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Neck Size of Ordered Cage-Type Mesoporous Silica FDU-12 and Origin of Gradual Desorption Kunimitsu Morishige* and Kenji Yoshida Department of Chemistry, Okayama UniVersity of Science, 1-1 Ridai-cho, Kita-ku, Okayama 700-0005, Japan ReceiVed: January 07, 2010; ReVised Manuscript ReceiVed: March 15, 2010
To evaluate the neck size of ordered cage-type mesoporous silica FDU-12 and also to elucidate the origin of the gradual desorption always observed for the ordered silicas with cage-like pores of large necks, we measured successive adsorption of water at 283 K and nitrogen or argon at 77 K on five kinds of FDU-12 samples. For all the materials examined here, the amount of nitrogen or argon condensed inside the large cavities decreased rapidly over a relatively small range of water filling with an increase in water filling. Eventually, it reached a small value at water filling, well before the onset of pore condensation of water inside the large cavities. This clearly indicates that most of the cavities are isolated from bulk nitrogen or argon gas with water frozen in the necks. With the assumption that the necks in cage-like pores are cylindrical in shape, we assessed the pore size distribution of the necks available for desorption from the equilibrium relative pressure of water, in which the amount of nitrogen or argon condensed in the large cavities drops considerably. The neck size in the materials is increased with an increase of the hydrothermal treatment temperature and time. For a sample hydrothermally treated at 373 K for 7 days, the hysteresis loop of argon at 77 K gradually closed above the corresponding lower limit of the adsorption hysteresis, as is often observed in cage-like pores of large necks. However, the wide distribution of the capillary evaporation pressure did not correspond directly to the pore size distribution of the necks in the material. I. Introduction Ordered silicas with large cage-like pores have recently attracted a great deal of interest because of their unique structures and potential applications in immobilization of biomolecules, separation, and catalysis.1-33 In these applications, size distribution of the necks interconnecting the large cavities as well as spatial distribution of neck sizes are especially important. There are several methods to examine the neck size in the ordered materials with cage-like pores. Electron crystallography is surely a most powerful method to elucidate the neck size, although the method appears to neglect a broad distribution of the neck sizes inherent to the ordered materials with cagelike pores and to be restricted to highly ordered samples with appreciable ordered domain sizes.34 The method is not suitable for the analysis of weakly ordered materials such as the ordered silicas prepared by hydrothermal treatment at low temperatures and not convenient for routine analysis of the neck size. Another method for the assessment of average neck size is based on the studies of pore accessibility after surface modifications with ligands of gradually increasing sizes.7,17,20 This method has many attractive features such as applicability to both ordered and disordered materials, cheap and readily available chemicals, and inexpensive equipment compared to the electron crystallography. However, the method involves tedious chemical operations and is based on a somewhat unreasonable assumption that alkyl chains of the ligands that cause pore inaccessibility are fully extended vertically from the pore wall surface. If the neck size is in a subnanometer range, it can be probed by examining molecular sieving effects of the materials against several molecules of different sizes.35 High-resolution pore size analysis of the adsorption branch often gives pore size distribution curves * To whom correspondence should be addressed.
