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J. Phys. Chem. C 2009, 113, 14927–14934

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Porous Structure of Ordered Silica with Cagelike Pores Examined by Successive Adsorption of Water and Nitrogen Kunimitsu Morishige* and Yoshinori Kanzaki Department of Chemistry, Okayama UniVersity of Science, 1-1 Ridai-cho, Okayama 700-0005, Japan ReceiVed: April 29, 2009; ReVised Manuscript ReceiVed: July 9, 2009

To examine the porous structure of ordered silica with cagelike pores, we measured successive adsorption of water at 283 K and nitrogen at 77 K on four kinds of SBA-16 and four kinds of KIT-5. For materials with smaller necks, we found the amount of nitrogen condensed inside the large cavities decreased rapidly with an increase in water filling and reached a small value at water filling, just before the onset of pore condensation of water inside the spherical cavities. This clearly indicates that most of the cavities are isolated from bulk nitrogen gas with water frozen in the necks. A comparison with the accessibility curve of voids expected for a random distribution of neck sizes throughout an ordered structure indicates the presence of correlation in the spatial distribution of neck sizes in the material. In addition, the relative pressure range of water, in which the amount of nitrogen condensed in the large cavities considerably drops, gave information about pore size distribution of the necks. However, for materials with larger necks, the amount of nitrogen condensed inside the large cavities decreased gradually with an increase in water filling because pore condensation of water occurs simultaneously in the necks and cavities. I. Introduction Percolation theory is often applied to the description of the desorption isotherm of a fluid in disordered mesoporous materials, where the pore space is treated as a lattice of voids interconnected by necks in a three-dimensional (3D) network.1-5 However, recent theoretical and simulation studies have shown that a hysteresis loop of type H2 in the International Union of Pure and Applied Chemistry (IUPAC) classification6 for fluid adsorption on disordered mesoporous silicas is reproduced without invoking pore blocking or percolation effects.7-10 The disordered mesoporous silicas do not necessarily take the pore structure consisting of cavities joined by constrictions that a model assumes.11-13 In this respect, ordered silicas with cagelike pores such as SBA-16 (with Im3m symmetry)14 and KIT-5 (with Fm3m symmetry)15 are regarded as the most suitable model adsorbents currently available for examination of the applicability of the theory to desorption because in these materials almost spherical cavities are arranged in a 3D lattice and connected through narrow necks. When the diameter of the necks is larger than a certain critical value, pore blocking controlled desorption is predicted to take place.16 Evaporation of capillary condensate in the pores is obstructed by liquid remaining condensed in the necks. Poreblocking controlled desorption from the interconnected network of the cavities in the ordered silicas is thought to depend not only on the size of the necks but also on the connectivity of the network and state of neighboring pores. This is a problem dealt with by the percolation theory. The theory tells us that the main part of desorption takes place over a narrow range of pressures, where the necks in a narrow range of pore size distribution are allowed to desorb when the pressure is decreased from saturation. Despite the wide distribution of neck size, one can obtain a steep desorption branch for the disordered mesoporous materials, namely, a hysteresis loop of type H2. However, the * To whom correspondence should be addressed. E-mail: morishi@ chem.ous.ac.jp.

desorption isotherms observed for ordered silicas with cagelike pores are often very gradual.15,17-28 It is evident that the percolation effect, expected for a large lattice with a noncorrelated distribution of cavity and neck sizes, is unable to account for the gradual desorption behavior of the liquid confined to the cavities of ordered silicas. The presence of correlated distribution in neck size was suggested.27 Ordered silicas with large cagelike pores have recently attracted a great deal of interest because of their unique structures and potential applications in immobilization of biomolecules, separation, and catalysis.14,15,17-36 In these applications, size distribution of the necks interconnecting the large cavities as well as spatial distribution of neck sizes are especially important. The nitrogen adsorption method at 77 K, which is normally employed for pore size analysis, reaches the limit of its applicability if the neck size is smaller than ∼5 nm.16,18,19 In that case, the experimental desorption branch of the nitrogen isotherm is unsuitable for neck size analysis because desorption occurs via spontaneous cavitation of liquid confined to the cavities in the relative pressure range of 0.41-0.48, regardless of neck size.16,22 In addition, even when neck size is larger than the critical size, the actual distribution of neck size is never obtained by a simple pore size analysis of the desorption branch because of the effects of the pore network.1-5 High-resolution pore size analysis of the adsorption branch often gives pore size distribution curves in the region of micropores and small mesopores relevant to the necks interconnecting the large cavities.17,21,22,24-26,28,31,32 However, most of the pores in this region are complementary pores located inside the framework walls of the ordered silicas with large cagelike pores.35 Electron crystallography can be used to determine neck size in the ordered silicas with cagelike pores,37 although the method appears to neglect a broad distribution of the neck sizes inherent to the ordered silicas with cagelike pores and to be restricted to highly ordered samples with appreciable ordered domain sizes. Jaroniec and co-workers tried to determine average neck size from studies of pore accessibility after surface modifications with ligands of

