Article pubs.acs.org/EF
Nanoclay-Based Solid Sorbents for CO2 Capture Elliot A. Roth, Sushant Agarwal, and Rakesh K. Gupta* Department of Chemical Engineering, West Virginia University, Morgantown, West Virginia 26506, United States ABSTRACT: A solid sorbent for carbon dioxide capture was developed on the basis of montmorillonite nanoclay, which is a low-cost and easily available bulk material. This high specific surface area, platelet-like nanoclay with hydroxyl groups on edges was treated with aminopropyltrimethoxysilane and polyethylenimine to provide sites for CO2 capture. CO2 sorption tests showed fast kinetics and capture capacities as high as 7.5 wt % at atmospheric pressure and about 17 wt % at 2.07 MPa pressure in the temperature range of 75−85 °C. The regeneration of these nanoclays can be achieved using nitrogen at 100 °C or CO2 (dry or humid) at 155 °C as the sweep gases. Furthermore, pressure swing operation, employing vacuum at 85 °C, is also effective in regenerating the sorbent. This work shows that amine-modified montmorillonite nanoclay has the potential to provide a highperforming solid sorbent for CO2 capture.
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INTRODUCTION The majority of the energy production in the world relies on combustion of carbon-containing fuels, such as oil, coal, natural gas, and biomaterials, leading to the release of approximately 28 gigatons of CO2 per year into the atmosphere.1 Because these fuels will continue to dominate as the main source of energy, a solution must be found to reduce the emission of carbon dioxide (CO2), a greenhouse gas, into the atmosphere. Carbon capture and sequestration (CCS) is considered as one of the most viable options to reduce the emissions of CO2 into the atmosphere from anthropogenic sources. It involves capturing CO2 from large point sources, such as coal-fired power plants, and then storing it permanently to isolate it from the atmosphere. For each ton of coal burned, there is almost 3 tons of CO2 produced, and a 500 MW coal-fired power plant alone generates about 3 million tons of CO2 per year.2 Thus, the most logical step to reduce the CO2 emissions would be to capture the CO2 from post-combustion flue gases from these large point sources. To achieve this objective, technology must be developed that can be applied not only to building new power plants but also used to retrofit old power plants. This technology should be efficient and cost-effective. The U.S. Department of Energy envisages that any viable solution should result in 90% CO2 capture with only a 30% increase in the cost of electricity.3 Currently, there are three main technologies that are being widely studied that include the use of scrubbing solutions, solid sorbents, and membranes to separate and capture CO2 from the flue stream. Use of scrubbing solutions, such as monoethanolamine (MEA), is the only technology that has been industrially applied successfully for operations, such as natural gas or syngas scrubbing,2 but it has not found any commercial application in post-combustion CO2 capture operations. That is because the liquid amine solution sorbent processes have been found to be very expensive because of the very large volume of flue gas that needs to be scrubbed. The heat of adsorption of CO2 in aqueous amine solutions is very high; thus, a very large of amount of heat energy is required to regenerate the liquid sorbents imposing a very high parasitic power requirement on the power plant, which makes the cost of generating electricity prohibitively high. Second, there are © XXXX American Chemical Society
also problems associated with equipment corrosion and loss of amines by evaporation. Membrane separation is also not very effective because the post-combustion flue gas is at a very low pressure and has a low concentration of CO2. An extra step of compression will be required to bring the flue gas to the required pressure for membrane separation, and that will add to the cost of electricity. Solid sorbents are being developed to overcome some of these challenges and to find an efficient and low-cost solution for CO2 capture. Solid sorbents typically consist of a very high specific area solid base, and when a CO2-containing gas mixture is passed through a bed of these solid sorbents, CO2 capture occurs by physisorption or chemisorption. Later, the solid sorbent can be regenerated by thermal or pressure swing methods. Solid sorbents offer several advantages over conventional (liquid) amine solution technology: (1) the energy requirement for regeneration is low; (2) there are no corrosion problems;4 (3) loss of amines to the gas stream is greatly reduced; and (4) separation can occur at low CO2 partial pressures. Many different kinds of solid sorbents have been developed for CO2 capture from a mixture of gases.5,6 These include activated carbons, zeolites, molecular baskets, metal− organic frameworks, and porous polymers. Their reported CO2 capture capacities vary from ∼200 to ∼4000 μmol/g (from ∼0.9 to ∼18 wt %).7 One approach in developing solid sorbents is the use of a solid base functionalized with amines. A high surface area solid base is chemically treated with an amine compound. When CO2-containing gas is passed through a bed of this sorbent, the immobilized amine reacts with CO2 forming carbamates, resulting in CO2 capture. The sorbent can be regenerated later via pressure or temperature swing methods. Amine immobilization on the particles of the solid base can be achieved in various ways. If the solid base is porous, wet Special Issue: Accelerating Fossil Energy Technology Development through Integrated Computation and Experiment Received: December 11, 2012 Revised: March 14, 2013
