1466
J. Phys. Chem. B 1998, 102, 1466-1473
Adsorption Equilibria of C1 to C4 Alkanes, CO2, and SF6 on Silicalite Matthew S. Sun, D. B. Shah, Heather H. Xu, and Orhan Talu* Chemical Engineering Department, CleVeland State UniVersity, CleVeland, Ohio 44115 ReceiVed: September 15, 1997; In Final Form: December 10, 1997
Adsorption isotherms of methane, ethane, propane, n-butane, isobutane, carbon dioxide, and sulfur hexafluoride were measured gravimetrically on silicalite crystals at temperatures ranging from 3 to 81 °C and at pressures up to 2000 kPa. A virial equation was used to correlate the experimental data and to calculate the isosteric heats of adsorption and the limiting heats of adsorption at zero loading. The adsorption isotherms of isobutane exhibit inflection points at loadings of 4-6 molecules/unit cell in a certain temperature range. This unusual adsorption behavior is attributed to adsorption of isobutane in different locations of the channel system.
Introduction The adsorption properties of MFI type zeolites, ZSM-5 and its aluminum-free-form silicalite, have been widely studied. The main channels of ZSM-5/silicalite are 10-member oxygen rings with a diameter of about 5.5 Å, which is close to the diameter of many hydrocarbon molecules. Recent studies1-3 have shown that adsorption isotherms of certain molecules on silicalite exhibit steps, a rather unusual behavior for zeolites. Silicalite is hydrophobic in nature, and its surface has been considered as homogeneous. However, the adsorption behavior also depends on the size of the channels. Different adsorbates may preferably adsorb on different sites in the silicalite structure, resulting in heterogeneous behavior even though cations do not exist in the pore structure of silicalite to cause energetic heterogeneity. Normal Alkane Adsorption on Silicalite. The unique adsorption behavior of hydrocarbons on silicalite has been observed since the time it was first synthesized. The adsorption of n-hexane in silicalite is very strong as reported in the pioneering paper by Flanigan et al.4 The isosteric heat of adsorption of n-hexane is 16-18 kcal/mol over a wide range of relative pressures. This value is substantially above the heat of evaporation of n-hexane (7.8 kcal/mol). Considering the volume, size, and geometry of the channels in the silicalite structure and the size of n-hexane molecules, it was shown that the molecules must be highly oriented in nearly linear strings. The fit of n-hexane in the channels is nearly perfect. Thus, n-hexane is in a low entropy and a highly ordered state in the silicalite lattice. The high affinity of silicalite for n-paraffins was also reported by Olson et al.5 Complete pore filling was achieved at about P/P0 ) 0.05 with n-paraffins. The volume adsorbed was nearly twice that of p-xylene, benzene, and 3-methylpentane. It was postulated that the larger molecules are apparently affected by the unfavorable packing in the channels. The high normal paraffin affinity was thought to result from the channel geometry that afforded very favorable paraffin-channel wall interactions. The isosteric heat of adsorption was found to be nearly constant with loading. This observation suggested a homogeneous behavior where paraffin molecule-zeolite interactions predominate over molecule-molecule interactions even at high loadings. * Corresponding author. Fax (216)687-9220, E-mail
[email protected].