in the region of micropores and small mesopores relevant to the necks interconnecting the large cavities.10,19-21,23-25,28,29,31 However, most of the pores in this region are complementary pores located inside the framework walls of the ordered silicas with large cage-like pores.30 Another way to estimate the neck size is based on the analysis of the desorption branch of an isotherm which exhibits a wide hysteresis loop.9,12,13,19,21,27 In cage-like pores, the capillary evaporation from a particular cavity is retarded until either the cavity becomes accessible from the bulk vapor phase through a continuous path of unfilled cavities and necks, or the lower pressure limit of hysteresis for the cavity is reached. The former of these behaviors corresponds to pore-blocking-controlled desorption, while the latter to cavitation-induced desorption. When the pore-blocking-controlled desorption is observed, it is believed that the capillary evaporation pressure provides information about the widest continuous path of necks connecting the given cavity with the bulk vapor phase, that is, about the smallest neck size along this widest path.13,19 In the case where the cavitation-induced desorption is observed, it is possible to determine only the upper limit of the possible neck size. From a comparison of the lower pressure limits of hysteresis with the capillary evaporation pressures from the cylindrical pores of ordered silicas, it has been suggested that nitrogen and argon adsorption at 77 K allow one to probe neck sizes down to ∼5.0 and 4.0 nm, respectively.9,13 However, the actual distribution of neck size is never obtained by a simple pore size analysis of the desorption branch because of the effects of pore network. The pore-blocking-controlled desorption from the interconnected network of the cavities is often accounted for by the percolation theory.36-40 For a random distribution of neck sizes throughout the ordered structure, a steep desorption branch is expected. On the other hand, the desorption isotherms
10.1021/jp100171n 2010 American Chemical Society Published on Web 03/30/2010
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observed for the ordered silicas with cage-like pores, in which the neck size is larger than a certain critical value, are always very gradual.12,13,16,19-22,27-29,31,33 The mechanism of poreblocking-controlled desorption from the interconnected network of the cavities in the ordered silicas is still unsettled and thus the relation between the capillary evaporation pressure and size of the necks available for desorption remains to be solved. In a previous study,33 we have shown that measurements of successive adsorption of water and nitrogen can provide important insights about a pore size distribution of the necks in a material with cage-like pores, as well as spatial correlation of neck sizes. On adsorption a fluid behaves quite independently in different parts of the pore system, regardless of the pore network.41-43 In ordered silicas with cage-like pores, necks of larger size are progressively filled with water with increasing pressure. The water condensed in the necks freezes by subsequent cooling and closes the window for movement of nitrogen from one cavity to an adjacent one. The percolation theory predicts that with an increase in water filling the amount of nitrogen condensed in the spherical cavities of ordered silicas drops rapidly over a narrow range of water filling for a random distribution of neck size in the pore network. If the neck size distributions in the materials are spatially correlated, the amount of nitrogen condensed in the cavities would start to decrease even at the small fractions of necks closed and would show a gradual decrease with increasing water filling. In addition, the relative pressure range of water, in which the amount of nitrogen condensed in the large cavities considerably drops, gives direct information about a pore size distribution of the necks. In the previous study, we examined the porous structures of ordered cage-type mesoporous silicas SBA-16 and KIT-5 by this method.33 For the materials with larger necks, however, the pore size distribution of the necks was not determined because pore condensation of water occurred simultaneously in the spherical cavities and a high fraction of the necks. The critical size of the necks, below which in cage-like pores cavitation occurs upon desorption, as well as the origin of the gradual desorption observed for the ordered silicas with cage-like pores, were not identified. The upper limit of the pore size distribution of the necks that can be evaluated by the present method is extended to larger sizes by increasing the size of the cavities in a material. FDU-12 is a highly ordered large cage-type mesoporous silica with a cubic Fm3jm close-packed structure.29 The size of the cavities in FDU-12 is larger than those in SBA-16 and KIT-5. The purpose of the present study is to identify the critical sizes of the necks below which in cage-like pores cavitation occurs upon desorption of nitrogen and argon at 77 K and to elucidate the origin of the gradual desorption always observed for the ordered silicas with cage-like pores of large necks. II. Experimental Section II.1. Materials and Characterization. FDU-12 was prepared by using Pluronic F127 triblock copolymer as a structuredirecting agent and mesitylene as a solubilizing agent according to the procedure of Yu et al.29 A 3.0 g sample of F127 and a 7.5 g sample of KCl were dissolved in 180 mL of 2 M HCl, then 6.6 g of mesitylene was added and the mixture was stirred at 288 K for 24 h in a capped bottle with use of a magnetic stirrer. Next 12.3 g of tetraethoxysilane was added to the resulting reaction mixture, which was left to stir for another 24 h at 288 K. Subsequently, the bottle containing the reaction mixture was heated at 313 or 353 K for 1 day, or 373 K for a period of time in the range from 1 to 7 days. The solid product was collected by filtration and dried at room temperature in air.
