Ind. Eng. Chem. Res. 2009, 48, 755–762
755
Controlled Fabrication of Porous Titania Beads by a Sol-Gel Templating Method Kai-Feng Du,†,‡ Dong Yang,† and Yan Sun*,† Department of Biochemical Engineering, School of Chemical Engineering and Technology, Tianjin UniVersity, Tianjin 300072, China, and Department of Pharmaceutical & Biological Engineering, School of Chemical Engineering, Sichuan UniVersity, Chengdu 610065, China
Porous titania beads were synthesized by a sol-gel templating method using agarose gel as template. The morphology and structure of the materials were characterized by X-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy, and N2 adsorption analysis, etc. It was found that the size and pore structure of the resultant titania beads could be readily controlled by using various sized templates and by changing the precipitation conditions. Under optimized conditions (i.e., impregnating agarose gel into titanium precursors for 6 h, three cycles of repeated impregnation and hydrolyzing process, and calcination at 450 °C), porous titania beads of 7-150 µm in mean diameters were fabricated by using different sized agarose beads as templates. The beads displayed perfect spherical shape, with an average pore size of 6 nm and specific surface area of 69.9 m2 g-1. The titania beads of 15 µm in average diameter were further characterized for application as liquid chromatographic packings. Flow hydrodynamic experiment indicated that they possessed high mechanical strength to withstand a pressure drop up to 12 MPa. The column efficiency reached 6165 plates m-1 for acetone and 4650 plates m-1 for N,N-dimethylaniline under nonretained condition. Moreover, baseline separation of three aniline derivatives featuring only small structural differences was realized by using this packed column. 1. Introduction In recent years, titania materials for high performance liquid chromatography (HPLC) have attracted considerable attentions. The popularity is attributed to their excellent chemical stability in comparison with silica, ability to selectively adsorb organophosphorylate compounds, and amphoteric ion-exchange properties for charged species, etc. 1 The column packed with titania beads has been developed for the analysis of nucleotides,2 phospholipids, 3 phosphorylated proteins and peptides,4-11 diesel fuels, 12 and basic fine chemicals and pharmaceuticals.13 Furthermore, there are already some titania packings, such as Sachtopore13 and Titansphere,12 commercially available in the market. Until now, several strategies have been used to produce titania beads, including polymerization-induced colloid aggregation (PICA), 14 oil emulsion method (OEM),15 and spray-dry method.16 It is notable that there are some disadvantages of these methods for the fabrication of titania beads. For example, the agglomeration process in the PICA method easily leads to products with low repeatability because their mechanical strength and porous structure can be seriously affected by heating treatment. Titania particles synthesized by the sol-gel, OEM, and spray-dry technologies always have a broad particle size distribution. Furthermore, the size of titania particles produced using these methods is usually small (below 10 µm), which limits their application in chromatographic separation. More recently, titania monoliths with controlled multiscale porosity have been prepared by a sol-gel process accompanied by phase separation17,18 and the vapor-phase leaching of zinc from a Zn2TiO4 precursor.19 Unfortunately, these two technologies cannot be used to synthesize titania beads. Therefore, it remains a challenge to explore a novel method to synthesize * To whom correspondence should be addressed. E-mail: ysun@ tju.edu.cn. Tel.:+86-22-27404981. Fax: +86-22-27406590. † Tianjin University. ‡ Sichuan University.
