Final 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 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 project was to develop a technology for the direct synthesis of acetic acid from carbon dioxide (CO2) and methane (CH4):
|CO2(g) + CH4(g) " CH3COOH(g)||(1)|
The emission of greenhouse gases, such as CO2, is of interest for environmental reasons. In response to this issue, a number of industrial nations have ratified the Kyoto Protocol treaty that calls for a voluntary worldwide reduction in the emissions of CO2 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 CO2. One such technology is the utilization of CO2 as a reactant in chemical synthesis.
Acetic acid is a vital industrial chemical; more than 6 million 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 CO2 and CH4 via a solid catalyst will contribute to the reduction of CO2 emissions and may provide an economical means of acetic acid production.
Thermodynamic calculations were performed on reaction (1). The RGIBBS reactor model in AspenPlus 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 CH4 increased with increasing temperature and increasing pressure. At 1,000 K and 150 atm, the fractional conversion of CH4 was calculated from the Peng-Robinson equation 1.6 x 10-6. The calculations show that the direct synthesis of acetic acid from CO2 and CH4 is thermodynamically limited at all conditions of practical interest.
Methods to Drive Equilibrium
Thermodynamic calculations on reaction (1) show that the direct synthesis of acetic acid from CO2 and CH4 is thermodynamically limited at all conditions of practical interest. Therefore, 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:
|CO2 + CH4 " CH3COOH||(1)|
|CH3COOH + C2H2 " CH3CO2CH3||(2)|
|CO2(g) + CH4(g) + C2H2(g) " CH3CO2CH3 (g)||(3)|
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 CH4 increases with increasing pressure and decreasing temperature. The maximum fractional conversion of CH4, 0.983, was achieved at 300 K and 150 atm. Even at lower pressures the fractional conversion was still high (e.g., at 300 K and 5 atm, the conversion was 0.971). The highest conversions are reached when the system has a liquid phase. Unfortunately, the conversion drops dramatically when the system is a vapor. At 350 K and 5 atm, the fractional conversion of methane was 0.0025.
These calculations suggest that the above system of reactions is thermodynamically favorable. Therefore, this is a feasible alternative to overcome the thermodynamic limitations of the direct synthesis reaction of acetic acid from CO2 and CH4.
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 results in the adsorbed acetate species and water:
|CO2 + CH4 + *-OH " CH3COO-* + H2O||(6)|
The water is removed from the system by the unreacted gases or a sweep gas. Once the catalyst is saturated with the acetate species, it is removed from the reactants and reaction conditions, leaving only the catalyst with the adsorbed species. The catalyst is then exposed to steam under conditions that are favorable to the removal of the acetate species as acetic acid. This regenerates the catalyst sites and yields the desired acetic acid product:
|CH3COO-* + H2O " *-OH + CH3COOH||(7)|
The catalyst would then be removed from the desorption conditions, and the cycle would begin again. This catalytic cycle is illustrated in Figure 1.
Figure 1. Diagram of Cyclic Regeneration of Catalyst
Because of the lack of thermodynamic data for solids in the AspenPlus database, no simulations could be run for this system. However, it still may be a feasible method to overcome the thermodynamics of reaction (1), which we will examine further.
Formation of Acetate from CO2 and CH4
DRIFTS Experiments. Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) studies show the formation of the acetate species from a mixture of CO2/CH4 on the catalyst. Experiments were conducted by first adsorbing acetic acid on the catalyst to identify the absorption bands corresponding to acetic acid. Then, a fresh sample of the catalyst was exposed to a CO2/CH4 mixture, resulting in the appearance of spectral bands matching those of pure acetic acid. Two catalysts were examined-5 percent Pd/Carbon and 5 percent Pt/Alumina. The palladium catalyst was prepared via precipitation by Calgon Carbon. The platinum catalyst was prepared via incipient wetness by Johnson-Matthey. The catalyst was pretreated at 500°C in flowing helium (He) prior to the experiments. The catalyst then was exposed to a flow of the experimental gas. The temperature was increased in 50°C increments up to 400°C. Each temperature was maintained for 30 minutes, with spectra being taken every 10 minutes.