In a recent paper by Olson and Reischman,3 the n-hexane and n-decane adsorption on ZSM-5 was studied using isobars. An inflection point was found in the adsorption of n-hexane, but not for n-decane. This inflection point corresponds to an adsorption capacity of n-hexane of 3.7 molecules/uc. This number is close to 4, a significant number for ZSM-5, which is equal to the number of intersections, the number of straight channel segments between intersections, and the number of zigzag channels in a unit cell. At low loadings, n-hexane is believed to be adsorbed in the zigzag channels. On the other hand, at high loadings, adsorption is governed by pore filling. Similar unusual adsorption behavior for normal hexane and heptane were also reported by Sun et al.2 Inflection points on the isotherms of n-hexane and n-heptane were reported at about 4-5 molecules/uc. Both the heats of adsorption and saturation capacities of normal hexane and heptane displayed unusual behavior outside the trend established by other n-alkanes. Unusual adsorption behavior was also found by van Well et al.1 and Olson et al.3 in the adsorption equilibrium study of normal alkanes in ZSM-5/silicalite. However, in another report, Richard and Rees6 showed that the isosteric heat of n-hexane adsorption in silicalite increases with loading without any unusual phenomena. The lateral interactions between the n-hexane molecules were offered as the reason for the increase in the heat of adsorption. Aromatic Adsorption on Silicalite. The structure of ZSM-5 was reported by Olson et al.5 in an early publication. With a channel size intermediate between small pores (8-ring) and larger pore (12-ring) zeolites, ZSM-5 possesses distinct sorption and diffusion properties. The adsorption isotherms of normal paraffins, m- and o-xylenes, 1,2,4-trimethylbenzene, and naphthalene on ZSM-5 were reported. These molecules have minimum diameters of ≈6.9 Å, which is larger than the nominal diameter of a 10-member oxygen ring. The indication of special packing arrangements for p-xylene was mentioned by Olson et al.5 At a constant relative pressure of 0.03 (P/P0 ) 0.03), the adsorbed phase changes from 4 to 6.5 molecules of p-xylene per unit cell of ZSM-5. The number 4 is again significant crystallographically and suggests an ordered packing of the p-xylene molecules. The higher density at 6.5 molecules/uc probably corresponds to pore filling rather than a stoichiometric or ordered p-xylene-ZSM-5 sorption complex.
S1089-5647(97)03019-8 CCC: $15.00 © 1998 American Chemical Society Published on Web 02/03/1998
C1-C4 Alkanes, CO2, and SF6 on Silicate This phenomenon was later extensively studied by Talu et al.7 and Guo et al.8 The shape of the isotherm of benzene and p-xylene adsorption on silicalite changed from type I to type IV as temperature was increased. With a further increase in temperature, the isotherm shape changed back to that of type I. The temperature range where these changes occurred was between 0 and 80 °C depending on the adsorbates. These changes were attributed to the tight fit of the molecules in the silicalite micropores. The steps in the isotherms were attributed to surface phase transition. The isosteric heats of adsorption for all the aromatics displayed a maximum at intermediate loadings. A two-patch model was proposed to explain the nature of the isotherm. Adsorption of Small Molecules on Silicalite. The nitrogen and carbon monoxide adsorption on two types of silicalite was studied by Zecchina et al.:9 a pure silicalite without Na and Al impurity, another with some Na and Al impurity. On both samples, the CO showed a step in the adsorption isotherms. The adsorption isotherm of N2 on the first sample was type I, while the isotherm was of type IV on the second sample. The adsorption isotherm of Ar in silicalite also showed steps as reported by Borghard et al.10 The step was attributed to different localization of Ar molecules at different values of P/P0. A total of 33 Ar molecules/uc is reported as the saturation capacity in silicalite. The authors placed 8 molecules in the zigzag channels, 8 in the straight channels, and another 8 at the channel intersections. These 24 molecules were claimed to have the same adsorption interaction energy. Additional adsorption of Ar molecules