Large-Pore Cagelike Silica with Necks of Molecular Dimensions - The

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Large-Pore Cagelike Silica with Necks of Molecular Dimensions Kunimitsu Morishige* and Tsubasa Yasuki Department of Chemistry, Okayama UniVersity of Science, 1-1 Ridai-cho, Kita-ku, Okayama 700-0005, Japan ReceiVed: March 25, 2010; ReVised Manuscript ReceiVed: May 17, 2010

The neck size of five kinds of FDU-12 silicas prepared without and with the hydrothermal treatment at low temperatures was assessed by examining the molecular sieving effect for several different molecules. The results clearly indicate that the ordered silicas with cagelike pores thus obtained possess necks of molecular dimensions, and the size of the necks available for molecular sieving is increased with an increase in the hydrothermal treatment temperature. The molecular sieving effect of the cagelike silica is discussed based on bond-controlled percolation. I. Introduction Molecular sieving is the selective adsorption of molecules into the pores of an adsorbent and the exclusion of others due to their dimensions being above the critical size. Adsorbents that exhibit the molecular sieving effect; namely, molecular sieves are mostly crystalline and utilized in many adsorptionbased separation processes. Exploitation of better adsorbents with the molecular sieving effect, especially those easily tailored, can be improved the performance of current industrial processes.1 Conventionally, molecular sieves are made of inorganic zeolites. Because of the ordered arrangement of the atoms and the rigidity of the bonds in such materials, a zeolite molecular sieve is made with a fixed pore size. This is beneficial when the pore size precisely fits the separation needs. However, when the size difference of the two gases is very small, a molecular sieve with the precise pore size is not always readily available. In such cases, pore-adjustable molecular sieves that can always meet the separation needs are highly desirable.2,3 In ordered cage-type mesoporous silicas such as SBA-164 and KIT-5,5 almost spherical cavities are arranged in a threedimensional (3D) lattice and connected through narrow necks. The size of the necks can be effectively tailored in a range from 98%) were obtained from Sumitomo Seika Chemicals, Inc. n-C4H10 (>98%) and c-C6H12 (>99.5%) were obtained from Tokyo Chemical Industry, Inc. III. Results III.1. Characterization. Figure 1 shows the XRD powder patterns of five kinds of FDU-12 samples. The materials were prepared according to the procedure of Yu et al.,12 except for the hydrothermal conditions. They have concluded from their extensive investigations that the materials possess a facecentered-cubic structure (space group Fm3m). It is well-known that hydrothermal treatment at low temperatures leads to the preparation of weakly ordered materials, as is the case for the present materials. The sample prepared with the hydrothermal treatment at 333 K exhibited four diffraction peaks that can be indexed as the 111, 311, 331, and 531 reflections in a cubic unit cell of 28.4 nm. The samples prepared without and with the hydrothermal treatment at lower temperatures exhibited only two diffraction peaks, indicating that the materials possess lessordered porous structures. The unit-cell parameter of FDU-12 tends to increase with an increase in the hydrothermal treatment

10.1021/jp102727x  2010 American Chemical Society Published on Web 06/01/2010

Large-Pore Cagelike Silica

Figure 1. X-ray diffraction patterns of FDU-12 samples prepared without and with the hydrothermal treatment at different temperatures for 24 h.

