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

A commercial heterogeneous Ni-Mo/Al2O3 catalyst was tested for the oxidative dehydrogenation (ODH) reaction of n-butane with different oxidant species: O-2, 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 O-2-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 O-2 favoured the formation of cracked hydrocarbon chains and COx. The highest turnover yield to 1,3-butadiene was achieved at an oxidant-to-butane molar ratio of 2 : 1 at temperatures of 350 degrees C and 450 degrees 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 alpha-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 O-2 as the oxidant. The former molecules promote Mo atom oxygen termination, while in an O-2 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 O-2, 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.