Dissertation > Industrial Technology > Chemical Industry > Basic Organic Chemistry Industry > The production of aromatic compounds > Aromatic hydrocarbons

Dehydrogenation of Ethylbenzene to Styrene in the Presence of Carbon Dioxide

Author YeXingNan
Tutor GaoZi
School Fudan University
Course Physical and chemical
Keywords carbon dioxide ethylbenzene dehydrogenation styrene hydrotalcite-like precursor chromia Al2O3 SiO2 support impregnation coprecipitation sol-gel mixed oxides transition metal oxide promoter Pd
Type PhD thesis
Year 2004
Downloads 254
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Styrene is one of the most important fundamental chemicals, which is used for the production of plastic, resin and synthetic rubber. The total output of styrene in the world is above 25 billion ton/a, and 90% of styrene is produced by catalytic dehydrogenation of ethylbenzene in the presence of steam, a high-energy-consuming and equilibrium-limited process. Oxidative dehydrogenation of ethylbenzene has attracted much attention since the discovery of certain catalysts for the reaction in the early 1970s. Nevertheless, a considerable decrease in styrene selectivity owing to deep oxidation of hydrocarbons to carbon oxide makes it unpractical in economical point of view. Dehydrogenation of ethylbenzene in the presence of CO2 has aroused widespread interest recently for its lower energy consumption and higher equilibrium yield of styrene. Numerous research works on this subject have been reported and the possibility of dehydrogenation of ethylbenzene in the presence of CO2 instead of O2 has been acknowledged, but people are still in search of a good catalyst for the process.The emphases of this thesis are to screen reasonable catalytic materials and preparation methods for the dehydrogenation of ethylbenzene in the presence of CO2, and attempt to understand the role of CO2 and the reaction characteristics over various catalysts. The main contents of this thesis are as follows:(1) The Mg/Fe hydrotalcite-like compounds at Mg: Fe molar ratio from 2:1 to 4:1 were synthesized by coprecipitation method. Mg/Fe mixed oxide catalysts prepared by calcination of these hydrotalcite-like precursors have high specific surface area and are active for the dehydrogenation of ethylbenzne to styrene in the presence of CO2. Mg/Fe(2/1) catalyst calcined at 823 K displays a maximum ethylbenzene conversion of 35.6% and a styrene selectivity of 99.5%. The activity of the catalysts decreases as the calcination temperature is raised above 1073 K because of the formation of MgFe2O4 spinel structure with low surface area. Incorporation ofAl3+, Ni2+, Co2+ and Zn2+ into the hydrotalcite-like precursors may further enhance thecatalytic activity of the calcined catalysts. The Mg/Zn/Al/Fe(3/3/l/2) catalyst affords the highest ethylbenzene conversion of 53.8% and a styrene selectivity of 96.7%. The amount of CO in the reaction product agrees well with the amount of styrene at the initial stage, but is reduced by a factor of 44% as the reaction goes on, indicating that the oxidative dehydrogenation reaction is probably accompanied with the simple dehydrogenation reaction to a minor extent. Acid-base measurements reveal that the existence of a large amount of stronge acid sites and a moderate amount of base sites accounts for the high dehydrogenation activity of Mg/Zn/Al/Fe catalyst. TPR studies show that the reduction of the iron oxide in the catalysts species is retarded after the incorporation of Zn2+ and Al3+, which favors the redox cycle and enhances the catalytic activity and stability of Mg/Zn/Al/Fe catalyst. After regeneration the activity of Mg/Zn/Al/Fe catalyst is almost fully restored.(2) Chromia supported on Al2O3 and SiO2 .catalysts were prepared by inpregnation method, and the chromia-based mixed oxide catalysts were prepared by coprecipitation and sol-gel methods. The dispersion threshold of Cr2O3 on Al2O3 is close to the theoretical monolayer surface coverage, whereas the dispersion threshold of Gr2O3 on SiO2 is much smaller than the theoretical monolayer surface coverage. The activities of the Cr/Al catalysts are higher than those of the Cr/Si catalysts, and their maximum ethylbenzene conversions are 59.0% and 22.7%, respectively, whereas the activities of the Cr-Si catalysts are higher than those of the Cr-Al catalysts, and their etylbenzene maximum conversions are 56.0% and 45.8%, indicating the influence of preparation methods on catalytic activities. The maximum activity of various catalysts appears at a Cr2O3 content close to its dispersion threshold, suggesting that the dispersed chromia species in the catalysts are more

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