Q Copyright 1993 American Chemical Society
The ACS Joumal of
Surfaces and Colloids MAY 1993 VOLUME 9, NUMBER 5
Letters Enhancement Effect of Micropore Filling for Supercritical Methane by MgO Dispersion Katsumi Kaneko,* Katsuyuki Murata, Kazuyuki Shimizu, Sory Camara, and Takaomi Suzuki Departmeut of Chemistry, Faculty of Science, Chiba University, 1-33 Yayoi, Inage, Chiba 263, Japan Received November 17,1992. In Final Form: January 19,1993
Pitch-based activated carbon fiber (ACF) was modified with MgO fine particles. The high-pressure methane adsorptionisothermsof ACF and MgO-dispersed ACF were determinedat 303K. These isotherms were Langmurian. The MgO dispersion markedly enhanced the methane adsorptivity of ACF. The saturated amount of methane adsorption for MgO-dispersedACF was 1.5 times greater than that of ACF in terms of milligrams per gram; even the methane adsorption per micrppore volume was remarkably enhanced by the MgO dispersion.
Introduction Methane is a primary component of natural gas, which is an important candidate for new transportation fuels. A good adsorbent for methane has been searched for with high-energy density storage. Activated carbon having micropores of high capacity is the most hopeful adsorbent for methane storage. Various kinds of activated carbons have been examined, and the optimum microporous structure of the activated carbon has been studied by molecular simulations.14 The adsorption of methane by microporous carbons at ambient conditions is not easy, becausemethane is a supercriticalgas at room temperature. Vapors can be easily adsorbed in micropores by the micropore filling mechanism. On the contrary even microporous carbons cannot adsorb sufficiently a supercritical gas due to small intermolecular interaction. Application of high pressure to methane adsorptivesleads to an increase of the intermolecular interaction, accompanied by methane adsorption by microporous carbons. (1) &malR. , K.; Amakwak, K. A. G.; Schwatrz, d. A. Carbon 1990, 28,169. Hayhuret, D. T. J. Phys. Chem. 1991,95, (2) Zhang, S.-Y.; Talu, 0.; 1722. (3) Tan, 2.; Gubbine, K. E. J. Phys. Chem. 1992,96,845. (4) Matranga, K. R.; Myem, A. L.; Glandt, E. D. Chem. Eng. Sci., in prw.
0743-746319312409-1165$04.OO/0
The capacity of methane retained at high pressure is govemed by the microporous field strength. Activated carbon fibers (ACFs) are highly microporous solids of greater micropore volume than granulated activated carbon^.^ ACF has a partly ordered structure of micrographite8along the fiber basic methane adsorption studies by ACF are desired from the practical and fundamental aspects. The increase of methane intermolecular interaction by a weak chemisorption should give rise to a marked enhancement of methane adsorption according to the previous studies on supercritical NO and NZ.~'OThis letter describes an enhancement effect of methane adsorption on ACFs by the MgO dispersion. This novel enhancement effect should stimulate the research on methane storage projects.
Experimental Procedure Pitch-based activated carbon fiber (ACF)was used. The ACF surface was modified with MgO fine particles. The preparation (5) Mataumoto, A.; Tsutsumi, K.; Kaneko, K. Longmuir 1992,8,2615. (6) Suzuki, T.; Kaneko, K. Chem. Phys. Lett. 1992,191,569. (7) Mataumoto, A., Kaneko, K.; Ramsay, J. D. F. In Fundamentab of Adsorption; Suzuki, M., Ed.; Koudansha: Tokyo; in press. (8) Kaneko, K. Langmuir 1987,3, 357. (9) Kaneko, K.; Shimizu, K.; Suzuki,T. J. Chem. Phys. 1992,97,8705. (10) Wang, Z.-M.;Suzuki, T.; Uekawa, N.;hakura, K.; Kaneko, K. J . PhYS. CheM. 1992, 96,10917.
