Oxidative dehydrogenation of ethylbenzene under industrially relevant conditions:on the role of catalyst structure and texture on selectivity and stability

Styreen (ST), een belangrijk chemisch product voor bijvoorbeeld het maken van polystyreen (piepschuim), wordt industrieel geproduceerd door middel van de directe dehydrogenering van ethylbenzeen (EB) behulp van stoom bij 580-630 °C. Het proces heeft een hoog energieverbruik en een lage reactor conversie vanwege chemische evenwicht beperkingen. Er wordt nog steeds veel onderzoek en ontwikkeling gedaan om te komen tot verbeterde styreen productie processen. Een voorbeeld is de oxidatieve dehydrogenering van EB. Echter, het proces wordt nog niet commercieel uitgevoerd en verder onderzoek en ontwikkeling op het gebied van de heterogene katalyse en proces technologie is noodzakelijk. In dit onderzoek is gekeken naar de ontwikkeling van verbeterde heterogene katalysatoren op basis van commerciële dragers zoals alumina, silica, aluminosilicaat, zeolieten, en koolstof gebaseerde materialen voor de oxidatieve dehydrogenering van ethylbenzeen tot styreen onder industrieel relevante omstandigheden. Hierbij lag de nadruk op het verbeteren van de styreen selectiviteit en de katalysator stabiliteit en het bepalen van katalysator-proces relaties. Er zijn verbeterde katalysatoren geïdentificeerd en er is een beter begrip verkregen van de werking van de katalysatoren

Oxidative dehydrogenation of ethylbenzene under industrially relevant conditions:on the role of catalyst structure and texture on selectivity and stability

Styreen (ST), een belangrijk chemisch product voor bijvoorbeeld het maken van polystyreen (piepschuim), wordt industrieel geproduceerd door middel van de directe dehydrogenering van ethylbenzeen (EB) behulp van stoom bij 580-630 °C. Het proces heeft een hoog energieverbruik en een lage reactor conversie vanwege chemische evenwicht beperkingen. Er wordt nog steeds veel onderzoek en ontwikkeling gedaan om te komen tot verbeterde styreen productie processen. Een voorbeeld is de oxidatieve dehydrogenering van EB. Echter, het proces wordt nog niet commercieel uitgevoerd en verder onderzoek en ontwikkeling op het gebied van de heterogene katalyse en proces technologie is noodzakelijk. In dit onderzoek is gekeken naar de ontwikkeling van verbeterde heterogene katalysatoren op basis van commerciële dragers zoals alumina, silica, aluminosilicaat, zeolieten, en koolstof gebaseerde materialen voor de oxidatieve dehydrogenering van ethylbenzeen tot styreen onder industrieel relevante omstandigheden. Hierbij lag de nadruk op het verbeteren van de styreen selectiviteit en de katalysator stabiliteit en het bepalen van katalysator-proces relaties. Er zijn verbeterde katalysatoren geïdentificeerd en er is een beter begrip verkregen van de werking van de katalysatoren

The Brunauer–Emmett–Teller model on alumino-silicate mesoporous materials. How far is it from the true surface area?

Determining the surface area of porous materials through the Brunauer–Emmett–Teller (BET) model is a common practice. The method is generally applied in commercial software packages, where the assumptions are sometimes accepted by the experimenter whilst they may sometimes require a deeper analysis. One element of debate is the molecular cross-sectional area of the adsorptive. There is not yet agreement about the correctness of the BET model using a certain value for cross-sectional area of N2; the conventionally-used parameter seems to overestimate the surface areas. In this work, a preliminary study of a modified method is presented, which introduces an ‘apparent’ cross-sectional area for N2, which is smaller to the typically-used value. This value was obtained after measuring a number of relevant mesoporous materials in N2 and Ar, using a model that considers an apparent value for the cross-sectional area. The model predicts outcomes very close to the Ar-based measurements in terms of low relative error. Then, we went one step further and looked into the geometrical surface areas, also referred to as true surface areas. By combining prior studies with our work, it was found that the surface area, using N2 and the conventionally-used cross section, can be ca. 50% higher than the geometrical surface area. Therefore, the significance of the BET surface area seems to be far from well understood, though it is widely applied. This approach also allowed to define an ‘effective’ cross section for N2, that relates it to the geometrical surface area. Its value agrees with prior considerations for an epitaxial orientation of the N2 molecule with a hydroxylated silica surface. As a final recommendation, critical thinking is needed about the default settings in standardised calculations, which may not represent a reliable measure of the true surface area.</p

