Langmuir 1999, 15, 6043-6049
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Experimental and Computer Simulation Studies of the Removal of Carbon Dioxide from Mixtures with Methane Using AlPO4-5 and MCM-41† C. A. Koh,* T. Montanari, R. I. Nooney, S. F. Tahir, and R. E. Westacott Chemistry Department, King’s College London, Strand, London WC2R 2LS, U.K. Received October 14, 1998. In Final Form: April 16, 1999 The selective adsorption of carbon dioxide-methane mixtures in AlPO4-5 and MCM-41 materials have been studied using gas chromatography mass spectrometry and Grand Canonical Monte Carlo computer simulations. AlPO4-5 and MCM-41 are molecular sieves consisting of uniform arrays of unidimensional pore channels. The materials were shown to be selective for carbon dioxide under all conditions studied. There was qualitative agreement between experimental and simulation adsorption and selectivity data. Any discrepancies that were observed were attributed to the effect of pore blocking by adsorbate molecules, which was not accounted for in the simulation model. In the case of MCM-41, quantitative differences in selectivity were the result of the simplistic models of MCM-41 used in the simulations. Partial interaction potential models were used in Monte Carlo calculations to determine the factors that effect carbon dioxide selectivity over methane. Molecular shape and fluid-wall interactions were found to have a significant effect on selectivity, while electrostatics were less important.
1. Introduction International concern about the adverse environmental effects arising from the release of volatile organic compounds (VOCs) into the atmosphere and industrial interest in removing trace pollutants and greenhouse gases from process and exhaust streams has led to significant research to address these problems.1 Several methods exist to accomplish these separations, but adsorption onto solid substrates provides one of the most effective means for the removal of specific compounds from gas mixtures. Significant experimental2 and theoretical work3 has already been concerned with the adsorption of single gases into microporous, carbonaceous, and zeolite materials. Some of this work has been concerned with the phase behavior of the fluid phase in the confined space of the pore,4 the effects of pore shape and size,3 and the nature of the fluid-wall interactions.5 The same materials can be used for gas separations through selective adsorption of one component over the others. Porous and zeolitic materials have previously been shown to be effective as molecular sieves through which molecules of smaller size than the pore can pass.6 In theory, these materials can be used as chemical as well as physical sieves. That is, the chemical properties of the adsorbate and adsorbent can be utilized for effective separation of molecules of similar physical size. In this paper we show experimental and computer simulation results for the adsorption of a mixture † Presented at the Third International Symposium on Effects of Surface Heterogeneity in Adsorption and Catalysis on Solids, held in Poland, August 9-16, 1998.
(1) Schneider, S. H. Global Warming. Are we entering the Greenhouse Century?; The Lutterworth Press: Cambridge, 1989. (2) Branton, P. J.; Hall, P. G.; Treguer, M.; Sing, K. S. W. J. Chem. Soc., Faraday Trans. 1995, 91, 2041. (3) Jiang, S.; Gubbins, K. E.; Balbuena, P. B. J. Chem. Phys. 1994, 98, 2403. (4) Cracknell, R.; Koh, C. A.; Thompson, S. M.; Gubbins, K. E. Mater. Res. Soc. Symp. Proc. 1993, 290, 135. (5) Cracknell, R. F.; Gubbins, K. E.; Maddox, M.; Nicholson, D. Acc. Chem. Res. 1995, 28, 281. (6) Wilson, S. T.; Messina, C. A.; Lok, B. M.; Flanigan, E. M. Intrazeolites, ACS Symposium Series 218; American Chemical Society: Washington, DC, 1983; p 79.
