Langmuir 1998, 14, 2415-2425
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Adsorption of Gases on Carbon Molecular Sieves Used for Air Separation. Spherical Adsorptives as Probes for Kinetic Selectivity C. R. Reid, I. P. O’koye, and K. M. Thomas* Northern Carbon Research Laboratories, Department of Chemistry, Bedson Building, University of Newcastle upon Tyne, Newcastle upon Tyne NE1 7RU, U.K. Received August 18, 1997. In Final Form: January 29, 1998 The adsorption of oxygen, nitrogen, and a series of noble gases (neon, argon, and krypton) on a carbon molecular sieve were studied over a range of temperatures above the critical temperature of the adsorptives as a function of pressure in order to understand further the mechanism of air separation. The noble gases were used as probes for the selective porosity in the carbon molecular sieve. The uptakes of all gases studied were virtually linear at low equilibrium pressures in agreement with Henry’s law, but deviation occurred at higher pressures. The isosteric enthalpies of adsorption were calculated from the variation in the Henry’s law constant with temperature. The adsorption kinetics were studied with different amounts of preadsorbed gas for pressure increments in the range 1-100 kPa. The adsorption kinetics obey a linear driving force mass transfer model for oxygen, nitrogen, argon, and krypton for the experimental conditions studied. The adsorption kinetics for neon deviate from this model, and the data fit a kinetic model which combines diffusion and barrier resistance characteristics. The ratios of the rate constants (k(O2)/k(N2)) for each pressure increment in the pressure range 0-9 kPa over the temperature range 303-313 K were typically 25, and this clearly demonstrates the molecular sieving characteristics. The activation energies for the adsorption process were in the order krypton > argon > nitrogen > oxygen ∼ neon. The results are discussed in terms of the mechanism of gas separation using carbon molecular sieves.
Introduction The use of carbon molecular sieves in the separation and purification of mixtures of gases is of considerable interest in the chemical and petrochemical industries. A wide range of commercial carbon molecular sieves (CMS) have been manufactured from a variety of precursors, for example, coal, coconut shell, polymers, and biomass materials.1,2 The pore size distribution of the CMS is controlled by varying the precursor, the temperature of carbonization, the activation procedure, the pore blocking method, and passivation techniques. These carbon molecular sieves have pores of molecular dimensions which give rise to selective adsorption characteristics, and the materials are used widely for gas separation3 and storage4 applications. A typical application is the industrial separation of air into oxygen and nitrogen by pressure swing adsorption (PSA). The PSA technique is based on the difference between the kinetics of adsorption of oxygen and nitrogen with oxygen adsorption being much faster than nitrogen adsorption. The equilibrium adsorption capacities of this class of molecular sieve for oxygen and nitrogen adsorption are very similar. When a carbon sample with molecular sieving characteristics is contacted with air, an oxygen enriched adsorbed phase and a corresponding nitrogen rich gas phase are produced initially while the adsorption capacities of oxygen and nitrogen are very similar at equilibrium. The kinetic selectivity of the CMS is usually produced by carbon deposition on a microporous substrate. This selectivity is gradually reduced by particle size reduction due to the production of nonselective pathways.5,6 Hence, only part of the porous structure has kinetic selectivity character* To whom correspondence should be addressed. (1) Metcalf, J. E.; Kawahata, M.; Walker, P. L. Fuel 1963, 42, 233. (2) Moore, S. V.; Trimm, D. L. Carbon 1977, 15, 177. (3) Sircar, S.; Golden, T. C.; Rao, M. B. Carbon 1996, 34, 1. (4) Verma, S. K.; Walker, P. L. Carbon 1992, 30, 837.
