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Adsorption Isotherm of Water Vapor and Its Large Hysteresis on Highly Ordered Mesoporous Silica
JOURNAL OF COLLOID AND INTERFACE SCIENCE ARTICLE NO.
180, 623–624 (1996)
0345
NOTE Adsorption Isotherm of Water Vapor and Its Large Hysteresis on Highly Ordered Mesoporous Silica The first and second adsorption–desorption isotherms of water vapor on a new mesoporous material derived from kanemite have been measured. The isotherms show unusual type V isotherms and large hysteresis. The type V isotherms, which have never been observed for the other adsorbates, suggest that the mesoporous material has a hydrophobic surface, although the hydrophobicity decreased after treatment with water vapor because of rehydration of the surface. The significantly large hysteresis could be explained by the difference in contact angle between adsorption and desorption. q 1996 Academic Press, Inc. Key Words: water vapor; mesoporous material; hydrophobicity; hysteresis. FIG. 1. X-ray powder diffraction pattern of FSM-16. The spectrum was recorded on a Rigaku RAD-B diffractometer with Cu Ka radiation.
INTRODUCTION The synthesis of highly ordered mesoporous materials (1–6) with uniform pore size has attracted much attention in recent years. MCM-41 (2, 3) and FSM-16 (4) mesoporous materials have been synthesized using surfactants from aluminosilicate gel or a layered silicate, kanemite, respectively. Since the materials are good models for mesoporous adsorbents because of their narrow pore size distributions, some investigations have focused on adsorption–desorption behavior of gases and vapors on those mesoporous materials. Franke et al. (7) have reported adsorption isotherms of cyclopentane vapor and nitrogen on MCM-41. Branton et al. have reported adsorption isotherms of nitrogen, oxygen, and argon on MCM-41 (8) and also have reported those of nitrogen on FSM-16 (9). The isotherms of nitrogen and cyclopentane show no hysteresis, while those of oxygen and argon show little hysteresis. However, Llewellyn et al. (10) and Schmidt et al. (11) reported large hysteresis for nitrogen and argon on MCM-41 with larger pore diameter over 4 nm. Llewellyn et al. proposed that hysteresis should accompany capillary condensation, the absence of hysteresis on the smaller pore MCM-41 materials might be a process resembling secondary micropore filling (10). They concluded that pore size has an important effect on the presence or absence of hysteresis. Water vapor has shown unique adsorption properties on various adsorbents. However, there is no precise adsorption–desorption isotherm of water on the mesoporous materials and no satisfactory interpretation of the hysteresis phenomena. It has been pointed out that both MCM-41 and FSM-16 have hydrophobic surfaces (12). Although the perfect adsorption–desorption isotherm of water vapor is important in discussing hydrophobicity, we have no reports yet. Here, we report the large hysteresis in adsorption isotherms of water on FSM-16 and the hydrophobicity of FSM-16, which was changed by treatment with water vapor.
in 1000 ml of a 0.1 mol dm03 hexadecyltrimethylammonium [C16H33N / (CH3 )3 ] chloride solution and heated at 707C for 3 h under stirring without pH adjustment. After the pH of the suspension was adjusted to 8.5 by the addition of a 2 mol dm03 HCl aqueous solution, the suspension was allowed to settle at 707C for another 3 h. The solid product was filtered and washed with water repeatedly. The dried sample was calcined at 5507C for 6 h to remove the organic fraction to yield FSM-16. X-ray powder diffraction of FSM-16 (Fig. 1), whose XRD peaks appear at d Å 3.79, 2.17, 1.84, and 1.41 nm, confirmed a hexagonal structure with lattice parameter 4.38 nm. The adsorption isotherms of nitrogen, benzene, and water were measured by means of a volumetric method using commercial automatic apparatus, BELSORP 28 for nitrogen and BELSORP 18 for benzene and water, respectively. The pressure gauge of the BELSORP 18 was heated to 1007C during the measurement to prevent adsorption on the inner wall of the apparatus. The samples were evacuated at 3007C for 3 h under a pressure õ10 04 Torr before measurement of the first isotherm. The second isotherm was also measured for water vapor. The measurements of the first adsorption–desorption isotherms required 1 day for nitrogen and benzene and 3 days for water.
