Selective Decarbonylation of Fatty Acids to Long-Chain Alkenes via PtSn/SnOₓ-Induced C–O Activation
2021
Zhao, Lei | Li, Bolong | Zhao, Chen
Long-chain olefins are an important class of chemicals and platform intermediates. A green and promising route for synthesizing long-chain olefins is the one-step decarbonylation and dehydration of biomass-derived fatty acids under hydrogen-free conditions. Major impediments for synthesizing olefins from fatty acids are (1) the difficulty in selectively cleaving C–O bonds (461.5 kJ/mol) of fatty acids and (2) the tendency to break the weaker C–C bonds (384.9 kJ/mol) to generate alkanes. Traditional homogeneous catalysts use a sacrificial anhydride to activate the C–O bonds of fatty acids. In this work, we prepared a bimetallic PtSn/SiO₂ catalyst achieving quantitative conversion of stearic acid and a heptadecene yield of 73% at 320 °C without exogenous hydrogen after 4 h of reaction. The PtSn/SiO₂ catalyst allowed changing the adsorption mode of carboxylic acid-containing molecules on SnOₓ while activating the C–O bond in the form of acyl adsorption. Reaction kinetics indicated that the direct decarbonylation of carboxylic acid to olefins was the primary reaction. In situ infrared (IR) spectroscopy measurements also supported this finding, indicating that carboxylic acids were adsorbed on SnOₓ surface species and dissociated into acyl and OH– over oxygen vacancies. In a subsequent step, C–C is broken on the active center of the Pt₁Sn₁ alloy to generate CO and alkyl cations, followed by deprotonation of the alkyl cations and binding with OH– to generate alkenes and water. The route described herein provides a competitive and green approach for long-chain olefin production from inexpensive biomass resources.
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