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Preparation of a Water-Resistant Siliceous MCM-41 Sample, through Improvement of Crystallinity, and Its Prominent Adsorption Features Toshinori Mori,† Yasushige Kuroda,*,† Yuzo Yoshikawa,† Mahiko Nagao,‡ and Shigeharu Kittaka§ Department of Fundamental Material Science, Division of Molecular and Material Science, Graduate School of Natural Science and Technology, Okayama University, Tsushima, Okayama 700-8530, Japan, Research Laboratory for Surface Science, Faculty of Science, Okayama University, Tsushima, Okayama 700-8530, Japan, and Department of Chemistry, Faculty of Science, Okayama University of Science, Ridaicho, Okayama 700-0005, Japan Received July 5, 2001. In Final Form: November 16, 2001 We have prepared MCM-41 samples having good crystallinity and possessing a small amount of surface hydroxyl groups (0.48 OH nm-2) through extensive aging of a mixed solution of silica and surfactant in the temperature range from 308 to 413 K. The MCM-41 sample thus prepared exhibits excellent resistance to water, and its structure survives even after treating it in boiling water for more than 1 week. The surface and acidic properties of this sample were examined by X-ray diffraction, 29Si magic-angle spinning NMR, adsorption isotherms of dinitrogen and water molecules, and Fourier transform infrared measurements to determine the factors governing an adsorption feature and affecting the acidic property of the MCM-41 samples. The synthesized sample gives a sharp band centered at 3745 cm-1 tailing toward the lower wavenumber side. This band was deconvoluted into three components (3749, 3736, and 3715 cm-1), which were assigned to the stretching vibrations of the respective OH groups: a free OH (silanol), a geminal OH, and a terminal OH group of the hydrogen-bonded species. The data obtained by utilizing CO as a probe molecule provide useful information on the acidic nature of the surface OH groups. A linear relationship between ∆νOH (shift in wavenumber of OH stretching vibration) and νCO (stretching vibration of adsorbed CO) has been found to hold in the present system. It is concluded that the newly formed OH groups, which give strong IR bands at 3736 and 3715 cm-1, after water vapor and boiling water treatments, are responsible for the sites that are generally recognized as the strongly acidic sites existing in MCM-41, compared with the acidity of free silanol groups. The freshly prepared sample shows characteristic behavior in the differential heat of adsorption of water, qdiff, due to lateral interaction of the adsorbed water molecules. This is a different behavior from water adsorbed on the hydrated and boiled samples, indicating the homogeneous nature of the surface of the as-prepared sample for adsorbing water molecules. All properties of such materials should be discussed by taking account of the types of surface OH groups; the surface of MCM-41 having a smaller number of acid sites exhibits a prominent feature for water adsorption. From our results, it was concluded that the surface crystallinity of the sample plays a pivotal role in protecting the sample against attack by water and also in the adsorption behavior of adsorbates, such as water and CO molecules.
Introduction In recent years, new materials have been prepared, such as MCM-41 and FSM, which have larger pore sizes than zeolite groups.1,2 These materials are characterized by having highly uniform mesopores and have attracted much interest because of their potential application as a reaction pot in the design of nanostructured materials for catalytic application or in separations involving bulky molecules.3-6 The crucial problem is that the MCM-41 material is very * To whom correspondence should be addressed. E-mail:
[email protected]. † Department of Fundamental Material Science, Division of Molecular and Material Science, Graduate School of Natural Science and Technology, Okayama University. ‡ Research Laboratory for Surface Science, Faculty of Science, Okayama University. § Department of Chemistry, Faculty of Science, Okayama University of Science. (1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Vartuli, J. C. Nature 1992, 359, 710. (2) Inagaki, S.; Fukushima, Y.; Kuroda, K. J. Chem. Soc., Chem. Commun. 1993, 680. (3) Corma, A. Chem. Rev. 1997, 97, 2373. (4) Ying, J. Y.; Mehnert, C. P.; Wong, M. S. Angew. Chem., Int. Ed. 1999, 38, 57. (5) Thomas, J. M. Angew. Chem., Int. Ed. Engl. 1999, 38, 3588. (6) Kageyama, K.; Tamazawa, J.; Aida, T. Science 1999, 285, 2113.
