HCN Synthesis from Methane and Ammonia:  Mechanisms of Pt⁺-Mediated C−N Coupling

Abstract

The Pt + -mediated coupling of methane and ammonia has been studied both experimentally and computationally. This system serves as a model for the Degussa process for the industrial production of the valuable feedstock hydrogen cyanide. Mass spectrometric studies demonstrate that C−N bond formation is catalyzed efficiently by Pt + . Details of the experimentally observed reaction channels have been explored computationally using the B3LYP hybrid DFT/HF functional. In the first reaction step, Pt + dehydrogenates CH 4 to yield PtCH 2 +; in contrast, dehydrogenation of ammonia by Pt + is endothermic and does not occur experimentally. Starting from PtCH 2 + and NH 3, C−N bond formation, which constitutes the crucial step in making HCN from CH 4 and NH 3, is achieved via two independent pathways. The major pathway is found to be exothermic by 23 kcal mol - 1 and yields neutral PtH and CH 2 NH 2 + . The second pathway involves a dehydrogenation to yield the aminocarbene complex PtC(H)NH 2 + (Δ r H = −36 kcal mol - 1 ); dehydrogenation of PtC(H)NH 2 + to PtCNH + is exothermic with respect to PtCH 2 + + NH 3 (Δ r H = −8 kcal mol - 1 ) but hindered by kinetic barriers. A comparison of Pt + with other transition metal cations (Fe +, Co +, Rh +, W +, Os +, Ir +, and Au + ) shows that Pt + is unique with respect to its ability to activate 1 equiv of CH 4 and to mediate C−N bond coupling.