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The sonochemical synthesis and characterization of mesoporous chiral titania using a chiral inorganic precursor
Ultrasonics Sonochemistry 17 (2010) 605–609
Contents lists available at ScienceDirect
Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultsonch
The sonochemical synthesis and characterization of mesoporous chiral titania using a chiral inorganic precursor Alexandra Gabashvili, Dan T. Major, Nina Perkas, Aharon Gedanken * Department of Chemistry, Kanbar Laboratory for Nanomaterials, Nanotechnology Research Center, Institute of Nanotechnology and Advanced Materials, Bar-Ilan University, Ramat-Gan 52900, Israel
a r t i c l e
i n f o
Article history: Received 10 August 2009 Received in revised form 25 October 2009 Accepted 27 October 2009 Available online 30 October 2009 Keywords: Sonochemistry Mesoporous titania Chirality Chiral resolution
a b s t r a c t The paper presents a successful sonochemical attempt to synthesize mesoporous chiral titania using a chiral inorganic precursor and dodecylamine, as the surfactant template. The resulting porous structure was characterized by nitrogen sorption experiments, transmission electron microscopy, and small-angle XRD. The enantioselectivity of this mesoporous titania after the extraction of the amine was examined by selective adsorption of enantiomers and racemic aqueous solution of camphor. The selective adsorption was measured by circular dichroism (CD) spectroscopy. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction The importance of mesoporous materials lies in their use in many applications, such as catalytic and separation technologies. In a recent review article, the synthesis of mesoporous silica was regarded as one of the most important discoveries in solid-state and material science in the last decade [1]. We have already demonstrated the good results obtained in the sonochemical synthesis of mesoporous silica, MCM-41 [2], YSZ (yttria stabilized zirconia) [3] and titania [4], which were all prepared by this method. Titania [4] was synthesized sonochemically using titanium isopropoxide as the precursor and dodecylamine as the templating agent. Recently, the preparation of chiral mesoporous materials has become a great interest for material scientists. It is known that for the preparation of mesoporous materials, two starting materials are necessary, an inorganic polymerizing material and an organic surfactant. All the successful preparations of chiral mesoporous structures have inserted the chirality into the mesostructure through the use of a chiral template. To the best of our knowledge, there is no report on the preparation of chiral mesostructures using a chiral inorganic compound. Examples of the fabrication of chiral mesoporous materials are as follows: Alvaro et al. used chiral binaphthyl with TEOS (tetraethoxysilane) for the preparation of optically-active porous silica [5]. In a series of articles, Avnir et al. showed that template molecules, * Corresponding author. E-mail address: [email protected] (A. Gedanken). 1350-4177/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.ultsonch.2009.10.019
such as propranolol, 2,2,2-trifluoro-1-(9-anthryl) ethanol, DOPA, or tyrosine, can be used to prepare a chiral imprint sol–gel matrix [6,7]. Similarly, TiO2 thin films imprinted by chiral carboxylic acids were also reported, and enantioselectivity was observed [8]. A variety of other imprinting approaches for the preparation of chiral porous materials [9–11], including polymers [12] and dendrimers [13], has been studied. In all above-mentioned examples, the chiral property is introduced into the chiral mesoporous material via an organic chiral templating component. In the current paper, we present results describing an attempt to synthesize the chiral mesoporous material using a chiral inorganic precursor, while the templating surfactant was a non-chiral molecule. This paper presents a successful attempt to synthesize mesoporous chiral titania. For this synthesis we employed an ultrasound-assisted procedure and completed the synthesis within a few hours. Mesoporous chiral titania with wormhole-like framework structures was prepared using a longchain organic amine (dodecylamine) as a structure-directing agent and a chiral Ti ligand as a precursor, and its optical properties were demonstrated.
2. Experimental section The synthesis of the chiral (R,R)-§-Lig2Ti(OiPr)2 was carried out by a method previously reported by Kol and co-workers [14]. This ligand, chiral-at-metal, was used as the Ti precursor for the following synthesis of the chiral mesoporous titanium.