sensitive to moisture; treating it either in steam or in hot water easily destroys its structure. Surface silylation is an efficient way to prevent disintegration of the surface.7,8 To put it the other way around, the active sites in various phenomena are lost during the silylation procedure. From such viewpoints, the preparation of mesoporous silica which exhibits good hydrothermal stability is one of the most important requirements.9-19 This is also essential (7) Koyano, K. A.; Tatsumi, T.; Tanaka, Y.; Nakata, S. J. Phys. Chem. B 1997, 101, 9463. (8) Zhao, X. S.; Lu, G. Q. J. Phys. Chem. B 1998, 102, 1556. (9) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmidtt, 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. (10) Kim, J. M.; Kwak, J. H.; Jun, S.; Ryoo, R. J. Phys. Chem. 1995, 99, 16742. (11) Ryoo, R.; Kim, J. M.; Shin, C. H. J. Phys. Chem. 1996, 100, 17718. (12) Ryoo, R.; Jun, S. J. Phys. Chem. B 1997, 101, 317. (13) Kruk, M.; Jaroniec, M.; Sayari, A. Microporous Mesoporous Mater. 1999, 27, 217. (14) Mokaya, R. J. Phys. Chem. B 1999, 103, 10204. (15) Das, D.; Tsai, C. M.; Cheng, S. Chem. Commun. 1999, 473. (16) Chen, L.; Horiuchi, T.; Mori, T.; Maeda, K. J. Phys. Chem. B 1999, 103, 1216. (17) Sayari, A.; Yang, Y. J. Phys. Chem. B 2000, 104, 4835. (18) Liu, Y.; Zhang, W.; Pinnavania, T. J. J. Am. Chem. Soc. 2000, 122, 8791.
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to ensure successful postsynthesis modification such as grafting and ion exchange, because such processes are generally carried out in aqueous solution. From the viewpoint of material as a microreactor, it is important to study the state of the sample surface as well as the adsorbed molecules in such a confined space.20-26 Such properties are expected to be different depending on the surface state of the samples, that is, the amount and kinds of hydroxyl groups. However, there have been only a few attempts to determine the effect of the surface state of such mesoporous materials on the adsorption properties, relating to the existence of surface acidic groups. In the present paper, we are concerned with preparing a sample that has excellent resistance to water and with elucidating the surface properties of this sample, as well as samples treated with water under various conditions, and we are also concerned with explaining the properties of the adsorbed molecules in a confined space, based on heats of adsorption of water, solid state 29Si NMR, and X-ray diffraction (XRD) measurements, as well as Fourier transform infrared (FT-IR) measurements using CO as a probe molecule. Experimental Section Samples. The MCM-41 sample was prepared according to the similar method reported by several authors.1,27,28 A fumed silica sample (aerosil 200) and hexadecyl-trimethylammonium bromide (CTABr) were used as the silicon source and as the template organic reagent, respectively. The silica-surfactant gel was aged at 308 K for 3 days and then treated at 413 K for an additional 3 days to improve surface crystallinity.29 The sample thus prepared was thoroughly washed with distilled water. The sample dried at ambient temperature was finally calcined at 873 K for 6 h in air to remove an organic template. This as-prepared sample, sample A, was treated under the circumstance of humidity of 80% for 12 h (abbreviated as sample B) or treated in boiling water for 3 days (sample C). X-ray Measurements. X-ray powder diffraction measurements were performed on a Rigaku RAD-1C diffractometer using Cu KR radiation. NMR Measurements. The solid-state 29Si NMR spectra of several samples were recorded on a Varian Unity Inova 300 spectrometer using magic-angle spinning (MAS) with or without cross-polarization (CP). Measurement conditions were as follows: resonance frequency, 59.59 MHz; pulse width, 4.2 s; pulse delay, 90 s for MAS and 10 s for CP-MAS; spinning rate, 3.5 kHz. The values of chemical shift were determined relative to the value of tetramethylsilane as an external standard. IR Measurement. IR spectra were measured by a JEOL JIR100 spectrophotometer with an MCT detector. The sample was pressed under a pressure of 40 kg cm-2 to a self-supporting disk and was evacuated at 300 or 623 K for 4 h under a reduced (19) Mokaya, R. Chem. Commun. 2001, 933. (20) Hansen, E. W.; Schmidt, R.; Stocker, M.; Akporiaya, D. J. Phys. Chem. 1995, 99, 4148. (21) Hansen, E. W.; Sto¨cker, M.; Schmidt, R. J. Phys. Chem. 1996, 100, 2195. (22) Schmidt, R.; Hansen, E. W.; Sto¨cker, M.; Akporiaye, D.; Ellestad, O. H. J. Am. Chem. Soc. 1995, 117, 4049. (23) Morishige, K.; Nobuoka, K. J. Chem. Phys. 1997, 107, 6965. (24) Holly, R.; Peemoeller, H.; Choi, C.; Pintar, M. M. J. Chem. Phys. 1998, 108, 4183. (25) Morishige, K.; Kawano, K. J. Chem. Phys. 1999, 110, 4867. (26) Takahara, S.; Nakano, M.; Kittaka, S.; Kuroda, Y.; Mori, T.; Hamano, H.; Yamaguchi, T. J. Phys. Chem. B 1999, 103, 5814. (27) Cheng, C.-F.; Park, D. H.; Klinowski, J. J. Chem. Soc., Faraday Trans. 1997, 93, 193. (28) Cheng, C.-F.; Zhou, W.; Park, D. H.; Klinowski, J.; Hargreaves, M.; Gladden, L. F. J. Chem. Soc., Faraday Trans. 1997, 93, 359. (29) Treatment of the MCM-41 sample at high temperature brings about the change in the crystallinity of the surface as well as in the structure of the bulk lattice. This process relates to both the number and nature of silanol groups and also local atomic ordering. It is expected that surface ordering provides a stable structure, resulting in the decrease in the number of surface OH groups.