MATERIALS AND METHODS Calcination of dried water glass (SiO2 /Na2O Å 2.00) at 7007C for 6 h gave d-Na2Si 2O5 crystals. Fifty grams of d-Na2Si 2O5 was dispersed in 500 ml water and stirred for 3 h. A filtered sample, wet kanemite, was dispersed
FIG. 2. Adsorption isotherms of nitrogen (77 K) and benzene (298 K) on FSM-16. Filled symbols denote desorption.
623
AID
JCIS 4174
/
6g10$$$301
05-24-96 06:14:41
0021-9797/96 $18.00 Copyright q 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
coidal
AP: Colloid
624
NOTE hysteresis was observed, according to the Kelvin equation. The smaller hysteresis for the second isotherm could be explained by the smaller contact angle due to the rehydration. When adsorbate molecules were adsorbed on a wet surface in type IV isotherms, hysteresis observed in those isotherms would be attributable to capillary condensation phenomena, because the contact angles at adsorption and desorption were almost the same. The large hysteresis of the water isotherms indicates that both the pore size and the surface property of the adsorbent have important effects on hysteresis in the adsorption isotherm.
REFERENCES
FIG. 3. Adsorption isotherms of water vapor at 298 K. s, The first isotherms; L the second isotherms. Filled symbols denote desorption.
RESULTS AND DISCUSSION Figure 2 shows the adsorption isotherms of nitrogen and benzene on FSM-16. They have the usual type IV isotherms. Hysteresis in the nitrogen isotherm at P/P0 Å 0.47–0.95 is attributable to the mesopores (d ú 10 nm) formed in interparticle spaces. The isotherm of benzene has no hysteresis, like those of nitrogen and cyclopentane on MCM-41. Although the lattice parameter is larger for FSM-16 than for MCM-41 (a Å 4.0 nm), the P/P0 at which the amount of adsorbed benzene increases steeply is lower for FSM-16 (P/P0 Å 0.12–0.20) than for MCM-41 ( P/P0 Å 0.21– 0.34) (3). This suggests a thicker pore wall of channels in FSM-16 than in MCM-41. Pore diameter, specific surface area, and pore volume of this sample, derived from the adsorption isotherm of nitrogen, were 2.3–2.8 nm, 990 m2 g 01 , and 0.77 cm3 g 01 , respectively. The tentative pore diameter was determined from the P/P0 of the sudden increase portion (P/P0 Å 0.26–0.34) by using the Kelvin equation with zero contact angle collecting multilayer adsorption. The thickness of multilayer adsorption was determined by using adsorption data on nonporous silica reported by Sing and Turk (13). The specific surface area was calculated by the BET method using the isotherm data between P/P0 Å 0.05 and 0.2, over which the capillary condensation occurred. The pore volume was obtained from the volume adsorbed at P/P0 Å 0.4 by assuming that the pores were filled with condensed liquid nitrogen at the P/P0 . Figure 3 shows adsorption isotherms of water vapor on FSM-16. There are two characteristics in these isotherms. One point is that the adsorption isotherms are type V, which is characterized by a small amount of adsorption at lower P/P0 in the type IV isotherm. The isotherms of the other adsorbates reported previously on MCM-41 or FSM-16 were type IV or I. The other point is that the isotherms have hysteresis of significant size. The observed type V isotherms suggest a weak interaction between the solid surface, which means the hydrophobic surface of FSM-16, and the water molecules. The same types of isotherms have also been observed on