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.

A descriptor of active CuO-ZnO(Al₂O₃) methanol-synthesis and water–gas-shift catalysts is the presence of high-temperature carbonates (HT-CO₃) in the oxidic catalyst precursor. Previous reports have shown that such HT-CO₃ lead to an appropriate interaction between the oxides; as a result, a high Cu surface area (or Cu-Zn or Cu/ZnO interphase areas) can be achieved. Yet, their nature is not well understood. In this study, the nature of these carbonates was investigated by experimental and theoretical methods in the oxidic precatalyst. A calcined Cu-Zn-Al hydrotalcite model compound revealed to have well-dispersed ZnO and CuO phases, together with highly stable HT-CO₃. It was hypothesized that these HT-CO₃ groups may be placed at critical locations at nano-scale as a glue, thus avoiding the growth of the oxide crystallites during calcination. This is an excellent pre-condition to achieve a high Cu surface area (or Cu-Zn or Cu/ZnO interphase areas) upon reduction, and therefore a high activity. To prove that, first-principles calculations were carried out based on the density functional theory (DFT); alumina was not considered in the model as the experimental data showed it to be amorphous but it may still have an effect. Comprehensive calculations provided evidence that such carbonate groups favourably bind the CuO and ZnO together at the interface, rather than being isolated on the individual oxide surfaces. The results strongly suggest that the HT-CO₃ groups are part of the structure, in the calcined precatalyst, where they would prevent thermal sintering through a bonding mechanism between CuO and ZnO particles, which is a novel interpretation of this important catalyst descriptor.

The authors regret that the original graphical abstract contained an error by accident. In particular, the values of the OH density for the calcined SBA-15 and solvent-&-Fenton-treated SBA-15 were swapped. These values contradict the main message of the study. The values contained in the paper are however correct. In summary, the paper's data are correct but the graphical abstract contained swapped data leading to a contradiction. Below, an amended graphical abstract can be found, as well as in the online journal website. The authors would like to apologise for any inconvenience caused. [Figure presented]

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.

This work reports initial results on the effect of low concentrations (ppm level) of a stabilizing agent (2,6-di-tert-butyl-4-methylphenol, BHT) present in an off-the-shelf solvent on the catalyst performance for the hydrogenolysis of γ-butyrolactone over Cu-ZnO-based catalysts. Tetrahydrofuran (THF) was employed as an alternative solvent in the hydrogenolysis of γ-butyrolactone. It was found that the Cu-ZnO catalyst performance using a reference solvent (1,4-dioxane) was good, meaning that the equilibrium conversion was achieved in 240 min, while a zero conversion was found when employing tetrahydrofuran. The deactivation was studied in more detail, arriving at the preliminary conclusion that one phenomenon seems to play a role: the poisoning effect of a solvent additive present at the ppm level (BHT) that appears to inhibit the reaction completely over a Cu-ZnO catalyst. The BHT effect was also visible over a commercial Cu-ZnO-MgO-Al2O3 catalyst but less severe than that over the Cu-ZnO catalyst. Hence, the commercial catalyst is more tolerant to the solvent additive, probably due to the higher surface area. The study illustrates the importance of solvent choice and purification for applications such as three-phase-catalyzed reactions to achieve optimal performance.

Deactivation of a Pd/alumina catalyst has been observed during the hydrogenation of α-methylstyrene and styrene. In both feedstocks, deactivation is caused by an additive, 4-tert-butylcatechol (TBC), a polymerization inhibitor, commonly employed at the ppm concentration level in the formulation of commercial monomers. It was found that the reaction rate in the α-methylstyrene fluctuated notably among the reactant vendors, and this was ascribed to the varying concentration of TBC, although other factors, such as the concentration of water, may play a role. The study was extended into the hydrogenation of styrene using a trickle bed reactor. The negative impact of the TBC present at the ppm level was obvious. The deactivation mechanism was complex to rationalize. A two-stage behavior was observed: a first stage of a relatively fast deactivation followed by a second stage of slow deactivation. A tentative explanation considers the presence of two types of Pd-sites, which are poisoned by TBC: the more active α-Pd-H sites and the less active β-Pd-H sites. Finally, in practical terms, it is important to emphasize that such an additive must be removed from the reactant to maximize the catalyst performance. This can be achieved by adsorption using a commercial F-200 Alcoa alumina.

