The oxidative dehydrogenation of ethylbenzene (EB) into styrene (ST) has been proposed as an alternative to the conventional energy-consuming synthesis of styrene. Various types of catalysts have been reported as promising for the reaction under industrially-relevant conditions. However, they show a complex deactivation behaviour. The EB conversion and the ST selectivity decay, with an increased CO_x selectivity. This phenomenon was investigated by means of experimental data and reaction model analysis, using two reference inorganic catalysts. The active catalyst is the deposited coke (ODH-coke) and not directly the inorganic material. The coke is formed in the initial reaction phase and promoted by the Lewis acid sites of the inorganic material. The reaction shows an activation period in which the ODH-coke is deposited and the EB conversion reaches a maximum where O₂ is fully converted. From that point onwards, the reaction model is applicable and the experimental data fit very well with low standard deviations. The model explains that the EB conversion′s decay with time on stream is associated to changes in the selectivity. Hence, EB conversion is not an independent parameter. This simplifies the understanding of this complex deactivation; the deactivating parameter is the selectivity. At this moment, we cannot discriminate between increased CO_x and decreased ST, or both effects, because both routes are competitive. This case represents a new type of catalyst deactivation behaviour, in which selectivity affects conversion.

Deposition of carbonaceous species on a solid catalyst́s surface during a catalysed processes is quite usual. This causes fouling, or depletion of the textural features, with a performance decay. In this work, preliminary results for an ex situ thermal oxidative reactivation of a fouled MWCNT are presented. The coke is nearly completely removed and, as a side effect, some of the MWCNT is combusted. Remarkably, the textural features are improved; 23 % higher BET area and 29 % higher total pore volume, while maintaining the isotherm shape. These improvements are attributed to two factors. Firstly, the removal during reactivation of non-porous (or less-porous) carbon domains present in the starting MWCNT, that positively influences the porosity in various ways; a control experiment employing the fresh MWCNT strongly suggests this hypothesis. Secondly, the new microporosity also contributes to the better BET.

The authors regret that Eq. (2) in the original manuscript was mistyped by accident. Below, an amended Eq. (2) is given. The values associated to this equation in the original publication are however correct and it does not change the discussion of the results. However, the wrong equation in the original publication can lead to confusion. Corrected equation: [Formula presented] Where [Formula presented]values (mol/h) are the molar flowrates, p is a stoichiometric factor (1 for styrene and 8 for CO_x). The term ‘EB’ refers to ethylbenzene. The term ‘X’ refers to the products which can be styrene or CO_x. The subscript ‘IN’ means the molar flow entering the reactor, whereas ‘OUT’ means the flow out of the reactor. The authors would like to apologise for any inconvenience caused.

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.

Ethylbenzene oxidative dehydrogenation over γ-alumina under in situ conditions has revealed that the catalyst recovers fully the original conversion and selectivity under steady state conditions. In the transition state, the reactivated catalyst achieved the steady state conditions faster. This was supported by the physico-chemical characterisation that revealed pore widening due to crystallite sintering during the reactivation, which has a beneficial effect. The excellent stability after the reactivation recycle, as well as along the run, shows the great promise of this catalyst.