When the He only pretreated catalyst sample was exposed to an equimolar mixture of CH4 and CO2, no acetate peaks were observed. CH4 peaks at 3,010 cm-1 and 1,300 cm-1 and CO2 peaks at 3,660 cm-1 and 2,350 cm-1 were the only peaks seen on the spectra. However, when the pretreatment included exposure to flowing CO2 at 500°C for 1 hour, the spectra did show acetate peaks. The results from this experiment are shown in Figure 2.
Figure 2. 50 Percent CO2/50 Percent CH4 Mixture Over 5 Percent Pd/Carbon Pretreated in CO2
In addition to the CH4 and CO2 peaks, four acetate peaks can be seen in the spectra. The peak at 1,790 cm-1 corresponds to the C = O stretch in a dimer of acetic acid, and the peak at 1,743 cm-1 corresponds to the C = O stretch in a monomer of acetic acid. The 1,420 cm-1 peak can be attributed to the O-H deformation in a dimer of acetic acid. Finally, the 1,600 cm-1 peak corresponds to the O-C = O stretch in a monodenate absorbed acetate.
When the 5 percent Pt/Alumina catalyst was used, the acetic acid adsorption showed slightly different peaks, because of a different interaction with the catalyst surface. When the 5 percent Pt/Alumina sample was exposed to the equimolar mixture of CH4 and CO2, distinct acetate peaks were observed. The results of this experiment can be seen in Figure 3. Unlike the 5 percent Pd/C catalyst, the pretreatment had no effect on the formation of the acetate. Additionally, the amount of acetate formed by the 5 percent Pt/Alumina catalyst was significantly more than that formed on the 5 percent Pd/C catalyst.
Figure 3. 50 Percent CO2/50 Percent CH4 Mixture Over 5 Percent Pt/Alumina Pretreated in He
TPR Experiments. To further confirm the formation of acetic acid in the gas phase, we used an Altamira AMI 1 temperature programmed reaction (TPR) system. In addition to a thermal conductivity detector, this system has an online mass spectrometer (MS). Because the DRIFTS experiments indicated that the 5 percent Pt/alumina catalyst performs better than the 5 percent Pd/carbon, we examined the 5 percent Pt/alumina catalyst in the TPR experiments.
For each experiment, the catalyst was pretreated in He at 500°C for 1 hour. The temperature then was reduced to 50°C, and the catalyst was soaked in CO2 for 1 hour. The catalyst then was exposed to the reaction gas, and the temperature was ramped up to 400°C and held there for 1 hour. The system then was allowed to cool to room temperature. MS data at 43 amu and 60 amu, and unique mass fragments of acetic acid, were taken throughout the entire experiment.
In the first experiment, after the pretreatment in He and CO2, the catalyst was exposed to pure CH4. Figure 4 shows the MS data for these peaks, starting at the time when CH4 was introduced into the system.
Figure 4. CH4 Experiment MS Data
As seen in Figure 4, both the 43 and 60 peaks begin to increase when the temperature reaches about 375°C. The peak signal increases to a maximum, and then sharply decreases, even though the temperature remains between 375°C and 400°C. We attribute this to the adsorbed CO2 reacting with CH4. As the CH4 flow continues, the adsorbed CO2 is gradually consumed. Because there is no additional CO2 in the system during the CH4 treatment, the used CO2 is not being replaced.
It also is important to note that both peaks are above the background noise, as indicated by the green line, which was taken at both 43 and 60 amu when no catalyst was present. From this, we conclude that the reaction required a catalyst.
In the second experiment, after the catalyst was pretreated in He and CO2, it was exposed to an equimolar mixture of CO2 and CH4. Figure 5 shows the data at 60 amu for three experiments: CH4, CO2 + CH4, and no catalyst.
Figure 5. Comparison of Mass 60 amu
From Figure 5, we can see that the 60 amu peak for the CO2 + CH4 experiment begins to increase when the temperature reaches about 375°C. Once the peak reaches a maximum, it remains there until the system starts to cool down from 400°C, at which point we see it decrease to the noise level.