Morishige and Yoshida The resulting powder was calcined at 823 K for 5 h in air to remove the copolymer template. The obtained FDU-12 silicas are denoted FDU12-x-y, where x and y correspond to the aging temperature (in K) and time (in day), respectively. Adsorption isotherms of nitrogen and argon at 77 K were measured volumetrically on a BELSORP-mini II (Bell Japan, Inc.) and homemade semiautomated instrument, respectively. II.2. Measurement of Successive Adsorption. Successive adsorption of water and nitrogen or argon was measured volumetrically on the homemade semiautomated instrument equipped with a Baratron capacitance manometer (Model 690A) with a full scale of 1000 Torr and an additional gas dosing volume of ∼2400 cm3. The adsorption isotherm of water was measured at 283 K by using the additional gas dosing volume. III. Results III.1. Adsorption Isotherms. It has been shown that a comparative study of nitrogen and argon adsorption at 77 K provides valuable structural information about the necks in ordered materials with cage-like pores.9,13,19 Figure 1 shows the adsorption-desorption isotherms of nitrogen and argon at 77 K on five kinds of FDU-12 samples. The sharp adsorption branches of both adsorbates observed for all samples indicate the presence of uniform cavities because capillary condensation during adsorption is controlled by the size of the cavities. The average diameter of the spherical cavities was estimated by using the relationship between the capillary condensation pressure of nitrogen and pore diameter reported by Broekhoff and de Boer.44 For all the samples, except for FDU12-373-7, the hysteresis loops of nitrogen closed sharply at a relative pressure of 0.48, corresponding to the lower limit of the adsorption hysteresis. This strongly suggests that desorption takes place via cavitation, and the diameter of the narrow necks in these materials is smaller than ∼5 nm. However, in the case of a sample hydrothermally treated at 373 K for 7 days, the onset (p/p0 ≈ 0.53) of capillary evaporation of nitrogen was slightly above the corresponding lower limit of hysteresis, although the hysteresis loop closed sharply at the lower limit of the adsorption hysteresis. A comparison with the capillary evaporation pressures of nitrogen from the cylindrical pores of the ordered silicas13 suggests that a pore size distribution, of the smallest neck along the widest continuous path of necks to the bulk vapor phase, in this sample is centered at around 5 nm in diameter and the upper boundary is ∼6.5 nm. On the other hand, for the same sample, the hysteresis loop of argon gradually closed above the corresponding lower limit of hysteresis, which suggests the neck diameter is above ∼4 nm. Therefore, it is inferred from the simple analysis of the desorption branches of nitrogen and argon at 77 K that the necks available for evaporation of liquid condensed in the cavities of this material are distributed in the size range of 4-6.5 nm in diameter. In the case of a sample hydrothermally treated at 373 K for 3 days, the onset (p/p0 ≈ 0.41) of capillary evaporation of argon was slightly above the corresponding lower limit of hysteresis, although the hysteresis loop closed sharply at the lower limit of the adsorption hysteresis. A comparison with the capillary evaporation pressures of argon from the cylindrical pores of the ordered silicas13 suggests that an appreciable fraction of the necks in this material are distributed in the small range of 4-4.2 nm in diameter. For other materials, the hysteresis loops of argon closed sharply at a relative pressure of 0.35, corresponding to the lower limit of the hysteresis. This suggests that the diameter of the narrow necks in these materials is smaller than ∼4 nm. The specific surface area (SBET) was calculated by using the
Ordered Cage-Type Mesoporous Silica FDU-12
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Figure 1. Adsorption-desorption isotherms of nitrogen and argon at 77 K on FDU-12 hydrothermally treated at several different conditions. Volumes of nitrogen adsorbed for FDU12-353-1, FDU12-373-1, FDU12-373-3, and FDU12-373-7 were incremented by 100, 200, 300, and 500 cm3 (STP) g-1, respectively, while volumes of argon adsorbed for FDU12-353-1, FDU12-373-1, FDU12-373-3, and FDU12-373-7 were incremented by 100, 200, 400, and 700 cm3 (STP) g-1, respectively. Empty and closed symbols denote adsorption and desorption points, respectively.
