Dehydrogenation of Ethylbenzene with Carbon Dioxide over Supported Vanadium- Antimony Oxide Catalysts: Effect of Zirconia Modification in Alumina Support

Abstract

Styrene, an important monomer for synthetic polymers, is commercially produced by the ethylbenzene catalytic dehydrogenation (EBDH) process, which is thermodynamically limited and energy consuming. Alternative methods using oxygen as an oxidant are free from these disadvantages but have their own sets of challenges related to safety (the operations with potentially explosive oxygen-hydrocarbons mixtures) and substantial loss of styrene selectivity due to hydrocarbons total oxidation by this strong oxidant. Dehydrogenation of ethylbenzene in the presence of soft oxidant, carbon dioxide (CO2-EBDH), allows to produce styrene selectively with an energy-saving and environmentally friendly process as well as to effectively utilize CO2, a greenhouse gas. Several catalysts, mainly based on supported (on active carbon, alumina, MgO, TiO2, silicates, ZSM-5, MCM-41) or mixed oxides of Fe, Cr, and V were found to be efficient in the CO2-EBDH. 2-14 Among the best catalytic systems for this reaction, alumina supported V-Sb oxide catalyst demonstrates a very high styrene selectivity (97-98%) at high and stable time-on-stream activity. Zirconium oxide has received considerable attention not only as a promising catalyst for a number of reactions including hydrocarbons oxidative dehydrogenation (see Refs. in), but also as an active and “tunable” support with practically important chemical, thermal, and mechanical features. ZrO2 has bifunctional properties of acid and base along with reducing and oxidizing ability. Its high thermal stability is important as the CO2-EBDH is performed at high temperature of about 550-600 C. Like γ-alumina, zirconia is an excellent support for the synthesis of highly dispersed oxides, including VOx-species. 16,17 Also, in contrast with weaker interacting supports, zirconia inhibits the sintering of supported oxides in the presence of water at high temperatures. In general, zirconia has low surface area which, nevertheless, can be substantially increased by use of some special preparation methods. However, zirconia is more expensive than the traditional oxide materials such as alumina, silica, etc., and this is one of the reasons why attempts were made to explore the inherent favorable properties of both alumina and zirconia supports in a mixed Al-Zr oxide. There are several examples when Al2O3ZrO2 supported catalysts exhibited better catalytic properties than the catalysts supported on pure Al2O3 and ZrO2. 21,22,26