Novel Polyethylenimine-Modified Mesoporous Molecular Sieve of

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Energy & Fuels 2002, 16, 1463-1469

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Novel Polyethylenimine-Modified Mesoporous Molecular Sieve of MCM-41 Type as High-Capacity Adsorbent for CO2 Capture Xiaochun Xu, Chunshan Song,* John M. Andresen, Bruce G. Miller, and Alan W. Scaroni Clean Fuels and Catalysis Program, The Energy Institute, and Department of Energy & Geo-Environmental Engineering, The Pennsylvania State University, 209 Academic Projects Building, University Park, Pennsylvania 16802 Received March 7, 2002

The objective of the work presented here is to develop a nanoporous solid adsorbent which can serve as a “molecular basket” for CO2 in the condensed form. Polyethylenimine (PEI)-modified mesoporous molecular sieve of MCM-41 type (MCM-41-PEI) has been prepared and tested as a CO2 adsorbent. The physical properties of the adsorbents were characterized by X-ray powder diffraction (XRD), N2 adsorption/desorption, and thermogravimetric analysis (TGA). The characterizations indicated that the structure of the MCM-41 was preserved after loading the PEI, and the PEI was uniformly dispersed into the channels of the molecular sieve. The CO2 adsorption/desorption performance was tested in a flow system using a microbalance to track the weight change. The mesoporous molecular sieve had a synergetic effect on the adsorption of CO2 by PEI. A CO2 adsorption capacity as high as 215 mg-CO2/g-PEI was obtained with MCM41-PEI-50 at 75 °C, which is 24 times higher than that of the MCM-41 and is even 2 times that of the pure PEI. With an increase in the CO2 concentration in the CO2/N2 gas mixture, the CO2 adsorption capacity increased. The cyclic adsorption/desorption operation indicated that the performance of the adsorbent was stable.

Introduction Fossil fuels will likely remain the mainstay of energy supply well into the 21st century. Availability of these fuels to provide clean, affordable energy is essential for the prosperity and the security of the world. However, increased CO2 concentration in the atmosphere due to emissions of CO2 from fossil fuel combustion has caused concerns about global warming.1-4 Improving the efficiency of energy utilization and increasing the use of low-carbon energy sources are considered to be potential ways to reduce CO2 emissions.4 Recently, CO2 capture and sequestration are receiving significant attention and being recognized as a third option.4 Also, enriched CO2 streams can be an important starting material for * Corresponding author. Tel: 814-863-4466. Fax: 814-865-3248. E-mail: [email protected]. (1) Azar, C.; Rodhe, H. Science 1997, 276, 1818-1819. (2) (a) Herzog, H.; Drake, E.; Adams, E. CO2 Capture, Reuse and Storage Technologies for Mitigating Global Climate Change. Report No. DOE/DE-AF22-96PC01257, U.S. Department of Energy: Pittsburgh, PA, 1999. (b) Ruether, J. A. FETC Programs for Reducing Greenhouse Gas Emissions. Report No. DOE/EFTC-98/1058, U.S. Department of Energy: Pittsburgh, PA, 1999. (3) (a) CO2 Conversion and Utilization; Song, C., Gaffney, A. M., Fujimoto, K., Eds.; ACS Symposium Series, American Chemical Society (ACS): Washington, DC, 2002; Vol. 809, 448 pp; (b) Environmental Challenges and Greenhouse Gas Control for Fossil Fuel Utilization in the 21st Century; Maroto-Valer, M. M., Song, C., Soong, Y., Eds.; Kluwer Academic/Plenum Publishers: New York, 2002; 447 pp. (4) (a) DOE/OS-FE. Carbon sequestration. State of the Science; Office of Science and Office of Fossil Energy; U.S. Department of Energy: Washington, DC, 1999. (b) U.S. Department of Energy. Carbon Sequestration-Research and Development; 1999. http://www.fe.doe.gov/ coal_power/sequestration/reports/ rd/index.html.