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impregnation can be used to immobilize amine into the pores8 or the amine compounds can be covalently bound to the solid support via hydroxyl functionality.9 In yet another approach, polyamine compounds can be directly polymerized on the surface of the solid.10 Comprehensive summaries of various solid sorbents under investigation by various research groups can be found in some excellent reviews.6,7 To be considered a viable candidate for industrial applications, a solid sorbent must also be low cost, easy to regenerate, and easy to produce in large enough quantities to be used in operating power plants. While some of the solid sorbents proposed may have high capture capacities, their high cost and difficulties in scaling up production levels may preclude their application on an industrial scale. In this work, a solid sorbent has been developed on the basis of montmorillonite nanoclay on which amine molecules have been immobilized. The montmorillonite nanoclay is an easily available material, and it is widely used in the polymer nanocomposite and drilling fluids industry because of its ability to swell, its high surface area, and its high surface reactivity.11 It is a cationic smectite clay that consists of two tetrahedral silicate layers with a central alumina or magnesia octahedral layer.11−13 This 2:1 ratio of tetrahedral/octahedral layers forms a onelayered sheet of clay with dimensions of about 1 nm thick and 30 nm to several micrometers in lateral dimensions. It has a surface area in the range of 750 m2/g when the individual platelets are completely exfoliated.11,13 Quite often, the montmorillonite nanoclays are organically modified before being used in aforementioned applications. Thus, for them, industrial processes already exist to perform chemical modifications as necessary. In this work, the montmorillonite nanoclay was chemically modified using two kinds of amine compounds and then CO2 sorption and desorption behavior was studied under various conditions.
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be noted that all of the experiments were performed with PEI of Mn = 423, unless noted otherwise. For the samples treated with both APTMS and PEI, the nanoclay was first treated with APTMS using the methods described earlier. The treated clay sample was then crushed using a mortar and pestle. PEI was dissolved in methanol in an 8:1 ratio using the same method as the wet impregnation with untreated clay. The treated clay was then added in a 1:1 ratio based on the weight of the treated clay with PEI to achieve a 50% loading of PEI onto the treated clay. Thermogravimetric Analysis. A TA Instruments TA-Q500 thermogravimetric analyzer (TGA) was used to determine the amount of amine content that was grafted onto the samples. The samples were first heated at 20 °C/min to 900 °C in nitrogen at a flow rate of 40 mL/min. The derivative temperature weight loss was used to determine different weight loss steps. The weight loss that occurred around 100 °C was assumed to be due to evaporation of water bound to the nanoclay. The weight loss after this step resulting from thermal decomposition of organic materials was used to determine the amount of amine grafted onto the clay. Another method used to further investigate different weight loss steps was to heat the sample in nitrogen with a flow rate of 40 mL/min to 100 °C and keep the sample at 100 °C for 30 min to remove all of the water. The sample temperature was then ramped to 600 °C at 20 °C/min and kept there for 30 min to remove all of the grafted amine. The difference in weight at 100 °C and that at the end of 600 °C reflected the amount of amine grafting. CO2 Sorption and Regeneration Experiments. Thermo Cahn Thermax 500 and TA Instruments TA-Q500 TGAs were used to study the CO2 sorption and desorption behavior of the solid sorbents at atmospheric pressure. The experimental procedure was the same for all of the samples and included an initial drying step in nitrogen at 100 °C for 30 min to remove any moisture or pre-adsorbed CO2 from the sample and to obtain a stable baseline for the calculation of the CO2 sorption. If the sorption temperature was above 100 °C, such as 125 or 150 °C, the initial drying step was completed using the