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Thermal stability and hydrophobicity of mesoporous silica FSM-16
Colloids and Surfaces A: Physicochemical and Engineering Aspects 203 (2002) 185– 193 www.elsevier.com/locate/colsurfa
Thermal stability and hydrophobicity of mesoporous silica FSM-16 Akihiko Matsumoto *, Tatsuo Sasaki, Nobuyuki Nishimiya, Kazuo Tsutsumi Department of Materials Science, Faculty of Engineering, Toyohashi Uni6ersity of Technology, Tempaku-cho, Toyohashi 441 -8580, Japan Received 5 January 2001; accepted 15 October 2001
Abstract Thermal stability and hydrophobic property of mesoporous silica, FSM-16, has been studied. The mesoporous structure of FSM-16 was stable against calcination up to 1073 K in vacuo, but collapsed at 1273 K. Adsorption isotherm and differential heat of adsorption measurements showed that the calcination temperature affected the hydrophobic nature of FSM-16, and that the surface changed to hydrophilic by hydroxylation of surface siloxane during water adsorption. Once the surface became hydrophilic, the hydrophobic character was never restored even after evacuation at 823 K. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Adsorption; Calorimetry; FSM-16; Heat of adsorption; Mesoporous silica
1. Introduction Mesoporous silica with highly ordered pores, such as FSM-16 and MCM-41, have attracted considerable interests in the field of adsorption science and catalysis chemistry [1 – 5]. FSM-16 is prepared by an intercalation of quarterly ammonium surfactant as a template in a layered sodium silicate, kanemite, followed by calcination to remove the template [1 – 3]. Because of its highly ordered mesoporous structure with unidimensional and hexagonal arrays like MCM-41, FSM16 is expected for the application such as catalytic reaction and adsorbent of large size molecules. * Corresponding author. Tel.: + 81-532-44-6811; fax: + 81532-48-5833. E-mail address: [email protected] (A. Matsumoto).
In molecular adsorption on a solid surface, chemical nature of the solid surface plays an important role. Especially, hydrophile –hydrophobe properties are important ones to control the adsorptivity for polar/non-polar molecules. The hydrophilic property of silica surface depends on the concentration of silanol groups, because they are able to bind water molecules by hydrogen bonding [6]. It was reported that the surfaces of MCM-41 and FSM-16 showed hydrophobic nature owing to a low concentrations of surface silanol groups [7–11]. However, the surface of FSM-16 becomes hydrophobic by repetition of water adsorption [10 –12]. Because the concentration of silanol groups on silica surface varies with heating temperatures 13a, 14, the calcination temperature would affect the hydrophobic nature of FSM-16.
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In this study, the stability of FSM-16 against different calcination temperatures was examined by X-ray diffraction (XRD) and nitrogen adsorption measurements. Effect of the calcination temperature on the hydrophile– hydrophobe properties of FSM-16 was investigated by aid of in situ IR spectroscopy and adsorption calorimetry of water.
2. Experimental
2.1. Synthesis of FSM-16 FSM-16 was prepared as follows according to the published method by using hexadecyltrimethylammonium chloride as a template [9]. A 120 mm of 0.27 mol l − 1 sodium hydroxide aqueous solution was added to 60 g of sodium silicate solution (SiO2/Na2O =2.12, Kishida Chemical) to adjust SiO2/Na2O =ca. 2. The mixture was stirred at 300 K for 3 h followed by drying at 373 K in a vacuum drying oven to remove water. The dried sodium silicate was calcined at 973 K for 6 h in air to obtain a layered sodium silicate, kanemite (d-Na2Si2O5). Kanemite was obtained as a white lump. The kanemite lump was crushed by use of a mortar. This kanemite powder was deliquescent and used for further treatment immediately. Thirty grams of kanemite was dispersed in 300 ml of water and then stirred for 3 h at 300 K. Then the suspension was filtered out to obtain wet kanemite paste. All of the kanemite paste was dispersed in 480 ml of an aqueous solution of n-hexadecyltrimethylammonium chloride (0.125 mol l − 1) and then stirred at 343 K for 3 h. The pH value of the suspension was 11.5– 12.5 at this stage. Afterwards, the pH value was adjusted carefully to 8.5 by adding 2 mol l − 1 hydrochloric acid with stirring. The suspension was kept stirring at 343 K for 3 h with keeping the pH value at 8– 9. After cooling to room temperature, the solid product was filtered out, washed with 1 l of water and dried in air to yield mesoporous silicate, FSM-16, with retaining the template. The FSM-16 was calcined at an appropriate temperature from 823 to 1273 K to burn off the surfac-
tant and immediately used for characterization and adsorption experiments. The FSM-16 sample is designated FSM-16-nK, where n stands for the calcination temperature in degree Kelvin.
2.2. Characterization The XRD pattern of freshly calcined sample was recorded by use of an automatic powder diffractometer (Rigaku, RINT2000) using CuKa radiation. The nitrogen adsorption isotherm was measured volumetrically at 77 K by use of a laboratory-made adsorption apparatus. Freshly calcined sample was immediately preevacuated at 298 K for 6 h in vacuo (1 mPa) before the adsorption measurement. IR spectra of FSM-16 were measured in situ with a resolution of 2 cm − 1 over a spectral range of 3000–4000 cm − 1 at 298 K by FT–IR spectrometer (JASCO, FT/IR420) attached with an in situ diffuse reflectance cell (SR-800) and an MCT detector, and 1500 consecutive scans were summed. An appropriate amount of freshly calcined sample was diluted in diamond powder and placed in the diffuse reflectance cell. The sample was preevacuated at 423 K for 12 h under 1 mPa before the measurement. IR spectra of FSM-16 were measured before water adsorption and after water adsorption at a relative pressure (P/P0)= ca. 0.8 followed by evacuation at 423 K for 2 h and optionally at 823 K.