well-defined porous titania beads with favorable porosity and mechanical stability for HPLC packings. The sol-gel templating is a good method for fabricating inorganic materials with controlled morphology and inner-pore structure. The fabricating process involves the infiltration of precursors into the interstice of preformed templates and subsequent removal of the templates by calcination or extraction.20,21 Using this method, Caruso and co-workers 22,23 and Qi and co-workers24 prepared porous titania beads, employing commercial porous polymer beads as template, and investigated their photocatalytic property. It is notable that these templates are usually costly, and their size and structure are unchangeable. So, the size and structure of the prepared titania beads are generally uncontrollable. In addition, the resultant titania beads are usually brittle. Recently, a novel titania material was successfully prepared using cuboid agarose gel block as template.25,26 Although the pore structure was perfectly replicated and controllable, the final material showed poor mechanical stability because the agarose fibers were only coated with a thin titania film. The shortcomings would seriously limit their application as chromatographic packings. In this work, porous titania beads were synthesized by an improved sol-gel templating procedure using agarose gel as template. To fabricate high-quality beads for chromatography, two key points must be taken into account. One is the choice of template. The agarose gel can be prepared by a water/oil emulsion method. The customized template is easily prepared and its size and agarose content can be readily adjusted, so the titania beads prepared with the template would be competent to the requirement of chromatographic packings. The other is the optimization of sol-gel procedure. In the present work, we have extensively investigated the important factors influencing the formation of porous material. As a result, porous anatase beads were obtained under optimized conditions. The fabricated porous titania beads were fully characterized and the chromatographic performance of the material was studied.
10.1021/ie8011165 CCC: $40.75 2009 American Chemical Society Published on Web 12/03/2008
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2. Experimental Details 2.1. Chemicals. Agarose powder was obtained from Biowest (Shanghai, China); polyoxyethylene sorbitanmonooleate (Tween 80) was purchased from Dengfeng Chemical Company (Tianjin, China); Sorbitan trioleate (Span 85) was purchased from China National Pharmaceutical Industry Company (Beijing, China); the metal oxide precursor, titanium(IV) tetraisopropoxide (TTIP, 99%), was purchased from Luchuan Chemical Company (Shandong, China). Other reagents were all of analytical grade and received from local sources. 2.2. Preparation of Agarose Gel. Agarose gel of 6% (w/v) was prepared by the water/oil emulsion method as described in the literature.27 To synthesize agarose beads of suitable particle size and pore structure, some modifications in preparation were carried out. In a typical procedure, 3 g agarose powder was added into 47 mL water and heated to 95 °C in a water bath to dissolve agarose (6%, w/v). During the warm-up period, care was taken to keep the agarose powder well suspended with occasional shaking until complete dissolution. Hot water was added during the heating process to keep the original weight of agarose solution unchanged. Then, the agarose solution was poured into a thermostatic three-neck flask containing a mixture of 100 mL of cyclohexane, 3.8 g of Span 85, and 1.1 g of Tween 80 at 60 °C under continuous stirring. After the mixture was emulsified by stirring at 800 rpm for 30 min at 60 °C, it was cooled down to 5 °C and kept stirring for another 30 min to solidify the agarose gel. Subsequently, the beads were washed with ethanol and distilled water, and screened with metal meshes. A solvent exchange procedure for the agarose gel was carried out by slowly increasing the ratio of 2-propanol to water until the beads were in high-purity 2-propanol (99.7%), similar to the procedure reported previously.28 Dehydrated agarose beads with different size ranges were obtained by the procedure and used as templates for the titania precipitation. 2.3. Fabrication of Porous Titania Beads. The titania beads were fabricated using the above agarose gel as template. Typically, the agarose gel was dipped into an 80 wt % TTIP solution in 2-propanol, and kept in a closed container for 2-6 h at 25 °C. The agarose gel filled with TTIP was washed with 10 portions of 2-propanol for 3 min in air, and then the residual TTIP on the gel surface was removed by suction filtration on a filter flask. The beads were then transferred into a water/2propanol (1:1, v/v) mixture, where hydrolysis and condensation reaction (here defined as precipitation) occurred and continued for 1 h. This is the sol-gel precipitation procedure. To investigate the effect of the procedure, the impregnation time of the template in TTIP solution was fixed at 6 h and one to four cycles of the precipitation processes were carried out. Finally, the titania/agarose hybrid beads were dried in air at room temperature and then calcined at 450 °C for 6 h in air to burn off the agarose and to crystallize the titania. 2.4. Characterization of Titania Beads. Optical microscopy (OM) images of agarose gel were recorded on a Nikon ECLIPSE E600 microscope. Scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) were employed to observe the microscopic morphology of the as-synthesized titania beads on an XL 30 ESEM (Philips, Netherlands) and a Tecnai G2 F20 (FEI Company, Netherlands) microscope, respectively. For analysis with HRTEM, the samples were crushed completely, and then dispersed using ethanol. The X-ray powder diffraction (XRD) patterns were collected on a Rigaku D/max 2500v/pc X-ray diffractometer (Rigaku, Japan) using Cu KR radiation (λ ) 0.1542 nm) to determine the crystallite size and identity of the prepared titania
Figure 1. OM (a,b) and SEM images (c,d) of agarose gel: swollen beads in 2-propanol (a); dried beads (b,c); broken debris of dried agarose beads (d).
