OXIDATIVE DEHYDROGENATION OF HYDROCARBONS USING CARBON DIOXIDE AS AN OXIDANT

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An attempt to use carbon dioxide as a diluent and oxidant in the dehydrogenation of ethylbenzene to stirene was carried out over an activated carbon supported iron catalysts (Fe 17 wt %) at 773-973 K,CO2/ ethylbenzene=50-70mol/mol and W/F=30-120 g h/mol. An addition of 20-30 mol % lithium nitrate to iron resulted in a significant increase in the catalytic activity. The highest yield stirene (40-45 %) with more than 90 % selectivity was obtained at a ratio of lithium to iron of 0.1-0.2 (mol/mol). In addition to stirene, carbon monoxide and water were formed as products. This indicated that the reaction proceeds via an oxidative dehydrogenaiton mechanism. Added lithium nitrate was converted into lithium ferrite during treatment of an iron -lithium co-loaded activated carbon catalyst under carbon dioxide at 973 K.The same catalyst was employed in the dehydrogenation of isobutane to isobutene. At 823 K and W/F= 8 g h/mol, conversion of isobutane was 24 % in the 80 % selectivity to isobutene. With increasing the W/F to 14, conversion of isobutane increased to 30 % with an increase in the selectivity to 85 %. Again deactivation of the catalyst during the reaction occurred. Slight recovery of the catalytic activity was achieved by regeneration of the catalyst with carbon dioxide flow without feeding isobutane. Additives such as lithium nitrate did neither improve catalytic activity and activity decay.In both reactions iron and iron oxide redox cycle was a key catalytic cycle.Catalytic activity of chromia loaded activated carbon was comparable to the iron loaded activated carbon in the dehydrogenation of ethylbenzene to stirene. However, in this case, no redox cycle of the catalyst was observed.