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Langmuir 1996, 12, 5888-5895
Calorimetric Heats of Adsorption and Adsorption Isotherms. 1. O2, N2, Ar, CO2, CH4, C2H6, and SF6 on Silicalite J. A. Dunne,† R. Mariwala,‡ M. Rao,§ S. Sircar,§ R. J. Gorte, and A. L. Myers* Department of Chemical Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104 Received May 21, 1996. In Final Form: August 1, 1996X A Tian-Calvet type calorimeter is applied to the simultaneous determination of adsorption isotherms and heats of adsorption. This is the first of a series of studies of the effect of adsorbate size and polarity on the energetics of adsorption in zeolites. The adsorbate gases used in this study are quadrupolar (N2 and CO2) and nonpolar (Ar, O2, CH4, C2H6, and SF6). The heats of adsorption of both polar and nonpolar gases are either constant or increase with coverage, so silicalite may be classified as a relatively homogeneous adsorbent compared to X type zeolite. Reversibility was established by comparing adsorption and desorption isotherms. Reproducibility was studied by comparing runs for different samples of the same adsorbent. The average experimental error in loading is (0.6%. The error in the isosteric heat of adsorption is (2% for heats larger than 20 kJ/mol and (5% for heats smaller than 20 kJ/mol.
1. Introduction Adsorption is well established as a unit operation for the production of pure gases and for the treatment of waste streams. Recent developments have, in certain cases, enabled adsorption to replace cryogenic distillation as a means of gas separation.1 Future progress in adsorption technology depends upon elucidating the factors which determine selective adsorption of a particular gas from a multicomponent mixture of gases. Such factors include the size and shape of adsorbate molecules and the topology of the adsorbent. Chemical factors such as the polarity of adsorbate molecules and the inclusion of nonframework ions in zeolitic adsorbents also have a major impact on the energetic heterogeneity of an adsorbent and its selectivity. The equilibrium adsorption of a particular gas is described by its adsorption isotherm and heat of adsorption. Experimental data are usually reported as a set of two or three adsorption isotherms for a particular gasadsorbent system,2 and the isosteric heat of adsorption qst is calculated from the adsorption isotherms by a Clapeyron equation:
[
]
∂ ln f qst ) -R ∂(1/T)
R
-h
(1)
n
For a perfect gas, the fugacity f ) P and the residual enthalpy hR ) 0. Isosteric heats determined by differentiation are very sensitive to errors in the adsorption isotherms. The heat of adsorption of a single gas is reported as a function of amount adsorbed at a given temperature. The limiting slope (Henry's constant, H) of the adsorption isotherm *Person to whom correspondence should be addressed. E-mail address:
[email protected]. † Allied Signal, Santa Clara, CA. ‡ TDA Research, Wheat Ridge, CO. § Air Products & Chemicals, Inc., Allentown, PA. X Abstract published in Advance ACS Abstracts, November 1, 1996. (1) C&E News Ed. Staff. Environment and economy boost demand, prices for industrial gases. C&EN 1994, 12 Dec, 15-16. (2) Valenzuela, D.; Myers, A. L. Adsorption Equilibrium Data Handbook; Prentice-Hall: Englewood Cliffs, NJ, 1989.
S0743-7463(96)00495-7 CCC: $12.00
Figure 1. Schematic diagram of combination calorimetervolumetric apparatus.
H ) lim Pf0
dn (Pn) ) lim(dP ) Pf0
(2)
can also be measured by gas chromatography. The temperature dependence of the Henry’s constant yields the isosteric heat at the limit of zero coverage:
[
q°st ) R
]
d ln H d(1/T)
(3)
This technique also suffers from the inaccuracy associated with differentiation, especially if the Henry’s constants are obtained by extrapolation according to eq 2. Another experimental method for studying a particular gas-adsorbent system is to measure the adsorption isotherm in a volumetric or gravimetric apparatus and the heat of adsorption in a calorimeter. Calorimetry has been applied extensively to characterize both the solid adsorbent and specific adsorbate-adsorbent interactions, e.g. the characterization of solid acid catalysts,3,4 by chemisorption. A thorough literature survey of chemisorption calorimetry has been published by Cardona(3) Chen, D. T.; Zhang, L.; Yi, C.; Dumesic, J. A. Methylamine synthesis over solid acid catalysts: microcalorimetric and infrared spectroscopic studies of adsorbed species. J. Catal. 1994, 146, 257267.
© 1996 American Chemical Society
Heats of Adsorption and Adsorption Isotherms
Figure 2. Thermopile response, voltage versus time. The total heat recorded by the pulse is 52.12 mV s × 0.0549 w/mV ) 2.860 J.
Figure 3. Comparison of adsorption from three separate experiments for adsorption of C2H6 on silicalite: (O) 23.03 °C; (0) 23.31 °C; (4) 22.95 °C. Solid line is the best fit of the experimental data.
Martinez and Dumesic.5 The principles of calorimetry in the case of physisorption have been set forth by Hill6 and Le´toquart et al.7 Hill defined various types of heats of adsorption corresponding to different experimental procedures. Differences between the various types of heats are of the order of RT, which is small or negligible for chemisorption but as much as 10% of the heat of physisorption. Thus for physisorption it is important to specify exactly which type of heat is being measured. Heats of adsorption have two important applications in addition to providing the temperature coefficient of (4) Parrillo, D. J.; Gorte, R. J. Characterization of stoichiometric adsorption complexes in H-ZSM-5 using microcalorimetry. Catal. Lett. 1992, 16, 17-25. (5) Cardona-Martinez, N.; Dumesic, J. A. Applications of adsorption microcalorimetry to the study of heterogeneous catalysis. Adv. Catal. Lett. 1989, 38, 149-244. (6) Hill, T. L. Statistical mechanics of adsorption. V. Thermodynamics and heat of adsorption. J. Chem. Phys. 1949, 17 (6), 520-535. (7) Le´toquart, C.; Rouquerol, F.; Rouquerol, J. Les chaleurs d’adsorption: expression des chaleurs d’adsorption physique, en termes d’e´nergie interne, a partir des donne´es experimentales. J. Chim. Phys. 1973, 70 (3), 559-573.