samples. Isothermal adsorption-desorption curves and pore size distribution were obtained from N2 adsorption measurement at -196 °C with a NOVA 2000 porosimeter (Quantachrome). The thermogravimetric-differential thermal analysis (TG-DTA) was performed using a Pyris Diamond TG-DTA (Perkin-Elmer Instruments, Boston, MA) at a heating rate of 10 °C min-1 and an air flow of 100 mL min-1. Particle-size distribution of the resultant samples was measured with a Mastersizer 2000 U (Malvern Instruments, U.K.). The density and water content of the wet beads were calculated by water replacement in a 25 mL pycnometer as described in the literature.29 2.5. Chromatography. Titania beads with a mean diameter of 15 µm were fabricated and packed in a stainless steel column of 50 mm × 4.6 mm without further sieving. A flow hydrodynamic experiment was carried out using distilled water as mobile ¨ KTA Explorer 100 system (GE Healthcare, phase on an A Uppsala, Sweden). Other chromatographic experiments were performed on an Agilent 1100 system (Agilent Technologies, DE) using the same packed column. Column efficiency was expressed as the height equivalent to a theoretical plate (HETP) and calculated by the half-peak height method.30 Acetone and N,N-dimethylaniline were used as tracers under nonretained condition. These parameters can to some extent reflect the relative quality of titania beads and provide a reference for preparing chromatography packings. The weakly acidic surface of the titania-based packings allows the separation of basic compounds. So, to further evaluate the chromatographic performance of the titania beads, liquid chromatography of a mixture containing aniline derivatives (N,N-dimethylaniline, N-methylaniline, aniline) was conducted using a mixed solvent of n-heptane and 2-propanol (99.5:0.5, v/v) as eluent. 3. Results and Discussion 3.1. Template. Typical images of 2-propanol-filled and dried agarose beads are shown in Figure 1. It is observed that the 2-propanol-filled beads exhibit a perfectly spherical shape and relatively uniform size of about 110 µm (Figure 1a). After drying, the size of agarose beads shrank to 35 µm (Figure 1b), only about 3.4% of the swollen beads in volume. From the SEM images (Figure 1c,d), we can see more clearly that the dried agarose beads were of spherical shape with the surface full of micrometer-sized wrinkles, and their exterior and interior were homogeneous (Figure 1d). This was also confirmed by the
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Figure 2. Schematic illustration of the sol-gel templating procedure for the fabrication of porous titania beads using agarose gel as template.
