J. Phys. Chem. C 2007, 111, 999-1004
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Super Hydrophobic Mesoporous Silica with Anchored Methyl Groups on the Surface by a One-Step Synthesis without Surfactant Template Dongjiang Yang, Yao Xu, Dong Wu, and Yuhan Sun* State Key Laboratory of Coal ConVersion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, P. R. China
Huaiyong Zhu* School of Physical and Chemical Sciences, Queensland UniVersity of Technology, Brisbane QLD 4001, Australia
Feng Deng State Key Laboratory of Magnetic Resonance and Atomic Molecular Physics, Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, P. R. China ReceiVed: September 7, 2006; In Final Form: October 19, 2006
Mesoporous silicas with methyl groups anchored on the pore wall by covalent bonds were prepared using polymethylhydrosiloxane and tetraethylorthosilicate as silica sources via a one-step synthesis approach. No surfactants were introduced in the synthesis and the pore size can be tuned readily by adjusting the amount of polymethylhydrosiloxane used in the synthesis. Generally, such a hybrid structure is achieved by two steps: synthesizing mesoporous silica substrate using surfactant as template and grafting the pore wall surface with an organic silane. The synthesis in this study is based on a nonsurfactant route and thus is distinctly different from other synthesis routes in reaction mechanism. The obtained silicas exhibit superior hydrophobicity: their contact angles with water are all beyond 150° because the surface is covered by methyl groups. The hydrophobic silicas can be used as efficient adsorbents for cleaning up spills of oil or organic chemicals on the water surface and toxic organic pollutants in water, such as alkylphenols, at very low concentrations.
Introduction The family of mesoporous hybrids of silica and organics generally includes mesoporous materials of silica-based frameworks and pore wall surface functionalized with chemically bonded organic groups, mesophases with organic/silica frameworks, and mesoporous silicas with occluded organic materials such as polymers.1 The mesoporous silicas functionalized with organic groups have profound potential for applications in the field of catalysis and environmental remediation.2 These hybrid solids are generally obtained by a template synthesis of the mesoporous silica framework and a subsequent grafting of organic groups on the silica surface.2-6 Ionic7,8 or neutral9-11 surfactants have been widely used as templates for the synthesis of silicas with uniform mesopores, which direct the mesophases formation by electrostatic or hydrogen bonding interactions. Organic compounds other than surfactants have also been employed as templates. Wei et al.12,13 used dibenzolyl-L-tartaric acid, D-maltose, and D-glucose as templates to prepare mesoporous silica materials and gold-silica nanocomposites through an HCl-catalyzed sol-gel reaction process. Jansen et al.14 obtained three-dimensional mesopore networks using triethanolamine as template. It is found that mesoporous silica materials could be prepared using tartaric acid in conjunction with metal chlorides as templates and tetraethylorthosilicate * To whom correspondence should be addressed. E-mail: yhsun@ sxicc.ac.cn.
(TEOS) as silicon source, and the pore volume and pore diameter could be tuned only by varying the template content.15 However, the ability to control the pore parameters is limited because the template will crystallize from the gel at high template content.15 In these syntheses, organic template molecules act simply as templates and are removed subsequently via solvent extraction or calcinations so that they are not a source of organic component for forming a hybrid structure. Recently, alkoxysilanes containing long-chain organic groups (alkyl, amino, mercapto, phenyl, or sulfoaryl) have been used as both silicasourcesandtemplatesforpreparingmesoporousmaterials,16-19 but the syntheses of these alkoxysilanes are complicated and these compounds are expensive. Polymethylhydrosiloxane (PMHS) is a byproduct of the silicon industry, which is inexpensive, nontoxic, and stable to air and moisture and mainly used as a reducing agent for halogens, ketones, ethers, imines, and phosphine oxides.20,21 In this study, we reported the synthesis of hybrid mesoporous silicas using PMHS and TEOS as silica sources without employing a surfactant as template. A prominent feature of this synthesis approach is that the hybrid structure is achieved in the synthesis, and the grafting experiment is not required. Therefore, it avoids solvent extraction or calcination operation to remove the organic templates in a conventional synthesis. The obtained mesoporous hybrids possess large surface areas and pore volumes and narrow pore size distributions. Moreover, we can tune the pore size in a relatively wide range from ∼4 to