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The sonochemical synthesis of mesoporous titania was performed according to a procedure described elsewhere [4], namely, 0.01 mol of a chiral ligand was dissolved in 10 mL of absolute ethanol. The molar ratio of the organic amine to the ligand was fixed at 1:3.3. Typically, the corresponding amount of dodecylamine (0.0033 mol) was dissolved in a 10 mL absolute ethanol solution of the chiral ligand. The resulting solution was added slowly to 40 mL of double-distilled water under sonication (volume ratio of ethanol:water = 1:4). After this addition, the suspension was sonicated at ambient temperature for 6 h by a high-intensity ultrasonic probe (Misonix, XL sonifier, 1.13 cm diameter Ti horn, 20 kHz, 100 W/cm2). The obtained powder was separated by centrifugation. A control reaction without sonication was conducted at 60 °C. The reaction mixture that was heated for 6 h did not yield any products. For the removal of the surfactant, the as-prepared sample was treated by extraction with a dilute solution of HNO3 in ethanol, washed three times with ethanol, and dried overnight under vacuum. 3. Results and discussion
Fig. 2. Low-angle XRD pattern of the extracted sample. Inset is the wideangle XRD patterns.
Isotherm Data 400
Volume Adsorbed (cm3/g STP)
Mesostructured TiO2 was obtained by ultrasound irradiation using dodecylamine as the structure-directing agent and a chiral Ti ligand as the precursor. Spherical particles with a diameter of 50–100 nm were obtained for the as-prepared and the extracted material synthesized by the above-described method. Fig. 1 shows the high-resolution transmission electron microscopy (HR-TEM) image of the extracted sample. It is clear that the spherical particles are aggregates of very small particles about 2–4 nm in size. The morphology of the sample is in good agreement with the XRD results. The low-angle XRD of chiral mesoporous titania shows only one broad diffraction band peaked at 2h = 2.8° for the sample after extraction (Fig. 2). The bulk structure of the mesoporous chiral titanium is characterized by wide angle XRD, and it is demonstrated that the extracted sample is amorphous (Fig. 2 inset). The similar XRD patterns are also observed for the as-prepared samples and indicate the disordered wormhole frameworks structure. All these measured characteristic features are similar to the results reported previously for the sonochemical synthesis of mesoporous titania [4]. Fig. 3 shows the adsorption/desorption isotherms of the samples after extraction. It reveals a typical shape of IV isotherm with a hysteresis curve characteristic for mesoporous materials with well organized cylindrical pores. Specific surface areas of 50 m2/g
360 320 280 240 200 160 120 80 40 0 0
0.2
0.4
0.6
0.8
1
1.2
Relative Pressure (P/P0) Fig. 3. Adsorption/desorption isotherms for mesoporous TiO2 after extraction.
Fig. 1. HR-TEM images of extracted mesoporous titania.
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Fig. 4. Dodecyl amine micelle.
Fig. 5. Dodecyl amine micelle with covalently attached chiral (R,R)-§-Lig2Ti(OiPr)2.