narrow pore size distribution of the dried agarose beads, which was measured by an N2 adsorption experiment (data not shown). When the dried agarose beads were immersed in water, and the water was repeatedly displaced with 2-propanol, they could nearly swell back to the original size of swollen beads in 2-propanol. This result indicates that there was large volume of cavity in the swollen agarose gel, and small molecules could easily diffuse into its inside. These properties of agarose gel endow its potential application as a template for the fabrication of inorganic porous beads, because the swelling property allows easy infiltration of reaction precursors, and the shrinking property enables the entanglement of precipitated solid. In addition, the porosity and size of agarose beads can be easily adjusted by respectively changing stirring speed and agarose and Tween 80 concentrations in the water/oil emulsification procedure. All these characteristics suggest that agarose beads would be a good template for the synthesis of spherical titania materials. 3.2. Sol-Gel Templating Procedure. A schematic diagram for the sol-gel templating method is shown in Figure 2. In the first step, the titanium precursor TTIP was impregnated into the swollen agarose beads consisting of an interwoven porous network. This was followed by a subsequent hydrolysis and polycondensation to produce titania/agarose hybrid beads. The hybrid beads are composed of two interconnected threedimensional networks: one is the interwoven fiber originated from the agarose template, and the other is the continuous amorphous titania formed within the pores of templates. It is worthwhile noting that water must be completely excluded from the agarose template in order to avoid the formation of hollow titania shells or heterogeneous structures.24 Furthermore, to prevent the aggregation of the hybrid beads, TTIP on the surface of the template should be washed away with 2-propanol before its hydrolyzation. In the second step, the hybrid beads were calcined at high temperature to obtain porous titania beads composed of titania nanocrystals. The pore structure of the titania beads may come from two sources: one is the pore left by the template upon calcination and the other is the pore accumulated by titania nanocrystals. 3.3. Porous Titania Beads. Typical SEM images of titania/ agarose hybrid beads and titania beads obtained by calcination at 450 °C are presented in Figure 3. Both types of the beads show perfectly spherical shape and relatively smooth surface (Figure 3a,c), indicating a good correlation with the original agarose gel shown in Figure 1. From the picture of the fractured titania/agarose hybrid bead (Figure 3b), a distinct gap in the hybrid bead is observed. This is a general phenomenon observed in many fractured beads. It is considered that repeated impregnation and washing in the fabrication procedure might be the main reason for the gap formation. The titania precursors can diffuse into the templates during impregnation and diffuse out in the washing process. The repeated processes would result in inhomogeneous distribution of amorphous titania in the templates after hydrolysis and drying. Under pressure, the hybrid beads were broken up, and the gaps were formed along with the most uneven parts in the hybrid beads. After calcination,
Figure 3. SEM images of titania/agarose hybrid beads (a,b) and titania beads after calcination (c,d). Conditions: the samples (a,b) were prepared by 6 h impregnation in 80 wt % TTIP solution for each precipitation and three cycles of precipitations; the samples (c,d) were obtained upon calcination of the dried titania/agarose hybrid beads at 450 °C for 6 h. Table 1. Size and Volume Ratio of Original Agarose Gel and Calcined Titania Beads original agarose gela dp (size range)b (µm)
calcined titania beadsa dp (size range)b (µm)
Vagarose c gel/Vtitania
344 (220-485) 236 (187-283) 110 (76-147) 17 (11-24)
150 (95-210) 102 (76-131) 47 (33-63) 7 (5.5-8.8)
12.1 12.4 12.8 14.3
a Particle-size distribution measured with a Mastersizer 2000 U (Malvern Instruments, U.K.). b dp is the volume-weighted mean diameter. The size range describes the volumetric diameter range of d10 to d90, which are defined as the points on the size distribution where, respectively, 10% and 90% by volume of the particles are smaller than the stated diameter. c Volume of samples calculated from the particle size (V ) 4πR3/3).
