Energy & Fuels 2009, 23, 1101–1106
1101
A Comparison Study of Carbon Dioxide Adsorption on Polydimethylsiloxane, Silica Gel, and Illinois No. 6 Coal Using in Situ Infrared Spectroscopy A. L. Goodman* U.S. Department of Energy, National Energy Technology Laboratory, Pittsburgh, PennsylVania 15236 ReceiVed September 22, 2008. ReVised Manuscript ReceiVed NoVember 26, 2008
Adsorption of supercritical carbon dioxide (CO2) on polydimethylsiloxane (PDMS), silica gel (SiO2), and Illinois No. 6 coal was compared using in situ attenuated total reflectance Fourier transform infrared (ATRFTIR) spectroscopy at pressures up to 14 MPa and temperatures at 40 °C and 50 °C. Only physical adsorption of CO2 was recorded for PDMS, SiO2, and Illinois No. 6. There was no evidence of the formation of carbonic acid, bicarbonates, carbonates, or any other reaction product between CO2 and PDMS, SiO2, and Illinois No. 6 coal. Carbon dioxide adsorption on PDMS and SiO2 produced a linear isotherm while a typical Langmuirlike isotherm was observed for Illinois No. 6 coal. Attempts to measure CO2 induced swelling of the three materials was unsuccessful due to the design of the ATR-FTIR cell.
Introduction Carbon dioxide (CO2) storage in coal seams has been identified as a potential option to reduce greenhouse gas emissions to the atmosphere. For a given coal seam, adsorption isotherm measurements provide information about the storage capacity, the overall economics of the process, and the operating conditions that can be used.1 The storage capacity of a coal seam is traditionally estimated from manometric, volumetric, or gravimetric isotherm measurements.2-4 These traditional techniques work well for materials that are rigid and do not change in volume when exposed to CO2. However, it may not be acceptable to consider coal as a rigid material. When a coal comes into contact with a fluid that is soluble in the coal, the coal can behave as a flexible, organic, polymeric solid.5 Because CO2 is soluble in coal and capable of swelling coal, treating the coal as a rigid solid may lead to errors.6-10 Typically, published isotherm measurements assume a constant coal volume that does not include corrections for coal shrinking and swelling 1,7,8,11-16 or structural changes 17-20 that * Author to whom correspondence should be addressed. Tel: 1-412-3864962, fax: 1-412-386-5870, e-mail:
[email protected]. (1) White, C. M.; Smith, D. H.; Jones, K. L.; Goodman, A. L.; Jikich, S. A.; LaCount, R. B.; Dubose, S. B.; Ozdemir, E.; Morsi, B. I.; Schroeder, K. T. Energy Fuels 2005, 19, 659–724. (2) Lu, X.-C.; Li, F.-C.; Watson, A. T. Fuel 1995, 74, 599–603. (3) Ruppel, T. C.; Grein, C. T.; Beinstock, D. Fuel 1972, 51, 297–303. (4) Humayun, R.; Tomasko, D. L. AIChE J. 2000, 46, 2065–2075. (5) Larsen, J. W. Int. J. Coal Geol. 2004, 57, 63–74. (6) Sethuraman, A. R.; Reucroft, P. J. Prepr. Pap. Am. Chem. Soc. DiV. Fuel Chem. 1987, 32, 259–264. (7) Reucroft, P. J.; Sethuraman, A. R. Energy Fuels 1987, 1, 72–75. (8) Reucroft, P. J.; Patel, H. Fuel 1986, 65, 816–820. (9) Reucroft, P. J.; Patel, K. B. Fuel 1983, 62, 279–284. (10) Nandi, S. P.; Walker, P. L., Jr. Fuel 1964, 43, 385-393. (11) Walker, P. L.; Verma, S. K.; Rivera-Utrilla, J.; Khan, R. Fuel 1988, 67, 719–726. (12) Ozdemir, E.; Morsi, B. I.; Schroeder, K. Fuel 2004, 83, 1085– 1094. (13) Ozdemir, E.; Morsi, B. I.; Schroeder, K. Langmuir 2003, 19, 9764– 9773. (14) St.George, J. D.; Barakat, M. A. Int. J. Coal Geol. 2001, 45, 105– 113.