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Figure 1. Powder XRD patterns of the respective samples. pressure of 1 mPa. The spectra were recorded under various pressures of CO at 123 K by using an in situ cell, which was capable of treating in vacuo at 873 K and adsorbing various gases at 100 K in situ conditions. Isotherm, Adsorption Heats, and Water Content Measurements. The measurement of the adsorption isotherm of N2 was carried out volumetrically. Before measurement, every sample was evacuated at 301 K for 4 h under a pressure of 1 mPa. The measurement of adsorption heats of water vapor was performed at 301 K by direct calorimetry.30 The water contents of the samples were determined by the successive ignition loss method.
Results and Discussion Basic Characterizations of the MCM-41 Samples. The XRD patterns of the samples (evacuated at 300 K) exhibit four well-resolved bands that can be indexed as 100, 110, 200, and 210 reflections associated with hexagonal symmetry of the lattice, although that of sample C gives a slight broadening of each band (Figure 1). Detailed examination of the XRD and N2 adsorption data (shown below) reveals that the pore size decreases on water treatment. Transmission electron microscope images of the samples revealed the typical hexagonal honeycomb structure of MCM-41, whether treated or not treated with boiling water (not shown). The most striking aspect of the sample used in this experiment is that the sample keeps its original honeycomb structure even after treatment in boiling water, that is, it shows remarkable stability against hydrothermal treatment, although the sample usually utilized loses its structure easily with this procedure. Three runs of N2 adsorption at 77 K on these three samples were conducted, and the corresponding isotherms are shown in Figure 2, together with the results of pore analysis performed by applying the Dollimore and Heal method.31 The adsorption behavior of samples A and B for N2 is similar; the jump appears in the relative pressure region of 0.35-0.40 due to the filling of uniform size pores composed of the hexagonal lattice with N2. Correspondingly, the pore size distribution shows a narrow uniform mesopore at a mean value of 17 Å. For sample C, the adsorbed amount is depressed in the initial adsorption stage (in the range of nearly zero pressures), and the (30) Matsuda, T.; Taguchi, H.; Nagao, M. J. Therm. Anal. 1992, 38, 1835. (31) Dollimore, D.; Heal, G. R. J. Appl. Chem. 1964, 14, 109.
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Figure 3. Water contents of the silica and MCM-41 samples: (0) original aerosil; (9) rehydrated aerosil; (~) aerosil sample treated in boiling water; (O) sample A; (b) sample B; (.) sample C.
Figure 2. Isotherms of N2 adsorption (open symbols) and desorption (filled symbols) on MCM-41 samples treated in various conditions and mesopore size distribution data for the respective samples. Table 1. Parameters Estimated from XRD and Adsorption Isotherm Data samples
d100/Å
rave/Å
wall thickness/Å
specific surface area/m2 g-1
sample A sample B sample C
42.1 41.6 36.3
16.9 16.7 15.0
14.8 14.6 11.9
968 862 865
relative pressure where the jump appears is lower (0.250.40) than that for the other two samples, as well as the isotherm having a gentle slope in the jump region. In addition, if examined in detail, in the final adsorption region the rate of increase in the amount adsorbed on sample C seems to be larger than that on the others. This indicates the collapse of crystal and simultaneous formation of amorphous particles that form the macropores. The last fact corresponds to the decrease in intensity observed in the XRD pattern. The thickness of the wall of the MCM-41 lattice can be calculated using both XRD and isotherm data, and the values are summarized in Table 1. The surface hydroxyl content of the MCM-41 and silica (aerosil 200) samples treated in various ways is shown in Figure 3 as a function of evacuation temperatures. Relating to the water-resistant feature of sample A, its
water content is evaluated to be 0.48 OH nm-2, being much smaller than that reported so far for MCM-41 and FSM samples,32,33 at 1.40 OH nm-2, and also for silica samples.34-36 This value increases for sample B and of course for sample C. However, the amounts are not as large for these as for silica (aerosil 200) samples treated under the same conditions. We should note that the desorption behavior in the MCM-41 samples is roughly separated into two regions: the lower temperature range of 300-573 K and the higher range from 573 to 1173 K. NMR Study. To obtain information on the state of the MCM-41 lattice during the hydration process, 29Si MAS NMR spectra for samples that were freshly calcined (sample A), treated with water vapor (sample B), or boiled in distilled water for 3 days (sample C) were measured; these are shown in Figure 4. The spectrum for sample A gives a broad band at -108 ppm, together with a faint shoulder at -98 ppm. These two bands can be assigned to Q4 silicon species forming the lattice and to Q3 silicon species existing in the vicinity of the surface.32,37 The latter band was observed for sample B as a definite shoulder. The intensities of the Q3 species increased with an increase in time of treatment with water. The Q3 signal is clearly seen as a resolved signal under hydrothermal conditions (sample C). These data indicate that a highly ordered surface exists on the sample and that the hydration proceeds when the sample is treated with water vapor or hydrothermally in water. 