synthetic clean fuels and chemicals.3,5,6 For carbon sequestration, the costs of capture and separation are estimated to make up about three-fourths of the total costs of ocean or geologic sequestration.4 It is therefore important to explore new approaches for CO2 separation.4,5 Adsorption is one of the promising methods that could be applicable for separating CO2 from gas mixtures, and numerous studies have been conducted on separation of CO2 by adsorption in the last two decades. Various adsorbents, such as activated carbons, pillared clays, metal oxides, and zeolites have been investigated.7-19 (5) Song, C. Am. Chem. Soc. Symp. Ser. 2002, 809, 2-30. (6) Song, C. Chem. Innovation 2001, 31, 21-26. (7) Ma, Y. H.; Mancel, C. AIChE J. 1972, 18, 1148-1153. (8) Ma, Y. H.; Roux, A. J. AIChE J. 1973, 19, 105-1059. (9) Valenzuela, D.; Myers, A. L. In Adsorption equilibrium data handbook; Prentice-Hall: Englewood, Cliffs, NJ, 1989; pp 39-59. (10) Wilson, R. J.; Danner, R. P. J. Chem. Eng. Data 1983, 28, 1418. (11) Hayhurst, D. T. Chem. Eng. Commun. 1980, 4, 729-735. (12) Han, C.; Harrison, D. P. Chem. Eng. Sci. 1994, 49, 5875-5883. (13) Kapoor, A.; Yang, R. T. Chem. Eng. Sci. 1989, 44 (8), 17231733. (14) Yong, Z.; Mata, V. G.; Rodrigues, A. E. Adsorption 2001, 7, 4150. (15) Siriwardane, R. V.; Shen, M. S.; Fisher, E. P.; Poston, J. A. Energy Fuels 2001, 15, 279-284. (16) Ding, Y.; Alpay, E. Chem. Eng. Sci. 2000, 55, 3461-3474. (17) Pereira, P. R.; Pires, J.; Carvalho, M. B. Langmuir 1998, 14, 4584-4588. (18) Yong, Z.; Mata, V.; Rodrigues, A. E. J. Chem. Eng. Data 2000, 45, 1093-1095. (19) Anand, M.; Hufton, J.; Mayorga, S.; Nataraja, S.; Sircar, S.; Gaffney, T. APCI report for DOE, 1995.

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At lower temperatures (e.g., room temperature), the zeolite-based adsorbents have generally been found to show higher adsorption capacity. Siriwardane et al.15 reported that the CO2 adsorption capacity of zeolite 13X, zeolite 4A, and activated carbon was about 160, 135, and 110 mg/g-adsorbent, respectively, at 25 °C and 1 atm CO2 partial pressure. However, their adsorption capacities rapidly decline with increasing temperature. Moreover, since all the gases are physically adsorbed into/onto these adsorbents, the separation factors (such as CO2/N2 ratio) are low. To operate at relatively high temperature and reach a high separation factor, chemical adsorption was adopted. Ding et al.16 investigated the adsorption performance of hydrotalcite, and this material showed a CO2 adsorption capacity of 22 mg/ g-adsorbent at 400 °C and 0.2 atm CO2 partial pressure. Anand et al.19 reported that MgO showed an adsorption capacity of 8.8 mg/g-adsorbent at 400 °C. Both types of adsorbents need high-temperature operation and have a low adsorption capacity, thus they are not suitable for practical use for CO2 separation. For practical applications, selective adsorbents with high capacity are desired. Many of the separations should preferably be operated at relatively higher temperature, i.e., higher than room temperature and up to ∼150 °C which is a typical value of power plant stack temperature. Developing an adsorbent with high CO2 selectivity and high CO2 adsorption capacity, which can also be operated at relatively high temperature, is desired for more efficient CO2 separation by an adsorption method. A new concept called “molecular basket” is being explored by us for developing a high-capacity, highly selective CO2 adsorbent. The objective of the work presented here is to explore a novel type of solid adsorbent, which can serve as a “molecular basket” for “packing” CO2 in condensed form in nanoporous channels. To capture a large amount of CO2 gas, the adsorbent needs to have large-pore channels filled with a CO2-capturing substance as the “basket”. To cause the “basket” to be a CO2 “molecular basket”, a substance with numerous CO2-affinity sites should be loaded into the pores of the support to increase the affinity between the adsorbent and CO2 and, therefore, to increase the CO2 adsorption selectivity and CO2 adsorption capacity. Mesoporous molecular sieve MCM-41, which has a large pore volume, was selected as the “basket”. The sterically branched polymer polyethylenimine (PEI), which has branched chains with numerous CO2-capturing amino groups, was immobilized into the channels of the mesoporous molecular sieve. Branched amines have lower heat of adsorption than that of the primary amines, and thus desorption can be easier and requires less energy. With the use of PEI, enhanced adsorption/ desorption of CO2 is expected.20 Satyapal et al.20 used PEI as a CO2 adsorbent by coating the PEI on a polymer surface. The composite material can effectively adsorb CO2 from the gas mixture and were successfully used in the space shuttle. The CO2 adsorption capacity was ∼40 mg/g-adsorbent at 50 °C and 0.02 atm CO2 partial pressure. By loading the PEI into the mesoporous molecular sieve MCM-41, it is shown in this paper that (20) Satyapal, S.; Filburn, T.; Trela, J.; Strange, J. Energy Fuels 2001, 15, 250-255.