10.1021/ef8008025
may alter the coal volume during the isotherm measurement. Some research groups have used models to correct coal volume changes due to swelling.13,21 Recently, independent swelling data were used to correct traditional CO2-coal isotherm measurements for Australian bituminous coals.22 Day et al. found coal swelling was proportional to the amount of CO2 adsorption up to 8 MPa at which point swelling leveled off. A maximum volumetric coal swelling of 1.9% for supercritical CO2 was reported.22 Ideally, CO2 adsorption and swelling data should be collected simultaneously for the same sample under the same conditions. While this is a challenging task, it has been done for CO2 interacting with polymers by Flichy et al.23,24 In their previously cited work, attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy was used to measure gas adsorption and swelling when polydimethylsiloxane (PDMS) was exposed to CO2.23,24 Under supercritical CO2, PDMS swelled by 60% at 11 MPa. In this work, we examined the interactions of supercritical CO2 with Illinois No. 6 coal using in situ ATR-FTIR spectroscopy. Carbon dioxide pressures up to 14 MPa were tested. We also followed the interaction of CO2 with PDMS, a flexible polymer that is known to swell in the presence of CO2 and silica gel (SiO2), a three-dimensional nonordered polymer that may (15) Shimizu, K.; Takanohashi, T.; Iino, M. Energy Fuels 1998, 12, 891–896. (16) Takanohashi, T.; Terao, Y.; Yoshida, T.; Iino, M. Energy Fuels 2000, 14, 915–919. (17) Goodman, A. L.; Favors, R. N.; Hill, M. M.; Larsen, J. W. Energy Fuels 2005, 19, 1759–1760. (18) Larsen, J. W.; Flowers, R. A.; Hall, P. J.; Carlson, G. Energy Fuels 1997, 11, 998–1002. (19) Hsieh, S. T.; Duda, J. L. Fuel 1987, 66, 170–178. (20) Kelemen, S. R.; Kwiatek, L. M.; Siskin, A.; Lee, A. G. K. Energy Fuels 2006, 20, 205–213. (21) Dutta, P.; Harpalani, S.; Prusty, B. Fuel 2008, 87, 2023–2036. (22) Day, S.; Fry, R.; Sakurovs, R. Int. J. Coal Geol. 2008, 74, 41–52. (23) Flichy, N. M. B.; Kazarian, S. G.; Lawrence, C. J.; Briscoe, B. J. J. Phys. Chem. B 2002, 106, 754–759. (24) Duarte, A. R. C.; Anderson, L. E.; Duarte, C. M. M.; Kazarian, S. G. J. Supercrit. Fluids 2005, 36, 160–165.
This article not subject to U.S. Copyright. Published 2009 by the American Chemical Society Published on Web 01/07/2009
1102 Energy & Fuels, Vol. 23, 2009
Figure 1. Schematic of modified ATR-FTIR tunnel cell designed by Axiom Analytical.
swell in the presence of CO2.25 The interactions between the three materials PDMS, SiO2, and Illinois No. 6 coal with supercritical CO2 were compared. Experimental Section Carbon dioxide (100%) and mixtures (50% CO2 with nitrogen balance and 13% CO2 with nitrogen balance) were used as supplied from Butler gas (99.999% supercritical grade). Ultra high purity nitrogen (UHP) was used as supplied from Butler gas. PDMS was purchased from Polymer source (Mw ) 156 000, Mw/Mn ) 1.25, Mn ) 125 000). This polymer is similar to the one used by Flichy et al. but not identical (Mw ) 188 000, Mw/Mn ) 1.17, Mn ) 161 000).23 Silica gel was purchased from Sigma-Aldrich (Davisil grade 633, 200-425 mesh, 60 Å pore diameter, surface area equals 480 m2/g). Illinois No. 6 coal (high volatile bituminous) was purchased from the Argonne premium coal bank. The Argonne premium coals are the highest quality samples available.26,27 These coals are well characterized, providing the research community with a large database of physical and chemical properties. The samples were obtained as powders (100 mesh). Experiments were conducted using two custom-designed tunnel ATR (attenuated total reflectance) cells from Axiom Analytical (Figure 1), which can accommodate pressures up to 14 MPa. The two stainless steel cells were connected in tandem via 0.16 cm stainless steel tubing so that each cell experienced the same CO2 pressure. A cylindrical Zinc Selenide (ZnSe) ATR crystal was placed inside of each cell. The length of the ATR crystal was 2.8 cm, the diameter of the ATR crystal