4950
Langmuir 1998, 14, 4950-4952
Adsorption of Benzene and Ethanol on MCM-41 Material
vapors offer a means to study the effects of temperature on the capillary condensation process.
C. Nguyen, C. G. Sonwane, S. K. Bhatia, and D. D Do*
II. Experimental Section
Chemical Engineering Department University of Queensland, Brisbane, QLD 4072, Australia Received November 4, 1997. In Final Form: May 29, 1998
I. Introduction In recent years, articles on MCM-41 can be found quite frequently in leading journals. The attention paid to this new family of adsorbents is mainly due to the unique mesopore structure, which is not shared by any other families of adsorbent. Pores in MCM-41 are believed to be straight and parallel to each other, without any interconnection between them. The size of pores in MCM41 can be tailored within the range 13-100 Å1 by using different templating surfactants or modifying the assynthesized sample by means of heat and/or chemical treatment. In terms of adsorption capacity, MCM-41 are comparable to other conventional adsorbents. They possess a rather high BET surface area, typically in the range of 600-1300 m2/g.2 The external surface area is very small compared to the mesopore area.3 Extensive studies of the structure of MCM-41 have made use of advanced characterization techniques such as TEM, XRD, NMR, IR, etc. Nevertheless, the process of characterization by analyzing the adsorption of known adsorbates such as nitrogen at 77 K is still a popular method.4-7 Not surprisingly, a majority of MCM-41 isotherms found in the literature have the characteristic shape associated with physisorption in mesopores, that is monolayer-multilayer adsorption followed by capillary condensation. A very narrow pore size distribution, previously displayed only by microporous adsorbents such as zeolites makes MCM-41 a suitable choice for studies of adsorption in mesoporous adsorbents. The mesopore system in MCM-41 materials is identified as an ideal structure to study some of the fundamental phenomena in adsorption such as hysteresis, pore size estimation, etc. The aim of this paper is to study the adsorption of benzene and ethanol in MCM-41 samples of differing pore diameters. The adsorption is used as a tool for characterizing the pore structure in addition to other methods such as adsorption of nitrogen at 77 K. Furthermore, being carried out at various temperatures, adsorption of these * Corresponding author. (1) Davis, M. E.; Saldarriaga, C.; Montes, C.; Garees, J.; Crowder, C. Nature 1988, 331, 698. (2) Maddox, M. W.; Olivier, J. P.; Gubbins, K. E. Langmuir 1997, 13, 1737. (3) Llewellyn, P. L.; Sauerland, C.; Martin, C.; Grillet, Y.; Coulomb, J.-P.; Rouquerol, F.; Rouquerol, J. In Characterization of porous solids IV; McEnaney, B., Yans, T. J., Rouquerol, J., Rodriguez-Reinoso, F., Sing, K. S. W., Unger, K. K., Eds.; The Royal Society of Chemistry: Cambridge, England, 1997; p 111. (4) Ratkoutsky, J.; Zukal, A.; Franke, O.; Schulz-Elkoff, G. J. J. Chem. Soc., Faraday Trans. 1994, 91, 2041. (5) Chen, C. Y.; Li, H. X.; Davis, M. E. Microporous Mater. 1993, 2, 17. (6) Schmidt, R.; Hansen, E. W.; Stocker, M.; Akporiaye, D.; Ellestad, O. H. J. Am. Chem. Soc. 1995, 117, 4049. (7) Alba, M. D.; Becerro, A. I.; Klinowski J. J. Chem. Soc., Faraday Trans. 1996, 92, 849.
A series of pure silicate MCM-41 samples was prepared by using surfactants with various carbon chain lengths (C8, C12, C16 and C18). Details of the synthesis procedure are available elsewhere.8 X-ray diffraction of the samples were carried out using cobalt KR radiation. Nitrogen adsorption was performed on micromeritics ASAP 2010 analyzer. Adsorption of benzene and ethanol were conducted on two manually operated volumetric rigs. The sample weight was measured on a dry basis, and before adsorption, samples were outgassed overnight at 300 °C under a vacuum of the order of 10-4 Torr. Benzene and ethanol adsorption was measured at three temperatures: 0, 15, and 30 °C. A sample of the commercial Aerosil-200 was used as the nonporous reference adsorbent.
