Scientists from the University of California, Berkeley, have made significant progress in making ammonia production more environmentally friendly, using green ammonia as a greener fertiliser.
Industrial production of ammonia – the fuel for the green revolution – is one of the world’s largest chemical markets, but also one of the most energy intensive.
The traditional Haber-Bosch production method contributes to around 1% of fossil fuels and 1% of all carbon emissions globally, making it a major contributor to climate change.
The new process results, ‘A ligand insertion mechanism for cooperative NH3 capture in metal-organic frameworks,’ were published in the journal Nature.
What are some of the challenges of decarbonising fertiliser production?
A major hurdle in producing green ammonia with less energy input has been separating the ammonia from the reactants – primarily nitrogen and hydrogen – without the large temperature and pressure swings required by Haber-Bosch.
During this process, the reaction takes place between 300 and 500°C , but ammonia is removed by cooling the gas to around -20°C, where the gaseous ammonia condenses as a liquid. Moreover, the process requires pressurising the reactants to around 150-300 times the atmospheric pressure. These steps all contribute to a significant rise in carbon emissions.
Alternative methods for ammonia separation could open the door to different processes, helping to produce green ammonia and operate under less extreme conditions. To address this issue, the chemists at UC Berkeley designed and synthesised porous materials, known as metal-organic frameworks (MOFs), that bind and release ammonia at moderate pressures and temperatures of around 175°C.
Because the MOFs do not bind to any of the reactants, the capture of green ammonia can be accomplished with smaller temperature swings, which saves energy. Benjamin Snyder, a UC Berkeley postdoctoral fellow and leader of the study, explained: “A big challenge to decarbonising fertiliser production is finding a material where you can capture and then release very large quantities of ammonia, ideally with a minimal input of energy.
“However, you don’t want to have to put a lot of heat in your material to force the ammonia to come off, and likewise, when the ammonia absorbs, you don’t want that to generate a lot of waste heat.”
Advantages of the new green ammonia method
A key advantage of a process that operates at lower temperatures and pressures is that green ammonia and green fertiliser could be produced at smaller facilities closer to farmers, rather than at large, centralised chemical plants.
“The dream here would be enabling a technology where a farmer in an economically disadvantaged area of the world now has much more ready access to the ammonia that they need to grow their crops,” Snyder said.
“To be clear, our material hasn’t gone and solved that problem outright. But we’ve put forward a new way of thinking about how you can use metal-organic frameworks in the context of green ammonia capture for a modified Haber-Bosch process. I think this study represents a really important conceptual advance in that direction.”
“This work is of fundamental importance because it reveals a new cooperative mechanism for selective gas capture,” said Jeffrey Long, the C Judson King Distinguished Professor at UC Berkeley and a faculty scientist at Lawrence Berkeley National Laboratory. “We are optimistic that the mechanism will extend to other molecules of industrial significance that have a strong affinity for binding metals.”
Can the Haber-Bosch process be made more sustainable?
Many researchers are working on ways to make the Haber-Bosch ammonia process – which dates from the early 20th century – more sustainable.
These attempts include producing one major reactant, usually hydrogen, using solar power to split water into hydrogen and oxygen. In the present day, hydrogen is typically obtained from natural gas in a reaction that releases carbon dioxide, the dominant greenhouse gas.
Other green ammonia modifications include novel catalysts, which operate at lower temperatures and pressures to react hydrogen with nitrogen. These gases would be typically captured from the air to form ammonia NH3.
However, removing ammonia from the mixture after the reaction process has remained difficult. Other porous materials, such as zeolites, are unable to absorb and release large quantities of ammonia. Furthermore, attempts to use other MOFs have often disintegrated in the presence of ammonia, which is highly corrosive.
Adapting metal-organic frameworks
To overcome challenges with MOFs, the team decided to try a relatively new variety of MOFs, which employs copper atoms linked by organic molecules called cyclohexane-dicarboxylate to create a rigid and highly-porous MOF structure.
To the team’s surprise, ammonia did not destroy this MOF and instead converted it into strands of copper and ammonia-containing polymer that has an extremely high density of green ammonia. Moreover, the polymer strands easily gave up their bound ammonia at relatively low temperatures, restoring the material to its initial rigid, porous MOF structure.
“When you expose this framework to green ammonia, it completely changes its structure,” Snyder commented. “It starts as a porous, three-dimensional material, and upon being exposed to ammonia, it actually unweaves itself and forms what I would call a one-dimensional polymer. Think of it like a bundle of strings. This really unusual adsorption mechanism allows us to uptake huge quantities of ammonia.”
In the reverse process, he added: “The polymer somehow will weave itself back into a three-dimensional framework when you remove the ammonia, which I think is one of the most interesting features of this material.”
Additionally, the team found that the MOF could be tuned to absorb and release green ammonia under a large range of pressures, making it more adaptable to whatever reaction conditions are the best for producing green ammonia from sustainable reactants.
Snyder said: “The benefit of our MOFs is that we’ve discovered that they can be rationally tuned, which means that if you end up locking in on a certain set of reaction conditions in a specific process, we can modify the MOF’s performance parameters – the temperature that you use and the pressure that you use for this adsorbent – to closely match up with the specific application.”
Snyder emphasised that ammonia capture is just one part of any modified process to make green ammonia, which is still a work in progress.
“There are lots of smart people thinking about catalyst and reactor design for a modified Haber-Bosch process that’s designed to operate under more moderate temperatures and pressures. Where we come in is, after you’ve made the ammonia, our materials are what you would try to use to separate and capture the ammonia under these new reaction conditions,” Snyder concluded.
The research was supported by the U.S. Department of Energy Office.