Details

Autor(en) / Beteiligte

• Zha, Zhaoru
• Deshlahra, Prashant

Titel

Mechanistic Framework and Effects of High Coverage in Vinyl Acetate Synthesis

Ist Teil von

• ACS catalysis, 2021-02, Vol.11 (3), p.1841-1857

Ort / Verlag

American Chemical Society

Erscheinungsjahr

2021

Quelle

Alma/SFX Local Collection

Beschreibungen/Notizen

• Solid catalysts often operate at high surface coverage, but fully analyzing and leveraging coverage effects remains challenging. Vinyl acetate (VA) synthesis via oxidatively acetoxylating ethylene on Pd-based catalysts requires multiple bimolecular activation steps involving C2H4-, CH3COOH-, and O2-derived species on acetate-covered surfaces. Titrations of pre-formed acetates with C2H4 and density functional theory (DFT) calculations have elucidated steps involving acetate–C2H4 coupling and H-abstraction to form VA but have left out acetate formation and alternate consumption steps that must occur simultaneously at steady state. This work utilizes steady-state experiment and DFT to obtain more complete mechanistic insights. Rates are inhibited by CO (cofed or produced by side reactions) and increase monotonically with the CH3COOH pressure but increase and decrease with O2/C2H4 pressure ratios at low and high ratios, respectively. The complex trends resulting from these effects are described well by catalytic cycles proposed based on DFT analysis, in which the acetate coverage is set by steps that irreversibly activate O–H bond in CH3COOH and those that consume acetates by reacting with either C2H4 or O2. The coverage affects the formation of acetate species more strongly than its consumption, leading to a formation barrier much lower at 5/16 acetate/Pd coverage but much higher at 6/16 coverage. For practical reactant pressure ratios, the balance between formation and consumption necessary for steady-state operation is achievable only over a narrow coverage range between these limits where their barriers are nearly equal. As a result, the identity of the rate-limiting step shifts from VA formation toward acetate formation when the reactant pressure changes increase the coverage slightly within this narrow range, which is consistent with measured changes in C2H4/C2D4 and CH3COOH/CH3COOD kinetic isotope effects. Promotion of coupling and inhibition of O2 dissociation by high coverage are also demonstrated. These insights provide a comprehensive framework for understanding the effects of conditions and compositions in high-coverage catalysis.