Separation of Organic Compounds by Spherical Mesoporous Silica

Oct 24, 2007 - Spherical mesoporous silica (SMS) was prepared in a two-phase ... quantity of organic compounds with as-made SMS increased with the ...
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Ind. Eng. Chem. Res. 2007, 46, 8152-8157

Separation of Organic Compounds by Spherical Mesoporous Silica Prepared from W/O Microemulsions of Tetrabutoxysilane Yoshikazu Miyake,* Masaki Hanaeda, and Masayuki Asada Department of Chemical Engineering and High-Technology Research Center, Faculty of Engineering, Kansai UniVersity, 3-3-35 Yamatecho, Suita 564-8680, Japan

Spherical mesoporous silica (SMS) was prepared in a two-phase system composed of hydrophobic tetrabutoxysilane (TBOS) as the silica source, and an aqueous phase contained dissolved hexadecyltrimethyl ammonium bromide (CTAB) as a surfactant. The adsorption properties of some organic compounds with the as-made and calcined SMS were evaluated by batch and column adsorption experiments. The solubilized quantity of organic compounds with as-made SMS increased with the hydrophobicity of these substances. The breakthrough time obtained by the use of column-packed as-made SMS also increased with the hydrophobicity of organic compounds. Mutual separation of phenol and aniline could also be demonstrated. Separation of organic compounds of different molecular sizes was possible with the column-packed calcined SMS. The pressure drop of the column-packed SMS was lower than that of molded commercial mesoporous silica powder, because of the spherical morphology of the SMS. The breakthrough time for calcined SMS became shorter than that for molded commercial silica powder with MCM41 nanostructure because of the smaller mesopore size. Introduction Mesoporous silica prepared by the use of surfactant as a molecular template has been the subject of much interest since it was first developed by Beck et al. in 1992.1 Until now, many papers have reported on the synthesis, characterization, stability, and application of these materials. The nanostructures of the mesoporous silicates are mainly classified into three types: hexagonal arrays of mesopores (as MCM-41), three-dimensional interconnected mesopores (as MCM-48), and lamellar nanostructures (as MCM-50). The synthesis and application of these materials have been investigated by many researchers because of their characteristics such as large specific surface area (over 1 000 m2/g), large pore volume (>0.8 cm3/g), and uniformity of mesopore size, which is easily controllable.2-10 These mesoporous materials have been used as adsorbents and catalytic carriers. Many investigations have focused on the properties as adsorption media for various gases and for metal ions and organic compounds in the water phase.11-23 However, most research has been limited to MCM-41 materials with a uniform, one-dimensional pore network, because of the simple method of preparation. It has been recognized that, among the novel mesoporous materials developed, the interconnected threedimensional mesopore network, such as MCM-48, has a greater potential for various applications than other mesoporous materials with a one-dimensional mesopore structure.24,25 When mesoporous silica materials are used as adsorbents and catalytic carriers, the spherical morphology of the materials is useful because the pressure drop in a column of packed spherical silica is decreased. Methods for preparing spherical mesoporous silica (SMS) have been developed. These are mainly classified into two categories: (1) analogies of the well-known Sto¨ber method in the homogeneous phase of alcohol solution and (2) two-phase systems composed of organic and water phases. As the diameter of spherical silica prepared by analogies of the Sto¨ber method range from several hundred nanometers to several * To whom correspondence should be addressed. Tel.: +81-66368-0950. Fax: +81-6-6388-8869. E-mail: [email protected].

Figure 1. Images of (a) nanostructure of as-made SMS and (b) solubilization of phenol into hydrophobic regions of the residual surfactants.

micrometers, the separation of spherical silica is difficult after its use as an adsorbent. The preparation method using a twophase system was first reported by Huo et al.,26 and hydrophobic tetrabutoxysilane (TBOS) as a silica source was mechanically dispersed in an aqueous solution dissolved surfactant. SMS of about 500 µm in diameter was obtained. The formation mechanism has been studied by Miyake and Kato,27 who found

10.1021/ie0705047 CCC: $37.00 © 2007 American Chemical Society Published on Web 10/24/2007

