Langmuir 2001, 17, 47-51
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Evaluation of the Hydrophobic Properties of Mesoporous FSM-16 by Means of Adsorption Calorimetry Akihiko Matsumoto,* Tatsuo Sasaki, Nobuyuki Nishimiya, and Kazuo Tsutsumi Department of Materials Science, Faculty of Engineering, Toyohashi University of Technology, Tempaku-cho, Toyohashi 441-8580, Japan Received November 9, 1999. In Final Form: October 10, 2000 The hydrophobicity of the mesoporous FSM-16 surface was studied by measurements of adsorption isotherms and heats of adsorption of water at 298 K. The hydrophobic-hydrophilic change on the surface derived by water adsorption was also studied. The water adsorption isotherm of freshly prepared FSM-16 was of type V and the initial heat of adsorption of water was ca. 20 kJ/mol, which is indicative of weak adsorbent-adsorbate interactions of a hydrophobic character. However, once water was adsorbed the water adsorptivity increased at a lower relative pressure region and the initial heat of adsorption increased to 75 kJ/mol. The surface became hydrophilic to a small extent, and the original hydrophobic behavior was never restored even after evacuation at high temperatures, 573 and 823 K. An in situ IR analysis revealed that silanol groups were formed and some of them remained after reevacuation at high temperatures once water had been adsorbed on the freshly calcined FSM-16.
Introduction Mesoporous materials with highly ordered pores, such as FSM-161-3 and MCM-41,4,5 have attracted interest in the fields of adsorption science and catalysis chemistry. FSM-16 is prepared by intercalation of surfactant micelles as a structural directing agent (SDA) in a layered sodium silicate, kanemite, followed by removal of the SDA.1-3 Because of its regular mesopore system, which consists of an array of unidimensional and hexagonally shaped mesopores like those of MCM-41, as well as because of its thermal stability, FSM-16 is expected to be useful for applications such as in catalytic reactions and as an adsorbent of large molecules. In molecular adsorption on a solid surface, the chemical nature of the solid surface plays an important role. In particular, hydrophilic-hydrophobic properties are important in controlling the adsorptivity for polar/nonpolar molecules. It was reported that the surfaces of MCM-41 and FSM-16 showed a hydrophobic nature with a low concentration of surface silanol groups.6-8 However, Inagaki et al. measured the water adsorption isotherms repeatedly for the same FSM-16 sample and found that the repetition of water adsorption brought about the hydrophilic property.8,9 * To whom correspondence should be addressed. Tel: +81-53244-6811. Fax: +81-532-48-5833. E-mail:
[email protected]. (1) Yanagisawa, T.; Shimizu, T.; Kuroda, K.; Kato, C. Bull. Chem. Soc. Jpn. 1990, 63, 988. (2) Inagaki, S.; Fukushima, Y.; Kuroda, K. J. Chem. Soc., Chem. Commun. 1993, 680. (3) Inagaki, S.; Koiwai, A.; Suzuki, N.; Fukushima, Y.; Kuroda, K. Bull. Chem. Soc. Jpn. 1996, 69, 1449. (4) Kregse, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (5) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kragse, C. T.; Schmitt, K. D.; Chu, C. T.-W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (6) Llwellyn, P. L.; Schu¨th, F.; Grillet, Y.; Rouquerol, F.; Rouquerol, J.; Unger, K. K. Langmuir 1995, 11, 574. (7) Branton, P. J.; Hall, P. G.; Treguer, M.; Sing, K. S. W. J. Chem. Soc., Faraday Trans. 1995, 91, 2041. (8) Inagaki, S.; Fukushima, Y. Microporous Mesoporous Mater. 1998, 21, 667. (9) Inagaki, S.; Fukushima, Y.; Kuroda, K.; Kuroda, K. J. Colloid Interface Sci. 1996, 180, 623.
