J. Phys. Chem. 1991, 95, 310-319
310
Room-temperature 129Xe NMR is a convenient means of studying heterogeneous distributions of HMB in NaY, providing the heterogeneity length scales are larger than about 10 ^tm. Such situations may lend themselves to NMR chemical shift imaging methods in which a linear magnetic field gradient might be used to image xenon profiles in samples possessing macroscopic adsorbate heterogeneities. The suitability of multiple-quantum NMR spectroscopy for probing adsorbate distributions quantitatively is due primarily to the sensitivity of the technique to the number of dipole-dipole coupled spins in a collection of isolated molecules. Counting the number of proton spins in clusters of chemisorbed
means by which a catalyst’s microscopic adsorbate structure can be correlated with its chemical reaction properties. Such information, used in conjunction with the macroscopic adsorbate distributions measured by 129Xe NMR spectroscopy, is key to characterizing intracrystalline mass transport and adsorption of reactant species within zeolitic cat-
tentially valuable
alysts.
Acknowledgment. We thank D. N. Shykind and M. Trecoske for assistance with the multiple-quantum experiments and with zeolite sample preparation. M. G. Samant kindly provided the NaY zeolite used. This work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Sciences Division of the U.S. Department of Energy, under Contract No. DE-AC03-76SF00098.
J. Phys. Chem. 1991.95:310-319. Downloaded from pubs.acs.org by EAST CAROLINA UNIV on 01/09/19. For personal use only.
organic species yields information on their spatial distributions and, thus, about microstructural features of the adsorption sites themselves. Because of the central importance of these sites to the reaction process, multiple-quantum NMR represents a po-
Preparation and Characterization of Highly Dispersed Cobalt Oxide and Sulfide Catalysts Supported on SI02 Yasuaki Okamoto,* * Kozo Nagata,* Toshinori Adachi,* Toshinobu Imanaka,* Kazuhiro Inamura,* and Toshiyuki Takyu* 1*
Department of Chemical Engineering, Faculty of Engineering Science, Osaka University, Toyonaka, Osaka 560, Japan, and Central Research Laboratories, Idemitsu Kosan Co., Ltd., 1280, Kamiizumi, Sodegaura-machi, Kimitsu-gun, Chiba 299-02, Japan (Received: June 13, 1990)
Physicochemical characterization of calcined and sulfided CoO/Si02 catalysts were carried out to reveal the interaction modes between cobalt and Si02 using XPS, TPR, TEM, DRS-VIS, and XRD techniques. The CoO/Si02 catalysts were prepared by an impregnation method using cobalt acetate as well as cobalt nitrate and by an ion-exchange technique. It was found that several kinds of cobalt species are formed on CoO/Si02. These species are assigned to Co304, Co-Si-O mixed oxide, surface Co3+ species, surface silicate, surface Co2+ species in the order of the TPR reduction temperature. Their proportions strongly depended on the starting salt, cobalt content, and preparation method. Cobalt acetate was found to
provide highly dispersed CoO/Si02 catalysts with a uniform distribution of cobalt species throughout the catalyst particles compared to conventionally employed cobalt nitrate. The proportion of Co3+ greatly decreased when cobalt acetate was used instead of cobalt nitrate. All the cobalt species interacting with Si02 were found to be sulfided at 673 K. It was demonstrated that sulfided CoO/Si02 catalysts prepared from cobalt acetate show several times higher hydrogenation activity than the catalysts from cobalt nitrate. On the basis of the XPS characterization of uncalcined precursors, the effects of starting salt on the cobalt-Si02 interaction modes and cobalt dispersion and distribution are discussed. as