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∼15 nm (in diameter, calculated by Barrett-Joyner-Halenda, BJH, method22) by varying the PMHS concentration. All the obtained hybrid structures displayed super hydrophobicity and adsorption ability for alkylphenols. They could be used as adsorbents for cleaning up spills of oil or organic chemicals on a water surface and removal of alkylphenols at very low concentration (ppm) from water. Experimental Section Synthesis of Samples. Hybrid mesoporous silicas were prepared via the following procedures. First, 0.47 mL (0.4676 g) of polymethylhydrosiloxane (PMHS, MW ≈ 2300, Aldrich) was dripped into 70 mL of ethanol in the presence of sodium hydroxide (NaOH) as catalyst at room temperature for 24 h. During this period the hydrogen atom of PMHS was replaced by -OC2H5. To this solution 5 mL (4.676 g) of tetraethylorthosilicate (TEOS, 99%, Acros) and deionized H2O were added and the TEOS:H2O molar ratio and the PMHS/TEOS mass ratio of this system were 4:1 and 1:5, respectively. This mixture was vigorously stirred for 3 h and aged under ambient conditions for 4-5 d. In this stage a sol formed and converted to a gel. The gel was heated in a vacuum oven at 333 K to remove the EtOH. The samples prepared with the PMHS/TEOS mass ratios of 1:5, 1:8, and 1:10 were M1, M2, and M3, respectively. Characterization. Powder X-ray patterns were measured using Ni-filtered Cu KR radiation (λ ) 1.5404 Å) and a Rigaku D Max III VC diffractometer equipped with a rotating anode operated at 40 kV and 30 mA. N2 adsorption isotherms were obtained at 77.3 K on a Tristar 3000 Sorptometer, using static adsorption procedures. Samples were degassed at 423 K and under a vacuum of 10-6 Torr for at least 12 h prior to the measurement. BET specific surface areas were calculated using the adsorption data in a relative pressure (P/P0) range from 0.05 to 0.20. Pore size distributions of the samples were derived using the conventional BJH method.22 TEM images were taken on a JEOL 100CX microscope operating at an accelerating voltage of 200 kV. SEM micrographs were acquired from Au-coated sample powders with a Philips SEM 505 microscope with a LaB6 filament, operating at 4 keV. 29Si MAS NMR spectra were obtained using a DOTY Scientific multinuclear probe and 5 mm zirconia rotors on a UNITY INOVA-500 Spectroscope: 29Si resonance frequency is 99.745 M Hz; pulse width, 4 µs; recycle delay time, 400 s; spinning speed, 8 kHz; and reference to tetramethylsilane (TMS) assigned 0 chemical shift. The contact angles for water of the tableted samples were measured by a contact angle meter (CA-A, Kyowa, Japan). The residual compounds were analyzed with an ultra-visible spectrometer (UV-vis 3150PC, Shimadzu). Adsorption of Phenols by the Samples. Ten milligrams of the adsorbents (M1, M2, and M3) were added into aqueous solutions (40 g) of organic compound (para-methylphenol, tertamylphenol, and nonylphenol) with the initial concentration at 8 ppm, respectively. After adsorption with stirring for a given time, the supernatant solutions were separated by centrifugation and the residual compounds were analyzed. Results and Discussion Pore Structure Determination. The N2 adsorption-desorption isotherms and the pore size distributions (PSD) of the samples prepared by the approach proposed in this study are shown in Figure 1. Clearly, samples M2 and M3 exhibited type IV isotherms with type H1 hysteresis loop, according to IUPAC.23 Such
Figure 1. N2 adsorption-desorption isotherms of samples M1, M2, and M3. Inset: BJH pore size distributions of samples M1, M2, and M3.