10.1021/jp903968q CCC: $40.75  2009 American Chemical Society Published on Web 07/28/2009

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gradually increasing sizes.38,39 This method has many attractive features such as easy chemistry, 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 surface. Therefore, there is a need to develop more convenient or more reliable methods for neck size elucidation. In the present study, we measure the adsorption isotherm of nitrogen at 77 K on ordered silicas with cagelike pores that adsorbed various amounts of water in advance. On adsorption, water behaves quite independently in different parts of the pore system. The filling of every neck on the adsorption branch of the isotherm is determined by their individual characteristics. With increasing pressure, necks of larger size are progressively filled with water, independent of the state of neighboring pores. The capillary-condensed water freezes by subsequent cooling and blocks the movement of nitrogen from one cavity to an adjacent one with the adsorption of nitrogen at 77 K. The percolation theory predicts that with an increase in water filling the amount of nitrogen condensed in spherical cavities drops rapidly over a narrow range of water filling for a random distribution of neck size in the pore network, where the necks in a narrow range of pore size distribution are allowed to be blocked by ice confined to the necks. A correlated distribution in neck size would be detected as deviations from such idealized behavior. The purpose of the present study is to examine the size distribution of the necks as well as the spatial distribution of the neck sizes in ordered silicas with cagelike pores and to elucidate the origin of the gradual desorption of nitrogen often observed on these materials by measuring successive adsorption of water and nitrogen on four kinds of SBA-16 and four kinds of KIT-5. The structure of SBA-16 consists of a body-centered cubic array of almost spherical cavities, while that of KIT-5 consists of a face-centered cubic array of the spherical cavities. The pore diameter of these materials can be enlarged by aging of the synthesis mixture18-22,24,25,27,28,35 or sulfuric acid treatment of the as-synthesized material.31 II. Experimental Section II.1. Materials and Characterization. SBA-16 was prepared by using Pluronic F127 triblock copolymer as a structuredirecting agent at an aging temperature of 373 K for 48 h according to the procedure of Kleitz et al.33 and subsequently treated with 48 wt % H2SO4 solution at 368 K.31 The obtained SBA-16 silicas are denoted SBA-16-x, where x corresponds to the H2SO4 treatment time. KIT-5 with expanded spherical cavities was prepared by using Pluronic F127 triblock copolymer as a structure-directing agent and benzene as a solubilizing agent. The preparation and characterization of KIT-5 samples have been described in detail elsewhere.27 The obtained KIT-5 silicas are denoted KIT-5-x, where x corresponds to the aging time at 393 K. Adsorption isotherms of nitrogen at 77 K were measured volumetrically on a BELSORP-mini II (Bell Japan, Inc.). TEM images were recorded on a JEOL JEM-2000 EX electron microscope, operating at 200 kV. II.2. Measurement of Successive Adsorption. Successive adsorption of water and nitrogen was measured volumetrically on a 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

Morishige and Kanzaki

Figure 1. Schematic illustration of adsorption of nitrogen in the pore network of ordered silicas with cagelike pores after water was preadsorbed and then frozen in the necks.

