Langmuir 1996, 12, 599-600
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Physisorption of Nitrogen by Mesoporous Modified Kanemite P. J. Branton, K. Kaneko,* and N. Setoyama Department of Chemistry, Faculty of Science, Chiba University, 1-33 Yayoi, Inage, Chiba 263, Japan K. S. W. Sing School of Chemistry, Bristol University, Bristol, U.K. S. Inagaki and Y. Fukusima Toyota Central R & D Labs, 41-1, Yokomichi, Nagakute, Aichi-gun 480-11, Japan Received August 1, 1995. In Final Form: September 28, 1995
Figure 1. Adsorption isotherm of nitrogen at 77 K on FSM-12. Filled symbols denote desorption.
Introduction There has been a great deal of interest in highly uniform mesoporous adsorbents following the disclosure in 1992 by Mobil scientists of the preparation of a new family of molecular sieves designated as M41S.1 One member of this family, MCM-41, has unusual adsorption properties and shows considerable promise as a model mesoporous adsorbent.2 In 1990, Kuroda et al.3 reported preliminary work on a modified form of kanemite (a polysilicate, NaHSi2O5‚3H2O), which indicated that it also had a regular honeycomb-type mesopore structure.4 However, the adsorption isotherms reported so far were not determined in sufficient detail to confirm the uniformity of the mesopore structure. The results of nitrogen adsorption measurements reported here were obtained on two kanemite-derived adsorbents with different pore structures. The aim of the work was to prepare and characterize modified forms of kanemite possessing uniform mesopore structures.5 It is well-known that classical capillary condensation, which generally involves hysteresis,6 takes place in the wider mesopores, but the pore-filling mechanisms in pores of width of 2-3 nm are still unclear. For this reason, it was decided to focus attention on the lower end of the mesopore range.
Figure 2. Adsorption isotherm of nitrogen at 77 K on FSM-16. Filled symbols denote desorption.
Experimental Section The new adsorbent samples were prepared at the Toyota Central R & D Labs in Japan and designated folded sheet mesoporous materials, FSM-12 and FSM-16. The number refers to the carbon chain length of the organic species used in the synthesis. FSM-12 was prepared by the ion exchange reaction of kanemite with dodecyltrimethylammonium [C12H25N+(CH3)3] bromide followed by calcination at 823 K, and FSM-16 was prepared by the ion exchange reaction of kanemite with hexadecyltrimethylammonium [C16H33N+(CH3)3] chloride followed by calcination at 823 K. Nitrogen isotherms were determined using * E-mail:
[email protected]. (1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (2) Branton, P. J.; Hall, P. G.; Sing, K. S. W. J. Chem. Soc., Chem. Commun. 1993, 1257. (3) Yanagisawa, T.; Shimizu, T.; Kuroda, K.; Kato, C. Bull. Chem. Soc. Jpn. 1990, 63, 988. (4) Inagaki, S.; Fukushima, Y.; Kuroda, K. J. Chem. Soc., Chem. Commun. 1993, 680. (5) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquerol, J.; Siemieniewska, T. Pure Appl. Chem. 1985, 57, 603. (6) Keizer, A.; Michalski, T.; Findenegg, G. Pure Appl. Chem. 1991, 63, 1495.
0743-7463/96/2412-0599$12.00/0
Figure 3. Rs plot for nitrogen adsorption on FSM-12. a computer-aided gravimetric apparatus. Outgassing temperatures of 383 K were used prior to isotherm determination.
Results and Discussion The adsorption isotherms of nitrogen on FSM-12 and FSM-16 are shown in Figures 1 and 2, respectively. They are both of type 4 in the IUPAC classification5 and of © 1996 American Chemical Society
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Langmuir, Vol. 12, No. 2, 1996
Notes Table 1. Specific Pore Parameters of FSM Series as(BET), m2 g-1
as (Rs), m2 g-1
Vp, cm3 g-1
range of dp, nm
d*p, nm
882 977
870 1045
0.56 0.91
1.9-2.6 2.8-3.9
2.5 3.7
FSM-12 FSM-16
Figure 4. Rs plot for nitrogen adsorption on FSM-16.
similar shape to the isotherm of nitrogen on MCM-41. However, although the FSM-12 isotherm is completely reversible, the FSM-16 isotherm exhibits some hysteresis at high relative pressure. The latter isotherm is similar to that reported7 for cyclopentane adsorption on MCM41, and it is generally accepted that this type of hysteresis is likely to be associated with the aggregate structure of the material. The form of the Rs plots in Figures 3 and 4 provides clear evidence that reversible pore filling has taken place over fairly narrow ranges of P/P0. The reduced Rs adsorption data are derived from the isotherm of nitrogen on nonporous hydroxylated silica.8 The initial practically linear section and zero intercept of each Rs plot together confirm that monolayer adsorption has preceded pore filling and that no significant primary micropore filling occurred at very low relative pressure (i.e., in pores of molecular dimensions). Derived values of surface area are given in Table 1: as expected, the values of as (Rs) (7) Franke, O.; Schilz-Ekloff, G.; Rathousky, J.; Starek, J.; Zukal, A. J. Chem. Soc., Chem. Commun. 1993, 724. (8) Bhambhani, M. R.; Cutting, P. A.; Sing, K. S. W.; Turk, D. H. J. Colloid Interface Sci. 1972, 38, 109.
calculated from the slopes of the Rs plots agree well with the corresponding BET areas, as(BET). By applying the Kelvin equation, assuming hemispherical meniscus formation, and correcting for the adsorbed layer thickness,9 we are able to calculate the ranges of pore width, dp, recorded in Table 1. The values of mean pore diameter, d*p, in Table 1 are obtained from the pore volume/surface ratio, i.e., by applying the principle of hydraulic radius and assuming the pores to be nonintersecting cylindrical capillaries.5 The total pore volume, Vp, is evaluated in the usual manner9 by converting the uptake of gas at P/P0 ) 0.95 into a liquid volume, and the BET area, as(BET) is assumed to represent the total surface. We recognize that the good agreement in the corresponding values for d*p and dp may be deceptive. It is unlikely that the Kelvin equation provides a reliable basis for the calculation of pore widths of around 6 molecular diameters. Furthermore, the application of the standard statistical multilayer thickness correction is probably an oversimplification. Tentatively, we may regard reversible pore filling in the range P/P0 ) 0.2-0.4 as a form of capillary condensation, but further work is required to establish a clear distinction between reversible capillary condensation and cooperative micropore filling.9 For the above reasons, the values of pore width recorded in Table 1 should be regarded as apparent rather than real pore sizes. However, it is already clear that the modified kanemite adsorbents possess pores of welldefined shape and narrow ranges of size and are therefore of special value for the investigation of reversible mesopore/micropore filling. Studies of organic vapor adsorption by these new materials are now in progress. LA950652E (9) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface area and Porosity, 2nd ed.; Academic Press: New York, 1982.