Langmuir, Vol. 12, No. 24, 1996 5889
Figure 4. Isosteric heat of adsorption of C2H6 on silicalite for three separate experiments. Legend is given for Figure 3. Solid line is the linear best fit of the data. Dashed lines show (2% variation from the solid line.
Figure 5. Adsorption isotherms for SF6 on silicalite: (O) 31.79 °C; (4) 40.41 °C; (0) 59.25 °C.
adsorption by eq 1. The first application is that the heat of adsorption profile reveals the degree of energetic heterogeneity of gas-solid interactions. An increase in heat of adsorption with gas loading is characteristic of nonheterogeneous adsorbents (e.g. graphitized carbon) with constant gas-solid energies of interaction. The increase is due to cooperative interactions between adsorbed molecules. A decrease in heat of adsorption with gas loading is characteristic of highly heterogeneous adsorbents (e.g. activated carbon) with a wide distribution of gas-solid interaction energies. A constant heat of adsorption with gas loading indicates a balance between the strength of cooperative gas-gas interactions and the degree of heterogeneity of gas-solid interactions. The second application of heats of adsorption is the calculation of energy balances in packed columns. Most columns operate adiabatically, and the heat of adsorption determines the temperature profile inside the column. The heat of adsorption is the energy required to regenerate the column, which is the major operating cost for thermalswing-adsorption (TSA) columns.
5890 Langmuir, Vol. 12, No. 24, 1996
Figure 6. Adsorption isotherms for C2H6 on silicalite: (O) 23.03 °C; (4) 40.57 °C; (0) 59.92 °C.
Figure 7. Adsorption isotherms for CO2 on silicalite: (O) 30.60 °C; (4) 40.69 °C; (0) 61.18 °C. Data of Golden and Sircar: (2) 31.40 °C; (1) 68.40 °C.
Calorimetic studies have been performed on commercial (SETARAM, DAK) and specially built calorimeters to study the subtle effects associated with the size and shape of adsorbate molecules and the energetic heterogeneity of adsorbents. Most experiments have been conducted at room temperature and moderately low pressure, but some work at low temperature8,9 and high pressure10,11 has been undertaken. Thamm and others12-14 have investigated the effect of adsorbate size on heat of adsorption for a series of n-alkanes and cycloalkanes in silicalite. They (8) Llewellyn, P. L.; Coulomb, J. P.; Grillet, Y.; Patarin, J.; Lauter, H.; Reichert, H.; Rouquerol, J. Adsorption by MFI-type zeolites examined by isothermal microcalorimetry and neutron diffaction. 1. Argon, krypton, and methane. Langmuir 1993, 9, 1846-1851. (9) Llewellyn, P. L.; Coulomb, J. P.; Grillet, Y.; Patarin, J.; Andre, G.; Rouquerol, J. Adsorption of MFI-type zeolites examined by isothermal microcalorimetry and neutron diffraction. 2. Nitrogen and carbon monoxide. Langmuir 1993, 9, 1852-1856. (10) Gusev, V.; Fomkin, A. High-pressure adsorption of Xe on NaX zeolite by microcalorimetry and isosteric analysis. J. Colloid Interface Sci. 1994, 162, 279-283. (11) Isirikyan, A. A.; Levin, M. A.; Serpinskii, V. V.; Fomkin, A. A. Gas adsorption heats at elevated pressures. Izv. Akad. Nauk SSSR, Ser. Khim. 1987, No. 7, 1658-1660.
Dunne et al.
Figure 8. Adsorption isotherms for CH4 on silicalite: (O) 23.07 °C; (4) 40.12 °C; (3) 61.24 °C. Data of Golden and Sircar: (2) 31.00 °C; (1) 69.60 °C.
Figure 9. Adsorption isotherms for N2 on silicalite: (O) 22.95 °C; (0) 61.46 °C; (3) 71.57 °C. Data of Golden and Sircar: (2) 31.90 °C; (1) 68.70 °C.
observed that as the adsorbate size approaches the size of the silicalite channel, the heat profiles change from a homogeneous type in the case of n-alkanes to complex heat profiles in the case of isobutane, cyclopentane, and cyclohexane. Benzene and toluene, which are about the same size as the silicalite channel, display a very complex heat profile.13,15,16 Adsorbent geometry also has a marked effect on heats of adsorption: benzene on AlPO4-517 displays a different profile than benzene on silicalite. The complex nature of the benzene-silicalite profile is believed to be (12) Stach, H.; Thamm, H.; Ja¨nchen, J.; Fiedler, K.; Schirmer, W. Experimental and theoretical investigations of the adsorption of n-paraffins, n-olefins, and aromatics on silicalite. Proceedings of the 6th International Zeolite Conference; Olson, D., Bisio, A., Eds.; Butterworths: Guildford, U.K., 1984; pp 225-231. (13) Thamm, H. Adsorption site heterogeneity in silicalite: a calorimetric study. Zeolites 1987, 7, 341-346. (14) Thamm, H.; Stach, H. Calorimetric studies of the adsorption of cycloalkanes on silicalite. Izv. Akak. Nauk SSSR, Ser. Khim. 1985, No. 4, 944-947. (15) Thamm, H.; Stach, H.; Regent, N. I. Calorimetric studies of adsorption of aromatic hydrocarbons on silicalite. Izv. Akad. Nauk SSSR, Ser. Khim. 1982, No. 12, 2820-2823. (16) Wendt, R.; Thamm, H.; Fiedler, K.; Stach, H. Z. Phys. Chem. (Leipzig) 1985, 266, 289-301.
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Figure 10. Adsorption isotherms for Ar on silicalite: (O) 32.60 °C. Data of Golden and Sircar: (2) 32.60 °C. Figure 12. Adsorption isotherms on silicalite: (O) C2H6 at 23.03 °C; (4) CO2 at 30.60 °C; (0) CH4 at 23.07 °C; (3) N2 at 22.95 °C.