by using the additional gas dosing volume. After a given amount of water was adsorbed, the valve separating the gas dosing volume and sample was closed, and then sample powder (∼0.2 g) was slowly cooled to 200 K with a cooling rate of ∼1 deg/ min. The adsorption isotherm of nitrogen on the sample with preadsorbed water was measured at 77 K without the use of the additional gas dosing volume. III. Theoretical Background It is well-known that on adsorption a fluid behaves quite independently in different parts of the pore system.40-42 In ordered silicas with cagelike 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 (Figure 1). The percolation theory deals with the transmission of a fluid to sites within a medium against randomly distributed barriers (bonds), which determines whether the fluid can move from one site to an adjacent one. To apply the theory to successive adsorption of water and nitrogen, we regard the voids in the porous medium as the sites and the necks between the voids as the bonds of the percolation theory. The conditions applied in such a medium are analogous to those of bond-controlled percolation in a 3D lattice. 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, where the necks in a narrow range of pore size distribution are allowed to be blocked by ice on the adsorption of nitrogen. This range of fractions of necks closed is about 0.7-0.8 and 0.8-0.9, respectively, for body-centered cubic and face-centered cubic lattices of voids.1 The pore structures of SBA-16 and KIT-5 correspond to body-centered cubic and face-centered cubic lattices, respectively. Before that, the amount of nitrogen condensed in the cavities would not drop appreciably over a large range of fractions of necks closed as shown by the curves of accessibilities of the voids in the 3D lattices as functions of the fractions of necks open.1 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. The relative pressure, at which pore condensation of water occurs, is related to the size of the necks. Therefore, the relative pressure range of water, in which the amount of nitrogen condensed in the large cavities considerably drops, gives information about pore size distribution of the necks. If the neck size is uniform, the amount of

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J. Phys. Chem. C, Vol. 113, No. 33, 2009 14929 TABLE 1: Physicochemical Parametersa

sample

surface area SBET (m2/g)

micropore volume Vmic (cm3/g)

total volume Vt (cm3/g)

cavity diameter D (nm)

SBA-16-0 h SBA-16-3 h SBA-16-12 h SBA-16-24 h KIT-5-1 day KIT-5-3 days KIT-5-5 days KIT-5-7 days

925 709 706 416 552 596 509 543

0.14 0.01 0 0 0.13 0.07 0.01 0.01

0.81 0.79 0.93 0.87 0.55 0.78 0.78 0.91

10.3 11.3 12.8 14.2 16.8 17.4 18.4 19.2

a Parameters are derived from nitrogen physisorption data at 77 K for SBA-16 samples prepared with different H2SO4 treatment times at 368 K and KIT-5 samples prepared with different hydrothermal treatment times at 393 K.

Figure 2. Adsorption-desorption isotherms of nitrogen at 77 K on SBA-16 treated with H2SO4 for different periods of time at 368 K. Volumes adsorbed for the samples prepared from the H2SO4 treatment for 3, 12, and 24 h were incremented by 250, 500, and 900 cm3(STP)g-1, respectively.

nitrogen condensed in the cavities would drop sharply in a certain small range of water vapor pressure in the isotherm when water was preadsorbed. IV. Results and Discussion IV.1. SBA-16. IV.1.1. Characterization. Figure 2 shows the adsorption-desorption isotherms of nitrogen at 77 K on four kinds of SBA-16 samples. The shape of the desorption branch changed as the H2SO4 treatment was prolonged. However, the shape of the adsorption branch remained almost unchanged, although its position was shifted into higher relative pressures, with an increase of the H2SO4 treatment time. Because capillary condensation during adsorption is controlled by the size of the cavities, the sharp adsorption branch observed for all samples indicates the presence of uniform cavities. The average diameter of the spherical cavities was estimated by using the relationship between the capillary condensation pressure and pore diameter reported by Broekhoff and de Boer.43 The specific surface area (SBET) was calculated by using the Brunauer-Emmet-Teller method.44 The micropore volume and total pore volume were estimated by using the t-plot method.45 Table 1 summarizes the main physicochemical parameters of the samples used in the present study. For two samples untreated and treated with H2SO4 for 3 h, the hysteresis loop closed sharply at a relative pressure of 0.48 corresponding to the lower limit of the adsorption hysteresis. This indicates that desorption takes place via cavitation, and the diameter of the narrow necks in these materials is smaller than ∼5 nm.16,18,19 However, in the cases of samples treated with H2SO4 for long times (12 and 24 h), the hysteresis loops gradually closed above the lower limit of hysteresis, which indicates the neck diameters are above ∼5 nm. With an increase in H2SO4 treatment time, the onset of evaporation shifted into higher relative pressures. This indicates a gradual enlargement of the narrow necks as well as the spherical cavities with prolonged H2SO4 treatment, being consistent with previous results.31 The isotherm of SBA-16-24 h exhibits the hysteresis loop of type H1, and the loop is very narrow. This suggests the presence of pores with wide necks