of carbon dioxide and methane into two porous materials: a microporous aluminophosphate and a mesoporous silicate. In gas fields, mixtures of hydrocarbons with carbon dioxide occur naturally. These are also important in enhanced oil recovery processes. Environmental considerations and the desire to improve the energy content of hydrocarbon products drive the need to separate carbon dioxide from natural gas mixtures (both carbon dioxide and methane have been identified as greenhouse gases). For removal of carbon dioxide from natural gas, an adsorbent that selects carbon dioxide over hydrocarbons is needed, whereas for the enhanced oil recovery applications the reverse applies, i.e., selectivity for hydrocarbons. Methane gas has commonly been used in the carbon dioxide-hydrocarbon mixtures to represent the hydrocarbons, and in most cases carbon dioxide has been reported as preferentially adsorbed. The literature identifies porous boron nitride as showing a strong preference for carbon dioxide over methane at 273 K and pressure around 1 bar.7 Aluminophosphates were the first type of molecular sieves to contain no silicon. These materials consist of alternating [AlO4]5- and [PO4]5- tetrahedra. Some of the known aluminophosphate structures are isomorphous with zeolites. Aluminophosphates are neutral materials and therefore contain no exchangeable cations (cf. microporous silica). One of the most commonly used molecular sieves of this class is AlPO4-5. It contains unidimensional channels that have edges formed by 12 alternating aluminum and phosphorus tetrahedra and an internal diameter of ca. 7.6 Å. The surface properties of AlPO4-5 have been extensively studied by Choudhary et al.8 MCM-41 is a purely siliceous material that contains mesoporous unidimensional channels. These channels can vary in diameter from 15 to 200 Å, depending on the template used during synthesis. The pore walls are (7) Janik, J. F.; Ackerman, W. C.; Paine, R. T.; Hua, D. W.; Maskara, A.; Smith, D. M. Langmuir 1994, 10, 514. (8) Choudhary, V. R.; Pandit, M. Y.; Sansare, S. D. Indian J. Technol. 1992, 30, 116.
10.1021/la9814337 CCC: $18.00 © 1999 American Chemical Society Published on Web 06/18/1999
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believed to resemble amorphous silica.9 Great interest has been shown recently in these materials for the adsorption of gases10 and for supports during catalytic processes11 because of its hydrothermal stability and tailored pore size. The work presented in this paper shows the use of experimental and computer simulation methods in the removal of carbon dioxide from mixtures with methane using AlPO4-5 and MCM-41. Both AlPO4-5 and MCM-41 are attractive adsorbents to be used in a combined experimental and simulation study since their uniform arrays of straight channels make them easy to model. Although these materials are chemically different it is hoped that the different properties required for selectivity can be determined. Experiments were performed on a purpose-built adsorption rig linked to a gas chromatograph/mass spectrometer (GCMS). Grand Canonical Monte Carlo (GCMC) simulations will show the important properties of the gas molecules that influence preferential adsorption and maximum selectivity. The properties of the gas that will be investigated are shape, intermolecular potential, and atomic point charges. The selectivity for a component was calculated using the following expression, in which x indicates the adsorbed phase, y indicates the gas phase, and the indices indicate the component of the gas mixture.
SCO2 )
( )( ) xCO2 yCH4
xCH4 yCO2
The remainder of this paper is organized as follows. In section 2 the experimental and simulation methods are described. In section 3 the results of the simulations on gas mixture adsorption are presented and compared to the experimental results. Reasons for any differences are discussed. The preferential adsorption of gases and the rationale for selectivity are discussed. Section 4 contains concluding remarks and suggestions for further investigations in this area. 2. Experimental and Simulation Details Materials. The zeolitic adsorbent materials were prepared according to the literature methodology: aluminophosphate, AlPO4-5 with pore diameter of 7.6 Å according to the method of Wilson et al.12 and MCM-41 with pore diameter of 33 Å according to the method described by Nooney.13 Carbon dioxide and methane gas mixtures with a purity higher than 99.99% were used as adsorbates (Linde). Experimental Procedure. An adsorption rig was designed and built in our laboratory that could measure high-pressure isotherms at various temperatures (303343 K) and pressures between 0 and 1500 kPa (Figure 1). For the work presented in this paper, the adsorption of methane and carbon dioxide from a mixture containing 50 mol % methane and 50 mol % carbon dioxide was investigated at 303 K and pressures up to 1250 kPa using this apparatus. The amount of each gas adsorbed and the (9) Chen, C. Y.; Li, H. X.; Davis, M. E. Micoporous Mater. 1993, 2, 17. (10) 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, 10834. (11) Corma, A.; Martinez, A.; Martinez-Soria, V. J. Catal. 1995, 153, 25. (12) Wilson, S. T.; Lok, B. M.; Messina, C. A.; Cannon, T. R.; Flanigan, E. M. J. Am. Chem. Soc. 1982, 104, 1146. (13) Nooney, R. I. Ph.D. Thesis, King’s College London, October 1998.