III. Results and Discussions a. Adsorption of Benzene and Ethanol. Benzene, a nonpolar aromatic adsorbate and ethanol, with a polar OH group were chosen for the adsorption. There are some advantages in using alternative adsorbates other than nitrogen. For example, since benzene condensation occurs at a lower relative pressure, larger pores can be readily characterized by benzene adsorption. Another point is that adsorption of a subcritical vapor provides a means to study the effects of temperature on the adsorption properties of porous adsorbents as they can be readily carried out at different temperatures. Most of all, being carried out at ambient temperatures, subcritical vapors are particularly useful when there is a possibility that the porous texture of the studied materials is affected by prolonged exposure to the liquid nitrogen temperature. Examples of benzene and ethanol adsorption isotherms are shown in Figures 1-3. Parameters obtained from benzene and ethanol adsorption data as well as from nitrogen adsorption at 77 K are summarized in Table 1. As seen in the table, the nitrogen volume capacity of the sample MCM-41-1, which has the smallest pore size is comparable with that calculated from benzene and ethanol adsorption, however the nitrogen volume capacities of other samples are consistently smaller than those obtained from the benzene and ethanol adsorption. Similar discrepancies were also observed by Branton et al.9 The reason may be the uncertainty in the adsorbed phase density, which was taken as the bulk liquid density at the experimental temperature, and/or that the pore structure of MCM-41 was effected by the extremely low temperature of liquid nitrogen. Adsorption of benzene onto four MCM-41 samples is shown in Figure 1. As seen in the figure, the shape of the isotherms changes from type I to type IV as the pore diameter increases, with the isotherm of the sample MCM41-2 representing the transition between the two types. The ethanol and benzene isotherms of samples MCM41-1 and -4, one with the smallest and the other with the largest pore size among the samples, are presented in Figure 2 to demonstrate the difference in the adsorption mechanism of the two adsorbates. As seen, ethanol exhibits a higher affinity toward the MCM-41 surface than benzene does. This may be due to the polarity of the ethanol molecules. It is interesting to see the difference (8) Sonwane, C. G.; Bhatia, S. K. Manuscript in preparation. (9) Branton, P.; Hall, P. G.; Sing, K. S. W. Adsorption 1995, 1, 77.
S0743-7463(97)01203-1 CCC: $15.00 © 1998 American Chemical Society Published on Web 07/25/1998
Notes
Langmuir, Vol. 14, No. 17, 1998 4951 Table 1. Adsorption of Nitrogen, Benzene, and Ethanol nitrogen
sample
P/Po at inf point
MCM-41-1 MCM-41-2
0.16
benzene
Vp (cm3/g)
BET (m2/g)
Hys. loop
0.38a
937
no
0.68
1318
no
MCM-41-3
0.35
0.94
1240
no
MCM-41-4
0.42
0.99
1123
yes
a
temp (°C) 0 15 30 0 15 30 0 15 30 0 15 30
ethanol
P/Po at inf point
Vp (cm3/g)
Hys. loop
P/Po at inf point
Vp (cm3/g)
no
0.13 0.15 0.16 0.17 0.19 0.2 0.24 0.25 0.26
0.36a 0.37a 0.38a 0.53 0.53 0.53 0.68 0.68 0.67 0.74 0.74 0.75
no yes no no
0.26 0.3 0.33 0.37 0.4 0.45
0.39a 0.39a 0.40a 0.57 0.57 0.57 0.68 0.68 0.68
yes
0.45 0.51
0.81 0.79
no no
hys loop no no no no no no no
Total pore volume.
Figure 1. Benzene adsorption at 30 °C onto MCM-41 samples (filled, adsorption; open, desorption).
between the benzene and ethanol isotherms of the sample MCM-41-1. While the former can be classified as type I, the later exhibits a linear portion in the relative pressure range of 0.1-0.3, which can be taken as a sign of capillary condensation. b. Hysteresis. Very frequently, nitrogen isotherms of MCM-41 are reported without any hysteresis. This kind of reversible adsorption is thought to be a consequence of the instability of the nitrogen meniscus in smaller pores.10-12 The presence of a hysteresis loop on an isotherm is indicated in Table 1. As seen, nitrogen, benzene and ethanol adsorption onto the samples MCM41-1 and -2 are all reversible. The nitrogen isotherm of the sample MCM-41-3 does not exhibit any hysteresis, while a loop is observed on isotherm of the sample MCM41-4 at a relative pressure of 0.42. This pressure is in good agreement with the threshold pressure reported in the literature,14 below which the hysteresis loop cannot exist on nitrogen isotherms. Since a hysteresis loop is observed on the benzene isotherm of MCM-41-4 at 30 °C, it is understood that the benzene isotherms at lower temperatures, 15 and 0 °C, must exhibit some hysteresis as well. It is interesting to see that none of the ethanol (10) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonewicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T.-W.; Olson, D. U.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B. and Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (11) Franke, O.; Shulz-Ekloff, G.; Rathousky, J.; Starek, J.; Zukal, A. J. Chem. Soc., Chem. Commun. 1993, 724. (12) Branton, P.; Hall, P. G.; Sing, K. S. W., J. Chem. Soc., Chem. Commun. 1993, 1257. (13) Ruthven, D. M. Principles of Adsorption and Adsorption Processes; John Wiley & Sons: New York 1984. (14) Kruk, M.; Jaroniec, M.; Sayari, A. J. Phys. Chem. B 1997, 101, 583.