Figure 11. Adsorption isotherms for O2 on silicalite: (O) 32.30 °C. Data of Golden and Sircar: (2) 31.50 °C.
due to reorientation of the molecule within the adsorbent channels. Studies18 of a series of alkanes on different crystalline adsorbents (mordenite, MFI type zeolites, AlPO4-5 and -11, VPI-5, and dealuminated faujasites) showed that heats of adsorption increase with the density of the adsorbent framework. The effect of energetic heterogeneity on heats of adsorption has been studied as well: the quadrupolar molecule CO2 on NaX and NaA,19 on Na-ZSM-5,20 and on a series of ion-exchanged zeolites (Li+, Na+, and K+ X and Y type faujasites).21 Typically there is a strong dependence of the initial heat of adsorption on the polarity and degree of heterogeneity of (17) Thamm, H.; Stach, H.; Jahn, E.; Fahlke, B. Calorimetric investigation of the adsorption properties of microporous aluminophosphate AlPO4-5. Adsorpt. Sci. Tech. 1986, 3, 217-220. (18) Stach, H.; Fiedler, K.; Ja¨nchen, J. Correlation between initial heats of adsorption and structural parameters of molecular sieves with different chemical compositionsa calorimetric study. Pure Appl. Chem. 1993, 65 (10), 2193-2200. (19) Avgul, A. A.; Aristov, B. G.; Kurdyukova, L. Ya.; Kiselev, A. V. Heats of adsorption of carbon dioxide on NaX and NaA zeolites and the variation of the extent of adsorption with gas pressure and temperature. Russ. J. Phys. Chem. 1968, 42 (10), 1424-1426. (20) Dubinin, M. M.; Rakhmatkariev, G. U.; Isirikyan, A. A. Heats of adsorption of CO2 on high-silicon zeolites ZSM-5 and silicalite. Izv. Akad. Nauk SSSR, Ser. Khim. 1989, No. 11, 2636-2638. (21) Vasileva, E. A.; Khvoschev, S. S.; Karetina, I. V. Heats of adsorption of CO2 and NH3 on synthetic zeolites of different structural types. Communication 3. CO2 adsorption on Li, Na, K forms of X and Y zeolites. Izv. Akad. Nauk SSSR, Ser. Khim. 1984, No. 9, 1942-1947.
Figure 13. Isosteric heats of adsorption on silicalite at 23-33 °C. Solid lines are linear best fit of experimental points.
the adsorbent. Alcohols and alkanes13,22-25 have also been used as probes on heterogeneous adsorbents. This paper describes the design and operation of a calorimeter which measures adsorption isotherms and heats of adsorption simultaneously. The measurements were made for a series of nonpolar (Ar, O2, CH4, C2H6, SF6) and quadrupolar gases (N2, CO2) on the homogeneous adsorbent silicalite. 2. Heats of Adsorption There are two main types of heats: differential and integral. The integral heat refers to the introduction of
5892 Langmuir, Vol. 12, No. 24, 1996
Dunne et al.
a large sample of gas, e.g. from zero loading to 50% of saturation capacity. The change in energy for adsorbing na moles of gas is
∆U ) na(ug - ua)
(4) Q ) 0.46Vd(1 - β)∆P
and the corresponding change in enthalpy is
∆H ) na(hg - ha)
(5)
where ug and ua are the molar internal energies in the bulk gas and adsorbed phases, respectively, and hg and ha are the molar enthalpies in the bulk gas and adsorbed phases, respectively. na is the total number of moles adsorbed, the so-called surface excess. For practical purposes ua ) ha because of the small volume of the adsorbed phase. The differential heat of adsorption is the differential of the integral heat with respect to the number of moles adsorbed (na). From eq 4, the differential energy is
∆u j ) ug - ua - na
∂ua ∂na
(6)
and from eq 5 the differential enthalpy is
∂ua ∆h h ) hg - ha - na a ∂n
(7)
∆u j ≡ qd is the differential energy of desorption, for which the standard terminology is differential heat of adsorption. ∆h h ≡ qst is the differential enthalpy of desorption, for which the standard terminology is isosteric heat of adsorption. The isosteric heat (qst) is the quantity required for energy balances in adsorption columns. The differential and isosteric heats of adsorption are related by
qst ) qd + zRT
ideal apparatus described by Hill6 except the cooling effect inside the dosing loop is not registered by the calorimeter. It was found experimentally that the heat Q absorbed by the dosing loop during the expansion is given by
(8)
where z ) Pv/RT is the compressibility factor in the bulk gas phase. For an ideal gas, z ) 1. The heat registered by an adsorption calorimeter depends upon its design. Hill6 showed that qd is measured directly in a closed system containing both the dosing chamber and the sample chamber within the calorimeter. Such an arrangement is conceptually simple but difficult to construct. Most commercial instruments are open systems, with the dosing chamber outside the calorimeter. It has been shown by Cardonna-Martinez and Dumesic5 that if the gas enters the calorimeter at the temperature of the sample cell and if the same number of moles of gas enters the sample cell and a reference cell wired in reverse polarity, the calorimeter measures qst directly. Our system is also open, but the gas expands rapidly and nonisothermally from an external dosing loop into the sample cell. In order to reduce the dead space to a minimum, gas is not introduced to the reference cell of our calorimeter. In design, our system is the same as the (22) Akhmedov, K. S.; Rakhmatkariev, G. U.; Dubinin, M. M.; Isirikyan, A. A. Heat of adsorption of methanol on the ultrahigh-silica zeolite silicalite. Izv. Akad. Nauk SSSR, Ser. Khim. 1987, No. 8, 17171721. (23) Dubinin, M. M.; Rakhmatkariev, G. U.; Isirikyan, A. A. Differential heats of adsorption and adsorption isotherms of alcohols on silicalite. Izv. Akad. Nauk SSSR, Ser. Khim. 1989, No. 9, 2117-2120. (24) Izmailova, S. G.; Karentina, I. V.; Khvoschev, S. S. Adsorption of methanol on sodium faujasites and mordenites. Izv. Akad. Nauk, Ser. Khim. 1993, No. 3, 476-479. (25) Thamm, H.; Stach, H.; Schirmer, W.; Fahlke, B. Charakterisierung von mikroporo¨sen SiO2 und Zeoliteischen Molekularsieben during Kalorimetrische Bestimmung der Adsorptionswechselwirkung mit n-Butan. Z. Phys. Chem. (Liepzig) 1982, 461-469.