In this study, the hydrophilic-hydrophobic properties of FSM-16 were characterized and the change in the chemical aspects of its surface by repetition of water adsorption was investigated with the aid of adsorption calorimetry, adsorption isotherms, and spectroscopic measurements. The sample was sometimes heated to a higher temperature than that sufficient to remove physisorbed water before the repetition of water adsorption in order to clarify the intrinsic change of the surface structure of FSM-16. Experimental Section Sample Preparation and Characterization. A mesoporous material, FSM-16, with retention of a SDA was kindly supplied by Toyota Central R&D Laboratory, which prepared the sample by the method shown in ref 8 by using hexadecyltrimethylammonium bromide as the SDA. The FSM-16 was heated at 723 K to burn off the SDA and immediately used for characterization by X-ray diffraction and nitrogen adsorption at 77 K. The X-ray diffraction was recorded by use of an automatic powder diffractometer (Rigaku, RINT2000) using Cu KR radiation. The adsorption isotherm of nitrogen on the sample was measured volumetrically at 77 K. The sample was preevacuated (1 mPa) for 12 h at 823 K prior to the nitrogen adsorption experiment. Water Adsorption. Heats of adsorption of water were measured at 298 K with a twin conduction type microcalorimeter (Tokyo Riko) equipped with a volumetric adsorption apparatus. The adsorption isotherm of water was consecutively measured with calorimetric measurements. Heats of immersion into water at 298 K were also measured by use of a twin conduction type microcalorimeter (Tokyo Riko). The solid-state 29Si magic angle spinning (MAS) NMR spectra were measured on a 400 MHz spectrometer (Varian, VNMR400P) with a MAS probe. IR spectral changes with water adsorption at 298 K were measured with a Fourier transform infrared spectrometer (JASCO, FT/IR-420) equipped with an in situ diffuse reflectance cell (DR-800/HS). The change in the porosity of FSM-16 before and after water adsorption was also checked by analysis of a nitrogen adsorption isotherm at 77 K. The samples were preevacuated (1 mPa) for 12 h at an appropriate temperature (298, 573, and 823 K) before each experiment.
Results and Discussion Porosity of FSM-16. The nitrogen adsorption isotherm of freshly calcined FSM-16 was of type IV in the BDDT
10.1021/la991468m CCC: $20.00 © 2001 American Chemical Society Published on Web 12/06/2000
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Figure 1. Adsorption isotherms of nitrogen on freshly calcined FSM-16 (O) and on the sample after water adsorption and reevacuation (b).
Figure 2. Adsorption isotherms of water at 298 K on freshly calcined FSM-16 preevacuated at 298 K (b) and at 823 K (0).
classification, as shown in Figure 1. The adsorbed amount increased stepwise at a relative pressure (P/P0) between 0.24 and 0.40, indicating capillary condensation in the mesopores. The specific surface area (as) and pore volume (vp) were 950 m2/g and 0.60 mL/g, obtained by applying the Rs plot on the nitrogen isotherm.10 The nitrogen adsorption data on nonporous crushed quartz were used to construct the Rs plot.11 The hydraulic pore diameter was estimated from as and vp as 2.5 nm.12 The X-ray diffraction pattern of freshly calcined FSM-16 exhibited a typical feature in mesoporous silica with hexagonal pore arrays.1-6 A sharp Bragg peak ascribed to the (100) reflection of the hexagonal structure of the mesopores was observed at 2θ ) 2.45°, corresponding to d ) 3.6 nm. Besides the strong peak, weak ones ascribed to the (110), (200), and (210) reflections were observed at 2θ ) 4.14, 4.75, and 6.26°, corresponding to d ) 2.1, 1.9, and 1.4 nm, respectively. These clear peaks indicated the formation of the long-range-order structure. Hydrophobic Properties of FSM-16. Figure 2 shows adsorption isotherms of water at 298 K on freshly calcined FSM-16 after preevacuation at 298 and 823 K. The water adsorption isotherms were of type V in the BDDT classification regardless of the preevacuation temperature, (10) Gregg, S. J.; Sing, K. S. W. Adsorption Surface Area and Porosity, 2nd ed.; Academic Press: London, 1982; Chapter 4. (11) Naono, M.; Hakuman, M. Hyomen 1991, 29, 362. (12) Rouquerol, F.; Rouquerol, J.; Sing, K. Adsorption by Powders and Porous Solids; Academic Press: London, 1999; Chapter 7.