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adsorption capacities as high as 133 mg-CO2/g-adsorbent can be obtained at 75 °C. This indicates that the novel “molecular basket” material yields a higher CO2 adsorption capacity than that of the PEI-polymer composite material and is functional at higher temperatures as described here. Experimental Section 1. Preparation of the Adsorbents. Mesoporous molecular sieve MCM-41 was hydrothermally synthesized from a mixture with the following composition: 50SiO2:4.32Na2O:2.19(TMA)2O: 15.62(CTMA)Br:3165H2O.22 The synthesis procedure was established in our laboratory,22,23 and is based on the method invented by researchers at Mobil.24,25 Cab-O-Sil fumed silica (Cabot Corporation), tetramethylammonium silicate solution (0.5 TMA/SiO2, 10 wt % silica, Sachem Inc.), sodium silicate (containing 14 wt % NaOH and 27 wt % silica, Aldrich), cetyltrimethylammonium bromide (Aldrich), and deionized water were used as raw materials. The synthesis was carried out at 100 °C for 40 h. After the synthesis, the solid product was recovered by filtration, washed several times with deionined water, dried at 100 °C overnight, and calcined at 550 °C for 5 h to remove the template. The PEI-modified MCM-41 was prepared by a wet impregnation method. In a typical preparation, the desired amount of PEI was dissolved in 8 g of methanol under stirring for about 15 min, after which 2 g of calcined MCM-41 was added to the solution. The resultant slurry was continuously stirred for about 30 min, and then dried at 70 °C for 16 h under 700 mmHg vacuum. The as-prepared adsorbent was denoted as MCM-41-PEI-X, where X represents the loading of PEI as weight percentage in the sample. 2. Characterization of the Adsorbents. The mesoporous molecular sieve MCM-41 before and after modification was characterized by X-ray diffraction (XRD) and N2 adsorption/ desorption. The X-ray diffraction patterns were obtained on a Rigaku Geigerflex using Cu KR radiation. The N2 adsorption/ desorption was carried out on a Quantachrome Autosorb 1 automated adsorption apparatus, from which the pore volume, the BET surface area, and the average pore size were obtained. The sample was outgassed at 75 °C for 48 h using a high vacuum line prior to adsorption. The pore volume of the mesoporous molecular sieve was calculated from the adsorbed nitrogen after complete pore condensation (P/P0 ) 0.995) using the ratio of the densities of liquid and gaseous nitrogen. The pore size was calculated by using the BJH method. The thermal chemical and physical properties of the MCM-41, PEI, and MCM-41-PEI-50 were characterized by thermal gravimetric analysis (TGA). The TGA was performed on a PE-TGA 7 analyzer. About 10 mg of the sample was heated at 10 °C/ min to 600 °C in air (100 mL/min). 3. Adsorption Measurement. The adsorption and desorption performance of the adsorbent was measured using a PETGA 7 analyzer. The weight change of the adsorbent was followed to determine the adsorption and the desorption performance of the materials. In a typical adsorption/desorption process, about 10 mg of the adsorbent was placed in a small sample cell, heated to 100 °C in N2 atmosphere at a flow of 100 mL/min, and held at that temperature (about 30 min) (21) Kohl, A.; Nielsen, R. Gas purification, 5th ed; Gulf Publishing Co.: Houston, TX, 1997. (22) Reddy, K. M.; Song, C. Catal. Lett. 1996, 36, 103-109. (23) Reddy, K. M.; Song, C. Stud. Surf. Sci. Catal. 1998, 117, 291299. (24) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710-712. (25) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T. W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114 (27), 10834-10843.