(9)
where Vd is the volume of the dosing loop, Vc is the volume of the sample cell, β ) Vd/(Vd + Vc) is the fractional volume of the dosing loop, and ∆P is the pressure difference between the dosing loop and the sample cell before the expansion. The spurious heat term Q is subtracted from the calorimeter signal to obtain qd differential heat of adsorption. The isosteric heat qst is obtained from eq 8. 3. Combination Calorimeter-Volumetric Apparatus The calorimeter is a Tian-Calvet type designed for chemisorption studies by Parrillo and Gorte.4 The volume of the calorimeter cell was reduced to 20 cm3 to improve accuracy and to facilitate diffusion in mixture experiments. A schematic diagram of the apparatus is shown in Figure 1. The cubical, Pyrex calorimeter cell (#14) is encased on five of its six sides by highly sensitive thermopiles (#15) which generate a voltage proportional to the heat released by adsorption. The cell encased in thermopiles sits inside an aluminum block (#1) which provides a large thermal mass for rapid heat dissipation and small temperature rise (less than 0.1 K). The five thermopiles in the sample cell are connected in series to the five thermopiles (#22) of the reference cell (#21) with reversed polarity to reduce background noise. The output signal from the thermopiles is input to a computer (#16) which stores the response curve and performs a numerical integration to obtain the heat effect corresponding to adsorption of a known increment of gas. The temperature of the calorimeter is measured by a type-K thermocouple (#20) placed on the aluminum block at the edge of the cubical cavity. The calorimeter unit is enclosed in an aluminum enclosure (#2) to maintain isothermal conditions. The Pyrex cell (#14) is connected to the dosing loop (#8) through a custom-made T-shaped adaptor (#13). A small-bore (0.03 cm) dosing tube (#10) passes through the adaptor, which is connected to the pressure transducer (#17) and the outlet valve (#19). The dosing system consists of inlet (#4) and outlet (#11) valves, a Valco gas-sampling valve (#7) with a small dead space (0.01 cm3), a calibrated dosing loop (#8), and a pressure transducer (#5). The sampling valve can be connected to either the calorimeter cell or the dosing system by automatic operations. The dosing procedure is to switch the sampling valve to the dosing side, flush the dosing system with adsorbate gas, and then close the inlet and outlet valves. The amount of gas introduced to the calorimeter cell is determined by the dosing-loop volume, the pressure, and the temperature. The outlet from the calorimeter cell and dosing system is connected to a mechanical pump (#24) through a liquid nitrogen trap (#23). This apparatus allows the adsorption isotherm and the heat of adsorption to be determined simultaneously and thus eliminates small errors associated with variability in the degassing procedure applied to the adsorbent. 4. Experimental Section The adsorption of seven gases (SF6, C2H6, CO2, CH4, N2, O2, Ar) on silicalite was determined using the combination calorimeter-volumetric apparatus. Materials. The silicalite (MFI) used was a commercial powder (Linde S115 manufactured by Union Carbide Corp.) consisting of crystals 10 µm in length. Silicalite is the ultrahigh silicon
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Langmuir, Vol. 12, No. 24, 1996 5893
Table 1. Isosteric Heats of Adsorption (kJ/mol) on Silicalite at the Limit of Zero Coveragea
a
gas
q°st, kJ/mol
Ar O2 N2 CH4 CO2 C2H6 SF6
15.8 16.3 17.6 20.9 27.2 31.1 34.4
Temperatures vary from 23 to 33 °C.
Table 2. Isosteric Heat of Adsorption of Carbon Dioxide on Silicalite at the Limit of Zero Coverage and at 300-330 K adsorbent
method
ref
qst, kJ/mol
made in house 0.01 wt % Al Linde S-115 0.3% Al2O2 Linde S-115 powder made in house
GC pulse isosteric volumetric calorimetric calorimetric
29 30 28 this work 20
21.7 24.6 24.1 27.2 29.0
Table 3. Isosteric Heat of Adsorption of Methane on Silicalite at the Limit of Zero Coverage and at 300-330 K adsorbent 0.01 wt % Al powder from Union Carbide made in house 0.3 wt % Al2O3 98.06 wt % SiO2, 0.44 wt % Al2O3 Linde S-115 pellets powder and pellets from Union Carbide Linde S-115 powder
method isosteric volumetric GC pulse volumetric GC pulse volumetric GC pulse
ref
qst, kJ/mol
30 31 29 28 32 33 34
20.0-20.5 16 ( 4 18.4 18.6 20.0 20.4 20.5
calorimetric this work 21.0
Table 4. Isosteric Heat of Adsorption of Ethane on Silicalite at the Limit of Zero Coverage and at 300-330 K adsorbent
method
ref
qst, kJ/mol
made in house powder and pellets from Union Carbide Si/Al ratio ) 1230 Laporte Inorganics 0.03 wt % Al Linde S-115 powder made in house Linde S-115 pellets
calorimetric GC pulse
18 34
29.0 29.8
gravimetric 35 isosteric 36 calorimetric this work calorimetric 13 volumetric 33
30.0 30.5 31.2 31.6 32.8
member of the pentasil family of zeolites and has the same crystal structure as ZSM-5.26 This structure consists of a threedimensional network of intersecting channels. Straight and sinusoidal channels consisting of 10-membered rings have cross sections of 5.4 × 5.6 and 5.1 × 5.4 Å2, respectively. TPD-TGA of isopropylamine on the hydrogen ion-exchanged form of the crystals revealed a Si/Al ratio of about 300, or 0.3 weight percent Al2O3. Adsorbents were regenerated prior to each experiment. The bakeout procedure was 6 h in the range from ambient temperature to 110 °C, followed by 3 h at 110 °C, 4 h in the range from 110 to 350 °C, and finally 12 h at 350 °C. Vacuum was maintained at 0.01-0.001 Torr during the degassing. Thermogravimetric analysis of the silicalite samples gave a dehydrated weight equal to 99% of the weight in air. The amount of sample varied from 0.5 to 2.4 g. Calibration of Thermopiles. The primary calibration of the calorimeter is based upon the Clapeyron equation applied to a series of high-precision adsorption isotherms measured in a separate volumetric apparatus for C2H6 on silicalite. The calibration constant of 0.05488 W/mV obtained from the Clapeyron equation for adsorption of C2H6 is confirmed by excellent agreement of the calorimetric data with the Clapeyron equation for SF6, CO2, CH4, and N2 (see Figure 14 below). The calibration constant is independent of the amount of adsorbent in the cell (0.5-2.4 g). (26) Olson, D. H.; Kokotailo, G. T.; Lawton, S. L.; Meier, W. M. Crystal structure and structure related properties of ZSM-5. J. Phys. Chem. 1981, 85, 2238-2243.