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which is indicative of weak adsorbent-adsorbate interactions.13 The adsorption amount was low at low P/P0, increased steeply from 4 to 28 mmol/g at a P/P0 value between 0.5 and 0.6, and then attained a value of 33 mmol/ g. This steep step is due to condensation of water caused by the polar interaction among adjacent water molecules rather than the interaction between the surface and water. In the case of nonporous silica and nonmicroporous silica, heating at 373 K for a prolonged period removes all physisorbed water.14,15 However, in the present case the water isotherm of FSM-16 preevacuated at 298 K agreed well with that for FSM-16 preevacuated at 823 K, which suggests that few water molecules were adsorbed on the surface of the freshly calcined FSM-16 surface even before preevacuation. These results show the intrinsic hydrophobic property of the FSM-16 surface. Because FSM-16 consists of a silica network, the intrinsic hydrophobic nature should be due to siloxane groups on the surface. Change in Hydrophobic/Hydrophilic Properties by Water Adsorption. Inagaki and Fukushima reported that the hydrophobic surface of FSM-16 became hydrophilic by repetition of water adsorption at 298 K.8 In their report, the authors concluded that the hydrophilic feature was attributable to irreversible adsorption of water on the surface. If the hydrophobic/hydrophilic change originated only from irreversibly adsorbed water, the surface would show the hydrophobic feature after preevacuation at a high temperature where physically adsorbed water molecules can be removed. The adsorption isotherms of water with and without the repetition of water adsorption were measured as follows to confirm the origin of the hydrophobic/hydrophilic change of the surface. First, freshly calcined FSM-16 was preevacuated at 298 K for 10 h and the adsorption isotherm of water was measured until ca. 0.8P0. After the first adsorption run was finished, the sample was evacuated at 1 mPa and 573 K for 10 h and the adsorption isotherm was measured for the second time. Then, the sample was evacuated at 1 mPa and 823 K for 10 h and the adsorption isotherm was measured for the third time. All adsorption measurements were carried out at 298 K. The change in water adsorption isotherms at 298 K by repetition of water adsorption is shown in Figure 3. The freshly calcined FSM-16 showed hydrophobic properties. The P/P0 value giving a condensation step was identical to that observed in the isotherm for the sample preevacuated at 298 K in Figure 2. The reproducibility was good, but a slight discrepancy of the adsorption amount was observed at P/P0 > 0.6. When the water adsorption was repeated after reevacuation of adsorbed water, the adsorbed amount became 2-3 times that of the first run in the lower P/P0 region (P/P0 < 0.3) despite preevacuation at a high temperature, 573 or 823 K, which is enough to remove physisorbed water on nonmicroporous silica.14,15 Furthermore, the condensation step shifted to a lower P/P0 value (P/P0 ) 0.3). The solid state 29Si MAS NMR spectrum of freshly calcined FSM-16, shown in Figure 4a, exhibited two broad signals at -111 and -102 ppm for the Q4 and Q3 environments, respectively.16,17 The signal at -102 ppm is attributed to silicon atoms with single silanol groups, being either free or vicinal silanol groups. The signal at (13) Rouquerol, F.; Rouquerol, J.; Sing, K. Adsorption by Powders and Porous Solids; Academic Press: London, 1999; Chapter 1. (14) Vansant, E. F.; van der Voort, P.; Vrancken, K. C. Stud. Surf. Sci. Catal. 1995, 93, 59. (15) Iler, R. K. Chemistry of Silica; Wiley: New York, 1979; Chapter 6.
Hydrophobic Properties of Mesoporous FSM-16
Figure 3. Change in water adsorption isotherms at 298 K by repetition of water adsorption: (b) the first run, (0) the second run, and (4) the third run.
Figure 4. 29Si MAS NMR spectra of FSM-16 before (a) and after (b) water adsorption. The dashed lines indicate deconvoluted signals for the Q3 and Q4 environments using a Gaussian line shape.
