A major scientific step towards the production of “green” ammonia and “green” fertilizers

A major scientific step towards the production of “green” ammonia and “green” fertilizers

The process of manufacturing ammonia –the main ingredient in fertilizers– is an energy-intensive process and a major contributor to greenhouse gas emissions, which in turn contribute to increasing the phenomenon of climate change and its related negative effects. To improve the energy efficiency of this process, chemists have designed and manufactured porous materials from metal-organic frameworks (MOFs) that can capture and separate the ammonia produced by the production process at moderate pressures and temperatures, compared to the pressures and temperatures required by the standard Haber-Bosch process. These porous materials are not bound to any of the reactants, making the ammonia capture and separation process less energy consuming and therefore greener.

As we know, the industrial production of ammonia is mainly directed towards the manufacture of fertilizers. This substance – which many consider to be the fuel of the agricultural revolution of the last century – constitutes one of the largest markets for chemicals in the world, but at the same time it is one of the most energy-consuming industries.

What is the Haber-Bosch process?

Globally, the Haber-Bosch process is used to make ammonia. This process consumes about 1% of all fossil fuels and produces 1% of all global carbon dioxide emissions, making it a major contributor to climate change.

For those who do not know, the Haber-Bosch process is the standard industrial practice for producing ammonia. It was invented by the German chemist “Fitz Haber” in 1909, and later the German chemist “Karl Bosch” also developed this method for industrial use, and it was then called “Haber-Bosch.” The process converts nitrogen from atmospheric air (N2) into ammonia (NH3), by reacting with hydrogen (H2), using a metal catalyst, under high temperatures (400-550 °C) and high pressures (100-250 atm), which requires the use of large quantities of fossil fuels, thus emitting huge amounts of Greenhouse Gases (GHGs).

Chemists at the University of California were able to take a big step towards making ammonia production more environmentally friendly, which necessarily means achieving the concept of “green ammonia” and “green fertilizers.” This contributes to making agriculture more sustainable.

The main obstacle facing researchers in making ammonia – with less energy input – is the process of capturing and separating the ammonia resulting from the reactants (mainly nitrogen and hydrogen), without resorting to the high temperatures and pressures required by the Haber-Bosch process, as this reaction occurs at high temperatures, which may exceed 500 degrees Celsius, but the ammonia separation process is done by cooling the gas To approximately -20 degrees Celsius, which allows gaseous ammonia to condense – at this point – into a liquid.

Innovative materialFor ammonia separation

Alternative methods of ammonia capture and separation could open the door to ammonia production processes operating under conditions less severe than those of the standard Haber-Bosch process; So chemists at the University of California, Berkeley, designed porous materials from metal-organic frameworks (MOFs) that can capture and separate ammonia at moderate pressures and temperatures (about 175 degrees Celsius). This is because these frameworks do not bind to any of the reactants and can therefore capture and release ammonia without having to cool the process to -20 degrees; Then reheat again from the beginning, which means significant energy savings.

MOFs – which were first revealed in the 1990s – can be described as crystalline systems consisting of metal ions, which are linked together via an atom or central ion, by organic ligands, which are absorbent materials characterized by a large surface area.

Benjamin Snyder, a postdoctoral fellow at the University of California, Berkeley, who led the research, said: “The big challenge in reducing carbon emissions associated with fertilizer production is to find a material where we can capture very large amounts of ammonia and then separate it perfectly, with minimal energy input, meaning we don’t have to use a lot of heat to get the ammonia out. Likewise, when we separate ammonia we don’t want to result in too much heat loss.”

One of the main advantages of the new developed process is that it operates at low temperatures and pressures; Therefore, ammonia – as well as fertilizers – can be produced in smaller facilities closer to farmers, instead of being produced in large central chemical plants.

“The goal here is to enable technology so that farmers in some economically disadvantaged areas around the world now have more easy access to the ammonia they need to grow their crops,” Snyder said. “We should be clear here that our research has not completely solved this problem, but we have presented a new way of thinking about how MOFs can be used to capture ammonia from a modified Haber-Bosch process. “I think this study represents a really important advance in that direction.”

It is worth noting that “Snyder” and “Jeffrey Long” – the senior author of the paper, and a professorChemical Engineering and Biomolecular Engineering at the University of California – published the details of the study in the journal “Nature”.

Long said: “This work is of fundamental importance, because it reveals a new mechanism for capturing ammonia gas, and we are optimistic that this mechanism will be extended to include a number of other important industries, in which MOFs can be adapted.”

Haber-Bosch “Green” Process

According to Snyder, several researchers are working on ways to make the Haber-Bosch process more sustainable. This includes the production of one of the main reaction materials – hydrogen – using solar energy, by decomposing water into the elements hydrogen and oxygen, as the current common knowledge in the ammonia industry is that the hydrogen needed for the process comes from the cracking reaction of natural gas (methane), but this last reaction produces large quantities of carbon dioxide gas.

Other green modifications to the ammonia production process include the use of new catalysts, which operate at low temperatures and pressures that allow hydrogen to react with nitrogen; To form ammonia. However, the process of removing ammonia from the reaction medium remained a major obstacle, and some have tried using a group of other porous materials – such as zeolite – to capture and separate ammonia, but they were unable to absorb and release large quantities. Some other metal-organic frameworks have also been tried by some researchers, but they were subjected to corrosion due to the strong properties of ammonia gas.

Snyder’s innovation was to experiment with a new – and relatively diverse – group of metal-organic frameworks that use copper atoms bonded to organic molecules called cyclohexanedicarboxylates. This is to create a solid, highly porous structure of MOFs. What is unique about this innovative tire is that it was not corroded when tested due to ammonia, but rather was transformed into threads of copper and polymer, which contains ammonia in a very high density. Moreover, these polymer strands can easily release the bound ammonia at relatively low temperatures, returning the material to its initial solid, porous structure used at the beginning of the process.

Snyder commented on this by saying: “When this MOF was exposed to ammonia, its entire structure changed from a three-dimensional, highly porous material to a group of threads that absorb ammonia in a huge way, but the interesting thing is that after the ammonia was separated from these threads, it formed itself again into a three-dimensional structure.”

Promising future uses

Snyder also discovered that these MOFs can be tuned to absorb and release ammonia under a wide range of pressures, making them more adaptable to any reaction conditions that turn out to be best for producing ammonia more efficiently and sustainably.

We conclude from the above that the innovative MOFs are unique in that they can be adjusted appropriately according to the reaction conditions. So if researchers find a specific set of reaction conditions, they can then modify the variables of the MOFs; To closely match this specific application.

Despite the promising results of the new material, Snyder emphasized that capturing ammonia is just one aspect of any future process for making greener ammonia, as the rest of the aspects of this process must also be considered comprehensively; This is to transform this industry into a truly sustainable industry.

Snyder concluded, saying: “There are many competent researchers working on redesigning the Haber-Bosch process, allowing it to operate under more moderate temperatures and pressures. What my research team has come up with is an innovative new method to effectively capture and separate ammonia resulting from the industrial process, thus making it a future part of any modified process to manufacture this important gas.”