Selective oxidation passing through η3-ozone intermediates: applications to direct propene epoxidation using molecular oxygen oxidant

Abstract

Computation was used to design a new catalytic route for selective oxidation using molecular oxygen as the oxidant without requiring a coreductant. Formation of η³-ozone intermediates is a key feature. Key steps in the catalytic cycle are: (a) the η³-ozone group adds an O atom to substrate (e.g., propene) to form substrate oxide (e.g., propylene oxide) plus a peroxo or adsorbed O₂ group, (b) the peroxo or adsorbed O₂ group adds an O atom to the substrate to form substrate oxide plus an oxo group, (c) an oxygen molecule adds to the oxo group to generate an η²-ozone group, and (d) the η²-ozone group rearranges to regenerate the η³-ozone group. Our Density Functional Theory (DFT) calculations reveal the first instances of this catalytic cycle for any material. We expect this catalytic cycle could be used to selectively oxidize a variety of substrates. As a commercially important example, we focus on applications to direct propene epoxidation. Existing commercial manufacture of propylene oxide uses propene oxidation with one or more co-reactants and produces co-products/by-products. Direct propene epoxidation (i.e., without co-reactants) is a potentially greener process with economic and environmental benefits due to eliminating or reducing co-product/by-product formation. The grand challenge is to identify catalysts that can efficiently activate an oxygen molecule and sequentially add the resulting O atoms to two propene molecules in a catalytic cycle. We use DFT to identify and study several catalysts. Our computations introduce two new classes of Zr organometallic complexes that have dinitrone and imine-nitrone based bis-bidentate ligands, respectively. For these and bis-diimine ligated Zr complexes, we study the stability of different catalyst forms as a function of oxygen chemical potential and compute complete catalytic cycles with transition states. A new homogeneous Zr catalyst is designed with a computed enthalpy energetic span (i.e., apparent activation energy for the entire catalytic cycle) of ∼28.3 kcal mol⁻¹—the lowest reported for any direct propene epoxidation catalyst to date. We propose an electrochemical cell process for assembling these catalysts and a preliminary chemical process flow diagram for direct propene epoxidation.