temperature (see Table 1). Figure 2 shows the TEM images of the two samples prepared without and with the hydrothermal treatment at 333 K taken along the [110] directions. It reveals that the materials possess 3D cubic mesoscopic order, in accord with the XRD patterns. The unit-cell parameter of the sample prepared with the hydrothermal treatment at 333 K, which was estimated from the TEM image, was ∼28 nm, being consistent with the value obtained from the analysis of the XRD pattern. Figure 3 shows the adsorption-desorption isotherms of nitrogen at 77 K on five kinds of FDU-12 samples. All the samples, except for the sample prepared without the hydrothermal treatment, exhibited wide hysteresis loops with sharp adsorption and desorption branches. Because capillary condensation during adsorption is controlled by the size of the cavities, the sharp adsorption branch observed 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.16 The specific surface area (SBET) was calculated by using the Brunauer-Emmett-Teller method.17 The micropore volume and total pore volume were estimated by using the t-plot method.18 Table 1 summarizes the main physicochemical parameters of the samples used in the present study. For all the samples, except for the sample prepared without the hydrothermal treatment, 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 ∼4 nm.11,19,20 The sample prepared without the hydrothermal treatment did not show at all the appearance of capillary condensation of nitrogen in the large cavities, although both the XRD pattern and the TEM image indicated the presence of the large cavities in the material. This strongly suggests that the size of the interconnecting necks in the material is smaller than the kinetic diameter of N2. III.2. Molecular Sieving Properties. In cagelike structures such as FDU-12, large cavities are arranged in a 3D lattice and connected through narrow necks. Molecular sieving properties of the materials can be examined by measuring the adsorption isotherms of several molecules of different sizes when the size

J. Phys. Chem. C, Vol. 114, No. 24, 2010 10911 of the necks is comparable to those of the molecules. We used ammonia, carbon dioxide, nitrogen, n-butane, isobutane, and cyclohexane as adsorbates: the kinetic diameters of NH3, CO2, N2, n-C4H10, iso-C4H10, and c-C6H12 are 0.29, 0.33, 0.37, 0.47, 0.53, and 0.61 nm, respectively.1 Figures 4-8 show the adsorption-desorption isotherms of these probe molecules on the samples prepared without and with the hydrothermal treatment at 288, 303, 313, and 333 K, respectively. Here, the volume adsorbed is expressed in a volume of the corresponding liquid because the saturated volume adsorbed in capillary condensation, when expressed as a volume of liquid should be the same for all adsorbates on a given porous solid (the Gurvitsch rule).21 For the sample prepared without the hydrothermal treatment, the adsorption capacities for N2, n-C4H10, iso-C4H10, and c-C6H12 were very low, whereas those for NH3 and CO2 were moderate. In particular, the isotherm of NH3 showed a pronounced hysteresis loop in a high relative pressure region. The adsorption capacity attained ∼0.2 cm3g-1, which is comparable to the total pore volume of the sample prepared with the hydrothermal treatment at 288 K. This suggests that NH3, the most smallest among the probe molecules used in the present study, penetrates the pore network almost completely, and thus the size of the necks is in the range of 0.29-0.37 nm. Small amounts of adsorption observed for N2, n-C4H10, iso-C4H10, and c-C6H12 are due to the adsorption of these molecules on the external surfaces. For the sample prepared with the hydrothermal treatment at 288 K, the adsorption isotherms of NH3 and N2 exhibited wide hysteresis loops typical of cagelike pores. On the other hand, the adsorption step due to capillary condensation in the cavities was not observed for CO2, although the kinetic diameter of CO2 is smaller than that of N2. The stable phase of bulk CO2 at 195 K is solid, and thus the vapor pressure of the bulk solid is lower than that of the bulk liquid. At 195 K, the capillary condensation pressure of CO2 in the large cavities is higher than the saturated vapor pressure of the bulk solid, although it is lower than that of the metastable bulk liquid.22 The adsorption capacity of N2 was comparable to that of NH3, whereas the use of n-C4H10 as a probe molecule led to a significant decrease of adsorption capacity. The use of iso-C4H10 and c-C6H12 with larger kinetic diameters resulted in almost complete loss of adsorption capacity in the pore network. This suggests that the diameter of the necks in the material is in the range of 0.37-0.53 nm. For the sample prepared with the hydrothermal treatment at 303 K, the adsorption capacity of N2 was almost identical to that of NH3. Adsorption capacity rapidly decreased with a further increase in the kinetic diameter of a probe molecule, and the use of c-C6H12 as a probe molecule led to almost complete loss of adsorption capacity in the pore network. This suggests that the diameter of the necks in the material is in the range of 0.37-0.61 nm. Since the decrease in the adsorption capacity when the probe molecule was changed from N2 to n-C4H10 for the sample prepared with the hydrothermal treatment at 303 K was much smaller than that for the sample prepared with the hydrothermal treatment at 288 K, it is inferred that the average size of the necks is increased with an increase in the hydrothermal treatment temperature from 288 to 303 K. For the sample prepared with the hydrothermal treatment at 313 K, the adsorption capacity of N2 was identical to that of NH3, and a further increase in the kinetic diameter of a probe molecule resulted in a gradual decrease in their adsorption capacity. The adsorption capacity of c-C6H12, the largest molecule, was appreciable. This indicates further enlargement of the necks.