-102 ppm increased by water adsorption at 298 K as shown in Figure 4b; the ratio of the integral signal intensity of the Q3 environment to the total signal intensity increased from 0.2 to 0.28 by water adsorption. It should be noted that the signal at -102 ppm is due to both surface and internal silanol groups for the Q3 environment; however, the change in the intensity of the Q3 signal indicates that the concentration of surface silanol groups was increased by water adsorption. Therefore, the enhancement of water adsorptivity at P/P0 < 0.3 in the second and third adsorption runs is due to an increase in the number of surface silanol groups. The condensation step in the adsorption isotherm in Figure 3 became gentle, and the adsorbed amount at P/P0 ) ∼0.8 was reduced by repetition of water adsorption. As shown in Figure 1, the nitrogen adsorption isotherm (16) For example: Maciel, G. E.; Ellis, P. D. In NMR Techniques in Catalysis; Bell, A. T., Pines, A., Eds.; Dekker: New York, 1994; Chapter 5. van Blaaderen, A.; Vrij, A. In The Colloid Chemistry of Silica; Bergna, H. E., Ed.; Advances in Chemistry Series 234; American Chemical Society: Washington, DC, 1994; Chapter 4. (17) Zhao, X. S.; Audsley, F.; Lu, G. Q. J. Phys. Chem. B 1998, 102, 4143.
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Figure 5. The differential heats of adsorption of water on FSM-16 at 298 K with and without the preadsorption of water: (b) the first run, (0) the second run, and (4) the third run.
of FSM-16 after water adsorption followed by reevacuation at 573 K exhibited a reduced saturation uptake and a slight shift of the condensation step to a lower P/P0. The as and vp of FSM-16 decreased from 950 to 905 m2/g and from 0.60 to 0.48 mL/g, respectively, by water adsorption and reevacuation at 573 K. It is reported that silanol formation by the hydrolysis of siloxane with adsorbed water and capillary forces by adsorbed water in mesopores would bring about the collapse of the pore structure of mesoporous silica MCM-41.17,18 Similarly, in the present case water adsorption on FSM-16 would bring about the hydrolysis of surface siloxane groups and the partial collapse of the pore structure, resulting in the decrease in the water uptake at P/P0 ) ∼0.8. The hydrophobic-hydrophilic change by water adsorption influenced the differential heats of adsorption of water on FSM-16 at 298 K, as shown in Figure 5. Heats of adsorption at the initial stage in the first adsorption run showed quite a different tendency from those of the second and third runs. The initial heat of adsorption in the first adsorption run was ca. 20 kJ/mol, and then the released heat increased gradually as the adsorption proceeded, attaining a constant value of ca. 45 kJ/mol. The adsorption amounts when heats of adsorption reached the constant value were found to be equal to those at the beginning of the condensation step in the isotherms shown in Figure 3. The low adsorption heat at the initial stage of adsorption indicates the hydrophobic character of the surface of FSM16, that is, less interaction between siloxane and water molecules. Afterward, the adsorbed water in the mesopore behaved as free water, judging from the fact that adsorption heats were similar to the heat of liquefaction of water vapor, 44 kJ/mol. On the other hand, the differential heats of adsorption at the initial stage of adsorption were ca. 70 kJ/mol in the second and third runs and then decreased gradually to 45 kJ/mol with an increase in the water uptake. The high heats at the initial stage of adsorption would be due to the strong interaction between water and the hydrophilic surface. The changes in the heat curves as well as the water adsorption isotherms suggest that the surface becomes hydrophilic by water adsorption. Once water was adsorbed, the hydrophilic property was retained even after evacuation at 823 K. The heat of immersion into water at 298 K of freshly calcined FSM-16 was 130 mJ/m2, which is comparable to the heat of immersion of high-silica ZSM-5 and faujasite, 110 mJ/m2,19 whereas in the case of FSM-16 after water (18) Ribeiro Carrot, M. M. L.; Esteˆva˜o Candeias, A. J.; Carrot, P. J. M.; Unger, K. K. Langmuir 1999, 15, 8895.
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Figure 6. IR spectral change of FSM-16 before and after water adsorption and reevacuation at various temperatures.