istics. The structure and characteristics of this selective part of the porosity are not well-understood. The adsorption capacities of microporous materials for gases and vapors are related to a number of factors, including the properties of the adsorptive (molecular size, shape, and electronic configuration), the experimental conditions (temperature and pressure), the surface structure, pore structure, and pore size distribution of the microporous material. The comparative dimensions of the adsorptive and adsorbent pore size distribution may lead to the exclusion of some adsorptives from parts of the porous structure, thereby leading to molecular sieving effects under equilibrium conditions. This has led to the use of probe molecules for the determination of micropore size distributions from the amounts and dimensions of the molecules which are adsorbed, but this method has limitations.7-9 It has been proposed that this difference in adsorption kinetics for oxygen and nitrogen on carbon molecular sieves is related to molecular size.6 The kinetic diameter of oxygen (0.346 nm) is slightly smaller than that of nitrogen (0.364 nm). The reason such a relatively small difference in molecular dimensions should have such a large effect on the rate of adsorption is unclear. The present study involved an investigation of the kinetics of oxygen and nitrogen adsorption on a carbon molecular sieve with various amounts of preadsorbed gas for a series of pressure steps. A series of noble gases were selected as probes for the selective porosity responsible for the (5) Chagger, H. K.; Ndaji, F. E.; Sykes, M. L.; Thomas, K. M. Carbon 1995, 33, 1411. (6) Armor, J. N. In Separation Technology; Vansat, E. F., Ed.; Elsevier Science B. V.: Amsterdam, 1994; p 163. (7) Ainscough, A. N.; Dollimore, D. Langmuir 1987, 3, 708. (8) Braymer, T. A.; Coe, C. G.; Farris, T. S.; Gaffney, T. R.; Schork, J. M.; Armor, J. N. Carbon, 1994, 32, 445. (9) Moyer, J. D.; Gaffney, T. R.; Armor, J. N.; Coe, C. G. Microporous Mater. 1994, 2, 229.
S0743-7463(97)00929-3 CCC: $15.00 © 1998 American Chemical Society Published on Web 04/09/1998
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molecular sieving characteristics in the CMS. The adsorption kinetics were studied as a function of temperature and pressure and the activation energies and preexponential factors for adsorption determined as a function of atom size. These gases were chosen because their spherical symmetry allows the size of the selective porosity to be probed unambiguously without the complication of shape factors. Experimental Section Materials Used. The commercial carbon molecular sieve (CMS) used in the present study was supplied by Air Products and Chemicals Inc. The CMS was prepared by carbon deposition on a microporous substrate. The gases used were supplied by BOC Ltd. and had the following purities: nitrogen, 99.999%; oxygen, 99.999%; neon, 99.99%; argon, 99.999%; and krypton, 99.999%. Measurement of Adsorption Kinetics. The kinetic measurements were carried out using the Intelligent Gravimetric Analyzer (IGA) supplied by Hiden Analytical Ltd. The IGA instrument allows the adsorption-desorption isotherms and the corresponding kinetics of adsorption or desorption at each pressure step to be determined.10 The system consists of a fully computerized microbalance which automatically measures the weight of the carbon sample as a function of time with the gas pressure and sample temperature under computer control. The microbalance had a long-term stability of (1 µg with a weighing resolution of 0.2 µg. The pressure was monitored by two pressure transducers with ranges of 0-10 kPa and 0-1 MPa. The accuracy of the set-point regulation was (0.02% of the range used. The sample temperature was measured at ∼5 mm from the sample and was controlled to (0.05 K. The carbon sample (∼0.5 g) was outgassed to a constant weight at 383 K and 10-5 Pa prior to measurement of the isotherms. The initial pressure increment from high vacuum ( neon > krypton. It is interesting that the kinetics of neon adsorption are relatively slow compared with argon adsorption and contrary to the order based on atomic size. Also, the adsorption kinetics for neon adsorption deviate from the linear driving force mass transfer model, and this will be discussed later. The activation energies for each pressure step are given in Tables 2-6. The data for argon adsorption on the CMS indicate that the activation energy decreases from ∼48 to 36 kJ mol-1 over the pressure range 0-900 kPa. This contrasts with oxygen adsorption where no significant change in activation energy was (14) Dacey, J. R.; Thomas, D. G. Trans. Faraday Soc. 1954, 50, 740.
Adsorption Isotherms. Most adsorption studies in the literature have been carried out at temperatures well below the gas critical temperature where the amounts adsorbed are much higher, and the pressures under which adsorption takes place can be compared relative to the saturated vapor pressures. Above the critical temperature, theoretical interpretation of the results is possible in terms of nonideal gas theory but comparisons of various gases are difficult. At low surface coverages the adsorption obeys the equation n ) K0p, where n is the amount adsorbed per unit weight of adsorbent at equilibrium pressure p and K0 is Henry’s law constant. Nicholson and Sing15 analyzed adsorption data using a virial equation and reported that at very low pressures virial expansions reduce to Henry’s law. The virial equation can be written in two forms:15
n/p ) K0 + K1p + K2p2 + ...