Thermal stability of porous sol-gel phosphosilicates and their surface area stabilisation by lanthanum addition

The thermal stability of porous sol-gel phosphosilicates was studied by comparing the textural features upon calcination between 400 and 550 degrees C. A significant loss of surface area and pore volume were observed; the first is due to thermal coarsening of the nanoparticles, and the pore volume reduction was ascribed to sintering of the most external nanoparticles producing less void volume. Lanthanum addition was investigated as thermal stabilizer. For the mesoporous phosphosilicate composition, lanthanum addition enhanced the surface area, showing a 45% and 50% improvement with respect to the La-free counterpart; the effect was much less visible for the macroporous composition. (C) 2016 Elsevier B.V. All rights reserved.</p

On the thermal stabilization of carbon-supported SiO2 catalysts by phosphorus:evaluation in the oxidative dehydrogenation of ethylbenzene to styrene and a comparison with relevant catalysts

A strategy to enhance the thermal stability of C/SiO2 hybrids for the O2-based oxidative dehydrogenation of ethylbenzene to styrene (ST) by P addition is proposed. The preparation consists of the polymerization of furfuryl alcohol (FA) on a mesoporous precipitated SiO2. The polymerization is catalyzed by oxalic acid (OA) at 160 °C (FA:OA = 250). Phosphorous was added as H3PO4 after the polymerization and before the pyrolysis that was carried out at 700 °C and will extend the overall activation procedure. Estimation of the apparent activation energies reveals that P enhances the thermal stability under air oxidation, which is a good indication for the ODH tests. Catalytic tests show that the P/C/SiO2 hybrids are readily active, selective and indeed stable in the applied reactions conditions for 60 h time on stream. Coke build-up during the reaction attributed to the P-based acidity is substantial, leading to a reduction of the surface area and pore volume. The comparison with a conventional MWCNT evidences that the P/C/SiO2 hybrids are more active and selective at high temperatures (450–475 °C) while the difference becomes negligible at lower temperature. However, the comparison with reference P/SiO2 counterparts shows a very similar yield than the hybrids but more selective to ST. The benefit of the P/C/SiO2 hybrid is the lack of stabilization period, which is observed for the P/SiO2 to create an active coke overlayer. For long term operation, P/SiO2 appears to be a better choice in terms of selectivity, which is crucial for commercialization

Selectivity-induced conversion model explaining the coke-catalysed O2-mediated styrene synthesis over wide-pore aluminas

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

Surface area per volumetric loading and its practical significance

The surface area of porous materials is a widely-used parameter in catalysis and adsorption. The most-commonly applied model, Brunauer–Emmett–Teller (BET), bases the parameter to the mass of material. Though the model does have its limitations, comparisons among materials are widely done to rank them from low to high surface areas, and to make correlations with other properties. However, in practical terms volume is often more appropriate since the design of reactors and adsorption columns aims at minimizing their volume. Thus, in such cases the surface areas can be better expressed per material bed volume using the bulk density. In this communication, we discuss the concept of volumetric surface area as m2/cm3MatBed, showing remarkable new insights. We applied this concept to two mesoporous silica-based materials with comparable pore size and C values. A material which was far superior in terms of conventional surface area (561 versus 227 m2/gMat), they both became comparable in terms of m2/cm3MatBed (ca. 110 both) due to the much lower density of the former material. In other words, being superior in gravimetric terms does not guarantee to be volumetrically superior. The results highlight the importance of considering the material bulk density when comparing surface areas of porous materials for applications where volume is critical