Epoxidation of propylene with nitrous oxide on Rb2SO4-modified iron oxide on silica catalysts

The catalytic activity of alkaline and earth alkaline-modified silica-supported metal oxide was investigated for epoxidizing propylene with nitrous oxide. Iron oxide gave the best results, and surprisingly chromium oxide also produced propylene oxide (PO). Unmodified iron oxide catalyst showed low oxidation activity and produced propanal (57% selectivity) in concert with small amounts of acrolein, allyl alcohol, and acetone. After modification, the oxidation rate increased significantly, with PO the principal product. PO selectivities up to 85-90% and spacetime yields of 0.25-0.53 mmol PO g(-1) h(-1) were obtained over supported iron oxide modified with Rb2SO4. A high throughput composition study revealed that other alkali and earth alkali salts were less effective modifiers. Isopropanol decomposition demonstrated that Rb2SO4 severely reduced the acidity of the catalyst. As a result of the neutralization, PO isomerization was drastically reduced. Accordingly, when feeding PO instead of propylene with N2O over the catalyst, a similar reduction of consecutive PO reactions was observed on Rb2SO4 modification. Despite the excellent epoxidation results, a catalytic process remains infeasible due to the restricted service time of the catalyst. Thermogravimetric analyses of a spent catalyst showed carbonaceous residues, suggesting that cokes deactivate the catalyst. Feeding PO indicates that PO itself is a source of cokes. Catalyst regeneration is possible without significant loss of performance. UV-vis DRS and EPR were used to determine the local environment of Fe3+ in the (un)promoted iron oxide catalyst; the findings suggest well-dispersed distorted tetrahedral Fe3+ sites for epoxidation activity. Fe dispersion is ruled by the promoter salts, with both anions and cations being essential. Along with the structural influences, inspection of the catalytic data in concert with XPS and Raman analyses provides evidence of a direct (electronic) promoter effect on the catalytic activity. (c) 2007 Elsevier Inc. All rights reserved.