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Langmuir 2006, 22, 1150-1155
Heats of Adsorption and Adsorption Heterogeneity for Methane, Ethane, and Carbon Dioxide in MCM-41 Yufeng He† and Nigel A. Seaton* Institute for Materials and Processes, School of Engineering and Electronics, UniVersity of Edinburgh, King’s Buildings, Mayfield Road, Edinburgh EH9 3JL, United Kingdom ReceiVed August 17, 2005. In Final Form: NoVember 4, 2005 We report the adsorption isotherms and the isosteric heats of adsorption of pure methane, ethane, and CO2 and a mixture of methane and CO2 in the periodic mesoporous silica MCM-41 using a multicomponent adsorption calorimeter of the Tian-Calvet type, looking in particular at the degree of heterogeneity in the adsorption of these species. The adsorption of methane and ethane in MCM-41 was found to be essentially homogeneous, while the adsorption of pure CO2 and of CO2 from a CO2/methane mixture was found to be significantly heterogeneous, reflecting the electrostatic interactions between CO2 and the adsorbent.
1. Introduction MCM-41 was the first periodic mesoporous silica (PMS) to be developed, by Mobil scientists in 1992.1,2 As well as being of interest for technological applications, the regular, geometrically simple pore structure of MCM-41 (an ordered, hexagonal array of parallel pores with approximately constant cross section) makes the material useful as a model adsorbent to test our understanding of adsorption at the molecular level and to evaluate methods for the prediction of multicomponent adsorption equilibrium. In an earlier paper,3 we reported a study of adsorption heterogeneity in MCM-41, in which we used three different atomistic models to predict adsorption in MCM-41 by Monte Carlo simulation. We compared the simulation predictions with experimental adsorption isotherms for ethane, CO2, and a mixture of these species on three pure-silica MCM-41 materials with different pore diameters, and found that a model with a rough, amorphous surface was required to accurately predict the experimental data.3 Figure 1 shows this model, which is generated by randomly placing the oxygen atoms in the matrix, subject to constraints reflecting the coordination of oxygen atoms in the real material, and then carrying out an energy minimization. The difference between surface hydroxyl groups and oxygen atoms is ignored in the potential calculation because the hydrogen atoms have a small effect on adsorption.3 The isosteric heat of adsorption is known to be a highly sensitive probe of adsorption heterogeneity (and hence of surface structure).4 In this paper we continue our investigation of adsorption heterogeneitysreflecting in turn the energetic heterogeneity of the adsorbent3sby a calorimetric investigation of adsorption in the same three MCM-41 samples we used in the earlier study, combined with Monte Carlo simulations (using the same atomistic model, shown in Figure 1). * To whom correspondence should be addressed. E-mail: n.seaton@ ed.ac.uk. † Present address: Department of Chemical Engineering, University of Bath, Bath BA2 7AY, U.K. (1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (2) 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. (3) He, Y. F.; Seaton, N. A. Langmuir 2003, 19, 10132. (4) Dunne, J. A.; Mariwala, R.; Rao, M.; Sircar, S.; Gorte, R. J.; Myers, A. L. Langmuir 1996, 12, 5888.
Figure 1. Atomistic model for MCM-41: red, oxygen; blue, silicon.
2. Experimental Section 2.1. Materials. Three pure-silica MCM-41 samples with different pore diameters were supplied by Chonnam National University, South Korea. The samples are designated M41Cn, where n is the number of carbon atoms in the alkyl chain of the surfactant molecules in the micellar template used to produce the materials; we have studied M41C14, M41C16, and M41C22. Standard nitrogen adsorption measurements were carried out at the nitrogen normal boiling temperature, 77.4 K, using a Micromeritics ASAP 2010 apparatus. The nitrogen adsorption isotherms of the three samples are shown in Figure 2. (Desorption isotherms were also measured; these coincided with the adsorption isotherms.) The rapid increase due to capillary condensation that is observed in each case suggests that these three MCM-41 samples have very narrow pore size distributions. An increase in the pore size leads to a shift in this condensation to higher relative pressures. The specific surface area and the average pore size were calculated from the nitrogen adsorption data by standard methodssBET and BJH, respectively; the results are shown in Table 1. The BET surface areas are all about 1000 m2/g for the three samples, a typical value for MCM-41. A standard comparative adsorption method, the Rs plot,5 was applied to study the mesoporosity of the MCM-41 samples. In this method, the amount adsorbed on the porous solid of interest is plotted as a function of the amount adsorbed on a reference solid. Usually, a macroporous reference adsorbent which has surface properties similar to those of the adsorbents under study is chosen. Macroporous silicas have been successfully used as a reference in the characterization of novel PMSs (see ref 5 and references therein). We used (5) Jaroniec, M.; Kruk, M.; Olivier, J. P. Langmuir 1999, 15, 5410.
