Mechanistic Insights on Coverage-Dependent Selectivity Limitations in Vinyl Acetate Synthesis

Area of Science:

• Catalysis
• Surface Science
• Sustainable Chemistry

Background:

• Improving catalysts for sustainable chemical processes requires understanding the fundamental atomistic origins of their activity, selectivity, and stability.
• Vinyl acetate (VA) synthesis is a key industrial process where catalyst selectivity is often limited by decomposition pathways.

Purpose of the Study:

• To investigate the elementary steps responsible for decomposition product formation that limit vinyl acetate selectivity during synthesis on palladium (Pd) surfaces.
• To elucidate the mechanisms of acetate and ethylene decomposition and their competition with vinyl acetate formation.

Main Methods:

• Utilized density functional theory (DFT) calculations to model reaction pathways.
• Employed steady-state kinetic analyses to understand reaction rates and coverages.
• Examined elementary steps including C-H activations, C-O formations, and C-C cleavages.

Main Results:

• Acetate formation and coupling with ethylene are central to the VA formation cycle, but both can decompose to CO2.
• Both acetate and ethylene decomposition initiate via C-H activation at acetate vacancies.
• Non-oxidative ethylene decomposition, particularly via an ethylidyne intermediate, shows significantly lower energy barriers compared to oxidative pathways, becoming competitive with VA formation at relevant steady-state coverages.

Conclusions:

• The decomposition of ethylene and acetate on Pd surfaces significantly impacts vinyl acetate synthesis selectivity.
• Ethylene decomposition is facile but coverage-dependent, influencing the balance between VA formation and decomposition at steady-state conditions.
• These findings provide a detailed mechanistic framework crucial for optimizing catalyst design and predicting VA synthesis rates and selectivity on metallic surfaces.