2.3. Characterization of hydrophobic–hydrophile properties Heats of adsorption of water were measured at 298 K by a twin conduction type microcalorimeter (Tokyo Riko Co.) equipped with a volumetric adsorption apparatus. Adsorption isotherm of water was consecutively measured with the calorimetric measurements. The sample that was freshly calcined at 823 or 1073 K was immediately placed in the sample cell and evacuated at 298 K under 1 mPa for 12 h prior to the measurements. After the first measurements, the sample was evacuated under 1 mPa at 823 K for 12 h and used for the second measurements.
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Fig. 1. Nitrogen adsorption isotherms on FSM-16 calcined at different temperatures.
3. Results and discussion
Fig. 2. XRD patterns of FSM-16-823 K and -1073 K.
3.1. Effect of calcination temperature on porosity of FSM-16 The nitrogen adsorption isotherms of FSM-16823 and -1073 K are of Type IV in the IUPAC classification, as shown in Fig. 1, which are characteristic for mesoporous materials [15]. These results show that FSM-16 retains its mesoporous structure even after the heat treatment at 823 or 1073 K. Specific surface areas, as,BET, and apparent pore diameters of FSM-16-823 and -1073 K were estimated by the BET plot and the BJH method, respectively, and shown in Table 1. Here it should be noted that the apparent pore diameters are underestimated because a thickness of adsorbed layer is not taken into account in the BJH method [16,17]. The XRD pattern of FSM-
16-823 K shown in Fig. 2 exhibits clear Bragg peaks attributable to the (100), (110) and (200) reflections of the hexagonal mesostructure at 2q= 2.48 (d= 3.56 nm), 4.22 and 4.84°, respectively. These clear peaks evidenced the formation of the long-range-order structure. FSM-16-1073 K also showed the XRD pattern attributable the hexagonal mesostructure (Fig. 2), the (100) reflection appeared at 2q= 2.52° (d= 3.50 nm), however, other peaks overlapped and decreased in intensity. Inaki et al. reported that FSM-16 is more thermally stable than MCM-41 and the hexagonal pore structure of FSM-16 was retained after evacuation at 1073 K [18]. In the present study, the result of XRD as well as nitrogen adsorption measurements for FSM-16-1073 K co-
Table 1 Characteristics of FSM-16 prepared in this study Sample
Specific surface areaa (m2 g−1)
FMS-16-823 K 940 FSM-16-1073 K 920 FSM-16-1273 K 260 a
Determined Determined c The median d Determined b
Specific pore volumeb (ml g−1)
Apparent pore diameter (nm)
0.58 0.54 0.09
2.7c 2.4c 0.6d
by the BET plot. by the hs plot shown in Fig. 3. of pore size distribution curve determined by BJH method. by the inflection point in the hs plot shown in Fig. 3.
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incides with their report although the hexagonal structure became less ordered comparing to FSM16-873 K. On the other hand, the nitrogen isotherm of FSM-16-1273 K is not of Type IV, as shown in Fig. 1. Because the hs method revealed that FSM16-1273 K has micropores as mentioned below, the nitrogen adsorption isotherm of FSM-16-1273 K would be regarded as a combination of Type I and II isotherms resulting from adsorption in the micropores and on the external surface, respectively. The change in the isotherm type suggests that the mesoporous structure was not retained through calcination at 1273 K. The as,BET of FSM-16-1273 K was 260 m2 g − 1, which also indicates decreasing the porosity. The structure of FSM-16 is retained after calcination at 1173 K [3], but becomes amorphous by heating at 1273 K [11]. In the present case, the mesoporous structure of FSM-16 would be collapsed by heating at 1273 K. The hs method is useful to analyze isotherm on porous materials [19]. The adsorption isotherms in Fig. 1 are transformed into the corresponding hs plots shown in Fig. 3. Standard data for the adsorption of nitrogen at 77 K on non-porous silica prepared by crushed quartz were used to obtain the hs plots [20], because the surface of FSM-16 is rather hydrophobic like crushed quartz than hydroxylated silica. In fact, regardless of different surface nature between these silica materials, the standard data for the nitrogen adsorp-
tion at 77 K on crushed quartz coincides well with that on non-porous hydroxylated silica in the P/P0 range between 0 and 0.9 [20]. The hs plots of FSM-16-823 K and -1073 K showed a linear relationship passing through the origin at hs B 0.84 and bent upward at hs = ca. 0.84, and then moderately increased at hs \ 1.0. The linear relationship at hs B 0.84 corresponds to monolayer– multilayer adsorption on both internal and external surfaces. The steep rise between 0.84B hs B 1 is due to capillary condensation in mesopores, and the moderate increasing at hs \ 1.0 is due to multilayer adsorption on external surface. These hs plots indicate that FSM-16-823 K and -1073 K have mesoporous structures. On the other hand, the hs plot of FSM-16-1273 K was linear and passed through the origin at hs B 0.6 and then moderately increased above hs = 0.6, which suggests that FSM-16-1273 K is microporous rather than mesoporous. The approximate pore size of FSM-16-1273 K was estimated as 0.6 nm from the inflection point of hs plot at hs = 0.6 considering the mean thickness of an adsorbed nitrogen layer. Calcination at 1273 K would bring about collapse of the mesoporous structure and formation of micropores. An apparent pore volume of each sample was estimated from the intercept of back-extrapolation of the linear part above hs = 1 to the ordinate, and shown in Table 1. FSM-16-823 K and -1073 K have higher pore volume than FSM-16-1273 K. Because FSM-16823 K and -1073 K retained the mesoporous structure, they were characterized through further experiments.
