RHODIUM PORPHYRINS AS CATALYSTS FOR AMMONIA BORANE HYDROLYSIS: PERFORMANCE AND EFFICIENCY

Efficient and safe hydrogen storage remains a major challenge for its widespread application, especially in portable and vehicular technologies. Among chemical hydrogen storage materials, ammonia borane (AB) stands out due to its high hydrogen content and stability. However, its thermal decomposition is limited by harsh conditions and byproduct formation. Alternatively, AB hydrolysis in water offers a cleaner and milder route for hydrogen release, especially in the presence of suitable catalysts. The reaction proceeds as follows: While many transition metal-based catalysts have been explored,1,2 rhodium has shown exceptional activity in promoting AB solvolysis.3,4 In this study, we present the first systematic investigation of AB hydrolysis catalyzed by four distinct rhodium porphyrin complexes, designed to function under either homogeneous or heterogeneous conditions. The catalysts were synthesized5–7 by coordinating RhCl₃·3H₂O with structurally diverse free-base porphyrins, including three water-soluble ionic derivatives and one neutral, water-insoluble porphyrin. All complexes were thoroughly characterized (NMR, UV-Vis, ATR-FTIR, ICP) and tested for catalytic performance in aqueous media. Our findings highlight how both peripheral substituents and pH contribute to tuning the catalytic behavior of rhodium porphyrins. The water-soluble complexes operate efficiently in homogeneous solution, while the water-insoluble complex enables a heterogeneous approach with practical advantages such as catalyst recovery and reuse. This work introduces a new class of rhodium porphyrin catalysts for AB hydrolysis and demonstrates the potential of porphyrin-based design strategies in developing efficient and versatile systems for hydrogen generation. These findings contribute to the advancement of tunable catalytic platforms for chemical hydrogen storage and release.