement reports the rational molecular design of single-layer metal-organic framework materials to assemble an enzyme-like catalytic system composed of catalytic sites, amino acids and cofactors, so as to achieve efficient artificial photosynthesis.

The corresponding author of the paper is Lin Wenbin; The co-first authors are Blu Xu, Fan Yingjie, and Shi Wenjie.

The fossil energy crisis is one of the major problems facing mankind in the 21st century. In order to expand the sources of renewable energy to cope with the energy crisis, artificial photosynthesis has received extensive research attention. In nature, photosynthesis is involved in enzyme catalysts with high efficiency and selectivity. Although many artificial systems have been developed to mimic the catalytic activity of natural enzymes, they can only mimic the metal catalytic center of the enzyme and cannot perform the functions of amino acids or cofactors.

In this work, Professor Lin Wenbin’s team assembled metal catalytic centers, amino acids and cofactors into metal-organic framework monolayer materials (J. Am. Chem. Soc. 2019, 40, 15767), synthesized metalloorganic enzyme, MOZ. The researchers designed and synthesized two classes of MOZ libraries for catalytic activity screening of photoinduced carbon dioxide reduction and water oxidation reactions, respectively. By adjusting the amino acids introduced in the MOZ, the activity and selectivity of the MOZ catalyst are systematically optimized. Finally, by combining the best catalytic CO2 reduction MOZ with water-oxidized MOZ, the researchers achieved efficient artificial photosynthesis of (1+n)CO2 + 2H2O → CH4 + nCO+ (2+n/2)O2.

Figure 1: MOZ design ideas.

Synthesis and characterization of MOZ.The hydrothermal reaction of hafnium tetrachloride with a photosensitive ligand produces a photosensitive metal-organic monolayer Hf-Ir. Through two-step post-surface modification, the carbon dioxide reduction catalyst blood matrix (haem) and 20 natural amino acids are successively attached to Hf-Ir, forming a carbon dioxide reduction MOZ library containing 20 grafted different amino acids. Morphological characterization (transmission electron microscopy and atomic force microscopy) showed that these MOZs were ultrathin nanosheets about 2.1 nm thick and about 150 nm in diameter. Chemical characterization (NMR and IR spectroscopy) demonstrates the successful introduction of haem and amino acids on MOZ. In the same way, cyclopentadienyl bipyridine iridium (MBA-Ir) is attached to Hf-Ir as a water oxidation catalyst to form a MOZ library that catalyzes water oxidation.

Figure 2: MOZ synthesis and library building screening.

Figure 3: Characterization of MOZ.

MOZ selection and optimization.MOZ’s photoreduction CO2 reactivity test demonstrated two mechanisms for promoting CO2 reduction through amino acids: acidic group-led proton-coupled electron transfer (PCET) and amide substrate-led intermediate hydrogen bond stabilization. Based on these two mechanisms, an artificial amino acid Ur is synthesized and grafted on the MAZ, showing a carbon dioxide photoreduction efficiency far exceeding that of other MOZs. Density functional theory (DFT) calculations also support the stabilizing effect of hydrogen bonds on carbon dioxide reduction intermediates. Through the same method, in the water oxidation reaction, the authors found that amino acids with suitable redox potentials can be used as redox regulators to promote the process of photo-promoting water oxidation, and accordingly synthesized MOZ that is most suitable for water oxidation for the optimization of the next full reaction.

Figure 4: MOZ-catalyzed photoinduced CO2 reduction reaction.

Optimization of the full reaction。 By simulating the Z scheme in photosynthesis, MOZ for carbon dioxide reduction and MOZ for water oxidation are coupled by electron transport intermediary Co(bpy)3Cl2, and artificial photosynthesis with CO2 reduction conversion frequency (TOF) of 98.7 h-1 and quantum yield of 1.1% is achieved under optimized catalyst ratio and concentration, and the reaction rate exceeds that of similar catalysts by an order of magnitude.

Figure 5: MOZ-catalyzed artificial photosynthesis.

This study demonstrates breakthroughs in molecular design and the potential of metal-organic materials as catalysts. Through reasonable design, this work realized the assembly of enzyme-like catalytic systems on metal-organic materials for efficient artificial photosynthesis. This article was selected as the November cover article of Nature catalysis. (Source: Web of Science)

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