TABLE 1: Physicochemical Parameters sample
surface area SBET (m2/g)
micropore vol Vmic (cm3/g)
total vol Vt (cm3/g)
cavity diameter D (nm)
FDU12-313-1 FDU12-353-1 FDU12-373-1 FDU12-373-3 FDU12-373-7
284 612 662 672 614
0.08 0.20 0.22 0.23 0.14
0.22 0.47 0.54 0.67 0.72
12.8 16.6 16.6 19.2 19.2
Figure 2. Adsorption isotherms of water at 283 K on FDU-12 hydrothermally treated at 313 K for 1 day (O), 353 K for 1 day (4), 373 K for 1 day (0), 373 K for 3 days (]), and 373 K for 7 days (b).
Brunauer-Emmet-Teller method45 for the nitrogen adsorption data. The micropore volume and total pore volume were estimated by using the t-plot method for the nitrogen isotherm.46 Table 1 summarizes the main physicochemical parameters of the samples used in the present study. Figure 2 shows the adsorption isotherms of water at 283 K for five kinds of FDU-12 samples. Here f denotes a pore filling. The pore filling is defined as the volume ratio f ) Vw/Vt, where Vt is the total pore volume of FDU-12 samples, and Vw is the volume occupied by the adsorbed water at 283 K. All isotherms show the type V curve of the BDDT definition47 of an adsorption isotherm, indicating some of the hydrophobic nature of the pore
surface. For all the samples, the pore condensation of water in the large cavities started to occur at pore fillings around f ) 0.6, although the pressure at which pore condensation of water occurs was shifted to slightly higher relative pressures with an increase in the hydrothermal treatment temperature and time. III.2. SuccessiveAdsorption.Figure3showstheadsorption-desorption isotherms of nitrogen at 77 K on FDU-12 hydrothermally treated at 353 K for 1 day on which various amounts of water were preadsorbed. The monolayer capacity of nitrogen decreased almost linearly with an increase in the amount of water preadsorbed up to fH2O ) 0.25, indicating that nitrogen molecules interact very weakly with a pore wall covered with water molecules. When the amount of water preadsorbed was increased beyond fH2O ) 0.08, the amount of nitrogen condensed inside the large cavities of FDU-12 decreased rapidly and reached a small value at fH2O ) 0.25, well below the onset of pore condensation of water inside the large cavities. This clearly indicates that most of the cavities are blocked for nitrogen adsorption with water frozen in the necks. Similarly, for other FDU-12 samples examined here, the amount of nitrogen condensed in the cavities decreased rapidly with an increase in water filling when the amount of water preadsorbed was increased beyond a certain critical value (Figures 1S-4S of the Supporting Information). When the cavitation-induced desorption takes place, the desorption branch is very sharp because liquid condensed in the individual cavities evaporates at almost the same pressure, irrespective of their neck and cavity sizes. The temperature dependency of cavitation pressure is distinctly greater than that of evaporation pressure from necks.9,27,48 Therefore, the de-
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Figure 3. Adsorption-desorption isotherms of nitrogen at 77 K on FDU-12 hydrothermally treated at 353 K for 1 day on which water was preadsorbed at pore fillings of 0 (b), 0.08 (O), 0.17 (4), 0.21 (0), and 0.25 (]). The isotherms at fH2O ) 0.11, 0.13, 0.16, and 0.20 are omitted for clarity.