activated carbons (14) and amorphous silicas (15). The hydrophobicity is correlated with steep increases in adsorption on the isotherms. The P/P0 at the sudden increase is smaller for the second adsorption isotherm than for the first, which suggests a decrease in the hydrophobicity after the first measurement. A larger amount of adsorption and unclosed hysteresis for the first isotherm suggests an irreversible adsorption of water and rehydration of the surface. The rehydration decreased the hydrophobicity of FSM-16 evacuated at 3007C. Such rehydration by treatment with water vapor has also been observed for a silica gel (15). The large hysteresis of the water isotherms could not be explained by capillary condensation phenomena alone. The difference in contact angle between adsorption and desorption could explain the large hysteresis qualitatively. Water molecules contacted unwetted surfaces in the first adsorption experiment, while they drew back from wetted surfaces in desorption. As the contact angle of the latter is smaller than that of the former, large
AID
JCIS 4174
/
6g10$$$301
05-24-96 06:14:41
1. Yanagisawa, T., Shimizu, T., Kuroda, K., and Kato, C., Bull. Chem. Soc. Jpn. 63, 988 (1990). 2. Kresge, C. T., Leonowicz, M. E., Roth, W. J., Vartuli, J. C., and Beck, J. S., Nature 359, 710 (1992). 3. Beck, J. S., Vartuli, J. C., Roth, W. J., Leonowicz, M. E., Kresge, C. T., Schmitt, K. D., Chu, C. T-W., Olson, D. H., Sheppard, E. W., McCullen, S. B., Higgins, J. B., and Schlenker, J. L., J. Am. Chem. Soc. 114, 10834 (1992). 4. Inagaki, S., Fukushima, Y., and Kuroda, K., J. Chem. Soc. Chem. Commun., 680 (1993). 5. Inagaki, S., Fukushima, Y., and Kuroda, K., in ‘‘Zeolite and Related Microporous Materials: State of the Art 1994’’ (J. Weitkamp, H. G. Karge, H. Pfeifer, and W. Holderich, Eds.), p. 125. Elsevier, Amsterdam, 1994. 6. Inagaki, S., Fukushima, Y., and Kuroda, K., in ‘‘Science and Technology in Catalysis 1994’’ (Y. Izumi, H. Arai H., and M. Iwamoto, Eds.), p. 143. Kodansha, Tokyo, 1994. 7. Franke, O., Schulz-Ekloff, G., Rathousky, J., Starek, J., and Zukal, A., J. Chem. Soc. Chem. Commun., 724 (1993). 8. Branton, P. J., Hall, P. G., and Sing, K. S. W., J. Chem. Soc. Chem. Commun., 1257 (1993); Branton, P. J., Hall, P. G., and Sing, K. S. W., J. Chem. Soc. Faraday Trans. 90, 2965 (1994). 9. Branton, P. J., Sing, K. S. W., Kaneko, K., Inagaki, S., and Fukushima, Y., Langmuir 12, 599 (1996). 10. Llewellyn, P. L., Grillet, Y., Schuth, F., Reichert, H., and Unger, K. K., Microporous Mater. 3, 345 (1994). 11. Schmidt, R., Stocker, M., Hansen, E., Akporiaye, D., and Ellestad, O. H., Microporous Mater. 3, 443 (1994). 12. Chen, C.-Y., Li, H.-X., and Davis, M. E., Microporous Mater. 2, 17 (1993). 13. Sing, K. S. W., and Turk, D. H., J. Colloid Interface Sci. 38, 109 (1972). 14. Gregg, S. J., and Sing, K. S. W., in ‘‘Adsorption, Surface Area and Porosity,’’ 2nd ed., Chap. 5. Academic Press, New York, 1982. 15. Naono, H., Hakuman, M., Joh, H., Sakurai, M., and Nakai, K., Porous Mater. 31, 203 (1993). SHINJI INAGAKI * ,1 YOSHIAKI FUKUSHIMA * KAZUO KURODA† KAZUYUKI KURODA† *Toyota Central Research & Development Laboratories, Inc. 41-1, Yokomichi, Nagakute-cho Aichi-gun 480-11, Japan †Department of Applied Chemistry Waseda University Ohkubo-3, Shinjuku-ku Tokyo 169, Japan Received July 7, 1995; accepted December 14, 1995
1
To whom correspondence should be addressed.
