Metal-free hydrogen generation from ammonia borane hydrolysis: A study on the effect of pKa of organic acid catalysts

Abstract

Catalytic hydrogen generation from chemical hydrogen storage materials has attracted significant attention as a clean, efficient, and controllable approach for on-demand hydrogen production. While various organic acids have been utilized as catalysts for hydrogen release, previous research has primarily focused on sodium borohydride systems and not on ammonia borane (AB) hydrolysis. In this work, the effect of acid strength (pK_a) on hydrogen generation from AB hydrolysis was systematically investigated using different organic acids. Among the acids tested, methanesulfonic acid (pK_a1 = –1.9) demonstrated the highest catalytic activity, yielding >90% hydrogen with a generation rate of 6.8 mL s^–1, whereas weaker acids such as acetic acid (pK_a = 4.75) led to incomplete conversion. The hydrogen generation rate showed an inverse correlation with the acid pK_a1, confirming that the availability of protons, particularly the most acidic proton in polyprotic acids, dictates the reaction kinetics. The apparent activation energy was found to be 42 kJ mol^–1, and a kinetic isotope effect (KIE = 3.54) for citric acid catalyzed AB hydrolysis indicated that O–H bond cleavage in water is the rate-determining step. These findings highlight the key role of acid strength in AB hydrolysis and identify organic acid as an efficient single-use catalyst for sustainable, low-cost, and metal-free hydrogen generation.

With decreasing the pK_a of organic acids, rate of hydrogen generation from ammonia borane hydrolysis increases.