Synthesis of new carboxyphosphonate metal-organic frameworks (mofs) and their potential as electrocatalysts for co2 reduction

Ogunbanjo, Oriyomi ORCID: 0000-0001-5240-4160 (2024). Synthesis of new carboxyphosphonate metal-organic frameworks (mofs) and their potential as electrocatalysts for co2 reduction. University of Birmingham. Ph.D.

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Abstract

The increasing concentration of CO2 in the atmosphere is alarming for modern society and the reduction of CO2 into valuable products would be an ideal solution. Metal–organic frameworks (MOFs), constructed from organic linkers interconnected with metal nodes, with high porosity and large surface area, are an emerging class of electrocatalysts for reduction of CO2. The research presented in this thesis aimed at developing new carboxyphosphonate metal-organic frameworks (MOFs) and exploring their potentials as electrocatalyst for the CO2 reduction reaction. This study started with the synthesis of the carboxyphosphonate MOF, BIRM-1, exploring its ion exchange capability with nickel metal, producing a new ion exchange material named Ni2+-BIRM-1. Findings revealed that nickel (II) cations were successfully incorporated into BIRM-1 after 24 hours of ion exchange experiment, maintaining its crystallinity as confirmed by powder XRD and SEM. The extent of nickel cation exchange was evaluated via SEM-EDX analysis, with the result showing that the calculated molar ratio of Ni:P:Zn in Ni2+-BIRM-1 matched the expected molar ratio, indicating 100% ion exchange efficiency. Single crystal XRD also revealed that Ni2+-BIRM-1 retained BIRM-1's crystallographic characteristics, with no ammonium cations in the pores, confirming full ion exchange and structure preservation. However, cyclic voltammetry indicated that Ni2+-BIRM-1 was not electrochemically active for CO2 reduction. New carboxyphosphonate materials were synthesised using 3-phosphonopropionic acid ligand and characterised using single crystal XRD, resulting in five new structures: BIRM-11(Zn), BIRM-12(Cu), BIRM-13(Cu), BIRM-14(Cu), and BIRM-15(Co) which are either layered compounds or 2D MOFs. The pH played a critical role in deprotonating the carboxylphosphonate ligand and achieving metal-ligand coordination. In BIRM-11(Zn), BIRM-14(Cu), and BIRM-15(Co), complete deprotonation was achieved, and their structures appeared as 2D frameworks with covalent bonds while in BIRM-12(Cu) and BIRM-13(Cu) structure, incomplete deprotonation led to hydrogen-bonded layered structures. Efforts to convert BIRM-13 (Cu) into 3D structures using 4,4-bipyridine resulted in the 2D MOF, BIRM-14. Preliminary electrochemical studies using cyclic voltammetry technique showed that BIRM-12 (Cu), BIRM-13 (Cu), and BIRM-15 (Co) were active for hydrogen evolution, while BIRM-14 was promising for CO2 reduction. A previously synthesized 2D nickel carboxyethylphosphonate also demonstrated significant potential as an electrocatalyst for the reduction of CO2 to syngas, giving an onset potential of -0.51 V vs RHE and a current-mass activity of 9.5 mA/mg. Long-term electrolysis at -0.8 V (vs. RHE) produced a CO and H2 mixture in a 30:70 ratio. DFT calculations indicated a 0.52 eV difference in limiting potentials for HER and CO2RR, favouring HER, highlighting the thermodynamic driving forces behind the predominant production of H2 over CO. The flexibility of 3-phosphonopropionic acid ligand makes the synthesis of 3-dimensional porous framework structures difficult, so a more rigid ligand, namely, 4-phosphonobenzoic acid was further explored for new MOF synthesis. A total of five novel framework materials (BIRM-17, BIRM-18, BIRM-19, BIRM-20 & BIRM-21) were synthesised and characterised via single crystal XRD. BIRM-17 is a 2D layered structure; optimising the synthesis temperature and time produced the 3D porous frameworks BIRM-18 and 19. Topological analysis shows that BIRM-18 and BIRM-19 exhibits a (4;2)-connected GIS-type topological structure seen in zeolites, with voids and channels ideal for gas adsorption and ion exchange. BIRM-20 and 21 crystalised as 2D layered nickel and cobalt-containing phosphonate MOFs.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):