with CoO/Al203 systems. Very recently, Castner et al.24,25 have shown by using TPR, XPS, AEM, XAS, and XRD that after
Introduction Supported cobalt-molybdenum sulfide catalysts have been widely employed for hydroprocessing of petroleum feedstocks and extensively investigated by many workers.1,2 Recently, Topsoe et al.3,4 have proposed that the formation of so-called Co-Mo-S phases generate catalytic synergies between Co and Mo. The Co sites in the Co-Mo-S phases are considered to form catalytically active centers. In addition, de Beer and Prins et al.5-8 have shown that carbon-supported cobalt sulfides exhibit very high hydrodesulfurization (HDS) activities as compared to carbon- or alumina-supported molybdenum sulfide catalysts, suggesting that sulfided cobalt species show a high HDS activity in a specific configuration as the Co species in the Co-Mo-S phases do. Besides hydrotreating processes, supported cobalt metal catalysts are extensively used for the hydrogenation of carbon monoxide to produce hydrocarbons.9,10 The physicochemical characterization of Al203-supported cobalt catalysts has shown the presence of several cobalt species in oxide states by using XPS,11"16 X-ray absorption spectroscopy (XAS),17 temperature-programmed reduction (TPR),18,19 and other techDetailed characterization of cobalt species on niques.20-23 SiOrsupported catalysts are, however, very scarce as compared
Massoth, F. E. Adv. Catal. 1978, 27, 265. Grange, P. Catal. Rev.-Sci. Eng. 1980, 21, 135. Topsoe, H.; Clausen, B. S. Catal. Rev.-Sci. Eng. 1984, 26, 395. Wievel, C.; Candia, R.; Clausen, B. S.; Morop, S.; Topsoe, H. J. Catal. 1981, 68, 453. (5) Prins, R.; de Beer, V. H. J.; Somorjai, G. A. Catal. Rev.-Sci. Eng. 1989, 31, 1. (6) Vissers, J. P. R.; de Beer, V. H. J.; Prins, R. J. Chem. Soc., Faraday Trans. I 1987, 83, 2145. (7) Duchet, J. C.; van Oers, E. M.; de Beer, V. H. J.; Prins, R. J. Catal. 1983, 80, 386. (8) Bouwens, S. M. A. M.; Koningsberger, D. C.; de Beer, V. H. J.; Prins, R. Catal. Lett. 1988, 1, 55. (9) Vannice, M. A.; Garten, R. L. J. Catal. 1979, 56, 236. (10) Anderson, R. B. The Fischer-Tropsch Synthesis; Academic Press: New York, 1984. (11) Okamoto, Y.; Nakano, H.; Imanaka, I.; Teranishi, S. Bull. Chem. Soc. Jpn. 1975, 48, 1163. (12) Okamoto, Y.; Imanaka, I.; Teranishi, S. J. Catal. 1980, 65, 448. (13) Grimblot, J.; Bonnelle, J. P.; Beaufils, J. P. J. Electron Spectrosc. Relat. Phenom. 1976, 8, 437. (14) Bonnelle, J. P.; Grimblot, J.; D’Huysser, A. J. Electron Spectrosc. Relat. Phenom. 1975, 7, 151. (15) Declerck-Grimee, R. I.; Canesson, P.; Friedman, R. M.; Fripiat, J. J. J. Phys. Chem. 1978, 82, 885. (16) Chin, R. L.; Hercules, D. M. J. Phys. Chem. 1982, 86, 360.
(1) (2) (3) (4)
’Osaka University. *
Idemitsu Kosan Co.
0022-3654/91 /2095-0310S02.50/0
©
1991
American Chemical Society
Si02-Supported Cobalt Oxide and Sulfide Catalysts
The Journal
of Physical Chemistry,
Vol. 95, No.
I,
1991
311
highly dispersed cobalt oxide and sulfide catalysts. It was found that several kinds of interaction species are present on Si02 surfaces in addition to crystalline Co304 and that cobalt acetate provides the catalysts with significantly higher degree of the cobalt dispersion on Si02 than cobalt nitrate. Similar results have been obtained for Co0/Al203 catalysts that will be reported elsewhere.32
Figure
1.
Pore size distribution
of Si02 (JRC-SIO-4).