was 0.6 cm, the incidence angle was 45°, and the number of reflections was five. One cell contained a ZnSe ATR crystal with no sample present and the other cell contained a ZnSe ATR crystal coated with sample. An 1.1 cm sample port was machined to the top of the cells to allow the upper portion of the ZnSe ATR crystal to be coated with a sample. Springloaded Teflon o-rings purchased from Saint Gobain Performance Plastics (230-202-A0108) were used to make a high-pressure seal between the stainless steel chamber and ATR crystal. The cells were placed on a pneumatic translator inside the FTIR spectrometer. The cells could then be moved to allow infrared light to probe either the blank ATR crystal or sample ATR crystal. As reported previously, minimum penetration depth of the infrared beam into the sample is expected to be 1 µm.30 This ensures that both the bulk and surface of the sample are interrogated. The cells were enclosed in a temperature jacket that allowed a recirculating bath fluid to flow through the external temperature jacket to either heat or cool the cells. All ATR-FTIR data were collected with a single beam FTIR spectrometer (Thermo Electron Nexus 670 FTIR ESP) equipped with a wide-band mercury cadium telluride (MCT) detector. Unless otherwise noted, 500 scans were collected with an instrument resolution of 4 cm-1 over the spectral range extending from 4000 to 600 cm-1. (25) (26) (27) Sample
Chukin, G. D.; Malevich, V. I. J. Struct. Chem. 1977, 18, 76–83. Vorres, K. S. Energy Fuels 1990, 4, 420–426. Vorres, K. S. Users Handbook For The Argonne Premium Coal Program. http://www.anl.gov/PCS/pcshome.html, 1993.
Goodman The ZnSe ATR crystal was coated with PDMS, SiO2, or Illinois No. 6 coal through the sample port machined on the top of the cell. The sample port allows only the upper portion of the ATR crystal to be coated with the sample of interest, leaving the bottom of the ATR crystal uncoated. The PDMS sample was directly cast onto the ATR crystal through the sample port. Silica gel was immersed in water to form a slurry and mixed for 1 h. The ATR crystal was then coated with a thin layer of the slurry (≈10 mg) through the sample port. This method has been used before with deposition of SiO2 on ZnSe ATR crystals and has been proven beneficial in accessing the spectral window between 1550 and 1100 cm-1 where SiO2 absorptions are typically observed in the infrared.28 Illinois No. 6 coal was handled in a nitrogen-filled glovebag to avoid surface oxidation. The coal was immersed in hexane to form a slurry and mixed for 1 h on a stir plate. The ATR crystal was then coated with a thin layer of the slurry (≈10 mg) through the sample port. Once the ATR crystal was coated with a sample, the sample port was sealed, and the samples were flushed with UHP nitrogen for at least 24 h. Next, the blank and sample ATR cells were thermally equilibrated by recirculating a silicon oil bath (NESLAB RTE 7) through the temperature jacket. PDMS was equilibrated at 50 °C to complement the data reported by Flichy et al.23 SiO2 and Illinois coal No. 6 were equilibrated at 40 °C. The blank and sample ATR cells were exposed to CO2 as a function of increasing pressure from 0 to 14 MPa. Two spectra were recorded: one from the cell containing both CO2 and sample and one from the cell containing CO2. By calculating the difference between the two spectra, an absorption spectrum of CO2 adsorbed on the sample of interest was obtained. This spectral subtraction procedure has been explained before.29 Carbon dioxide adsorption was monitored with ATR-FTIR by following the growth of the ν3 stretching band of adsorbed CO2 (2342-2333 cm-1) versus time and pressure. Equilibrium was reached within minutes for PDMS and SiO2. As discussed in previous work, Illinois No. 6 coal was allowed to interact with CO2 (0.5 MPa) for three days before collection of CO2 adsorption isotherm data.30 After pretreatment of the coal, equilibrium was reached within minutes, but CO2 adsorption was monitored for 24 h.
