Cocrystallization can mitigate troublesome chemical properties that impact environment

Developing greener alternatives for industrial chemicals that have a smaller impact on the environment isn’t always straightforward. In fact, engineering environmentally friendly compounds usually involves tradeoffs: eco-friendly characteristics often come at the expense of other unfavorable properties, like chemical instability, for a specific application and condition.

To address these challenges, a team of chemical engineers from Yokohama National University, the National Institute of Occupational Safety and Health, Japan (JNIOSH) and the National Institute of Advanced Industrial Science and Technology (AIST) explored a novel approach. Their findings were published on November 6, 2024, in the journal Chemical Communications.

One such compound the team investigated was ammonium nitrate (NH₄NO₃, AN), a widely used fertilizer and excellent oxidizer, or a chemical that supplies oxygen during combustion and releases energy. Oxidizers are commonly used as a blasting agent, in military explosives and as propellants and pyrotechnic devices. AN could be a more green alternative to current oxidizing compounds that contain residual metal atoms, such as potassium (K), strontium (Sr) and copper (Cu) that can harm the environment.

Unfortunately, AN is a hygroscopic compound, meaning it tends to absorb moisture from the air, which could change the burning behavior of combustible materials using AN as an oxidizer. Additionally, AN can change phases and consequently volumes, much like water can change phases from ice to water, which complicates storage and impacts burning behavior.

To overcome these issues, the researchers applied a cocrystallization technique to AN to decrease the hygroscopicity of the compound and eliminate phase transitions.

“Cocrystallization has the potential to null or ease drawbacks of a substance. It can contribute to the utilization of materials which have not been previously employed and facilitate the design of materials with desired characteristics, such as more environmentally friendly compounds,” said Mieko Kumasaki, professor in the Faculty of Environment and Information Sciences at Yokohama National University and an author of the research paper.

For this particular cocrystallization process, the engineers combined the amino acid glycine (Gly), a building block of proteins, with AN to form cocrystals. The molecular interactions between AN and glycine molecules in the crystals decrease the interaction of either molecule with water in the air, decreasing hygroscopicity, and increase the stability of the molecules, eliminating phase transitions. Alternatively, other strategies, such as coating or phase stabilization with salt or organic molecules, would solve only hygroscopicity or phase transitions—not both.

Another lab had previously cocrystallized AN with sarcosine (Sar), another amino acid. This AN/Sar cocrystal, however, has a highly negative oxygen balance (OB) of -61%, meaning the AN/Sar cocrystal contains less oxygen than is required to fully oxidize its carbon and hydrogen atoms during combustion. Ideally, the novel AN cocrystals developed by the team would have an OB as close to 0 as possible, making it more suitable as an energetic material.

The team chose Gly to combine with AN as a cocrystal because of its few carbon and hydrogen atoms: Gly contains only one carbon and three hydrogen atoms compared to sarcosine’s three carbon atoms and seven hydrogen atoms. This strategy reduced the amount of oxygen required to fully oxidize the carbon and hydrogen atoms in the AN/Gly cocrystal, bringing the OB down to -17%.

Fortunately, the cocrystallization of AN and Gly succeeded in both decreasing the hygroscopicity of AN and increasing the stability of the molecules, eliminating temperature-induced phase transitions. Importantly, AN/Gly cocrystals are also stable enough to pass friction sensitivity tests, meaning the crystals are sufficiently safe for handling.

“Our research aims to create safe, low-sensitivity and environmentally friendly energetic materials with desired energy-release performance characteristics. Our ultimate goal is to develop the methodology required to design and manufacture such materials,” said Kumasaki.

First author of the research paper is Kazuki Inoue from the Graduate School of Environment and Information Sciences at Yokohama National University in Yokohama, Japan. Other contributors include Yosuke Nishiwaki from the National Institute of Occupational Safety and Health, Japan (JNIOSH); Shinya Matsumoto from the Faculty of Environment and Information Sciences at Yokohama National University; and Ken Okada from the Research Institute of Science for Safety and Sustainability at the National Institute of Advanced Industrial Science and Technology (AIST) in Tsukuba, Japan.