Highly efficient Co-Al₂O₃ catalysts for oxidative dehydrogenation of ethylbenzene to styrene with CO₂

nCo-Al₂O₃ catalysts with different Co contents (n=2%, 5%, 10%, 15%, 20%) were prepared by a sol-gel approach. The effect of Co content on the nCo-Al₂O₃ catalyst structure and performance in the oxidative dehydrogenation of ethylbenzene to styrene by CO₂ was investigated. The results showed that the isolated Co^{2 +} ions on the nCo-Al₂O₃ catalysts had a positive influence on the catalytic activity, where the isolated tetrahedral Co^{2 +} species were considered as the active sites. Co contents on the Co-Al₂O₃ catalyst greatly affected the structure of Co species and the catalytic performance. The isolated tetrahedral Co^{2 +} species are preferentially generated on the resultant nCo-Al₂O₃ catalyst when the content of Co (n) is less than 10%; as a result, an increase of Co content here leads to the formation of more isolated Co^{2 +} sites and then improves the catalytic activity of nCo-Al₂O₃ in the dehydrogenation of ethylbenzene. When Co content exceeded 10%, crystalline Co₃O₄ particles were obtained on the formed catalyst, which resulted in the decline of the isolated Co^{2 +} sites and catalytic activity. Among various nCo-Al₂O₃ catalysts, 10Co-Al₂O₃ exhibited the best catalytic performance, with 64.4% conversion rate for ethylbenzene and 99.3% selectivity for styrene at 550 ℃. This catalyst remained stable without obvious deactivation for 30 h of reaction, which suggests that the isolated Co^{2 +} species as active sites presented excellent structural stability and excellent anti-coke deposition.