Fertilizers are a good thing to celebrate, as they help feed countless people. But there is also justification for complaining, because too much fertilizer pollutes the water and the production consumes vast amounts of energy. The production of ammonia for nitrogen fertilizer alone contributes at least one percent to global greenhouse gas emissions. Researchers around the world are therefore working to reduce this proportion. A team from the University of California, Berkeley, reported how a forest-green, copper-containing substance can help recently in the trade magazine Nature.
Ammonia has been produced from nitrogen, the main component of air, and hydrogen in the so-called Haber-Bosch process for more than 100 years. In order for the pungent-smelling gas to be harvested in the largest possible quantities, harsh temperature changes are necessary again and again. While the chemical reaction requires temperatures of up to 500 degrees Celsius, the product ammonia can best be separated from the gas mixture at minus 20 degrees, because it liquefies in the cold. While the necessary heat can be covered by the waste heat from the process, cooling requires an extra portion of energy.
This is where the idea of Benjamin Snyder, who has been researching and teaching at the University of Illinois since the beginning of the month, comes in. The chemist does not want to liquefy the ammonia gas, but to extract it from the mixture with a porous substance. More specifically, it’s a copper-containing tangle of hydrocarbon molecular chains called metal-organic frameworks (MOFs) that can soak up large amounts of ammonia, Snyder says.
If the ammonia is released, the tangle purrs back together
What happens at the molecular level surprised the researcher himself. “This porous, actually three-dimensional structure changes its structure. Think of it as a bundle of strings that unravel,” he says. The color also changes, going from forest green to blue. If the ammonia is then released and collected, the strings of molecules purr back into a green ball.
Snyder’s team and other scientists have studied other porous materials for ammonia harvesting, including other frameworks and aluminum- and silicon-containing zeolites. However, the researcher emphasizes that the forest-green MOF variant is particularly stable and energy-efficient when it comes to releasing the ammonia. According to the study, the process works at moderate pressure and temperatures of around 175 degrees Celsius.
However, by far the largest share of the CO₂ footprint of ammonia production is due to the starting substance hydrogen, which has so far mostly been obtained from fossil raw materials such as natural gas. “For sustainable fertilizer production, for example, it can be produced by electrolysis from water using electricity from renewable energies,” says Steffen Reichle from the Max Planck Institute for Coal Research in Mülheim an der Ruhr. Researchers are also working on new catalysts for hydrogen production and on converting ammonia synthesis with alternative energy inputs, for example electrically, with light or mechanically in a ball mill, as Reichle is currently investigating.
The goal of many research approaches is climate-friendly ammonia production in smaller, decentralized plants, where the gas is needed. The industry is also working on it. Thyssenkrupp Uhde in Dortmund, for example, already offers small plants for ammonia production with a reduced CO₂ footprint. The company sees not only the use of ammonia in agriculture, but also as a chemical hydrogen storage and energy source.
Benjamin Snyder also has decentralized production facilities in his sights. “However, this requires further advances in adsorbent design and more effective catalysts for ammonia and hydrogen production.” His team’s successful laboratory tests on ammonia harvesting are just the beginning and just one piece in a larger puzzle. Until a “green” chemical ammonia synthesis is ready for the market, the most effective method of fertilizing in a climate-friendly manner remains: apply artificial fertilizer sparingly and only where it is needed. In addition, clever crop rotations, such as legumes, can ensure a natural nitrogen input into the soil.