3.2. Characterization of surface by IR measurements
Fig. 3. hs plots of nitrogen adsorption isotherms on FSM-16 calcined at different temperatures.
As shown in Fig. 4(a), the IR spectrum of the freshly calcined FSM-16-823 K exhibited a strong absorption band at 3746 cm − 1 which was slightly asymmetric toward the lower wavenumber side [12]. The band at 3746 cm − 1 is attributable to the OH stretching vibrations of isolated (single) silanol groups, and the asymmetry would be due to either isolated or geminal silanol groups [13b,21,22]. Upon water adsorption followed by evacuation at 423 K, a shoulder band at 3720
A. Matsumoto et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 203 (2002) 185–193
Fig. 4. IR spectral change of FSM-16-823 K by water adsorption and evacuation at different temperatures: (a) before water adsorption; (b) after water adsorption at P/P0 = 0.8 and subsequent evacuation at 423 K; (c) further evacuated at 823 K.
cm − 1 and a wide band centered at 3600 cm − 1 appeared (Fig. 4(b)). At this stage, physisorbed water molecules would be desorbed [6,21]. Therefore, it is likely that siloxane bonds cleaved by hydroxylation in the following steps even at room temperature: physical adsorption of water molecules on hydrophilic silanol sites and hydrolysis of siloxane near the silanol sites, to yield a pair of new silanol groups [6,13a]. Such silanol groups interact with each other and show the absorption bands of the OH stretching vibrations of hydrogen-bonded silanols at hydrogen and oxygen atoms at 3520 and 3720 cm − 1, respectively [21]. Therefore, the shoulder at 3720 cm − 1 would be due to the variations of hydrogenbonded silanol groups at oxygen atoms. The wide bands centered at 3600 cm − 1 would be due to overlapping of the bands attributable to the OH stretching vibrations of hydrogen-bonded silanols at hydrogen atoms and those from perturbed silanol groups at points of interparticle contact centered at 3650 cm − 1 [21]. The IR spectrum did not restore the initial appearance before water loading by evacuation at 823 K as shown in Fig. 4(c): the broad band
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around 3500 cm − 1 disappeared, but the band at 3650 cm − 1 assigned to the OH stretching vibrations of the perturbed silanol groups was observed. Furthermore, the strong band at 3746 cm − 1 became more asymmetric towards the lower wavenumber side. These results suggest that the silanol groups that formed by hydroxylation of siloxane were not completely removed by evacuation at 823 K. The IR spectral changes of FSM-16-1073 K by water adsorption and desorption were similar to those of FSM-16-823 K as shown in Fig. 5, however, some differences were observed. The 3746 cm − 1-band in the spectrum before water loading was narrower than that of FSM-16-823 K (Fig. 5(a)). This result suggests that the concentration of isolated or geminal silanol groups on FSM-16-1073 K was lower than that on FSM-16823 K. Since the concentration of silanol groups on silica decreases with increasing pretreatment temperature [6,13b,14], the calcination at 1073 K would more severely dehydoxylate silanol groups than that at 823 K. The IR spectrum after water adsorption followed by evacuation at 423 K exhibited a shoulder at 3720 cm − 1 and broad bands
Fig. 5. IR spectral change of FSM-16-1073 K by water adsorption and evacuation at different temperatures: (a) before water adsorption; (b) after water adsorption at P/P0 =0.8 and subsequent evacuation at 423 K; (c) further evacuated at 823 K.
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Fig. 6. Water adsorption isotherms at 298 K on FSM-16-823 K and -1073 K. Closed and open symbols stand for FSM-16823 K and -1073 K, respectively. Circular and square symbols denote the first and the second adsorption runs, respectively.
at 3650 and 3550 cm − 1 (Fig. 5(b)), which shows that hydrogen-bonded silanol groups formed on inter/intraparticles. Upon further evacuation at 823 K, these bands disappeared, but the 3746 cm − 1-band was still asymmetric toward the lower wavenumber side in the same manner as the case of FSM-16-823 K (Fig. 5(c)), showing the presence of isolated or geminal silanol groups.
3.3. Water adsorption Fig. 6 shows adsorption isotherms of water at 298 K on FSM-16-823 K and -1073 K. Regardless of the calcination temperature, the adsorption isotherms for the first adsorption run were of Type V in the IUPAC classification, which is indicative of weak adsorbent– adsorbate interactions [15]. However, the adsorption isotherm for the second run were of Type IV, suggesting rather hydrophilic nature of the surfaces. The change of the surface property of FSM-16 with water adsorption was accord well with that reported by Inagaki et al. [10]. In the case of FSM-16-823 K, the adsorption amount for the first run changed from 0 to 4 mmol g − 1 at 0B P/P0 B0.5 and steeply increased from 4 to 28 mmol g − 1 at 0.5 BP/P0 B0.6, as already reported in [12]. This steep rise is due to capillary condensation of water caused by the polar interaction among adjacent water molecules
rather than the interaction between the surface and water. However, in the second run, the adsorbed amount at the lower P/P0 region (P/P0 B 0.4) was 1.5–2 times as high as that in the first run. Furthermore, a steep rise of adsorbed amount took place at lower P/P0 values from 0.45 to 0.60. In the first adsorption run, the concentration of silanol groups of freshly calcined FSM-16 was rather low and showed hydrophobic nature. Whereas, once water molecules were condensed in pores, the siloxane bonds would be hydrolyzed to give silanol groups. Some of these silanol groups, mainly isolated silanols, were thermally stable and remained even after preevacuation with heating up to 823 K [14], so that the surface became hydrophilic. Because the multilayer adsorption would take place at the lower P/P0 region (P/ P0 B 0.4) in the second run so that the narrow the effective radius (pore radius minus the thickness of ordinary multilayer adsorption expected at that P/P0), the capillary condensation of water was observed at the lower P/P0. After the water condensation completed, the adsorbed amount monotonously increased with P/P0 by condensation on external surface. In the case of the first adsorption run on FSM16-1073 K, the adsorbed amount at lower P/P0 before water condensation was smaller than that on FSM-16-823 K, and the water condensation began at a higher P/P0 value of 0.62. These results indicate that the surface of FSM-16-1073 K is more hydrophobic than that of FSM-16-823 K, which is consistent with the results of IR measurements. Because the apparent mesopore size determined by nitrogen adsorption was similar for FSM-16-823 K and -1073 K, as shown in Table 1, the difference in the P/P0 value at the capillary condensation would indicate more hydrophobic nature or lower concentration of silanol groups on FSM-16-1073 K. Ishikawa et al. also measured the water adsorption isotherms on FSM-16 samples that were calcined at different temperatures from 823 to 1273 K [11], and found the water adsorptivity of FSM-16 decreased with increasing the calcination temperature. They suggested that the change in water adsorptivity is due to the decrease of silanol concentration with the increase of the calcination temperature [11], which coincides with the present results.
