Effect of porosity of carbogenic molecular sieve catalysts on ethylbenzene oxidative dehydrogenation

The conversion of ethylbenzene to styrene by oxidative dehydrogenation is compared over several carbogenic molecular sieves. At 300 degrees C, Carbosieve G deactivated rapidly due to its nanoporous structure. During deactivation, the apparent activation energy dropped nearly a factor of 2 with a corresponding pore volume decrease from 87 to 15 mg/g. The carbogenic Ambersorb adsorbents have meso- and macroporosity in addition to nanoporosity. With 1% oxygen in the feed and at 400 degrees C, the rates of coke deposition on Ambersorb 563 were below measurable levels. The initial yield of styrene was over 70% and slowly fell to 55% with time on stream. For Carbosieve G under the same conditions, the initial yield rose rapidly to 70% but then fell to approximately 10%. The differences in these materials can be attributed to the effects of differences in pore structure and their influence on the coupled reaction and diffusion phenomena. These results indicate that carbogenic catalysts can produce commercially relevant levels of styrene with minor deactivation, providing the pore structure includes substantial amounts of transport porosity and the oxidizing potential of the gas phase is mediated properly.