the templates were decomposed to CO2 and water, while the amorphous phase of titania was transformed to anatase phase and realigned. All of these made the resultant titania beads more homogeneous and compact, and the gaps were eliminated (Figure 3d). The change in size of the beads is directly revealed by the particle-size distribution measurement. The average size of 2-propanol-filled titania/agarose hybrid beads is about 110 µm and has the same size distribution with that of original agarose gel; it decreased to 70 µm after drying, and further to 47 µm after calcination. Thus, compared with the dried hybrid beads, the average size of the titania beads decreased more than 33% in diameter. However, the shrinking did not cause any cracking of the beads or leave any flaw on the surface, as observed in Figure 3c,d. The shrinkage is a common phenomenon observed in the sol-gel process of titania.21-24 It is attributed to the condensation of titanium precursors with unreacted alkoxy or hydroxyl groups and the crystallization of amorphous titania particles. In the study, we have also generated micrometer-sized titania bead (mean size of 7 µm) by using agarose gel of about 17 µm in size and investigated the effect of size of the agarose gel on that of the calcined titania beads. The variation of size and volume of original agarose gel and calcined titania beads were summarized in Table 1. As seen here, the size of prepared titania beads was directly proportional to that of the original agarose gel. Moreover, the volume ratio of original template to final
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Figure 4. SEM images of titania beads prepared at different impregnation times: 2 h (a,b); 4 h (c,d); 6 h (e,f). Other conditions: the samples were prepared by one cycle of precipitation in 80 wt% TTIP solution and calcined at 450 °C for 6 h.
calcined titania beads was calculated as a relatively constant value of 13 ( 1 under a fixed sol-gel process regardless of altering the size of agarose gel. For this reason, the size of titania beads can be easily adjusted via changing the template size. 3.4. Effect of Impregnation Time on Bead Structure. In the precipitation step, the duration of agarose beads impregnation in the TTIP solution in 2-propanol may influence the morphology and structure of the resulting titania beads. To investigate this effect, a series of titania beads were prepared with different impregnation time, and their SEM images are shown in Figure 4. When the impregnation time was not longer than 2 h, the resulting titania material not only shrank seriously, but also was prone to become deformed and fragile because of its hollow structure (Figure 4a,b). When the impregnation time reached 4 h, solid beads were obtained because of complete infiltration of the titanium precursors, but there were still some collapsed places on their surface (Figure 4c,d). When an impregnation time of 6 h was applied, the titania beads exhibited better spherical shape and smoother surface (Figure 4e,f). Further impregnating agarose gel in titania precursor did not perfect the structure and morphology according to the further experimental verification. The impregnation time mainly reflects the diffusion degree of TTIP into agarose gel. According to these results, 6 h impregnation was required to make TTIP molecules saturate the template inside in this work. 3.5. Effect of Precipitation on Bead Structure. The precipitation is another important factor influencing the structure of titania beads. So, titania beads were synthesized at different precipitation cycles, and their typical SEM images are presented in Figure 5. Because large-sized agarose gel was used as template, the mean size of the beads was about 150 µm. From the figure, it can be seen that the titania material prepared by one cycle of precipitation shows a spherical morphology, but there are some dents on the surface (Figure 5a,b). It shows that even though the precursor molecules were saturated in the agarose beads by the 6 h impregnation, serious
Figure 5. SEM images of titania beads and their surface prepared at different precipitation cycles: one (a,b); two (c,d); three (e,f); four (g,h). The mean size of the particles was about 150 µm. Other conditions: the samples were prepared by 6-h impregnation in 80 wt % TTIP solution for precipitation and finally calcined at 450 °C for 6 h.