100 D
98 96 94
Mass loss %
and 323 m2/g were found for the as-prepared and the extracted materials, respectively. Pore diameters of 5.5 nm were calculated from the desorption branch of the BET. In all previous work where dodecylamine was used as the template-directing agent for the synthesis of mesoporous materials, the pore size of the mesostructure was smaller (2.2–2.4 nm). It is evident from the above results that the pore size calculated from the absorption/desorption curves is 5.5 nm, which is much larger than that expected for dodecylamine. We therefore suggest that in the current case the mesostructure pore is influenced by both the dodecylamine surfactant, and the chiral Ti ligand. To calculate the pore’s width, a theoretical model of micelles based on bare dodecyl amine (DDA) molecules and of a dodecylamine bonded to the chiral (R,R)-§-Lig2Ti(OiPr)2 was developed. In Figs. 4 and 5 we present the structures as calculated by the model. A micelle based on SDS [15] was mutated into a DDA micelle using the Materials Studio Visualizer (Accelrys, Inc.), subsequently, the DDA micelle was minimized in the gas phase using the COMPASS forcefield [16] and the Forcite module of Materials Studio (Accelrys, Inc.). The surfactants were assumed to be neutral. Fig. 4 indicates that the diameter of a micelle made of pristine dodecylamine is about 4 nm. After the reaction with chiral ligand, the diameter is estimated to be around 5.5 nm (Fig. 5). The increase in diameter size can be explained by the ligand-assisted templating approach. In the first step, nitrogen from the amine group of the surfactant is covalently linked to the chiral Ti atom in the precursor. Subsequently, the Ti-amine complex is treated with water to force condensation. The aqueous environment in the condensation step is likely to force the hydrophobic ligand of the Ti-complex into the interior of the micelle, causing its swelling. This swelling increases the diameter of the micelles, and upon extraction yields larger pores. The aqueous environment in the condensation step is likely to force the hydrophobic ligand of the Ti-complex into the interior of the micelle, causing its swelling. This swelling increases the diameter of the micelles, and upon extraction yields larger pores. The increase in the surface area demonstrates that the extraction process successfully removes the surfactant molecules from the pores. The complete removal of the surfactant from the pores was confirmed by TGA measurements (Fig. 6). It is known, that the dodecylamine is degraded at 160 °C (m.p. 121 °C). The weight of dodecylamine in the mesoporous material
92 90 88 86 84 82 80
50
100
150
200
250
300
350
400
450
500
Temperature (C) Fig. 6. Thermogravimetric analyses of the mesoporous titania after removal of the surfactant.
is lost at a higher temperature than in the pure dodecylamine. According to the results of the Wang Yu-de, TGA of the as-synthesized mesoporous Ti with dodecylamine as the templating agent shows a loss of water below 153 °C and the surfactant loss in the range of 200–336 °C [17]. Fig. 6 presents the thermogravimetric analyses of the mesoporous titania obtained after polymer removal, showing approximately 15% weight loss in a temperature range of 50–130 °C due to loss of water and ethanol. No further weight loss at higher temperatures (up to 500 °C) due to loss of carbon residues was observed. In order to further prove the the full removal of the template via the solvent extraction process, we also performed elemental analysis measurements on the extracted chiral titania. The results of elemental analysis measurements showed no indication for the presence of nitrogen and very small amounts of carbon (ca. 1.5% carbon) proving a good removal of the polymer. In the next stage, we have examined the chiral recognition ability of our chiral titania. In this work, circular dichroism (CD) spectroscopy was selected as the method for exploring the chiral recognition of our chiral material.