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TABLE 1: Physicochemical Parameters hydrothermal treatment temperature (K) untreated 288 303 313 333

unit-cell constant a (nm)

surface area SBET (m2/g)

micropore volume Vmic (cm3/g)

total volume Vt (cm3/g)

cavity diameter D (nm)

26.8 26.8 27.6 26.8 28.4

12 210 252 277 322

0 0.04 0.05 0.06 0.07

0.01 0.17 0.21 0.23 0.28

13 13 12 14

Figure 2. Transmission electron microscopy images of FDU-12 samples prepared (a) without and (b) with the hydrothermal treatment at 333 K for 24 h viewed from the [110] direction.

Figure 3. Adsorption-desorption isotherms of nitrogen at 77 K on FDU-12 samples prepared without and with the hydrothermal treatment at different temperatures for 24 h.

Finally, for the sample prepared with the hydrothermal treatment at 333 K, a decrease of adsorption capacity with an increase in the kinetic diameter of a probe molecule up to 0.61 nm was small, suggesting that most of the necks are larger than 0.61 nm in diameter. IV. Discussion IV.1. Origin of Necks of Molecular Dimensions. Largepore cagelike silicas with necks of molecular dimensions were obtained by the templating of spherical block copolymer micelles at a low temperature. The copolymer used as a template contains poly(ethylene oxide) as a hydrophilic block and poly(propylene oxide) as a hydrophobic block. In the structure of a block copolymer/silica composite, the cores of spherical micelles are mainly constituted by PPO blocks, whereas the micelle corona, which consists of PEO blocks, interacts with the silica framework.23-25 The hydrophilicity of the PEO blocks increases with decreasing temperature. At lower temperatures, the PEO chains favorably interact with hydrophilic silica species and thus have a tendency to be intimately mixed with the silica framework.26 As a result, a certain fraction of the PEO chains are expected to be tightly embedded inside the silica walls as a consequence of the molecular templating of single PEO chains.

Since the PEO chains of adjacent micelles may be highly entangled, the penetration of the PEO chains within the pore walls of as-synthesized FDU-12 may provide connectivity between the spherical micellar templates.23-25,27-30 Hence, the removal of the block copolymer by calcination leads to ordered silica with spherical cavities connected through narrow necks of molecular dimensions, the size of which is determined by the diameter of single PEO chains. Of course, the size of the necks is likely to be smaller than the size of the single PEO chains because the template removal leads to shrinkage of the silica framework. Figure 9 illustrates the schematic porous structure of the large-pore cagelike silica with necks of molecular dimensions that was fabricated by the templating of the spherical copolymer micelles bridged by the single PEO chains. When the copolymer/silica composite is subjected to the hydrothermal treatment at higher temperatures, these interactions become less favorable, which is expected to lead to a higher degree of aggregation of the PEO blocks in the silica wall. This aggregation, that is, the formation of bundles of the PEO chains, is likely to lead to the increase in the diameter of bridges of the PEO chains between the adjacent micelles in the ordered silica framework.9 This may lead to the increase in the neck size with an increase in the hydrothermal temperature because the space occupied by these bridges becomes the pore space that connects the adjacent cavities after the removal of the copolymer. IV.2. Percolation-Controlled Molecular Sieving. The ordered silicas with cagelike pores exhibited the molecular sieving effect depending on the conditions of hydrothermal treatment. Because the ordered silicas are not crystalline in their atomic arrangement, however, this does not necessarily indicate that the necks in the material are uniform in size. In the ordered silicas with cagelike pores, almost spherical cavities are arranged in a 3D lattice and connected through narrow necks. All the cavities are of almost the same size, whereas the necks possess a finite size distribution inherent to the ordered silicas with cagelike pores.10,11,31 Molecules with kinetic diameter smaller than all the necks penetrate the pore network completely. When the kinetic diameter of a probe molecule is larger than some of the necks, movement of the molecule from one cavity to an adjacent one through the smaller necks is prohibited. Some of the necks that are inaccessible to the molecule are nevertheless large enough to accommodate the molecules but simply isolated from the surface by necks that are too small to allow the molecule to pass. The cavities, in contrast, are all large enough to accommodate the molecule, but some will be isolated by necks that are smaller than the molecules.31 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. The cavities and necks in the porous medium are regarded as the sites and bonds, respectively, of the percolation theory. We assume that pore volume is associated solely with sites. The conditions applied in such a medium are analogous to those of bond-controlled percolation in a 3D