isotherms exhibits a hysteresis loop, even with the isotherm of the sample MCM-41-4 (cf. Figure 2). This observation indicates that adsorption of subcritical vapors may be a useful means to study the hysteresis of adsorption on MCM-41. The benzene isotherms of sample MCM-41-3 can be used to demonstrate the evolution of the hysteresis loop with temperature (cf. Figure 3): At 15 and 30 °C, benzene adsorption is reversible, but at 0 °C when there is an appropriate combination of all factors (surface tension, molar volume, etc.), a hysteresis loop of type H1 according to IUPAC classification is observed. Thus, an argument about the absence of a hysteresis loop can be offered as follows. In MCM-41 samples, pores are believed to be very long compared to their diameters.10 It is then very likely that the radius is not constant along the pore length, but rather there may be some constrictions caused by imperfections of the pore wall. It is possible that such defects may act as the nuclei for the adsorbate to adsorb at the constrictions and this may divide the through pore into two blind pores. It is understood that adsorption in blind pores is reversible because as adsorption proceeds, a hemispherical meniscus is formed at the bottom of the pore, and subsequent adsorption will occur through this gas-liquid interface. In pores of larger radii, the formation of a meniscus during the adsorption step due to the imperfections of the pore wall is less likely; therefore, adsorption in MCM-41 with larger pores is more often accompanied by a hysteresis loop. c. Porous Properties from Adsorption Data. Due to the mesopore condensation occurring in MCM-41 samples, the validity range of BET equation has to be adjusted accordingly. For example for sample MCM-41-3 the upper limit of the relative pressure interval in which the BET equation is applicable is 0.2 for nitrogen, 0.13 for benzene, and 0.36 for ethanol. This is because in the relative pressure scale, the condensation pressure increases in the order benzene < nitrogen < ethanol. The BET area determination using benzene and ethanol isotherms was tested for sample MCM-41-3. The calculated quantities of adsorbates (benzene or ethanol) forming a monolayer on MCM-41-3 surfaces at different temperatures are within a few percents of each other, which is an acceptable tolerance in surface area measurement.13 However, their BET surface areas are significantly smaller than that obtained from nitrogen adsorption at liquid nitrogen temperature. The pore diameter of MCM-41 samples is calculated from the benzene and ethanol adsorption data using the Kelvin equation in conjunction with the statistical film thickness “t”. An optimization procedure in which data
4952 Langmuir, Vol. 14, No. 17, 1998
Notes
Figure 2. Ethanol (b) and benzene ([) isotherms of sample MCM-41-1 (left) and MCM-41-4 (right) (filled: adsorption; open: desorption).
Figure 3. Ethanol (b) and benzene ([) adsorption on MCM-41-3 (filled: adsorption, open: desorption). Table 2. Pore Diameters from Different Methods nitrogen
benzene
ethanol
sample
d (Å)
eq 1 (Å)
d (Å)
eq 1 (Å)
d (Å)
eq 1 (Å)
MCM-41-1 MCM-41-2 MCM-41-3 MCM-41-4
18.7 28.9 33.7
23.6 27.4 37.7 43.5
14.2 24.4 29.6 36.5
22.1 25.9 35.5 41.3
16.8 22.8 31.4 36.8
23.3 26.1 35.2 42.1
at all temperatures are optimized simultaneously is used in the diameter determination. The diameter also can be estimated using the following equation:14
d ) 1.2128
(
)
FVp 1 + FVp
1/2
d100
(1)
F is the density of the amorphous silica (2.2 Vp is the mesopore volume and d100 is the X-ray lattice spacing. g/cm3),
The results are presented in Table 2, where pore diameters from nitrogen adsorption are also included. The values obtained from the optimization using benzene and ethanol adsorption data are quite comparable, and larger than those obtained from nitrogen data. On the other hand, the diameters calculated by using eq 1 for different adsorbates are within 1-2 Å of each other. IV. Conclusions The adsorption of subcritical vapors at ambient temperatures can be a useful tool to characterize MCM-41 in addition to nitrogen adsorption. It provides a means to study the effect of temperature on the capillary condensation process and the existence of hysteresis in MCM-41. There are some discrepancies between the results of nitrogen adsorption analysis and those of benzene and ethanol. More work is required to resolve these discrepancies. Acknowledgment. Support from the Australian Research Council (ARC) is gratefully acknowledged. LA971203C