0 1993 American Chemical Society
Letters
1166 Langmuir, Vol. 9, No. 5, 1993 120 100
I
0
0.4
I
I
0.8 1.2 a, value
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1
I
1.6
2.0
Figure 1. High-resolution a, plots: (0) MgO-ACF, ( 0 )ACF. is as follows. The ACF sample was immersed in the saturated Mg(N03)Zsolutionat 303 K after evacuation,and then the solution with ACF was adjusted to pH 9 and 10 with NaOH solution. The ACF samples with Mg(OH)* deposits were washed and dried, then heated at different temperatures of 303-773 K in air. In this letter, results on MgO-dispersed ACF prepared by heating at 573 K after deposition will be mainly shown; the ACF sample will be designated MgO-ACF. The thermal change of bulk Mg(OH)z powder was examined by a thermal analyzer (Rigaku, Thermoflex). The microporosity of MgO-ACF was determined by Nz adsorption at 77 K. The methane adsorption at 303 K was determined until the pressure reached 8 MPa with the aid of gravimetric equipment. The buoyancy effect was corrected by use of the true density (1.95 g mL-l for ACF) determined by the He high-pressure measurement. In this paper, the pressure is not converted into the fugacity.
Results and Discussion Micropore Structures. The Nz adsorption isotherms of ACF and MgO-ACF were of typical type I. Those isotherms were analyzedby the high-resolutionananalysis, as shown in Figure 1. Both anplota have a remarkable deviation from the linear plot below as = 0.5. We can subtract the enhancement contribution by the micropore field to evaluate the monolayer capacity using the highresolution asplot. Then, the high-resolution asanalysis provides the correct surface area, the micropore volume, and the micropore width of the slit-shaped pore. The micropore width was determined by the modified Wicke method using the micropore volume and the surfacearea.g The detailed procedure of the asanalysis was reported in previous papers.11J2 These micropore structural parameters are shown in Table I. The MgO dispersion increases slightly both the surface area and the micropore volume. On the other hand, it brings about a slight decrease of the micropore width. As the size of a methane molecule from the second virial coefficient is 0.382 nm,l3 methane molecules can be adsorbed bilayerly in the micropores of both ACF samples. The MgO dispersion does not block the micropores because of the slight increase of the micropore volume; almost fine MgO particles should be dispersed on the external surface of the ACF. Micropore Filling of Supercritical Methane. Figure 2 shows the methane adsorption isotherms at 303 K. The adsorption isotherm can be basically described by the Langmuir equation. The methane adsorption by MgOACF is much greater than that by ACF. The saturated (11)Kaneko, K.; Ishii, C.; Ruike, M.; Kuwabara, H. Carbon 1992,30, 1075. (12)Kaneko, K.;Ishii, C. Colloid Surf. 1992,67, 203. (13)Hirschfelder, J. 0.; Curtiee, C. F.; Bird, R. B. Molecular Theory of Cases and Liquids; John Wiley & Sons: New York, 1964; p 165.
0
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8
C H,Pressure/MPa
Figure 2. Methane adsorption isotherms of ACF &d MgOMgO-ACF, (0)ACF. ACF at 303 K: (0) 2.5 2.0
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cg
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1.0 0.5
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Figure 3. Extended DR plots for the methane adsorption MgO-ACF, ( 0 )ACF. isotherms: (0)
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0 300 400 500 600 700 800 TemperaturelK
Figure 4. Relationships of the saturated amount of methane adsorption at 303 I( and the heating temperature of the ACF samples having magnesium oxides dispersed at pH 9 and 10 (0) pH 10, ( 0 )pH 9. Table I. mcroDore Structures of ACF Samelea surface micropore area volume micropore (mL l3-9 width (nm) sample (m213-9 ACF 1138 0.45 0.83 MgO-ACF 1230 0.49 0.80 ~
amount of methane adsorption, WL, was determined by the Langmuir plot. The W Lof MgO-ACF is 1.5 times greater than that of ACF in terms of mg gl.We propose the supercritical gas to vapor transition mechanism by the micropore field for high-pressure adsorption of supercritical gases in the preceding work.g The micropore field and the high-pressure application induce the enhancement in the intermolecular interaction, leading to