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The chiral recognition ability of the MSP titania was probed using a racemic solution of D/L-camphor, as well as pure D-camphor and L-camphor solutions. The evaluation of the amount of adsorbed molecules onto the chiral mesoporous titania was determined by circular dichroism (CD) spectroscopy. We chose camphor, a chiral molecule, as a representative case to demonstrate the chiral recognition ability of the mesoporous Ti. This choice is based on the relatively large g value, (g = De/e, the anisotropy factor) of the n?p* transition in chiral carbonyls. The detection of the enantiomeric excess of the camphor would therefore be easier. CD measurements were carried out by the following steps: to each of the six portions of a racemic solution of D/L-camphor (0.75 g/10 ml), 2.5 mg of mesoporous TiO2 was added. These solutions were stirred at room temperature for different periods of time. 1 ml of the solution was withdrawn from each sample after 1, 4, 6, 10, 12, and 16 h. The same procedure was carried out for the solutions of the l or the d-enantiomer, and the optical activity of the solutions was tested as a function of time. In Fig. 7, we present the CD measurements of the racemic solution of camphor with chiral mesoporous titania. As can be seen from this figure, the excess of the L-enantiomer increased with time relative to the excess of the D-enantiomer in the solution. This was observed by the rise in the positive signal, which is the function of the content of the Lenantiomer in the solution. This means that the concentration of Dcamphor in the racemic solution decreases because of its selective adsorption into the pores of mesoporous titania The selective adsorption becomes evident after 6 h stirring of a camphor racemic solution with mesoporous titania. The following CD measurements of the pure enantiomer solutions (Figs. 8 and 9) proved that the chiral Ti is enriched with the d-enantiomer, namely, the D-enantiomer is preferentially adsorbed to the mesoporous Ti. The results demonstrate the enantioselective discrimination of the chiral Ti for the enantiomer pairs. In Fig. 8, which illustrates the CD measurements of the solution of the Lenantiomer with the mesoporous Ti, we did not observe an exchange of the positive signal, and thus we can conclude that the L-enantiomer was not adsorbed into the pores of Ti. On the other hand, in Fig. 9, which depicts the CD measurements of a solution of a D-enantiomer with the mesoporous Ti, we observed an increase in the negative signal, which is a function of the concentration of the D-enantiomer in solution. This result demonstrates the decrease in the amount of D-camphor in the solutions during the time.
Fig. 7. CD measurements of the camphor racemic solution with mesoporous titania after (A) 1 h, (B) 4 h, (C) 6 h, (D) 10 h, (E) 12 h, (F) 16 h.
Fig. 8. CD measurements of the camphor solution of L-enantiomer with mesoporous titania after (A) 1 h, (B) 10 h, (C) 16 h.
Fig. 9. CD measurements of the camphor solution of D-enantiomer with mesoporous titania after (A) 1 h, (B) 4 h, (C) 6 h, (D) 10 h, (E) 12 h, (F) 16 h.
References [1] J. Beck, J. Vartuli, W. Roth, M. Leonowicz, C. Kresge, K. Schmitt, C. Chu, D. Olson, E. Sheppard, S. Mccullen, J.B. Higgins, J. Schlenker, A new family of mesoporous molecular-sieves prepared with liquid-crystal templates, J. Am. Chem. Soc. 114 (1992) 10834–10843. [2] X. Tang, S. Liu, Y. Wang, W. Huang, Y. Koltypin, A. Gedanken, Rapid synthesis of high quality MCM-41 silica with ultrasound radiation, Chem. Commun. (2000) 2119–2120. [3] Y. Wang, L. Yin, O. Palchik, Y. Rosenfeld-Hacohen, Y. Koltypin, A. Gedanken, Rapid synthesis of mesoporous yttrium–zirconium oxides with ultrasound irradiation, Langmuir 17 (2001) 4131–4133. [4] Y. Wang, X. Tang, L. Yin, W. Huang, Y. Rosenfeld-Hacohen, A. Gedanken, Sonochemical synthesis of mesoporous titanium oxide with wormhole-like framework structures, Adv. Mater. 12 (2000) 1183–1186. [5] M. Alvaro, M. Bentez, D. Das, B. Ferrer, H. Garcia, Synthesis of chiral periodic mesoporous silicas (ChiMO) of MCM-41 type with binaphthyl and cyclohexadiyl groups incorporated in the framework and direct measurement of their optical activity, Chem. Mater. 16 (2004) 2222–2228. [6] S. Fireman-Shoresh, D. Avnir, S. Marx, Chem. Mater. 15 (2003) 3607. [7] S. Fireman-Shoresh, I. Popov, D. Avnir, S. Marx, Enantioselective chirally templated sol–gel thin films, J. Am. Chem. Soc. 127 (2005) 2650–2655. [8] S. Fireman-Shoresh, I. Turyan, D. Mandler, D. Avnir, S. Marx, Langmuir 21 (2005) 7842. [9] M. Lahav, A. Kharitonov, I. Willner, Imprinting of chiral molecular recognition sites in thin TiO2 films associated with field-effect transistors: novel functionalized devices for chiroselective and chirospecific analyses, Chem. Eur. J. 7 (2001) 3992–3997. [10] A. Thomas, M. Antonietti, Silica nanocasting of simple cellulose derivatives: towards chiral pore systems with long-range order and chiral optical coatings, Adv. Funct. Mater. 13 (2003) 763–766. [11] A. Seddon, H. Patel, S. Burkett, S. Mann, Chiral templating of silica–lipid lamellar mesophase with helical tubular architecture, Angew. Chem., Int. Ed. 41 (2002) 2988–2991. [12] S. Polarz, A. Kuschel, Preparation of a periodically ordered mesoporous organosilica material using chiral building blocks, Adv. Mater. 18 (2006) 1206.