Results and Discussion First, adsorption of CO2 on PDMS, SiO2, and Illinois No. 6 was monitored in situ as a function of increasing CO2 pressure by examining ATR-FTIR spectroscopic data between 2450 and 2250 cm-1 (Figure 2a-c). The samples were exposed to a known pressure of CO2 and then sealed off from the CO2 source. After spectral subtraction of gaseous CO2, a positive absorption band appeared in the spectrum at 2338 cm-1 for PDMS, 2342 cm-1 for silica, and 2333 cm-1 for Illinois No. 6. This absorption band has been observed before and is indicative of physical adsorbed CO2 (ν3 antisymmetric stretching mode).23,24,29,31-34 Observation of only one distinct symmetrical absorption band suggested that there was one type of adsorption site for the CO2 for these materials under this pressure regime.35 If more than one type of adsorption mechanism were available for CO2, (28) Ninness, B. J.; Bousfield, D. W.; Tripp, C. P. Appl. Spectrosc. 2001, 55, 655–662. (29) Goodman, A. L.; Campus, L. M.; Schroeder, K. T. Energy Fuels 2005, 19, 471–476. (30) Goodman, A. L.; Favors, R. N.; Larsen, J. W. Energy Fuels 2006, 20, 2533–2543. (31) Fastow, M.; Kozirovski, Y.; Folman, M. J. Electron Spectrosc. Relat. Phenom. 1993, 64, 843–848. (32) Mawhinney, D. B.; Rossin, J. A.; Gerhart, K.; Yates, J. T., Jr Langmuir 1999, 15, 4617–4621. (33) Little, L. H. Infrared Spectra of Adsorbed Species; Academic Press: New York, 1966. (34) Davydov, A. A. Infrared Spectroscopy of Adsorbed Species on the Surface of Transmission Metal Oxides; Wiley and Sons: Chichester, 1990. (35) Nelson, M. R.; Borkman, R. F. J. Phys. Chem. 1998, 102, 7860– 7863.
Carbon Dioxide Adsorption
Energy & Fuels, Vol. 23, 2009 1103
for Illinois No. 6 coal (2333 cm-1) increased as pressure increased to 5 MPa and then began to level off as pressure was further increased. Beyond 6 MPa, subtractions of the gaseous CO2 component were unsuccessful for Illinois No. 6 coal because the absorption of the gaseous CO2 continued to increase with pressure while the amount of CO2 adsorbed to the coal remained virtually constant. The adsorbed CO2 signal, therefore, was buried in the gaseous absorption signal. When a less concentrated CO2 gas mixture was used with Illinois No. 6 coal up to 14 MPa, subtractions were again fruitful. Two CO2 mixes (50% CO2 in nitrogen and 13% CO2 in nitrogen) were recorded with ATR-FTIR spectroscopy for Illinois No. 6 coal with no spectral subtraction problems. The ν3 CO2 antisymmetric stretching mode spectral data for the 50% CO2 mixture in nitrogen is shown in Figure 2c. Using these two gas mixtures, the CO2 absorption band for Illinois No. 6 coal (2333 cm-1) increased in intensity with pressure up to 5 MPa and then the absorption band began to level off in intensity as pressure reached 14 MPa. According to the Beer-Lambert Law,37 the spectral absorption intensity of the ν3 CO2 antisymmetric stretching mode is directly proportional to the absolute amount of CO2 adsorbed in PDMS, SiO2, and Illinois No. 6 by eq 1 A ) N × ε × c × de,u
Figure 2. Subtracted ATR-FTIR spectra of ν3 stretching mode of adsorbed CO2 (a) 100% CO2 on PDMS at 50 °C, (b) 100% CO2 on SiO2 at 40 °C, and (c) 50% CO2/50% N2 on Illinois No. 6 coal at 40 °C up to 14 MPa.
distinct absorption bands corresponding to each type of adsorption site would be expected.32,36 Spectra were then recorded until equilibrium was reached, i.e. until the ν3 CO2 absorption band no longer changed with time. For PDMS and SiO2, equilibrium was reached within minutes. For Illinois No. 6 coal, equilibrium was reached within minutes but was monitored for an additional 24 h. The above procedure was repeated by increasing CO2 pressure in small increments until 14 MPa was reached. The ν3 absorption band for PDMS shifted from 2338 cm-1 to 2335 cm-1 as CO2 pressure was increased to 14 MPa (Figure 2a). This band shift was also detected by Flichey et al. in their study of CO2 adsorption on PDMS.23 The ν3 absorption bands for SiO2 and Illinois No. 6 coal remained constant at 2342 and 2333 cm-1, respectively (Figure 2b,c). The intensity of the CO2 absorption band of PDMS (2338 cm-1) and SiO2 (2342 cm-1) increased as CO2 pressure increased. The CO2 absorption band intensity (36) Gallei, E.; Stumpf, G. J. Colloid Interface Sci. 1976, 55, 415–420.