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Figure 1. Schematic of the adsorption rig used in these experiments.
selectivity for carbon dioxide were determined using a gas chromatography system with a thermal conductivity detector attached to a mass spectrometer. In addition, pure methane and pure carbon dioxide were tested under similar conditions for comparison purposes. High-pressure adsorption isotherms were constructed by determining the equilibrium pressure from readings obtained from a high-pressure transducer (PT1 in Figure 1). The number of moles of adsorbate is not directly proportional to pressures of this magnitude, and therefore the ideal gas law does not apply. Rather than use a more complicated equation of state that is valid at these pressures, a pipet volume was used. A small amount of the gas at high pressure was isolated and allowed to expand into a larger volume. Therefore, the number of moles of gas in the pipet volume could be determined using the ideal gas equation. This value was then multiplied by the volume ratio between the high pressure section and the pipet volume to obtain the number of moles of adsorptive. This method was chosen because of the ease of application: moles adsorbed could easily be calculated without need to digress into modified equations of state at high pressure. The gases studied were assumed to behave ideally at low pressure and high temperature. Computer Simulations. The AlPO4-5 model was created from the crystallographic unit cell identified by Wilson et al.,6 which is hexagonal, space group P6cc, and has dimensions a ) b ) 13.7 Å and c ) 8.5 Å. The unit cell contains 12 aluminum atoms, 12 phosphorus atoms, and 48 oxygen atoms, arranged such that each aluminum and phosphorus atom was bonded to four oxygen atoms and each oxygen atom was bonded to one phosphorus and one aluminum atom. The bond lengths were equal to the equilibrium bond length for each pair of atoms. A portion of the single cylindrical pore was located at each corner of the unit cell, one-sixth in each corner of one pair of opposite corners and one-third in each of the other two corners. The simulation cell was constructed from a 2 × 2 × 2 array of these unit cells, such that a pore, of radius 7.6 Å, was at the center of the simulation cell. The walls of the MCM-41 structure are believed from experimental studies to be amorphous and have similar structure and properties to amorphous silica.9 The com-
Removal of CO2 from Mixtures
plete structure of MCM-41, therefore, cannot be derived from X-ray diffraction experiments. In effect, these experiments only identify the a and b unit cell parameters (the c parameter can take any value due to the disordered structure in the direction of the pore), the R, β, and γ unit cell angles, and the spacing between pores (which is the only symmetry related property of the unit cell). There are several approaches to creating MCM-41 structures for use in computer simulations. The simplest approach to modeling MCM-41 is to use a cylindrically averaged potential in which the interaction energy is only dependent on the distance of the sorbate molecule from the pore wall. The properties of the silicon and oxygen atoms in the wall are in effect averaged. This approach also has the substantial benefit that only one interaction needs to be calculated for every sorbate, and therefore the computational time required for each simulation is greatly reduced. The main disadvantage with this approach is that it assumes the pore wall surface is homogeneous and therefore does not identify the features of MCM-41 that are important for adsorption. The remaining approaches use explicit adsorbent-adsorbate interactions, which make them computationally more expensive but more realistic. It was decided that the best approach was to create a model of this type, while keeping the method to generate the structure as simple as possible. To accomplish this, silicon and oxygen atoms were inserted at random into a unit cell of the size determined by X-ray diffraction until the correct pore wall density was obtained. Two criteria were used to decide if each randomly generated position was acceptable: the position was not within the free volume of the pore and the position was not within 3 Å of any of the other atoms. This approach provides a simple starting point for the development of an MCM-41 structure for use in computer simulations, but a more realistic model is currently under development in our laboratory. An important character of the MCM-41 model to be used in simulations is the diameter of the pore. For a crystalline material, the size of the pore can be obtained directly as a result of the X-ray diffraction experiments used to determine the structure of the material. For MCM41, however, the pore diameter must be obtained through other means. For the work presented here, the pore diameter has been calculated using the Kelvin equation, which relates the pore size to the molar volume of the adsorbate and the surface tension. Characterization of the MCM-41 material used in this work has led to a pore diameter of 33 Å being used in the simulations.13 The properties of each atom required to describe the potential energy of the adsorbate were the Lennard-Jones collision diameter, σ, and well depth, , and the Coulombic point charge, q. For AlPO4-5 the point charges were calculated using the Qeq method.14,15 Point charges for the atoms in MCM-41 were taken from the literature.16 The numerical values for these parameters can be found in Table 1, in which the values of q are averages for each atom. Two types of models were used for the adsorbates. First, full atom models were used for all adsorbates in simulations with both AlPO4-5 and MCM-41. Second, both carbon dioxide and methane were modeled as a combination of partial models and single Lennard-Jones spheres in a set of simulations with AlPO4-5 to investigate the effects of (14) Castonguay, L. A.; Rappe, A. K. J. Am. Chem. Soc. 1992, 114, 5832. (15) Rappe, A. K.; Goddard, W. A. J. Phys. Chem. 1991, 95, 3358. (16) Burchart, E. D.; Vanbekkum, H.; Vandegraaf, B.; Vogt, E. T. C. J. Chem. Soc., Faraday Trans. 1992, 88, 2761.
