H₂ Activation, Reversibly

In a promising development for industrial hydrogenation and the storage and production of hydrogen for fuel cells, researchers have synthesized a lightweight, nonmetal compound that readily breaks apart and recombines H₂ molecules.

A nonmetal species having both these talents is rare and highly sought after, as the metal-based hydrogenation catalysts used in myriad industrial processes can be toxic and environmentally unfriendly, to say nothing of heavy and expensive.

Chemistry professor Douglas W. Stephan, graduate student Gregory C. Welch, and colleagues at the University of Windsor, in Ontario, synthesized the compound, a phosphonium borate that, when heated, readily gives off H₂ to form a phosphine borane. The borane then reacts with H₂ at room temperature, regenerating the borate (Science 2006, 314, 1124).

On the phosphonium borate, a proton is bound to the phosphorus, and a hydride sits a distance away on the boron. The researchers believe the proton migrates across the molecule's arene linker to the hydride, and the two combine and sail off as H₂. In the reverse reaction, the H₂ likely attaches to the boron, then a proton splits off and migrates to the phosphorus.

"Regardless of the mechanism, the discovery is important because of the reversible nature of the hydrogen activation," notes chemist Gregory J. Kubas at Los Alamos National Laboratory in a commentary accompanying the report.

The compound binds only 0.25% by weight of H₂—a far cry from the 6-9 wt % that would make it a practical hydrogen-storage material, the authors note.

Other nonmetal systems such as ammonia boranes also have shown promise, as they hold a lot of hydrogen for their weight. But when these compounds lose all their hydrogen, the result is boron nitride, which is very difficult to convert back to ammonia borane, Stephan notes. The phosphonium borate system, on the other hand, "goes both ways but doesn't store very much," he says.

The group is investigating variants of the system that might hold more H₂. But an even more promising avenue, Stephan says, might be to use such species as a catalyst in tandem with other systems.