Ind. Eng. Chem. Res., Vol. 46, No. 24, 2007 8153 Table 1. Parameters of Langmuir Isotherm and Breakthrough Time for Organic Compounds with Different Hydrophobicities solute

λmax (nm)

 (1/(cm‚mM))

log(POW)

q∞ (mmol/g)

KL (1/mM)

Cin (mM)

breakthrough time (min)

k (1/min)

τ (min)

aniline phenol o-cresol 1-naphthol

280 270 272 292

1.38 1.42 1.47 4.7

0.94 1.46 1.95 2.85

0.20 0.69 0.78 1.72

0.28 2.28 3.76 2.87

2.0 3.0 3.0 0.5

56 850 1 100 6 840

0.045 0.025 0.020 0.003

85 900 1 200 8 000

that the surfactants in the aqueous phase were extracted together with water molecules into the hydrophobic phase of TBOS and that hydrolysis and condensation of TBOS proceeded in the water-in-oil microemulsion. The silica particles formed in waterpools of the water-in-oil microemulsion were connected to each other by reactions, and a regularly arranged nanostructure was formed by the self-assembly of surfactants. As the hydrophobic phase was mechanically dispersed in the aqueous phase, SMS with a large diameter was obtained. The nanostructure of the as-made SMS was considered that the silica rods were in hexagonal array in the continuous phase composed of surfactant, butanol, and TBOS, as shown in Figure 1.27 Because the continuous surfactant layers in the as-made SMS have an advantage for separation fields, we have reported, using a batch experimental method, that the hydrophobic organic compounds in aqueous phase can be solubilized in the hydrophobic regions composed of the residual surfactants in as-made SMS.28 In this study, we first evaluated the solubilization of some hydrophobic organic compounds in aqueous phase by using asmade SMS in a batch experimental system. Next, the as-made SMS was packed in a column, and breakthrough curves of hydrophobic organic compounds were obtained. The breakthrough curves for mutual separation systems were also obtained. The solubilized hydrophobic organic compounds could be eluted using water as an elution. Furthermore, calcined SMS was prepared by calcination of as-made SMS, though the nanostructure of as-made SMS may be destroyed by the removing the surfactants. To demonstrate the size selectivity of calcined SMS, we evaluated the adsorption properties for organic compounds of different molecular sizes in batch and column experiments. Finally, we compared the breakthrough curves for the calcined SMS and the commercial silica powder with MCM-41. Experimental Section Reagents. Hexadecyltrimethyl ammonium bromide (CTAB; Wako Co. Ltd.) as a template and tetrabutoxysilane (TBOS; Shinetsu Co. Ltd.) as a silica source were used. The properties of solubilization of several organic compounds in the hydrophobic region of as-made SMS, which was formed by the selfassembly of the residual CTAB, were evaluated. Several organic compounds of different hydrophobicity (aniline, phenol, ocresol, and 1-naphthol) were used. The hydrophobicity of these compounds was evaluated by the distribution coefficient between water and octanol phases, POW, and these values are summarized in Table 1. Rhodamine B, cyanocobalamin (vitamin B12), and hemoglobin, which have different molecular sizes, were used to evaluate the size selectivity of calcined SMS. The concentrations of these organic compounds in water solution were determined by measuring the spectrum in the UV wavelength range (2500PC; Shimadzu Ltd.). The wavelengths and extinction coefficients for these organic compounds are also shown in Table 1. Preparation of SMS. Methods for the preparation of SMS were reported in previous papers.27,28 The typical procedures for SMS are as follows. CTAB, 4.3 g, and 25 cm3 of 2N NaOH were dissolved in 250 cm3 of distilled water, yielding a transparent solution of pH 13.0 measured by pH meter. Next,