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The adsorption isotherm of FSM-16-1073 K in the second run was of Type IV similar to FSM16-823 K; the capillary condensation of water began at lower P/P0 value, 0.41. Although FSM16-1073 K showed more hydrophobic behavior than FSM-16-823 K in the first run, the surface of FSM-16-1073 K changed to hydrophilic by water adsorption and hydrophobicity was not recovered by evacuation as well as in the case of FSM-16823 K. The P/P0 value at the beginning of capillary condensation for FSM-16-1073 K was slightly lower than that for FSM-16-873 K, which could be due to narrower pore sizes in FSM-161073 K. Difference in the adsorption capacity of water between FSM-16-823 K and -1073 K (ca. 0.04 ml g − 1) would be ascribed to that in the mesopore volumes (ca. 0.05 ml g − 1).
3.4. Heats of adsorption The differential heats of adsorption of water on FSM-16-823 K and -1073 K are shown in Fig. 7. The heats of adsorption at initial stage in the first adsorption run showed different tendency from those in the second run for each sample.
Fig. 7. The differential heats of adsorption of water at 298 K on FSM-16-823 K and -1073 K. Circular and square symbols denote the first and the second adsorption runs, respectively.
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In the case of FSM-16-1073 K, the initial heats of adsorption in the first run was ca. 20 kJ mol − 1, which is comparable to the heat of hydroxylation of siloxane surface to form silanol [6]. Then the released heat gradually increased as the adsorption proceeded, attaining the constant value of 45–50 kJ mol − 1. Slightly adsorbed water on surface silanol groups hydroxylates siloxane near the silanol groups to give rise to silanol groups in succession [6]. Since the silanol groups would strongly interact with water molecules than siloxane, the adsorption heats would gradually increase with the progress of adsorption. The adsorption amounts where heats of adsorption reached the constant value were found to be equal to those where the amounts exhibited steep rise in the isotherms in Fig. 6. Afterwards 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 − 1 [23]. On the other hand, the differential heats of adsorption in the second run were ca. 70 kJ mol − 1 at the initial stage of adsorption, and then decreased gradually to 45–50 kJ mol − 1 with increase in the adsorbed amount of water. As already mentioned above, the adsorption isotherm of water suggests that the surface changes to more hydrophilic by the first water adsorption. Furthermore, IR results showed that a considerable amount of silanol groups formed on FSM-161073 K by exposure to water and remained even after evacuation at 823 K. Therefore, the high heats at initial stage in the second adsorption run would be due to a strong interaction between water molecules and surface silanol groups. These calorimetric measurements also suggest that once the water is being adsorbed, the hydrophilic property was retained even after evacuation at 823 K. In the case of FSM-16-823 K, the change in initial heats of adsorption at the first adsorption run was different from that of FSM-16-1073 K. Although the initial heats were ca. 20 kJ mol − 1 as well as in FSM-16-1073, the heats immediately reached the constant value of 45–50 kJ mol − 1. Klier and Zettlemoyer suggested the water adsorption mechanism on silica surface as follows [24]: water adsorbs with ‘oxygen down’ on the
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silanol groups (SiOH) at least at the beginning stages of adsorption. Then hydrogen-bonded clusters of water molecules may be formed even before all the silanol groups have adsorbed water molecules to form SiOHOH2 groups [6,24]. In the present study, the results of IR measurements and adsorption isotherm of water indicate that the surface of FSM-16-823 K is covered with more silanol groups than that of FSM-16-1073 K even before water adsorption. Therefore, water adsorption on both SiOH and SiOHOH2 would simultaneously take place as well as hydroxylation of siloxane by water adsorption. The adsorption energies of water molecule on a silanol group (SiOH) of dehydrated silica surface and on silanol group with adsorbed water (SiOHOH2) are 25 and 44 kJ mol − 1, respectively [6,23]. Therefore, the heat evolution at the initial stage immediately increased to 45–50 kJ mol − 1. The differential heats of adsorption of the second run showed the similar tendency as observed in FSM-16-1073 K, because the surface became hydrophilic by the first water adsorption.
4. Conclusion Pore structure of mesoporous silica FSM-16 is stable against heat treatment up to 1073 K in vacuo but collapsed by calcination at 1273 K. FSM-16 calcined at 1073 K has less silanol groups than that calcined at 823 K, although both FSM16 exhibit hydrophobic surface character regardless of calcination temperatures. The difference in calcination temperature affects adsorption behavior of water as well as rehydroxylation of FSM16. Once water is adsorbed on the freshly calcined FSM-16 surface, the surface becomes hydrophilic and original hydrophobic character is never restored even after evacuation at 823 K.