10.1021/la052237k CCC: $33.50 © 2006 American Chemical Society Published on Web 12/22/2005
Adsorption Heterogeneity for CH4, C2H6, and CO2
Langmuir, Vol. 22, No. 3, 2006 1151
Figure 2. Adsorption isotherms of nitrogen in three MCM-41 samples at 77.4 K.
Figure 4. Adsorption isotherms of pure methane, ethane, and CO2 in M41C14 at 298 K. Lines are fits to eq 2. Table 2. Results from the rs Plot Analysis sample
total surface area, m2/g
external surface area, m2/g
mesopore vol, cm3/g
pore diam, Å
M41C14 M41C16 M41C22
984 1060 945
29 49 100
0.729 0.833 0.930
30.6 32.9 44.0
mesopore volume. We assume the pores to be approximately cylindrical. The pore diameter is then given by D ) 4V/S
Figure 3. Rs plot of nitrogen adsorption in M41C14 at 77.4 K. Table 1. Properties of MCM-41, Obtained by the Analysis of Nitrogen Adsorption sample
BET surface area, m2/g
pore diam, Å
pore vol, cm3/g
M41C14 M41C16 M41C22
1014 1047 947
27.0 30.1 38.9
0.88 1.01 1.19
nitrogen adsorption data on LiChrospher Si-1000 silica,5 since this material is known to be an essentially amorphous material. Figure 3 shows, as an example, the Rs plot for nitrogen adsorption in M41C14 at 77.4 K. In the case of mesoporous materials such as MCM-41, the initial part (region A) of the comparative Rs plot indicates the existence of surface roughness, which can be alternatively thought of as very fine micropores in the adsorbent (similar to the molecules in size). A high degree of linearity shows the absence of larger micropores; this was so for all three samples. Then the plot shows an upward deviation from linearity in the capillary condensation region and levels off before showing another linear section (region B) when the mesopores are filled with the adsorbate.5,6 The slope of the first linear part (region A) is used to calculate the total surface area, and the slope of the second linear part (region B) gives the external surface area. The difference between the total surface area and the external surface area is the surface area of the mesopores. The intercept of the second linear part indicates the (6) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity; Academic Press: London, 1982.
(1)
where D is the diameter of the pore and V and S are the mesopore volume and the surface area of the mesopores, respectively. The results of the total and external surface areas, the mesopore volume, and the estimated pore diameter, obtained from the Rs plot, are given in Table 2. The pore diameter obtained from the Rs plot is bigger than the results from N2 adsorption at 77 K by the BJH method (shown in Table 1), which always underestimates the pore diameter of MCM-41 samples.7 The MCM-41 samples were produced from the original powder using the method described in ref 3. The methane, ethane, and CO2 gases were supplied by BOC with a purity of 99.99%. The gases were dried with 5A molecular sieves packed in cylinders before they enter the adsorption system. 2.2. Apparatus. A multicomponent adsorption calorimeter of the Tian-Calvet type, constructed in our laboratory8 following the design of Dunne et al.,4 was used to measure the adsorption isotherms and the isosteric heats of adsorption of pure methane, ethane, and CO2 in the three MCM-41 samples at 298 K and at pressures up to 100 kPa. The same apparatus was used to measure the adsorption of binary methane/CO2 mixtures in three MCM-41 samples. More details of the description of the apparatus and operating procedures are given by He et al.8
3. Results and Discussion 3.1. Pure-Gas Isotherms and Isosteric Heats. Figure 4 shows results, from two different runs, for the experimental pure-gas adsorption isotherms for M41C14. The fit of the isotherms to the Toth equation
n)
mP (b + pt)1/t
(2)
(7) Ravikovitch, P. I.; Wei, D.; Chueh, W. T.; Haller, G. L.; Neimark, A. V. J. Phys. Chem. B 1997, 101, 3671.
1152 Langmuir, Vol. 22, No. 3, 2006
He and Seaton
Figure 5. Calorimetric isosteric heat of adsorption of pure ethane, CO2, and methane in M41C14 at 298 K. Lines are fits to eq 3.
Figure 7. Adsorption isotherms of pure ethane in the three MCM41 samples at 298 K. Lines are fits to eq 2.
