Reduction Mechanism And Kinetics Study Of A Multicomponent Molybdate Catalyst For Propylene Ammoxidation

Propylene ammoxidation into Acrylonitrile is a typical selective catalytic oxidation process of hydrocarbons which obeys "Redox" mechanism.Content of[O]is from the lattice oxygen in the catalyst.In the reduction stage,the selective oxidation using lattice oxygen is a key step of the process of propylene ammoxidation. Therefore,to investigate the consumption rule of lattice oxygen and the reduction kinetics is important for the improvement of the catalyst use efficiency.The experiments are carried out in a fixed bed reactor where the catalyst reacts with propylene/ammonia.In absence of gaseous oxygen, catalysts with different reduction time were prepared.The lattice structure and surface properties of the catalysts with different reduction degrees were tested by using XRD,Laser Raman spectroscopy(Raman),and X-ray photoelectron spectroscopy(XPS) methods.Following these characterization,the reduction mechanism is discussed.The as-prepared catalyst was then put into a thermo-analyze.The thermal reduction behavior of the catalyst can be studied by measuring the mass variety. The reaction kinetics was investigated according to the thermal analysis data and a most suitable reaction mechanism model was obtained.The results of XRD,XPS,and Raman showed that there are four reactions happened in orders during the reduction processes and the lattice oxygen migrates from iron/cobalt/nickel molybdates to bismuth molybdate through bulk diffusion.The oxygen storage capacity of catalyst is not only related to the content of bismuth molybdate,but also to the content of Fe₂Mo₃O₁₂ and(Fe/Co/Ni)MoO₄.The XPS results showed that FeMoO₄,MoO₂ and Bi are mainly present in the bulk of the catalyst.Mo and Bi are concentrated on the surface and then migrate from the surface to the bulk.Fe,Co and Ni migrate from the bulk to the surface during the ammoxidation reaction.Several methods for estimating the kinetic parameters and identifying the mechanism of the reaction were used to treat the non-isothermal kinetic data.The results obtained suggestted that the reduction process of the catalyst follows a single mechanism,best expressed by Chemical Reaction(n=2).The reaction rate equation can be expressed as dα/ dt = Aβ^-1 exp(-E / RT)×(1-α)²,where the activation energy E=142 kJ/mol,the frequency factor A=9.84×10⁹min^-1.