2000 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. , Jang, Ben W-L. , Spivey, James J. , Wilcox, Esther
Current Investigators: Roberts, George W. , Spivey, James J. , Wilcox, Esther
Institution: North Carolina State University , Desert Research Institute
Current 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, 1999 through September 30, 2000
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):
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 millions tons are produced per year 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 Pd/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.
The progress to date is summarized below.
Methods to Drive Equilibrium
Thermodynamic calculations on reaction (1) show that the direct synthesis of acetic acid from carbon dioxide and methane is thermodynamically limited at all conditions of practical interest. Thus, methods have been examined to drive the unfavorable equilibrium by either reacting the acetic acid further, or removing it from the reaction as it is formed. Among the most promising reactions using the first of these methods is the synthesis of vinyl acetate from a mixture of CO2, CH4 and C2H2. The thermodynamics of this overall reaction are favorable, as shown below.
Vinyl Acetate. One possible way to drive the equilibrium of reaction (1) is to produce vinyl acetate with a two-reaction system:
Equilibrium thermodynamic calculations (Gibbs free energy minimization) were performed on the above reaction system using the Peng-Robinson equation of state and stoichiometric inlet composition of the reactants, CO2, CH4 and C2H2.
For this system, the fractional conversion of methane increases with increasing pressure and decreasing temperature. The maximum fractional conversion of methane, 0.983, was achieved at 300K and 150 atm. Even at lower pressures the fractional conversion was still high; e.g., at 300K, and 5 atm, the conversion was 0.971. The highest conversions are reached when the system has a liquid phase. Unfortunately, the conversion drops of dramatically when the system is a vapor. At 350K and 5 atm, the fractional conversion of methane was 0.0025.
These calculations suggest that the above system of reactions is thermodynamically favorable. Thus, this is a feasible alternative to overcome the thermodynamic limitations of the direct synthesis reaction of acetic acid from carbon dioxide and methane.
Formation of Acetate on the Catalyst. A second method to overcome the unfavorable equilibrium would be to react CO2 and CH4 to form acetate on a metal oxide catalyst with surface hydroxyl groups. This reaction will result in the adsorbed acetate species and water:
The water will be removed from the system by the unreacted gases or a sweep gas. Once the catalyst is saturated with the acetate species, it will be removed from the reactants and reaction conditions, leaving only the catalyst with the adsorbed species. The catalyst will then be exposed to steam under conditions that are favorable to the removal of the acetate species as acetic acid. This will regenerate the catalyst sites, and yield the desired acetic acid product:
The catalyst would then be removed from the desorption conditions, and the cycle would begin again. This catalytic cycle is illustrated in Figure 1.
Due to the lack of thermodynamic data for solids in the AspenPlusa database, no simulations could be run for this system. However, it still may be a feasible method to overcome the thermodynamics of reaction (1) and will be examined in this research project.
Formation of Acetate from CO2 and CH4
Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) experiments were performed to further explore the reaction of CH4 and CO2 to form acetic acid on the catalyst. Both a pre-reduced and an un-reduced 5 percent Pd/BPL catalyst were examined. The catalysts were first used without further reduction, and then in later experiments both catalysts were used after further reduction. After the pretreatment, either reduction in flowing hydrogen or no reduction with just cleansing using helium, the catalyst was exposed to an equimolar mixture of CO2/CH4. Each hour the temperature was increased by 100?C up to 400?C and spectra were taken every 15 minutes during the experiment.
In these experiments, no acetate nor acetic acid peaks were observed. The only peaks seen on the spectra were those of the CO2 and CH4. At the end of the reporting period it was discovered that the key to the acetate formation is the pretreatment of the catalyst. Oxidation of the catalyst, rather than reduction will yield acetate. Since the data has not been completely analyzed, more detail will be given in the next annual report.