(1)
where A is the net absorbance of the ν3 CO2 antisymmetric stretching mode near 2333 cm-1, N is the number of internal reflections of the ATR cell, ε is the molar absorptivity of carbon dioxide, c is the concentration of adsorbed carbon dioxide, and de,u is the effective path length of the ATR cell that would give the same absorbance if using transmission infrared spectroscopy. The effective path length is a dependent upon light polarization, refractive index of the ATR crystal and sample of interest, and angle of the incidence infrared light. In order for the Beer-Lambert equation to be accurate, the sample of interest must completely and uniformly cover the ATR crystal and make good contact with the ATR crystal. With this ATR system, only the upper portion of the ATR crystal is coated and the sample is not pressed upon the crystal to make good contact. Thus, application of Beer’s law to the data measured here would underestimate the amount of CO2 interacting with the sample. Calculations using the Beer-Lambert Law were not pursued. However, the direct observation of CO2 adsorption can be used to derive the isotherm shape by plotting the net spectral absorbance of adsorbed CO2 versus pressure.38 This requires only a constant amount of sample to be in contact with the ATR crystal. The knowledge of the shape of the absolute adsorption isotherm can provide useful information even if the amount of CO2 adsorbed is not quantitative. The CO2 ν3 antisymmetric stretching mode was integrated from 2420 to 2260 cm-1 for the three materials, and the area was plotted as a function of increasing pressure. The adsorption isotherms generated for PDMS, SiO2, and Illinois No. 6 are shown in Figures 3, 4, and 5 on the y1 axis. The plots show the shape of the CO2 isotherm path for PDMS, SiO2, and Illinois No. 6 coal using integrated CO2 absorbance units as a function of pressure. For PDMS, CO2 adsorption increased linearly as a function of increasing pressure up to 12 MPa (Figure 3, open circles, y1 (37) Harrick, N. J. Internal Reflection Spectroscopy; Harrick Scientific Corporation: New York, 1979. (38) Yarwood, J. Anal. Proc. 1993, 30, 13–18. (39) Giovanni, O. D.; Dorfler, W.; Mazzotti, M.; Morbidelli, M. Langmuir 2001, 17, 4316–4321. (40) Hocker, T.; Rajendran, A.; Mazzotti, M. Langmuir 2003, 19, 1254– 1267.
1104 Energy & Fuels, Vol. 23, 2009
Figure 3. y1-ATR-FTIR adsorption isotherm of CO2 on PDMS at 50 °C and pressure up to 14 MPa. Open circles (O) represent the integrated absorbance of the ν3 stretching mode of adsorbed CO2 at 2338 cm-1. y2-ATR-FTIR adsorption isotherm of CO2 on PDMS at 50 °C and pressure up to 14 MPa.23 Crosses (×) represent the % mass of the ν3 stretching mode of adsorbed CO2 at 2338 cm-1.
Figure 4. y1-ATR-FTIR adsorption isotherm of CO2 on silica at 40 °C and pressure up to 8.5 MPa. Open circles (O) represent the integrated absorbance of the ν3 stretching mode of adsorbed CO2 at 2342 cm-1. y2-gravimetric adsorption isotherm of CO2 on PDMS at 39 °C and pressure up to 8.5 MPa.39,40 Crosses (×) represent the excess adsorption of CO2 adsorbed on silica (g/g).
axis). This data was compared to ATR-FTIR data obtained by Flichy et al. for CO2 adsorption on PDMS at 50 °C using a similar ATR-FTIR technique (Figure 3, crosses, y2 axis).23 The data obtained by Flichy et al. are in terms of % mass CO2 and are shown on the y2 axis of Figure 3. That fact that the y-axis scales, CO2 integrated area (y1) for this work and % mass CO2 (y2) for Flichy et al., are the same is merely coincidental. Both data sets showed the same linear trend for CO2 adsorption on PDMS. For SiO2, CO2 adsorption also increased linearly as a function of increasing pressure (Figure 4, open circles, y1-axis). The error bars plotted on the open circle data points represent the range (41) Mavor, M. J.; Owen, L. B.; Pratt, T. J. Measurement and Evaluation of Coal Sorption Isotherm Data. Presented at the 65th Annual Technical Conference and Exhibition, Society of Petroleum Engineers, Sept 23-26, 1990, New Orleans, LA, SPE paper 20728, pp 157-70. (42) Goodman, A. L.; Busch, A.; Bustin, R. M.; Chikatamarla, L.; Day, S.; Duffy, G. J.; Fitzgerald, J. E.; Gasem, K. A. M.; Gensterblum, Y.; Hartman, R. C.; Jing, C.; Krooss, B. M.; Mohammed, S.; Pratt, T. J.; Robinson, R. L.; Romanov, V. N.; Sakurovs, R.; Schroeder, K.; White, C. M. Int. J. Coal Geol. 2007, 72, 153–164. (43) Adamson, A. W. Physical Chemistry of Surfaces; John Wiley & Sons Inc.: New York, 1990; p 662.