TABLE 1. Specific Surface Areas, Pore Volumes, and Pore Diameters of the Obtained Hybrid Mesoporous Solids
sample M1 M2 M3
PMHS:TEOS (mass ratio)
BET surface area (m2 g-1)
total pore volume (STP) (cm3 g-1)
BJH pore diameter (nm)
1:5 1:8 1:10
680 580 470
0.65 1.00 1.07
4 9 15
isotherms are associated with capillary condensation that takes place in connected mesopores of similar size.1 Their hysteresis loops appear in a relatively high P/P0 range (0.6-0.9), indicating large mesopore size as also illustrated in the inset of Figure 1, in which the PSD is calculated by the BJH method. The isotherm of M1 is distinctly different from those of M2 and M3 samples, exhibiting a shape of type IV isotherm with a hysteresis of type H2, and the hysteresis loop appears in the P/P0 range between 0.4 and 0.8, due to the existence of the small mesopores with a diameter of ∼4 nm. The observation is obviously different from the template syntheses of mesoporous silicas using surfactants. In the template syntheses the pore size depends on hydrocarbon chain length of the surfactant tail group,24 rather than the surfactant content. It is also found that the concentration of PMHS has a strong influence on the pore size distribution. When the PMHS/TEOS mass ratio is increased from 1:10 (sample M3) to 1:5 (sample M1), the PMHS concentration is increased, and the dominant pore diameter decreases from about 12 nm to as low as 2 nm. The detailed structure information of the samples is listed in Table 1.The mesoporous structure of the samples was also investigated by transmission electron microscopy (TEM) technique, and a representative TEM image of sample M1 is illustrated in Figure 2. The sample has a wormhole-like pore structure as shown in the image, which is very similar to the pore arrays of MSU-X type mesoporous silicas prepared using a neutral surfactant template.9,10,25-28 In such a structure mesopores have similar diameters, but there is no long-range ordering in the mesopore packing. The powder XRD pattern of sample M1 is also given in Figure 2. There is a broad diffraction peak (due to 100 reflection) in the pattern, which corresponds to a d-spacing of 6.8 nm. XRD patterns with such a broad peak were also observed from some mesostructured silica materials.9,10,12 The predominant pore diameter can be estimated by deducting the
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Figure 2. HRTEM image (a) and XRD pattern (b) of sample M1 prepared at the PMHS/TEOS mass ratio of 1:5. Inset: The diffraction pattern of M1 at high 2θ values.
Figure 3. Solid-state
29
Si MAS NMR spectra of M1, M2, and M3. Figure 4. FTIR spectra of M1, M2, and M3.
thickness of the pore walls from the d-spacing value of the 100 reflection.9,10 The pore diameter thus obtained appears slightly larger than the BJH pore diameter (∼4 nm). Nonetheless, the pore diameter derived by the method based on Kelvin equation only, like the BJH method, is smaller than the real diameter,29 so most of mesopores in the solid have diameters between 4 and 5 nm. The presence of a single broad peak centered at a 2θ value of ∼23° (Figure 2, inset) is attributed to the amorphous pore wall of the silica framework.12 29Si MAS NMR Characterization. Figure 3 shows the 29Si MAS NMR spectra of the samples M1, M2, and M3. Noticeably, all spectra exhibit four distinct signals at -111 ppm, -100 ppm, -64 ppm, and -56 ppm, corresponding to Q4 (Si*(OSi)4), Q3 ((HO)Si*(OSi)3),29 T3 ((SiO)3Si*CH3), and T2 ((OH)(SiO)2Si*CH3)16 environments of silicon atoms, respectively. The presence of T3 and T2 signals confirms that the methyl groups (-CH3) in the products are chemically bonded to the silica framework. The -CH3 stretching band at 2900 cm-1 and the Si-C stretching peak at 1275 cm-1 are also observed in the FTIR spectra of these samples (see Figure 4), which is in good agreement with the result of 29Si MAS NMR spectra. Particle Morphology. The particle morphology of these mesoporous materials synthesized by the proposed approach without surfactant can be observed from field emission scanning electron microscopy (FESEM) images in Figure 5. As can be seen, the samples are composed of glass particles
with irregular shapes, being analogous to the conventional silica xerogel prepared in the absence of surfactant.30,31 The size of the particles is independent of the preparation conditions, but depends only on the grinding after the synthesis. In such xerogel solids the interparticle voids should be marcropores (>50 nm) and have little contribution to the porosity measured by nitrogen adsorption. Thus, it is concluded that the porosity of the hybrid solids are predominantly from framework mesopores observed in TEM image, rather than the interparticle voids of the silica particles.31-33 Tuning the Pore Structure. The pore size of the samples can be enlarged by simply introducing a swelling agent, such as 1,3,5-trimethylbenzene (TMB). A sample was synthesized by introducing TMB into the synthesis system of M1 sample, and the obtained xerogel was designated as Ms. The N2 adsorption-desorption isotherms and pore size distributions of M1 and Ms are compared in Figure 6. The adsorption capacity of Ms is considerably larger than that of M1, reflecting a marked increase in the pore volume, which resulted from the TMB addition. The hysteresis on the isotherm of Ms begins at P/P0 of about 0.6, being higher than the beginning of the hysteresis for M1, at about 0.43. This reveals that the mesopores in Ms are considerably large than those in M1, so that adding TMB in the synthesis of M1 swells the mesopores. The PSDs in the inset of Figure 6, derived from nitrogen adsorption data, indicate directly the pore swelling by
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Figure 5. SEM images of the samples obtained via the nonsurfactant pathway: (a) M1, (b) M2, and (c) M3.
