Direct Synthesis of Acetic Acid from Carbon Dioxide and Methane
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Date
2005-07-26
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Abstract
The catalytic activation of carbon dioxide in reactions with methane has been investigated. Of specific concern was the direct synthesis of acetic acid from methane and carbon dioxide using a heterogeneous catalyst:
CO2 + CH4 -> CH3COOH
DGr0 = + 71 kJ/mol DHr0 = + 36 kJ/mol
Equilibrium thermodynamic calculations were performed on this reaction to understand its potential behavior in the laboratory. This reaction was found to be severely limited by thermodynamics at all conditions of practical interest. Equilibrium calculations were performed on other reaction systems to investigate potential methods for overcoming the thermodynamic limitations of the direct synthesis reaction. Promising approaches include the synthesis of acetic anhydride from ketene, the synthesis of vinyl acetate from ethylene and oxygen, and the synthesis of vinyl acetate from acetylene.
Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) experiments definitively showed the formation of adsorbed acetates, monomeric and dimeric acetic acid from carbon dioxide and methane over solid Pt and Pd catalysts. Further reaction experiments, conducted using a micro-reactor system with an online mass spectrometer, confirmed the formation of gas phase acetic acid from carbon dioxide and methane using either a 5% Pd/alumina or 5% Pt/alumina catalyst. In addition to acetic acid, carbon monoxide, hydrogen and water were observed under some conditions.
The effective catalysts for the direct synthesis reaction adsorbed both methane and carbon dioxide. Methane appeared to adsorb on the metal, and carbon dioxide on the alumina support. The effective catalysts also had small metal clusters dispersed on the support, which increases the number of adjacent carbon dioxide adsorbing and methane adsorbing sites.
Preliminary experiments showed that vinyl acetate could be synthesized from carbon dioxide, methane and acetylene. The most effective catalyst appeared to be an admixture of Pt or Pd/Al2O3 and Zn-acetate/carbon. These experiments suggest that the direct synthesis of acetic acid can be driven by coupling it with a thermodynamically favorable reaction that consumes acetic acid.
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Carbon Dioxide, Acetic Acid, Greenhouse Gases, Methane, Vinyl Acetate
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Degree
PhD
Discipline
Chemical Engineering