CO2 sorption temperature instead of 100 °C. For CO2 adsorption at temperatures lower than 100 °C, the sample was cooled in nitrogen. Once the TGA temperature reached the designated reaction temperature, the reaction gas was switched to CO2 and the weight gain of the sample was monitored until an equilibrium value was reached. Experiments were performed to see the effects of the sorption temperature and reaction gas composition, which was either 100% CO2 or CO2 diluted with 90% N2. TGAs were also used to regenerate the samples using the temperature swing method and to study the multiple sorption and regeneration cycles. Once a sample reached an equilibrium value during the sorption step, the reaction gas was switched to either nitrogen or CO2 and the temperature was set to the desired regeneration temperature. Once the weight loss of the sample because of regeneration reached an equilibrium value, the temperature was changed again to the sorption temperature and the reaction gas was switched to CO2. These steps were repeated multiple times, and, at each step, the CO2 capture capacity and desorption efficiency were determined. The sorption temperatures ranged from 50 to 150 °C, and the regeneration temperatures ranged from 100 to 155 °C. CO2 Sorption at High Pressures. A stainless-steel autoclave reactor vessel rated for 37.2 MPa (5400 psi) was used to study the CO2 sorption behavior at elevated CO2 pressures that may be relevant for pre-combustion CO2 capture. Known amounts of nanoclay samples were loaded in preweighed small glass cuvets, which were then placed in the reactor. The reactor was sealed and then pressurized with the desired pressure of CO2 by connecting it to a CO2 gas cylinder fitted with a pressure gauge. After the desired period of reaction time, the reactor was opened and the glass cuvets were weighed using a Mettler-Toledo sensitive balance. The weight difference between the initial and sorbed samples showed the CO2 capture by the nanoclay samples. Most of the high-pressure sorption experiments were performed from room temperature to 85 °C, for which the pressurized reactor was immersed in a water bath set at the desired temperature.
EXPERIMENTAL SECTION
Materials. The montmorillonite nanoclay, Cloisite Na+, was obtained from Southern Clay Products, Inc. Polyethylenimine (PEI), oligomer mixture (Mn = 423), and 97% 3-aminopropyltrimethoxysilane (APTMS) were procured from Sigma Aldrich and used as obtained. Various solvents, such as 99% N,N-dimethylformamide (DMF), methanol, 99% acetone, and toluene, obtained from Sigma Aldrich, and deionized water were used as reaction solutions. Reactant gases, such as nitrogen, CO2, and a mixture of 10% CO2/90% N2, were purchased from Airgas, Morgantown, WV. Amine Grafting. To carry out the APTMS grafting, the nanoclay was dispersed in deionized water and stirred for 24 h using a magnetic stirrer to swell it. Then, DMF was added to the clay/water suspension and stirred for another 24 h, sonicated for 30 s, filtered, and washed. The filtered clay was then added to fresh DMF and stirred for approximately 15 min to redisperse the clay. APTMS was then added in different concentrations. The sample was then stirred for 24 h, sonicated for 30 s, filtered, and washed with copious amounts of DMF. The samples were then dried overnight in an oven at 60 °C. After the sample was dry, it was crushed using a mortar and pestle to achieve a fine powder for testing. To immobilize PEI, a wet impregnation method was used. This method involved dissolving PEI into methanol at a ratio of 8:1 methanol/PEI. The desired amount of nanoclay was added to achieve a specified percent loading of PEI onto the clay, followed by stirring for 24 h. Samples were loaded with 33, 50, or 66% PEI. An example of 50% loaded PEI onto the clay support would be adding 1 g of clay to 1 g of PEI dissolved in 8 g of methanol. The solvent was then evaporated in a vacuum oven at 60 °C overnight. These samples were easily broken into a powder without the use of a mortar and pestle. It should B
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Vacuum Regeneration. To study the vacuum regeneration after the CO2 capture, in either the TGA or the pressure vessel as described before, the preweighed nanoclay samples were placed in a vacuum oven at 85 °C and vacuum pressure of 93 kPa. The samples were then removed after a designated period of time, and the weight difference was measured. The samples were then placed back into the TGA or pressure vessel to measure the amount of CO2 adsorbed after regeneration. The amount of CO2 initially adsorbed was then compared to the amount adsorbed after the vacuum regeneration step. Fourier Transform Infrared (FTIR) Spectroscopy Characterization. FTIR spectroscopy was used to determine if any amine was attached to the clay support. A Thermo Scientific Nicolet iS10 FTIR with the attenuated total reflectance (ATR) attachment and a diamond crystal was used using a resolution of 1 cm−1 and 40 scans per sample.