Figure 3. Transmission electron microscopy images of SBA-16 treated with H2SO4 for different periods of time at 368 K viewed from the [111] direction. Treatment time with sulfuric acid is as follows: (a) 0 h, (b) 12 h, and (c) 24 h.

of more uniform size and thus a pseudocylindrical pore geometry, consisting of arrays of regularly undulating channels rather than discrete spherical cavities. Figure 3 shows the TEM images of SBA-16 untreated as well as the samples treated with H2SO4 for 12 and 24 h. It reveals that all materials possess excellent 3D cubic mesoscopic order, and the Im3m structure is still kept after prolonged treatment with H2SO4. The lattice parameters of SBA-16-0 h, SBA16-12 h, and SBA-16-24 h estimated from the TEM images were ∼15, 16, and 19 nm, respectively, being consistent with the previous study.31 IV.1.2. SuccessiWe Adsorption. Kinetics of water adsorption was extremely slow, especially in the capillary condensation region. It usually took about 3-4 days for a measurement of the adsorption isotherm of water. Figure 4 shows the adsorption isotherms of water at 283 K on four kinds of SBA-16 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 SBA-16 samples, and Vw is the volume occupied by the adsorbed water at 283 K. All isotherms show the type V curve of the

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Figure 4. Adsorption isotherms of water at 283 K on SBA-16 treated with H2SO4 for 0 h (O), 3 h (4), 12 h (0), and 24 h (]) at 368 K.

Figure 5. Adsorption-desorption isotherms of nitrogen at 77 K on SBA-16 untreated with H2SO4 on which water was preadsorbed at pore fillings of 0 (b), 0.22 (O), 0.34 (4), 0.42 (0), and 0.47 (]) The isotherms at fH2O ) 0.11 and 0.52 are omitted for clarity.

BDDT definition of an adsorption isotherm, indicating some of the hydrophobic nature of the pore surface.46 The pressure at which pore condensation of water occurs was shifted to higher relative pressures with an increase in H2SO4 treatment time. Figure 5 shows the adsorption-desorption isotherms of nitrogen at 77 K on SBA-16 untreated with H2SO4 on which various amounts of water were preadsorbed. Preadsorption of a small amount of water resulted in a large decrease of the monolayer capacity of nitrogen, indicating that nitrogen molecules interact very weakly with a pore wall covered with water molecules. When the amount of water preadsorbed was further increased beyond fH2O ) 0.34, the amount of nitrogen condensed inside the spherical cavities of SBA-16 decreased rapidly and reached a small value at fH2O ) 0.47, before 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. For SBA-16 treated with H2SO4, however, the amount of nitrogen condensed in the cavities decreased slowly with an increase in water filling (Figures 1S-3S of the Supporting Information). Figure 6 shows the ratio of the amount of nitrogen condensed in the spherical cavities of SBA-16 with preadsorbed water, Vc, and without preadsorbed water, Vc0, as a function of water filling for four kinds of SBA-16 samples. Vc was estimated from the

Morishige and Kanzaki

Figure 6. Ratio Vc/Vc0 of capillary condensate of nitrogen at 77 K inside the large cavities of SBA-16 treated with H2SO4 for 0 h (O), 3 h (4), 12 h (0), and 24 h (]) as a function of water filling. Dotted line denotes accessibility of the cavities from the surface as a function of water filling.

difference between the saturated amount of adsorbed nitrogen and amount of adsorbed nitrogen 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 between the cavities, before the onset of capillary condensation inside the cavities. Therefore, quantitative analysis of the data based on the percolation theory is impossible. For SBA-16 untreated, the amount of nitrogen condensed in the cavities dropped rapidly over a narrow range of fH2O ) 0.3-0.45. If water adsorption in the necks occurs with a constant probability over a whole range of water filling before the onset of pore condensation of water inside the large cavities, the curve of Vc /Vc0 versus fH2O can be directly compared with the curve of accessibility of the voids in a body-centered cubic lattice as a function of the fraction of necks open for a random distribution of neck sizes in the pore network.5 In Figure 6, the accessibility of the voids for the body-centered cubic lattice was plotted against the pore filling of water under the assumption that all of the necks in this material are completely filled with water at fH2O ) 0.5. When pore condensation of water in the necks occurs in a certain limited range of water filling, the accessibility of the voids would change more steeply with an increase in water filling. As shown in Figure 6, the decrease of Vc/Vc0 with an increase in water filling for SBA-16-0 h was gradual compared to the accessibility curve of the voids expected for a random distribution of neck sizes throughout the ordered structure. The small size of the pore network and/or the presence of the correlated distribution of neck sizes would be responsible for it. For SBA-16 treated with H2SO4 for 3 h, Vc/Vc0 started to decrease with preadsorption of a small amount of water. This indicates the presence of a correlation in the spatial distribution of neck sizes in the material. The necks of smaller sizes are concentrated in a part of the sample particle, and thus, the cavities in this region are preferentially isolated from the surface by ice formed in these small necks even at small fillings of water. Such a correlation in neck sizes would have already existed in SBA-16 untreated with H2SO4. With a further increase in water filling, Vc/Vc0 gradually decreased and attained a low value at fH2O ) 0.7, after the onset of pore condensation of water inside the spherical cavities. This indicates that a small fraction of the necks were enlarged by the H2SO4 treatment for 3 h to such an extent that capillary condensation of water takes place

