Rhodium porphyrin complexes as catalysts for ammonia borane hydrolytic dehydrogenation

Abstract

In this study, we report for the first time rhodium porphyrins as single-site catalysts for the hydrolytic dehydrogenation of ammonia borane (AB), overcoming the traditional role of porphyrin-based materials as stabilizing supports for metal nanoparticles. Rhodium complexes were synthesized by reaction of RhCl₃·3H₂O with four free-base porphyrins. To explore homogenous conditions, we chose two cationic (TMPyP_H₂ and TMAP_H₂) and one anionic (TSP_H₂) water-soluble core, whereas water-insoluble neutral TAP_H₂ was used as a heterogeneous catalysis. Rhodium porphyrins were fully characterized by UV-vis, ATR-FTIR and NMR spectroscopy, and the content of rhodium was estimated by ICP-MS. Considerably, all the investigated rhodium porphyrins catalyze effectively the hydrolytic dehydrogenation of ammonia borane at 303 K and 1 atm, with varying efficiency. Anionic TSP_Rh showed the best performance (70.7% H₂ yield in 9 min, TOF = 15.72 × 10⁶ h⁻¹), whereas the two cationic catalysts differed in activity and yield (TMPyP_Rh: 68.7% H₂ yield in 7 min, TOF = 3.41 × 10⁶ h⁻¹; TMAP_Rh: 57.5% H₂ yield in 21 min, TOF = 11.50 × 10⁶ h⁻¹). The stability tests pointed out a progressive deactivation of ionic rhodium porphyrins, mainly due to the reaction environment. Indeed, its impact is likely greater on water-soluble complexes, as suggested by a detailed spectroscopic investigation performed on the homogeneous catalysts after use. Conversely, the performances of heterogeneous TAP_Rh remained stable over seven successive runs, although the catalyst exhibited lower activity (65.4% H₂ yield in 28 min, TOF = 3.65 × 10⁶ h⁻¹). These results strongly demonstrate the efficiency of Rh-based porphyrins as homogeneous and heterogeneous catalysts for the ultra-pure hydrogen production from ammonia borane.