Mesoporous materials are of vital importance for use in separation, adsorption, and catalysis. The first step in their preparation consists of synthesizing an organic-inorganic hybrid in which a structuring directing agent (SDA, normally a surfactant) is used to provide the desired porosity. The most common method to eliminate the SDA, and generate the porosity, is high-Temperature calcination. Such a process is energy-intensive and slow. In this study, we investigated alternative nonthermal surfactant removal methods on a soft MCM-41 material, aiming at reducing the processing time and temperature, while maximizing the material's properties. The choice of a soft MCM-41 is critical since it is hydrothermally unstable, whereas the SDA removal is troublesome. Microwave processing yielded outstanding performance in terms of surfactant removal, structural preservation, and textural features; the surfactant was fully removed, the hexagonal structure was preserved, and the surface was highly rich in Si-OH groups. It is suggested that H2O2 is the dominant oxidant. In terms of the process features, the processing time is significantly reduced, 14 h (calcination) versus 5 min (microwaves), and the applied temperature is much lower. The energy savings were estimated to be 72% lower as compared to calcination; therefore, this approach contributes to the process intensification of a very relevant material's production.

An efficient process is reported for preparing a state-of-the-art Fe-ferrierite catalyst for N₂O decomposition under industrial tail-gas conditions. In the synthesis procedure, we evaluate the very demanding constraints for scale-up; i.e. large reactor volumes are typically needed, and long processing times and considerable amounts of wastewater are generated. The proposed synthesis minimizes the amount of water used, and therefore, the amount of produced wastewater is minimal; in this approach there is no liquid residual water stream that would need intensive processing. This has remarkable benefits in terms of process design, since the volume of equipment is reduced and the energy-intensive filtration is eliminated. This route exemplifies the concept of process intensification, with the ambition to re-engineer an existing process to make the industrial catalyst manufacture more sustainable. The so-obtained catalyst is active, selective, and very stable under tail-gas conditions containing H₂O, NO, and O₂, together with N₂O, keeping a high conversion during 70 h time on stream at 700 K, with a decay of 0.01%/h, while the standard reference catalyst decays at 0.06%/h; hence, it deactivates 6 times more slowly, with ∼5% absolute points of higher conversion. The excellent catalytic performance is preliminarily ascribed to the differential speciation.

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.

A novel route to prepare highly active and stable N₂O decomposition catalysts is presented, based on Fe-exchanged beta zeolite. The procedure consists of liquid phase Fe(III) exchange at low pH. By varying the pH systematically from 3.5 to 0, using nitric acid during each Fe(III)-exchange procedure, the degree of dealumination was controlled, verified by ICP and NMR. Dealumination changes the presence of neighbouring octahedral Al sites of the Fe sites, improving the performance for this reaction. The so-obtained catalysts exhibit a remarkable enhancement in activity, for an optimal pH of 1. Further optimization by increasing the Fe content is possible. The optimal formulation showed good conversion levels, comparable to a benchmark Fe-ferrierite catalyst. The catalyst stability under tail gas conditions containing NO, O₂ and H₂O was excellent, without any appreciable activity decay during 70 h time on stream. Based on characterisation and data analysis from ICP, single pulse excitation NMR, MQ MAS NMR, N₂ physisorption, TPR(H₂) analysis and apparent activation energies, the improved catalytic performance is attributed to an increased concentration of active sites. Temperature programmed reduction experiments reveal significant changes in the Fe(III) reducibility pattern with the presence of two reduction peaks; tentatively attributed to the interaction of the Fe-oxo species with electron withdrawing extraframework AlO₆ species, causing a delayed reduction. A low-temperature peak is attributed to Fe-species exchanged on zeolitic AlO₄ sites, which are partially charged by the presence of the neighbouring extraframework AlO₆ sites. Improved mass transport phenomena due to acid leaching is ruled out. The increased activity is rationalized by an active site model, whose concentration increases by selectively washing out the distorted extraframework AlO₆ species under acidic (optimal) conditions, liberating active Fe species.