Mixed-Metal Promotion in a Manganese-Molybdenum Oxynitride as Catalyst to Integrate C─C and C─N Coupling Reactions for the Direct Synthesis of Acetonitrile from Syngas and Ammonia.

Abstract

Transition metal nitrides and their oxy- and carbo-nitride analogues attract interest as catalysts in various sustainability areas. They offer potential in the development of conversion routes for the synthesis of nitrogenated organic compounds, via the integration of C─C and C─N coupling reactions from sustainable C₁ and N₁ feedstocks, linking carbon and nitrogen renewable cycles. Herein, a series of structurally uniform (NH₄)nH_mM_xM_oyO_z·2H₂O ammonium metal molybdate materials (C 2/m) serve as catalyst precursors, providing atomic-level metal mixing. Among various 3d transition metals, Mn proves the most effective promoter. A [MnMo] mixed-metal catalyst exhibits superior selectivity to acetonitrile (56 % among all organic products) and stability, significantly outperforming monometallic [Mo] and [Mn] counterparts. Multimodal physicochemical characterization, including temperature-resolved X-ray diffraction, Xray absorption near-edge structure and extended X-ray absorption fine structure, and depth-profiling X-ray photoelectron spectroscopy, shows partial nitridation to a cubic (Fm-3m) mixed-metal oxynitride as the working catalyst under operation conditions. Consistent with first-principles thermochemistry, manganese is found to disrupt extended molybdenum nitride domains, preserving manganese-enriched oxynitride domains under the markedly nitriding reaction conditions. Density functional theory predicts these partially oxidic domains to facilitate C─C coupling reactions compared to fully nitrided surfaces, enhancing conversion of the stable reaction intermediate HCN to acetonitrile.