Catalytic Vapor Phase Epoxidation of Propene with Nitrous Oxide as an Oxidant: Investigations on Catalyst Composition and Reaction Conditions

The vapor phase epoxidation of propene with nitrous oxide (N₂O) was experimentally investigated in a fixed bed reactor using different Cs_x/Fe_y/SiO₂ catalysts. This was done with a systematic approach which comprises the derivation of kinetic parameters for directly comparing catalyst performance. Therefore, kinetic measurements were made for each catalyst by variation of the residence time. It was found that the addition of an alkali promoter to the Fe_y/SiO₂ catalyst is essential for the formation of propylene oxide and a proper alkali/Fe molar ratio is crucial for both activity and selectivity. Maxima in both activity and selectivity were observed for alkali/Fe ratios in the region 1.2−1.7. A further increase in activity without any loss in selectivity was obtained by adjusting the calcination temperature to 783 K. The conversion of PO was used as a tool to measure the product stability and a minimum reaction rate was also found for alkali/Fe ratios in the region 1.2−1.7. The promoter is responsible for the formation of active centers and it reduces surface acidity which leads to an increased stability of PO through the inhibition of the consecutive conversion. Maximum selectivities to PO of about 40% at 5−10% conversion were achieved at moderate reaction temperatures of 648 K. Because of parallel and consecutive formation of carbonaceous deposits on the catalyst, the catalyst deactivated within 2 h of operation to a remaining activity of around 40%. Neglecting the carbonaceous deposits as a reaction product and considering only the vapor phase products, PO selectivity is more than 75% at 5−10% propene conversion. The attempt to slow down the deactivation through the addition of supplementary gases (H₂, O₂, NH₃, H₂O) was partially successful, but unfortunately this is always accompanied by lower PO selectivity.