MCM-41-PEI

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Figure 1. Comparison of the XRD patterns of MCM-41 and MCM-41-PEI-50. until no weight loss was observed. The temperature was then adjusted to the design temperature and 99.8% bone-dry CO2 adsorbate was introduced at a flow rate of 100 mL/min. In some experiments, CO2/N2 gas mixtures with different CO2 concentrations were also used. After adsorption, the gas was switched to 99.995% pure N2 at a flow rate of 100 mL/min to perform the desorption at the same temperature. The time for both the adsorption and the desorption was 150 min. Adsorption capacity in mg-adsorbate/g-adsorbent and desorption capacity in percentage were used to evaluate the adsorbent and were calculated from the weight change of the sample in the adsorption/desorption process. The desorption capacity in percentage was defined as the ratio of the amount of the gas desorbed over the amount of gas adsorbed. The impact of adsorption/desorption temperature and the CO2 concentration in the adsorbate on the adsorption and desorption performance of the adsorbent were also studied. Cyclical adsorption and desorption were investigated to evaluate the stability of the adsorbent.

Results and Discussion 1. Preparation and Characterization of MCM41-PEI Adsorbent. The structure of the MCM-41 before and after the loading of 50 wt % PEI was characterized by XRD and the results are compared in Figure 1. The diffraction patterns of the MCM-41 did not change after the PEI was loaded, which indicated that the structure of the MCM-41 was preserved. However, the intensity of the diffraction peaks of the MCM-41 did change. After the PEI was loaded, the intensity of the diffraction peaks of the MCM-41 decreased, which was possibly caused by the pore filling by PEI. Reddy et al.22 reported that the XRD patterns of the calcined MCM-41 exhibited peaks with increased intensity and a shift to lower diffraction angle compared to the un-calcined MCM-41. In this study, the diffraction intensity of the MCM-41 decreased substantially after modification using the PEI. Also, the diffraction angle of the (100) plane increased from 2.265 for MCM-41 to 2.325 for MCM-41-PEI-50. These results indicate that the PEI was loaded into the pore channels of the MCM41 support. The nitrogen adsorption/desorption isotherms of the MCM-41 and the MCM-41-PEI-50 are shown in Figure 2, which further confirm that the PEI was loaded into the pore channels of the MCM-41 support. Completely degassed MCM-41 shows a type IV isotherm (Figure 2). The surface area, pore volume, and pore diameter were 1480 m2/g, 1.0 mL/g, and 2.75 nm, respectively. After

Figure 2. Adsorption and desorption isotherms of MCM-41 and MCM-41-PEI-50.

Figure 3. TGA profile for PEI.

loading the PEI, the mesoporous pores were completely filled with PEI, resulting in a type II isotherm (Figure 2), restricting the access of nitrogen into the pores at the liquid nitrogen temperature. The residual pore volume of the MCM-41-PEI-50 is only 0.011 mL/g, the surface area was estimated to be 4.2 m2/g and the average pore diameter was smaller than 0.4 nm. These results correlate with the pore filling effect of the PEI which was also reflected by the XRD characterization. The thermochemical and physical properties of the MCM-41, PEI, and MCM-41-PEI-50 were measured by TGA. As expected, there was no weight loss for MCM41 up to 600 °C (not shown here), which indicated that the hydrophobic MCM-41 was free from adsorbed water or other gases at room temperature. Figure 3 shows the TGA profile and the corresponding differential thermogravimetry (DTG) profile for the PEI alone. The PEI lost 3.8% of its original mass at 100 °C, which can be mainly ascribed to the desorption of CO2 and moisture. This was proved by analyzing the effluent gas by GC. This also indicates that PEI has a low vapor pressure, unlike the commercially used amines such as monoethanolamine (MEA), which makes PEI suitable for long-term use at relatively high temperature. The PEI began to decompose above 150 °C and a sharp weight loss appeared at 205 °C. When the temperature was increased above 225 °C, the rate of weight loss decreased, indicating that a different decomposition process took place. At 600 °C, the PEI was completely decomposed and removed as volatiles. For MCM-41-PEI-50 (shown in Figure 4), there was a ∼10.5% weight loss at 100 °C,

