Mechanistic and kinetics insights into structure sensitivity of 2,6-Diamino-3,5-Dinitropiridine hydrogenation over Ni catalysts

Designing cost-effective Ni-based catalysts with satisfactory performance for the hydrogenation of 2,6-diamino-3,5-dinitropyridine (DADNP) to 2,3,5,6-tetraaminopyridine (TAP) is highly essential for producing high-performance poly (pyridobisimidazole) but remains a great challenge due to the limited mechanistic and kinetics understandings. Herein, a series of differently sized Ni catalysts but similar electronic properties are synthesized to explore the size-dependent kinetics behaviors of the hydrogenation of DADNP via two competitive routes. The results unravel that the hydrogenation of DADNP to TAP via the route 2, i.e., the formation of TAP from the direct hydrogenation of DADNP without being intermediated by the 2,3,6-triamino-5-nitropyridine (TANP), appears to be more prominent on larger sized Ni catalysts. Microkinetics studies based on the Langmuir-Hinshelwood-Hougen-Watson model indicate that the ratio of the TAP formation rates via these two routes, i.e., k´TAP2/k´TAP1, increases with the increasing Ni particle size, confirming the more favorable formation of TAP via the route 2 on larger sized Ni catalysts. These size-dependent kinetics behaviors are further rationalized by DFT calculations, which reveal that the simultaneous activation of both nitro groups of DADNP via flat adsorption is more likely to occur on the larger size Ni catalysts than that on the smaller sized ones.