Figure 14. Comparison of calorimetric heats (experimental points and solid lines) with heats from the Clapeyron equation (dashed lines). The calorimetric data are reproduced from Figure 13. The dashed lines were calculated by eqs 1 and 8 from adsorption isotherms measured independently in a volumetric apparatus at 40 and 60 °C (see Figures 5-8). The primary calibration constant was also compared with the rate of heat dissipation dQ/dt in a platinum filament wrapped around the outside of the cell and in good thermal contact with the cell wall and thermopiles using the relation
dQ ) I2R dt
(10)
The power-voltage relationship is linear and equal to 0.0590 ( 0.0010 W/mV. The reason for the 7.5% discrepancy is unknown. Errors of the same magnitude were observed by Handy et al.27 Procedure. The heat liberated by the adsorption of an increment ∆n moles of gas was determined by integrating the area under the response curve generated by the thermopiles. The noise level on this signal is 1 µV, which corresponds to a resolution of 54 µW. A typical response curve is shown in Figure 2. A sample calculation of the heat of adsorption is illustrated by the response curve in Figure 2, which was recorded by the incremental adsorption of 0.0950 mmol of C2H6 on 0.8752 g of silicalite at 296.09 K. The data acquisition system records the voltage output at a frequency of 2.3 kHz, and the average of each 1000 data points is computed and written to disk storage at the rate of 2.3 points per second. The curve in Figure 2 contains 950 averaged points from the time of departure (t ) 135 s) from the baseline (0 mV) to the time of return to baseline (t ) 550 s). Integration of the response curve using the trapezoidal rule yields 52.12 mV s. On the basis of the calibration constant k ) 0.05488 W/mV, the heat recorded by the pulse is 52.12 × 0.05488 ) 2.860 J. The experimental correlation for the spurious heat effect given by eq 9 was 0.084 J. The net heat assigned to adsorption is 2.860 - 0.084 ) 2.776 J. The differential heat of adsorption is 2.776/ 0.0950 ) 29.22 J/mmol or 29.22 kJ/mol. The isosteric heat of adsorption from eq 8 is 29.22 + 2.46 ) 31.68 kJ/mol. The adsorption isotherm was obtained by the volumetric method, on the basis of the difference between the amount of gas introduced into the cell and the amount of gas remaining in the dead space of the cell at equilibrium. Adsorption isotherms for SF6, C2H6, CO2, and CH4 were also measured at approximately 40 and 60 °C in a separate volumetric apparatus. (27) Handy, B. E.; Sharma, S. B.; Spiewak, B. E.; Dumesic, J. A. A Tian-Calvet heat flux microcalorimeter for the measurement of differential heats of adsorption. Meas. Sci. Technol. 1993, 4, 13501356.
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Table 5. Isosteric Heat and Amount Adsorbed
Table 6. Adsorption Isotherms
P, n, Qst, P, n, Qst, P, n, Qst, Torr mmol/g kJ/mol Torr mmol/g kJ/mol Torr mmol/g kJ/mol
P, Torr
n, mmol/g
8.66 27.60 63.60 74.06
0.216 0.508 0.828 0.894a
31.49 31.06
5.16 11.12 18.50 29.14
0.071 0.140 0.213 0.303
14.09 37.03 70.09 101.70
0.187 0.430 0.697 0.886
C2H6 on Silicalite (0.539 g) at 40.57 °C 163.60 1.128 411.0 1.521 787.8 241.00 1.311 443.1 1.546 798.0 319.30 1.428 553.1 1.616 910.3 352.40 1.466a 669.1 1.665
1.706 1.710a 1.738
19.26 40.32 62.82 85.51
0.131 0.255 0.373 0.478
C2H6 on Silicalite (0.539 g) at 59.92 °C 116.50 0.605 312.2 1.079 606.2 149.30 0.717 399.3 1.197 719.5 184.70 0.820 427.0 1.226a 832.0 240.00 0.950 501.5 1.297
1.371 1.434 1.482
44.40 59.37 75.95 91.08 107.71 132.80
0.0363 0.0476 0.0600 0.0711 0.0834 0.1013
16.97 17.06 16.19
21.24 49.95 82.13
27.92 27.90 28.06 28.10 28.06
1.09 1.80 2.54 3.53 5.59 7.83 11.31 15.02 19.08 24.57
0.031 0.051 0.072 0.099 0.156 0.213 0.298 0.382 0.468 0.574