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RESULTS AND DISCUSSION Amine Grafting. Weight loss experiments using a TGA showed that the untreated Cloisite nanoclay exhibited 5.7 wt % reduction in weight when heated to 900 °C in nitrogen. This loss in weight was taken into account when determining the amount of amine grafting on the amine-treated nanoclay samples. Figure 1 shows the amount of amine compound immobilized on the nanoclay for the samples treated with APTMS, PEI, and
Figure 2. FTIR spectra comparing the differences in amine treatments: (a) untreated clay, (b) clay treated with PEI, (c) clay treated with APTMS, and (d) clay treated with APTMS + PEI.
asymmetric and symmetric vibrations of the methylene groups on APTMS.13,16,17 The broad peak at 3300 cm−1 is observed because of N−H stretching from the amine in APTMS.18 The broad peak at 3275 cm−1 is attributed to symmetric NH2 stretching.18 It can be seen that the samples treated with PEI have a much more intense peak around 3275 cm−1, and this indicates that there are more NH bonds because of the presence of more amine groups. The 2940 and 2812 cm−1 peaks are also more intense with the PEI-treated samples, indicating that there is more CH stretching because of the increase in loading of PEI. The peak at 1590 cm−1 for samples treated with only PEI is contributed by a NH scissoring vibration similar to the peak at 1560 cm−1 for the APTMS sample.19,20 The sample treated with both APTMS and PEI had a more intense peak around 1560 cm−1, and this is attributed to a NH scissoring vibration overlap of APTMS and PEI.20 The peaks in the 1400 cm−1 region are attributed to CH3 and CH2 deformation.16 Thus, FTIR analysis confirms the fact that both APTMS and PEI are present on the nanoclay. CO2 Sorption in a TGA. To determine the CO2 capture capacity of nanoclay-based sorbents, experiments were performed in a TGA using pure CO2 as the reaction gas. Figure 3 shows CO2 capture capacity of various nanoclay samples as a function of amine treatment and sorption temperature. All data are an average of at least three repeat experiments, and the error bars show a spread of ±1 standard
Figure 1. Weight percent amine grafting on nanoclay treated with APTMS, PEI, and APTMS + PEI as determined from a TGA.
a combination of the two. It can be seen that, in case of APTMS alone, a maximum of 15.7 wt % amine grafting is achieved. For PEI, about 37.4 wt % amine immobilization is achieved when the nanoclay and PEI were added in 1:1 ratio while preparing the samples. When both amine treatments are employed, only a marginal increase to 40.5 wt % in the amine immobilization is observed. This could be due to the fact that APTMS, which is already present on the nanoclay, interferes with the PEI immobilization on the nanoclay surface. Second, APTMS-treated nanoclay, because of its relatively hydrophobic nature, does not disperse in methanol as well as untreated nanoclay. FTIR Analysis. Figure 2 shows FTIR spectra of untreated nanoclay and nanoclay treated with APTMS, PEI, or a combination of both. The large peak at 985 cm−1 indicates a Si−O in-plane stretching, which is similar to results in the literature.14 The broad peak around 3440 cm−1 that is not seen with much intensity in our spectrum is associated with O−H stretching from water that is not bound but adsorbed on the surface.14,15 The peak at 1659 cm−1 is also associated with O− H stretching in water.14 The peak around 3630 cm−1 is assigned to an asymmetric H2O stretching for bound water in the clay.15 The peaks around 2870 and 2930 cm−1 represent C−H
Figure 3. Effect of the sorption temperature and amine treatment on the nanoclay CO2 capture capacity at atmospheric pressure. C
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deviation. As seen from the figure, the untreated nanoclay shows very little CO2 capture, while amine-treated nanoclays show considerably higher CO2 capture capacities, demonstrating the effectiveness of the amine treatment. The CO2 sorption capacity increases as the temperature is increased from 50 to 85 °C, and it reaches as high as 7.5% at 85 °C for the nanoclay treated with both APTMS and PEI, although the nanoclays treated with either APTMS or PEI show about 6% CO2 capture capacity. Thus, Figure 3 shows that treating the nanoclay with both APTMS and PEI is beneficial and results in increased capture capacity. Additionally, in Figure 3, it can be seen that, as the temperature is increased above 100 °C, the adsorption of carbon dioxide is reduced for the samples treated with PEI, whereas for the clay treated with APTMS, it decreased when the temperature was above 85 °C. Because the highest capture capacities were observed at 85 °C, most of the other experiments were also performed at this temperature. In addition to high capture capacity, the kinetics of the CO2 sorption are also important in evaluating a solid sorbent. Figure 4 shows CO2 sorption profiles in a TGA for various amine-