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Figure 7. Ratio Vc/Vc0 of capillary condensate of nitrogen at 77 K inside the large cavities of SBA-16 treated with H2SO4 for 0 h (O), 3 h (4), 12 h (0), and 24 h (]) as a function of the relative pressure in the isotherm when water was preadsorbed at 283 K. Vertical bars at the bottom denote the relationship between the pore diameter and capillary condensation pressure (p/p0) of water for the cylindrical pores of ordered mesoporous silicas.47

simultaneously in the necks and spherical cavities. If the spatial distribution of neck sizes was kept random and more than ∼40% of the necks were enlarged by the prolonged H2SO4 treatment to such an extent that pore condensation of water occurs simultaneously in the necks and cavities, pore blocking by the ices confined to the necks would not take place at all. In this case, the ratio Vc/Vc0 decreases almost linearly with an increase in water filling. For the two samples treated with H2SO4 for 12 and 24 h, it is almost certain that a high fraction of the necks were enlarged by the prolonged treatment to such an extent. IV.1.3. Neck Size Distribution. Very recently, Ja¨hnert et al. have reported on the adsorption isotherms of water on MCM41 with cylindrical pores.47 Their data give the relationship between the pore diameter and capillary condensation pressure of water for the cylindrical pores of silicas. When we assume that the necks 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 this relationship. Figure 7 shows Vc/Vc0 as a function of the equilibrium relative pressure in the isotherm when water was preadsorbed. Figure 7 includes their relationship for the cylindrical pores of silicas. For SBA-16 untreated with H2SO4, it is indicated that the necks, which cause inaccessibility of the cavities from the surface, are distributed in the range of 2.5-3.7 nm in diameter. Because the cavities become inaccessible from the bulk vapor phase after more than ∼70% of the necks are closed by ice for a random distribution of the neck sizes, a major fraction of the necks in this material would have smaller sizes. Such an estimate of the neck sizes in SBA-16 is consistent with a neck diameter of ∼2.3 nm determined by the electron crystallography method.37 However, it is evident that the necks in SBA-16 are not uniform in size. It is well-known that the critical size of necks below which capillary evaporation of nitrogen at 77 K in cagelike pores takes place via cavitation is ∼5 nm in diameter.16,18,19 The relation between the pore diameter and capillary condensation pressure of water for the cylindrical pores of silica suggests that capillary condensation of water in the necks of diameter ∼5 nm may take place at a relative pressure slightly larger than ∼0.7. Such a relative pressure is close to the pressure at which capillary condensation of water starts to occur in the cavities of SBA-16 treated with H2SO4 for 3 h. For this material, the necks that cause the inaccessibility

Figure 8. Adsorption-desorption isotherms of nitrogen at 77 K on KIT-5 hydrothermally treated for different periods of time at 393 K. Volumes adsorbed for the samples prepared from the hydrothermal treatment for 3, 5, and 7 days were incremented by 200, 500, and 800 cm3(STP)g-1, respectively.