A. Gabashvili et al. / Ultrasonics Sonochemistry 17 (2010) 605–609 [13] J.H. Jung, Y. Ono, K. Hanabusa, S. Shinkai, Creation of both right-handed and left-handed silica structures by sol–gel transcription of organogel fibers comprised of chiral diaminocyclohexane derivative, J. Am. Chem. Soc. 122 (2000) 5008–5009. [14] I.A. Yeori, S. Groysman, I. Goldberg, M. Kol, Diastereoisomerically selective enantiomerically pure titanium complexes of Salan ligands: synthesis, structure and preliminary activity studies, Inorg. Chem. 44 (2005) 4466–4468.
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[15] A. MacKerell, Molecular dynamics simulation analysis of a sodium dodecyl sulfate micelle in aqueous solution decreased fluidity of the micelle hydrocarbon interior, J. Phys. Chem. B 99 (1995) 1846–1855. [16] H. Sun, J. Phys. Chem. B 102 (1998) 7338. [17] Y. Wang, C. Ma, X. Sun, H. Li, Synthesis and characterization of mesoporous TiO2 with wormhole-like framework structure, Appl. Catal. A 246 (2003) 161– 170.
Contents lists available at ScienceDirect
Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultsonch
The sonochemical synthesis and characterization of mesoporous chiral titania using a chiral inorganic precursor Alexandra Gabashvili, Dan T. Major, Nina Perkas, Aharon Gedanken * Department of Chemistry, Kanbar Laboratory for Nanomaterials, Nanotechnology Research Center, Institute of Nanotechnology and Advanced Materials, Bar-Ilan University, Ramat-Gan 52900, Israel
a r t i c l e
i n f o
Article history: Received 10 August 2009 Received in revised form 25 October 2009 Accepted 27 October 2009 Available online 30 October 2009 Keywords: Sonochemistry Mesoporous titania Chirality Chiral resolution
a b s t r a c t The paper presents a successful sonochemical attempt to synthesize mesoporous chiral titania using a chiral inorganic precursor and dodecylamine, as the surfactant template. The resulting porous structure was characterized by nitrogen sorption experiments, transmission electron microscopy, and small-angle XRD. The enantioselectivity of this mesoporous titania after the extraction of the amine was examined by selective adsorption of enantiomers and racemic aqueous solution of camphor. The selective adsorption was measured by circular dichroism (CD) spectroscopy. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction The importance of mesoporous materials lies in their use in many applications, such as catalytic and separation technologies. In a recent review article, the synthesis of mesoporous silica was regarded as one of the most important discoveries in solid-state and material science in the last decade [1]. We have already demonstrated the good results obtained in the sonochemical synthesis of mesoporous silica, MCM-41 [2], YSZ (yttria stabilized zirconia) [3] and titania [4], which were all prepared by this method. Titania [4] was synthesized sonochemically using titanium isopropoxide as the precursor and dodecylamine as the templating agent. Recently, the preparation of chiral mesoporous materials has become a great interest for material scientists. It is known that for the preparation of mesoporous materials, two starting materials are necessary, an inorganic polymerizing material and an organic surfactant. All the successful preparations of chiral mesoporous structures have inserted the chirality