sorption mechanism of liquid condensed in the cavities can be altered from cavitation to pore blocking with a decrease of temperature.9,27 The crossover temperature is believed to be dependent upon the smallest neck size along the widest continuous path of necks connecting the individual cavities with the bulk vapor phase. In the vicinity of the crossover temperature of the system, the desorption branch becomes slightly gradual because in a certain fraction of the cavities the capillary evaporation due to pore-blocking-controlled desorption begins above the corresponding lower limit of the adsorption hysteresis. The shape of the desorption branch of nitrogen at 77 K for FDU12-373-7 strongly suggests that for this system the crossover temperature in the desorption mechanism is close to 77 K. If the measurement of the nitrogen adsorption isotherm was conducted at a lower temperature, we could observe a more gradual desorption branch because the capillary evaporation in a major fraction of the cavities takes place via pore blocking. The relative pressures of cavitation for nitrogen, argon, oxygen, and carbon dioxide are identical at the same reduced temperature T/Tc,14 where Tc denotes the bulk critical temperature. Tc of nitrogen and argon are 126.2 and 150.7 K, respectively. Therefore, argon adsorption at 77 K has an effect similar to a decrease in the adsorption temperature of nitrogen, as is indicated in Figure 1. Figure 4 shows the adsorption-desorption isotherms of argon at 77 K on FDU-12 hydrothermally treated at 373 K for 7 days on which various amounts of water were preadsorbed. For this sample, the capillary evaporation of argon at 77 K began at relative pressures higher than the corresponding lower limit of the adsorption hysteresis. This earlier onset of capillary evaporation might be attributed to the existence of percolation pathways consisting of necks of size larger than ∼4 nm. When the amount of water preadsorbed was increased beyond fH2O ) 0.21, the amount of Ar condensed inside the large cavities decreased rapidly, while the shape of the adsorp-
Morishige and Yoshida
Figure 4. Adsorption-desorption isotherms of argon at 77 K on FDU12 hydrothermally treated at 373 K for 7 days on which water was preadsorbed at pore fillings of 0 (b), 0.15 (O), 0.27 (4), 0.38 (0), and 0.45 (]). The isotherms at fH2O ) 0.21 and 0.33 are omitted for clarity.
Figure 5. Ratio Vc/Vc0 of capillary condensate of nitrogen at 77 K inside the large cavities of FDU-12 hydrothermally treated at 313 K for 1 day (b), 353 K for 1 day (O), 373 K for 1 day (4), 373 K for 3 days (0), and 373 K for 7 days (]) as a function of water filling. Closed tilted squares ([) denote the ratio Vc/Vc0 of capillary condensate of argon at 77 K inside the large cavities of FDU-12 hydrothermally treated at 373 K for 7 days.
tion hysteresis remained almost unchanged. This behavior is puzzling because necks of larger size are progressively blocked for argon adsorption with frozen water with an increase in water filling. Figure 5 shows the ratio of the amount of nitrogen or argon condensed in the spherical cavities of FDU-12 with preadsorbed water, Vc, and without preadsorbed water, Vc0, as a function of water filling for five kinds of FDU-12 samples. Vc was estimated from the difference between the saturated amount of adsorbed nitrogen or argon and the amount of adsorbed nitrogen or argon at the lower closure point. The ratio Vc/Vc0 is thought to represent the fraction of the pore volume of the cavities still accessible from the surface after water was preadsorbed and then frozen. Water forms a monolayer on the pore wall and condenses in the complementary pores located inside the pore walls, besides the occurrence of pore condensation in the necks interconnecting the cavities, before the onset of capillary condensation inside
Ordered Cage-Type Mesoporous Silica FDU-12
Figure 6. Ratio Vc/Vc0 of capillary condensate of nitrogen at 77 K inside the large cavities of FDU-12 hydrothermally treated at 313 K for 1 day (b), 353 K for 1 day (O), 373 K for 1 day (4), 373 K for 3 days (0), and 373 K for 7 days (]) as a function of the relative pressure in the isotherm when water was preadsorbed at 283 K. Closed tilted squares ([) denote the ratio Vc/Vc0 of capillary condensate of argon at 77 K inside the large cavities of FDU-12 hydrothermally treated at 373 K for 7 days.