of the cavities for nitrogen are distributed in the range of 2.5-5.0 nm in diameter, and pore condensation of water occurs simultaneously in the largest necks and a fraction of the cavities. The size distribution of the necks was shifted to larger sizes by the H2SO4 treatment for 3 h at 368 K. Mayagoitia, Roja, and Kornhauser have considered the network effects during capillary condensation on the basis of the classical Kelvin equation.48 Their consideration suggests that if the diameters of most of the necks surrounding a given spherical cavity are larger than half the diameter of the cavity, then filling of the cavity should follow automatically once these necks are filled with condensate, otherwise condensation in the cavity will occur at a higher relative pressure. Actually, the diameters of a small fraction of the necks in SBA-16 were enlarged up to nearly half the average diameter of the spherical cavities by the H2SO4 treatment for 3 h. The fraction of necks enlarged to such an extent that capillary condensation of water occurs simultaneously in the necks and cavities was increased by prolonged H2SO4 treatment. For both samples treated with H2SO4 for 12 and 24 h, a large decrease in the accessibility of the cavities was observed only after capillary condensation of water started to occur in the large cavities. The relative pressures at which pore condensation of water starts to occur in the large cavities of the SBA-16 samples treated with H2SO4 for 12 and 24 h are ∼0.78 and 0.81, respectively. The adsorption isotherms of water on SBA-15 silicas with cylindrical pores of diameter ∼9 nm indicates that pore condensation of water in these cylindrical pores occurs at a relative pressure of ∼0.8.49,50 Therefore, for SBA-16 treated with H2SO4 for 24 h, it is suggested from the pore condensation pressure of water that a high fraction of the necks enlarged in this material attain ∼9 nm or above in diameter. The diameters of these enlarged necks are exceedingly larger than half the diameters of the cavities, and thus, pore condensation of water would occur simultaneously in the large necks and spherical cavities. The material seems to consist of arrays of regularly undulating channels rather than discrete spherical cavities. IV.2. KIT-5. IV.2.1. Characterization. Figure 8 shows the adsorption-desorption isotherms of nitrogen at 77 K on four

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Figure 9. Adsorption isotherms of water at 283 K on KIT-5 with expanded spherical cavities hydrothermally treated for 1 day (O), 3 days (4), 5 days (0), and 7 days (]) at 393 K.

kinds of KIT-5 samples with expanded cavities. The shape of the desorption branch changed drastically as the hydrothermal treatment time was prolonged. For KIT-5 hydrothermally treated only for 1 day, the hysteresis loop of the nitrogen adsorption isotherm at 77 K closed sharply at a relative pressure of 0.47, corresponding to the lower limit of the adsorption hysteresis. This indicates that the diameter of the necks is smaller than ∼5 nm. With an increase in hydrothermal treatment time, however, the onset of evaporation was shifted to higher relative pressures, and evaporation took place more gradually above the lower limit of hysteresis. This indicates a gradual enlargement of the neck diameter above ∼5 nm. The spherical cavities and necks were enlarged on prolonged hydrothermal treatment. The sizes of the spherical cavities of the present KIT-5 samples are larger than those of the SBA-16 samples. The main physicochemical parameters of the KIT-5 samples used in the present study are also summarized in Table 1. The broadness of the relative pressure range at which evaporation took place has been sometimes ascribed to a broad distribution of neck sizes in the material.18,19,31 However, the percolation effect that was expected for a noncorrelated distribution of neck sizes is unable to account for the gradual desorption of the liquid confined to the cavities of ordered mesoporous silicas.27 IV.2.2. SuccessiWe Adsorption. Figure 9 shows the adsorption isotherms of water at 283 K on four kinds of KIT-5 samples. All isotherms show a type V curve of the BDDT definition of the adsorption isotherm. The pressure, at which pore condensation of water occurs, was shifted to slightly higher relative pressures with an increase in hydrothermal treatment time. Figure 10 shows the adsorption-desorption isotherms of nitrogen at 77 K on KIT-5 hydrothermally treated for 7 days on which various amounts of water were preadsorbed. When the amount of water preadsorbed was increased, the amount of nitrogen condensed inside the large cavities of KIT-5 decreased gradually, while the shape of the adsorption hysteresis remained almost unchanged. However, for KIT-5 hydrothermally treated for 1 day and 3 days, the amount of nitrogen condensed in the cavities decreased rapidly with an increase in water filling (Figures 4S-6S of the Supporting Information). Figure 11 shows Vc/Vc0 for capillary condensation of nitrogen at 77 K as a function of water filling for four kinds of KIT-5 samples. For all samples, Vc/Vc0 decreased with preadsorption of a small amount of water, indicating the presence of correlation in the spatial distribution of neck sizes in these materials. For KIT-5 hydrothermally treated for 1 and 3 days, Vc/Vc0 dropped rapidly over narrow ranges of water filling. In Figure 11, the curve of accessibility of the voids in a face-centered cubic lattice5 is

Figure 10. Adsorption-desorption isotherms of nitrogen at 77 K on KIT-5 hydrothermally treated for 7 days at 393 K on which water was preadsorbed at pore fillings of 0 (b), 0.30 (O), 0.41 (4), and 0.73 (0). Isotherms at fH2O ) 0.04, 0.08, 0.16, and 0.60 are omitted for clarity.