was attributed to the packing changes that occurred on further adsorption. However, in a report by Golden and Sircar,11 the adsorption isotherms of H2, CH4, N2, CO, Ar, CO2, and Kr were reported to be all type I isotherms without steps or inflection points. The isosteric heats of adsorption of these compounds were used to show that these systems were homogeneous. Adsorption of N2, O2, H2S, CO2, C3-C10 n-paraffins, isoC4, iso-C5, neopentane, monomethylnonane, and xylene isomers was reported by Jacobs et al.12 Again, a step change was found in the isotherm of N2. However, the adsorption isotherm of O2 was type I without steps. The adsorption of C3, C4, and C5 n-paraffins in the ZSM-5 was regarded by the authors as simple pore filling. For C6 to C8 n-paraffins, the authors claim that the molecules occupy the straight channels first and that the adsorption in the zigzag channel is less dense. Pore filling was proposed for the adsorption of n-C9 and n-C10 alkanes. In a recent study by Coulomb et al.,13 the N2 and CO adsorption in silicalite was shown to have two phase transitions or three different adsorbed phases. The adsorption of Ar, Kr, O2, and H2 has one phase transition or two adsorbed phases, and other small molecules such as CH4, C2H6, Xe, and SiF4 have no phase transition. Three different adsorbed phases of N2 on silicalite were proposed: a fluid, lattice fluid, and a solid phase. The first two phases were believed to be in the zigzag channels and intersections and the solid phase in the straight channels. The sorbed phase in silicalite was characterized by a great disorder whatever the loading regime as a consequence of the weakness of sorbate-sorbate interactions. At low and medium loadings, a mobile sorbed phase results in a dynamic disorder. At the high loading regime, the disorder becomes static. Phase Transition of ZSM-5/Silicalite Structure. It has also been shown that MFI zeolites can undergo temperature- and sorbate-induced symmetry transformations. Fyfe et al.14 showed
J. Phys. Chem. B, Vol. 102, No. 8, 1998 1467
Figure 1. Adsorption isotherms of methane on silicalite.
by MAS NMR and XRD investigation that sorption of some molecules including p-xylene causes symmetry changes and some reversible lattice distortion in highly siliceous ZSM-5 zeolites. In a relatively recent study, this work was extended by Kokotailo et al.15 to ZSM-5 having low Si/Al ratios. It again confirmed that room-temperature sorption of p-xylene promotes the monoclinic-to-orthorhombic crystal transition and changes lattice parameters. The lattice parameters and unit cell volume at ambient temperature were higher for the p-xylene-loaded sample than for the unloaded sample. The transformation of ZSM-5 structure caused by adsorption can be either permanent or reversible. For example, Karsli et al.16 reported that a ZSM-5 zeolite sample that underwent p-xylene adsorption/desorption cycle exhibited faster diffusion rates of m- and o-xylene than a fresh sample. The above literature summary clearly shows that the adsorption of different molecules on silicalite displays widely varying behavior. The adsorbates studied include small molecules such as H2 and N2 to large molecules such as aromatics. The adsorption systems displayed homogeneous to heterogeneous behavior depending on the adsorbates and conditions. The very tight fit of aromatics in silicalite micropores can explain the step changes in their adsorption isotherms. However, very small gas molecules, like H2, N2, and O2, also show step changes in their isotherms. In some cases, adsorption at different locations was used to explain this behavior. In others, phase transition in the adsorbed phase was offered as the cause. To further complicate the picture, structural transformation of silicalite by adsorption may also be the cause of these unusual adsorption systems. The objective of this work is to present our measurements of adsorption equilibrium data for C1 to C4 alkanes, CO2, and SF6 on silicalite. The adsorption equilibrium data are reported for a wide range of temperatures and pressures. In addition, the rather unusual adsorption behavior of different molecules on silicalite is explained based on our measurements and literature data.
1468 J. Phys. Chem. B, Vol. 102, No. 8, 1998
Sun et al.