Acknowledgements 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.
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Thermal stability and hydrophobicity of mesoporous silica FSM-16 Akihiko Matsumoto *, Tatsuo Sasaki, Nobuyuki Nishimiya, Kazuo Tsutsumi Department of Materials Science, Faculty of Engineering, Toyohashi Uni6ersity of Technology, Tempaku-cho, Toyohashi 441 -8580, Japan Received 5 January 2001; accepted 15 October 2001
Abstract Thermal stability and hydrophobic property of mesoporous silica, FSM-16, has been studied. The mesoporous structure of FSM-16 was stable against calcination up to 1073 K in vacuo, but collapsed at 1273 K. Adsorption isotherm and differential heat of adsorption measurements showed that the calcination temperature affected the hydrophobic nature of FSM-16, and that the surface changed to hydrophilic by hydroxylation of surface siloxane during water adsorption. Once the surface became hydrophilic, the hydrophobic character was never restored even after evacuation at 823 K. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Adsorption; Calorimetry; FSM-16; Heat of adsorption; Mesoporous silica
1. Introduction Mesoporous silica with highly ordered pores, such as FSM-16 and MCM-41, have attracted considerable interests in the field of adsorption science and catalysis chemistry [1 – 5]. FSM-16 is prepared by an intercalation of quarterly ammonium surfactant as a template in a layered sodium silicate, kanemite, followed by calcination to remove the template [1 – 3]. Because of its highly ordered mesoporous structure with unidimensional and hexagonal arrays like MCM-41, FSM16 is expected for the application such as catalytic reaction and adsorbent of large size molecules. * Corresponding author. Tel.: + 81-532-44-6811; fax: + 81532-48-5833. E-mail address: [email protected] (A. Matsumoto).
In molecular adsorption on a solid surface, chemical nature of the solid surface plays an important role. Especially, hydrophile –hydrophobe properties are important ones to control the adsorptivity for polar/non-polar molecules. The hydrophilic property of silica surface depends on the concentration of silanol groups, because they are able to bind water molecules by hydrogen bonding [6]. It was reported that the surfaces of MCM-41 and FSM-16 showed hydrophobic nature owing to a low concentrations of surface silanol groups [7–11]. However, the surface of FSM-16 becomes hydrophobic by repetition of water adsorption [10 –12]. Because the concentration of silanol groups on silica surface varies with heating temperatures 13a, 14, the calcination temperature would affect the hydrophobic nature of FSM-16.
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In this study, the stability of FSM-16 against different calcination temperatures was examined by X-ray diffraction (XRD) and nitrogen adsorption measurements. Effect of the calcination temperature on the hydrophile– hydrophobe properties of FSM-16 was investigated by aid of in situ IR spectroscopy and adsorption calorimetry of water.
2. Experimental
2.1. Synthesis of FSM-16 FSM-16 was prepared as follows according to the published method by using hexadecyltrimethylammonium chloride as a template [9]. A 120 mm of 0.27 mol l − 1 sodium hydroxide aqueous solution was added to 60 g of sodium silicate solution (SiO2/Na2O =2.12, Kishida Chemical) to adjust SiO2/Na2O =ca. 2. The mixture was stirred at 300 K for 3 h followed by drying at 373 K in a vacuum drying oven to remove water. The dried sodium silicate was calcined at 973 K for 6 h in air to obtain a layered sodium silicate, kanemite (d-Na2Si2O5). Kanemite was obtained as a white lump. The kanemite lump was crushed by use of a mortar. This kanemite powder was deliquescent and used for further treatment immediately. Thirty grams of kanemite was dispersed in 300 ml of water and then stirred for 3 h at 300 K. Then the suspension was filtered out to obtain wet kanemite paste. All of the kanemite paste was dispersed in 480 ml of an aqueous solution of n-hexadecyltrimethylammonium chloride (0.125 mol l − 1) and then stirred at 343 K for 3 h. The pH value of the suspension was 11.5– 12.5 at this stage. Afterwards, the pH value was adjusted carefully to 8.5 by adding 2 mol l − 1 hydrochloric acid with stirring. The suspension was kept stirring at 343 K for 3 h with keeping the pH value at 8– 9. After cooling to room temperature, the solid product was filtered out, washed with 1 l of water and dried in air to yield mesoporous silicate, FSM-16, with retaining the template. The FSM-16 was calcined at an appropriate temperature from 823 to 1273 K to burn off the surfac-
tant and immediately used for characterization and adsorption experiments. The FSM-16 sample is designated FSM-16-nK, where n stands for the calcination temperature in degree Kelvin.
2.2. Characterization The XRD pattern of freshly calcined sample was recorded by use of an automatic powder diffractometer (Rigaku, RINT2000) using CuKa radiation. The nitrogen adsorption isotherm was measured volumetrically at 77 K by use of a laboratory-made adsorption apparatus. Freshly calcined sample was immediately preevacuated at 298 K for 6 h in vacuo (1 mPa) before the adsorption measurement. IR spectra of FSM-16 were measured in situ with a resolution of 2 cm − 1 over a spectral range of 3000–4000 cm − 1 at 298 K by FT–IR spectrometer (JASCO, FT/IR420) attached with an in situ diffuse reflectance cell (SR-800) and an MCT detector, and 1500 consecutive scans were summed. An appropriate amount of freshly calcined sample was diluted in diamond powder and placed in the diffuse reflectance cell. The sample was preevacuated at 423 K for 12 h under 1 mPa before the measurement. IR spectra of FSM-16 were measured before water adsorption and after water adsorption at a relative pressure (P/P0)= ca. 0.8 followed by evacuation at 423 K for 2 h and optionally at 823 K.