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Figure 4. Adsorption-desorption isotherms of probe molecules on the sample prepared without the hydrothermal treatment. (a) Circles, triangles, and squares denote the data points for NH3, CO2, and N2, respectively. (b) Circles, triangles, and squares denote the data points for n-C4H10, iso-C4H10, and c-C6H12, respectively. Open and closed symbols denote adsorption and desorption points, respectively.

Figure 5. Adsorption-desorption isotherms of probe molecules on the sample prepared with the hydrothermal treatment at 288 K for 24 h. (a) Circles, triangles, and squares denote the data points for NH3, CO2, and N2, respectively. (b) Circles, triangles, and squares denote the data points for n-C4H10, iso-C4H10, and c-C6H12, respectively. Open and closed symbols denote adsorption and desorption points, respectively.

lattice.31-33 For a give pore network, defined by a size distribution of necks and a coordination number of cavities, the percolation theory provides the relationship between two variables: the fraction of necks in a network that are large enough to accommodate the probe molecules, namely the bond occupation probability, and the fraction of cavities that are actually accessible to this adsorbate, namely the accessibility. The theory predicts that with an increase in the kinetic diameter of a probe molecule the accessibility of the cavities to the gas drops rapidly over a narrow range of the kinetic diameter for a random distribution of neck sizes in the pore network, where the necks in a narrow range of pore size distribution are allowed to be blocked for the access of the molecules to the pore network due to the molecular size exclusion. This range of fractions of necks closed is about 0.83-0.88 for a face-centered-cubic lattice of voids appropriate to FDU-12 silica.31-33 Before that, the

accessibility of the cavities would not drop appreciably over a large range of fractions of necks closed. Even if the size distribution of necks should be large, a small increase in the kinetic diameter of a probe molecule may lead to a rapid loss in adsorption capacity for the ordered silicas with necks of molecular dimensions. This imposes a window on the size distribution of necks, where with an increase in the kinetic diameter of a probe molecule the accessibility of the cavities drops rapidly over a narrow range of kinetic diameter. The window is bounded on the left by the accessibility of ∼0.8 and on the right by the percolation threshold of the pore network. The accessibility of the cavities to the gas steeply changes over a small range of the kinetic diameter of a probe molecule. When the kinetic diameter of a probe molecule is decreased just below the neck size corresponding to the percolation threshold, the number of cavities accessible to a probe molecule that penetrate

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from the external surface of the pore network is suddenly increased. The window of molecular sieving is extended as the network size is decreased.32 We are able to estimate such a critical range of kinetic diameter when a neck size distribution for a material is assumed. Figure 10 shows the results of simulations demonstrating the window in the neck size distribution where the volume of a liquid condensed in the pore network drops rapidly with an increase in the kinetic diameter of a probe molecule. The size distributions of necks are assumed to be log-normal:33,34

φ(d) ) exp{-[ln(d/dj)]2 /(2σ2)}/[dσ√2π]

(1)

where dj and σ are the neck median and dispersion, respectively. Neck diameter distributions were calculated with dj ) 0.20, 0.28,

