1999 Progress Report: Synthesis of Acetic Acid via Carboxylation of MethaneEPA Grant Number: R827124
Title: Synthesis of Acetic Acid via Carboxylation of Methane
Investigators: Roberts, George W. , Spivey, James J. , Wilcox, Esther
Institution: North Carolina State University
EPA Project Officer: Hahn, Intaek
Project Period: September 30, 1998 through September 30, 2001 (Extended to June 30, 2002)
Project Period Covered by this Report: September 30, 1998 through September 30, 1999
Project Amount: $118,119
RFA: Exploratory Research - Environmental Engineering (1998) RFA Text | Recipients Lists
Research Category: Sustainability , Land and Waste Management , Engineering and Environmental Chemistry
The objective of this research is to develop a technology for the direct synthesis of acetic acid from carbon dioxide (CO2) and methane (CH4):
CO2(g) + CH4(g) < CH3COOH(g)
The emission of greenhouse gases, such as carbon dioxide, is of interest for environmental reasons. In response to this issue, a number of industrial nations have ratified the Kyoto treaty that calls for a voluntary worldwide reduction in the emissions of carbon dioxide and other greenhouse gases. Although the United States has not yet ratified this treaty, there is national interest in its goals. It is essential to develop new technologies that will contribute to the reduction of carbon dioxide. One such technology is the utilization of CO2 as a reactant in chemical synthesis.
Acetic acid is a vital industrial chemical; over 6 million tons per year are produced worldwide. The current industrial process is based on the reaction of carbon monoxide with methanol. Most existing plants use a homogeneous rhodium catalyst, while newer ones are using a recently developed homogeneous iridium catalyst. Although this is a mature technology, there are incentives for a new process. The use of a solid catalyst and inexpensive and benign reactants would substantially reduce the production costs as well as the occupational and environmental risks.
The direct synthesis of acetic acid from carbon dioxide and methane via a solid catalyst will contribute to the reduction of CO2 emissions and may provide an economical means of acetic acid production. The feasibility of the use of a solid catalyst for the direct synthesis of acetic acid is supported by a DRIFTS (Diffuse Reflectance Infrared Fourier Transform Spectroscopy) experiment. The spectra of a 5 percent palladium/carbon catalyst exposed to an equimolar mixture of CO2 and CH4 displayed peaks corresponding to adsorbed acetate species. Our current research is directed at exploring this exciting lead by evaluating the roles of the active metal and the catalyst support, and by studying ways to drive the equilibrium of the reaction towards acetic acid.
Thermodynamic Analysis. Thermodynamic calculations were performed on reaction (1). The RGIBBS reactor model in AspenPlusa engineering simulation software was used to perform a Gibbs free energy minimization on the system to give the chemical and phase equilibrium composition. Three equations of state were used: Ideal gas, Peng-Robinson, and Redlich-Kwong. At low pressures and high temperatures, the equilibrium compositions using all three methods agreed to within 3 percent. As expected, at low temperatures and high pressures, the ideal gas equation was not in good agreement with the other two. The Peng-Robinson and Redlich-Kwong equations were in good agreement with each other, except at extreme low temperatures and extreme high pressure, 300?K and 100 atm.
For a feed of 95 percent CO2 and 5 percent CH4, the equilibrium fractional conversion of methane increased with increasing temperature and increasing pressure. At 1000?K and 150 atm, the fractional conversion of methane was calculated from the Peng-Robinson equation of state to be 1.6 x 10-6. The calculations show that the direct synthesis of acetic acid from carbon dioxide and methane is thermodynamically limited at all conditions of practical interest.
Recently, our work has been focusing on developing a method to drive the equilibrium to the acetic acid product. To do so, the acetic acid must be removed from the system. This can be accomplished by physically removing it, or chemically by reacting it in a second reaction. The second reaction should be thermodynamically favorable, thus driving the equilibrium of the first reaction.
One system we examined was the synthesis of methyl acetate from acetic acid and methanol. The overall process consists of two reactions, equation (1) and:
CH3OH + CH3COOH ? CH3COO CH3 + H2O
Thermodynamic calculations were preformed on this system using the RGIBBS reactor model in Aspen. For a system containing six compounds, and three elements, there are three independent chemical reactions that can occur. These are:
CO2 + CH4 ? CH3COOH
CH3OH + CH3COOH? CH3COOCH3 + H2O
|4 CH3OH ? CO2 + 3CH4 + 2H2O||(5)|
The extent of the last reaction was set to zero, thus allowing Aspen to let only the first two reactions to occur. In other words, the decomposition of methanol is not allowed in this analysis.
The fractional conversion of methane, reaction (3), increased with increasing temperature and increasing pressure. At 1000?K and 25 atm, the equilibrium fractional conversion of methane was calculated to be 2 x 10?4. Although this is a significant improvement over the equilibrium conversion of reaction (1), this system is not thermodynamically favorable.
We currently are examining other systems that are more favorable, such as the synthesis of acetic anhydride and vinyl acetate.