32 g of TBOS was added to the solution drop by drop under stirring with a two-paddle agitator at 700 rpm agitation speed. After stirring for ∼8 h, spherical particles dispersed in aqueous phase were obtained. The spherical particles were filtered off under reduced pressure, washed with distilled water, and dried in air at room temperature. As-made SMS of ∼500 µm diameter was obtained. The dried, as-made SMS was calcined in air for 5 h at 773 K to remove all residual surfactants. The calcined SMS was sifted out in the range of 300-500 µm. Commercial silica powder (SILFAM-A; Nippon Chem. Ind. Co. Ltd.) has an irregular morphology of 60 µm average diameter and a hexagonal array of mesopores (MCM-41). Commercial silica powder as-received was molded into tablet form; subsequently, the molded silica was crushed and sieved out to 300-500 µm using a mesh. The molded MCM-41 commercial silica and the calcined SMS were used in packed columns to compare their adsorption performances. The N2 adsorption-desorption isotherms of the calcined SMS and the commercial silica were measured by BELLSOAP (Nippon Bell Co. Ltd.) at 77 K. The pore volumes and pore distributions were calculated by the Cranston-Inkley method from the desorption isotherm. The specific surface areas were also calculated by the Brunauer-Emmett-Teller (BET) method from the volume adsorbed at the low relative pressure. Batch Adsorption Experiments. Weighed SMS and aqueous solution containing a dissolved organic compound were mixed and shaken for 24 h at 303 K in a test tube. The amount of adsorption, q, was calculated by the mass-balance relationship as

q)

Vaq(C0 - Ce) W

(1)

where q is the adsorption quantity (mmol/g), Vaq (dm3) is the volume of aqueous solution, C0 and Ce are the initial and equilibrium concentrations (mmol/dm3) of organic compound, respectively, and W is the weight of silica (g). Column Adsorption Experiments. Fixed-bed adsorption experiments were conducted in a glass column of volume 5.0 cm3 (8 mm inner diameter, 100 mm height). Silica particles were close packed in the column, and the weights of silica were 3.06 g (as-made SMS), 1.78 g (calcined SMS), 0.99 g (commercial MCM-41), and 1.74 g (molded commercial MCM-41). Aqueous solutions of organic compounds of controlled concentration were fed into the column at a constant volume rate of 1.0 cm3/min. The pH and ionic strength in feed solution were not adjusted. The pressure drop was measured. The outlet solution was sampled by use of a fraction collector (EYELA, DC-1500), and the absorbance of the outlet solution was measured. The separation properties of these mesoporous silica particles were evaluated for several organic compounds by drawing breakthrough curves. Results and Discussion Adsorption Properties of As-Made SMS. The CTAB used as a template are left behind in as-made SMS. The silica

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Figure 2. Langmuir isotherm for some organic compounds. Key: (]) for aniline, (b) for phenol, (0) for o-cresol, and (9) for 1-naphthol. The data shown by (O) and the broken line were published in ref 28. Solid lines are correlated from eq 2 with parameters summarized in Table 1.

Figure 3. Breakthrough curves for some organic compounds. Key: (O) for aniline, (b) for phenol, (0) for o-cresol, and (9) for 1-naphthol. Solid lines show the theoretical curves calculated by eq 3. Breakthrough times and parameters in eq 3 are summarized in Table 1.

Figure 6. Relationship between adsorption quantity and the concentration at equilibrium of several organic compounds: rhodamine B (b), C16PyrCl (0), cyanocobalamin (O), and hemoglobin (9).

Figure 7. Breakthrough curves for some organic compounds of different molecular size in a column packed with calcined SMS: hemoglobin (O), cyanocobalamin (9), and rhodamine B (b). The solid lines were calculated by eq 3.

centration in aqueous phase for several organic compounds with different hydrophobicities. The experimental data can be correlated by the Langmuir isotherm equation as follows,

q)

Figure 4. Breakthrough curves for mutual separation of phenol (b) and aniline (O). Water as an eluant was fed at 850 min, which corresponded to the breakthrough time for phenol.

Figure 5. Breakthrough curves for mutual separation of phenol (b) and methylene blue (O). Water as an eluant was fed at 850 min, which corresponded to the breakthrough time for phenol.