into the mesostructure through the use of a chiral template. To the best of our knowledge, there is no report on the preparation of chiral mesostructures using a chiral inorganic compound. Examples of the fabrication of chiral mesoporous materials are as follows: Alvaro et al. used chiral binaphthyl with TEOS (tetraethoxysilane) for the preparation of optically-active porous silica [5]. In a series of articles, Avnir et al. showed that template molecules, * Corresponding author. E-mail address: [email protected] (A. Gedanken). 1350-4177/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.ultsonch.2009.10.019
such as propranolol, 2,2,2-trifluoro-1-(9-anthryl) ethanol, DOPA, or tyrosine, can be used to prepare a chiral imprint sol–gel matrix [6,7]. Similarly, TiO2 thin films imprinted by chiral carboxylic acids were also reported, and enantioselectivity was observed [8]. A variety of other imprinting approaches for the preparation of chiral porous materials [9–11], including polymers [12] and dendrimers [13], has been studied. In all above-mentioned examples, the chiral property is introduced into the chiral mesoporous material via an organic chiral templating component. In the current paper, we present results describing an attempt to synthesize the chiral mesoporous material using a chiral inorganic precursor, while the templating surfactant was a non-chiral molecule. This paper presents a successful attempt to synthesize mesoporous chiral titania. For this synthesis we employed an ultrasound-assisted procedure and completed the synthesis within a few hours. Mesoporous chiral titania with wormhole-like framework structures was prepared using a longchain organic amine (dodecylamine) as a structure-directing agent and a chiral Ti ligand as a precursor, and its optical properties were demonstrated.
2. Experimental section The synthesis of the chiral (R,R)-§-Lig2Ti(OiPr)2 was carried out by a method previously reported by Kol and co-workers [14]. This ligand, chiral-at-metal, was used as the Ti precursor for the following synthesis of the chiral mesoporous titanium.
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The sonochemical synthesis of mesoporous titania was performed according to a procedure described elsewhere [4], namely, 0.01 mol of a chiral ligand was dissolved in 10 mL of absolute ethanol. The molar ratio of the organic amine to the ligand was fixed at 1:3.3. Typically, the corresponding amount of dodecylamine (0.0033 mol) was dissolved in a 10 mL absolute ethanol solution of the chiral ligand. The resulting solution was added slowly to 40 mL of double-distilled water under sonication (volume ratio of ethanol:water = 1:4). After this addition, the suspension was sonicated at ambient temperature for 6 h by a high-intensity ultrasonic probe (Misonix, XL sonifier, 1.13 cm diameter Ti horn, 20 kHz, 100 W/cm2). The obtained powder was separated by centrifugation. A control reaction without sonication was conducted at 60 °C. The reaction mixture that was heated for 6 h did not yield any products. For the removal of the surfactant, the as-prepared sample was treated by extraction with a dilute solution of HNO3 in ethanol, washed three times with ethanol, and dried overnight under vacuum. 3. Results and discussion
Fig. 2. Low-angle XRD pattern of the extracted sample. Inset is the wideangle XRD patterns.