Langmuir, Vol. 15, No. 18, 1999 6045 Table 1. Interaction Potential Parameters for the Adsorbent Materials AlPO4-5
MCM-41
Simulation Cell Contents 384 96 96 n/a
O Al P Si
1303 n/a n/a 651
Interaction Parameters σ (Å) 3.118 /kB (K) 30.196 [14] q (e) average -0.4363 [15] σ (Å) 4.008 /kB (K) 254.151 q (e) average +1.0921 σ (Å) 3.695 /kB (K) 153.497 q (e) average +0.6531 σ (Å) /kB (K) q (e) average
O Al P Si
2.708 [16] 101.6 -0.18
3.385 18.67 +0.36
Table 2. Interaction Potential Parameters for Adsorbate Moleculesa adsorbate
σ (Å)
/kB (K)
q (e)
spherical carbon dioxide spherical methane carbon dioxide C O q1 q2 methane C H
4.486 3.812 2.824 3.026 n/a n/a 3.473 2.846
189.1 148 26.3 75.2 n/a n/a 47.87 7.65
n/a [18] n/a [19] 1.0400 [20] 0.0000 -0.64 0.12 -0.1360 [21] +0.0340
a q1 and q2 are point charges of no mass used to approximate the quadrupole in carbon dioxide.
Table 3. Additional Methane and Carbon Dioxide Models Used, Where “Full” Indicates that All Atoms Were Included Explicitlya model 1 model 2 model 3 model 4 model 5
methane
carbon dioxide
spherical LJ spherical LJ full, uncharged full, charged full, charged
spherical LJ full, charged full, uncharged spherical LJ full, uncharged
a The interatomic potential parameters are the same as those described in Table 2.
shape on adsorption in AlPO4-5. In the full atom models each atom was assigned Lennard-Jones σ and parameters and point charges, q. The values of the point charges were either taken from the literature (see Table 2) or calculated using the MNDO method. The Lennard-Jones parameters were taken from the literature (see Table 2). For the partial models, point charges were excluded, to investigate the importance of these parameters. The parameters used in this work are listed in Table 2. For interactions with the aluminophosphate materials, cross-terms for the LennardJones potentials were calculated using the LorentzBerthelot combining rules.17 To study the important adsorbate properties required for selectivity in aluminophosphates, several other adsorbate models were used to investigate adsorption of methane and carbon dioxide in AlPO4-5. These additional models are described in Table 3. For example, methane and carbon dioxide were both approximated as spheres having a single Lennard-Jones site to investigate the effect of adsorbate shape on adsorption and selectivity. Point (17) Allen, M. P.; Tildesley, D. J. Computer Simulation of Liquids; Oxford University Press: Oxford, 1987.
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Figure 2. Adsorption isotherms of carbon dioxide and methane from a 50:50 mixture of the two gases in AlPO4-5 measured at 303 K and over the pressure range 50-600 kPa.