nanorods were self-assembled in a hexagonal array in the continuous surfactant region, as shown in Figure 1a, and this is the nanostructure of reversed MCM-41 described in the previous paper.27 Organic compounds in aqueous phase are solubilized in the hydrophobic regions, in which the residual surfactants are self-assembled, because of hydrophobic interactions, as shown in Figure 1b. Figure 2 shows the relationship between the solubilization quantity and the equilibrium con-

q∞KLCe 1 + K LC e

(2)

where q∞ and KL are the saturated adsorption quantity and Langmuir constant, respectively. The two parameters were determined by fitting the data. The parameters of eq 2 for some organic compounds are summarized in Table 1. The results are shown by the solid lines in Figure 2. These can be compared with the data for phenol used in the previous paper,28 which were shown by the dotted line and open circles in Figure 2. The values for phenol in this study were lower than those in previous data. The difference was caused by the low quantity of CTAB solubilized in as-made SMS because of the lower feed concentration of CTAB and the lower pH of the aqueous solution. The hydrophobicity of the organic compounds was evaluated by the distribution coefficient between octanol and water, POW, which is shown in Table 1. The saturated quantity, q∞ (mmol/g), increased linearly with the hydrophobicity of the organic compounds. However, the slope of the linear relationship was greater than that reported in the previous paper.28 This difference may be caused by the lower quantity of residual CTAB in as-made SMS. The as-made SMS was packed in a glass column (8 mm inner diameter, 100 mm height); the weight was 3.06 g. The feed solution containing dissolved organic compound at a concentration of Cin (mM), as shown in Table 1, was fed into the column at a constant volume rate of 1.0 cm3/min. The outlet solution was sampled using a fraction collector. The concentration of the outlet solution was analyzed using a spectrophotometer. The wavelength at maximum absorption, λmax (nm), and the extinction coefficient,  (1/(cm mM)), at the wavelength for used organic compounds are also shown in Table 1. Figure 3 shows

Ind. Eng. Chem. Res., Vol. 46, No. 24, 2007 8155 Table 2. Separation Properties of Some Organic Compounds by Calcined SMS molecular size solute

MW (g/mol)

max (nm)

min (nm)

q∞ (mmol/g)

KL (1/mM)

Cin (mM)

breakthrough time (min)

k (1/min)

τ (min)

rhodamine B C16PyrCl cyanocobalamin hemoglobin

479 340 1 355 64 534

1.51 2.7 2.21 10.9

0.59 0.58 0.58 10.2

0.60 0.125 0.02

1.15 9.75 10.5

50

3 360

0.0008

6 000

50 5

180 20

0.013 0.5

370 20

Table 3. Comparison of Some Properties between Calcined SMS and Commercial MCM-41

as-made SMS calcined SMS commercial MSM-41 (60 µm) commercial MCM-41 (500 µm)

pore diameter (nm)

specific surface area (m2/g)

pore volume (cm3/g)

packed weight (g)

pressure drop (kg/cm2)

q∞ (mmol/g)

KL (1/mM)

k (1/min)

τ (min)

2.16 4.10 (4.10)

850 970 (970)

0.53 0.99 (0.99)

3.06 1.78 0.99 1.74

0.4 0.4 2.0 0.9

0.02 0.09 0.042

10.5 4.5 6.0

0.012 0.03 0.005

350 510 700

the breakthrough curves for several organic compounds, in which the time is plotted against the logarithmic value. The breakthrough times, which were determined by the time at Cout/ Cin ) 0.05, are summarized in Table 1. Breakthrough time increased with the hydrophobicity of the organic compounds. The breakthrough time for aniline was significantly short; this may have been due to a repulsive interaction between the quaternary ammonium salt surfactant and aniline, which existed in neutral form in the feed solution because the pKa of aniline is 4.65. The experimental data of the breakthrough curves can be simulated by a simple model, which is derived by assuming that the adsorption rate is proportional to residual capacity of the sorbent and the concentration of the sorbing species.29,30 The model equation with two parameters is expressed as29,30

Cout 1 ) Cin 1 + exp[k(τ - t)]