TABLE 1: Adsorption Isotherm Data T ) 3.8 °C
T ) 34.8 °C
T ) 79.6 °C
T ) 3.8 °C
P (kPa)
N (mol/kg)
P (kPa)
N (mol/kg)
P (kPa)
N (mol/kg)
41.35 82.69 142.65 223.27 369.37 516.84
0.598 0.953 1.292 1.580 1.892 2.079
85.45 138.51 216.38 359.72 518.90 692.56
0.482 0.692 0.940 1.260 1.490 1.670
82.00 143.34 212.94 355.58 514.77 694.63
0.191 0.317 0.445 0.672 0.866 1.050
0.69 1.72 3.45 6.89 13.09 20.33 34.46 66.84 97.51 133.00 206.05
1.089 1.325 1.433 1.624 1.779 1.877 1.987 2.117 2.193 2.256 2.341
2.76 6.89 10.34 17.23 26.88 40.66 68.22 102.68 137.13 205.36 343.87
0.662 0.916 1.070 1.297 1.477 1.635 1.796 1.910 1.979 2.065 2.169
6.89 13.78 20.67 34.46 50.99 70.98 104.75 133.69 199.84 347.31 515.46
0.319 0.460 0.587 0.792 0.980 1.149 1.344 1.461 1.626 1.811 1.918
1.72 2.76 3.45 5.51 6.89 11.03 17.92 25.50 41.35
1.652 1.728 1.764 1.804 1.835 1.877 1.917 1.941 1.973
2.07 2.76 3.45 5.51 7.58 11.03 17.92 23.43 38.59
1.138 1.319 1.393 1.518 1.588 1.665 1.742 1.785 1.845
3.45 5.51 7.92 11.03 17.92 24.81 37.90 51.68 73.05
0.572 0.715 0.878 1.034 1.229 1.345 1.480 1.564 1.643
0.04 0.08 0.15 0.28 0.53 0.79 1.06 2.03 3.58
1.274 1.379 1.437 1.491 1.531 1.557 1.577 1.590 1.613
0.06 0.09 0.17 0.28 0.41 0.67 0.94 1.30 1.38
0.616 0.805 1.035 1.163 1.249 1.331 1.375 1.414 1.409
0.13 0.28 0.55 0.80 1.08 1.44 2.07 3.45 5.17
0.153 0.295 0.487 0.613 0.715 0.817 0.942 1.098 1.198
0.07 0.11 0.20 0.31 0.56 0.80 1.07 1.46 3.51
0.695 0.752 0.797 0.825 0.860 0.882 0.904 0.923 0.976
0.06 0.09 0.16 0.28 0.54 0.82 1.07 1.44 6.89
0.290 0.381 0.479 0.563 0.649 0.690 0.721 0.757 0.782
0.15 0.29 0.55 0.82 1.08 1.34 2.14 3.45 7.17
0.090 0.167 0.262 0.330 0.377 0.411 0.498 0.558 0.623
3.46 6.98 13.90 20.81 27.73
0.094 0.167 0.265 0.332 0.389
6.98 12.10 17.36 27.73 41.70
0.051 0.087 0.119 0.177 0.244
7.30 14.95 27.50 41.42 55.06 68.22 81.87 95.72 108.60
0.551 0.917 1.346 1.662 1.879 2.030 2.149 2.244 2.333
8.89 16.40 27.84 41.42 55.13 69.12 81.66 94.34 104.06
0.268 0.426 0.630 0.834 1.008 1.158 1.275 1.376 1.447
13.85 27.70 41.48 62.16 82.90 105.50 126.11 178.48 241.19
0.133 0.233 0.322 0.439 0.543 0.649 0.798 0.997 1.190
P (kPa)
T ) 34.8 °C
T ) 79.6 °C
N (mol/kg)
P (kPa)
N (mol/kg)
P (kPa)
N (mol/kg)
Methane 722.19 1057.10 1430.60 1749.67 2052.19
2.246 2.416 2.531 2.607 2.661
1052.97 1407.17 1768.96 2061.15
1.918 2.077 2.184 2.263
1041.94 1383.06 1725.55 2071.83
1.303 1.488 1.619 1.728
Ethane 337.67 503.05 679.47 1027.47 1372.03 2108.69
2.432 2.497 2.539 2.581 2.589 2.598