coidal
AP: Colloid
180, 623–624 (1996)
0345
NOTE Adsorption Isotherm of Water Vapor and Its Large Hysteresis on Highly Ordered Mesoporous Silica The first and second adsorption–desorption isotherms of water vapor on a new mesoporous material derived from kanemite have been measured. The isotherms show unusual type V isotherms and large hysteresis. The type V isotherms, which have never been observed for the other adsorbates, suggest that the mesoporous material has a hydrophobic surface, although the hydrophobicity decreased after treatment with water vapor because of rehydration of the surface. The significantly large hysteresis could be explained by the difference in contact angle between adsorption and desorption. q 1996 Academic Press, Inc. Key Words: water vapor; mesoporous material; hydrophobicity; hysteresis. FIG. 1. X-ray powder diffraction pattern of FSM-16. The spectrum was recorded on a Rigaku RAD-B diffractometer with Cu Ka radiation.
INTRODUCTION The synthesis of highly ordered mesoporous materials (1–6) with uniform pore size has attracted much attention in recent years. MCM-41 (2, 3) and FSM-16 (4) mesoporous materials have been synthesized using surfactants from aluminosilicate gel or a layered silicate, kanemite, respectively. Since the materials are good models for mesoporous adsorbents because of their narrow pore size distributions, some investigations have focused on adsorption–desorption behavior of gases and vapors on those mesoporous materials. Franke et al. (7) have reported adsorption isotherms of cyclopentane vapor and nitrogen on MCM-41. Branton et al. have reported adsorption isotherms of nitrogen, oxygen, and argon on MCM-41 (8) and also have reported those of nitrogen on FSM-16 (9). The isotherms of nitrogen and cyclopentane show no hysteresis, while those of oxygen and argon show little hysteresis. However, Llewellyn et al. (10) and Schmidt et al. (11) reported large hysteresis for nitrogen and argon on MCM-41 with larger pore diameter over 4 nm. Llewellyn et al. proposed that hysteresis should accompany capillary condensation, the absence of hysteresis on the smaller pore MCM-41 materials might be a process resembling secondary micropore filling (10). They concluded that pore size has an important effect on the presence or absence of hysteresis. Water vapor has shown unique adsorption properties on various adsorbents. However, there is no precise adsorption–desorption isotherm of water on the mesoporous materials and no satisfactory interpretation of the hysteresis phenomena. It has been pointed out that both MCM-41 and FSM-16 have hydrophobic surfaces (12). Although the perfect adsorption–desorption isotherm of water vapor is important in discussing hydrophobicity, we have no reports yet. Here, we report the large hysteresis in adsorption isotherms of water on FSM-16 and the hydrophobicity of FSM-16, which was changed by treatment with water vapor.
in 1000 ml of a 0.1 mol dm03 hexadecyltrimethylammonium [C16H33N / (CH3 )3 ] chloride solution and heated at 707C for 3 h under stirring without pH adjustment. After the pH of the suspension was adjusted to 8.5 by the addition of a 2 mol dm03 HCl aqueous solution, the suspension was allowed to settle at 707C for another 3 h. The solid product was filtered and washed with water repeatedly. The dried sample was calcined at 5507C for 6 h to remove the organic fraction to yield FSM-16. X-ray powder diffraction of FSM-16 (Fig. 1), whose XRD peaks appear at d Å 3.79, 2.17, 1.84, and 1.41 nm, confirmed a hexagonal structure with lattice parameter 4.38 nm. The adsorption isotherms of nitrogen, benzene, and water were measured by means of a volumetric method using commercial automatic apparatus, BELSORP 28 for nitrogen and BELSORP 18 for benzene and water, respectively. The pressure gauge of the BELSORP 18 was heated to 1007C during the measurement to prevent adsorption on the inner wall of the apparatus. The samples were evacuated at 3007C for 3 h under a pressure õ10 04 Torr before measurement of the first isotherm. The second isotherm was also measured for water vapor. The measurements of the first adsorption–desorption isotherms required 1 day for nitrogen and benzene and 3 days for water.
