| Acrylonitrile (AN) is a versatile petrochemical intermediate used extensively in the production of acrylic fibers, resins, rubbers and specialty products. Propylene ammoxidation into AN is a typical selective catalytic oxidation process of hydrocarbons, and the AN is the intermediate product of the consecutive reaction, which obeys "Redox" mechanism, i.e., the lattice oxygen in the muticomponent metal oxide catalyst reacts with propylene/ammonia to generate AN, and the reduced catalyst is continuously reoxidized by gaseous oxygen, while the gaseous or adsorbed oxygen lead into COx and the other organic byproducts.In recent years, selective oxidation of hydrocarbons using lattice oxygen of catalyst has been attempted, the theory of which is that reduction and reoxidation of the catalyst are performed in the two reactors, respectively. There are many advantages in the new technology, such as controlling the reaction degree, increasing the selectivity of reaction, saving resource and protecting environment. The new technology of propylene ammoxidation using lattice oxygen of catalyst has been studied, where lattice oxygen of catalyst and propylene/ammonia react in a reactor, and catalyst reoxidation is performed in another separate regenerator, the whole catalytic process is completed. Therefore, the study on the performance of the reduction and reoxidation process respectively is very important for improving the new technology of propylene ammoxidation using lattice oxygen of catalyst.In this work, the kinetics and mechanism of reoxidation/reduction of MB-98 catalyst, which is used widely in industry in China, was studied by XRD, Laser Raman, XPS and thermal analysis methods.Firstly, an experimental setup was built for separating reduction and reoxidation process of the catalyst. The fresh catalyst was reduced to different degrees in a fixed bed reactor by propylene/ammonia or hydrogen; and then the fresh and reduced catalysts were characterizated by using X-ray powder diffraction, Laser Raman spectroscopy and X-ray photoelectron spectroscopy. The results showed that there are four reactions happened sequentially 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. FeMoO4. MoO2 and Bi are mainly existed in the bulk phase in the catalyst. Mo and Bi are concentrated on the surface and then migrate from the surface to the bulk phase. Fe, Co and Ni diffuse from the bulk phase to the surface during the ammoxidation reaction. The catalyst partially reduced by hydrogen has the similar lattice and surface structure as the catalyst partially reduced by propylene/ammonia.Secondly, intrinsic kinetics of reduction of the catalyst was studied by thermogravimetric analysis method. The kinetic triplet of this reaction is obtained by comparison method of Achar-Sharp differential and Coats-Redfern integral equation. The activation energy and pre-exponential factor (A) are 142.0kJ/mol and 9.84×109min-1, respectively. The kinetic model is best expressed by Avrami-Erofeyev equation, i.e., g(a)=(1-α)-1-1, suggesting Chemical Reaction mechanism (n=2).Thirdly, along with XRD and XPS used to detect the structure of the catalyst after reoxidation, the results of the thermogravimetric analysis showed that the reoxidation of the catalyst is attributed to the replenishment of the bulk lattice oxygen in the lower temperature which is respectively combined with Bi (280℃), Fe (340℃) and Mo (440℃), and the replenishment of the surface lattice oxygen in the higher temperature (510℃). Combustion of the deposited carbon started tempestuously around 370℃and finished before 460℃. The favorable reoxidation temperature is around 440℃. Finally, intrinsic kinetics of reoxidation of the catalyst was obtained by Kissinger-Akahira-Sunose isoconversional and master-plot method. The activation energy and pre-exponential factor (1nA) are 140.01kJ/mol and 21.527 min-1, respectively. The kinetic model is best expressed by Avrami-Erofeyev equation, i.e., g(α)=[-ln(1-α)]1/(0.922), suggesting a nucleation and growth mechanism. |
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