the cavities. Therefore, quantitative analysis of the data based on the percolation theory is impossible.33 For all the samples, the amount of nitrogen or argon condensed in the cavities dropped rapidly over narrow ranges of water filling, well below the onset (fH2O ≈ 0.6) of pore condensation of water inside the large cavities. This clearly indicates that cage-like character is preserved for all the samples examined here and thus structural information about the necks available for desorption can be fully explored by the successive adsorption method. For all the samples, however, the decrease of Vc/Vc0 with an increase in water filling was gradual compared to the accessibility curve of the voids expected for a random distribution of neck sizes throughout the ordered structure, similarly to the cases of SBA16 and KIT-5.33 The small size of the pore network and/or the presence of the correlated distribution of neck sizes would be responsible for this. Figure 6 shows Vc/Vc0 as a function of the equilibrium relative pressure in the isotherm when water was preadsorbed. For a sample hydrothermally treated at 313 K for 1 day, preadsorption of a very small amount of water resulted in a large decrease of the amount of nitrogen condensed in the cavities. This strongly suggests that the necks, which cause inaccessibility of the cavities from the surface, are microporous rather than mesoporous. The relative pressure range of water, in which the amount of nitrogen condensed in the large cavities drops considerably, shifted to higher pressures with an increase of the hydrothermal treatment temperature and time, indicating rapid enlargement of the necks. In addition, the finite range of relative pressure of water, in which the amount of nitrogen or argon condensed in the cavities drops considerably, indicates appreciable size distributions of the necks. IV. Discussion IV.1. Neck Size Distribution. Several research groups have reported on the adsorption isotherms of water on ordered mesoporous silicas with cylindrical pores.49-56 When we assume that the necks in cage-like pores are cylindrical in shape, the equilibrium relative pressure of water in our measurements can be related to the maximum pore diameter below which the necks are closed for the movement of nitrogen in the pore network using the relationship between the pore diameter and capillary condensation pressure of water for the cylindrical pores of silicas. We extracted the relationship between the pore diameter and condensation pressure (p/p0) of water for the cylindrical
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Figure 7. Ratio Vc/Vc0 of capillary condensate of nitrogen at 77 K inside the large cavities of FDU-12 hydrothermally treated at 353 K for 1 day (O), 373 K for 1 day (4), 373 K for 3 days (0), and 373 K for 7 days (]) as a function of the maximum pore diameter below which the necks are closed for the movement of nitrogen in the pore network of FDU-12. Closed tilted squares ([) denote the ratio Vc/Vc0 of capillary condensate of argon at 77 K inside the large cavities of FDU-12 hydrothermally treated at 373 K for 7 days.
pores of silicas from the isotherm data of Kittaka et al.54 and Ja¨hnert et al.56 (Figure 5S of the Supporting Information). The numerical relation is given by
p/p0(H2O) ) 0.0028D3 - 0.0596D2 + 0.4394D - 0.3349 (1) where D is the pore diameter (in nm). Figure 7 shows Vc/Vc0 as a function of the maximum pore diameter below which the necks are closed for the movement of nitrogen in the pore network of FDU-12. For FDU-12 hydrothermally treated at 353 K for 1 day, it is indicated that the necks, which cause inaccessibility of the cavities from the surface, are distributed in the range of 1.3-1.8 nm in diameter. This does not necessarily represent a real pore size distribution of the necks in the material, however, because the cavities become inaccessible from the bulk vapor phase after more than ∼70% of the necks were closed by ice for a random distribution of the neck sizes. The neck size in the materials is increased with an increase of the hydrothermal treatment temperature and time. The necks available for desorption in the materials prepared by the hydrothermal treatment at 373 K for 1, 3, and 7 days are distributed in the range of 1.7-2.5, 2.2-3.7, and 2.6-8.0 nm in diameter, respectively. Such an estimate of the neck sizes in the