Isotherm Data 400
Volume Adsorbed (cm3/g STP)
Mesostructured TiO2 was obtained by ultrasound irradiation using dodecylamine as the structure-directing agent and a chiral Ti ligand as the precursor. Spherical particles with a diameter of 50–100 nm were obtained for the as-prepared and the extracted material synthesized by the above-described method. Fig. 1 shows the high-resolution transmission electron microscopy (HR-TEM) image of the extracted sample. It is clear that the spherical particles are aggregates of very small particles about 2–4 nm in size. The morphology of the sample is in good agreement with the XRD results. The low-angle XRD of chiral mesoporous titania shows only one broad diffraction band peaked at 2h = 2.8° for the sample after extraction (Fig. 2). The bulk structure of the mesoporous chiral titanium is characterized by wide angle XRD, and it is demonstrated that the extracted sample is amorphous (Fig. 2 inset). The similar XRD patterns are also observed for the as-prepared samples and indicate the disordered wormhole frameworks structure. All these measured characteristic features are similar to the results reported previously for the sonochemical synthesis of mesoporous titania [4]. Fig. 3 shows the adsorption/desorption isotherms of the samples after extraction. It reveals a typical shape of IV isotherm with a hysteresis curve characteristic for mesoporous materials with well organized cylindrical pores. Specific surface areas of 50 m2/g
360 320 280 240 200 160 120 80 40 0 0
0.2
0.4
0.6
0.8
1
1.2
Relative Pressure (P/P0) Fig. 3. Adsorption/desorption isotherms for mesoporous TiO2 after extraction.
Fig. 1. HR-TEM images of extracted mesoporous titania.
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Fig. 4. Dodecyl amine micelle.
Fig. 5. Dodecyl amine micelle with covalently attached chiral (R,R)-§-Lig2Ti(OiPr)2.
100 D
98 96 94
Mass loss %
and 323 m2/g were found for the as-prepared and the extracted materials, respectively. Pore diameters of 5.5 nm were calculated from the desorption branch of the BET. In all previous work where dodecylamine was used as the template-directing agent for the synthesis of mesoporous materials, the pore size of the mesostructure was smaller (2.2–2.4 nm). It is evident from the above results that the pore size calculated from the absorption/desorption curves is 5.5 nm, which is much larger than that expected for dodecylamine. We therefore suggest that in the current case the mesostructure pore is influenced by both the dodecylamine surfactant, and the chiral Ti ligand. To calculate the pore’s width, a theoretical model of micelles based on bare dodecyl amine (DDA) molecules and of a dodecylamine bonded to the chiral (R,R)-§-Lig2Ti(OiPr)2 was developed. In Figs. 4 and 5 we present the structures as calculated by the model. A micelle based on SDS [15] was mutated into a DDA micelle using the Materials Studio Visualizer (Accelrys, Inc.), subsequently, the DDA micelle was minimized in the gas phase using the COMPASS forcefield [16] and the Forcite module of Materials Studio (Accelrys, Inc.). The surfactants were assumed to be neutral. Fig. 4 indicates that the diameter of a micelle made of pristine dodecylamine is about 4 nm. After the reaction with chiral ligand, the diameter is estimated to be around 5.5 nm (Fig. 5). The increase in diameter size can be explained by the ligand-assisted templating approach. In the first step, nitrogen from the amine group of the surfactant is covalently linked to the chiral Ti atom in the precursor. Subsequently, the Ti-amine complex is treated with water to force condensation. The aqueous environment in the condensation step is likely to force the hydrophobic ligand of the Ti-complex into the interior of the micelle, causing its swelling. This swelling increases the diameter of the micelles, and upon extraction yields larger pores. The aqueous environment in the condensation step is likely to force the hydrophobic ligand of the Ti-complex into the interior of the micelle, causing its swelling. This swelling increases the diameter of the micelles, and upon extraction yields larger pores. The increase in the surface area demonstrates that the extraction process successfully removes the surfactant molecules from the pores. The complete removal of the surfactant from the pores was confirmed by TGA measurements (Fig. 6). It is known, that the dodecylamine is degraded at 160 °C (m.p. 121 °C). The weight of dodecylamine in the mesoporous material
92 90 88 86 84 82 80
50
100
150
200
250
300
350
400
450
500
Temperature (C) Fig. 6. Thermogravimetric analyses of the mesoporous titania after removal of the surfactant.