(1)
ln(n/p)) A0 + A1n + A2n2 + ...
(2)
where n is the amount adsorbed at pressure p, and the first coefficient of the two equations are related by
K0 ) exp(A0) ) Henry’s law constant
(3)
K0 (Henry’s law constant) is totally dependent on the interaction between the adsorbent surfaces and the adsorbed gas molecules. In this paper the Henry’s law constants were calculated by two methods: (1) from the gradient of the gas uptake (n) versus pressure (p) graphs over the pressure range 0-9 kPa for oxygen, nitrogen, and argon and (2) from extrapolation of the virial graphs of ln(n/p) versus n excluding the low-pressure data for oxygen, argon, and neon. In the case of the former the pressure range was chosen to ensure that Henry’s law was obeyed, while in the case of the latter the errors in the virial graphs at low (15) Nicholson, D.; Sing, K. S. W. In Colloid Science; Everett, D. H., Ed.; Chemical Society: London, 1979; Vol. 3, pp 1-62.
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Table 2. Kinetic Data (k/(10-4 s-1) for Nitrogen Adsorption on CMSA (Temperature Range, 303-343 K; Pressure Range, 0-9 kPa)a value for given pressure increment temp/K
a
0.005-2 kPa
2-4 kPa
4-6 kPa
6-8 kPa
8-9 kPa
303 308 313 318 323 328 333 338 343
4.01 ( 0.22 6.20 ( 0.30 6.79 ( 0.33 8.26 ( 0.15 11.83 ( 0.49 14.16 ( 0.29 19.49 ( 0.99 23.43 ( 0.22
3.07 ( 0.14 4.61 ( 0.21 6.03 ( 0.18 6.96 ( 0.19 8.52 ( 0.14 11.40 ( 0.23 14.52 ( 0.19 18.47 ( 0.18 22.36 ( 0.20
3.16 ( 0.15 4.08 ( 0.14 5.36 ( 0.13 6.76 ( 0.12 8.17 ( 0.11 10.14 ( 0.23 13.94 ( 0.18 18.71 ( 0.32 22.36 ( 0.29
3.15 ( 0.11 3.85 ( 0.12 5.21 ( 0.10 6.82 ( 0.10 8.04 ( 0.13 10.59 ( 0.22 14.18 ( 0.36 18.53 ( 0.33 22.68 ( 0.34
3.05 ( 0.15 3.39 ( 0.09 4.95 ( 0.12 6.53 ( 0.17 7.72 ( 0.15 10.21 ( 0.30 13.79 ( 0.43 18.84 ( 0.53 23.65 ( 0.44
ln A Ea/(kJ mol-1)
9.17 ( 0.75 43.44 ( 2.02
Arrhenius Graph Data 8.54 ( 0.49 8.84 ( 0.43 41.72 ( 1.30 42.66 ( 1.16
9.19 ( 0.36 44.05 ( 1.10
10.02 ( 0.57 45.93 ( 1.52
Activation energy, Ea ) 43.54 ( 0.98 kJ mol-1.
Figure 7. Variation of ln(1 - wt/we) against time for the adsorption of neon at 313 K for pressure increments 0-50 kPa (O) and 200-300 kPa (4) (s, calculated from barrier resistance/ diffusion model22).