charges were removed from the full adsorbate models to investigate the importance of electrostatic interactions. Five pairs of models were investigated, these are listed in Table 3. Monte Carlo simulations were performed using the Grand Canonical ensemble. In these simulations, the adsorbate molecules (in this case a binary mixture of carbon dioxide and methane) are inserted into the adsorbent material. Four different types of Monte Carlo trials were used in these simulations: creation of a new adsorbate molecule at a random position in the adsorbent, removal of a randomly chosen adsorbate molecule from the adsorbent, translation of a randomly chosen adsorbate molecule within the adsorbent, and rotation of a randomly chosen adsorbate molecule within the adsorbent. For each trial the energy difference between the new and old configurations was calculated. The new configuration was accepted if the new energy was less than the old energy or if the Boltzmann factor calculated from the energy difference was less than a random number generated at each trial. The composition of the gas mixture was approximated using relative partial pressures. The simulations were run for at least 3 × 106 trials and using periodic boundaries to approximate the infinite solid. The long-range interaction cutoff was set to 12 Å. Simulations were run at several total pressures at constant temperature in order to create isotherms. 3. Results and Discussion The results are separated into two parts: first the comparison of experiment and simulation for the selective adsorption of methane and carbon dioxide in AlPO4-5 and the computer simulations to determine the important properties of the molecules that are required for correct selectivity to be observed; second, the comparison of experiment and simulation for the selective adsorption of these gases in MCM-41. The Selective Adsorption of Methane and Carbon Dioxide in AlPO4-5. Adsorption isotherms of carbon dioxide and methane in AlPO4-5 were measured at 303 K and over the pressure range 50-600 kPa (Figure 2) from a 50:50 mixture of the two gases. The equilibration time
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Figure 3. Selectivity for carbon dioxide over methane in AlPO4-5 at 303 K.
Figure 4. Adsorption isotherms for methane and carbon dioxide from a 50:50 mixture of the two gases for comparison with experiment: O and b represent the full models for carbon dioxide and methane, respectively.
was varied between 2, 24, and 48 h. In all cases more carbon dioxide was adsorbed than methane. The total number of moles adsorbed was independent of the equilibration time. However, increasing the equilibration time from 2 to 24 h altered the adsorbate phase composition: more carbon dioxide was adsorbed and more methane was desorbed, thus changing the selectivity. Figure 3 shows the change in selectivity for carbon dioxide with pressure. Low selectivity was observed after 2 h; however, selectivity increased dramatically after 24 h. This may be attributed to the process of replacing a methane molecule with a more favorable carbon dioxide molecule being diffusion limited within the pores. As the pressure was increased, selectivity decreased. Figure 4 shows adsorption isotherms for methane and carbon dioxide from a 50:50 mixture of two gases in AlPO4-5 using a full model simulation in order to compare with the experimental results. These simulations show that more gas is adsorbed than in the experiment at all pressures. This could be for a number of reasons. The experiments may not have been performed for long enough for complete equilibration; however, this seems unlikely since the difference in total adsorption between 24 and 48
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Figure 5. Selectivity for carbon dioxide over methane in AlPO4-5 calculated from Grand Canonical Monte Carlo simulations.
h is very small. Other experimental problems could arise from pore blockages that occur through incomplete removal of the organic template or the creation of carbonaceous deposits in the pore during calcination that lead to reduced adsorption of the gas. Diffuse reflectance FT-IR spectroscopy has indicated, however, that no organic material remained in the pore after removal of the template,13 so the latter is more than likely one of the problems. Additionally, different results from the simulations could be obtained through a different choice of interaction potential models. It is encouraging that the difference in total adsorption between simulation and experiment is only around a factor of 2 or less at all pressures. Figure 5 shows the selectivity for carbon dioxide over methane calculated from Grand Canonical Monte Carlo simulations on a mixture of these gases in AlPO4-5. The results shown in Figure 5 were calculated from the same data as used in Figure 4. It is obvious that AlPO4-5 is selective for carbon dioxide. The calculated selectivity is much higher than that obtained from the experimental results for the shorter experiments (Figure 3). It is interesting that the experimental results approach those of the simulation for longer experiments even though the total amount adsorbed during the experiments changes very little between the 2 and 48 h experiments. This indicates that selectivity is diffusion controlledsinitially both gases are adsorbed, but over time adsorbed methane molecules are replaced by carbon dioxide molecules. However, simulations predict that selectivity increases with increasing pressure, while experiments indicate the opposite. The second part of the simulations on this system was to investigate the molecular properties important for adsorption of carbon dioxide and methane in AlPO4-5 and how these properties affect the selectivity for carbon dioxide. Figure 6A shows the adsorption for the models used in this study. The gas mixture used here was 95 mol % carbon dioxide. The details have been enlarged in Figure 6B. For all the partial models shown in Figure 6, the adsorption is similar to that obtained in experiment (Figure 2) and full model simulation (Figure 4), although it is not expected to be identical in either case because the gas mixture used was different. However, the overall trend is the samesmore carbon dioxide was adsorbed than methanesin all cases. The amount of each gas adsorbed in each case varies due to the different fluid-wall interactions. A more direct view of the effects of fluidwall interactions and the effect of molecular properties can be seen in Table 4, which shows the selectivity for carbon dioxide for these simulations.
