Oxidative dehydrogenation of ethylbenzene as a test reaction for carbon-based catalyst
The development of green sustainable energy technologies has been one of the biggest challenges in the chemical industry nowadays. Many processes must be enhanced in order to avoid high waste of energy and natural resources. Undoubtedly, the industrial production of styrene is a method that due to its high endothermicity and harsh conditions, consumes large amounts of energy. The waste of water is significant in this technology, since it is used as steam to meet the requirements of reaching high temperatures in order to perform the reaction. Many other drawbacks such as low equilibrium conversion and high production costs increase the necessity of developing new alternatives for this process. Among many new technologies applied in order to develop new systems that allow decreasing the waste of energy and misuse of water in a large degree, the progress on the ODH of ethylbenzene studies has been playing an important role in this matter. The addition of oxygen to the catalytic dehydrogenation turns this reaction in exothermic, which means that it can be carried out at lower temperatures and milder conditions. The use of steam is omitted and it can reach higher conversions since it is a non-equilibrium reaction. However, so far the big challenges in the ODH research have been to find a proper catalyst that gives good results regarding activity and stability in order to be able of being used at industrial scale. The evaluation of carbon-based catalysts for this reaction is not a new topic in this field. Normally, these materials are active due to the availability of oxygen functional groups on their surface. Nevertheless, the main drawback is the tendency of total oxidation of the catalyst, mainly due to their structure and the interaction with the oxygen present in the reaction stream. Therefore, to get the right morphology is a critical step for this catalytic system. Nanocarbons are considered among the most promising catalysts due to its unique structure that unlike amorphous carbon materials has the property of keeping high stability towards total oxidation. In this work, catalytic measurements of different carbon-based catalysts were performed. These catalysts were MWCNTs functionalized by means of acid treatment and afterwards vanadium was deposited with different concentrations on the carbon surface using the ALD method. Vanadium is a well-known metal with good catalytic properties for many reactions including the ethylbenzene ODH. Previous studies point out that vanadium could be active for this reaction only when it is well dispersed on the catalyst surface. A uniform and effective deposition was possible by using the ALD device developed in our group. The first step in this work was to design and built-up a reactor setup able to perform the catalytic test. Afterwards, the evaluation of all MWCNTs took place and it was mainly focused on the study of the activity and stability of the carbon structures itself and to determine whether the vanadium species on the surface have an influence on the catalytic properties of the nanotubes. The kinetic results suggested that all the materials were active and stable at the chosen reaction conditions even at long time on stream, confirming the already known good properties of these materials. Regarding the influence of vanadium there was an improvement on the activity with low loadings and a negative influence with higher loadings. The contribution of vanadium active sites only takes place when there are single sites that form the active V5+ species which are able to react with the ethylbenzene and form styrene. Nevertheless, higher vanadium loadings lead to agglomerations and deactivation of the catalyst. Further characterizations of the tested materials in the ODH reaction were carried out in order to get more information regarding modifications on the structure and content of the active groups before and after the reaction. Thermogravimetry combined with mass spectrometry (TG-MS), temperature program oxidation (TPO) and transmission electron microscopy images (TEM) provided valuable information regarding the modification that the nanocarbon structures undergo during the reaction. It was found that there is always the presence of an active coke layer on the surface that is formed during the reaction and remains stable after long time on stream. The most active catalysts showed on the TEM images a thicker layer of this amorphous carbon which was almost negligible for the less active samples. Further analysis confirmed that most of the active groups were presumably located on this coke layer. Therefore, the formation of active amorphous carbon was the critical step where vanadium at low concentrations might play a role of enhancing building up of this layer.