514.08 687.74 860.02 1028.16 1202.51 1369.96 1547.75 1721.11 1889.56 2057.70
2.248 2.304 2.343 2.373 2.394 2.411 2.423 2.432 2.434 2.433
708.41 1022.65 1423.71 2020.49
1.992 2.070 2.130 2.184
Propane 72.36 104.75 145.40 213.63 354.21 425.18 530.62 537.51
2.005 2.020 2.033 2.047 2.065 2.238 2.250 2.266
52.37 79.24 108.19 141.26 238.43 351.45 556.12 695.32 804.89
1.878 1.911 1.945 1.964 1.997 2.015 2.033 2.038 2.042
101.30 149.54 234.30 345.59 550.60 693.94 789.03
1.710 1.776 1.840 1.887 1.935 1.954 1.963
n-Butane 6.58 13.95 33.08 48.27 65.85 82.31 101.85
1.635 1.659 1.681 1.692 1.710 1.717 1.732
2.03 3.48 6.89 13.71 20.67 40.66 61.95 82.69 103.44
1.443 1.483 1.525 1.561 1.579 1.605 1.618 1.627 1.632
6.96 10.41 17.37 34.73 57.64 71.32 83.11 103.16
1.261 1.334 1.399 1.466 1.503 1.513 1.520 1.528
Isobutane 6.96 12.06 17.26 27.56 41.42 55.20 68.95 86.17
1.212 1.371 1.469 1.535 1.577 1.602 1.619 1.632
13.92 21.09 34.52 51.68 65.40 82.28 102.61
0.850 0.914 1.036 1.165 1.246 1.311 1.371
13.99 27.70 37.49 55.13 68.98 86.35 110.19
0.661 0.710 0.730 0.759 0.777 0.799 0.835
Isobutanea 41.56 60.37 82.91 103.86 130.69
0.456 0.500 0.538 0.559 0.574
62.23 82.91 103.72 130.27
0.310 0.354 0.388 0.427
CO2 133.00 208.80 328.71 521.66 676.02 1013.69 1357.56 1704.87 2001.19
2.502 2.709 2.881 3.026 3.099 3.195 3.253 3.284 3.296
136.10 204.32 352.14 517.18 693.25 1035.05 1376.16 1724.86 2043.92
1.683 1.969 2.301 2.497 2.598 2.779 2.867 2.927 2.965
348.00 523.04 693.94 1037.46 1385.12 1736.57 2038.40
1.440 1.724 1.915 2.165 2.326 2.444 2.519
C1-C4 Alkanes, CO2, and SF6 on Silicate
J. Phys. Chem. B, Vol. 102, No. 8, 1998 1469
TABLE 1 (Continued) T ) 3.8 °C
T ) 34.8 °C
T ) 79.6 °C
T ) 3.8 °C
P (kPa)
N (mol/kg)
P (kPa)
N (mol/kg)
P (kPa)
N (mol/kg)
1.03 2.48 3.79 5.79 8.27 11.72 17.92 24.81 32.39
0.812 1.203 1.397 1.559 1.677 1.770 1.846 1.885 1.915
3.10 5.86 8.27 11.72 17.23 24.12 36.52 70.63 137.82
0.503 0.816 0.955 1.145 1.323 1.463 1.613 1.774 1.882
7.58 11.37 17.23 25.50 39.28 74.42 145.40 208.11 287.36
0.317 0.412 0.531 0.668 0.838 1.122 1.404 1.535 1.633
T ) 34.8 °C
T ) 79.6 °C
P (kPa)
N (mol/kg)
P (kPa)
N (mol/kg)
P (kPa)
N (mol/kg)
37.90 55.13 73.05 106.12 141.96 274.27 408.65 578.17
1.931 1.961 1.977 1.996 2.007 2.025 2.033 2.034
275.65 418.29 549.23 709.79
1.951 1.978 1.991 1.997
417.60 556.81 708.41
1.728 1.788 1.831
SF6
a
The data in the first two columns are at T ) 117 °C, and the data in the last two columns are at T ) 149 °C.