MATERIALS AND METHODS Calcination of dried water glass (SiO2 /Na2O Å 2.00) at 7007C for 6 h gave d-Na2Si 2O5 crystals. Fifty grams of d-Na2Si 2O5 was dispersed in 500 ml water and stirred for 3 h. A filtered sample, wet kanemite, was dispersed
FIG. 2. Adsorption isotherms of nitrogen (77 K) and benzene (298 K) on FSM-16. Filled symbols denote desorption.
623
AID
JCIS 4174
/
6g10$$$301
05-24-96 06:14:41
0021-9797/96 $18.00 Copyright q 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
coidal
AP: Colloid
624
NOTE hysteresis was observed, according to the Kelvin equation. The smaller hysteresis for the second isotherm could be explained by the smaller contact angle due to the rehydration. When adsorbate molecules were adsorbed on a wet surface in type IV isotherms, hysteresis observed in those isotherms would be attributable to capillary condensation phenomena, because the contact angles at adsorption and desorption were almost the same. The large hysteresis of the water isotherms indicates that both the pore size and the surface property of the adsorbent have important effects on hysteresis in the adsorption isotherm.
REFERENCES
FIG. 3. Adsorption isotherms of water vapor at 298 K. s, The first isotherms; L the second isotherms. Filled symbols denote desorption.
RESULTS AND DISCUSSION Figure 2 shows the adsorption isotherms of nitrogen and benzene on FSM-16. They have the usual type IV isotherms. Hysteresis in the nitrogen isotherm at P/P0 Å 0.47–0.95 is attributable to the mesopores (d ú 10 nm) formed in interparticle spaces. The isotherm of benzene has no hysteresis, like those of nitrogen and cyclopentane on MCM-41. Although the lattice parameter is larger for FSM-16 than for MCM-41 (a Å 4.0 nm), the P/P0 at which the amount of adsorbed benzene increases steeply is lower for FSM-16 (P/P0 Å 0.12–0.20) than for MCM-41 ( P/P0 Å 0.21– 0.34) (3). This suggests a thicker pore wall of channels in FSM-16 than in MCM-41. Pore diameter, specific surface area, and pore volume of this sample, derived from the adsorption isotherm of nitrogen, were 2.3–2.8 nm, 990 m2 g 01 , and 0.77 cm3 g 01 , respectively. The tentative pore diameter was determined from the P/P0 of the sudden increase portion (P/P0 Å 0.26–0.34) by using the Kelvin equation with zero contact angle collecting multilayer adsorption. The thickness of multilayer adsorption was determined by using adsorption data on nonporous silica reported by Sing and Turk (13). The specific surface area was calculated by the BET method using the isotherm data between P/P0 Å 0.05 and 0.2, over which the capillary condensation occurred. The pore volume was obtained from the volume adsorbed at P/P0 Å 0.4 by assuming that the pores were filled with condensed liquid nitrogen at the P/P0 . Figure 3 shows adsorption isotherms of water vapor on FSM-16. There are two characteristics in these isotherms. One point is that the adsorption isotherms are type V, which is characterized by a small amount of adsorption at lower P/P0 in the type IV isotherm. The isotherms of the other adsorbates reported previously on MCM-41 or FSM-16 were type IV or I. The other point is that the isotherms have hysteresis of significant size. The observed type V isotherms suggest a weak interaction between the solid surface, which means the hydrophobic surface of FSM-16, and the water molecules. The same types of isotherms have also been observed on activated