Figure 11. Ratio Vc/Vc0 of capillary condensate of nitrogen at 77 K inside the large cavities of KIT-5 with expanded cavities hydrothermally treated at 393 K for 1 day (O), 3 days (4), 5 days (0), and 7 days (]) as a function of water filling. Dotted line denotes accessibility of the voids from the surface as a function of water filling.

compared with the experimental curve of Vc/Vc0 versus fH2O under the assumption that water adsorption in the necks occurs with a constant probability over a whole range of water filling before the onset of pore condensation of water inside the large cavities at fH2O ) 0.4. The decrease of Vc/Vc0 with an increase in water filling for the KIT-5-1day and KIT-5-3days samples was very gradual compared to the accessibility curve of the voids for a random distribution of neck sizes. Therefore, the spatial distribution of neck sizes in the materials is correlated to a considerable extent. For KIT-5 hydrothermally treated for 5 and 7 days, Vc/Vc0 decreased very gradually over a wide range of water filling. An appreciable fraction of the necks are enlarged by the prolonged hydrothermal treatment to such an extent that capillary condensation of water takes place simultaneously in the necks and large cavities. IV.2.3. Neck Size Distribution. Figure 12 shows Vc/Vc0 as a function of the equilibrium relative pressure in the isotherm when water was preadsorbed. For KIT-5 hydrothermally treated for 1 day, the necks that cause the inaccessibility of the cavities are distributed over a wide range below ∼5 nm in diameter. The size distribution of the necks was shifted to larger sizes by hydrothermal treatment for 3 days at 393 K. However, for KIT-5

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Figure 12. Ratio Vc/Vc0 of capillary condensate of nitrogen at 77 K inside the large cavities of KIT-5 with expanded cavities hydrothermally treated at 393 K for 1 day (O), 3 days (4), 5 days (0), and 7 days (]) as a function of the relative vapor pressure in the isotherm when water was preadsorbed at 283 K. Vertical bars at the bottom denote the relationship between the pore diameter and capillary condensation pressure of water (p/p0) for the cylindrical pores of ordered mesoporous silicas.47

hydrothermally treated for 5 or 7 days, a large decrease in the accessibility of the cavities was observed only when pore condensation of water occurred in the large cavities of diameter 19 nm at a relative pressure above ∼0.8. The adsorption isotherms of water on SBA-15 silicas with cylindrical pores of diameter ∼9 nm indicate that pore condensation of water in these cylindrical pores occurs at a relative pressure of ∼0.8.49,50 Therefore, for KIT-5 hydrothermally treated for 5 or 7 days, it is suggested from the pore condensation pressure of water that an appreciable fraction of the necks enlarged in these materials attain ∼9 nm or above in diameter. If we assume a random distribution in neck size throughout the ordered structure of the sample particle, it is concluded that at least more than ∼20% of the necks were enlarged by prolonged hydrothermal treatment to such an extent that pore condensation of water occurs simultaneously in the necks and large cavities. IV.2.4. Origin of Gradual Desorption. When the neck size is smaller than ∼5 nm in diameter, the capillary evaporation of liquid nitrogen confined to the large cavities takes place via cavitation at the lower pressure limit of adsorption hysteresis, irrespective of neck sizes. As shown in Figure 8, capillary evaporation for KIT-5 hydrothermally treated for 5 and 7 days at 393 K began at relative pressures higher than the lower limit of the adsorption hysteresis. This earlier onset of capillary evaporation can be attributed to the existence of percolation pathways consisting of necks of sizes larger than ∼5 nm. Taking into account the connectivity of the entire pore system, it is conceivable that the pressure at which the delayed capillary evaporation takes place from a given cavity reflects the size of the narrowest neck along the widest path that connects the given cavity with the surfaces.18,19 Therefore, a very broad distribution of neck sizes in the KIT-5 samples hydrothermally treated for long times can be expected from the shape of the desorption branch of nitrogen at 77 K. The adsorption isotherms of water on SBA-15 with cylindrical pores of diameter ∼9 nm49,50 suggest that all necks of diameter smaller than ∼9 nm are closed for the movement of nitrogen from one cavity to an adjacent one, when water was preadsorbed at a relative pressure of ∼0.8 and then frozen in the necks by subsequent cooling. Nevertheless, for the KIT-5-5days and KIT-5-7days samples, the shape of the desorption branch of nitrogen at 77 K remained almost unchanged even after the necks of smaller sizes in the materials were preferentially blocked with ice confined in the necks. This