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Figure 5. The CO2 adsorption and desorption curve of MCM41-PEI-50.

Figure 4. TGA profile for MCM-41-PEI-50. Table 1. Adsorption and Desorption Performance of MCM-41, MCM-41-PEI, and PEI under Pure CO2 Atmosphere (CO2 flow rate ) 100 mL/min)

adsorbents

temperature (°C)

adsorption capacity (mg/g-adsorbent)

desorption capacity (%)

MCM-41 only MCM-41 only MCM-41 only MCM-41-PEI-15 MCM-41-PEI-30 MCM-41-PEI-50 MCM-41-PEI-50 MCM-41-PEI-50 MCM-41-PEI-75 PEI

50 75 100 75 75 50 75 100 75 75

14.3 8.6 6.6 19.4 68.7 44 112 110 133 109

100 100 99 101 98.3 24.7 99.8 84.1 101 56.4

which was higher than that of the pure PEI and can also be ascribed to the desorption of CO2 and moisture. A sharp weight loss took place at 125 °C, much lower than that of the pure PEI. At the temperature range of 125-140 °C, the sample lost 30% of its original weight. The total weight loss was ∼56% at 600 °C. If the adsorbed moisture or CO2 is excluded from the total weight, the PEI loading is calculated to be about 50 wt %. This is in accordance with the designed PEI loading and indicates that there was no PEI loss during the preparation process. The decrease in the decomposition temperature of PEI after loading into the MCM-41 can be ascribed to the uniform dispersion of the PEI in the nanoporous support pores, since the melting or decomposition temperature will decrease when the particle size of a substance decreases.26 2. Adsorption Properties of MCM-41-PEI. 2.1. Effect of PEI Loading. The influence of PEI loading on the CO2 adsorption performance of the MCM-41-PEI was investigated in pure CO2 atmosphere at 75 °C and the results are shown in Table 1. A typical adsorption and desorption curve is shown in Figure 5. Before the PEI was loaded, the MCM-41 support alone showed a CO2 adsorption capacity of 8.6 mg/g-adsorbent. The low adsorption capacity was caused by the weak interaction between the CO2 and the MCM-41 at relatively high temperature. To strengthen the interaction between the CO2 and the MCM-41, the branched polymeric substance PEI with numerous CO2-capturing sites was loaded into the channels of the MCM-41. After loading the PEI, the adsorption capacity of the MCM-41 in(26) Zeng, P.; Zajac, S.; Clapp, P. C.; Rifkin, J. A. Mater. Sci. Eng., AsStruct. 1998, 252 (2), 301-306.

Figure 6. Adsorption capacity weighted on PEI in MCM-41PEI adsorbent.