29 Si CP-MAS NMR measurements were performed to obtain detailed information on the change in the state in the vicinity of the surface. As shown in Figure 5, the spectrum of sample A gives a weak single band at -98 ppm (Q3 species), which was accompanied by extremely weak shoulders at around -108 and -89 ppm; these two (32) Zhao, X. S.; Lu, G. Q.; Whittaker, A. K.; Millar, G. J.; Zhu, H. Y. J. Phys. Chem. B 1997, 101, 6525. (33) Ishikawa, T.; Matsuda, M.; Kandori, K.; Inagaki, S.; Fukushima, T.; Kondo, S. J. Chem. Soc., Faraday Trans. 1996, 92, 1985. (34) Zhuravlev, L. T. Langmuir 1987, 3, 316. (35) Burneau, A.; Barres, O.; Gallas, J. P.; Lavalley, J. C. Langmuir 1990, 6, 1364. (36) Legrand, A. P. The Surface Properties of Silicas; John Wiley & Sons: Chichester, 1998; p 61. (37) Chuang, I.-S.; Maciel, G. E. J. Phys. Chem. B 1997, 101, 3052.
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Figure 4.
29
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Si MAS NMR spectra of the respective samples.
Figure 6. IR spectra in the region of the OH stretching vibration of the respective MCM-41 samples. The deconvoluted curves of the spectra are also indicated. All samples were evacuated at 300 K.
Figure 5.
29Si CP-MAS NMR spectra of the respective samples.
components can be ascribed to Q4 and Q2 silicon species. For sample B, a new band was observed at -89 ppm due to a Q2 species, in addition to the strong Q3 band and a distinctive shoulder at -108 ppm. The three-band nature is clearly seen for sample C. A large portion of the band is composed of Q3 species as for sample A, indicating that the surface species are dominantly composed of the singletype silanol species. For the rehydrated sample, the band of the Q2 species appears weakly, indicating the existence of geminal species. In addition, it was shown that the signal intensity of the Q2 species for sample C becomes strong, compared with that for sample B. These data clearly indicate that the condensation reaction in the surface region proceeds to form a stable siloxane bond for the original sample A. As for sample C, the hydrolysis of the siloxane bond took place on the surface and resulted in the formation of a new hydroxylated species, the geminal-type silanol group due to cleavage of siloxane bonds, as well as an increase in quantity of the singletype silanol groups. These considerations are also substantiated by the results deduced from water content data. Spectroscopic Characterization Using CO Molecules as a Probe. The IR spectrum of the original sample
evacuated at 300 K gives a sharp band at around 3745 cm-1, which is accompanied by tailing toward lower wavenumbers (Figure 6). This band was resolved into at least three components by applying the deconvolution technique, 3749, 3736, and 3715 cm-1; the resultant respective spectra are also shown in Figure 6.38 In addition to these bands, a weak and broad band distinctively appears in the 3600-3400 cm-1 region, indicating the existence of hydrogen-bonded species. The three resolved bands are assigned to the single OH, geminal OH, and terminal OH groups of hydrogen-bonded species by reference to the papers so far reported.39-44 The first species is clearly seen, and the other two components are also suggested by the NMR experiments described above. It is also clearly seen in the IR spectrum of sample B that the amounts of geminal and hydrogen-bonded species increase significantly. These tendencies correspond well to the appearance of the distinctive band and shoulder due to the Q3 and Q2 species found in the CP-MAS NMR data. As for the boiled sample, IR data also demonstrate the formation of hydrogen-bonded and geminal species, being consistent with the increase in the intensity of the Q3 and (38) The deconvolution of the IR bands observed in both stretching vibrational regions of OH and CO was performed on the basis of the overall knowledge on the location of the OH and CO stretching vibrational bands of the respective samples. (39) Morrow, B. A.; Cody, I. A. J. Phys. Chem. 1975, 79, 761; 1976, 80, 1995, 1998. (40) Hoffmann, P.; Kno¨zinger, E. Surf. Sci. 1987, 188, 181. (41) Morrow, B. A.; McFarlan, A. J. Langmuir 1991, 7, 1695: J. Phys. Chem. 1992, 96, 1395. (42) Voort, P. V. D.; Gillis-D’Hamers, I.; Vrancken, K. C.; Vansant, E. F. J. Chem. Soc., Faraday Trans. 1991, 87, 3899. (43) Ghiotti, G.; Garrone, E.; Morterra, C.; Boccuzzi, F. J. Phys. Chem. 1979, 83, 2863. (44) Zecchina, A.; Bordiga, S.; Spoto, G.; Marchese, L.; Petrini, G.; Leofanti, G.; Padovan, M. J. Phys. Chem. 1992, 96, 4985, 4991.