than 50 min for the PEI-containing sample. This suggests that the CO2 sorption that occurs because of the reaction between amine groups and CO2 is diffusion-controlled for PEIcontaining samples. This can be expected because APTMS is covalently bound with −OH groups present on the edges of the nanoclay platelets, and thus, the amine groups are easily accessible to CO2 molecules, whereas PEI physically coats the nanoclay surface and that coating could be considerably thick, providing a mass transfer barrier to CO2 molecules once the amine groups present at the topmost surface are exhausted. This is suggested by the fact that, for 50% equilibrium capacity, the time of sorption for APTMS and PEI is not that significantly different but becomes quite large for near equilibrium values. Because post-combustion flue gas contains only about 10− 15% CO2, the sorption tests were also performed using a dilute CO2 stream, which contained only 10% CO2 with the balance being 90% nitrogen. Figure 5 shows a comparison of CO2
Figure 4. CO2 sorption profiles for nanoclays treated with various amines as a function of time to show the kinetics of the sorption process at 85 °C in a TGA at atmospheric pressure. Figure 5. CO2 adsorption capacity in pure CO2 and 10% CO2/90% nitrogen, at 85 °C and atmospheric pressure for various amine-treated nanoclays.
treated nanoclays. It can be qualitatively observed that the nanoclay treated with APTMS alone shows fast adsorption of CO2 and reaches near equilibrium very quickly. The nanoclay treated with PEI and the combination of PEI and APTMS demonstrates an initial quick adsorption, followed by a slow adsorption profile. To compare the kinetics of the CO2 sorption process, the samples were reacted at 85 °C with CO2 and it was assumed that the equilibrium was achieved in 90 min, which represented 100% CO2 sorption, and then the times for 50, 75, 90, and 95% of equilibrium CO2 adsorption were determined. At least four runs were used to measure the adsorption times for each case, and their averages are given in Table 1. It can be seen that CO2 sorption is fastest for APTMStreated samples but slower for samples treated with PEI. It takes only about 18.7 min to reach 95% of the equilibrium sorption capacity for the APTMS sample, but it takes more
capture capacities for pure CO2 gas steam with 10% CO2 gas stream showing similar values within the experimental error. This suggests that these solid sorbents are equally effective in capturing CO2 even in a dilute CO2 stream, which is more representative of an actual flue gas stream. Regeneration in N2. Any successful solid sorbent should be easily regenerable so that it could be used over multiple cycles of sorption and desorption without any significant decline in its capture capacity. The regeneration of these nanoclay samples was accomplished using nitrogen as the sweep gas at 100 °C. Figure 6 shows the capture capacities for 10 sorption− desorption cycles. It can be seen that, for APTMS-treated samples, almost no decline in capture capacity is observed, whereas PEI + APTMS samples show some decrease but it is less than for samples treated with PEI alone. This suggests that not only these solid sorbents are easily recyclable but also the presence of APTMS is helpful in delaying any loss in capture capacity because of multiple uses. Regeneration in CO2. Although very frequently used in the literature, using N2 to regenerate the solid sorbent is not a practical option because one again ends up with a mixture of CO2 and N2, the same as the initial flue gas achieving no
Table 1. Time Taken To Achieve Percentage of Equilibrium CO2 Capture Capacity percent adsorption
50%
75%
90%
95%
nanoclay + APTMS (min) nanoclay + PEI (min) nanoclay + APTMS + PEI (min)
0.7 1.1 2.0
2.2 8.2 11.7
8.1 32.2 33.8
18.7 53.8 58.8 D
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Figure 6. CO2 capture capacities over 10 cycles of sorption and desorption. Sorption is at 85 °C in CO2, whereas desorption is performed at 100 °C using N2 as sweep gas.