(3)

where k is the rate constant (min-1), τ is the time required for 50% adsorbed breakthrough (min), and t is the sampling time (min). The value of τ depends on the saturated adsorption quantity, the column length, and the feed rate of solution. The k value is an index for the overall adsorption rate. The solid lines in Figure 3 show the curves calculated by eq 3 with the values of parameters as shown in Table 1. The value of τ increased with the hydrophobicity of the organic compounds because of the increase in hydrophobic interaction between the compounds and the hydrophobic zone composed of the hydrocarbon of the surfactant. The value of the rate constant, k, depends on the slope of the breakthrough curve; when k increases, the slope of the breakthrough curve becomes steep. The value of k decreases with the hydrophobicity of the organic compounds; that is, the breakthrough curve becomes flatter. Next, we examined the mutual separation of aniline and phenol, the concentrations of which, in the feed solution, were both 2.0 mM. Because the spectra of phenol (λmax ) 270 nm) and aniline (λmax ) 280 nm) overlap, the concentrations of these compounds were evaluated from the absorbance at 270 and 280 nm. The breakthrough curves are shown in Figure 4. The breakthrough times for aniline and phenol were 90 and 850 min, respectively, which are nearly equal to that of the single component as shown in Figure 3. As the aniline concentration of the output solution became equal to the feed concentration at ∼100 min, and the breakthrough time for phenol was 850 min, the feed solution was changed to water as an eluant at 850 min. Until ∼1 000 min, the concentration of aniline was almost equal to the feed concentration and the phenol concentration slowly increased. In this range, the aniline and phenol

solutions held in the column were eluted. After this, the aniline concentration in the eluant decreased rapidly and the phenol concentration decreased from a maximum value at 1 500 min. The results show that these weak basic and acidic organic compounds can be mutually separated using as-made SMS. As, generally, phenol adsorbed by polymer beads cannot be eluted with water, hydrophilic organic solvents such as alcohol and acetone were used as eluants.31 An advantage of the as-made SMS was that water could be used as an eluant. It is a disadvantage that the residual surfactants in the SMS were slowly eluted into the water phase, though the loss of surfactant could be neglected until about 10 000 min (7 days), as shown by Figure 3. To solve the problem of loss of surfactants, it is necessary that the surface of the SMS is covalently modified using a reagent such as an octyltriethoxisilane with a long alkyl group.32 Figure 5 shows the breakthrough curves for mutual separation of methylene blue and phenol, the feed concentrations of which were 0.05 and 2 mM, respectively. Methylene blue has an azo group, and because it was hardly solubilized into as-made SMS, the breakthrough time was very short. When, at 850 min, the feed solution was changed to water as the eluant, the concentration of methylene blue rapidly decreased to zero. In contrast, the elution of phenol took longer than that of the phenol-aniline mixture. This result can be qualitatively interpreted as an increase of the solubilization quantity of phenol, because the methylene blue was not solubilized into as-made SMS. Adsorption Properties of Calcined SMS. Calcined SMS was prepared by calcination of the as-made SMS in air for 5 h at 773 K. The adsorption quantities of some organic compounds of different molecular sizes were evaluated from the concentration differences at initial and equilibrium states. The results are shown in Figure 6. The saturated adsorption quantity was greatest for rhodamine B, and decreased in the order of C16PyrCl, cyanocobalamin, and hemoglobin. The solid lines show the correlation curves with the Langmuir isotherm of eq 2. The parameters in the Langmuir isotherm are evaluated and summarized in Table 2 with molecular size. Because the pore size of calcined SMS was ∼2.2 nm, hemoglobin, which is larger than the mesopores, could not be adsorbed into the mesoporous silica, and cyanocobalamin, which is nearly the same size as the mesopores, was slightly adsorbed into the calcined SMS. Cationic solutes such as rhodamine B and C16PyrCl were adsorbed by electrostatic interaction with the silica surface. The results suggest that the adsorption property of the calcined SMS is due to size-selective adsorption depending on mesopore and molecular size. The calcined SMS was packed in a column as described above. The organic compound solutions were fed at a constant

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Figure 8. SEM images of commercial MCM-41 (a), molded commercial MCM-41 (b), and calcined SMS (c).

