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.