pressure were greater and therefore not used in the extrapolation. The maximum gas adsorption in the pressure range was typically argon > neon. This same order was observed for the isosteric enthalpy of adsorption for theses gases on graphitized carbon blacks16 and microporous carbons at low surface coverage.12,17,18 The order for rates of surface diffusion is expected to be the reverse, with neon being the fastest. In both cases of diffusion through a size selective barrier and surface diffusion along a pore, the adsorption kinetics are expected to be in the same order: neon > argon > krypton. The rate constants for adsorption determined from the linear driving force mass transfer model and kb from the barrier resistance/diffusion model are in the order argon > neon . krypton when compared on the same basis, with those for argon and neon being similar. Comparison of the activation energies at low surface coverage shows the following order: krypton > argon > neon, although the values for krypton and argon do not differ markedly. The latter order is in agreement with the theoretical predictions for diffusion through a size selective barrier and surface diffusion. However, there is a variation in activation energy calculated for each pressure step with the pressure, and this is clearly shown for the argon adsorption data. Therefore the activation energies for specific surface coverages were calculated, and the results are shown in Table 8. It is apparent that the activation energies are virtually constant for argon and oxygen adsorption on the CMS when calculated on this basis and similar to the activation energies obtained in the Henry’s law region. However, surface coverage is an equilibrium property, whereas activation energy is a kinetic property. The former measures the average loading over all pore (30) Everett, D. H. Trans. Faraday Soc. 1950, 46, 453.
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Table 9. Molecular and Atomic Dimensions (pm) of Adsorptive Molecules Determined by Various Methods and Calculated Critical Pore Dimensions (pm)a LennardJones (1σ) helium neon argon oxygen nitrogen krypton
van der Waals
calcd crit pore dimens after Rao et al.25,26 508
275 340 346 364 360
280 300
575 544 572
a A more extensive list of molecular sizes derived by a number of methods is given in ref 6.
sizes at a given pressure, while the latter measures the average energy barrier for diffusion through pore apertures. Therefore a correlation between surface coverage and activation energy is not necessary. In the case of the adsorption of oxygen, nitrogen, argon, and krypton the activation energies (Ea) for the kinetic process are much higher than the corresponding isosteric enthalpies of adsorption (∆Hi). In contrast, in the case of neon the situation is more complex. Comparisons of the activation energies (see Table 8) calculated from both the barrier resistance (kb) and diffusion (kd) rate constants derived from fitting the experimental data to the barrier resistance/diffusion model with the ∆Hi values shows that the differences are much smaller. The slow rates of adsorption for neon compared with argon are accompanied by a higher isosteric heat of adsorption and a change in the kinetic mechanism. Hence, it is unlikely that activated diffusion is the only factor in the case of neon adsorption. The adsorption uptakes of neon on the CMS at a given temperature and pressure are one-fifth that of argon on a mole basis under the same conditions. Therefore while the adsorption of neon is lower, there is sufficient overlap with the argon adsorption data for comparison purposes. The adsorption capacities of neon on the carbon molecular sieve for the 0-50 kPa pressure step over the temperature range 313-343 K are similar to that observed for argon adsorption in the pressure range 0-9 kPa and temperature range 318-343 K where Henry’s law behavior has been demonstrated for argon adsorption. The rate of adsorption of neon is relatively slow but begins to increase markedly above 200 kPa pressure, suggesting that molecular sieving effects are less significant at higher surface coverage. The results suggest that neon is more strongly adsorbed in the porous structure than either argon or krypton under the low surface coverage conditions used in this study and that this is possibly a factor influencing the adsorption kinetics. The most likely explanation is that the neon is able to access part of the heterogeneous ultra-microporous structure not available to argon due the much smaller size of neon (275 pm) compared with argon (340 pm) (see Table 9) and this influences the adsorption kinetics at low surface coverages. This is supported by kinetic data which show that the activation energy is ∼30-40 kJ mol-1 in the low surface coverage region. Adsorption initially occurs on the most energetic sites which are formed by the edge carbon atoms and by the ultra-microporosity. Hence, at very low surface coverage a higher isosteric heat of adsorption is expected, and this is observed for the adsorption of neon. Surface diffusivity also increases with increasing surface coverage. The consequence of the stronger interactions and the access of the smaller neon atoms to porous structure not accessible to the larger gas molecules/atoms is the slower adsorption kinetics. Comparison of the activation energies at specific surface coverages shows that they are in the order krypton >