Figure 6. Adsorption isotherms of pure methane in the three MCM41 samples at 298 K. Lines are fits to eq 2.
Figure 8. Adsorption isotherms of pure ethane in the three MCM41 samples at 264.6 K.
are also shown. Similarly, Figure 5 shows the results for the isosteric heats for pure methane, ethane, and CO2 in this material. The isosteric heat is fitted with the following equation of the virial type:
in pore size between the first pair. The same trend is seen for ethane adsorption, shown in Figure 7. Although the adsorption of ethane in M41C14 is nearly the same as that in M41C16 at this temperature, at lower temperatures the adsorption of ethane in M41C14 is significantly greater than that in M41C16. This was confirmed by measuring adsorption at a lower temperature (264.6 K) in a separate adsorption apparatus3 (the calorimeter being restricted to temperatures of not less than 298 K); the data are shown in Figure 8. Figure 9 shows the adsorption isotherms of pure CO2 in the three MCM-41 samples. In contrast to the case of methane and ethane, the CO2 isotherms in the three samples are almost identical. This suggests the adsorption mechanism of methane and ethane is different from that of CO2 in these materials. The calorimetric isosteric heats, shown in Figures 10-12, confirm this difference. The isosteric heats of adsorption of methane and ethane in MCM-41 decrease as the pore size of the sample increases, while the isosteric heat of adsorption of CO2 in MCM41 is almost independent of the pore size. The behavior of the isosteric heat as a function of loading also differs between the adsorptives. The isosteric heat of methane and ethane shows no clear trend for any of the samples, while the heat for CO2 decreases with loading. We conclude that the adsorption of methane and ethane in these materials is essentially homogeneous, with an isosteric heat that is independent of loading,
qst ) R(k1 + b1n + c1n2 + d1n3)
(3)
The experimental error is less than 0.2% and 1.5% for the pure-gas and binary isotherm data, respectively. The error in the isosteric heat is 3% for ethane and CO2 and 5% for methane. (The larger relative error for methane is due to the smaller absolute value, and thus the larger noise.8) Both the amount adsorbed and the isosteric heat are reproducible from run to run. Experimental results for the adsorption of the three pure gases in the three MCM-41 materials are shown in Figures 6-12. In all cases, the data from two different runs for each adsorption system are combined. Figure 6 shows the pure-methane adsorption isotherms of the three samples. The adsorption of methane in MCM-41 decreases as the pore size of the material increases. The decrease in adsorption between M41C14 and M41C16 is less than that between M41C16 and M41C22, reflecting the smaller difference (8) He, Y. F.; Yun, J.-H.; Seaton, N. A. Langmuir 2004, 20, 6668.
Adsorption Heterogeneity for CH4, C2H6, and CO2
Langmuir, Vol. 22, No. 3, 2006 1153
Figure 9. Adsorption isotherms of pure CO2 in the three MCM-41 samples at 298 K. Lines are fits to eq 2.
Figure 12. Calorimetric isosteric heat of adsorption of pure CO2 in the three MCM-41 samples at 298 K. Lines are fits to eq 3.
Figure 10. Calorimetric isosteric heat of adsorption of pure methane in the three MCM-41 samples at 298 K. Lines are fits to eq 3.
heterogeneous, with the higher energy sites being occupied first, giving a reduction in the isosteric heat with loading but, because of the dominance of surface heterogeneity, little effect of the pore size on either the isosteric heat or the amount adsorbed. This observation supports evidence from our earlier study of the adsorption of pure CO2 in this material,3 in which we used molecular simulation to analyze adsorption isotherms and found evidence of significant surface heterogeneity. In our model for MCM-41, shown in Figure 1, the interfacial region (in which the density of silica is less than the bulk value) is about 3 Å in depth.3 Since the CO2 molecule has a smaller cross section than the others, and has a strong quadrupole moment (which interacts with the charges on the atoms in the adsorbent), it can penetrate more easily into the MCM-41. So it is not the shape of the pores that dominates the adsorption of CO2 in MCM41 at low pressures, but the detailed structure of the interfacial region of the pore wall. In other words, CO2 adsorption is a more sensitive probe of the surface heterogeneity of MCM-41 than methane and ethane. We carried out Monte Carlo simulations of the adsorption of ethane in M41C14, evaluating the isosteric heat using the method of Vuong and Monson.9,10 The excess isosteric heat was obtained using the method given in ref 9. In Figure 13, the simulation results are compared with experimental data obtained by two routes: calorimetrically and using the Clausius-Clapeyron equation to analyze adsorption isotherm data. (Note that two slightly different temperatures are used: 273 K for the simulation and the Clausius-Clapeyron equation using the data from ref 3 and 298 K for the calorimetric data.) The three isosteric heats are broadly in agreement, with the Clausius-Clapeyron equation and the simulation methods suggesting that the isosteric heat of ethane in MCM-41 decreases with loading at very low pressures, suggesting a small degree of surface heterogeneity in ethane adsorption which was not evident in the calorimetric results. 3.2. Binary Isotherms and Isosteric Heats. The selectivity of component A over component B is defined by
Figure 11. Calorimetric isosteric heat of adsorption of pure ethane in the three MCM-41 samples at 298 K. Lines are fits to eq 3.