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Figure 7. IR spectra of the as-synthesized sample (sample A) in the OH stretching vibrational region (a), difference spectra after progressive CO introduction at 123 K (b), and the perturbed stretching vibrational band of CO, νCO, during the successive CO adsorption (c): (1) zero pressure; (2) 67 Pa; (3) 133 Pa; (4) 399 Pa; (5) 2.00 kPa; (6) 3.33 kPa; (7) 6.65 kPa; (8) 13.3 kPa. The sample was preevacuated at 623 K.
Figure 8. IR spectra of the rehydrated MCM-41 (sample B) in the OH stretching vibrational region (a), difference spectra after progressive CO introduction at 123 K (b), and the perturbed stretching vibrational band of CO, νCO, during the successive CO adsorption (c): (1) zero pressure; (2) 67 Pa; (3) 133 Pa; (4) 399 Pa; (5) 2.00 kPa; (6) 3.33 kPa; (7) 6.65 kPa; (8) 13.3 kPa. The sample was preevacuated at 623 K.
Q2 bands. These observations help to account for the result on the water content. As pointed out earlier, the water content data show the existence of at least two kinds of site: the range between 300 and 573 K and the region higher than 573 K. Both single and geminal silanols participating in hydrogen bonding are most easily dehydroxylated under evacuation at temperatures between 300 and 573 K to form low-strain siloxane bonds. The free and geminal silanols resist dehydroxylation and heat treatment between 573 and 873 K. The treatment at the temperature above 573 K causes dehydroxylation from these species, forming highly strained siloxane bridges after evacuation at higher temperatures. It is now worthwhile examining how the acid strength depends on the surface states of the MCM-41 samples. A polar molecule, carbon monoxide, is a suitable probe for delineating the role played by acid sites formed in host materials, such as zeolites and aluminophosphates.45,46 It
has been extensively employed to analyze the states of silanols, as well as Brønsted acid sites, which act as binding sites for a CO molecule.47-49 Hence, we intend to discriminate the states of the acidic (adsorption) sites of MCM-41 by utilizing the position of both vibrational bands of the adsorbed CO molecules and of OH formed on siliceous MCM-41, as well as the relation between them. In Figures 7-9, the spectra of the respective MCM-41 samples previously evacuated at 623 K and successively contacted with various pressures of CO at 123 K are depicted in both the OH and CO stretching vibrational (45) Smith, L.; Cheetham, A. K.; Gianotti, E. Catal. Lett. 1996, 41, 13. (46) Makarova, M. A.; Ojo, A. F.; Karim, K.; Hunger, M.; Dwyer, J. J. Phys. Chem. 1994, 98, 3619. (47) Beebe, T. P.; Gelin, P.; Yates, T., Jr. Surf. Sci. 1984, 148, 526. (48) Lercher, J. A.; Gru¨ndling, C.; Eder-Mirth, G. Catal. Today 1996, 27, 353. (49) Busca, G. Phys. Chem. Chem. Phys. 1999, 1, 723.
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Figure 9. IR spectra of the MCM-41 sample treated in boiling water (sample C) in the OH stretching vibrational region (a), difference spectra after progressive CO introduction at 123 K (b), and the perturbed stretching vibrational band of CO, νCO, during the successive CO adsorption (c): (1) zero pressure; (2) 67 Pa; (3) 133 Pa; (4) 399 Pa; (5) 2.00 kPa; (6) 3.33 kPa; (7) 6.65 kPa; (8) 13.3 kPa. The sample was preevacuated at 623 K.