Figure 7. Schematic model of the changes in the chemical structure of the FSM-16 surface before and after water adsorption. The ellipses represent water molecules. The corresponding isotherm at each stage is also shown.
adsorption followed by reevacuation at 573 or 823 K, the heat of immersion increased to 170 mJ/m2. These results also suggest the formation of hydrophilic adsorption sites. IR spectral changes of FSM-16 with water adsorption followed by reevacuation at various temperatures, shown in Figure 6, elucidate silanol formation by water adsorption and the stability of silanol to heating. In the spectrum of freshly calcined FSM-16, a sharp absorption band attributable to the stretching vibration of the O-H bond of free (isolated) silanol groups was observed.14,15 The presence of free silanol groups was also shown by the 29Si NMR result. This band was observed even after evacuation at 823 K because of the thermal stability of the free silanol groups. Upon water adsorption, the broad bands around 3450-3650 and 3200-3450 cm-1 appeared, which are due to the O-H vibrations of isolated pairs of adjacent silanol groups (vicinal form) and adsorbed water, respectively.14,15 Taking the 29Si NMR spectral change by water adsorption into consideration as well, the formation of vicinal silanol results from hydrolysis of the siloxane bridge by adsorbed water. Apparent spectral changes were not observed between the spectrum after water adsorption and that after evacuation at 298 K, because neither removal of adsorbed water nor condensation of silanol groups takes place at 298 K.15 The bands due to vicinal silanol became clear after the removal of physically adsorbed water by reevacuation at 423 K. The intensities of the bands decreased when the reevacuation temperature was increased to >573 K. This result suggests that the reformation of siloxane bonds by dehydroxylation of the vicinal
silanol groups takes place by heating above 573 K. However, the bands due to vicinal silanol groups still remained after reevacuation at 823 K and the spectrum did not coincide with that of freshly calcined FSM-16 before water adsorption. It is reported that silanol groups on the FSM-16 surface are retained after evacuation at 673 K.20 Similarly, in the present study, once the silanol groups had formed by hydrolysis with adsorbed water on the surface, some of them would remain after reevacuation at 823 K. Therefore, the surface of FSM-16 at the beginning of the second and third adsorption runs would be more hydrophilic than that of the freshly calcined sample. The repetition of water adsorption would increase the concentration of silanol groups at the initial stage of each adsorption run; however, it would decrease the intrinsic mesoporous regularity of FSM-16. The changes in the chemical structure of the FSM-16 surface before and after water adsorption are schematically shown in Figure 7. Most of the surface of freshly calcined FSM-16 consisted of siloxane and isolated silanols, which showed a hydrophobic nature (Figure 7a). However, once water molecules were condensed in pores the siloxane bonds would be hydrolyzed to give silanol groups (Figure 7b). Some of the silanol groups remained even after preevacuation up to 823 K (Figure 7c) so that the surface became hydrophilic. Consequently, the water adsorptivity was enhanced in the lower P/P0 region in the second and
(19) Tsutsumi, K.; Kawai, K.; Yanagihara, T. Stud. Surf. Sci. Catal. 1994, 83, 217.
(20) Ishikawa, T.; Matsuda, M.; Yasukawa, A.; Kandori, K.; Inagaki, S.; Fukushima, T.; Kondo, S. J. Chem. Soc., Faraday Trans. 1996, 92, 1985.
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third runs (Figure 7d). In these runs, the surface is more hydrophilic than in the first adsorption run; hence, multilayer water adsorption occurred easily on the surface at 0 < P/P0 < 0.3. Because the multilayer adsorption would narrow the effective radius (the pore radius minus the thickness of ordinary multilayer adsorption expected at that P/P0), the steep rise of the adsorption amount was observed at a lower P/P0 value. Conclusion Adsorption calorimetry and adsorption measurements revealed that freshly prepared FSM-16 exhibited a hydrophobic surface character. Once water was adsorbed, the surface became hydrophilic to a small extent and the original hydrophobic behavior was never restored even
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after evacuation at high temperatures, 573 and 823 K. Solid state 29Si NMR and in situ IR measurements showed that the hydrophilic feature is due to silanol groups formed by hydrolysis of siloxane bridges with adsorbed water and that this feature remained even after evacuation at 823 K. Acknowledgment. The authors are indebted to Drs. S. Inagaki and Y. Fukushima, Toyota Central R&D Laboratory, Inc., for their kindness in supplying the FSM16 sample. The financial support by a Grant-in-Aid for Science Research from the Ministry of Education, Science, Sports and Culture, Japanese Government, is greatly appreciated. LA991468M