creased substantially (Table 1). The adsorption capacities at 75 °C were 19 mg/g-adsorbent and 69 mg/gadsorbent for MCM-41-PEI-15 and MCM-41-PEI-30, respectively. When the PEI loading was 50 wt %, the adsorption capacity of 112 mg/g-adsorbent was higher than that of the pure PEI (109 mg/g-adsorbent). The highest adsorption capacity of 133 mg/g-adsorbent was obtained when the PEI loading was 75 wt %, which was 15.5 times higher than that of the MCM-41 support alone. The desorption was complete for all the MCM41-PEI adsorbents as well as the MCM-41 support. However, the desorption for pure PEI was slow and was not complete compared to the desoption time of the MCM-41-PEI absorbents. The fast desorption of CO2 from the MCM-41-PEI absorbents can be explained by the high dispersion of the PEI into the MCM-41 channels as shown by the XRD, N2 adsorption/desorption and TGA characterization. To evaluate the effect of the MCM-41, the adsorption capacity of the PEI only in the MCM-41 supported absorbents can be calculated. Since the contribution from MCM-41 at 75 °C is only 8.6 mg/g-adsorbent, its contribution to the PEI-loaded absorbents will be very small. Subtracting the theoretical adsorption from the MCM-41 part and weighing the adsorption due to PEI only on its loading will show if supporting the PEI on the MCM-41 support increases the effectiveness of the PEI. Figure 6 shows the calculated adsorption capacity weighted (i.e., based) on the PEI amount for the adsorbents with different PEI loadings. The adsorption capacity weighted on PEI increases with the increase

MCM-41-PEI

of PEI loading, and reaches peak capacity at 50 wt % loading corresponding to 215 mg/g-PEI. This value is 2 times that of the pure PEI. This clearly shows that the MCM-41 has a synergetic effect on the CO2 adsorption when loading PEI into its porous structure. However, the calculated adsorption capacity of PEI decreases when the PEI loading was further increased to 75 wt %. The synergetic effect of the MCM-41 is related to the pore structure of the adsorbents with different PEI loadings. Since the pore volume of MCM-41 is about 1.0 mL/g and the density of PEI is about 1.0 g/mL, the theoretically largest amount of PEI that can be loaded into 1.0 g MCM-41 is 1.0 mL, i.e., 50 wt % PEI loading. Interestingly, the highest calculated adsorption capacity weighed on PEI is also at the 50 wt % PEI loading. There are two possible reasons for the synergetic effect of the MCM-41, i.e., the high surface area and the uniform mesoporous channel of the MCM-41. When the PEI was loaded on the materials with high surface area, more CO2 affinity sites were exposed to the adsorbate and thus the adsorption capacity increased. This was also observed by the desorption rate of CO2 from the MCM-41-PEI and the PEI. After sweeping in N2 flow for 150 min, the CO2 was completely desorbed for MCM-41-PEI and only 56% CO2 desorbed for PEI alone. The channels of the MCM-41 may play an important role on the increase of the CO2 adsorption capacity. When the channels of the MCM-41 are filled with the PEI, the apparaent (measured) pore size of the MCM-41 will be decreased. At the same time, more CO2 affinity sites are introduced into the channel. These two effects may combine together and result in the further increment of the adsorption capacity. The highest calculated adsorption capacity weighed on PEI was obtained when the channels of the MCM-41 are fully filled with the PEI. When the PEI loading was further increased and the PEI was coated on the external surface of the MCM-41, the calculated adsorption capacity weighed on PEI decreased. The adsorption capacity weighed by PEI is 215 mg/g for MCM-41-PEI-50 and 174 mg/g for MCM-41-PEI-75. If we assume that 25 wt % of the PEI is loaded into the pores of the MCM-41 and the other 50 wt % of the PEI is coated on the external surface of the MCM-41 for MCM-41-PEI-75, we can calculate that the adsorption capacity of the PEI that was coated on the external surface of MCM-41 is only 154 mg/g, slightly higher than that of the pure PEI of 110 mg/g and much lower than that of the MCM-41PEI-50 of 215 mg/g when the PEI was fully loaded into the channels of the MCM-41. These results indicate that, although the dispersion of the PEI on the high surface area materials can increase the adsorption capacity, it is the “molecular basket” concept that largely increases the adsorption capacity. However, even for the best adsorbent of MCM-41-PEI-50, only about 44% of the amine groups react with CO2. Improving the adsorption performance in view of preparation is worth exploring. 2.2 Influence of Operating Temperature. The adsorption and desorption performance of the MCM-41-PEI50 were measured at different temperatures in pure CO2 atmosphere and the results are shown in Table 1. With increasing temperatures, the adsorption capacity of the MCM-41-PEI-50 becomes larger and reaches its maxi-

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Figure 7. Schematic diagram of PEI status in MCM-41 at (A) low temperature, and (B) high temperature. (b) Active CO2 adsorption sites; (O) hidden CO2 adsorption sites.