Figure 7. Percent remaining capture capacity when using pure CO2 at 155 °C for 30 min for regeneration. The sorption was carried out at 85 °C in a TGA.
through a flask of water, which saturated CO2 at room temperature before it was introduced in the TGA for regeneration at 155 °C for 30 min. Figure 8 shows the percent remaining capture capacity for the nanoclay samples for 2 cycles of regeneration. It can be seen that, for the nanoclay treated with APTMS, the remaining capacity decreased only to
separation. Thus, alternate ways of regenerating the solid sorbents must be developed. One such possibility is the use of a temperature swing method while using CO2 as the sweep gas. Experiments were performed for 3 cycles of regeneration, where the sorption took place at 85 °C and desorption took place at 155 °C, both under the flow of pure CO2. Figure 7 shows percent regeneration capacities for these nanoclay solid sorbents. It can be seen that the capture capacity drops to about 80% of the initial capacity for all three amine treatments after only 3 cycles of regeneration. This decline is faster than the one observed in regeneration using nitrogen. This can be explained by the fact that dry CO2 can react with amines at high temperature to produce urea in an irreversible way, which does not easily decompose to release CO2, causing incomplete regeneration of the solid sorbent.21 At low sorption temperatures, only carbamates are formed, which decompose more easily, leading to almost complete regeneration of the sorbents. It is known that, in the presence of water, there is reduced formation of urea.21 Thus, CO2 saturated with water vapor should be more effective than CO2 alone for the regeneration step. To explore this idea, regeneration experiments were performed, where CO2 was saturated with water vapor at room temperature. To accomplish this, CO2 was first bubbled
Figure 8. Percent remaining capture capacity when using CO2 saturated with water for regeneration at 155 °C for 30 min. The sorption was carried out 85 °C in a TGA. E
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The highest pressure used corresponded to a CO2 pressure generally observed in integrated gasifier combined cycle (IGCC) power plants. Figure 10 presents the results for the nanoclay treated with only APTMS, where it can be seen that the adsorption capacity
95% of the original, whereas for nanoclay with PEI, it dcreased to 95 and 92% for the first and second cycles, respectively. The regeneration of samples treated with both APTMS + PEI went down to 92% over 2 cycles. Thus, humid CO2 causes less deterioration in capture capacity because of regeneration, and therefore, it could be better than using pure CO2. At the end of the capture stage, water can be easily condensed out, generating pure CO2 stream for sequestration. Vacuum Regeneration. When using any inert gas, such as nitrogen, the difference in partial pressure of CO2 works as the driving force for the regeneration. The same driving force can be realized if vacuum is used to regenerate the solid sorbents. To study this, at first, the CO2 sorption step was carried out in a TGA as described earlier, and then the spent samples were placed in a vacuum oven at 85 °C. Vacuum was set at 93 kPa. Samples were removed after 1 and 17 h and weighed to determine the weight loss as a result of regeneration, and then these samples were again placed in a TGA for sorption cycle to determine their remaining capture capacity. Figure 9 shows
Figure 10. Effect of the CO2 pressure and sorption time on CO2 capture capacity of the nanoclay treated only with APTMS. Sorption tests were conducted at room temperature (25 °C).