TABLE 2: Virial Constants for the Adsorption Isothermsa adsorbate
a0
std dev
a1
std dev
b0
std dev
b1
std dev
c0
std dev
c1b
d0
std dev
d1b
methane ethane propane n-butane iso-butane SF6 CO2
12.88 15.11 15.01 18.0 18.26 14.24 12.58
0.132 0.431 0.884 1.74 4.46 0.46 0.272
-2448.7 -3980.6 -4915.1 -6083.9 -6707.8 -4263.5 -2874.4
45.92 143.5 297.8 609.5 1600. 162.9 90.73
0.714 0.311 5.70 5.11 1.59 4.98 2.038
0.118 0.338 0.964 1.94 5.33 0.613 0.19
-59.83 141.3 -422.0 152.5 217.1 -275.8 -187.47
33.7 86.6 250.2 167.2 1887 108.5 48.51
-0.12 -0.90 -5.8 -8.21 -5.35 -3.74 -1.03
0.068 0.19 0.74 1.35 2.96 0.558 0.087
0 0 0 0 0 0 0
0.115 0.54 2.60 4.35 12.58 1.41 0.27
0.015 0.048 0.23 0.56 1.12 0.158 0.017
0 0 0 0 0 0 0
a
Henry’s constant ) K0 exp(-∆H/RT) ) exp(-a0 - a1/T). b c1 and d1 values are insignificant.
Experimental Section The adsorption equilibrium isotherms were measured gravimetrically using a Cahn-1000 microbalance. The details of the experimental setup are given elsewhere.23 Three pressure gauges, with a range of 0-10 Torr, 0-15 psia, and 0-300 psia, were used to accurately measure the adsorption in different pressure ranges. This was necessary because of the importance of the data in the low-pressure region for accurate determination of Henry’s law constants. The temperature range covered was 3-81 °C. Three temperature levels were used for each adsorbate to enhance the accuracy of isosteric heat calculation. Most measurements were repeated, and both adsorption and desorption experiments were performed to confirm reversibility. The silicalite crystals were synthesized in our laboratory. The average size of the crystals was 35 × 35 × 50 µm. The crystals were calcined for more than 12 h at 550 °C. The sample was activated in the balance before each experiment for at least 3 h at 300 °C with helium flow at a pressure of 10 Torr until no weight loss was observed. Before each run, the sample was vacuumed using a turbomolecular pump at the experimental temperature for at least 2 h until the recorded sample weight and pressure were constant. Equilibrium was achieved usually within 30 min for most of the adsorbates. The sample weight was continuously monitored during the experiments; equilibrium was assumed when the sample weight changed less than 0.1 mg in a 1 h period. The total sample weight was of the order of 400 mg. The accuracy of the measurements was (0.5 °C and (0.1 mg. The adsorbates used were all high purity (>99.5%). Buoyancy corrections become significant at higher pressures. The buoyancy force was calculated using the displacement volume of the sample assembly as shown in the following equation. The actual weight (Wa) is expressed in terms of measured weight (Wm) and the buoyancy force:
Wa ) Wm + Vsdfg
(1)
where Vs is the volume of the sample assembly, df is the density
of the adsorbate gas at the experimental conditions, and g is the gravitational acceleration. The assembly volume Vs is obtained by measuring the weight change that occurs when helium is used at low pressures. Helium is assumed to be nonadsorbing in silicalite. Results and Discussion Molecular Dimensions and Silicalite Pore System. ZSM5/silicalite has a three-dimensional channel structure with straight channels of 5.4 × 5.6 Å in the y direction and zigzag channel of 5.6 Å in the x-z plane. The zigzag channels connect subsequent layers of straight channels. The unit cell dimension of ZSM-5/silicalite is 20.07 × 19.93 × 13.40 Å. The molecular diameters of methane to n-butane and carbon dioxide are about 4 Å, and the molecular diameter for isobutane is about 5 Å. SF6 is a big molecule with a diameter of 6.1 Å. Although the diameters of SF6 and isobutane are close to the channel diameters, they adsorb on silicalite as fast as smaller molecules. Adsorption Isotherms. The adsorption isotherms of methane through normal butane, isobutane, carbon dioxide, and SF6 are given in Figures 1-7, respectively. The figures also contain plots in the “Virial” domain. These plots highlight the lowpressure region. All the equilibrium data are also listed in Table 1. The isotherms for all gases were fitted by the Virial equation as suggested by Talu et al.18 The Virial coefficients and Henry’s law constants were expanded in a power series of amount adsorbed with temperature-dependent coefficients. The values of Virial constants are given in Table 2. The following equation was used to determine the temperature-independent parameters by regression of all isotherms of a gas in a single step:
()
ln
(
) (
)
a1 b1 c1 P ) a0 + + b0 + N + c0 + N2 + N T T T d1 d0 + N3 + ... (2) T
(
)
Here P is the pressure, N is the amount adsorbed, and a0, a1, b0, b1, c0, c1, d0, and d1 are the Virial constants.