calcination at 723-813 K major cobalt species are Co304 on CoO/Si02 prepared from cobalt nitrate (5-10 wt % Co) and that the Co304 phase is moderately dispersed with a uniform particle size of 5—20 nm, depending on the pore size of Si02. These cobalt oxide species were completely reduced by H2 below 723 K. Pepe et al.26 have reported that at a lower calcination temperature (523 K) Co304 of a particle size of 12-15 nm is formed on CoO/Si02 prepared from cobalt nitrate. These observations suggest that the dispersion of cobalt is moderate on Si02. On the other hand, Pepe et al.26 have noticed that CoO/Si02 catalysts prepared by using cobalt oxalate contains X-ray amorphous phases besides crystalline Co304 after calcination at 523 K. Recently, Sugi et al.27,28 have shown that in contrast to cobalt-Si02 catalysts prepared from cobalt nitrate, cobalt catalysts (uncalcined) from cobalt acetate are not reduced by H2 at 723 K. Their brief XPS characterization suggested the presence of Co2+ after the H2 reduction.29 The formation of cobalt species unreducible at 673 K has also been reported by Reuel and Bartholomew30 with a CoO/Si02 catalyst (3 wt % Co) prepared by a pH-controlled precipitation method.31 Accordingly, it is inferred that CoO/Si02 catalysts contain several cobalt species interacting with Si02 besides crystalline Co304 particles and that the cobalt-Si02 interaction modes depend strongly on preparation variables such as a starting material. No detailed investigation of CoO/Si02 has been carried out concerning the characterization of the interaction species on CoO/Si02 and the reduction and sulfiding behaviors of these species. In the present study, CoO/Si02 catalysts were prepared by an impregnation of cobalt acetate as well as conventional cobalt nitrate and by an ion-exchange technique and characterized by TPR, XPS, transmission electron microscope (TEM), diffuse reflectance visible spectroscopy (DRS-VIS), and XRD to reveal the effects of preparation variables on cobalt-Si02 interaction modes and on the dispersion and distribution of cobalt species. Reduction and sulfidation behaviors and catalytic properties of cobalt were investigated to provide a clue for the preparation of (17) Greegor, R. B.; Lytle, F. W.; Chin, R. L.; Hercules, D. M. J. Phys. Chem. 1981, 85, 1232. (18) Arnoldy, P.; Moulijn, J. A. J. Catal. 1985, 93, 38. (19) Arnoldy, P.; de Brooys, J. L.; Scheffer, B.; Moulijn, J. A. J. Catal. 1985, 96, 122. (20) Chung,.K. S.; Massoth, F. E. J. Catal. 1980, , (c) 10 wt %, and (d) 20 wt % CoO.
and 470 nm can be assigned to the transitions T 1(4 — 4A2(, |T)g, and 3A|g, respectively. However, the color of the sample changed from light pink to blue on grinding using mortar and pestle (ca. 10 min) at room temperature. The DR spectrum (b) of the ground sample is characteristic of Co2+ in a tetrahedral configuration and
500
600
700
800
900
Reduction Temperature/K 5. Figure Temperature-programmed reduction profiles of CoO/Si02 and CojO* (d = 2.5 K min'1): (a) 5 wt % CoO/Si02(A), (b) 10 wt % CoO/Si02++> + >±> (not detected). -
surface Co2+ species in a (distorted) tetrahedral symmetry. The formation of surface cobalt silicate is also possible due to an expected high stability of the silicate and hence Co V is tentatively assigned to surface silicate. A considerable proportion of the ion-exchanged Co2+ species is converted to the surface Co2+ species in a tetrahedral symmetry on calcination at 673 K. It is inferred, however, that a majority of the surface Co2+ species on CoO/ Si02(A) are produced from well-dispersed Co2+ acetate species during the calcination, since the amount of ion-exchanged cobalt was only 4.1 wt % CoO for 20 wt % CoO/Si02(A) under the present impregnation conditions. The cobalt species on CoO/Si02(N), -(A), and -(I) are qualitatively summarized in Table III. Their proportions depend on CoO content as deduced from the TPR results in Figures 4-6. The assignment of cobalt species in Table III is in conformity with the Co(2p3/2) BEs in Table I and Co(2p)/Si(2p) intensity ratios (vide infra). It is deduced that the above remarkable differences in the cobalt species between CoO/Si02(A) and -(N) result from the presence of N03" anions on CoO/Si02(N), which effectively oxidize Co2+ to Co3+ and hence facilitate the formations of Co304, Co-Si-0 mixed oxide, and Co3+ interaction species. 2. Dispersion and Distribution of Cobalt on CoO/Si02. The XRD and TEM observations in Figures 2 and 3 indicate that cobalt species are much more highly dispersed on CoO/Si02(A) than on CoO/Si02(N). The Co(2p)/Si(2p) XPS intensity ratios in Figure 11 agree with these observations, taking into account eq 1.
