Catalytic Ammonia Synthesis Over Metal-Free Silicon Disubstituted Cyclo[18]carbon: A Density Functional Theory Study

In this work, we investigated silicon disubstituted cyclo[18]carbon as a metal-free catalyst for nitrogen activation and ammonia synthesis using density functional theory at the ωB97XD/def2-TZVPP level. A systematic comparison of the structural features of pristine C₁₈, monosubstituted C₁₇Si, and nine C₁₆Si₂ isomers (C₁₆Si₂(0)-C₁₆Si₂(8)) identifies C₁₆Si₂(8) as the most stable configuration, attributed to the maximum Si-Si separation and minimal distortion of the conjugated carbon framework. Electronic structure analysis indicates that dual Si substitution creates active sites that are highly electron-deficient and a more symmetrically distributed reactive environment, thereby promoting strong N₂ adsorption and effective activation via Si → N₂ back-donation. The activated C₁₆Si₂-N₂ complex undergoes stepwise hydrogenation via C₁₆Si₂-NH₂ and C₁₆Si₂-(NH₂)₂ intermediates, ultimately yielding two NH₃ molecules through an overall energetically favorable pathway. Importantly, the resulting C₁₆Si₂-H₂ adduct re-enters the catalytic cycle by reacting with the remaining C₁₆Si₂-N₂, producing C₁₆Si₂-(NH)₂ and restoring the catalytic pathway, thereby establishing a self-sustaining mechanism. Overall, our findings show that multisite silicon engineering of cyclo[18]carbon frameworks is an effective strategy for developing efficient, metal-free catalysts for ammonia synthesis under mild conditions.