Researchers Develop Two EMOFs to Enhance Ammonium Perchlorate Decomposition

In a recent study published in Small, researchers have developed two energetic metal-organic frameworks (EMOFs) that significantly lower the thermal decomposition temperature of ammonium perchlorate (AP), concentrate its heat release, and contribute additional energy during decomposition.

Using a structural co-assembly strategy, the researchers combined nitrogen-rich aromatic ligands, strongly reductive anions, and metal nodes to synthesize the two EMOFs. Adding 5 wt.% of one compound to AP reduced its high-temperature decomposition peak by 116 °C and narrowed the exothermic peak width from 141 °C to 10 °C.

The study was conducted by a research team led by Profs. GUO Guocong, ZHENG Fakun, and XU Jiangang from the Fujian Institute of Research on the Structure of Matter of the Chinese Academy of Sciences.

Ammonium perchlorate is an important oxidizer in composite solid propellants because of its high oxygen content, excellent thermal stability, and high energy output. However, its high thermal decomposition temperature, dispersed exothermic process, and insufficient heat release limit the development of high-performance solid propellants.

Previous approaches have included nanoscale metal oxides, which can increase contact area and catalytic activity but tend to aggregate, and inert organic dyes, which can enhance AP decomposition and mechanical safety but often result in insufficient and dispersed heat release. Metal complexes offer the combined advantages of metal catalytic activity and energy contributions from ligands. Nevertheless, simultaneously lowering decomposition temperature, concentrating heat release, and providing additional energy remains challenging.

In this study, the researchers assembled 3,5-diamino-1,2,4-triazole (Hdatz) ligands, cyanoborohydride (CBH⁻) anions, and Zn2+/Cd2+ nodes to synthesize two EMOFs: [Zn2(datz)(CBH)2·H2O]n (compound 1) and [Cd(Hdatz)2(CBH)2]n (compound 2).

Structural analysis showed that compound 1 crystallizes in the monoclinic I2/a space group. Zn(II) adopts a four-coordinate distorted tetrahedral geometry, and datz⁻ bridges adjacent Zn centers in a µ3-κN11:κN12:κN13 bridging mode, forming a 2D network parallel to the ac plane.

Compound 2 crystallizes in the orthorhombic Pbcn space group. Cd(II) adopts a six-coordinate octahedral geometry, and Hdatz bridges in a µ2-κN11:κN13 mode to form a 2D planar structure.

Powder X-ray diffraction (PXRD) confirmed that both compounds are pure phases and retain their crystalline phases after air exposure, water immersion, and thermal treatment, demonstrating good environmental robustness.

Design strategy and catalytic performance of 2D EMOFs for ammonium perchlorate thermal decomposition. (Image by Prof. GUO's group)

Catalytic performance evaluation showed that incorporating 5 wt.% of 1 into AP (5%-1-AP) reduced the high-temperature decomposition (HTD) peak from 439 °C for pure AP to 323 °C, a decrease of 116 °C. The apparent activation energy Ea decreased from 202.6 to 84.5 kJ·mol−1.

The exothermic peak width narrowed from 141 °C to 10 °C, only 1/14 that of pure AP. The total heat release Q reached 979.7 J·g−1, 1.75 times that of pure AP and higher than the theoretical physical mixture value of 566.3 J·g−1, indicating that the EMOF itself contributes additional energy upon decomposition.

The researchers added that 5%-2-AP also performed well, but 1 was superior to 2.

Real-time TG-FTIR analysis revealed that 5%-1-AP generated more high-valent nitrogen oxides at 270-370 °C, with NO2 as the dominant gas-phase product, indicating that 1 significantly promotes NH3 oxidation and AP decomposition.

According to the researchers, the findings demonstrate an "energy concentration" effect, in which the catalyst simultaneously accelerates decomposition, narrows the energy release window, and contributes additional heat.

The study provides a potential design approach for developing catalytic materials for high-performance solid propellants, they said.