Effect of O2, CO2 and N2O on Ni–Mo/Al2O3 catalyst oxygen mobility in n-butane activation and conversion to 1,3-butadiene

Abstract

A commercial heterogeneous Ni–Mo/Al₂O₃ catalyst was tested for the oxidative dehydrogenation (ODH) reaction of n-butane with different oxidant species: O₂, CO₂ and N₂O. The effect of the lattice oxygen mobility and storage in Ni–Mo/Al₂O₃ on catalytic conversion performance was investigated. Experiments indicated that a high O₂-storage/release is beneficial for activity, however at the expense of selectivity. A significant amount of butadiene with no oxygenated compound products was formed upon using carbon dioxide and nitrous oxide, while O₂ favoured the formation of cracked hydrocarbon chains and CO_x. The highest turnover yield to 1,3-butadiene was achieved at an oxidant-to-butane molar ratio of 2 : 1 at temperatures of 350 °C and 450 °C. With CO₂, significant amounts of hydrogen and carbon monoxide have evolved due to a parallel reforming pathway. Partial nickel/molybdenum oxidation was also observed under CO₂ and N₂O atmospheres. TPR revealed the transformation of the high valence oxides into structurally distinct metal sub-oxides. In TPRO, three distinct peaks were visible and ascribed to surface oxygen sites and two framework positions. With N₂O, these peaks shifted towards a lower temperature region, indicating better diffusional accessibility and easier bulk-to-surface migration. XRD revealed the presence of an α-NiMoO₄ active phase, which was used in DFT modelling as a (110) plane. Theoretical ab initio calculations elucidated fundamentally different reactive chemical intermediates when using CO₂/N₂O or O₂ as the oxidant. The former molecules promote Mo atom oxygen termination, while in an O₂ environment, Ni is also oxygenated. Consequently, CO₂ and N₂O selectively dehydrogenate C₄H₁₀ through serial hydrogen abstraction: butane → butyl → 1-butene → 1-butene-3-nyl → butadiene. With O₂, butane is firstly transformed into butanol and then to butanal, which are prone to subsequent C–C bond cleavage. The latter is mirrored in different mechanisms and rate-determining steps, which are essential for efficient butadiene monomer process productivity and the optimisation thereof.