(48a) New Catalysts for Selective C-N Coupling Reactions During HCN Synthesis From Ammonia and Methane

In the Degussa (BMA) and the Andrussow process for HCN synthesis, NH₃ and CH₄ are catalytically converted to HCN and H₂ over Pt gauze catalysts at very high reaction temperatures up to 1573 K. The reason for these high reaction temperatures is the competing ammonia decomposition reaction 2 NH₃ → N₂ + 3 H₂ (ΔH⁰ = 46 kJ/mol(NH₃), which is at lower temperatures thermodynamically favored over the highly endothermic main reaction NH₃ + CH₄ → HCN + 3 H₂ (ΔH⁰ = 251 kJ/mol(NH₃)). In the Andrussow process oxygen is added to the feed and reacts with the produced hydrogen to provide the necessary heat, while in the Degussa process energy is provided through the reactor walls.¹

Various different C-N coupling mechanisms during HCN synthesis have been proposed previously, e.g. complete CH₄ and NH₃ dehydrogenation followed by coupling of atomic C and N or coupling between various CH_x and NH_y fragments, and also the role of homogeneous gas phase reactions has been discussed.¹ In this work, we use recently established scaling relations² to estimate the stability of CH_x and NH_y intermediates and use periodic density functional theory (DFT) calculations to derive Brønsted-Evans-Polanyi (BEP) type relations³ for the transition state energies of various CH_x-NH_y (x,y = 0,1,2) coupling reactions. Using these scaling and BEP relations derived from first principles as input to a microkinetic model, we can not only address the question of the dominant reaction mechanism on different transition metal surfaces, but we can also predict the activity, and particularly the selectivity, for HCN production as a function of simple descriptors, in this case the adsorption energies of atomic C and N. These descriptors can be used for the rational design of new alloy catalysts with improved selectivity for HCN production, which may allow decreasing the reaction temperature and reducing the production cost.

1. J. Sauer, M. Bewersdorf, M. Köstner, M. Rinner, D. Wolf, in Handbook of Heterogeneous Catalysis, 2 ed. (Eds.: G. Ertl, H. Knözinger, F. Schüth, J. Weitkamp), Wiley-VCH Verlag GmbH & Co. KGaA, 2008, pp. 2592.

2. F. Abild-Pedersen, J. Greeley, F. Studt, J. Rossmeisl, T. R. Munter, P. G. Moses, E. Skúlason, T. Bligaard, J. K. Nørskov, Physical Review Letters 2007, 99, 016105.

3. J. K. Nørskov, T. Bligaard, A. Logadottir, S. Bahn, L. B. Hansen, M. Bollinger, H. Bengaard, B. Hammer, Z. Sljivancanin, M. Mavrikakis, Y. Xu, S. Dahl, C. J. H. Jacobsen, Journal of Catalysis 2002, 209, 275