mum of 112 mg/g-adsorbent at 75 °C. When the temperature was increased to 100 °C, the adsorption capacity slightly decreased to 110 mg/g-adsorbent. The desorption was not complete at low temperature. When the temperature was 50 °C, about 20% of the CO2 desorbed. The desorption was complete when the temperature was 75 °C. However, the desorption capacity decreased when the temperature was 100 °C. The adsorption of CO2 into PEI or MCM-41 is an exothermic process.21 Accordingly, the adsorption capacity should decrease with the increase of temperature. However, the adsorption capacity increased with increasing temperature in this study. Figure 7 shows the proposed hypothetical explanation for this seemingly abnormal phenomenon. At low temperature (A), the PEI exists in the channels of MCM-41 like nanosized particles. In this case, only the CO2 affinity sites on the surface of the particles can readily react with the CO2. The affinity sites inside the nanosized particles can only react with the CO2 when the CO2 is diffused into the particles. This is a kinetically (diffusion)-controlled process, since the access of CO2 to the affinity sites inside the PEI particles will be subject to the diffusion limitation, and CO2 may reach more affinity sites if the diffusion time is sufficiently long. In this case, more affinity sites will be exposed to the CO2 and thus the adsorption capacity will increase when using a short adsorption time.

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Figure 8. Temperature-programmed adsorption of CO2 by MCM-41-PEI-50. (Pure CO2 atomosphere, CO2 flow rate: 100 mL/min).

Figure 9. The influence of CO2 concentration in the CO2/N2 mixture on the adsorption performance of MCM-41-PEI-50 (Operation temperature 75 °C, flow rate 100 mL/min).

Table 2. Comparison of the CO2 Adsorption Performance of MCM-41-PEI-75 and Other Adsorbents

of CO2 concentration in a CO2/N2 gas mixture on the adsorption and desorption performance of MCM-41-PEI50 was investigated at 75 °C. The results are shown in Figure 9. Since the adsorption of N2 is negligible in the presence of CO2, the adsorption quantity measured for the CO2/N2 mixture was considered as the CO2 adsorption capacity. For pure CO2, the adsorption capacity was 112 mg/g-adsorbent. With the decrease in the CO2 concentration, the adsorption capacity of the MCM-41PEI-50 also decreased somewhat. When the CO2 concentration in the feed decreased to 30%, the adsorption capacity decreased to 102 mg/g-adsorbent and was about 9% lower than that in the pure CO2 atmosphere. When the CO2 concentration further decreased to 8% and 2%, the adsorption capacities decreased to 90.4 and 67.5 mg/ g, respectively. Even when the CO2 concentration was 0.5% (i.e., 3.8 mmHg), the MCM-41-PEI-50 can still effectively capture the CO2 from the gas mixture. The adsorption capacity was 47.6 mg/g-adsorbent at the CO2 concentration of 0.5%. The desorption of the CO2 was complete at the CO2 concentration range investigated. The main reaction responsible for CO2 chemical interaction with amine (chemical adsorption) is believed to be carbamate formation:20

adsorbents MCM-41-PEI-75 MCM-41-PEI-75 PEI-polymer 13X activated carbon 4A Zr-pillared clay basic alumina hydrotalcite MgO

temperadsorption ature pressure capacity (°C) (atm) (mg/g-adsorbent) reference 75 75 ∼50 25 25 25 20 20 400 400

1 0.02 0.02 1 1 1 1 1 0.2 -

133 67.5 ∼40 160 135 110 ∼30 44 22 8.8

this study this study [20] [15] [15] [15] [17] [18] [16] [19]