We developed a simple and reliable method to elucidate the porous structures of ordered silicas with cagelike pores. In this method, we measure the adsorption-desorption isotherms of nitrogen at 77 K on ordered mesoporous materials with cagelike pores on which various amounts of water were preadsorbed at 283 K. Ordered silicas with cagelike pores such as SBA-16 and KIT-5 consist of uniform cavities arranged in a 3D lattice and necks connecting the cavities. The necks in SBA-16 and KIT-5 are not uniform in size. For SBA-16 and KIT-5 samples with smaller necks, most of the cavities are blocked for nitrogen adsorption at 77 K with water frozen in the necks, which indicates the cagelike character of the porosity. The presence of the correlated distribution of neck sizes is indicated from the dependence of the amount of nitrogen condensed in the large cavities on water filling. The extent of the spatial correlation of neck sizes in KIT-5 is higher than SBA-16. A pore size distribution of the necks can be estimated from the relative pressure range of water, in which the amount of nitrogen condensed in the large cavities considerably drops, even when the diameter of the necks is smaller than ∼5 nm. The pore size distribution of the necks in SBA-16 without the H2SO4 treatment estimated by the present method is consistent with the neck size determined by electron crystallography.37 For SBA-16 and KIT-5 samples with larger necks, pore condensation of water occurs simultaneously in the large cavities and a high fraction of the necks. This indicates an almost complete loss of the cagelike character of the porosity. Acknowledgment. This work was supported by 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 SBA-16 and KIT-5. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Wall, G. C.; Brown, R. J. C. J. Colloid Interface Sci. 1981, 82, 141. (2) Mason, G. Proc. R. Soc. London, Ser. A 1983, 390, 47. (3) Neimark, A. V. Colloid J. 1984, 46, 813. (4) Parlar, M.; Yortsos, Y. C. J. Colloid Interface Sci. 1988, 124, 162. (5) Zhdanov, V. P. AdV. Catal. 1993, 39, 1. (6) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouque´rol, J.; Siemieniewska, T. Pure Appl. Chem. 1985, 57, 603. (7) Kierlik, E.; Monson, P. A.; Rosinberg, M. L.; Sarkisov, L.; Tarjus, G. Phys. ReV. Lett. 2001, 87, 055701. (8) Woo, H. -J.; Sarkisov, L.; Monson, P. A. Langmuir 2001, 17, 7472. (9) Pellenq, R. J. -M.; Rousseau, B.; Levitz, P. E. Phys. Chem. Chem. Phys. 2001, 3, 1207. (10) Gelb, L. D.; Gubbins, K. E. In Fundamentals of Adsorption 7; Kaneko, K., Kanoh, H., Hanzawa, Y., Eds.; IK International: Chiba, Japan, 2002; p 333. (11) Levitz, P.; Ehret, G.; Sinha, S. K.; Drake, J. M. J. Chem. Phys. 1991, 95, 6151. (12) Eschricht, N.; Hoinkis, E.; Ma¨dler, F.; Schubert-Bischoff, P.; Ro¨hlKuhn, B. J. Colloid Interface Sci. 2005, 291, 201. (13) Salazar, R.; Gelb, L. D. Langmuir 2007, 23, 530. (14) Zhao, D.; Huo, Q.; Feng, J.; Chmelka, B. F.; Stucky, G. D. J. Am. Chem. Soc. 1998, 120, 6024. (15) Kleitz, F.; Liu, D.; Anilkumar, G. M.; Park, I. -S.; Solovyov, L. A.; Shmakov, A. N.; Ryoo, R. J. Phys. Chem. B 2003, 107, 14296. (16) Ravikovitch, P. I.; Neimark, A. V. Langmuir 2002, 18, 9830. (17) Matos, J. R.; Mercuri, L. P.; Kruk, M.; Jaroniec, M. Langmuir 2002, 18, 884.

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