2.3. Characterization of hydrophobic–hydrophile properties Heats of adsorption of water were measured at 298 K by a twin conduction type microcalorimeter (Tokyo Riko Co.) equipped with a volumetric adsorption apparatus. Adsorption isotherm of water was consecutively measured with the calorimetric measurements. The sample that was freshly calcined at 823 or 1073 K was immediately placed in the sample cell and evacuated at 298 K under 1 mPa for 12 h prior to the measurements. After the first measurements, the sample was evacuated under 1 mPa at 823 K for 12 h and used for the second measurements.
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Fig. 1. Nitrogen adsorption isotherms on FSM-16 calcined at different temperatures.
3. Results and discussion
Fig. 2. XRD patterns of FSM-16-823 K and -1073 K.
3.1. Effect of calcination temperature on porosity of FSM-16 The nitrogen adsorption isotherms of FSM-16823 and -1073 K are of Type IV in the IUPAC classification, as shown in Fig. 1, which are characteristic for mesoporous materials [15]. These results show that FSM-16 retains its mesoporous structure even after the heat treatment at 823 or 1073 K. Specific surface areas, as,BET, and apparent pore diameters of FSM-16-823 and -1073 K were estimated by the BET plot and the BJH method, respectively, and shown in Table 1. Here it should be noted that the apparent pore diameters are underestimated because a thickness of adsorbed layer is not taken into account in the BJH method [16,17]. The XRD pattern of FSM-
16-823 K shown in Fig. 2 exhibits clear Bragg peaks attributable to the (100), (110) and (200) reflections of the hexagonal mesostructure at 2q= 2.48 (d= 3.56 nm), 4.22 and 4.84°, respectively. These clear peaks evidenced the formation of the long-range-order structure. FSM-16-1073 K also showed the XRD pattern attributable the hexagonal mesostructure (Fig. 2), the (100) reflection appeared at 2q= 2.52° (d= 3.50 nm), however, other peaks overlapped and decreased in intensity. Inaki et al. reported that FSM-16 is more thermally stable than MCM-41 and the hexagonal pore structure of FSM-16 was retained after evacuation at 1073 K [18]. In the present study, the result of XRD as well as nitrogen adsorption measurements for FSM-16-1073 K co-
Table 1 Characteristics of FSM-16 prepared in this study Sample
Specific surface areaa (m2 g−1)
FMS-16-823 K 940 FSM-16-1073 K 920 FSM-16-1273 K 260 a
Determined Determined c The median d Determined b
Specific pore volumeb (ml g−1)
Apparent pore diameter (nm)
0.58 0.54 0.09
2.7c 2.4c 0.6d
by the BET plot. by the hs plot shown in Fig. 3. of pore size distribution curve determined by BJH method. by the inflection point in the hs plot shown in Fig. 3.
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incides with their report although the hexagonal structure became less ordered comparing to FSM16-873 K. On the other hand, the nitrogen isotherm of FSM-16-1273 K is not of Type IV, as shown in Fig. 1. Because the hs method revealed that FSM16-1273 K has micropores as mentioned below, the nitrogen adsorption isotherm of FSM-16-1273 K would be regarded as a combination of Type I and II isotherms resulting from adsorption in the micropores and on the external surface, respectively. The change in the isotherm type suggests that the mesoporous structure was not retained through calcination at 1273 K. The as,BET of FSM-16-1273 K was 260 m2 g − 1, which also indicates decreasing the porosity. The structure of FSM-16 is retained after calcination at 1173 K [3], but becomes amorphous by heating at 1273 K [11]. In the present case, the mesoporous structure of FSM-16 would be collapsed by heating at 1273 K. The hs method is useful to analyze isotherm on porous materials [19]. The adsorption isotherms in Fig. 1 are transformed into the corresponding hs plots shown in Fig. 3. Standard data for the adsorption of nitrogen at 77 K on non-porous silica prepared by crushed quartz were used to obtain the hs plots [20], because the surface of FSM-16 is rather hydrophobic like crushed quartz than hydroxylated silica. In fact, regardless of different surface nature between these silica materials, the standard data for the nitrogen adsorp-
tion at 77 K on crushed quartz coincides well with that on non-porous hydroxylated silica in the P/P0 range between 0 and 0.9 [20]. The hs plots of FSM-16-823 K and -1073 K showed a linear relationship passing through the origin at hs B 0.84 and bent upward at hs = ca. 0.84, and then moderately increased at hs \ 1.0. The linear relationship at hs B 0.84 corresponds to monolayer– multilayer adsorption on both internal and external surfaces. The steep rise between 0.84B hs B 1 is due to capillary condensation in mesopores, and the moderate increasing at hs \ 1.0 is due to multilayer adsorption on external surface. These hs plots indicate that FSM-16-823 K and -1073 K have mesoporous structures. On the other hand, the hs plot of FSM-16-1273 K was linear and passed through the origin at hs B 0.6 and then moderately increased above hs = 0.6, which suggests that FSM-16-1273 K is microporous rather than mesoporous. The approximate pore size of FSM-16-1273 K was estimated as 0.6 nm from the inflection point of hs plot at hs = 0.6 considering the mean thickness of an adsorbed nitrogen layer. Calcination at 1273 K would bring about collapse of the mesoporous structure and formation of micropores. An apparent pore volume of each sample was estimated from the intercept of back-extrapolation of the linear part above hs = 1 to the ordinate, and shown in Table 1. FSM-16-823 K and -1073 K have higher pore volume than FSM-16-1273 K. Because FSM-16823 K and -1073 K retained the mesoporous structure, they were characterized through further experiments.