To verify the above hypothesis, the following experiment was designed. First, the adsorption of CO2 was carried out at 75 °C in CO2 flow for 150 min, then the temperature was decreased to 25 °C and held at that temperature for another 150 min both in CO2 flow. If the adsorption capacity increases at low temperature, we can conclude that the low adsorption capacity at low temperature was caused by the kinetic limitation. If the adsorption capacity decreases, other reasons may cause the low adsorption capacity at low temperature. The experimental results are shown in Figure 8. The adsorption capacity was 110 mg/g-adsorbent at 75 °C. When the temperature was decreased to 25 °C and held for 150 min, the adsorption capacity increased to 112 mg/g-adsorbent, which means that the adsorption is kinetically controlled. The low adsorption capacity at low temperature is therefore a result of the low adsorption rate. The adsorption capacity at low temperature will be larger than that at high temperature if the adsorption time is long enough to ensure that it reaches equilibrium. Table 2 compares the CO2 adsorption capacity of MCM-41-PEI-75 with the values reported in the literature for zeolites, activated carbons, and clays as well as PEI alone. It is clear that MCM-41-PEI-75 shows superior performance. Furthermore, MCM-41-PEI-75 shows superior selectivity to CO2 against N2, which is not apparent just by observing the data in Table 2. 2.3. Influence of CO2 Concentration. Because the concentration of CO2 varies in different gas mixtures, from about 13% in flue gas from coal-fired power plants5 to approximately 0.6% in a spaceship,20 the influence

CO2 + 2R2NH ) R2NH2+ + R2NCOOSince the experiment was carried out in a flow system, the amount of CO2 was in large excess for the reaction. However, the adsorption capacity was related to the CO2 concentration in the gas mixture at a given temperature, which implied that the reaction was partially thermodynamically controlled. To increase the adsorption capacity, the partial pressure of the CO2 must be increased. But the experimental data showed that the adsorption capacity seems to be related to the logarithm of the CO2 pressure. Increasing the CO2 pressure will lead to less benefit on the adsorption capacity and may increase the operating cost. The other optional way is making the MCM-41-PEI materials in a membrane form. In this case, the reaction (chemical adsorption) will happen at the feed side and the inverse reaction (desorption after chemisorption) happens at the permeate side. Thus, the CO2 can be recovered. In addition, the separation can be operated continuously. 2.4. Cyclical Operation. For practical use, the adsorbent should not only possess high selectivity and high

MCM-41-PEI

Energy & Fuels, Vol. 16, No. 6, 2002 1469

Conclusion

Figure 10. Cyclical adsorption and desorption of CO2 by MCM-41-PEI-50 (Operation temperature 75 °C, CO2 flow rate 100 mL/min).

adsorption capacity, but also show stable adsorption and desorption performance during long-term cyclical operation. The cyclical adsorption (at 75 °C in pure CO2 atmosphere) and desorption were carried out using MCM-41-PEI-50, and the results are shown in Figure 10. The adsorption capacity was constant and the desorption was complete after 7 cycles of adsorption and desorption, which indicates that the adsorption and desorption performance was stable under the condition investigated. The stable adsorption and desorption performance suggests that this adsorbent is promising for further study toward practical applications. A more detailed study on the effect of the MCM-41 is in progress and the results will be reported in the future.

The concept of novel CO2 “molecular basket” has been successfully developed using a PEI-modified mesoporous molecular sieve of MCM-41 (MCM-41-PEI). The use of MCM-41 showed a synergetic effect on the adsorption of CO2 by PEI. By loading the PEI into the MCM-41 pore channels, the adsorption capacity of the MCM-41 was significantly increased. Further, the desorption rate of CO2 for MCM-41-PEI was faster than that for pure PEI. The novel “molecular basket” concept explored in this work resulted in a CO2 adsorption capacity of 215 mg-CO2/g-PEI achieved with MCM-41-PEI-50 at 75 °C, which was 24 times greater than that of the MCM-41 and about 2 times that of pure PEI. The novel “molecular basket” material can effectively adsorb CO2 at very low CO2 concentration, e.g., 0.5%, and it is stable in cyclical operations at relatively high temperatures. By loading different substances with affinity to different gases to the mesoporous molecular sieve, the new idea can be used to develop different types of highselective, high-adsorption-capacity “molecular baskets”. Acknowledgment. Funding for the work was provided by the U.S. Department of Defense (via an interagency agreement with the U.S. Department of Energy) and the Commonwealth of Pennsylvania under Cooperative Agreement No. DE-FC22-92PC92162. EF020058U