1470 J. Phys. Chem. B, Vol. 102, No. 8, 1998
Figure 2. Adsorption isotherms of ethane on silicalite.
Sun et al.
Figure 4. Adsorption isotherms of n-butane on silicalite.
Figure 3. Adsorption isotherms of propane on silicalite.
The adsorption equilibrium data for isobutane at lower three temperatures could not be fitted by eq 2 because of the inflections in the isotherms clearly displayed in Figure 5 in the Virial domain. Similar behavior has been found for normal hexane and heptane by Sun et al.2 The measurements were repeated several times to confirm the results. The pressure at the inflection point varies with temperature. On the other hand, the changes set in at about 4 molecules/uc as is apparent in Figure 5. All unusual adsorption behavior observed for silicalite appears to occur at this loading. There are 4 zigzag channels, 4 channel intersections, and 4 straight channel segments between intersections in a unit cell of silicalite. It is obvious that isobutane is adsorbed in a specific region of the channel system at low loading. Yet, it is not possible to clearly state where this localization occurs from only the isotherm data. Other
Figure 5. Adsorption isotherms of isobutane on silicalite.
microscale experimental techniques and/or molecular simulation might help in identifying these sites. The Virial equation fits are shown in all figures as solid lines. Judging from the figures and the statistics given in Table 2, eq 2 accurately follows all the isotherm data for all adsorbates. Only the two higher temperature isotherms of isobutane were used in the Virial regressions due to the reasons explained above. A comparison of our equilibrium data with the data reported from the literature is compiled into Table 3. The measurement conditions of pressure and temperature and the measurement techniques are also given. Considering the differences in the temperature and pressure, loadings for methane, ethane, propane,
C1-C4 Alkanes, CO2, and SF6 on Silicate
J. Phys. Chem. B, Vol. 102, No. 8, 1998 1471
TABLE 3: Comparison of Adsorption Equilibrium Measurements from Different Sources method
temp (°C)
P (kPa)
N (mol/kg)
ref
gravimetric isosteric volumetric volumetric
3 0 31 2
Methane 2000 30 736.7 1578.2
2.7 0.5 1.7165 2.141
this work 22 11 17
gravimetric gravimetric volumetric isosteric
3 30 2 0
Ethane 2000 32 328.6 10
2.65 1.7 2.055 1.5
this work 8 17 19
gravimetric gravimetric volumetric volumetric isosteric
3 19 -78 2 0
Propane 350 50 max 360.59 20
2.05 2.0 2.25 1.784 1.8
this work 8 15 17 19
gravimetric gravimetric volumetric TPDa volumetric
3 30 0 20 2
Butane 100 32 max max 76.15
1.75 1.6 1.87 1.8 1.482
this work 8 15 1 17
gravimetric volumetric isosteric volumetric
4 20 0 31.4
CO2 2000 max 15 1698
3.35 2.67 1.1 3.003
this work 15 22 11
Isobutane 100 max
1.65 1.92
this work 15
SF6 700
2.0
this work
gravimetric volumetric gravimetric a
4 -14 34.8
Figure 6. Adsorption isotherms of carbon dioxide on silicalite.