regions, 3800-3400 and 2250-2050 cm-1. They are also depicted with the difference spectra in the OH stretching region between the respective CO-adsorbed spectra and the spectrum for the sample evacuated at 623 K. In the first dose of CO at an equilibrium pressure of 67 Pa on sample A, the spectrum shows a decrease in the band intensity at around 3740 cm-1, accompanied by the simultaneous appearance of a band in the region of 37003600 cm-1. If we examine the spectrum in detail, the decrease in intensity of the lower wavenumber band (3736 cm-1) occurs in preference to the higher wavenumber side (3749 cm-1). The band intensity at 3740 cm-1 decreases at the beginning of CO adsorption, accompanied by the appearance of a band at 3750 cm-1. This behavior is clearly seen in the difference spectra (spectra 2 and 3) shown in Figure 7b. After passing these adsorption regions, the 3749 cm-1 band gradually loses its intensity. It is accompanied by a new strong and broad band at around 3670 cm-1 that increases with the pressure of CO; the OH band gives a smaller shift in wavenumber (∆νOH) compared with that in the initial adsorption stage. The decrease in the band intensity at 3736 cm-1 that is observed in the initial adsorption process is well explained by considering the occurrence of the interaction of the OH species with an approaching CO molecule through hydrogen bonding of type II shown in Scheme 1. One of the geminal-type OH species interacts with CO molecules leaving another OH species free, accompanying the appearance of the 3750 cm-1 band at the initial adsorption stage. This consideration is also supported by the NMR data; the geminal species was detected weakly in the CP-MAS spectrum. Hoffmann and Kno¨zinger,40 as well as Morrow and McFarlan,41 also proposed a similar assignment of the band observed at 3736 cm-1 for a silica sample. The band at 3749 cm-1, which is assigned to the stretching vibration of free silanol groups, decreases in intensity through interaction with the approaching CO molecules, especially in the higher equilibrium pressure regions. The ν value of OH species interacting with CO in the initial adsorption region is slightly but distinctly lower than that for free OH (single-type) species, as well as giving larger shifts of the ν value when CO was adsorbed. This leads us to conclude that the acidity of the proton positioned on the
Scheme 1
geminal silanol is stronger than that on the single silanol group. To focus on the CO band, the adsorbed CO band was simultaneously observed at around 2158 cm-1. This band increases in intensity with increasing pressure in the initial adsorption region; simultaneously, the wavenumber of this band slightly shifts toward a lower wavenumber with increasing equilibrium pressures. In the higher pressure region, a strong new band appears at around 2140 cm-1, which is ascribable to the physisorbed CO species. The spectrum of sample B has a broadened nature at around 3736 cm-1, in addition to the sharp band at 3745 cm-1 (Figure 8). The shoulder band at 3715 cm-1 was
Water-Resistant Siliceous MCM-41 Sample
clearly seen, which may be due to the terminal hydrogenbonded species (type III in Scheme 1). When CO was adsorbed on the sample, a new shoulder was discernible at around 3600 cm-1. Other adsorption features seem to be similar to those observed for sample A. The behavior in the shift of the top of the OH stretching band with the pressure of CO is shown in the inset of Figure 8. The spectrum for sample C is more complicated due to the existence of a large number of OH species (Figure 9). Taking into account the interpretation of the data of samples A and B, the most characteristic feature is the appearance of the strong band at 3715 cm-1. In this case, the 3600 cm-1 band is clearly seen after CO adsorption. By considering both the increase in the amount of OH species and the NMR data, we are able to assign this band to the interaction of CO with the terminal hydrogenbonded OH species. With regard to samples B and C, the intensity of the band at around 3730 cm-1 increases, resulting in the appearance of the distinct band at 3640 cm-1 after CO adsorption. To understand these points easily, the behavior in the shift of the top of the OH stretching band of sample C with the pressure of CO is also shown in the inset of Figure 9. In addition, several bands described above in the difference spectra depending on the equilibrium pressures can be seen. As a result, we can recognize at least three types of OH groups giving absorption bands at 3715, 3736, and 3749 cm-1. They interact with CO to different extents, resulting in the band shift to 3600, 3640, and 3670 cm-1, respectively. The assignment for the respective bands is summarized in Scheme 1. These facts indicate that the geminal and terminal OH species increase their amounts by the rehydration procedure. The other important point for the adsorption properties related to these samples is that the amount of physisorbed CO seems to decrease with increasing surface roughness, with an increase in the degree of surface hydration. However, the band intensities due to the strongly adsorbed species (at around 2158 cm-1) show an opposite tendency; the surface electric field is stronger for the more hydrated sample. The perturbation of the hydroxyl groups existing in acidic materials with various kinds of weak bases was used to establish the correlation of the bathochromic shift in the OH stretching vibration with the basicity of the adsorbed molecules by Dwyer et al.46 The acidity of surface hydroxyl groups should also relate to the position of the CO stretching vibrational band, as well as to the relation between ∆νOH and νCO. In the case of the CO molecule used as a weak basic probe, the stretching frequency of CtO is changed upward (with respect to the 2143 cm-1 value for gas-phase CO)50 in C-bonded species, because of the positive electric charge localized on the acidic proton. The appearance of a CO band at around 2158 cm-1 in the present systems is associated with the formation of hydrogen-bonded species, tSi-OH‚‚‚CtO. The observed band position for the CO species gradually shifted to the lower wavenumber side with increasing coverage, indicating that CO interacts initially with the species of higher acidity and then with increasingly weaker species. Olivier Cairon et al. have performed interesting work on the zeolite systems and have proposed that a linear relationship holds between ∆νOH and νCO, when CO was used as the probe molecule.51,52 We described above the existence of at least three types of OH sites. Hence, we deconvoluted the bands (50) Ewing, G. E. J. Chem. Phys. 1962, 37, 2250. (51) Cairon, O.; Chevreau, T. J. Chem. Soc., Faraday Trans. 1998, 94, 323. (52) Cairon, O.; Chevreau, T.; Lavalley, J.-C. J. Chem. Soc., Faraday Trans. 1998, 94, 3039.