Figure 7. Photographs of a water droplet on the tablets of (a) M1, (b) M2, and (c) M3, respectively, and the photograph of (d) M1 in the mixture of benzene and water. Figure 6. N2 adsorption-desorption isotherms for M1 and Ms. Inset: BJH pore size distributions of M1 and Ms.
TMB addition. The PSD of Ms shifts to larger pore size by several nanometers. Generally for the surfactant-templated process, the pore size expansion caused by TMB molecules could be explained as that the TMB molecules migrate to the inside space of the micelle of the template,34,35 and the existence of the TMB molecules in the inside space is favored because of the hydrophobic nature of the hydrocarbon chains of the template. It appears that, during the formation of the mesopores in this work, the inside of the mesopores were hydrophobic. Probably the methyl groups were attached to the inside walls of the mesostructures through chemical bonds, forming a hybrid structure. Thus, the procedure of grafting organic compounds on the silica surface is not necessary, which is usually required to create mesoporous hybrid structures. One of the outstanding features of the present synthesis route is that no templates are introduced in the synthesis of the mesoporous framework and removed after the synthesis. TEOS is a widely used silicon source without any structure-directing function for forming mesostructures.36 Therefore, PMHS not only is a silicon source but also plays a crucial role in structure directing. Hydrophobicity. The most outstanding feature of the hybrid mesoporous structures is their hydrophobicity.37,38 The samples synthesized in this study exhibit superior hydrophobicity as shown in the photographs in Figures 7a-c. The water droplet with a diameter of ∼3 mm on the tablets of a sample looks like a round ball, and the contact angle exceeds 150 °, indicating the super hydrophobic nature of the mesoporous hybrid materials. This hydrophobicity is attributed to the existence of a large amount of methyls that disperse well among the hybirds. The
super hydrophobicity of the samples is demonstrated by another experiment: Sample M1 was added to the mixture of benzene and distilled water. As shown in Figure 7d, the M1 powder disperses in the benzene and floats on the surface of the water, although the density of the M1 powder (∼2 g cm-3) is far beyond that of the water. The solids with such excellent hydrophobicity can be used directly to remove the organic pollutants on the water surface, which are often caused by accidents of oil or chemical spills and results in enviromental disasters. Adsorption of Trace Amount of Alkylphenols in Water. Alkylphenols in water even at very low concentration (parts per million, ppm) could cause serious environmental problems.39,40 Removal of trace amount of harmful organic pollutants is a challenging task. The hybrid mesoporous solids have been found to be potential adsorbents which can remove the organic pollutants from water efficiently.40,41 The hydrophobic internal pore space has strong affinity to organic compound and thus the solids are able to attract the organic pollutants from aqueous media. In this study we investigated the performance of the prepared mesoporous solids as adsorbents for removal of three alkylphenols, para-methylphenol, tert-amylphenol, and nonylphenol, from the aqueous solutions with a trace amount of the alkylphenols (8 ppm). The changes in the concentrations of the phenols during the adsorption course are depicted in Figure 8. As can be seen in Figure 8, generally the adsorption equilibrium is reached within about 2 h. In terms of the alkyphenols, the adsorption of nonylphenol by the three solids is the most efficient. This alkylphenol has the longest alkyl chain compared to the other two phenols. Samples M1 and M2 can remove all the nonylphenol in the solution within an hour. Nonylphenol is a biodegradation product of alkylphenol polyethoxylate resin and is regarded as a very toxic pollutant in
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Figure 8. Time courses of different organic compounds concentration in aqueous solution during adsorption: (a) para-methylphenol, (b) tertamylphenol, and (c) nonylphenol.