TPD ) temperature-programmed desorption.
n-butane, CO2, and isobutane agree with each other reasonably well. Data for SF6 are not available in the literature. The adsorption equilibrium data of methane and carbon dioxide on silicalite by Golden and Sircar11 and of methane by Abdul-Rehman et al.19 are given in Figure 8. The points in the figure represent the data from the literature, and the lines represent the calculation using our Virial parameters at the same temperatures. Our calculations fit the data of Golden and Sircar11 very well for both methane and carbon dioxide as shown in the figure. The data by Abdul-Rehman et al.19 for methane seem to be somewhat lower than both our results and the data of Golden and Sircar.11 A comparison of our data and data of Abdul-Rehman et al.19 for ethane, propane, and butane is given in Figure 9. Golden and Sircar11 did not report equilibrium data for ethane, propane, and butane. As is apparent in Figure 9, our results are higher than the data from Abdul-Rehman et al.19 by about 20%. These differences may be related to the samples; pure crystals were used in this study while AbdulRehman et al.19 used silicalite pellets. Heat of Adsorption. The isosteric heat of adsorption is calculated from the Virial equation as given by the following:
Qst ∂ ln P )| ) a1 + b1N + c1N2 + d1N3 + ... R ∂1/T n
(3)
The isosteric heats of adsorption versus loading for methane to butane, CO2, isobutane, and SF6 are given in Figure 10. The isosteric heats of adsorption are in the order of methane < carbon dioxide < ethane < SF6 < propane < n-butane < isobutane.
Figure 7. Adsorption isotherms of sulfur hexafluoride on silicalite.
Isosteric heats of adsorption for methane and carbon dioxide increase slightly as the loading increases. This indicates a homogeneous adsorption system although the trends are well within experimental uncertainty considering the mathematical manipulations necessary to relate isotherm data to the heat of adsorption. The isosteric heats of adsorption for propane and sulfur hexafluoride increase more substantially with increasing loading. This also suggests a homogeneous adsorption system with adsorbate-adsorbate interactions becoming more significant. The increase in isosteric heat of adsorption is caused by the lateral interactions among adsorbate molecules, while vertical interactions with the silicalite surface stay constant. The isosteric heats of adsorption for ethane and n-butane decrease slightly with increasing loading. However, the decreases are
1472 J. Phys. Chem. B, Vol. 102, No. 8, 1998
Figure 8. Comparison of CH4 and CO2 isotherms on silicalite to data from the literature.
Sun et al.
Figure 11. Limiting heats of adsorption on silicalite.
The limiting heats of adsorption as an indication of gassolid interaction are given in Figure 11. The limiting heat of adsorption is plotted against the polarizability of the adsorbates. The limiting heats of adsorption from both Abdul-Rehman et al.19 and Hufton and Danner20 agree very well with our results. The limiting heat of adsorption increases linearly with polarizability and carbon number in the n-alkanes. The limiting heat of adsorption for CO2 does not follow the trend probably because of its appreciable quadrupole moment. The limiting heat of adsorption for isobutane also stands out of the trend established by n-alkanes while that for SF6 is lower. These two gases have quite different molecular structures and size in comparison to n-alkanes. Conclusions
Figure 9. Comparison of ethane, propane, and butane isotherms on silicalite to data from the literature.
Adsorption equilibrium data for methane, ethane, propane, n-butane, isobutane, carbon dioxide, and sulfur hexaflouride on silicalite are reported. The data were fitted by Virial equation, except for the adsorption data of isobutane at lower temperatures. Virial constants, isosteric heats of adsorption, and limiting heats of adsorption are reported for all adsorbates. The adsorption isotherms for isobutane in Virial domain exhibit inflection points at loading of about 4 molecules/uc; the isotherms of isobutane have steps. The silicalite surface exhibits a near homogeneous surface for the systems studied. The limiting heat of adsorption increases linearly with the polarizability of n-alkanes. Acknowledgment. This work was supported by a grant from the National Science Foundation (CTS-9313661). References and Notes
Figure 10. Isosteric heats of adsorption on silicalite.
very small and well within the experimental uncertainty. These systems may also, therefore, be classified as a homogeneous adsorption system. The isosteric heat of adsorption for isobutane was determined from high-temperature isotherms. Therefore, the loading range is very small, and a trend cannot be deduced.
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