Development of Hydrogen Storage Materials Based on Amine-Boranes

Abstract

The production of coke oven gas (COG) from Port Talbot steelmaking generates several side products, (e.g. CH4, CO, CO2, NH3, H2, H2S, organic aromatic compounds, cresols, etc), that can enable possibilities for the recovery of energy (e.g. via utilisation of generated H2 and potential utilisation of ammonia gas as energy vectors) and valuable compounds. Particularly, the mandatory removal of ammonia, as either NH4+ (aq) or NH3 (g), from COG provides a foundation to be utilised as a precursor for hydrogen storage in the form of amine-boranes; where ammonia as a Lewis base forms Lewis adducts with boranes. In the Lewis adducts involving ammonia and boranes, the control of substituents on the boron can serve to influence the hydrogen storage characteristics of the Lewis pair. In this study, boron-methyl and boron phenyl amine boranes were selected as targets for their potential exploration as hydrogen storage materials. Meanwhile, literature analysis indicates the possibility of boron-diethyl and boron-bis(trifluoromethyl) phenylamine-borane as synthetic targets with improved product selectivity and potential for reversibility. Such characteristics could show an improved practicality for application at TATA Steel; although these are very reactive species and limited success in the preparation of these compounds was achieved. In addition to investigating the thermolytic character of synthesised boron-substituted amine boranes, the activation of H2 was also investigated via catalytic methods involving ruthenium complexes. Catalytic reactions showed H2 release was possible at room temperature, and in loadings reduced to 0.1 mol% at 70oC. Further, 11B and 1H NMR spectroscopy showed the catalytic release of all available H2 making them advantageous as hydrogen storage materials at TATA Steel. The potential direct involvement of the ligand in these catalytic methods, allowed us to speculate on a plausible mechanism of action via a metal ligand cooperation regime. This was possible by utilising phosphine ligands containing an OH functional group i.e. phosphinous acids [Ru-P(OH)] in RuCl2(p-cymene)PR2(OH)-type complexes. Thus, drawing on similar literature precedents, a bifunctional ligand co-operative mechanism was anticipated. This comprised of the transfer of hydridic BH to the metal center and NH proton activation to the basic oxygen functionality. Accordingly, the inclusion of phosphorous acid (PA) as a ligand was assessed and comparison studies were carried out with catalysts containing analogous functionality (Ru-HN-Propyl) or without (Ru-PPh3) as a benchmark. The activation of H2 from low catalytic loadings served to better understand the catalytic activity as well as explore cost implications for TATA Steel. Probing experiments such as stoichiometric reactions or independent hydride preparations 4 which relate to detailing the mechanism of dehydrogenation were also explored based on initial NMR spectroscopy (31P, 1H, 11B) observations.

Date of Award	2024
Original language	English
Supervisor	Nildo Costa (Supervisor), Gareth Owen (Supervisor) & Alan Guwy (Supervisor)