argon > neon. The sizes of these probe molecules vary systematically, but estimates of the atomic sizes have been made by a variety of methods.7 The atomic and molecular sizes for the gases used in this study and the calculated critical pore dimensions are given in Table 9. Comparison of oxygen and nitrogen adsorption with the noble gases shows some discrepancies on the basis of molecular and atomic sizes alone. However, it is interesting to note that the rate constants of adsorption of krypton and argon differ by a factor of ∼50 but their activation energies only differ by ∼3 kJ mol-1 while the atomic sizes differ by 20 pm. A comparison of the sizes of the noble gases with oxygen and nitrogen (see Table 9) and the corresponding adsorption kinetic rate constants illustrate the problem of relating the size of the adsorptive to the adsorption kinetics. The sizes on the Lennard-Jones values are in the order nitrogen (364 pm) > krypton (360 pm) > oxygen (346 pm) > argon (340 pm) > neon (275 pm). In comparison calculated critical pore dimensions are as follows: oxygen (544 pm), nitrogen (572 pm), and argon (575 pm). The rate constants are in the relative order oxygen (∼×1250) > nitrogen (∼×60) > argon (∼×40) > neon (∼×25 (for kb)) > krypton (×1), based on adsorption at 313 K with the rate of adsorption estimated for krypton. The activation energies for adsorption are in the order krypton > argon > nitrogen > oxygen ∼ neon. The calculated critical pore dimensions27,28 (see Table 9) are consistent with the similarities in the above kinetic data for nitrogen and argon. In the case of the noble gases where the order of size is beyond dispute, the rate constants do not follow the expected trend on the basis of size, whereas the activation energies are in the expected order on the size basis. Furthermore nitrogen and oxygen have a shape factor which is probably detrimental to the adsorption kinetics since these molecules must pass through the selective porosity lengthwise and there will be loss of rotational freedom in the process. It is apparent from comparison of the adsorption of water vapor on a CMS and a nonselective microporous carbon that molecular sieving effects are not apparent in the former, suggesting that the selective porosity is slit shaped.13,31 Therefore it is reasonable to conclude that while the estimates of atomic and molecular size vary, other factors are also significant in determining the adsorption kinetics. It is possible that heterogeneity in the porous structure plays a role. When a comparison is made of the adsorption characteristics of probe molecules, the surface coverages and accessibility to the porous structure are found to vary, which may make comparisons difficult in extreme situations such as very low surface coverage. The pore size and pore volume appropriate to each probe molecule needs to be considered in relation to the pore size distribution of the adsorbent.32 However, in the situation where molecular sieve characteristics are predominant, the characteristics are difficult to quantify. Conclusions The adsorption characteristics of oxygen and nitrogen on the carbon molecular sieve material are substantially different. The rate constants for oxygen and nitrogen adsorption on the CMS differ by a factor of ∼×25, while the adsorption capacities are similar indicating a kinetic rather than thermodynamic effect. The rate constants of adsorption for oxygen and nitrogen increase slightly with (31) Foley, N. J.; Forshaw, P. L.; Thomas, K. M.; Stanton, D.; Norman, P. R. Langmuir 1997, 13, 2083. (32) Everett, D.H.; Powl, J. G. J. Chem. Soc., Faraday Trans. 1, 1976, 72, 619.
Gas Adsorption on Carbon Molecular Sieves
increasing amounts of initial preadsorbed gas for a given pressure increment. The adsorption kinetics for argon and krypton are similar to oxygen and nitrogen following a linear driving force mass transfer model. In contrast, the adsorption kinetics for neon follow a combined barrier resistance/diffusion model. Hence, it is apparent that the kinetic model for adsorption depends on the size of the adsorbate relative to the selective porosity in the CMS. The rate constants for the adsorption of the series of noble gases on the carbon molecular sieve shows the following order argon > neon . krypton, which is contrary to that expected on the basis of atomic size. However, the activation energies for adsorption are in the order krypton > argon > neon, which corresponds to the atomic size. It is apparent that the adsorption kinetics are determined
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by a number of factors including interaction of the adsorptive with the adsorbent surface and the relative size of the adsorptive in relation to the pore size distribution of the adsorbent, in particular, the selective part of the porous structure. Differences in the accessibility of molecules to the carbon porous structure and heterogeneity in the carbon surface may also be factors. The variation in the kinetic rate constants with pressure and surface coverage needs to be considered when the adsorption kinetics for a range of adsorptives is compared. It is apparent that comparisons of sizes of gas atoms and molecules are not a reliable guide to the adsorption kinetics on heterogeneous carbon molecular sieves. LA9709296