but which decreases as the pore size increases, reflecting the reduction in the adsorption potential as the pore walls become further apart, giving a clear reduction in the amount adsorbed as the pore size increases. In contrast, the adsorption of CO2 is
SA,B )
xA/xB yA/yB
(4)
where x and y refer to the adsorbed- and gas-phase mole fractions, respectively. Since the selectivity contains the ratio of the (9) He, Y. F.; Seaton, N. A. Langmuir 2005, 21, 8297. (10) Vuong, T.; Monson, P. A. Langmuir 1996, 12, 5425.
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Figure 13. Isosteric heats of ethane in M41C14 obtained by the three different methods.
He and Seaton
Figure 15. Selectivity of CO2 over methane in M41C14 at 298 K: experimental data and IAST predictions. Table 3. Calorimetric Data for Binary Methane (1)/CO2 (2) in M41C14 at 298 K: Experimental Data
Figure 14. Amount adsorbed for binary methane/CO2 mixtures in M41C14 at 298 K, together with the IAST predictions.
adsorbed-phase mole fractions, it is a very sensitive measure of the accuracy of the prediction of the adsorbed-phase composition. We used ideal adsorption solution theory (IAST)11 to predict binary adsorption variables: the amounts adsorbed of each component, the selectivity, and the isosteric heats of each component. The inputs to the IAST predictions are the purecomponent isotherms and isosteric heats. IAST is based on the assumption that the adsorbed mixture is an ideal solution at constant spreading pressure and temperature. Deviations from IAST might result from the chemical dissimilarity of the adsorptive or from the heterogeneity of the adsorbent.12 The principles of IAST and calculation processes for the predictions for the binary adsorption equilibrium can be found in refs 11, 13, and 14. Figures 14 and 15 show experimental data for the amount adsorbed from binary methane/CO2 mixtures, and SCO2,methane in M41C14 at 298 K, together with the IAST predictions. Note that the gas-phase composition is not fixed in the experiment, but rather varies gradually from point to point as the experiment proceeds from zero loading;13 the methane gas-phase mole fraction (11) Myers, A. L.; Prausnitz, J. M. AIChE J. 1965, 11, 121. (12) Davies, G. M.; Seaton, N. A. Langmuir 1999, 15 (19), 6263. (13) Dunne, J. A.; Mariwala, R.; Sircar, S.; Gorte, R. J.; Myers, A. L. Langmuir 1997, 13, 4333. (14) Karavias, F.; Myers, A. L Langmuir 1991, 7, 3118.
P, kPa
Ntotal, mmol/g
y1
x1
14.01 21.02 35.42 43.15 59.48 68.14 82.87 90.90 105.70
0.020 0.081 0.101 0.159 0.181 0.238 0.258 0.308 0.327
1.000 0.695 0.814 0.668 0.759 0.655 0.728 0.651 0.708
Run 1 1.000 0.207 0.377 0.240 0.331 0.265 0.301 0.275 0.302
11.46 18.15 33.35 40.92 54.82 62.50 76.45 82.50 97.95
0.017 0.076 0.097 0.154 0.173 0.226 0.244 0.283 0.303
1.000 0.667 0.810 0.663 0.746 0.652 0.723 0.658 0.721
Run 2 1.000 0.173 0.373 0.229 0.316 0.245 0.287 0.268 0.299
qst,2, kJ/mol
qst,1, kJ/mol
S2,1
14.7 14.7 15.4 15.4 9.50 9.50 10.5 10.5 12.6
25.1 25.1 21.5 21.5 21.2 21.2 23.5 23.5
8.72 7.24 6.38 6.38 5.27 6.22 4.92 5.60
13.3 13.3 unavailable unavailable 13.6 13.6 unavailable unavailable 10.9
23.2 23.2 22.3 22.3 20.5 20.5 20.3 20.3
9.55 7.18 6.62 6.36 5.76 6.49 5.26 6.06
varies from 0.65 to 0.81 in Figures 14 and 15. Tables 3 and 4 show the experimental and IAST results for binary methane/ CO2 isosteric heat at 298 K, respectively. IAST gives accurate predictions of the amount adsorbed of methane and CO2 and the total amount adsorbed across the gasphase composition and pressure range. The IAST predictions of SCO2,methane are in reasonable agreement with the experimental data over most of the pressure range, but nevertheless show a different trend. The experimental selectivity decreases with pressure, up to about 60 kPa, while the selectivity predicted with IAST is almost constant. This shows that the methane/CO2 mixture is actually somewhat nonideal in MCM-41. As adsorption heterogeneity manifests itself in deviations from IAST,11,15 this is further evidence of the heterogeneity of MCM-41 with respect to adsorption of CO2. Figure 16 shows the individual heats of adsorption in the mixture (that is, partial molar enthalpies of adsorption of the individual components)13 of methane and CO2 in binary methane/ CO2 mixtures in M41C14 at 298 K. As for the case of pure methane, the mixture heat of adsorption for methane shows no (15) Yun, J. H.; Du¨ren, T.; Keil, F. J.; Seaton, N. A. Langmuir 2002, 18, 2693.