The difference between CH4 and CO2 + CH4 peak widths can be explained by the CO2 in the system. In the first experiment, there was no additional CO2 to replace what was used in the reaction. In the second experiment, there was additional CO2 in the feed, which could replace the reacted CO2. We see that the CO2 continuously is being replaced during the CO2/CH4 experiment.
In a full mass spectrum taken at 400°C, we observed peaks that corresponded to CH4 and CO2. We also observed a small but distinct peak at 60 amu. No other peaks were observed. This indicates that only acetic acid is formed. We have no evidence of any other reaction products.
Our research has focused on forming acetic acid from CO2 and CH4 over a solid catalyst. Thermodynamic analysis of the direct synthesis of acetic acid shows that although increasing pressure and temperature improve the equilibrium conversion of CH4, the reaction is thermodynamically limited at all conditions of practical interest.
To drive the equilibrium, methods to remove the acetic acid were examined. Of these, a few have conversions that quantitatively are reasonable under accessible conditions. We chose two to investigate further.
The first was the synthesis of vinyl acetate from acetic acid and acetylene. Because more than 60 percent of acetic acid is used to produce vinyl acetate, this would be a reasonable and industrially useful approach. This reaction would be coupled with the direct synthesis reaction, thus driving the conversion of CH4 to acetic acid. At 300 K and 5 atm, the equilibrium conversion of CH4 is more than 99 percent.
The second approach is the cyclic regeneration of the catalyst. This method would involve the formation of acetate on the surface of the catalyst under reaction conditions, and then the removal of the acetate as acetic acid under separate desorption conditions.
The formation of acetate on the surface of the catalyst and the formation of gas phase acetic acid from CO2 and CH4 have been demonstrated in both DRIFTS and TPR experiments, respectively.
DRIFTS experiments were performed using both 5 percent Pd/Carbon and 5 percent Pt/Alumina catalysts. When exposed to an equimolar mixture of CO2 and CH4, both catalysts showed the formation of an acetate. With the 5 percent Pd/C catalyst, CO2 pretreatment of the catalyst was required. The acetate was not formed when the catalyst was pretreated in He only. The acetate was only formed when the catalyst was pretreated with CO2. The 5 percent Pt/Alumina catalyst showed the formation of the acetate regardless of the pretreatment. Additionally, the acetate peaks were larger on the 5 percent Pt/Alumina spectra, thus indicating that this is the better of the two catalysts for this reaction.
Using a TPR system with an online MS, we tested for the formation of gas phase acetic acid from the reaction of CO2 with CH4 using the 5 percent Pt/Alumina catalyst. For each experiment, the catalyst was pretreated in He, soaked in CO2, then exposed to either pure CH4 or an equimolar mixture of CO2 and CH4.
In both experiments, we observed that the MS peak corresponding to acetic acid (60 amu) began to be observable around 375°C. This peak was above the background noise, which was observed when no catalyst was used.
Additionally, when the full mass spectrum taken during the maximum was examined, we found only peaks corresponding to CH4, CO2, and acetic acid. We have no evidence of any byproducts.
Therefore, we conclude that gas phase acetic acid can be formed from CO2 and CH4. This reaction requires a catalyst, as no acetic acid was formed in the absence of the catalyst. We have definitively shown the formation of both adsorbed acetate and gas phase acetic acid from CO2 and CH4 using a solid catalyst.
Journal Articles on this Report : 3 Displayed | Download in RIS Format
|Other project views:||All 6 publications||3 publications in selected types||All 2 journal articles|
||Wilcox EM, Gogate MR, Spivey JJ, Roberts GW. Direct synthesis of acetic acid from methane and carbon dioxide. In: Natural Gas Convesion VI Series. E Iglesia, JJ Spivey, TH Fleisch, Eds. Elsevier, Amsterdam, The Netherlands. 2001;136:259-264.||
||Wilcox EM, Roberts GW, Spivey JJ. Thermodynamics of light alkane carboxylation. Applied Catalysis A - General 2002;226(1-2):317-318.||
||Wilcox EM, Roberts GW, Spivey JJ. Direct catalytic formation of acetic acid from CO2 and methane. Catalysis Today 2003;88(1-2):83-90.||