is not significantly affected by the pressure or the adsorption time. This would indicate that a 2 h adsorption time is sufficient to achieve the maximum CO2 adsorption on the sample treated with APTMS. Additionally, the CO2 adsorption capacity, at room temperature, for all of the different pressures and time averages to 6.93 wt %, which is only slightly higher than the average adsorption capacity of the same sorbent in the TGA at 85 °C. This would indicate that the sorption of CO2 on clay treated with APTMS is not due to physical sorption but chemical sorption. CO2 sorption behavior for nanoclay samples treated with PEI alone and ATMS + PEI is shown in Figures 11 and 12,
Figure 9. Remaining capture capacity (%) after 1 cycle of regeneration in vacuum (93 kPa) and 85 °C temperature for nanoclays treated with PEI, APTMS, and APTMS + PEI.
percent remaining capture capacities after 1 cycle of vacuum regeneration. The nanoclay treated with APTMS showed a reduction in CO2 capacity to 87.1% of the original, but after 17 h, the CO2 capacity was again almost 100%. This would indicate that the regeneration time of 1 h might not be enough time for the sample to fully regenerate, but the sample is not degraded by a desorption time of 17 h in the vacuum oven. Samples treated with PEI and the combination of PEI and APTMS had a cycle adsorption capacity near 100% after the first vacuum desorption for 1 h but had an average decrease in the CO2 adsorption capacity after the 17 h vacuum step. This indicates that there might be some loss of the amines or degradation to the sample during long vacuum regeneration at 85 °C. More investigation will be needed to optimize the vacuum regeneration performance, but it shows that, for these solid sorbents, vacuum regeneration is also a potential option. Effect of the Pressure on CO2 Sorption. Sorption experiments performed using a TGA and presented in previous sections showed CO2 sorption behavior at atmospheric pressure. CO 2 adsorption at high pressures was also investigated to see if an increase in CO2 pressure had any effect on the adsorption capacity of the adsorbent. If there is enhanced adsorption capacity and if it is pressure-dependent, that may suggest other possible applications for the nanoclay sorbents. The procedure for the high-presssure tests was described in the Experimental Section. The pressure that was used varied from 0.275 MPa (40 psi) to 2.07 MPa (300 psi).
Figure 11. Effect of the CO2 pressure and sorption time on CO2 capture capacity of the nanoclay treated only with PEI. Sorption tests were conducted at room temperature (25 °C).
respectively. The results shown in Figure 12 show that the highest average adsorption capacity is 11.36% for the nanoclay treated with APTMS + PEI at 2.07 MPa (300 psi), which is much higher than the adsorption capacity of 7.2% for the same nanoclay at 85 °C and atmospheric pressure. Here again, there is no significant effect of CO2 pressure on the capture capacity. Also, the capture capacity does not seem to be dependent upon the sorption time. This is unexpected, because the experiments in the TGA showed that, for PEI-containing samples, the F
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Figure 12. Effect of the CO2 pressure and sorption time on CO2 capture capacity of the nanoclay treated with APTMS + PEI. Sorption tests were conducted at room temperature (25 °C).
diffusion of CO2 through PEI coating may be influencing the kinetics of the CO2 sorption process. This could be due to the fact that, because of the high pressure of CO2, there is larger driving force as a result of the high concentration at the surface overcoming the diffusive resistance. Sorption experiments at atmospheric pressure using the TGA showed that the sorption capacity increased with an increasing temperature up to 85 °C. Therefore, high-pressure sorption tests were also performed as a function of the temperature. Figure 13 shows CO2 capture capacities for the nanoclays with
Figure 14. CO2 capture capacities after 1 cycle of vacuum regeneration at 85 °C and 93 kPa vacuum for 2 h. High-pressure CO2 sorption occurred at 2.07 MPa and (a) room temperature (25 °C) and (b) 50 °C.
for 2 h, and in Figure 14b, the sorption was carried out at 50 °C. In both cases, the vacuum desorption was carried out at 85 °C for 2 h at vacuum of 94 kPa. Although only 1 cycle of desorption was studied, it can be seen that PEI-containing samples show very little degradation, whereas APTMS-treated samples show more significant degradation in their sorption capacity. Thus, it can be concluded that the regeneration of these samples is possible even when the CO2 capture occurs under high-pressure conditions. More testing will be required to see the effect of multiple recycling steps.
