MOF rocket fuel

Today, most rockets are fuelled by hydrazine, a toxic and hazardous chemical comprised of nitrogen and hydrogen. Those who work with it must be kitted up in protective clothing. Even so, around 12,000t of hydrazine is released into the atmosphere every year by the aerospace industry.

Image: US Navy/Getty

Now, researchers are in the process of developing a greener, safer rocket fuel based on metal organic frameworks (MOFs), a porous solid material made up of clusters of metal ions joined by an organic linker molecule. Hundreds of millions of connections join in a modular structure.

Robin Rogers, formerly at McGill University, has worked with the US Air Force on hypergolic liquids that will burn when placed in contact with oxidisers, to try get rid of hydrazine. He teamed up with Tomislav Friščić at McGill who has developed ways to react chemicals ‘mechanochemically’ – without the use of toxic solvents.

The pair were interested in a common class of MOFs called zeolitic imidazole frameworks, or ZIFs, which show high thermal stability and are usually not thought of as energetic materials. However, their structure and electronics suggested their bonds contained latent energy that can be released upon combustion.

‘We talked about what would happen if we used ZIFs, which we knew was not very reactive, with the imidazolate linkers containing trigger groups,’ says Friščić. ‘These trigger groups allowed us take advantage of the usually not accessible energetic content of these MOFs.’

They succeeded in unlocking this energy by introducing triggers of hypergolic behaviour, such as reactive groups like acetylene or vinyl (Science Advances, 2019, doi: 10.1126/sciadv.aav9044). The resulting ZIF is safe and does not explode, and it does not ignite unless placed in contact with certain oxidising materials, such as nitric acid, in this case.

Six hypergolic MOFs were developed, based on zinc, cobalt and cadmium, to show how varying the metal and linker could change the ignition and combustion properties. Ignition could start after just two milliseconds.

‘ZIFs are attractive for making energetic materials,’ comments materials chemist Adam Matzger at the University of Michigan, US, who has worked on them as potential new explosives, but was impressed by their use as hypergolic materials: ‘Nobody has done this before,’ he says.

‘One thing they haven’t discussed is that they are trying to compete with things like hydrazine fuels with oxidisers, which is really, really cheap stuff,’ Matzger adds. ‘Cost is going to be important.’

Authorities continue to use hydrazine because it could cost millions of dollars to requalify new rocket fuels, says Rogers. MOF fuel would not work in current rocket engines, so he and Friščić would like to get funding or collaborate with another company to build a small prototype engine that can use it. ‘Nothing is ever perfectly safe, but for these materials – when we scaled up to grams – we haven’t encountered any problems,’ says Friščić.