water. It can induce feminizing effect in fishes even at very low concentrations (1 ppm or below).39,40 The results in Figure 8c demonstrate that the solids prepared in this study are efficent adsorbents for removal of the toxic nonylphenol from water. In terms of the adsorbents, the order for the ability to remove tert-amylphenol and nonylphenol by adsorption is M1 > M2 > M3. It appears that the solid with smaller pore size and larger specific surface area (Table 1) have higher adsorption ability. Nonetheless, such an order is not observed for the adsorptions of para-methylphenol by the three solids. The para-methylphenol adsorptions are similar (in a narrow band in Figure 8a). According to the results of the adsorption study, the solids prepared in this study are effective adsorbents for the removal of the phenols from water. This adsorption ability is mainly due to the hydrophobicity of the methyl groups on the mesopore wall, and the structural parameters of the mesopores also have significant influence on the adsorption. The solid with small mesopores and large specific surface area is the most effective adsorbent for the removal of the phenols with large alkyl chains. Conclusions In summary, organic-inorganic hybrid mesoporous silicas can be achieved by the reported one-step synthesis approach in which PMHS and TEOS are used as silica source and no template is introduced. In the products, methyl groups remain chemically bonded to the silica framework. Therefore, this new approach does not require additional grafting reaction which is usually employed for creating hybrid mesoporous structures. The obtained materials possess wormhole-like pore frameworks, large specific surface areas, and pore volumes and narrow pore size distributions. The pore size and its distribution in the products can be tuned simply by varying the concentration of PMHS. The mesoporous materials not only exhibit super hydrophobic properties but also are able to effectively remove phenols from the water of a phenol concentration below 10 ppm. For instance, the solid M1 with small mesopores (∼4 nm) can remove nonylphenol completely (to a concentration below detection limit). These solids have great potential to be used for cleaning up spills of oil or organic chemicals on a water surface, and trace amount of very toxic organic pollutants in water. This study highlights new opportunities for the preparation of hybrid mesostructured functional materials. Acknowledgment. Financial support from the national key native science foundation (20133040) and State Key Program
for Development and Research of China (2005CB221402) and Australian Research Council are gratefully acknowledged. References and Notes (1) Kruk, M.; Jaroniec, M. Chem. Mater. 2001, 13, 3169, and references therein. (2) Sayari, A.; Hamoudi, S. Chem. Mater. 2001, 13, 3151. (3) Sellinger, A.; Weiss, P. M.; Nguyen, A.; Lu, Y.; Assink, R. A.; Gong, W.; Brinker, C. J. Nature 1998, 394, 256. (4) Stein, A.; Melde, B. J.; Schroden, R. C. AdV. Mater. 