carbons (14) and amorphous silicas (15). The hydrophobicity is correlated with steep increases in adsorption on the isotherms. The P/P0 at the sudden increase is smaller for the second adsorption isotherm than for the first, which suggests a decrease in the hydrophobicity after the first measurement. A larger amount of adsorption and unclosed hysteresis for the first isotherm suggests an irreversible adsorption of water and rehydration of the surface. The rehydration decreased the hydrophobicity of FSM-16 evacuated at 3007C. Such rehydration by treatment with water vapor has also been observed for a silica gel (15). The large hysteresis of the water isotherms could not be explained by capillary condensation phenomena alone. The difference in contact angle between adsorption and desorption could explain the large hysteresis qualitatively. Water molecules contacted unwetted surfaces in the first adsorption experiment, while they drew back from wetted surfaces in desorption. As the contact angle of the latter is smaller than that of the former, large
AID
JCIS 4174
/
6g10$$$301
05-24-96 06:14:41
1. Yanagisawa, T., Shimizu, T., Kuroda, K., and Kato, C., Bull. Chem. Soc. Jpn. 63, 988 (1990). 2. Kresge, C. T., Leonowicz, M. E., Roth, W. J., Vartuli, J. C., and Beck, J. S., Nature 359, 710 (1992). 3. Beck, J. S., Vartuli, J. C., Roth, W. J., Leonowicz, M. E., Kresge, C. T., Schmitt, K. D., Chu, C. T-W., Olson, D. H., Sheppard, E. W., McCullen, S. B., Higgins, J. B., and Schlenker, J. L., J. Am. Chem. Soc. 114, 10834 (1992). 4. Inagaki, S., Fukushima, Y., and Kuroda, K., J. Chem. Soc. Chem. Commun., 680 (1993). 5. Inagaki, S., Fukushima, Y., and Kuroda, K., in ‘‘Zeolite and Related Microporous Materials: State of the Art 1994’’ (J. Weitkamp, H. G. Karge, H. Pfeifer, and W. Holderich, Eds.), p. 125. Elsevier, Amsterdam, 1994. 6. Inagaki, S., Fukushima, Y., and Kuroda, K., in ‘‘Science and Technology in Catalysis 1994’’ (Y. Izumi, H. Arai H., and M. Iwamoto, Eds.), p. 143. Kodansha, Tokyo, 1994. 7. Franke, O., Schulz-Ekloff, G., Rathousky, J., Starek, J., and Zukal, A., J. Chem. Soc. Chem. Commun., 724 (1993). 8. Branton, P. J., Hall, P. G., and Sing, K. S. W., J. Chem. Soc. Chem. Commun., 1257 (1993); Branton, P. J., Hall, P. G., and Sing, K. S. W., J. Chem. Soc. Faraday Trans. 90, 2965 (1994). 9. Branton, P. J., Sing, K. S. W., Kaneko, K., Inagaki, S., and Fukushima, Y., Langmuir 12, 599 (1996). 10. Llewellyn, P. L., Grillet, Y., Schuth, F., Reichert, H., and Unger, K. K., Microporous Mater. 3, 345 (1994). 11. Schmidt, R., Stocker, M., Hansen, E., Akporiaye, D., and Ellestad, O. H., Microporous Mater. 3, 443 (1994). 12. Chen, C.-Y., Li, H.-X., and Davis, M. E., Microporous Mater. 2, 17 (1993). 13. Sing, K. S. W., and Turk, D. H., J. Colloid Interface Sci. 38, 109 (1972). 14. Gregg, S. J., and Sing, K. S. W., in ‘‘Adsorption, Surface Area and Porosity,’’ 2nd ed., Chap. 5. Academic Press, New York, 1982. 15. Naono, H., Hakuman, M., Joh, H., Sakurai, M., and Nakai, K., Porous Mater. 31, 203 (1993). SHINJI INAGAKI * ,1 YOSHIAKI FUKUSHIMA * KAZUO KURODA† KAZUYUKI KURODA† *Toyota Central Research & Development Laboratories, Inc. 41-1, Yokomichi, Nagakute-cho Aichi-gun 480-11, Japan †Department of Applied Chemistry Waseda University Ohkubo-3, Shinjuku-ku Tokyo 169, Japan Received July 7, 1995; accepted December 14, 1995
1
To whom correspondence should be addressed.
coidal
AP: Colloid