Adsorption Heterogeneity for CH4, C2H6, and CO2
Langmuir, Vol. 22, No. 3, 2006 1155
Table 4. Calorimetric Data for Binary Methane (1)/CO2 (2) in M41C14 at 298 K: IAST Results P, kPa
Ntotal, mmol/g
y1
x1
qst,2, kJ/mol
qst,1, kJ/mol
S2,1
21.02 35.42 43.15 59.48 68.14 82.87 90.90 105.70
0.080 0.101 0.163 0.183 0.249 0.260 0.321 0.330
0.695 0.814 0.668 0.759 0.655 0.728 0.651 0.708
Run 1 0.268 0.414 0.247 0.339 0.236 0.304 0.233 0.283
13.8 14.1 14.0 14.0 13.2 13.0 11.9 11.6
25.2 25.2 24.8 24.7 23.9 23.8 22.9 22.8
6.23 6.20 6.15 6.14 6.13 6.13 6.14 6.14
18.15 33.35 40.92 54.82 62.50 76.45 82.50 97.95
0.074 0.097 0.157 0.175 0.233 0.245 0.292 0.303
0.667 0.810 0.663 0.746 0.652 0.723 0.658 0.721
Run 2 0.243 0.407 0.242 0.323 0.234 0.299 0.239 0.296
13.8 14.1 14.1 14.0 13.5 13.3 12.5 12.2
25.2 25.2 24.8 24.7 24.1 23.9 23.4 23.1
6.25 6.21 6.15 6.14 6.13 6.13 6.14 6.14
trend with pressure; IAST and experiment are consistent to within experimental error. For CO2, the effect of pressure is also inconclusive, although the IAST predictions repeat the gradual decrease seen in the pure-component data. There is a small but significant error between the experimental heats and the IAST predictions over most of the pressure range for CO2, providing further evidence that the system is nonideal.
4. Conclusions The surface heterogeneity of MCM-41 was studied by the experimental measurement of the adsorption isotherms and isosteric heats of pure methane, ethane, and CO2 and a mixture of methane and CO2 in MCM-41 using a Tian-Calvet microcalorimeter, combined with Monte Carlo simulation of adsorption using an amorphous pore model which was previously shown to give a good representation of the extent of adsorption. The adsorption of the pure gases showed the adsorption of methane to be highly homogeneous, and the adsorption of ethane to be only very slightly heterogeneous. For CO2, in contrast, the adsorption was highly heterogeneous, which we believe to be
Figure 16. Individual heat of adsorption for binary methane/CO2 in M41C14 at 298 K: experimental data and IAST predictions.
due to the electrostatic interactions between the CO2 and the surface atoms. Adsorption heterogeneity was also probed by studying the adsorption of a mixture of methane and CO2. We found that the adsorption of this binary mixture showed a significant deviation from ideal solution behavior (and in particular a different trend with pressure), which is further evidence of adsorption heterogeneity. A general finding from this work is that the polarity of the adsorptive has a very big effect on the degree of heterogeneity experienced by the adsorptive molecules in MCM-41. Acknowledgment. We are grateful to Prof. G. Seo (Chonnam National University, Korea) for providing MCM-41 samples. The financial support of the U.K. Engineering and Physical Sciences Research Council and an ORS reward from Universities UK are gratefully acknowledged. LA052237K
