Insight into the Catalytic Effects of Open Metal Sites in Metal-Organic Frameworks on Hydride Dehydrogenation via Nanoconfinement

Chemical hydride ammonia borane (AB, NH₃BH₃) draws attentions as a hydrogen storage medium for its high hydrogen capacity (19.6 wt %) and good thermal stability at ambient environments. However, high hydrogen generation temperatures and slow kinetics limit AB practical applications. One way to overcome the obstacles is the nanoconfinement effect: AB incorporated with porous materials has facilitated dehydrogenation process. However, the mechanism is still under debate, and several factors have been proposed, like hydride nanosize or catalytic environments controlled/provided by microporous supports. In this research, metal-organic frameworks (MOFs) of Cu-BDC (BDC = benzenedicarboxylate) with/without manipulated active open metal sites by solvent removal/capping are applied for AB thermolysis via nanoconfinement. Both AB@MOFs show the same dehydrogenation peaked temperature regardless of catalytic environments, strengthening the theory that high surface tension from hydride nanosize controlled by MOF microporosity results in reduced dehydrogenation temperature. In addition, compared to solvent-capped MOFs, Cu-BDC with copper open metal sites eliminates byproduct emission and hence increases hydrogen yield from AB and also decreases dehydrogenation activation energy considerably. However, short cycle life due to copper reduction is observed in desolvated Cu-BDC, while lowered dehydrogenation temperature can be kept in solvated MOFs. In general, we clarify possible mechanisms and factors to hydride nanoconfinement and catalysis that improve AB dehydrogenation temperature and kinetics and provide new strategies for future hydride composite materials design.