shrinkage occurred in the condensation and calcination process, leading to the formation of defects in the titania beads. So, two or more precipitation processes were tested to avoid the defects. As can be seen from Figure 5c-h, with two or more repeated precipitation processes, the titania beads with smoother surface were obtained. Despite this, their surface structure observed at larger magnification displayed different morphology (Figure 5d,f,h). The surface of bead prepared by two cycles of precipitations (Figure 5d) shows a nonporous morphology with abundant wrinkles, similar to the template beads shown in Figure 1. In contrast, the beads prepared by three cycles of precipitations display a coarse surface with uniform pore channels (Figure 5f). It indicates that they might have large specific surface area and adsorption capacity. When four cycles of precipitations were conducted, the pore size of the resultant beads became smaller (Figure 5h), and many submicrometer-sized particles on the surface of titania beads were formed (Figure 5g). It is considered that the templates adsorbed excessive titania precursors, and titania crystalline precipitated after hydrolization and condensation. To further study the effect of precipitation on the bead structure, additional data of particle-size distribution, water content, and wet density were measured (Table 2). It was found that the particle-size distribution of the hybrid beads changed little with increasing the precipitation cycles. However, the wet density increased monotonically from 1.32 g cm-3 to 1.96 g cm-3, and the water content of the wet beads decreased correspondingly from 85.5% to 68.3% (v/v). This suggests that
Ind. Eng. Chem. Res., Vol. 48, No. 2, 2009 759 Table 2. Effect of Precipitation on the Properties of Titania/Agarose Hybrid Beads precipitation cycle
particle-size distributiona (µm)
water contentb (v/v, %)
wet densityb (g mL-1)
1 2 3 4
99 ( 2 99 ( 2 99 ( 2 99 ( 2
85.5 83.1 72.6 68.3
1.32 1.44 1.63 1.96
a Particle-size distribution measured with a Mastersizer 2000 U (Malvern Instruments, U.K.). b Water content and wet density calculated by water replacement in a 25 mL pycnometer.29
Figure 7. XRD patterns of titania beads calcined at different temperatures in air: 300 (a); 450 (b); 600 (c); 700 (d); 900 °C (e). Other conditions were the same as in Figure 6. Table 3. Physical Properties of Titania Beads Calcined at Different Temperatures SBETb temperature Vpc (°C) da (nm) (m2 g-1) (mL g-1) Dd (nm) Fe (%) P f (%) 450 600 700 900 Figure 6. Thermogravimetric analysis of dried agarose beads (a) and titania/ agarose hybrid beads (b) in air atmosphere. The hybrid beads were prepared by 6 h impregnation in 80 wt % TTIP solution for each precipitation and three cycles of precipitations.
titania content significantly increased with an increase in the precipitation cycle. 3.6. Effect of Calcination Temperature on Bead Structure. Calcination was performed to remove agarose in the hybrid beads and to modify the crystal phase of the titania material. Before calcination, thermogravimetric (TG) analysis of dried agarose beads and titania/agarose hybrid beads was performed to assess the removal condition of the template. As shown in Figure 6a, the TG plot of dried agarose beads reveals two weight loss processes from room temperature to 450 °C. Below 105 °C, there was a weight loss of about 15%. It is attributed to the loss of water and 2-propanol. Between 240 and 450 °C, a weight loss of the other 85% was observed. It indicates that agarose molecules had been pyrolyzed to CO2 and H2O completely. From the TG plot of titania/agarose hybrid beads (Figure 6b), a different phenomenon at 120-230 °C was observed. There was a weight loss of about 5% in this process, possibly arisen from the further condensation of titania oligomers and unreacted titanium precursors. When the calcination temperature surpassed 450 °C, no further weight loss was observed, leaving nearly 55% of the titania solid. The XRD patterns of the titania beads calcined at different temperature are presented in Figure 7. It can be seen that the XRD curve of titania beads calcined at 300 °C lacks crystalline peaks (Figure 7a), indicating an amorphous state of the titania. When the calcination temperature increased to 450-600 °C, the titania beads displayed a crystal phase of pure anatase. The transformation to the rutile phase occurred when the calcination temperature increased further. The rutile content of titania samples calcined at 900 °C reached 68.6% (Table 3). According to the line width analysis of the (101) reflection based on the Scherrer formula,33 the average crystallite size of titania beads changed from 10 to 33 nm with the increase of calcination temperature, in accordance with the results reported by Qi et al.24
