Challenges: green ammonia
Ammonia (NH₃) is a crucial compound for agriculture. In 2021, the global production of ammonia reached 185 million metric tons, with more than 80% of it used for fertilizer production. As the global population increases, so does the demand for ammonia. Ammonia is also a promising material as a transportation fuel and energy storage. Liquid ammonia has a higher energy density of 11.5 MJ/L compared to liquid hydrogen with an energy density of 8.491 MJ/L and compressed hydrogen at 690 bar and 15 ºC with an energy density of 4.5 MJ/L.
The primary method of industrial ammonia production is the Haber-Bosch process, which converts hydrogen (H₂) and nitrogen (N₂) into ammonia. The process can only be run economically in large, capital-intensive plants. They are usually situated in remote areas with cheap natural gas. The produced ammonia is transported considerable distances to customers and contributes to carbon emissions.
The Haber-Bosch process requires very high purity hydrogen and nitrogen and high temperature and pressure (above 450 ºC and 200 bar, respectively), which are highly optimized for the iron-based catalysts to attain significant yields of ammonia. The energy-intensive nature of the process results in the consumption of about 2% of worldwide fossil fuels and an annual emission of over 420 million tons of carbon dioxide (CO₂). The production of ammonia contributes to 1.5% of global greenhouse gas emissions.
Therefore, there is an urgent need to develop technologies for ammonia synthesis in a sustainable manner.
New technology for ammonia synthesis
A research group from CNRS in France, led by Dr. Nicolas Mézailles, has developed a simple and economical method for ammonia synthesis at room temperature and low pressure. The process uses a tropical boron-centered salt and a reducing agent dissolved in an organic solvent, forming boron-centered radicals. The high-energy boron-centered radicals react with nitrogen gas to produce borylamines. Bolyamines then undergo a hydrolysis reaction utilizing an acid solution to produce ammonium and a tropical boron-centered salt.
The process of ammonium synthesis is illustrated in the diagram below. The energy required for the process can be provided by renewable electricity.
A nitrogen-filled reactor has a reaction medium that is made up of an organic solvent, a reducing agent, and a tropical boron-centered molecular catalyst. At room temperature, the reaction medium is vigorously stirred under an anhydrous nitrogen atmosphere. The strong stirring makes the nitrogen gas come into contact with the reaction medium, which speeds up the reaction. The anhydrous atmosphere avoids the formation of byproducts such as hydrogen gas.
The tropical boron-centered salt is represented by the formula R₁R₂BX. It can be chlordicyclohexylboran (Cy₂BCl), which is represented by the following formula:
The reducing agent can be potassium metal (K), and the organic solvent can be tetrahydrofuran (THF) or hexane.
At ambient temperature and atmospheric pressure, Cy₂BCl reacts with the reducing agent of potassium and forms a high-energy boron-centered radical, Cy₂B•. The boron-centered radicals provide a kinetically and thermodynamically favorable pathway to nitrogen functionalization. The reaction of boron-centered radicals with nitrogen gas form stable borylamines. This process can be represented by the following reactions:
After the reaction is done, which takes about 6 to 12 hours, an excess of hydrochloride (HCl) dissolved in diethylether (Et₂O) is added to the reactor under an inert atmosphere. Through a hydrolysis reaction, borylamines of N(Cy₂B)₃ and NH(Cy₂B)₂ are transformed to ammonium (NH₄⁺) and Cy₂BCI according to the chemical reaction:
After the hydrolysis reaction, solvents and volatile compounds are removed by evaporation, leaving a solid residue. This solid is extracted with hexane, allowing the separation of Cy₂BCI from ammonium. Ammonium is obtained as a white solid. The yield of ammonium production is over 40%, which can be improved to over 90% by increasing the nitrogen gas pressure to 80 bar.
Ammonium chloride can be converted to ammonia by heating.
- FR3123065B1 Ammonia nitrogen production process