0.30, and 0.35 nm for a fixed σ of 0.5. For clarity, the distribution with dj ) 0.30 nm is omitted in the figure. The critical ranges of kinetic diameter of a probe molecule for the neck size distributions with dj ) 0.20, 0.28, 0.30, and 0.35 nm are 0.31-0.35, 0.44-0.50, 0.48-0.53, and 0.55-0.62 nm, respectively. Therefore, the cagelike solid with the neck size distribution of dj ) 0.20 nm is capable of adsorbing both NH3 and CO2 molecules with kinetic diameter between 0.29 and 0.33 nm but not N2 molecules of kinetic diameter 0.37 nm, as is the case for the FDU-12 sample prepared without hydrothermal treatment. Similarly, the cagelike solid with the neck size distribution of dj ) 0.28 nm is not capable of adsorbing at all iso-C4H10 molecules of kinetic diameter 0.53 nm, although n-C4H10 molecules of kinetic diameter 0.47 nm may be adsorbed in the solid to some extent, as is the case for the FDU-12 sample


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Figure 9. Schematic illustration of large-pore cagelike silica with necks of molecular dimensions.

because incomplete percolation path along which the molecules penetrate in the pore network may be formed for the probe molecule with kinetic diameter close to the neck size corresponding to the percolation threshold. The sample hydrothermally treated at 303 K seems to indicate such a molecular sieving effect. c-C6H12 molecules may be moderately adsorbed in the cagelike solid with the neck size distribution of dj ) 0.35 nm because the kinetic diameter of the molecule is just below the neck size corresponding to the percolation threshold, as is the case for the sample hydrothermally treated at 313 K. The molecular sieving properties of the materials prepared without and with the hydrothermal treatment at 288, 303, and 313 K seem to be well accounted for by the neck size distributions of dj ) 0.20, 0.28, 0.30, and 0.35 nm, respectively. If the neck size distributions in the materials are spatially correlated, the volume of a liquid condensed in the large cavities would start to decrease even at the small fractions of necks closed and would show a gradual decrease with an increase in the kinetic diameter of the probe molecule. The probe molecule always showed a gradual decrease in the adsorption capacity with an increase in the kinetic diameter even in the size region smaller than the lower limit of the window for molecular sieving in the neck size distribution. This seems to indicate the presence of the correlated distribution of neck size in space. V. Conclusions

Figure 10. Neck size distributions with dj ) 0.20, 0.28, and 0.35 nm for a fixed σ of 0.5 and the corresponding windows for molecular sieving. Solid, broken, and dotted lines denote the size distributions with dj ) 0.20, 0.28, and 0.35 nm, respectively.

hydrothermally treated at 288 K. The cagelike solid with the neck size distribution of dj ) 0.30 nm is not capable of adsorbing at all c-C6H12 molecules of kinetic diameter 0.61 nm. However, iso-C4H10 molecules may be adsorbed in the solid to some extent

We have shown that the large-pore cagelike silicas with necks of molecular dimensions can be prepared without and with the hydrothermal treatment at low temperatures of ordered large cage-type mesoporous silica FDU-12. The molecular sieving properties of these materials seem to be well accounted for by the bond-controlled percolation of a gas in the interconnected cavities. The neck size available for the molecular sieving can be effectively tailored by properly adjusting the conditions of the hydrothermal treatment. This feature makes the large-pore cagelike silica with necks of molecular dimensions promising candidates for a new type of molecular sieves.