The decreases in the Co(2p)/Si(2p) ratio for CoO/Si02(N) grinding strongly suggest that cobalt species considerably segregate on the outer surface of Si02. The intense segregation of cobalt on CoO/Si02(N) is not caused during the impregnation processes, since cobalt species on uncalcined CoO/Si02(N) distributes in a considerably uniform manner throughout the Si02 particles as suggested by a comparison of the Co(2p)/Si(2p) ratios (Figure 12) measured before and after grinding. The Co(2p)/ Si(2p) ratios of uncalcined CoO/Si02(N) samples significantly diminished on calcination at 673 K. This indicates intense agglomeration of cobalt, forming Co304 and Si^2Co3_^04. It is obvious that these strong surface segregation and agglomeration of cobalt on CoO/Si02(N) are brought about by the calcination and hence by the oxidation of Co2+ to Co3+ by N03“ anions. The downward deviation of the Co(2p)/Si(2p) ratio from the linear correlation (Figure 11) implies that the extent of the agglomeration of cobalt strongly increases at 20 wt % CoO on the calcination. In contrast to CoO/Si02(N), no significant surface segregation and agglomeration of cobalt are observed for 2-10 wt % CoO/ Si02(A), indicating a uniform cobalt distribution and a high dispersion of cobalt species. The formation of the surface Co2+ species (Co VI) is responsible for the high dispersion of cobalt. A significant surface segregation of cobalt acetate observed for uncalcined 20 wt % CoO/Si02(A) is considered to result from a surface deposition of cobalt acetate due to a low solubility as compared to cobalt nitrate. An increase in the Co(2p)/Si(2p) ratio on grinding might suggest that cobalt acetate is deposited in pores slightly deeper than the surface layer sampled by XPS (2-5 nm). On calcination at 673 K, the distribution of cobalt becomes considerably uniform and the dispersion of cobalt is kept on
Okamoto et al.
Vol. 95, No. 1, 1991
high, suggesting a considerable surface mobility of Co2+ cations at 673 K. With CoO/Si02(I), it is apparent from Figure 12 that ionexchange takes place from the outer surface to the inner surface of Si02. A surface segregation of cobalt is still remarkable after the calcination. On the basis of the XPS ratio after grinding (Figure 11), it is concluded that the dispersion of cobalt is very high on calcined CoO/Si02(I). This is consistent with the TPR results in Figures 5 and 6. 3. Sulfided CoOjSi02 Catalysts. It is evident from Figure 10 that all the cobalt species on Si02, Co I—VII, are sulfided under the present sulfiding conditions. Taking into account the fact that the reduction temperatures of the surface Co2+ species in a tetrahedral symmetry and ion-exchanged Co2+ species are significantly higher than the sulfiding temperature (673 K), it is deduced that sulfiding of cobalt species takes place via O-S anion exchanges rather than via H2 reductions of Co2+ followed by sulfidation. Arnoldy et al.19 reached the same conclusion in the temperature-programmed sulfidation (TPS) study of Co0/Al203. With supported molybdenum oxide, a similar sulfidation mechanism has been proposed by Massoth.46 From comparisons of the XPS and XRD results between oxidic and sulfided catalysts, it is concluded that sulfidation of CoO/ Si02(A) and -(N) did not appreciably alter the distribution and dispersion of cobalt in the range 2-10 wt % CoO. At 20 wt % CoO, however, considerable decreases in the Co(2p)/Si(2p) ratio for both catalyst systems suggest enhanced agglomeration of cobalt on the sulfidation, probably, due to a conjestion of the aggregated cobalt phases in the oxidic state. It is evident that an intense agglomeration of cobalt is induced by sulfidation also for CoO/Si02(I), in which a surface accumulation of cobalt is very remarkable in the oxidic form as stated above. However, the extent of dispersion of cobalt is still high on sulfided CoO/Si02(I) even after the agglomeration of cobalt because of inherently high dispersion of cobalt as compared with that of CoO/Si02(N). The high catalytic activities of CoO/Si02(A) and -(I) for the hydrogenation of butadiene in Figure 14 obviously result from high dispersions of sulfided cobalt species as compared with those on CoO/Si02(N). The leveling off of the activity at 20 wt % CoO for CoO/Si02(A) and -(N) is a consequence of the increase in the extent of agglomeration of sulfided cobalt and hence of the leveling off in the number of active sites. This was corroborated by NO adsorption experiments.47 The hydrogenation activity of CoO/Si02(I) was very close to that of CoO/Si02(A) in spite of a high Co(2p)/Si(2p) ratio for CoO/Si02(I). This is ascribed to the surface segregation of cobalt as discussed above. Conclusions
The interaction modes between cobalt and Si02 were characterized by using XPS, TPR, TEM, DRS-VIS, and XRD techniques. CoO/Si02 was prepared by an impregnation method or an ion-exchange technique. It was found that several kinds of cobalt species are formed on Co0/Si02. These cobalt species were assigned to Co304, Si^2Co3-x04, surface Co3+ species, surface silicate, surface Co2+ species in tetrahedral coordinations, and ion-exchanged Co2+ in the order of the TPR reduction temperature. Their proportions were found to strongly depend on starting salt, CoO content, and preparation method. It was revealed that CoO/Si02(A) prepared from cobalt acetate contains predominantly surface Co2+ species in tetrahedral symmetries, while conventional CoO/Si02(N) prepared from cobalt nitrate contains ill-dispersed Co304 and Co-Si-O mixed oxide phases. The cobalt species on CoO/Si02(A) were demonstrated to be highly dispersed and uniformly distributed throughout the catalyst particles as compared with those on CoO/Si02(N). A considerable proportion of ion-exchanged Co2+ species in CoO/Si02(I) was found to transform to the Co2+ species in a tetrahedral symmetry. All the cobalt species interacting with Si02 were found to be sulfided at 673 K. Sulfided CoO/SiQ2(A) and -(I) exhibited several times (46) Massoth, F. E. J. Catal. 1975, 36, 164. (47) Okamoto, Y.; et al., unpublished work.