C2H6 on Silicalite (0.8752 g) at 23.03 °C 31.18 0.686 31.40 168.05 1.541 37.22 0.780a 31.98 194.43 1.602a 38.93 0.800 31.46 199.64 1.607 45.89 0.892 31.50 229.95 1.662 31.32 55.95 1.000 31.67 235.04 1.666 30.13 67.41 1.102 31.84 269.01 1.715 31.51 81.50 1.203 32.10 311.43 1.761 31.69 97.80 1.297 32.00 352.96 1.805a 31.29 117.29 1.385 32.19 355.45 1.802 31.51 140.12 1.465 32.04 402.98 1.837
2.66 5.50 8.60 11.86 15.58 19.42
0.077 0.154 0.234 0.312 0.395 0.474
C2H6 on Silicalite (0.6332 g) at 23.31 °C 31.44 23.48 0.551 30.61 59.59 1.019 31.07 28.27 0.634 31.58 67.83 1.089 31.24 33.16 0.711 31.53 77.10 1.157 31.28 39.24 0.796 31.78 88.13 1.226 45.21 0.870 31.38 99.18 1.284 30.88 52.06 0.946 31.88 111.63 1.341
32.23 31.90 31.64 32.16 32.56 32.30
2.53 5.29 8.21 11.30 14.74 18.33
0.075 0.152 0.228 0.303 0.381 0.458
C2H6 on Silicalite (0.6332 g) at 22.95 °C 31.31 22.45 0.538 31.31 57.05 1.006 31.11 26.88 0.618 31.23 64.89 1.075 31.17 31.78 0.698 31.62 74.34 1.148 31.23 37.51 0.781 31.69 83.98 1.211 30.97 42.54 0.854 31.25 95.07 1.274 31.22 49.62 0.930 32.18 107.85 1.336
32.27 31.85 32.21 31.90 32.30 31.72
25.07 50.34 76.34 102.37 128.53 156.64 182.78 209.14 239.65 269.32
0.0055 0.0108 0.0162 0.0217 0.0272 0.0331 0.0385 0.0439 0.0503 0.0562
33.42 53.32 74.27 96.30 119.54 143.72 168.64
0.183 0.265 0.343 0.419 0.494 0.567 0.637
O2 on Silicalite (3.26 g) at 32.30 °C 16.37 299.04 0.0621 568.62 15.51 330.08 0.0683 15.08 575.07 15.38 360.38 0.0743 16.44 599.09 16.62 391.74 0.0805 16.30 630.77 15.55 421.06 0.0865 662.23 15.24 451.74 0.0924 17.60 694.15 16.27 482.86 0.0982 16.62 715.78 15.26 512.25 0.1038 17.38 724.78 15.86 539.06 0.1089a 16.31 756.26 16.13 543.99 0.1098 16.49
CO2 on Silicalite (2.3556 g) at 30.60 °C 195.55 0.709 27.20 414.09 1.159 223.70 0.780 27.40 450.08 1.216 27.70 252.48 0.848 27.28 488.81 1.273 27.62 282.18 0.913 27.90 504.00 1.290a 27.38 314.79 0.980 27.62 526.17 1.325 27.76 347.85 1.043 28.24 566.89 1.376 27.50 380.38 1.101 27.64
20.81 48.98 79.24 112.52 116.97 150.09 182.54 215.82 248.53
0.0069 0.0156 0.0248 0.0347 0.0362 0.0460 0.0553 0.0648 0.0741
N2 on Silicalite (1.0239 g) at 22.95 °C 281.98 0.0836 17.03 513.39 16.84 309.09 0.0917a 18.79 548.21 18.09 314.69 0.0927 17.85 583.09 17.74 342.35 0.1008a 17.04 618.08 348.22 0.102 17.15 630.48 17.95 375.97 0.110 18.22 655.10 17.50 409.52 0.119 17.41 689.89 17.10 443.69 0.128 18.21 17.49 477.23 0.137 16.49
49.22 74.20 99.33 124.54 149.80 174.78 199.60 224.34 249.19
0.0112 0.0169 0.0227 0.0285 0.0342 0.0398 0.0454 0.0510 0.0566
Ar on Silicalite (2.356 g) at 32.60 °C 15.94 274.77 0.0622 16.37 479.37 15.22 299.41 0.0676 15.60 481.32 15.34 325.60 0.0732 15.78 504.87 14.91 350.52 0.0786 15.89 530.46 16.23 376.03 0.0841 15.82 556.41 15.90 401.82 0.0896 15.47 583.00 15.02 428.32 0.0952 15.69 608.14 17.13 454.01 0.1008a 17.66 15.75 454.76 0.1008 16.15
0.97 1.34 1.72 2.17 2.64 3.23 3.89 4.57 5.22 19.47 37.82 61.12 84.62 107.26 131.15 135.00 155.03 158.75 182.41 207.01 a
0.1144a 0.1157 0.1202 0.1261 0.1319 0.1379 0.1424a 0.1435 0.1495
17.20 16.27 16.52 16.80 17.16
0.146 0.154 0.163 0.172 0.175a 0.181 0.190
18.27 18.60 17.56 17.86 18.61 17.98 17.39
0.1061a 0.1065 0.1115 0.1169 0.1223 0.1278 0.1331
15.68 15.87
0.044 0.060 0.076 0.093 0.111 0.134 0.158 0.181
SF6 on Silicalite (2.3556 g) at 31.79 °C 5.31 0.205 33.84 41.58 0.847 36.00 6.08 0.229 33.94 50.60 0.941 33.64 6.87 0.253 35.72 61.20 1.032 33.65 10.67 0.356 73.97 1.125 15.45 0.454 36.54 88.07 1.210 36.78 20.60 0.553 37.12 105.33 1.294 35.22 26.58 0.652 37.18 34.06 33.64 0.750 37.54
0.007 0.025 0.048 0.076 0.103 0.129 0.156 0.161a 0.182 0.187 0.212 0.238
CH4 on Silicalite (0.5542 g) at 23.07 °C 21.46 232.26 0.264 20.64 540.11 0.530 21.36 257.11 0.288 20.78 566.59 0.550 20.66 282.17 0.312 20.92 566.87 0.550a 20.84 307.33 0.336 21.02 592.83 0.569 21.01 332.52 0.359 21.22 593.57 0.570 357.90 0.382 21.08 620.59 0.589 20.75 382.95 0.404 647.06 0.608 21.67 409.14 0.426 20.66 673.65 0.626 20.88 434.91 0.447 21.06 674.21 0.628a 20.98 460.56 0.468 21.39 701.04 0.645 21.14 486.76 0.489 21.30 20.98 513.43 0.510 21.36
Indicates desorption point.