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Figure 10. IR spectra of CO adsorbed on sample C under the pressure of 13.3 kPa. On the basis of the assumption described in text, the spectrum was deconvoluted into three components: (1) observed spectrum; (2) spectrum due to physisorbed CO species; (3) difference spectrum between (1) and (2); (4)-(6) the resulting deconvoluted curves (three respective components).
observed in the CO stretching vibrational region using the following procedure; the results are shown in Figure 10. (1) The band due to the physisorbed CO species (Figure 10, spectrum 2) was subtracted from the original spectrum according to the method proposed by Yates et al.47 (2) We assumed that the band due to the strongly adsorbed CO species consists of three components as deduced from the NMR data on the Si species as well as IR bands observed in the OH stretching region. In the deconvolution procedure, the respective three bands due to respective adsorbed CO species were obtained from the subtracted spectra described in (1): one distinctive peak and two shoulder bands (Figure 10, spectrum 3). (3) The half-width of each band was assumed to be proportional to that of the respective bands obtained in the OH stretching region. We initially applied this procedure to the spectrum for sample C, because the contribution due to physisorbed CO species is the smallest among the samples studied. The resultant spectra are shown in Figure 10 and are composed of three components: 2160, 2156, and 2152 cm-1. One of the separated bands (2160 cm-1) thus obtained was consistent in its position with the band that appears in the initial adsorption region. The same procedures were applied to the other two samples, and the observed spectra were well reproduced from these three deconvoluted spectra. After deconvolution of the spectra, we applied Cairon’s relation to the MCM-41 system, and the resulting data are shown in Figure 11, together with their data for the zeolite system. The plot almost obeys the linear relationship proposed by them: the acidic property of the MCM-41 samples used in the experiment was well explained on the similar basis of the acidity of the zeolite system. The pair of bands, 3600 cm-1 (∆νOH ) -115 cm-1) and 2160 cm-1, results from the H-bonding interaction between the terminal OH and CO of the type tSiOH‚‚‚ CtO (Scheme 1, type III). The next one is ascribed to the interaction between the geminal OH and CO (type II) and the last one, to the free silanol with CO (type I). In fact, the largest shift, ∆νOH ) -115 cm-1, observed in the present system is smaller than the shift observed in the zeolite system (Figure 11). Furthermore, the wavenumber of the OH band is higher than that for ZSM-5, indicating that the acidity of the MCM-41 sample is weak compared with that of ZSM-5 (Brønsted acid site). It is seen that the species giving a relatively high acidic property53 increases
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Figure 11. Correlation between the decrease in wavenumber of νOH bands of respective components and νCO after CO adsorption on various samples: (b) terminal band; (9) geminal; ([) free silanol. (Other data were quoted from ref 52.)
in amount with an increase in the degree of hydration. Hence, we need to obtain information on the acidic property of the samples through measurement of the IR spectra at lower temperatures, by using an adsorbate such as a CO molecule when we discuss the adsorption properties of MCM-41 samples or we use the MCM-41 sample efficiently as a catalyst in various processes. It can be concluded from the above discussion that (1) the OH species indicating strong acidity are scarcely found on the original sample, although free single-type silanol exists; (2) the hydrated and boiled samples bring about a relatively strong acidic nature, although not as strong as the Brønsted acid sites on a ZSM-5 sample; (3) the amount of acidic sites increases through the watertreatment procedures; and (4) the amount of physisorbed CO species seems to be large for sample A and small for sample C, depending on the surface states. The Water Molecule as a Useful Surface-Analyzing Probe. Figure 12 shows the adsorption isotherms and differential heats of adsorption, qdiff, of water on the MCM41 samples after evacuating at 301 K. Every isotherm exhibits a typical adsorption curve of type IV. When physisorption on sample A begins, the adsorbed amount increases with increasing equilibrium pressures. This increase flattens off, followed by a sudden increase at a relative pressure of 0.6, and finally gives an almost constant value at a relative pressure of 0.8.54 It is clearly seen that the heat of adsorption is about 40 kJ mol-1 at the initial stage, increases steeply up to 80 kJ mol-1 corresponding to the appearance of the step in the adsorption isotherm, and finally converges to a value of ca. 85 kJ mol-1 in the final adsorption stage. The initial heat of adsorption is slightly but distinctly lower than the heat of liquefaction of water (44.5 kJ mol-1), indicating that the MCM-41 surface has a hydrophobic nature, in contrast to the case of ordinary metal oxide samples or even the usual silica samples.55-57 The phenomenon, such (53) The term “high acidity or strongly acidic” used in the present article means that the acidity is stronger, compared with the acidity of free silanol groups. (54) Zhao, X. S.; Audsley, F.; Lu, G. Q. J. Phys. Chem. B 1998, 102, 4143. (55) Arnett, E. M.; Ahsan, T. J. Am. Chem. Soc. 1991, 113, 6861.
