K- and Ca-promoted ferrosilicates for the gas-phase epoxidation of propylene with O 2

Abstract In the propylene epoxidation reaction with Fe-SiO 2 catalysts the presence of iron oxide particles has a detrimental effect due to the total combustion of propylene on these iron species. Thus, the complete elimination of the iron oxide particles is presented as a preliminary strategy in order to increase the selectivity towards propylene oxide in iron-based catalysts. In this sense, a simple post-treatment of the catalysts with alkali or alkaline-earth elements (such as K or Ca, respectively) has proven effective in the total elimination of these iron oxide particles. Furthermore, the addition of K and Ca has modified the physico-chemical properties of the catalysts, decreasing their superficial acidity and (for higher K or Ca loadings) masking/blocking the active sites responsible for the catalytic reaction. With all this, it is shown that K has a higher efficiency removing the iron oxide particles compared with Ca (for the same molar ratios) and that a higher amount of K (compared to Fe) is required for the complete elimination of the iron oxide particles. A considerable propylene oxide selectivity enhancement (up to 65%) has been obtained for the K-promoted Fe 0.005 SiO 2 and Fe 0.01 SiO 2 catalysts using O 2 as sole oxidant.