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Explaining the conversion and selectivity of a heterogeneous catalyst is often done separately. The general rule is that with high temperatures conversion increases but the catalyst becomes less selective to the desired product, as other pathways to side products are activated. However, this study shows that conversion and selectivity can be correlated. Ethylbenzene (EB) can be converted into styrene under oxidative dehydrogenation conditions. This process is catalysed by the coke formed on an acidic solid material (alumina in this study) at the beginning of the reaction. The process also produces unwanted CO and CO2 (denoted as COx). In a previous study, the alumina calcination temperature influenced the coke selectivity; it increased the styrene selectivity. The EB conversion was also enhanced and it was explained in qualitative terms. In this work, we developed a quantitative model explaining the changes in EB conversion related to the selectivity. The model was obtained by setting an O2 mass balance and showed to explain the experimental reaction data well. The enhanced EB conversion, observed in the alumina series, was explained by the fact that the coke becomes less selective to the unwanted COx, and more to styrene. The COx-forming reactions consume more O2 than the styrene reaction, and have a highly negative impact on the EB conversion. The study provides evidence for the difficulty in achieving a high EB conversion under ODH conditions when COx is formed. For instance, to achieve an EB conversion above 80%, the selectivity to COx should be below 4% at O2/EB of 0.6 (mole) for a typical alumina catalyst. The model proves the link between conversion and selectivity and, secondly, the demanding conditions to achieve a high EB conversion for this reaction. To the best of our knowledge, this is the first example showing a quantitative model relating selectivity and conversion for a heterogeneously catalysed reaction. |
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