Figure 9. Breakthrough curves for cyanocobalamin with various mesoporous silica particles: calcined SMS (b), commercial MCM-41 (O), and molded commercial MCM-41 (9). The solid lines were calculated by eq 3.

volume rate of 1.0 cm3/min, and the feed concentrations, Cin, are summarized in Table 2. The concentration of the outlet solution was measured. The breakthrough curves for some organic compounds are shown in Figure 7. The breakthrough times as shown in Table 2 decreased with molecular size. The results suggest that size-selective separation can be performed using a column packed with calcined SMS. The solid lines in Figure 7 show the curves calculated by eq 3 with the values of parameters as shown in Table 2. The value of τ corresponds to the breakthrough time; that is, it decreased with the molecular size of the organic compounds because of the size selectivity. The value of the rate constant, k, decreased in the order hemoglobin, cyanocobalamin, and rhodamine B. The k value decreased with the strength of the interaction between the calcined SMS and the organic compounds, and the breakthrough curves became flatter. Comparison of Adsorption Performances between Calcined SMS and Commercial MCM-41. As mentioned in the previous paper,27 the as-made SMS prepared in oil-in-water emulsion possesses a reversed MCM-41 nanostructure comprising silica nanorods and continuous mesopores. Though the nanostructure may be destroyed by removing surfactants, the mesopores are formed by the surfactant regions between the arrays of silica nanorods as shown in Figure 1a. To evaluate the characteristics of separation for the calcined SMS, we compared it with commercial mesoporous silica particles with MCM-41. Commercial mesoporous silica particles, as shown in Figure 8, have an irregular shape of ∼60 µm particle size. The pore characteristics of commercial MCM-41 and calcined SMS are summarized in Table 3. The pore diameter of commercial particles was 4.1 nm, which is larger than that of the calcined SMS. The specific surface areas were almost the same. To obtain a particle size of 500 µm (which is that of the calcined SMS) for the commercial MCM-41, the commercial silica was molded by pressure. The molded powder was crushed and winnowed to prepare a powder size of 300-500 µm. A scanning electron microscopy (SEM) image of the molded powder is shown in Figure 8b. The shape of the particles was irregular and angular.

The mesoporous silica powders were packed in a column of 5 cm3 volume (8 mm inner diameter and 100 mm height). The packed weights of the particles are summarized in Table 3. The weight of as-made SMS was greater than that of the calcined SMS, because of the residual surfactants. The weights of the molded commercial MCM-41 and the calcined SMS were almost the same because of their similar powder sizes; the weight of the smaller commercial MCM-41 was lower for lower packing density. The pressure drop at a constant flow rate of 1.0 cm3/min depends mainly on the size and shape of the packed silica particles. The pressure drop for the packed calcined SMS was much lower than that for the molded commercial silica particles, irrespective of the fact that their particle sizes were the same, because the calcined SMS is spherical, as shown in Figure 8c. The breakthrough curves for cyanocobalamin (vitamin B12) as a solute are shown in Figure 9 with various silica particles. The solid lines were calculated by eq 3, and the values of the parameters are summarized with the Langmuir parameters for cyanocobalamin in Table 3. The saturated quantity and the Langmuir constant of cyanocobalamin for commercial silca particles were larger than those for calcined SMS because of the larger mesopore size. The breakthrough time for calcined SMS was shorter than that for the molded commercial MCM41, because of the differences of mesopore size. However, the value of k for the calcined SMS was larger than that for the molded commercial MCM-41; that is, the breakthrough curve for the SMS was sharp. This result may be caused by a weak interaction between the cyanocobalamin and calcined SMS because of the smaller mesopore size. The breakthrough time for the commercial MCM-41 was longer and the breakthrough curve was sharper. These results can be explained by the smaller particle size, because the diffusion path is shorter and the time to reach equilibrium adsorption was shorter than that for larger particles. To prolong the breakthrough time for the separation of cyanocobalamin by calcined SMS, the mesopore size needs to be increased. This issue will be addressed in a future study. Conclusion The adsorption properties of as-made and calcined SMS, which were prepared in two phases comprising an oil phase of TBOS as a silica source and an aqueous phase of dissolved surfactants, were evaluated by batch and column experiments, and the following conclusions were obtained. (1) The solubilized quantity of organic compounds with asmade SMS increased with the hydrophobicity of these substances. The breakthrough time also increased with the hydrophobicity. Mutual separation of weak acidic and basic compounds can be demonstrated using a column packed with as-made SMS and water as an eluant.