sample hydrothermally treated at 373 K for 3 days is in reasonable agreement with a neck diameter of ∼2.4 nm determined by the electron crystallography method for FDU-12 prepared under the same hydrothermal conditions.29 However, it is evident that the necks in FDU-12 are not uniform in size. The size distribution of the necks was drastically increased by the prolonged hydrothermal treatment at 373 K. The size distributions of the necks available for desorption in the materials assessed by the successive adsorption method of water and nitrogen are apparently inconsistent with those obtained from the simple analysis of the desorption branches that were described in a previous section. The main reason can be attributed to the inadequate assessment of the critical sizes of the necks below which in cage-like pores cavitation occurs upon desorption of nitrogen and argon at 77 K. The commonly accepted critical diameters of ∼4 and 5 nm, respectively, for argon and nitrogen adsorption at 77 K are obtained from a simple comparison of the cavitation pressures with the capillary evaporation pressures from the cylindrical pores of the ordered
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silicas. The cavitation pressure itself depends slightly on the size of the cavities20,48 and there is no guarantee that the desorption pressure of liquid condensed in the necks of a given size is equal to the capillary evaporation pressure of liquid condensed in the open-ended cylindrical pores of the same size. The present method of neck size determination is based on the relation between the capillary condensation pressure of water and pore diameter for the cylindrical pores of the ordered silicas. It is well-known that capillary condensation occurs depending solely on the pore size, independently in different parts of the pores.41-43 Therefore, the neck size evaluated by the present method is more reliable. The capillary evaporation of argon at 77 K in the cavities of FDU12-373-1, which possesses the necks of size in the range of 1.7-2.5 nm in diameter, took place via cavitation, while in a certain fraction of the cavities of FDU12373-3 that possesses the necks of size in the range of 2.2-3.7 nm the capillary evaporation occurred via pore-blockingcontrolled desorption. On the other hand, the capillary evaporation of nitrogen at 77 K in the cavities of FDU12-373-3 took place via cavitation, while in an appreciable fraction of the cavities of FDU12-373-7 that possesses the necks of size in the range of 2.6-8.0 nm the capillary evaporation occurred via pore-blocking-controlled desorption. These results strongly suggest that the critical sizes of the necks, below which in cagelike pores cavitation occurs upon desorption of nitrogen and argon at 77 K, are ∼4.0 and 3.0 nm, respectively. IV.2. Origin of Gradual Desorption. For the FDU-12 prepared by the hydrothermal treatment at 373 K for 7 days, the size of the necks is widely distributed between 2.6 and 8.0 nm. Therefore, in this material, the capillary evaporation of argon at 77 K mainly occurs via pore-blocking-controlled desorption. However, the wide distribution of neck size is not enough to account for the gradual desorption branch in the isotherm because a steep desorption branch is expected for a random distribution of neck sizes throughout the ordered structure from the percolation theory due to pore blocking. If the neck size distribution in the material is spatially correlated, the pore-blocking-controlled desorption would take place gradually with decreasing pressure. For example, when the necks in the outer shell of each particle are expanded preferentially compared to those in the inner shell by the prolonged hydrothermal treatment at 373 K, the capillary evaporation of liquid condensed in the cavities in the outer shell of each particle would occur earlier than in the cavities in the inner shell with decreasing pressure, which results in the gradual desorption branch. However, the gradual desorption branch of argon observed at 77 K on FDU12-373-7 is not directly related to the wide distribution of neck size in this material. In the present experiment, the necks of larger size are progressively filled with water with increasing pressure and the water condensed in the necks freezes by subsequent cooling and closes the window for movement of argon from one cavity to an