Mori et al.
Figure 12. Differential heats of adsorption (upper part) and adsorption isotherms (lower part) of water at 301 K on the respective samples.
high adsorption heats as 80 kJ mol-1, observed in the present system (for the sample A) is very strange and also very interesting. This phenomenon admits two interpretations: condensation of water, that is, lateral interaction, or an onset of hydrophilicity. If hydrophilicity is the main reason for the present investigation, the initial heats in the second adsorption process should give such higher heats as 80 kJ mol-1. However, this is not the case; we observed no such higher heats in the initial region and, of course, even higher heats for the samples B and C (shown below). In addition, it is well-known that the time taken to reach the equilibrium of the reaction of water with the silica surface is long. In the present study, the equilibrium of the adsorption of water was established within 30 min, and any additional heat was not observed after passing 30 min. Therefore, the observed larger values of heat of adsorption compared with the heat of liquefaction of water in the region where the steep increase of the adsorbed amount appears suggest the existence of lateral interaction among adsorbed water molecules, in addition to the interaction between water molecules and the pore wall.58 The last stage of the adsorption after filling in the pore can be interpreted in terms of the adsorption of water onto the external surface. The adsorption isotherm and differential heat values for the rehydrated sample, sample B, show different adsorption behavior from that of sample A: (1) both the initial adsorption amount and heat of adsorption are slightly larger than those for sample A; and (2) the pressure bringing about the steep rise in the adsorbed amount is lower than that for sample A, indicating that adsorbed water molecules feel a stronger potential from the wall than those in sample A. The heats of adsorption arising from the lateral interaction can be barely observed (qdiff ) ca. 55 kJ mol-1). Quite different behavior in qdiff values is observable for sample C, the sample treated in boiling water, in addition to the fact that the isotherm step appears at a slightly lower pressure for the case of sample B. The initial heats of adsorption give a value of 50 kJ mol-1, which is larger than that of the liquefaction of water. With regard to the lateral interaction, it is almost absent; the (56) Bolis, V.; Fubini, B.; Marchese, L.; Martra, G.; Costa, D. J. Chem. Soc., Faraday Trans. 1991, 87, 497. (57) Fubini, B.; Bolis, V.; Cavenago, A.; Ugliengo, P. J. Chem. Soc., Faraday Trans. 1992, 88, 277. (58) Thomy, A.; Duval, X.; Regnier, J. Surf. Sci. Rep. 1981, 1, 1.
Water-Resistant Siliceous MCM-41 Sample Chart 1
heats of adsorption are about 50-55 kJ mol-1 in all adsorption regions. The systematic decrease in the relative pressure where the sharp increase in adsorption occurs with increasing treatment of water indicates that the surface of the pore wall disintegrated readily during treatment with water, coinciding with the result derived from both XRD and N2 isotherms.
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The prominent adsorption behavior of water on sample A, the existence of lateral interaction, is explained by noting that water molecules adsorbed on sample A interact with each other through the weak interaction with surface OH groups and concurrently through the strong interaction with the neighboring water molecules, in contrast to that of ordinary metal oxides and usual silica samples. This is because the surface of this MCM-41 sample is slightly hydrophobic, as well as having an energetically homogeneous surface. Such a hydrophobic surface changes to a hydrophilic surface through treatment with water vapor and also in boiling water, reducing the degree and magnitude of the lateral interaction among water molecules. We can transform the adsorbed amount, corresponding to the jump height, to the number of molecules, and it gives ca. 12 molecules nm-2. In addition, the number of OH groups existing on the surface of sample A, chemisorbed species, is 0.48 OH nm-2. Therefore, the ratio of H2O/OH is calculated to be about 25, the average size of a cluster that is formed on this surface. The ratio for samples B and C was evaluated to be 15 and 8, respectively. Therefore, on sample A, the adsorbed water molecules interact weakly with the chemisorbed OH species and simultaneously the lateral interaction operates among adsorbed molecules, resulting in the formation of water clusters in the pore that probably takes an icelike structure. However, the samples treated with water vapor or in boiling water fix the water molecules by the interaction with the surface OH groups existing on the surface in a large amount, depressing the formation of hydrogen bonding among water molecules confined in the pore. Such states may be tentatively represented by the models presented in Chart 1. From these considerations, it is reasonable to suppose that the surface having the smaller number of acid sites is a necessary condition for the appearance of the prominent feature of heat of adsorption of water on MCM-41. As a result, it can be seen that the adsorption isotherm and heats of adsorption of water provide important information on the surface properties of the MCM-41 sample. They also provide a method to discriminate the surface states, depending on the samples. Acknowledgment. This work was supported in part by a grant from the Foundations of Wesco Co. We are grateful to the SS-NMR Division of the VB Laboratory of Okayama University for 29Si MAS NMR experiments. LA011019Y
