More efficient access to hydrogen as a fuel

Dr. Neil Canter, Contributing Editor | TLT Tech Beat January 2012

A new ruthenium catalyst has been developed to convert ammonia borane to hydrogen.

KEY CONCEPTS
• Hydrogen is difficult to use as a fuel because it’s very flammable and can only be stored in high-pressure or cryogenic tanks.
• The solid ammonia borane is a potential hydrogen storage material because of its high hydrogen density.
• A new ruthenium catalyst efficiently converts ammonia borane to hydrogen by an anhydrous dehydrogenation process in air and can be reused multiple times.

THE ABILITY TO USE HYDROGEN AS A FUEL remains a challenge because of the difficulty in finding a stable derivative that can act as a storage medium. Hydrogen has a number of advantages such as easy conversion to electricity in a fuel cell and the fact that it contains no carbon so the conversion does not generate such detrimental byproducts as carbon dioxide.

But hydrogen is highly flammable and remains a very difficult fuel to handle. Travis Williams, assistant professor of chemistry at the University of Southern California in Los Angeles, says, “Being a gas, hydrogen’s biggest problem is that it can only be stored in high-pressure or cryogenic tanks.”

In a previous TLT article, a potential aluminum hydride (alane) was found to be potentially very useful as a hydrogen storage material because this derivative contains 10% by weight (1). Researchers were able to develop an electrochemical process that generates alane from sodium aluminum hydride. The alane can then be converted to hydrogen under elevated temperature conditions such as the heat produced in an automotive internal combustion engine.

Another storage material that has attracted attention is ammonia borane, also known as AB. Williams says, “AB is a solid material, not a gas. This compound has a high hydrogen density of 19.6% that is four times larger than the density of the gasoline. AB also can easily be converted to hydrogen under mild thermal and catalytic conditions through a process known as catalytic dehydrogenation.”

In contrast, Williams points out that diesel, gasoline or methanol must be reacted at high temperatures with the use of a catalyst to release hydrogen.

One of the problems with efficiently converting AB to hydrogen is that a competitive catalytic hydrolysis process also can occur to convert AB to hydrogen. Williams says, “The issue with catalytic hydrolysis is that ammonia formed is poisonous to a fuel cell. Other byproducts include borax that can be reduced under severe high temperature conditions, which is energy intensive and very expensive.”

Various catalysts have been tried to convert AB to hydrogen but have suffered from two limitations. Williams explains, “There are catalysts that go very quickly but do not produce more than one equivalent of hydrogen. Other catalysts produce sufficient levels of hydrogen (above one equivalent), but they are all pretty water sensitive. This means they will only work during a single pass.”

A more attractive approach to producing hydrogen from AB is through an anhydrous dehydrogenation process. Such an approach has now been developed.

RUTHENIUM CATALYST
Williams and his research associates have developed a new ruthenium catalyst that is very effective at converting AB to hydrogen. Equally important, the catalyst is able to facilitate dehydrogenation in air and is sufficiently durable to be reused multiple times.

A crystal structure of the ruthenium catalyst is shown in Figure 3. The researchers found that reacting AB in the presence of the ruthenium catalyst at 70 C led to the production of more than two equivalents of hydrogen.


Figure 3. A new ruthenium catalyst shown here can catalyze the dehydrogenation of ammonia borane to hydrogen multiple times and in air. This process may lead to a more effective way to use hydrogen as a fuel. (Courtesy of the University of Southern California)

Williams says, “AB is a solid, which meant that we needed to add the solvent diglyme so that it could be reacted as a slurry with the consistency of oatmeal. As the reaction proceeds, hydrogen gas comes off and the slurry is converted into a gum eraser, which consists of the boron, hydrogen polymer polyborazylene.”

The choice of the reaction conditions was arbitrary. He adds, “If we increase the temperature above 70 C, the reaction moves faster. At lower temperatures, the process is slower.”

Further work by the researchers shows the durability of the ruthenium catalyst. A sequence of four runs made over six hours each produced greater than two equivalents of hydrogen. Additional solvent needed to be added to keep the viscosity of the reaction mixture manageable.

The mechanism for how the ruthenium catalyst is so effective is not known at this point. Williams says, “Based on isotope studies, we know that boron-hydrogen and nitrogen-hydrogen bonds of AB are involved in a transition state with the ruthenium catalyst. The bridging site connecting the ruthenium atom with the boron species in the catalyst is also suspected to contribute to the catalysis. Substitution of a hydroxyl group for a carboxyl group improves the effectiveness of the catalyst.”

Applications will involve preparing fuel cartridges containing AB and the catalyst that can be plugged into electronic devices such as golf carts, scooters and radios. Williams says, “We envision that the low-weight profile of AB will make it useful in small electronic applications. The main problem will be finding stations where spent cartridges can be regenerated.”

Future work will focus on how the catalyst can be regenerated in a cost-effective manner as possible. Additional information about the ruthenium catalyst can be found in a recent publication (2) or by contacting Williams at travisw@usc.edu.

REFERENCES
1. Canter, N. (2009), “Aluminum Hydride: Potential Hydrogen-Storage Material,” TLT, 65 (11), pp. 14-15.
2. Conley, B., Guess, D. and Williams, T. (2011), “A Robust, Air-Stable, Reusable Ruthenium Catalyst for Dehydrogenation of Ammonia Borane,” Journal of the American Chemical Society, 133 (36), pp. 14212-14215.


Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.