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David Bradley ISSUE #62
February 2007
Alumination at last!
Kit Bowen

More powerful solid rocket fuel could be the product of research into a new class of compounds containing aluminum and hydrogen compounds, according to an international research team. The new materials might also be used eventually in hydrogen-powered vehicles or other energy applications.

Scientists at the Universities of Konstanz and Karlsruhe in Germany, and the Johns Hopkins and Virginia Commonwealth universities in the US, have combined theoretical and experimental work to develop the new aluminum hydrides. These compounds are relatively stable and are analogs of the more familiar boranes, which are composed of boron and hydrogen atoms, and themselves analogs of organic, carbon compounds, the alkanes. There are dozens of boranes, but relatively few aluminum hydrides. The relative stability of the new "alanes" could be key to their viability as rocket fuel, explains JHU's Kit Bowen. The large gap between the highest occupied and the lowest unoccupied molecular orbitals of the compound Al4H6, together with its exceptionally high heat of combustion, further suggests that it may be an important energetic material if it can be prepared in bulk, say the researchers.

"It's always tough to predict how things will play out in the future," he says, "but our research finding is interesting enough for me to be willing to say that this synthesis may have the potential for some possibly very useful future applications, including the development of solid rocket fuel with more thrust."

Aluminum is a common additive in most rocket fuels, but incorporating the metal into a hydrogen-containing molecule might make it even more efficient as a co-fuel. Bowen is cautious as to whether that will be the case. "These complexes are a new class of thing that, because of their various properties, can at this point only be imagined to have uses in propulsion, or even in the forecasted hydrogen economy."

Nevertheless, given the recent recommendations by the Intergovernmental Panel on Climate Change (IPCC), our so-called "fossil fuel economy" has essentially had its day. While a "hydrogen economy" will be no panacea, it does offer us the chance to reduce carbon emissions provided chemists can develop efficient hydrogen fuels that can be produced using renewable, non-fossil fuel energy sources, such as solar and wind power. VCU's Puru Jena explains that developing the new alanes is one of the technological objectives of research into the hydrogen economy. "Our work has demonstrated that a synergy between experiment and theory can go a long way in meeting these challenges," he says, "particularly in developing novel nano-materials for storing and releasing hydrogen as well as for high-energetic materials applications."

alane structure

Science, 2007, 315, 356-358; http://dx.doi.org/10.1126/science.1133767