3.2. Characterization of surface by IR measurements
Fig. 3. hs plots of nitrogen adsorption isotherms on FSM-16 calcined at different temperatures.
As shown in Fig. 4(a), the IR spectrum of the freshly calcined FSM-16-823 K exhibited a strong absorption band at 3746 cm − 1 which was slightly asymmetric toward the lower wavenumber side [12]. The band at 3746 cm − 1 is attributable to the OH stretching vibrations of isolated (single) silanol groups, and the asymmetry would be due to either isolated or geminal silanol groups [13b,21,22]. Upon water adsorption followed by evacuation at 423 K, a shoulder band at 3720
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Fig. 4. IR spectral change of FSM-16-823 K by water adsorption and evacuation at different temperatures: (a) before water adsorption; (b) after water adsorption at P/P0 = 0.8 and subsequent evacuation at 423 K; (c) further evacuated at 823 K.
cm − 1 and a wide band centered at 3600 cm − 1 appeared (Fig. 4(b)). At this stage, physisorbed water molecules would be desorbed [6,21]. Therefore, it is likely that siloxane bonds cleaved by hydroxylation in the following steps even at room temperature: physical adsorption of water molecules on hydrophilic silanol sites and hydrolysis of siloxane near the silanol sites, to yield a pair of new silanol groups [6,13a]. Such silanol groups interact with each other and show the absorption bands of the OH stretching vibrations of hydrogen-bonded silanols at hydrogen and oxygen atoms at 3520 and 3720 cm − 1, respectively [21]. Therefore, the shoulder at 3720 cm − 1 would be due to the variations of hydrogenbonded silanol groups at oxygen atoms. The wide bands centered at 3600 cm − 1 would be due to overlapping of the bands attributable to the OH stretching vibrations of hydrogen-bonded silanols at hydrogen atoms and those from perturbed silanol groups at points of interparticle contact centered at 3650 cm − 1 [21]. The IR spectrum did not restore the initial appearance before water loading by evacuation at 823 K as shown in Fig. 4(c): the broad band
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around 3500 cm − 1 disappeared, but the band at 3650 cm − 1 assigned to the OH stretching vibrations of the perturbed silanol groups was observed. Furthermore, the strong band at 3746 cm − 1 became more asymmetric towards the lower wavenumber side. These results suggest that the silanol groups that formed by hydroxylation of siloxane were not completely removed by evacuation at 823 K. The IR spectral changes of FSM-16-1073 K by water adsorption and desorption were similar to those of FSM-16-823 K as shown in Fig. 5, however, some differences were observed. The 3746 cm − 1-band in the spectrum before water loading was narrower than that of FSM-16-823 K (Fig. 5(a)). This result suggests that the concentration of isolated or geminal silanol groups on FSM-16-1073 K was lower than that on FSM-16823 K. Since the concentration of silanol groups on silica decreases with increasing pretreatment temperature [6,13b,14], the calcination at 1073 K would more severely dehydoxylate silanol groups than that at 823 K. The IR spectrum after water adsorption followed by evacuation at 423 K exhibited a shoulder at 3720 cm − 1 and broad bands
Fig. 5. IR spectral change of FSM-16-1073 K by water adsorption and evacuation at different temperatures: (a) before water adsorption; (b) after water adsorption at P/P0 =0.8 and subsequent evacuation at 423 K; (c) further evacuated at 823 K.
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Fig. 6. Water adsorption isotherms at 298 K on FSM-16-823 K and -1073 K. Closed and open symbols stand for FSM-16823 K and -1073 K, respectively. Circular and square symbols denote the first and the second adsorption runs, respectively.
at 3650 and 3550 cm − 1 (Fig. 5(b)), which shows that hydrogen-bonded silanol groups formed on inter/intraparticles. Upon further evacuation at 823 K, these bands disappeared, but the 3746 cm − 1-band was still asymmetric toward the lower wavenumber side in the same manner as the case of FSM-16-823 K (Fig. 5(c)), showing the presence of isolated or geminal silanol groups.
3.3. Water adsorption Fig. 6 shows adsorption isotherms of water at 298 K on FSM-16-823 K and -1073 K. Regardless of the calcination temperature, the adsorption isotherms for the first adsorption run were of Type V in the IUPAC classification, which is indicative of weak adsorbent– adsorbate interactions [15]. However, the adsorption isotherm for the second run were of Type IV, suggesting rather hydrophilic nature of the surfaces. The change of the surface property of FSM-16 with water adsorption was accord well with that reported by Inagaki et al. [10]. In the case of FSM-16-823 K, the adsorption amount for the first run changed from 0 to 4 mmol g − 1 at 0B P/P0 B0.5 and steeply increased from 4 to 28 mmol g − 1 at 0.5 BP/P0 B0.6, as already reported in [12]. This steep rise is due to capillary condensation of water caused by the polar interaction among adjacent water molecules
rather than the interaction between the surface and water. However, in the second run, the adsorbed amount at the lower P/P0 region (P/P0 B 0.4) was 1.5–2 times as high as that in the first run. Furthermore, a steep rise of adsorbed amount took place at lower P/P0 values from 0.45 to 0.60. In the first adsorption run, the concentration of silanol groups of freshly calcined FSM-16 was rather low and showed hydrophobic nature. Whereas, once water molecules were condensed in pores, the siloxane bonds would be hydrolyzed to give silanol groups. Some of these silanol groups, mainly isolated silanols, were thermally stable and remained even after preevacuation with heating up to 823 K [14], so that the surface became hydrophilic. Because the multilayer adsorption would take place at the lower P/P0 region (P/ P0 B 0.4) in the second run so that the narrow the effective radius (pore radius minus the thickness of ordinary multilayer adsorption expected at that P/P0), the capillary condensation of water was observed at the lower P/P0. After the water condensation completed, the adsorbed amount monotonously increased with P/P0 by condensation on external surface. In the case of the first adsorption run on FSM16-1073 K, the adsorbed amount at lower P/P0 before water condensation was smaller than that on FSM-16-823 K, and the water condensation began at a higher P/P0 value of 0.62. These results indicate that the surface of FSM-16-1073 K is more hydrophobic than that of FSM-16-823 K, which is consistent with the results of IR measurements. Because the apparent mesopore size determined by nitrogen adsorption was similar for FSM-16-823 K and -1073 K, as shown in Table 1, the difference in the P/P0 value at the capillary condensation would indicate more hydrophobic nature or lower concentration of silanol groups on FSM-16-1073 K. Ishikawa et al. also measured the water adsorption isotherms on FSM-16 samples that were calcined at different temperatures from 823 to 1273 K [11], and found the water adsorptivity of FSM-16 decreased with increasing the calcination temperature. They suggested that the change in water adsorptivity is due to the decrease of silanol concentration with the increase of the calcination temperature [11], which coincides with the present results.
