Nitrogen fixation and reduction using a porous coordination polymer with bridging dinitrogen anions

Time:2022-12-16Viewed:10

Nitrogen molecules have high stability and are not easy to be reduced due to the large bond dissociation energy. The conversion of nitrogen into industrial products (such as ammonia, hydrazine, etc.) has important social benefits and economic value.Inspired by biological nitrogenase, there are two necessary conditions for nitrogen fixation and reduction with molecular catalysts: a) the molecular center has variable valence metals with unsaturated coordination; b) sufficient electrons and protons must be provided.

The research groups of Profs. Jing-Lin Zuo, Zhong Jin, Jing Ma, Shuhua Li, etc. from the School of Chemistry and Chemical Engineering of Nanjing University, have recently successfully synthesized a zinc-based coordination polymer that features bridging dinitrogen anionic ligands ({[Zn(L)(N2)0.5(TCNQ–TCNQ)0.5]·(TCNQ)0.5}n). Through single crystal structure, infrared spectrum, Raman spectrum, paramagnetic resonance and isotope exchange experiments, it is proved that the porous metal coordination polymer (NJUZ-1) has a clear dinitrogen ligand. The free electron acceptor TCNQ is embedded in the electron donor TTF interlayer through hydrogen bond and p-p interaction, forming a stable sandwich structure.

Fig. 1 crystal structure of NJUZ-1.

Variable physical and chemical characterization results show that NJUZ-1 has excellent light absorption property and nitrogen adsorption capacity. The results of photocatalytic nitrogen fixation experiments show that NJUZ-1 has excellent nitrogen fixation performance, especially under ambient atmosphere, and the catalytic stability is very good. Further analysis of the structure and physical properties of the catalyst after long-time catalytic reaction confirmed that the catalyst itself has good physical and structural stability.

Fig. 2Light absorption and photocatalytic nitrogen fixation performance of NJUZ-1.

Theoretical and experimental studies show that the charge transfer on NJUZ-1 is through TTF-TCNQ intramolecular and metal-to-ligand (MLCT) after light absorption, which promotes the separation of electron-hole pairs.

 

Fig. 3 Nitrogen fixation process and possible reaction pathways for NJUZ-1. 

The catalytic process and mechanism study shows that the coordination unit {Zn2+-(N≡N)-Zn2+} of NJUZ-1 acts as the catalytic site, and forms a stable unsaturated ligand [Zn2+···Zn+] intermediate which supported by a three-dimensional framework structure through the formation and the detachment of the NH3. Then the [Zn2+···Zn+] intermediate is refilled by external nitrogen to reform {Zn2+-(N≡N)-Zn2+} coordination saturation state. This nitrogen fixation process is similar to the Mars-van Krevelen (MvK) process. Theoretical studies suggest that the overall reaction pathway may be through an alternating pathway via protonated *N2H gradually forms *NHNH, *NHNH2, and *NH2NH2, eventually generates NH3.

This work first develops and investigates the dinitrogen functional coordination polymer with clear molecular active sites under mild conditions for nitrogen fixation, which provides a new idea and theoretical basis for further studies and application of molecular catalysts in photosynthetic nitrogen fixation. The work has been published in Nature Chemistry (DOI: 10.1038/s41557-022-01088-8) with title of Photocatalytic nitrogen fixation under an ambient atmosphere using a porous coordination polymer with bridging dinitrogen anions.

The co-first authors of the paper are Yan Xiong, Bang Li and Yuming Gu. Tong Yan and Zhigang Ni participated in this work. The above research work has been supported by the National Key Research and Development Program, the National Natural Science Foundation of China, the Jiangsu Province Carbon Peak and Carbon Neutral Science and Technology Innovation Fund, the Fundamental Research Funds for the Central Universities, the State Key Laboratory of Coordination Chemistry, the Key Laboratory of Mesoscopic Chemistry of the Ministry of Education, and Collaborative Innovation Center of Advanced Microstructures, etc.