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CONCLUSION In this work, an amine-containing solid sorbent for CO2 capture was developed on the basis of montmorillonite nanoclay, which is a low-cost easily available high specific surface area material. APTMS and PEI were used to impart amine functionality to the nanoclay. CO2 sorption experiments show fast kinetics and high CO2 capture capacities with easy regeneration. Experiments in pure CO2 and nitrogen-diluted 10% CO2 gas streams showed that the nanoclay treated with only one of the amine compounds exhibits about 6 wt % capture capacity. However, the nanoclay treated with both amines shows about 7.5 wt % CO2 capture capacity at 85 °C and atmospheric pressure. This solid sorbent can be regenerated at 100 °C using nitrogen over multiple cycles of sorption and desorption. More interestingly, the regeneration can also be accomplished using either dry or humid CO2 at 155 °C. Furthermore, regeneration is also possible using vacuum desorption. The latter two regeneration methods may be more industrially relevant. CO2 sorption experiments at high pressure of 2.07 MPa showed capture capacities as high as 17 wt % at 85 °C, and here also, regeneration could be achieved by vacuum desorption. Thus, this study shows that a high capacity solid sorbent can be developed on the basis of a low-cost nanoclay, which works over a wide range of sorption conditions and is easily regenerable under very practical conditions. This initial study employed simple gas compositions consisting of CO2 and/or CO2 in nitrogen. However, coal-derived flue gas is a
Figure 13. Effect of the temperature on CO2 capture capacity of the amine-treated nanoclays at 2.07 MPa (300 psi) CO2 pressure for 24 h of sorption time.
various amine treatments as a function of the temperature, where CO2 sorption was carried out at 2.07 MPa (300 psi) for 24 h. One can see that sorption capacities increase with an increasing temperature, but there is only a slight increase from 75 to 85 °C. The samples containing PEI show the highest capture capacities of 17.2 and 18.7 wt % at 75 and 85 °C, respectively, which are much higher than that observed at atmopsheric pressure, but such is not the case with nanoclays with only APTMS. This suggests that most of the grafted APTMS reacts easily and completely with CO2 even at low temperatures, but more PEI becomes available for CO2 capture as the temperature increases. CO2 reacts with surface PEI and then encounters diffusion, which limits the amount of CO2 that can diffuse into PEI. Increasing the temperature helps expand PEI and increase the diffusion, which increases the CO2 adsorption capacity. These sorbents can also be regenerated using vacuum regeneration. Figure 14 shows the adsorption capacities of fresh solid sorbent and after 1 cycle of vacuum regeneration. In Figure 14a, the sorption was carried out at room temperature G
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complex mixture also containing many acid gases, moisture, fly ash particles, and carbon monoxide. A typical untreated flue gas derived from the combustion of a U.S. low-sulfur Eastern bituminous coal can contain 5−7% H2O, 3−4% O2, 15−16% CO2, 1 ppb total Hg, 20 ppm CO, 10 ppm hydrocarbons, 100 ppm HCl, 800 ppm SO2, 10 ppm SO3, 500 ppm NOx, and balance N2.22,23 There can be potential impacts of the acid gases, such as SO2, SO3, HCl, NO, and NO2, upon the sorbent process, and this could be a fruitful area for future study and research. Future research will also focus on development of a detailed reaction mechanism describing the sorbent interactions with carbon dioxide. The possible loss of sorbent capacity and potential changes in the sorbent surface area and morphology after numerous regeneration cycles will also be examined in future work. A patent [U.S. Non-Provisional (utility) Patent Application, U.S. Serial Number 13/666, 37] has been filed on the basis of this work.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Funding for this work was provided, in part, by the Center for Advanced Separation Technologies at Virginia Polytechnic Institute and State University. Elliot Roth thanks the Bayer Foundation for a fellowship for the duration of his graduate studies. The authors also acknowledge useful discussions with and help provided by Dr. Evan Granite of the National Energy Technology Laboratory, Pittsburgh, PA and Dr. Karl Haider of Bayer MaterialScience, Pittsburgh, PA.
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dx.doi.org/10.1021/ef302017m | Energy Fuels XXXX, XXX, XXX−XXX
