Selective Chemical Looping Combustion of Terminal Alkynes in Mixtures with Alkenes

Area of Science:

• Chemical Engineering
• Materials Science
• Catalysis

Background:

• Alkynes are potent inhibitors in olefin polymerization, necessitating their removal from industrial feedstocks.
• Bismuth oxide (Bi2O3) has shown potential in catalytic applications, but its role in selective alkyne combustion requires further investigation.

Purpose of the Study:

• To demonstrate the selective combustion of terminal alkynes in the presence of alkenes using bismuth oxide (Bi2O3).
• To elucidate the reaction mechanism and kinetics governing the selective alkyne combustion on Bi2O3 surfaces.
• To explore the potential of this method for purifying olefin streams.

Main Methods:

• Selective combustion experiments were conducted using various alkyne-alkene mixtures (e.g., phenylacetylene in styrene) over bulk Bi2O3 during anaerobic reduction half-cycles.
• Reaction orders for hydrocarbon consumption were determined to understand the rate-determining steps.
• Density functional theory (DFT) calculations were employed to model initial C-H activation pathways on α-Bi2O3 surfaces.

Main Results:

• Bi2O3 achieved high selectivities (>96%) for alkyne combustion across multiple alkyne-alkene pairs.
• Near-unity reaction orders indicated rate-determining initial C-H activation.
• DFT calculations revealed that heterolytic C-H activation, favored by alkyne C-H acidity, is the kinetically preferred pathway on Bi2O3.

Conclusions:

• Selective chemical looping combustion over Bi2O3 offers an effective method for removing alkyne impurities from olefin streams.
• The selectivity arises from kinetic favorability of alkyne C-H bond activation due to intrinsic differences in acidity.
• This approach provides a novel route for olefin purification in industrial processes.