Surface interactions and radical generation in TCD decomposition: a DFT approach

Exo-tetrahydrodicyclopentadiene (exo-TCD) is a key component of Jet Propellant-10 (JP-10), a high-density hydrocarbon fuel extensively used in aerospace applications. The addition of aluminum particles enhances fuel performance and reactivity, making the understanding of initial decomposition pathways crucial. This study used density functional theory (DFT) calculations to investigate the initial hydrogen abstraction reactions in the decomposition of exo-TCD, with emphasis on radical formation processes. A significant aspect of this work is the role of the γ-Al₂O₃ surface in facilitating these reaction pathways, especially considering surface defects modeled by removing hydrogen from active hydroxyl groups. Five known active hydroxyl sites on γ-Al₂O₃ (A_Ia, A_Ib, A_IIA, B_IIb, and B_III) were used to construct complexes with exo-TCD. The formed complexes are primarily van der Waals interactions, with energies ranging from -11 to -20 kcal/mol and no substantial energy differences between configurations. The results indicate that hydrogen abstraction from the R4 site of exo-TCD is the most energetically favorable, owing to the molecular structure. Surface defects can boost reactivity by facilitating hydrogen abstraction, as seen in spontaneous H transfer to the active A_Ib site and low energetic barrier to the transition state of the H-abstraction of the B_IIb site. These findings improve the understanding of TCD decomposition and the catalytic role of γ-Al₂O₃, aiding the development of better propulsion fuels and energetic materials.

The calculations used the Perdew-Burke-Ernzerhof PBE exchange-correlation functional with split-valence polarization (SVP) and triple-zeta valence polarization (TZVP) basis sets, combined with the resolution of identity (RI) method to accelerate four-center electron repulsion integrals. The PBE results were benchmarked with the hybrid meta-GGA functional M06-2X. Dispersion correction D3 was applied throughout. All computations were performed using the Turbomole program.