Study Of Fe-doped Spinel Catalysts For Ethylbenzene Dehydrogenation With CO₂

Styrene is one of the most important basic chemicals as a monomer of polymers and it is industrially produced by the dehydrogenation of ethylbenzene (EB) over promoted iron oxide catalysts in the presence of a large quantity of steam at high temperature. Currently, dehydrogenation of EB in the presence of CO2has been extensively investigated because of its well-documented advantages over the steam-based process, such as alleviating thermodynamic equilibrium limitations, and also the energy losses due to steam condensation can be circumvented, making the overall process energy saving.In present work, composite oxides of MgFexAl2-xO4(x=0.1,0.3,0.5) were prepared by sol-gel method, and were tested in ethylbenzene dehydrogenation with CO2. The influence of the content of iron species on the structure of the catalysts and catalytic activity and stability was investigated. Moreover, microporous MgFe0.1Al1.9O4and mesoporous MgFe0.1Al1.9O4were prepared by different methods, the difference of the catalytic activity and stability were discussed. In the chapter4, Fe2O3-MgO/y-Al2O3containing the same element molar ratio with MgFe0.1Al1.9O4was prepared by impregnation method. The catalysts were reduced at450℃and650℃, and were characterized by Mossbauer spectroscopy (MS). The catalytic activity of Fe2+was discussed. The active species of the reaction and possible mechanism of EB dehydrogenation were investigated. Results and conclusions are shown as follows:(1) The content of iron species of MgFexAl2-xO4(x=0.1,0.3,0.5) influence the structure of the catalysts and the catalytic activity. With the increasing of the content of iron species, the crystallinity of the catalysts decrease, the stability of the catalysts decrease, while the initial activity of the catalysts increase.(2) The pore size of MgFe0.1Al1.9O4influence the catalytic activity and stability. Compared with microporous MgFe0.1Al1.9O4, mesoporous MgFe0.1Al1.9O4prepared by hydrothermal method has lower initial activity, but has much higher stability.(3) Synergy effect between Fe2+and Fe3+is beneficial to increase the catalytic activity of the iron-based catalyst for the EB dehydrogenation reaction with CO2. The reduced catalysts shows higher activity, which suggests that certain amount of Fe2+benefits for the reaction. The possible reason is Fe2+-O2-formed in the reduction process of the catalysts possesses moderate basicity, which is more favorable for desorption of CO2compared with Mg2+-O2-. However, excessive Fe2+substitutes the active sites of Fe3+will weaken the catalytic activity. The results suggest that MgFe0.1Al1.9O4contains about25%of Fe2+shows best catalytic activity, improves styrene yield nearly2times to45.0%.