Organocatalyst reactivation with improved performance in O₂-mediated styrene synthesis

An in situ thermal reactivation of a multi-walled carbon-nanostructure (MWCNT) is feasible with improved performance in the O₂-mediated styrene synthesis, also called oxidative dehydrogenation of ethylbenzene (EB). The actual catalyst is the coke deposited at the beginning of the reaction on the MWCNT, denoted as ODH-coke, i.e., forming a supported organocatalyst. The deactivation mechanism is the continuing and severe coking that reduces the surface area and eventually plugs the catalyst bed. The reactivation was carried out in situ after studying the combustion kinetics of the ODH-coke, which combusts at lower temperature than the MWCNT. The reactivation generated a different catalyst than the original, formed by porous ODH-coke, unmodified ODH-coke and the MWCNT backbone. The performance of such reactivated organocatalyst was improved, becoming more selective to styrene and less to CO_x. This was explained by the increased concentration of ketonic and phenol groups, during the reactivation. Explaining the conversion enhancement is not straightforward because the reaction is limited in O₂. For this, a model was applied that explains that the increased EB conversion is caused by the lowered CO_x selectivity. The model also explains the EB conversion decay with time on stream, due to the enhanced CO_x selectivity; though the nature of the latter effect is not yet fully understood. This work brings three main messages: (a) survival of the MWCNT backbone against gasification/combustion, (b) a better organocatalyst is formed after reactivation, and (c) a model which explains the conversion-selectivity changes (improvement and decay). The latter represents a change in paradigm since conversion and selectivity are considered independent parameters, in heterogeneous catalysis.