Goodman
Figure 5. y1-ATR-FTIR adsorption isotherms of CO2 on Illinois No. 6 coal at 40 °C and pressure up to 14 MPa. Open circles (O) represent the integrated absorbance of the ν3 stretching mode of adsorbed CO2 at 2333 cm-1for 100% CO2. Open squares (0) represent the integrated absorbance of the ν3 stretching mode of adsorbed CO2 at 2333 cm-1for 50% CO2. Plus signs (+) represent the integrated absorbance of the ν3 stretching mode of adsorbed CO2 at 2333 cm-1for 13% CO2. The solid lines represent the adsorption isotherm predicted by the Langmuir equation (eq 2). y2-volumetric adsorption isotherm of CO2 on Illinois No. 6 coal at 55 °C and pressure up to 14 MPa.42 Crosses (×) represent the absolute adsorption of CO2 adsorbed on Illinois No. 6 coal (mmol/ g, daf). The dashed lines represent the adsorption isotherm predicted by the Langmuir equation (eq 2).
between two separate ATR-FTIR isotherm measurements. The ATR data were compared with supercritical CO2 adsorption on silica using a gravimetric technique at 39 °C (Figure 4, crosses, y2-axis).39,40 Again, both data sets showed the same linear trend for CO2 adsorption on SiO2. For Illinois No. 6 coal, three CO2 isotherms are shown for three different gas mixtures: 100%, 50%, and 13% CO2 in nitrogen balance (Figure 5, open circles, open squares, and plus signs, y1-axis). Rapid CO2 uptake was observed at low pressure followed by gradual CO2 uptake at higher pressure. This Langmuir-type isotherm shape is typical of CO2-coal isotherms reported in the literature.41 As discussed above, CO2 uptake for the 100% gas stream could not be followed past 5 MPa due to spectral subtraction difficulties. The error bars plotted on the open circles in Figure 5 for 100% CO2 adsorption on Illinois No. 6 coal represent the standard deviation between four separate isotherm measurements. The ATR data were compared with CO2 adsorption on moisture-equilibrated Illinois coal reported in the literature using volumetric equipment at 55 °C (Figure 5, crosses, y2axis).42 Although the coal moisture levels are different and there is a 15 °C temperature difference between the two measurements, both data sets show the same Langmuir-type trend for CO2 adsorption on Illinois No. 6 coal. The lines in Figure 5 represent the fit predicted by the Langmuir eq 2: solid lines for the ATR-FTIR data and a dashed line for the volumetric data. bP (2) 1 + bP where b is the Langmuir constant, P is pressure, and θ is the fractional adsorption.43 The Langmuir equation, while very simple, is traditionally used to fit CO2-coal isotherm measurements. The energy of adsorption (Q) was estimated from the Langmuir constant b in eq 2 using eq 3, θ)
b ) boeQ/RT
(3)
Carbon Dioxide Adsorption
Energy & Fuels, Vol. 23, 2009 1105
where R is the gas constant, T is the temperature, and bo is a constant defined as bo )
Nστo (2πMRT)1⁄2
(4)
where N is Avogadro’s constant, σ is the molecular area of CO2 (0.22 nm2),44 τo equals 1 × 10-13 s, and M is the molecular weight of CO2. The Langmuir parameters and energy of adsorption values are listed in Table 1. The heat of adsorption ranging between 17.6 and 19.7 kJ/mol were consistent with physisorption and compared favorably with values reported in the literature.12,13,29,42,45,46 Next, the spectral region between 2000 and 670 cm-1 was monitored as a function of increasing pressure by using ATRFTIR spectroscopy to examine the interaction of CO2 with PDMS, SiO2, and Illinois No. 6. As discussed