Letters
Langmuir, Vol. 9, No.5, 1993 1167
Table 11. Inherent Micropore Volume for Methane WL, PSa,and a.urlls
ACF MgO-ACF
79 118
176 241
12 23
17.1 17.6
the transition of supercritical gas to quasi-vapor. The saturated amount of adsorption, WL, represents the inherent micropore volume which haa the micropore field strong enough to induce quasi-vaporization. Hence, even high-pressure adsorption of the supercritical gas by micropores can be described by the following extended DR equation proposed by us+ [ln(wL/w)l'/2 = (RT/BEo)(lnPw- In PI (1) Here, the WLand Pw are the inherent micropore volume and the saturated vapor pressure of the quasi-vaporized supercritical methane, respectively. The inherent microporevolume is governed by the methanecarbon surface interaction. The characteristic adsorption energy BE0 can be aeeociatedwiththe isosteric heat of adsorption,qst,ecl/e, at the fractional filling of 8 = l / e by eq 2.14 Here, AHvis
the heat of vaporization of liquid methane at the boiling temperature (8.17 k J mol-'). Both extended DR plots are linear in the wide range of pressure. These liriear plots provide both PW and AHv values. WL, PW,and AHv are listed in Table 11. The qst,ePl/eof 17-18 kJ mol-l is close to that of molecular sieve carbon.l5 The PW of MgO-ACF is greater than that of ACF; the qlt,eP1 e of MgO-ACF is also larger than that of ACF by 0.5 kf mol-'. A similar tendency was observed in other MgO-dispersed ACF samples prepared at different conditions. The slight increase of qst,e=l/e suggests weak interaction between methane molecules and the MgO surfaces. However, the (14) Kawazoe, K.; Astakhov, V. A.; Kawai, T.; Eguchi, Y. Kagaku Kogaku 1971,35, 1006. (15) Agarwal, R. K.; Schwarz, J. A. J. Colloidlnterfaee Sci. 1989,130, 137.
increase in PW is not anticipated by a chemisorptionassisted microporefilling mechanism of supercriticalNO.l0 In the case of the NO chemisorption-assistedmicropore filling, the molecule-eurface interaction enhances the lateral interaction to produce the NO dimer. With methane adsorption by MgO-dispersed ACF, only the vertical interaction may be associated with the enhancement; the MgO dispersionincreasesthe inherent micropore volume for supercritical methane. The methane intermolecular interaction is not enhanced, although the adsorption isotherm can be well described by the exptended DR equation. MgO has good catalytic properties for hydrogen atoms in alkane molecules at high temperature.16J7 The surface chemical property of MgO should play an important role in the enhancement of the vertical interaction. Figure 4 shows the relationships between WL and the heating temperature of Mg(OH)2 dispersed at pH 9 and 10. The WLdoes not change with the heating temperature below 600K in both pH cases. The drop above 600K is presumed to come from the decrease in the micropore volume due to partial burning of ACF according to the preceding related study.18 Consequently,the enhancement effect is insensitive to the preparation condition of the dispersed Mg oxides. Bulk Mg(OH)2powder decomposesinto MgO above 673 K by thermal analysis. Not only MgO but also Mg(OH)2 is effective for the enhancement. Possibly the enhancement effect is associated with chemicalproperties of surface oxygens and hydroxyls. So far we have no definite enhancement mechanism. The dispersed magnesium oxide state will be characterized by X-ray photoelectron spectroscopy and other surface analytical methods in order to elucidate the role of MgO in the enhancement of methane adsorption. Acknowledgment. Finanical support by the Science Research Grant from the Ministry of Education, Japanese Government, and Osaka Gas Research Center is greatly appreciated. (16) Boudart,M.;Delbouille,A.; Derouane, E. G.; Indovina,V.; Waltem, A. B. J. Am. Chem. SOC.1972,94,6622. (17) Lin, C.-H.; Campbell, K. D.; Wang, J.-X.;Lunsford,J. H. J. Phys. Chem. 1986,90, 534. (18) Kaneko, K.; Katori, K.; Shimizu, K.; Shindo, N.; Maeda, T. J. Chem. SOC.,Faraday Trans. 1992,88, 1305.