32.14 33.33 32.17 32.04 32.25 32.52 31.52
16.51 15.88 14.81 15.06
37.46 37.94 38.14 38.16 38.18 38.58
21.25 21.16 21.09 21.06 21.88 21.62 21.35 21.58 21.87 20.48
P, Torr
n, mmol/g
P, Torr
n, mmol/g
P, Torr
SF6 on Silicalite (0.539 g) at 40.41 °C 100.47 1.028 214.6 1.337 555.1 607.1 101.25 1.031a 296.2 1.443 146.70 1.189 383.2 1.519 674.5 150.55 1.203 448.4 1.558 SF6 on Silicalite (0.539 g) at 59.25 °C 49.22 0.439 260.0 1.069 623.0 82.64 0.608 329.3 1.163a 699.0 150.90 0.840 420.2 1.259 836.6
n, mmol/g 1.606 1.623a 1.643
1.387 1.420a 1.470
CH4 on Silicalite (0.566 g) at 40.12 °C 152.90 0.1156 292.9 0.208 516.8 178.20 0.1325 333.5 0.233 518.4 205.30 0.1512 371.7 0.256 582.0 236.30 0.1716 408.3 0.278 669.5 266.80 0.1912 450.3 0.302 737.5
0.339 0.340a 0.374 0.418 0.452
0.0192 0.0413 0.0650
CH4 on Silicalite (0.539 g) at 40.67 °C 137.1 0.1042 344.3 0.2372 592.4 198.5 0.1457 421.2 0.2823 695.1 273.8 0.1944 499.8 0.3262 865.0
0.3752 0.4260 0.5050
21.30 50.06 78.30 121.55
0.0190 0.0412 0.0622 0.0935
CH4 on Silicalite (0.539 g) at 40.73 °C 166.2 0.1248 353.0 0.2435 606.4 220.8 0.1606 389.9 0.2650a 703.0 285.1 0.2022 433.9 0.2900 918.4 514.4 0.3442
20.38 46.45 60.96 77.53 96.28 115.7
0.0104 0.0227 0.0294 0.0370 0.0454 0.0537
37.77 61.32 88.53 130.10
0.3820 0.4298 0.5300
CH4 on Silicalite (0.566 g) at 61.24 °C 128.9 0.0595 242.8 0.1090a 523.9 150.9 0.0697 278.9 0.1247 602.7 178.2 0.0822 315.8 0.1391 654.4 200.6 0.0920 380.0 0.1636 699.9 236.7 0.1074 447.2 0.1895 803.2
0.2184 0.2462 0.2637a 0.2804 0.3130
0.151 0.217 0.287 0.384
CO2 on Silicalite (0.539 g) at 40.69 °C 173.60 0.476 448.3 0.923 732.8 227.00 0.580 496.0 0.984a 839.4 303.40 0.712 546.0 1.041 885.3 395.40 0.850 631.2 1.132 945.2
1.230 1.319 1.359a 1.399
84.2 128.8 163.1 213.9
0.166 0.232 0.280 0.346
CO2 on Silicalite (0.539 g) at 61.18 °C 260.9 0.403 532.5 0.678 734.4 310.4 0.459 568.1 0.708a 803.7 360.4 0.513 639.7 0.767 939.8 430.9 0.584 691.4 0.805
25.58 67.24 156.25
0.0045 0.0109 0.0235
50.68 68.00 86.12 103.36
0.837a 0.886 0.975
N2 on Silicalite (0.566 g) at 61.46 °C 301.5 0.0435 520.6 0.0734 729.2 445.8 0.0629 605.4 0.0836 824.6
0.1001 0.1117
0.0070 0.0092 0.0114 0.0134
N2 on Silicalite (0.566 g) at 71.57 °C 120.5 0.0154 235.4 0.0288 146.8 0.0185 300.3 0.0359 173.6 0.0217 388.1 0.0456 203.6 0.0250 464.6 0.0531
539.8 618.2 693.2 807.2
0.0611 0.0689 0.0761 0.0871
122.3 230.3 333.5
0.0266 0.0493 0.0703
Ar on Silicalite (0.603 g) at 33.36 °C 434.6 0.0895 639.1 0.1270 844.9 536.4 0.1085 738.5 0.1447 975.1 800.9a 0.1613
0.1629 0.1941
123.05 231.8 336.1
0.0196 0.0361 0.0517
Ar on Silicalite (0.603 g) at 51.63 °C 442.3 0.0667 641.1 0.0931 844.0a 0.1206 539.85 0.0798 739.15 0.1063 943.1 0.1322 840.9 0.1191
94.29 208.7 337.0
0.0112 0.0238 0.0373
a
Ar on Silicalite (0.603 g) at 70.51 °C 467.0 0.0507 667.4 0.0699 792.7 564.95 0.0600 745.0a 0.0777 934.3
Indicates desorption point.