above, the samples were exposed to a known pressure of CO2 and then sealed off from the CO2 source. Spectra were then recorded until CO2 equilibrium was reached, i.e. until the ν3 absorption band no longer changed with time. This procedure was repeated by increasing CO2 pressure in small increments until 14 MPa was reached. For clarity, two spectra of each material are shown in Figure 6a-c. The dashed spectra represent PDMS, SiO2, and Illinois No. 6 before exposure to CO2. The solid spectra show PDMS, SiO2, and Illinois No. 6 in the presence of CO2 at 10.6, 8.0, and 10.2 MPa, respectively. No new absorption bands were detected upon exposure of CO2 to the three materials. Thus, there was no evidence for formation of carbonic acid, bicarbonates, carbonates, or any other reaction product between CO2 and PDMS, SiO2, and Illinois No. 6 coal. PDMS showed spectral bands at 1259, 1086, 1018, and 800 cm-1 (Figure 6a). The band at 1259 cm-1 is the δ(CH3) mode of PDMS and is typically used for quantitative measurements. This spectrum is identical to the one obtained by Flichy el al. for their ATR measurement of CO2 adsorption on PDMS.23 The spectral bands for SiO2 gel showed absorption peaks at 1632, 1201, 1146, and 1090 cm-1 (Figure 6b). This is consistent with SiO2 spectra reported in the literature.25 The band labeled at 1090 cm-1 is due to the Si-O lattice vibrations.28 The intense band at 1632 cm-1 is due to the bending mode of water molecules (ν2 H2O) and has been assigned previously.33 Absorptions detected at 1201 and 1146 cm-1 are due to the Teflon o-rings used to make the high pressure seal between the ATR crystal and stainless steel chamber. The Teflon O-ring vibrations are likely masking part of the Si-O lattice vibrations. Illinois No. 6 coal showed spectral bands at 1734, 1604, 1447, 1201, 1146, 1032, 1007, and 915 cm-1 (Figure 6c). Infrared spectra of coals are complex with many overlapping absorption bands.47-49 The ATR-FTIR spectra of Illinois No. 6 coal was typical of complex, heterogeneous coals containing multiple functionalities. The following band assignments were made for the organic components of Illinois No. 6 coal: 1734 cm-1, carbonyl stretch; 1604 cm-1, aromatic ring CdC stretch; and (44) McClellan, A. L.; Harnsberger, H. F. J. Colloid Interface Sci. 1967, 23, 577–599. (45) Day, S.; Duffy, G. J.; Sakurovs, R.; Weir, S. Int. J. Greenhouse Gas Control 2008, 2, 342–352. (46) Glass, A. S.; Larsen, J. W. Energy Fuels 1994, 8, 284–285. (47) Charcossett, H. Advance Methodologies in Coal Characterization; Elsevier: New York, 1990; pp 399-417. (48) Thomasson, J.; Coin, C.; Kahraman, H.; Fredericks, P. M. Fuel 2000, 79, 685–691. (49) Cai, M. F.; Smart, R. B. Energy Fuels 1994, 8, 369–374. (50) Painter, P. C.; Snyder, R. W.; Starsinic, M.; Coleman, M. M.; Kuehn, D. W.; Davis, A. Appl. Spectrosc. 1981, 35, 475–485. (51) Rochdi, A.; Landais, P. Fuel 1991, 70, 367–371.
Table 1. Langmuir Constants and Energies of Adsorption As Measured by ATR-FTIR Spectroscopy for Illinois No. 6 Coal technique
carbon dioxide, %
Q (kJ/mol)
b (1/Mpa)
R2 value
ATR-FTIR ATR-FTIR ATR-FTIR
100 50 13
19.7 18.5 17.6
0.589 0.432 0.303
0.9961 0.9903 0.9768
Figure 6. ATR-FTIR spectra between 2000 and 670 cm-1 (a) PDMS at 50 °C, (b) SiO2 at 40 °C, and (c) Illinois No. 6 coal at 40 °C. The dashed spectra are before exposure to CO2, and the solid spectra are in the presence of CO2. CO2 pressure (MPa) is given in parentheses in the legend.