J. Phys. Chem. 1991, 95, 319-324
319
Acknowledgment. We gratefully acknowledge Dr. Y. Nitta, Department of Chemical Engineering, Osaka University, for the supply of Co2Si04 and Mr. M. Odawara, Department of Chemical Engineering, Osaka University, for the assistance in the XRD and catalytic activity measurements. We are deeply indebted to Mr. K. Nakai, BEL Japan, Inc., for the measurements of the pore size distribution of Si02 (JRC-SIO-4).
higher catalytic activity for the hydrogenation of butadiene than
CoO/Si02(N). On the basis of the XPS characterization including uncalcined precursors, it is proposed that the differences between CoO/Si02(A) and -(N) in the cobalt species and in the dispersion and distribution of cobalt reside in the oxidation of Co2+ to Co3+ during calcination by residual N03~ anions on CoO/
Si02(N) precursors.
Pressure Dependence of the Excess Properties of Simple Molecular Mixtures. The CCI4 + CH2CI2 System A. Compostizo, A. Crespo Colin, M. R. Vigil, R. G. Rubio, and M. Diaz Pena* Departamento de Quimica Fisica, Facultad de Ciencias Qu'imicas, Universidad Complutense, 28040 Madrid, Spain (Received: March 7, 1990; In Final Form: May 17, 1990)
The equation of state of CC14 + CH2C12 has been studied over the whole composition range and for 298.15 < T/K < 318.15 and 0.1 < p/MPa < 40.0. As for the CC14 + CS2 or CC14 + CH2C12 systems, Vs- decreases with increasing pressure, its magnitude for x = 0.5 being similar to that for CC14 + CS2 and less than half the value for CS2 + CH2C12. HE has been calculated as a function of p, and a reasonably good agreement with experimental data was found. //E(x~0.5) is again almost half that of CS2 + CH2C12 but twice that of CC14 + CS2. The change of GE over the pressure range follows a trend similar to F6, but now GE increases with p. The results have been analyzed in terms of two semiempirical generalized van der Waals equations of state, one due to Deiters and another due to Kohler and co-workers. Although both equations describe quite well the (dp/dp)T, they fail in predicting (dp/6T)p for both pure components and binary mixtures. For the binary systems formed with either CC14 or CH2C12, the binary adjustable parameters show almost negligible composition and temperature dependences.