adjacent one. If the capillary evaporation pressure is indeed controlled by the smallest neck size along the widest continuous path of necks connecting the individual cavities with the bulk vapor phase, the shape of the desorption branch due to pore-blockingcontrolled desorption should change with increasing water filling. When the amount of water preadsorbed was progressively increased, however, the shape of the desorption branch for argon adsorption at 77 K remained almost unchanged, although the amount of argon condensed in the spherical cavities of this material dropped rapidly over a relatively narrow range of water filling. This clearly indicates that the gradual desorption often observed for the ordered silica with cage-like pores does not
Morishige and Yoshida necessarily correspond to the wide distribution in size of the necks available for desorption. The capillary evaporation pressure in the range of the poreblocking-controlled desorption did not reflect directly the smallest neck size along the widest continuous path of necks connecting the given cavity with the bulk vapor phase, as opposed to the concept of pore blocking. Recently, Woo, Porcheron, and Monson have examined the desorption mechanism of fluids in disordered mesoporous glasses using Monte Carlo simulations of a coarse-grained lattice model.57 They have suggested that the pore-blocking mechanism does not seem to play an important role in the desorption process of fluids confined in the disordered mesoporous glasses. The desorption proceeds via an advancing front separating regions of high and low fluid density that moves via bubble formation in the pores near the boundaries to the bulk vapor phase rather than the motion of a meniscus as has been commonly assumed in the pore-blocking-controlled desorption. In such a mechanism, neck size does not seem to be related directly to the capillary evaporation pressure. When the size of the necks in the material with cage-like pores is smaller than a certain critical value, bubble formation (cavitation) occurs in the pores deep inside as is often observed. On the other hand, for the material with cage-like pores of larger necks, it may occur only in the pores near the microscopic meniscus of the emptied pores from the bulk vapor phase to the interior of the material. This desorption mechanism might lead to the gradual desorption that always has been observed for the ordered silicas with cage-like pores of large necks. Acknowlegdment. . This work was supported by a matching fund subsidy for private universities from MEXT (Ministry of Education, Culture, Sports, Science and Technology). Supporting Information Available: Adsorption-desorption isotherms of nitrogen at 77 K on FDU-12 and the relation between the pressure of capillary condensation of water and pore size in cylindrical pores of ordered silicas. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Zhao, D.; Huo, Q.; Feng, J.; Chmelka, B. F.; Stucky, G. D. J. Am. Chem. Soc. 1998, 120, 6024. (2) Kra¨mer, E.; Fo¨rster, S.; Go¨ltner, C.; Antonietti, M. Langmuir 1998, 14, 2027. (3) Yu, C.; Yu, Y.; Zhao, D. Chem. Commun. 2000, 575. (4) Yu, C.; Yu, Y.; Miao, L.; Zhao, D. Microporous Mesoporous Mater. 2001, 44-45, 65. (5) Smarsly, B.; Go¨ltner, C.; Antonietti, M.; Ruland, W.; Hoinkis, E. J. Phys. Chem. B 2001, 105, 831. (6) Tattershall, C. E.; Jerome, N. P.; Budd, P. M. J. Mater. Chem. 2001, 11, 2979. (7) Kruk, M.; Antochshuk, V.; Matos, J.; Mercuri, L. P.; Jaroniec, M. J. Am. Chem. Soc. 2002, 124, 768. (8) Tattershall, C. E.; Aslam, S. J.; Budd, P. M. J. Mater. Chem. 2002, 12, 2286. (9) Ravikovitch, P. I.; Neimark, A. V. Langmuir 2002, 18, 9830. (10) Matos, J. R.; Mercuri, L. P.; Kruk, M.; Jaroniec, M. Langmuir 2002, 18, 884. (11) Van Der Voort, P.; Benjelloun, M.; Vansant, E. F. J. Phys. Chem. B 2002, 106, 9027. (12) Matos, J. R.; Kruk, M.; Mercuri, L. P.; Jaroniec, M.; Zhao, L.; Kamiyama, T.; Terasaki, O.; Pinnavaia, T. J.; Liu, Y. J. Am. Chem. Soc. 2003, 125, 821. (13) Kruk, M.; Jaroniec, M. Chem. Mater. 2003, 15, 2942. (14) Morishige, K.; Tateishi, N. J. Chem. Phys. 2003, 119, 2301. (15) Thomas, A.; Schlaad, H.; Smarsly, B.; Antonietti, M. Langmuir 2003, 19, 4455. (16) Fan, J.; Yu, C.; Gao, F.; Lei, J.; Tian, B.; Wang, L.; Luo, Q.; Tu, B.; Zhou, W.; Zhao, D. Angew. Chem., Int. Ed. 2003, 42, 3146.
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