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Acknowledgment. This work was supported by matching fund subsidy for private universities from MEXT (Ministry of Education, Culture, Sports, Science and Technology). References and Notes (1) Li, J.-R.; Kuppler, R. J.; Zhou, H.-C. Chem. Soc. ReV. 2009, 38, 1477. (2) Kuznicki, S.; Bell, V. A.; Nair, S.; Hillhouse, H. W.; Jacubinas, R. M.; Braunbarth, C. M.; Toby, B. H.; Tsapatsis, M. Nature 2001, 412, 720. (3) Ma, S.; Sun, D.; Yuan, D.; Wang, Y.-S.; Zhou, H. -C. J. Am. Chem. Soc. 2009, 131, 6445. (4) Zhao, D.; Huo, Q.; Feng, J.; Chmelka, B. F.; Stucky, G. D. J. Am. Chem. Soc. 1998, 120, 6024. (5) 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. (6) Kruk, M.; Antochshuk, V.; Matos, J. R.; Mercuri, L. P.; Jaroniec, M. J. Am. Chem. Soc. 2002, 124, 768. (7) Garcia-Bennett, A. E.; Williamson, S.; Wright, P. A.; Shannon, I. J. J. Mater. Chem. 2002, 12, 3533. (8) 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. (9) Kim, T. -W.; Ryoo, R.; Kruk, M.; Gierszal, K. P.; Jaroniec, M.; Kamiya, S.; Terasaki, O. J. Phys. Chem. B 2004, 108, 11480. (10) Morishige, K.; Kanzaki, Y. J. Phys. Chem. C 2009, 113, 14927. (11) Morishige, H.; Yoshida, K. J. Phys. Chem. C 2010, 114, 7095. (12) Yu, T.; Zhang, H.; Yan, X.; Chen, Z.; Zou, X.; Oleynikov, P.; Zhao, D. J. Phys. Chem. B 2006, 110, 21467. (13) Zapilkp, C.; Anwander, R. Chem, Mater. 2006, 18, 1479. (14) Traa, Y.; Sealy, S.; Weitkamp, J. Mol. SieVes 2007, 5, 103. (15) Morishige, K.; Ito, M. J. Chem. Phys. 2002, 117, 8036.

Morishige and Yasuki (16) Broekhoff, J. C. P.; de Boer, J. H. J. Catal. 1968, 10, 153. (17) Brunauer, S.; Emmett, P. H.; Teller, E. J. Am. Chem. Soc. 1938, 60, 309. (18) Lippens, B. C.; de Boer, J. H. J. Catal. 1965, 4, 319. (19) Ravikovitch, P. I.; Neimark, A. V. Langmuir 2002, 18, 9830. (20) Kruk, M.; Jaroniec, M. Chem. Mater. 2003, 15, 2942. (21) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity; Academic Press: London, 1982; p 113. (22) Morishige, K.; Tateishi, N. J. Chem. Phys. 2003, 119, 2301. (23) Smarsly, B.; Polarz, S.; Antonietti, M. J. Phys. Chem. B 2001, 105, 10473. (24) Tattershall, C. E.; Jerome, N. P.; Budd, P. M. J. Mater. Chem. 2001, 11, 2979. (25) Tattershall, C. E.; Aslam, S. J.; Budd, P. M. J. Mater. Chem. 2002, 12, 2286. (26) Paul, S. M. D.; Zwanziger, J. W.; Ulrich, R.; Wiesner, U.; Spiess, H. W. J. Am. Chem. Soc. 1999, 121, 5727. (27) Kramer, E.; Forster, S.; Goltner, C.; Antonietti, M. Langmuir 1998, 14, 2027. (28) Ryoo, R.; Ko, C. H.; Kruk, M.; Antochshuk, V.; Jaroniec, M. J. Phys. Chem. B 2000, 104, 11465. (29) Galarneau, A.; Cambon, H.; Renzo, F. D.; Fajula, F. Langmuir 2001, 17, 8328. (30) Voort, P. V. D.; Benjelloun, M.; Vansant, E. F. J. Phys. Chem. B 2002, 106, 9027. (31) Perez-Mendoza, M.; Gonzalez, J.; Wright, P. A.; Seaton, N. A. Langmuir 2004, 20, 9856. (32) Wall, G. C.; Brown, R. J. C. J. Colloid Interface Sci. 1981, 82, 141. (33) Zhdanov, V. P. AdV. Catal. 1993, 39, 1. (34) Foley, H. C. Microporous Mater. 1995, 4, 407.

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Large-Pore Cagelike Silica with Necks of Molecular Dimensions - The

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