Insights into Cr/SiO2 catalysts during dehydrogenation of propane: an operando XAS investigation

In situ and operando XAS spectroscopic methods were applied to monitor the variations in the oxidation state and in the local structure of the chromium sites in a 2.0Cr/SiO2-DHS catalyst during propane dehydrogenation under non-oxidative and different oxidative conditions. The spectroscopic data have been compared to the catalytic performances. Although the majority of the specialized literature ascribes the propane dehydrogenation activity mainly to CrIII species and only a few works report the presence of small amounts of CrII species, our data unequivocally demonstrate the co-presence of both CrIII and CrII species during the propane dehydrogenation reaction. The relative amount of the two chromium phases has been estimated by analyzing both the XANES and EXAFS spectra with a two-phase fitting approach. It was found that the CrIII-phase prevails when oxygen is present in the reaction mixture and when the reaction is performed at low flow or under static conditions. The amount of the CrIII-phase correlates well with the propane conversion. In contrast, the selectivity to propene correlates with the amount of the CrII-phase, at least when propane dehydrogenation is performed under oxidative conditions. Although other phenomena surely contribute to the overall catalytic performances of Cr/SiO2 catalysts, our results indicate that the relative proportion of the CrII and CrIII phases plays a role in influencing the selectivity of the propane oxidative dehydrogenation reaction.