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The adsorption isotherm of FSM-16-1073 K in the second run was of Type IV similar to FSM16-823 K; the capillary condensation of water began at lower P/P0 value, 0.41. Although FSM16-1073 K showed more hydrophobic behavior than FSM-16-823 K in the first run, the surface of FSM-16-1073 K changed to hydrophilic by water adsorption and hydrophobicity was not recovered by evacuation as well as in the case of FSM-16823 K. The P/P0 value at the beginning of capillary condensation for FSM-16-1073 K was slightly lower than that for FSM-16-873 K, which could be due to narrower pore sizes in FSM-161073 K. Difference in the adsorption capacity of water between FSM-16-823 K and -1073 K (ca. 0.04 ml g − 1) would be ascribed to that in the mesopore volumes (ca. 0.05 ml g − 1).
3.4. Heats of adsorption The differential heats of adsorption of water on FSM-16-823 K and -1073 K are shown in Fig. 7. The heats of adsorption at initial stage in the first adsorption run showed different tendency from those in the second run for each sample.
Fig. 7. The differential heats of adsorption of water at 298 K on FSM-16-823 K and -1073 K. Circular and square symbols denote the first and the second adsorption runs, respectively.
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In the case of FSM-16-1073 K, the initial heats of adsorption in the first run was ca. 20 kJ mol − 1, which is comparable to the heat of hydroxylation of siloxane surface to form silanol [6]. Then the released heat gradually increased as the adsorption proceeded, attaining the constant value of 45–50 kJ mol − 1. Slightly adsorbed water on surface silanol groups hydroxylates siloxane near the silanol groups to give rise to silanol groups in succession [6]. Since the silanol groups would strongly interact with water molecules than siloxane, the adsorption heats would gradually increase with the progress of adsorption. The adsorption amounts where heats of adsorption reached the constant value were found to be equal to those where the amounts exhibited steep rise in the isotherms in Fig. 6. Afterwards 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 − 1 [23]. On the other hand, the differential heats of adsorption in the second run were ca. 70 kJ mol − 1 at the initial stage of adsorption, and then decreased gradually to 45–50 kJ mol − 1 with increase in the adsorbed amount of water. As already mentioned above, the adsorption isotherm of water suggests that the surface changes to more hydrophilic by the first water adsorption. Furthermore, IR results showed that a considerable amount of silanol groups formed on FSM-161073 K by exposure to water and remained even after evacuation at 823 K. Therefore, the high heats at initial stage in the second adsorption run would be due to a strong interaction between water molecules and surface silanol groups. These calorimetric measurements also suggest that once the water is being adsorbed, the hydrophilic property was retained even after evacuation at 823 K. In the case of FSM-16-823 K, the change in initial heats of adsorption at the first adsorption run was different from that of FSM-16-1073 K. Although the initial heats were ca. 20 kJ mol − 1 as well as in FSM-16-1073, the heats immediately reached the constant value of 45–50 kJ mol − 1. Klier and Zettlemoyer suggested the water adsorption mechanism on silica surface as follows [24]: water adsorbs with ‘oxygen down’ on the
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silanol groups (SiOH) at least at the beginning stages of adsorption. Then hydrogen-bonded clusters of water molecules may be formed even before all the silanol groups have adsorbed water molecules to form SiOHOH2 groups [6,24]. In the present study, the results of IR measurements and adsorption isotherm of water indicate that the surface of FSM-16-823 K is covered with more silanol groups than that of FSM-16-1073 K even before water adsorption. Therefore, water adsorption on both SiOH and SiOHOH2 would simultaneously take place as well as hydroxylation of siloxane by water adsorption. The adsorption energies of water molecule on a silanol group (SiOH) of dehydrated silica surface and on silanol group with adsorbed water (SiOHOH2) are 25 and 44 kJ mol − 1, respectively [6,23]. Therefore, the heat evolution at the initial stage immediately increased to 45–50 kJ mol − 1. The differential heats of adsorption of the second run showed the similar tendency as observed in FSM-16-1073 K, because the surface became hydrophilic by the first water adsorption.
4. Conclusion Pore structure of mesoporous silica FSM-16 is stable against heat treatment up to 1073 K in vacuo but collapsed by calcination at 1273 K. FSM-16 calcined at 1073 K has less silanol groups than that calcined at 823 K, although both FSM16 exhibit hydrophobic surface character regardless of calcination temperatures. The difference in calcination temperature affects adsorption behavior of water as well as rehydroxylation of FSM16. Once water is adsorbed on the freshly calcined FSM-16 surface, the surface becomes hydrophilic and original hydrophobic character is never restored even after evacuation at 823 K.
Acknowledgements 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.
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