2000, 12, 1403. (5) Lu, Y.; Fan, H.; Doke, N.; Loy, D. A.; Assink, R. A.; LaVan, D. A.; Brinker, C. J. J. Am. Chem. Soc. 2000, 122, 5258. (6) Inagaki, S.; Guan, S.; Ohsuna, T.; Terasaki, O. Nature 2002, 416, 304. (7) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (8) Kim, J. M.; Kwak, J. H.; Jun, S.; Ryoo, R. J. Phys. Chem. 1995, 99, 16742. (9) Bagshaw, S. A.; Prouzet, E.; Pinnavaia, T. J. Science 1995, 269, 1242. (10) Tanev, P. T.; Pinnavaia, T. J. Science 1995, 267, 865. (11) Attard, G. S.; Glyde, J. C.; Go¨ltner, C. G. Nature 1995, 378, 317. (12) Wei, Y.; Jin, D. L.; Ding, T. Z.; Shih, W. H.; Liu, X. H.; Cheng, S. Z. D.; Fu, Q. AdV. Mater. 1998, 3, 313. (13) Cheng, S.; Wei, Y.; Feng, Q. W.; Qiu, K. Y.; Pang, J. B.; Jansen, S. A.; Yin, R.; Ong, K. Chem. Mater. 2003, 15, 1560. (14) Jansen, J. C.; Shan, Z.; Marchese, L.; Zhou, W.; Puil, N.v.d.; Maschmeyer, Th. Chem. Commun. 2001, 713. (15) Pang, J. B.; Qiu, K-Y.; Wei, Y. Chem. Mater. 2001, 13, 2361. (16) Shimojima, A.; Kuroda, K. Angew. Chem., Int. Ed. 2003, 42, 4057. (17) Ruiz-Hitzky, E.; Letaı¨ef, S.; Pre´vot, Dr. V. AdV. Mater. 2002, 14, 439. (18) Moreau, J. J. E.; Vellutini, L.; Man, M. W. C.; Bied, C.; Bantignies, J.-L. J. Am. Chem. Soc. 2001, 123, 7957. (19) Huo, Q. S.; Margolese, D. I.; Stucky, G. D. Chem. Mater. 1996, 8, 1147. (20) Lawrence, N. J.; Bushell, S. M. Tetrahedron Lett. 2000, 41, 4507. (21) Bette, V.; Mortreux, A.; Lehmann, C. W.; Carpentier, J. F. Chem. Commun. 2003, 332. (22) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity, 2nd Ed.; Academic Press: New York, 1982; Chapter 3. (23) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquerol, J.; Siemieniewska, T. Pure Appl. Chem. 1985, 57, 603. (24) Raman, N. K.; Anderson, M. T.; Brinker, C. J. Chem. Mater. 1996, 8, 1682. (25) Bagshaw, S. A.; Pinnavaia, T. J. Angew. Chem., Int. Ed. 1996, 35, 1102. (26) Prouzel, E.; Pinnavaia, T. J. Angew. Chem., Int. Ed. 1997, 36, 516. (27) Bagshaw, S. A. Chem. Commun. 1999, 271. (28) Tanev, P. T.; Chlbwe, M.; Pinnavaia, T. J. Nature 1994, 368, 321. (29) Zhu, H. Y.; Cool, P.; Vansant, E. F.; Su, B. L.; Gao, X. P. Langmuir 2004, 20, 10115. (30) Oviatt, H. W., Jr.; Shea, K. J.; Small, J. H. Chem. Mater. 1993, 5, 943. (31) Wang, X.; Li, W.; Zhu, G.; Qiu, S.; Zhao, D.; Zhong, B. Microporous Mesoporous Mater. 2004, 71, 87. (32) Lin, H. P.; Tsai, C. P. Chem. Lett. 2003, 32, 1092.
1004 J. Phys. Chem. C, Vol. 111, No. 2, 2007 (33) Zhai, S. R.; Zhang, Y.; Shi, X.; Wu, D.; Sun, Y. H.; Shan, Y. K.; He, M. Y. Catal. Lett. 2004, 93, 225. (34) Blin, J. L.; Su, B. L. Langmuir 2002, 18, 5303. (35) Boissie`re, C.; Martines, M. A. U.; Tokumoto, M.; Larbot, A.; Prouzet, E. Chem. Mater. 2003, 15, 509. (36) de A. A. Soler-Illia, G. J.; Sanchez, C.; Lebeau, B.; Patarin, J. Chem. ReV. 2002, 102, 4093-4138. (37) Matsumoto, A.; Sasaki, T.; Nishimiya, N.; Tsutsumi, K. Langmuir 2001, 17, 10115.
Yang et al. (38) Kruk, M.; Jaroniec, M.; Sayari, V. A. J. Phys. Chem. B 2002, 106, 10096. (39) Muller, S.; Schlatter, C. Pure Appl. Chem. 1998, 70, 1847, and references therein. (40) Inumaru, K.; Kiyoto, J.; Yamanaka, S. Chem. Commun. 2000, 903. (41) Inumaru, K.; Murashima, M.; Kasahara, T.; Yamanaka, S. Appl. Catal. 2005, 52, 275.