0.0815 0.0944
Heats of Adsorption and Adsorption Isotherms
5. Results and Discussion Adsorption isotherms were measured in the combination calorimetric-volumetric apparatus for Ar, O2, N2, CH4, CO2, C2H6, and SF6 on silicalite in the interval 23-33 °C (see Table 5). Separate isotherms at about 40 and 60 °C were measured independently in a volumetric apparatus for CH4, CO2, C2H6, and SF6 (see Table 6). Measurements were made in the interval 0-1000 Torr. To check the reproducibility of the calorimeter, three separate experiments were performed for C2H6 on silicalite: two runs on the same sample (0.6332 g) and one run on a different sample (0.8752 g). Figures 3 and 4 show the isotherms and heats of adsorption, respectively, for these runs. The isotherms were highly reproducible, as shown by Figure 3; the attainment of equilibrium was proven by the coincidence of desorption with adsorption. The heats of adsorption for different experiments agree within (2%. Figures 5-11 show isotherms obtained with the combination calorimeter-volumetric apparatus. Figures 5-9 also show isotherms obtained in a separate volumetric apparatus in the range 40-70 °C. These log-log plots are useful for examining the low-pressure portion of the adsorption isotherm. The Henry’s law region where eq 2 applies corresponds to a slope of unity on a log-log plot. The gases Ar, O2, and N2 are nearly in the Henry’s law regime over the pressure range studied. Figures 7-11 also show isotherms reported by Golden and Sircar;28 these data, upon interpolation with respect to temperature, are in good agreement with our results. A linear plot of the adsorption isotherms of C2H6, CO2, CH4, and N2 is shown in Figure 12. The SF6 isotherm is not shown because it intersects the C2H6 isotherm; the O2 and Ar isotherms overlap the N2 data. Figure 13 shows the isosteric heats of adsorption for N2, CH4, CO2, C2H6, and SF6. Data for O2 and Ar are not plotted because they overlap the N2 data. For N2 the uncertainty in the solid line plotted in Figure 13 is (1 (28) Golden, T. C.; Sircar, S. Gas adsorption on silicalite. J. Colloid Interface Sci. 1994, 162, 182-188. (29) Choudhary, V. R.; Mayadevi, S. Adsorption of methane, ethane, ethylene, and carbon dioxide on high silica pentasil zeolites and zeolitelike materials using gas chromatography pulse technique. Sep. Sci. Technol. 1993, 28, 2197-2209. (30) Rees, L. V. C.; Hampson, J.; Bru¨ckner, P. Sorption of single gases and their binary mixtures in zeolites. In Zeolite Microporous Solids: Synthesis, Structure and Reactivity; Derouane, E. G., et al., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1992; pp 133-149. (31) Otto, K.; Montreuil, C. N.; Todor, O.; McCabe, R. W.; Gandhi, H. S. Adsorption of hydrocarbons and other exhaust hydrocarbons on silicalite. Ind. Eng. Chem. Res. 1991, 30, 2333-2340. (32) Chiang, A. S.; Dixon, A. G.; Ma, Y. H. The determination of zeolite crystal diffusivity by gas chromatography. II. Experimental. Chem. Eng. Sci. 1984, 39 (10), 1461-1468. (33) Abdul-Rehman, H. B.; Hasanain, M. A.; Loughlin, K. F. Quaternary, ternary, binary, and pure component sorption on zeolites. I. Light alkanes on Linde S-115 silicalite at moderate to high pressures. Ind. Eng. Chem. Res. 1990, 29, 1525-1535.
Langmuir, Vol. 12, No. 24, 1996 5895
kJ/mol. The uncertainty in the heat of adsorption for O2 and Ar is comparable to that for N2. For the lighter gases O2, N2, and Ar, the uncertainty in the heat of adsorption is between 5 and 10%. As discussed previously, for higher heats of about 30 kJ/mol, the uncertainty in the heat is about (2% (see Figure 4). Figure 14 compares heats of adsorption determined calorimetrically (solid lines) with heats obtained by eq 1 and from the isotherms plotted in Figures 5-8 (dashed lines). The excellent agreement of the Clapeyron equation with calorimetry for CH4, CO2, and SF6 supports the thermodynamic consistency of our results and confirms the value of the calibration constant (0.05488 W/mV) determined from C2H6. Isosteric heats of adsorption based upon extrapolation of the calorimetric data to the limit of zero coverage are reported in Table 1. A review of experimental data reported in the literature for adsorption of methane, ethane, and carbon dioxide on silicalite is summarized in Tables 2-4. What is apparent is the wide variation of almost 40% in heats of adsorption reported by different investigators. Our result for the increase in isosteric heat for the addition of a methyl group of 10.2 kJ/mol agrees well with figures of 10.0-10.4 kJ/mol.18,34,37 Our heats of adsorption increase in the order Ar, O2, N2, CH4, CO2, C2H6, SF6. The heat profiles either increase slightly with coverage (CH4, CO2, C2H6) or markedly with coverage (SF6) (see Tables 5 and 6). For Ar, O2, and N2 the coverage is too low to determine the variation of heat with coverage, but the profiles are flat. In summary, these profiles indicate a low degree of energetic heterogeneity for both nonpolar and quadrupolar (CO2, N2) gases. The heats of adsorption increase roughly in proportion to the polarizability of the adsorbate molecules, indicating the predominance of dispersion compared to electrostatic interactions in silicalite. Acknowledgment. This research was supported by National Science Foundation Grant CTS 9213882 and by Air Products and Chemicals, Inc. LA960495Z (34) Hufton, J. R.; Danner, R. P. Chromatographic study of alkanes in silicalite: equilibrium properties. AIChE J. 1993, 39 (6), 954-961. (35) Bu¨low, M.; Schlodder, H.; Rees, L. V. C.; Richards, R. E. Molecular mobility of hydrocarbon ZSM5/silicalite systems studied by sorption uptake and frequency response methods. New Developments in Zeolite Science and Technology; Proceedings of the 7th International Zeolite Conference; Murakami, Y., Iijima, A., Ward, J. W., Eds.; Elsevier: Amsterdam, 1986; pp 579-586. (36) Hampson, J. A.; Rees, L. V. C. Adsorption of ethane and propane in silicalite-1 and zeolite NaY: Determination of single components, mixture and partial adsorption data using an isosteric system. J. Chem. Soc., Faraday Trans. 1993, 89 (16), 3169-3176. (37) Dubinin, M. M.; Rakhmatkariev, G. U.; Isirikyan, A. A. Adsorption energetics of hydrocarbons on silicalite. Izv. Akad. Nauk SSSR, Ser. Khim. 1989, No. 10, 2333-2335.