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(2) Breakthrough curves for different molecular sizes were obtained using a column packed with calcined SMS, and the size separation due to the mesopore size can be performed. (3) The pressure drop of the column packed with calcined SMS was lower than that for molded commercial mesoporous silica powder with MCM-41. The breakthrough curve for calcined SMS was sharper than that for molded commercial silica powder. We concluded that the mesopores in SMS are effective as adsorbents. Acknowledgment This study was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture, Japan (No. 16560662), and by a Tokyo Ohka Foundation for the promotion of Science and Technology (2005). Literature Cited (1) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T. W.; Olson, D. H., Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. A New Family of Mesoporous Molecular-Sieves Prepared with Liquid-Crystal Templates. J. Am. Chem. Soc. 1992, 114, 10834-10843. (2) Bagshaw, S. A.; Prouzet, E.; Pinnavaia, T. J. Templating of Mesoporous Molecular-Sieves by Nonionic Polyethylene Oxide Surfactants. Science 1995, 269, 1242-1244. (3) Feng, X.; Fryxell, G. F.; Wang, L. Q.; Kim, A. Y.; Liu, J.; Kemner, K. M. Functionalized Monolayers on Ordered Mesoporous Supports. Science 1997, 276, 923-926. (4) Lu, Y. F.; Ganguli, R., Drewien, C. A.; Anderson, M. T.; Brinker, C. J.; Gong, W. L., Guo, Y. X.; Soyez, H.; Dunn, B.; Hugng,. H.; Zink, J. I. Continuous formation of supported cubic and hexagonal mesoporous films by sol gel dip-coating. Nature 1997, 389, 364-368. (5) Zhao, D.; Fend, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Triblock copolymer syntheses of mesoporous silica with periodic 50 to 300 angstrom pores. Science 1998, 279, 548552. (6) Brinker C. J.; Lu, Y. F.; Sellinger, A.; Fan, H. Y. EvaporationInduced Self-Assembly: Nanostructures Made Easy. AdV. Mater. 1999, 11, 579-585. (7) Margolese, D.; Melero, J. A.; Christiansen, S. C.; Chmelka, B. F.; Stucky, G. D. Direct Syntheses of Ordered SBA-15 Mesoporous Silica Containing Sulfonic Acid Groups. Chem. Mater. 2000, 12, 2448-2459. (8) Huang, M. H.; Choudrey, A.; Yang, P. D. Ag nanowire formation within mesoporous silica. Chem. Commun. 2000, (12), 1063-1064. (9) Jung, J. H.; Ono, Y.; Hanabusa, K.; Shinkai, S. Creation of Both Right-Handed and Left-Handed Silica Structures by Sol-Gel Transcription of Organogel Fibers Comprised of Chiral Diaminocyclohexane Derivatives. J. Am. Chem. Soc. 2000, 122, 5008-5009. (10) Inagaki, S.; Guan, S.; Ohsuna, T.; Terasaki, O. An ordered mesoporous organosilica hybrid material with a crystal-like wall structure. Nature 2002, 416, 304-307. (11) Kruk, M.; Jaroniec, M.; Sayari, A. Application of Large Pore MCM41 Molecular Sieves To Improve Pore Size Analysis Using Nitrogen Adsorption Measurements. Langmuir 1997, 13, 6267-6273. (12) Sonwane, C. G.; Bhatia, S. K.; Calos, N. Experimental and Theoretical Investigations of Adsorption Hysteresis and Criticality in MCM41: Studies with O2, Ar, and CO2. Ind. Eng. Chem. Res. 1998, 37, 22712283. (13) Qiao, S. Z.; Bhatia, S. K.; Nicholson, D. Study of Hexane Adsorption in Nanoporous MCM-41 Silica. Langmuir 2004, 20, 389-395.