is lost at a higher temperature than in the pure dodecylamine. According to the results of the Wang Yu-de, TGA of the as-synthesized mesoporous Ti with dodecylamine as the templating agent shows a loss of water below 153 °C and the surfactant loss in the range of 200–336 °C [17]. Fig. 6 presents the thermogravimetric analyses of the mesoporous titania obtained after polymer removal, showing approximately 15% weight loss in a temperature range of 50–130 °C due to loss of water and ethanol. No further weight loss at higher temperatures (up to 500 °C) due to loss of carbon residues was observed. In order to further prove the the full removal of the template via the solvent extraction process, we also performed elemental analysis measurements on the extracted chiral titania. The results of elemental analysis measurements showed no indication for the presence of nitrogen and very small amounts of carbon (ca. 1.5% carbon) proving a good removal of the polymer. In the next stage, we have examined the chiral recognition ability of our chiral titania. In this work, circular dichroism (CD) spectroscopy was selected as the method for exploring the chiral recognition of our chiral material.
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A. Gabashvili et al. / Ultrasonics Sonochemistry 17 (2010) 605–609
The chiral recognition ability of the MSP titania was probed using a racemic solution of D/L-camphor, as well as pure D-camphor and L-camphor solutions. The evaluation of the amount of adsorbed molecules onto the chiral mesoporous titania was determined by circular dichroism (CD) spectroscopy. We chose camphor, a chiral molecule, as a representative case to demonstrate the chiral recognition ability of the mesoporous Ti. This choice is based on the relatively large g value, (g = De/e, the anisotropy factor) of the n?p* transition in chiral carbonyls. The detection of the enantiomeric excess of the camphor would therefore be easier. CD measurements were carried out by the following steps: to each of the six portions of a racemic solution of D/L-camphor (0.75 g/10 ml), 2.5 mg of mesoporous TiO2 was added. These solutions were stirred at room temperature for different periods of time. 1 ml of the solution was withdrawn from each sample after 1, 4, 6, 10, 12, and 16 h. The same procedure was carried out for the solutions of the l or the d-enantiomer, and the optical activity of the solutions was tested as a function of time. In Fig. 7, we present the CD measurements of the racemic solution of camphor with chiral mesoporous titania. As can be seen from this figure, the excess of the L-enantiomer increased with time relative to the excess of the D-enantiomer in the solution. This was observed by the rise in the positive signal, which is the function of the content of the Lenantiomer in the solution. This means that the concentration of Dcamphor in the racemic solution decreases because of its selective adsorption into the pores of mesoporous titania The selective adsorption becomes evident after 6 h stirring of a camphor racemic solution with mesoporous titania. The following CD measurements of the pure enantiomer solutions (Figs. 8 and 9) proved that the chiral Ti is enriched with the d-enantiomer, namely, the D-enantiomer is preferentially adsorbed to the mesoporous Ti. The results demonstrate the enantioselective discrimination of the chiral Ti for the enantiomer pairs. In Fig. 8, which illustrates the CD measurements of the solution of the Lenantiomer with the mesoporous Ti, we did not observe an exchange of the positive signal, and thus we can conclude that the L-enantiomer was not adsorbed into the pores of Ti. On the other hand, in Fig. 9, which depicts the CD measurements of a solution of a D-enantiomer with the mesoporous Ti, we observed an increase in the negative signal, which is a function of the concentration of the D-enantiomer in solution. This result demonstrates the decrease in the amount of D-camphor in the solutions during the time.
Fig. 7. CD measurements of the camphor racemic solution with mesoporous titania after (A) 1 h, (B) 4 h, (C) 6 h, (D) 10 h, (E) 12 h, (F) 16 h.
Fig. 8. CD measurements of the camphor solution of L-enantiomer with mesoporous titania after (A) 1 h, (B) 10 h, (C) 16 h.
Fig. 9. CD measurements of the camphor solution of D-enantiomer with mesoporous titania after (A) 1 h, (B) 4 h, (C) 6 h, (D) 10 h, (E) 12 h, (F) 16 h.
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