highly polarizable molecules these effects were more intense as a consequence of many-body interactions.10 Using p-V-T data for pure fluids ranging from Ar to CC14, benzene, CH2C12, or CHC13, we have confirmed the validity of the prediction of the Gubbins-Gray theory for the bulk modulus.11 Similar conclusions were reached for CS2 + CH2C12.12 However, the analysis of the excess properties at low pressures of several binary mixtures of simple molecular fluids, formed by molecules that were rigid and had different electrical moments and degrees of shape anisotropy, lead to the conclusion13 that the whole set of experimental data could not be fitted within the qualitative trends found by Flytzany-Stephanopoulos et al.9 This might be due to the fact that molecules such as benzene, CH3I, CH2C12, etc. are too aspherical to be described by a perturbation theory that uses a spherical
Introduction The technological importance of having good theoretical tools for predicting the thermophysical properties of fluids is obvious. However, so far, rigorous statistical mechanical theories can only In fact, only for those molecular systems with relatively low anisotropies in the intermolecular potential can semiquantitative thermodynamic surfaces or computer be obtained from different theoretical approaches1 simulations.5 6789Moreover, the simultaneous existence of anisotropy of shape of the molecular core and multipolar moments has not been treated satisfactorily with any of the current perturbation theories, except for very low anisotropies of shape. The effect of electrical moments upon the thermodynamic properties of fluids with a spherical reference potential has been described in detail by Gubbins’ group.6"8 More specifically, Flytzany-Stephanopoulos et al.9 have studied the effect of polar, quadrupolar, and overlap forces upon the excess properties of binary mixtures using a perturbation theory.2,6 They found that these anisotropic forces led to positive deviations from ideality and caused the excess function curves to depart from symmetry with respect to the equimolar mixture, even leading to S-shaped curves for anisotropy strengths large enough. For mixtures with be applied to quite simple systems.1
2
3"4
reference system. The shape effects have been extensively discussed by Kohler’s group using a perturbation theory based on the/expansion.4,14,15 In a recent paper, Bohn et al.16 have studied the excess properties of mixtures formed by fluids like CC14, CF4, noble gases, etc. They concluded that, in general, when the parameters of the interaction between molecules of different species were fitted to an excess property, the predictions of other excess functions were not different whether a spherical or a site-site reference system was used. This result is in agreement with the fact that a good description of p-V-T data for several pure fluids was obtained using a Lennard-Jones fluid equation of state obtained by computer simulation.11 However, when combining rules were used for calculating the mixed interactions, excluding binary parameters, the inclusion of the anisotropy of shape through the use of site-site
(1) Hansen, J. P.; McDonald, I. R. Theory of Simple Liquids-, Academic London, 1986. (2) Gray, C. G.; Gubbins, K. E. Theory of Molecular Fluids-, Clarendon Press: Oxford, 1984. (3) Chandler, D. In The Liquid State of Matter, Montrol, E. W., Lebowitz, J. L., Eds.; North-Holland Co.: Amsterdam, 1982. (4) Kohler, F.; Marius, W.; Quirke, N.; Perram, I. W.; Hoheisel, C.; Breithenfelder-Manske, H. Mol. Phys. 1979, 38, 2057. (5) Allen, M. P.; Tildesley, D. J. Computer Simulation of Liquids-, Clarendon Press: Oxford, 1987. (6) Gubbins, K. E.; Shing, K. S.; Streett, W. B. J. Phys. Chem. 1983, 87, Press:
(10) Venkatasubramanian, V.; Gubbins, K. E.; Gray, C. G.; Joslin, C. G.
Mol. Phys. 1984, 52, 1411. (11) Compostizo, A.; Crespo Colin, A.; Vigil, M. R.; Rubio, R. G.; Diaz Pena, M. Chem. Phys. 1989, 130, 177. (12) Compostizo, A.; Crespo Colin, A.; Vigil, M. R.; Rubio, R. G.; Diaz Pena, M. J. Phys. Chem. 1989, 93, 4973.
4573.
(7) Clancy, P.; Gubbins, K. E.; Gray, C. G. Discuss. Faraday Soc. 1978, 66, 116. (8) Lobo, L. Q.; Staveley, L. A. K.; Clancy, P.; Gubbins, K. E. J. Chem. Soc., Faraday Trans. I 1980, 76, 1174. Calado, J. C. G.; Gomez de Acevedo, E. J. S.; Clancy, P.; Gubbins, K. E. J. Chem. Soc., Faraday Trans. I 1983, 79, 2657. (9) Flytzany-Stephanopoulos, M.; Gubbins, K. E.; Gray, C. G. Mol. Phys. 1975, 30, 1649.
0022-3654/91 /2095-0319S02.50/0
(13) Crespo Colin, A.; Lezcano, E. G.; Compostizo, A.; Rubio, R. G.; Diaz Pena, M. J. Chem. Soc., Faraday Trans. 1 1989, 85, 4295. (14) Bohn, M.; Lustig, R.; Fischer, J. Fluid Phase Equilib. 1986, 25, 251. (15) Bohn, M.; Fischer, J.; Kohler, F. Fluid Phase Equilib. 1986, 31, 233. (16) Bohn, M.; Lustig, R.; Fischer, J.; Kohler, F. Mol. Phys. 1988, 64, 595.
©
1991
American Chemical Society