(14) Mercier, L.; Pinnavaia, T. J. Access in Mesoporous Materials: Advantages of a Uniform Pore Structure in the Design of a Heavy Metal Ion Adsorbent for Environmental Remediation. AdV. Mater. 1997, 9, 500503. (15) Brown, J.; Richer, R.; Mercier, L. One-step synthesis of high capacity mesoporous Hg2+ adsorbents by non-ionic surfactant assembly. Microporous Mesoporous Mater. 2000, 37, 41-48. (16) Antochshuk, V.; Olkhovyk, O.; Jaroniec, M.; Park, I. S.; Ryoo, R. Benzoylthiourea-Modified Mesoporous Silica for Mercury(II) Removal. Langmuir 2003, 19, 3031-3034. (17) Bois, L.; Bonhomme, A.; Ribes, A.; Pais, B.; Raffin, G.; Tessier, F. Functionalized silica for heavy metal ions adsorption. Colloids Surf., A 2003, 221, 221-230. (18) Kang, T.; Park, Y.; Yi, J. Highly Selective Adsorption of Pt2+ and Pd2+ Using Thiol-Functionalized Mesoporous Silica. Ind. Eng. Chem. Res. 2004, 43, 1478-1484. (19) Wei, Y.; Xu, J.; Dong, H.; Dong, J. H.; Qiu, K.; Jansen-Varnum, A. A. Preparation and Physisorption Characterization of D-GlucoseTemplated Mesoporous Silica Sol-Gel Materials. Chem. Mater. 1999, 11, 2023-2029. (20) Inumaru, K.; Kiyoto, J.; Yamanaka, S. Molecular selective adsorption of nonylphenol in aqueous solution by organo-functionalized mesoporous silica. Chem. Commun. 2000, 903-904. (21) Newalkar, B. L.; Choudary, N. V.; Kumar, P.; Komarneni, S.; Bhat T. S. G. Exploring the Potential of Mesoporous Silica, SBA-15, as an Adsorbent for Light Hydrocarbon Separation. Chem. Mater. 2002, 14, 304309. (22) Ho., K. Y.; Mckay, G.; Yeung, K. L. Selective Adsorbents from Ordered Mesoporous Silica. Langmuir 2003, 19, 3019-3024. (23) Vinu, A.; Murugesan, V.; Hartmann, M. Adsorption of Lysozyme over Mesoporous Molecular Sieves MCM-41 and SBA-15: Influence of pH and Aluminum Incorporation. J. Phys. Chem. B 2004, 108, 7323-7330. (24) Matsumoto, A.; Misran, H.; Tsutsumi, K. Adsorption Characteristics of Organosilica Based Mesoporous Materials. Langmuir 2004, 20, 71397145. (25) Yang X.-L.; Dai, W.-L.; Gao, R.; Chen, H.; Li, H.; Cao, Y.; Fan, K. Synthesis, characterization and catalytic application of mesoporous W-MCM-48 for the selective oxidation of cyclopentene to glutaraldehyde. J. Mol. Catal. A: Chem. 2005, 241, 205-214. (26) Huo, Q.; Feng, J.; Schuth, F.; Stucky, G. D. Preparation of Hard Mesoporous Silica Spheres. Chem. Mater. 1997, 9, 14-17. (27) Miyake, Y.; Kato, T. The Formation Process of Spherical Mesoporous Silica with Reverse Nanostructure of MCM41 in a Two-Phase System. J. Chem. Eng. Jpn. 2005, 38, 60-66. (28) Miyake, Y.; Yumoto, T.; Kitamura, H.; Sugimoto, T. Solubilization of Organic Compounds into As-Synthesized Spherical Mesoporous Silica. Phys. Chem. Chem. Phys. 2002, 4, 2680-2684. (29) Ozturk, N.; Kavak, D. Adsorption of Boron from Aqueous Solutions Using Fly Ash: Batch and Column Studies. J. Hazard. Mater. 2005, B127, 81-88. (30) Singh, T. S.; Pant, K. K. Experimental and modeling studies on fixed bed adsorption of As(III) ions from aqueous solution. Sep. Purif. Technol. 2006, 48, 288-296. (31) Gupta, V. K.; Mohan Suhas, D.; Singh, K. P. Removal of 2-Aminophenol Using Novel Adsorbents. Ind. Eng. Chem. Res. 2006, 45, 1113-1122. (32) Inumaru, K.; Nakano, T.; Yamanaka, S. Molecular selective adsorption of alkylphenols and alkylamines from for water by alkyl grafted mesoporous alumina: A comparative study to alkyl grafted mesoporous silica. Microporous Mesoporous Mater. 2006, 95 (1-3), 279-285.

ReceiVed for reView April 10, 2007 ReVised manuscript receiVed August 3, 2007 Accepted August 20, 2007 IE0705047