Figure 6. (A) Adsorption isotherms for a carbon dioxide and methane gas mixture using partial interaction potential models. Key: O and b represent a mixture of methane and carbon dioxide modeled as Lennard-Jones spheres, 4 and 2 represent a gas mixture composed of methane modeled as a LennardJones sphere and a full carbon dioxide model, 0 and 9 represent a gas mixture composed of a full methane model and carbon dioxide modeled as a Lennard-Jones sphere, ] and [ represent a mixture of full, uncharged models of methane and carbon dioxide, × in box and × represent a mixture of a full methane model and a full but uncharged carbon dioxide model. (B) Enlargement of part A showing the values of the simulations in greater detail. Table 4. Selectivity for Carbon Dioxide from Grand Canonical Monte Carlo Simulations Using Partial Models for the Interaction Potentialsa P/kPa
model 1
model 2
model 3
model 4
model 5
100 200 300 400 500
1.00 0.59 0.75 1.04 0.92
1.45 1.88 1.12 1.91 1.62
2.14 3.42 2.55 3.65 2.75
0.15 0.23 0.15 0.12 0.15
3.14 3.17 2.37 3.23 3.09
a
The models are described in Table 3.
It appears from the results illustrated in Figure 6 and Table 4 that use of the most complete interaction potential models leads to the greatest selectivity for carbon dioxide over methane in AlPO4-5. It is also apparent that use of the simplest models, i.e., Lennard-Jones spheres, to represent either or both adsorbates gives rise to no selectivity, or for the case of the mixture of Lennard-Jones carbon dioxide and full methane models, selectivity for methane. The information provided in Table 4 shows that molecular shape and van der Waals-type interactions are
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Figure 8. Selectivity for carbon dioxide over methane in MCM41 at 303 K and over the pressure range 50-1250 kPa. Figure 7. Adsorption isotherms of carbon dioxide and methane in MCM-41 measured at 303 K and over the pressure range 50-1250 kPa.
the overriding factors that determine selectivity. Electrostatics are important molecular properties for the magnitude of calculated selectivity, but the shape alone is enough to distinguish the preferred adsorbate. This has important implications for the use of computer simulations as a tool for the prediction of selective adsorption properties of porous materials: not including electrostatics greatly reduces the computer time required. The transferability of this effect to other gas mixtures and porous materials needs to be investigated. The opposite trends of selectivity with pressure predicted by simulation and shown by experiment may be due to a number of factors. However, the most dominant factor must be the size of the pore and how this affects adsorption of gas molecules in the experiment and simulation. When a gas molecule is adsorbed on the pore wall, the cross-sectional area of the pore is reduced at this point. In AlPO4-5, the adsorption of a large molecule or several smaller molecules in close proximity reduces the available space through which other molecules can move. This is another type of pore blocking. This means that molecules that would not normally be preferentially adsorbed, and thus pass through the pore, become trapped. This has the effect of reducing the selectivity for the preferred gas. In the GCMC simulations this effect does not occur. The destruction or removal of an adsorbed molecule occurs if it is energetically favorable for the trial to be accepted. This means that the trapping of the less preferred molecule that happens in the experiment can be reduced or avoided completely in the simulation. It is expected that this effect is more apparent with increasing pressure. The effect of pore blocking cannot be introduced into simulations of this type but could be investigated using nonequilibrium methods such as Grand Canonical Molecular Dynamics. The Selective Adsorption of Methane and Carbon Dioxide in MCM-41. Figure 7 shows the adsorption isotherms of a gas mixture for a methane (50%)/carbon dioxide (50%) mixture on MCM-41 at a temperature of 303 K and at different pressures between 50 and 1250 kPa. The equilibration time was varied between 2 and 24 h. In all cases more carbon dioxide was adsorbed than methane. The total number of moles of gas adsorbed was independent of the equilibration time; however, increasing the equilibration time from 2 to 24 h changed the adsorbate phase composition; more carbon dioxide was adsorbed and more methane was desorbed. Figure 7 also shows the adsorption isotherms of pure carbon dioxide and pure methane. Pure carbon dioxide adsorbed more than pure methane on MCM-41. Pure carbon dioxide adsorbed five
Figure 9. Fixed pressure GCMC simulated adsorption isotherm of carbon dioxide and methane in MCM-41 at 298 K and the pressure varied between 0 and 500 kPa.