1447 cm-1, CH2-CH3 deformation.49,50 Mineral components of Illinois coal were observed at 1032 and 1007 cm-1.49,51 Teflon o-rings absorptions were detected at 1201 and 1146 cm-1. Teflon O-ring absorptions are less prominent in the PDMS sample than for the SiO2 or Illinois No. 6 coal. This may be linked to fact that the PDMS polymer spectral signature was strong because it was able to make good contact with the ATR crystal. The ATR spectral signatures of SiO2 and Illinois No. 6 coal powders
1106 Energy & Fuels, Vol. 23, 2009
Figure 7. Percent swelling as calculated by eq 6 for PDMS this work (O), PDMS published (b),23 SiO2 (0), and Illinois No. 6 (×) up to 14 MPa CO2.
were weak because they did not make as good of contact with the ATR crystal. For PDMS, all spectral absorption bands consistently decreased with increasing CO2 pressure (Figure 6a, solid line). In addition, the band maximum of all the PDMS absorption bands increased in wavenumber as CO2 pressure was increased. Flichy et al. reported similar observations in their ATR study for CO2 adsorption on PDMS. The decrease in intensity of the PDMS absorption bands indicated that the PDMS was swelling in the presence of CO2.23 Swelling caused the PDMS sample to pull away from the ATR crystal causing less PDMS sample to be in contact with the ATR crystal, which in turn decreased the PDMS spectral absorption band signal. Flichy et al. extended the Beer-Lambert law (eq 1) to quantify the extent of swelling of the PDMS polymer by assuming that the sample occupies a volume V before exposure and V + ∆V during exposure of CO2 co V + ∆V ) )1+S (5) c V By combining eq 5 with eq 1 and assuming that the effective path length does not change in the presence of CO2, the amount of swelling can be determined from eq 6
(
)
Ao - 1 × 100 (6) A where A° is the net absorbance of PDMS before CO2 exposure and A is the net absorbance of PDMS during CO2 exposure.23,24 The swelling calculations were then applied to the data measured here. The spectral PDMS absorption band at 1259 cm-1 was integrated, and PDMS percent swelling was calculated according to eq 6 (Figure 7, open circles). The swelling data reported by Flichy et al. for their ATR study of CO2 adsorption on PDMS was added for comparison (Figure 7, closed circles). In this study, a maximum of 40% swelling was observed. In contrast, a maximum of 60% swelling was reported by Flichy et al. %S )
Goodman
The swelling data calculated here appears to underpredict the amount of PDMS swelling. This is expected given the design of the ATR system used in this study. In Flichy’s study, the ATR crystal is planar and can be completely covered with the sample of interest. Thus, when the PDMS sample begins to swell, it can swell in only one direction, away from the ATR crystal. In this study, the ATR crystal is a cylindrical rod and can be only partially coated with the sample. When the PDMS sample begins to swell, therefore, it can swell in two directions, away from the ATR crystal (decreased infrared absorption signal) and sideways on the ATR crystal to fill uncovered portions of the crystal (increased infrared absorption signal). Swelling in competing directions on the ATR crystal can lead to vague results. A nonuniform sample coverage may also play a role in masking the amount of swelling. For this PDMS sample, swelling is underpredicted when compared to the data reported by Flichy et al. One can speculate that the polymer swelled in both directions on the ATR crystal, causing the resultant net absorbance of the ATR absorption bands to be ambiguous. In the case of SiO2 and Illinois No. 6 coal, all spectral absorption bands remained virtually unchanged in spectral intensity and in band position as CO2 pressure was increased (Figure 6b,c). This could mean that these materials did not swell in the presence of CO2. As discussed above, interpretation of intensity changes of the ATR absorption bands in this study can lead to suspect conclusions. It is unlikely that the Illinois No. 6 coal did not swell because many have observed coal swelling previously.1,7,8,11-16 Although our attempts here were not successful, it would be beneficial to carbon capture and storage technologies to address CO2 storage capacity of coals by simultaneously addressing CO2 adsorption and swelling during characterization measurements. Conclusions Supercritical CO2 interactions between PDMS, SiO2, and Illinois No. 6 coal were compared using ATR-FTIR spectroscopy. Physical adsorption of CO2 was recorded for PDMS, SiO2, and Illinois No. 6. There was no evidence for formation of carbonic acid, bicarbonates, carbonates, or any other reaction product between CO2 and PDMS, SiO2, and Illinois No. 6 coal. CO2 interaction with PDMS and SiO2 produced a linear isotherm while a typical Langmuir-like isotherm was observed for Illinois No. 6 coal. Quantitative and swelling calculations proved unsuccessful for the three materials due to the design of the ATR cell. Acknowledgment. The author thanks Adam Bimle for collecting a portion of this data during the summer of 2007. The author is also grateful for numerous and profitable discussions with John W. Larsen throughout the course of this work. EF8008025