times more than pure methane on MCM-41 at 2 h equilibration. This result mirrored the findings when we tested the adsorption on the mixture of methane/carbon dioxide on MCM-41; i.e., the carbon dioxide showed greater adsorption. Selectivity for carbon dioxide from a carbon dioxide and methane mixture versus pressure is shown in Figure 8. Selectivity was found to depend on the equilibration time. In the 2 h experiment little selectivity was observed (1.11.2) above 500 kPa, while selectivity increased to 2.0-1.8 below 200 kPa. In the 24 h experiment selectivity increased to 3-3.5 at 200 kPa, indicating that selectivity for the more favored carbon dioxide molecule was diffusion limited. Binary adsorption isotherms of carbon dioxide and methane in MCM-41 were calculated using fixed pressure GCMC simulations (Figure 9). The adsorbate models used were selected because they were able to reproduce experimental selectivities in AlPO4-5 at 100 kPa (above) in good quantitative agreement. In addition, a spherical model for methane reduces the number of trials required in the GCMC simulation and thereby reduces the computational expense. The total gas adsorbed in MCM-41 at 500 kPa was 1.3 mmol g-1 compared to the predicted value at 500 kPa from simulation of 3.27 mmol g-1. However, extrapolation of the isotherm from simulation to 1100 kPa shows that the experimental isotherm approaches the simulated isotherm at higher pressure. In other words, the model of MCM-41 used in the simulations reaches maximum loading at much lower pressure than the real system. There are several possibilities that may explain this effect. However, the most likely explanation is as a consequence of the approximations made during the development of the simple model for MCM-41 and the
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simplistic representation of MCM-41 used in the simulations. However, it is interesting that the trend of decreasing selectivity with increasing pressure is successfully reproduced. 4. Conclusions
Figure 10. Selectivity for carbon dioxide over methane in MCM-41 calculated from fixed pressure GCMC simulations at 298 K and over the pressure range 0-500 kPa.
homogeneous nature of the pore surface used in these simulations. This has the effect of making the pore walls highly attractive, and therefore more adsorbates are attracted to the pore at lower pressures. A more accurate model for MCM-41 is currently being developed in our group. The simulated adsorption isotherm was of type I in the IUPAC classification system22 and was close to the complete adsorption capacity of the adsorbent. The predicted selectivity (Figure 10) was much higher than the experimental value or the selectivity predicted in AlPO4-5, at all pressures. At low pressure, selectivity increased dramatically. The quantitative difference between experiment and simulation is probably due to the diffusion-limited carbon dioxide adsorption and also the (18) Hirschfelder, J. O.; Curtiss, C. F.; Bird, R. B. Molecular theory of gases and liquids; Wiley: New York, 1954. (19) Steele, W. A. The Interaction of Gases with Solid Surfaces; Pergamon Press: Oxford, 1974. (20) Murthy, C. S.; O’Shea, S. F.; McDonald, I. R. Mol. Phys. 1990, 50, 531. (21) Mayo, S. L.; Olafson, B. D. J. Phys. Chem. 1990, 94, 8897. (22) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity; Academic Press: New York, 1982.
The separation of carbon dioxide and methane using AlPO4-5 and MCM-41 has been investigated using GCMS adsorption experiments and Grand Canonical Monte Carlo simulations. Both methods have been in qualitative agreement, showing that the materials are selective for carbon dioxide at all conditions studied. The discrepancies observed between the experimental and simulation study for AlPO4-5 can be explained by the effect that pore blocking by adsorbates in the experiment is not a factor in the simulations. Different types of Monte Carlo simulations may reduce these differences. The quantitative differences in selectivity observed between the experiments and simulations on MCM-41 have been attributed to the simplistic model for MCM-41 used in the simulations. The important molecular properties for selectivity of carbon dioxide over methane have been determined from Grand Canonical Monte Carlo simulations using partial interaction potential models for the two adsorbates. It was found that molecular shape is important for the prediction of selectivity, while electrostatics are less important in predicting selectivities using computer simulations. This indicates not only that are the fluidwall interactions important for selectivity but that adsorbate packing at monolayer and multilayer coverage has a large effect. Acknowledgment. The authors thank the Engineering and Physical Sciences Research Council and NATO for funding. We would also thank Viviane Boissel and Valeria Tricarico for their contribution in undertaking experiments for this research. LA9814337