10 13 16 33
69.9 12.9 6.7 5.2
0.1248 0.0404 0.0226 no
6 13 14 no
0 0 7.2 68.6
34.4 14.6 8.7 no
a Crystallite size calculated from XRD patterns based on the Scherrer formula. b BET surface area taken from the linear part of the BET plot (P/P0 ) 0.05-0.225). c Total pore volume estimated from the volume of nitrogen adsorbed at P/Po ) 0.986. d Average pore diameter obtained using the desorption branch of the isotherm by the BarrettJoyner-Halenda (BJH) method.26 e Rutile fraction calculated with the empirical equation:31 R(t) ) 0.678(IR/(IR + IA)) + 0.312(IR/(IR + IA))2. f Porosity calculated by the volume of the liquid nitrogen corresponding to the total pore and the volume of titania.32
The titania beads were further characterized by TEM and HRTEM to get more details about their microstructure and crystal size dependences on the calcination temperature. From the TEM images (Figure 8a,c,e), it is found that the titania beads were composed of titania nanoparticles. The size of titania nanoparticles increased from about 10 to 15 nm when the temperature increased from 450 to 700 °C, in good agreement with the corresponding XRD results (Table 3). As shown in Figure 8b,d,f, typical HRTEM images of the three samples all revealed the presence of nanocrystals and displayed clear anatase lattice fringes. It indicates that the titania beads consisted of anatase nanocrystals. The nitrogen adsorption-desorption isotherms were measured to determine the specific surface area, porosity, pore volume, and pore size distribution of the titania beads calcined at different temperature. As shown in Figure 9, the isotherm of titania samples obtained at 450 °C exhibits type IV with an H2-type hysteresis loop, and a well-defined step with a final saturation plateau occurs at P/P0 ) 0.6-0.9. The results suggest the presence of an interconnected uniform mesoporous system.34 This point is also proven by its pore size distribution plot in the figure inset, which shows a single peak at about 6 nm. The titania samples calcined at 600 and 700 °C exhibit much smaller isotherms than that shown in Figure 9 (data not shown), indicating lower adsorption capacity for nitrogen. It is inferred that the pores disappeared gradually with increasing calcination temperature. However, it is observed that there is a little increase in the adsorption curve until P0, for the beads calcined at 700 °C, suggesting that some macropores appeared, though their porosity was very small (about 8.7% v/v, data not shown). The variations of the BET (Brunauer-Emmett-Teller) surface area
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Figure 10. Effect of flow rate on back pressure: column, 50 × 4.6 mm; media, porous titania beads of 15 µm; mobile phase, distilled water. The beads were prepared by the method described in Figure 3.
Figure 8. TEM (a,c,e) and HRTEM (b,d,f) images of titania beads calcined at 450 (a,b); 600 (c,d); 700 °C (e,f). Other conditions were the same as in Figure 6.
Figure 9. Nitrogen adsorption-desorption isotherm of titania beads. The dash curve in the figure represents the adsorption isotherm and the solid curve represents the desorption isotherm. The corresponding pore size distribution (inset) was obtained from the desorption branch of the isotherm using the Barrett-Joyner-Halenda (BJH) method. P0 ) 101325 Pa; Ds represents differential specific area by BJH method. The beads were prepared by the method described in Figure 3.
and BJH (Barrett-Joyner-Halenda) pore volume of the titania beads with calcination temperature are listed in Table 3. It shows that with an increasing temperature from 450 to 700 °C, the surface area and pore volume monotonically decrease from 69.9 to 6.7 m2 g-1 and from 0.1248 to 0.0022 mL g-1, respectively. Obviously, the increase of calcination temperature brought about the growth of titania nanocrystals to produce accessible macropores and the mesopores disappeared.
Figure 11. Effect of the flow rate on HETP for acetone with water as mobile phase (a) and N,N-dimethylaniline with the mixture of n-heptane and 2-propanol (97:3, v/v) as mobile phase (b). Column was the same as in Figure 10.
3.7. Chromatographic Characteristics. 3.7.1. Flow Hydrodynamics. During chromatographic process, the back pressure has significant influence on the life-span of pump used in HPLC and packing materials. 35 Usually, HPLC pump allows a maximal pressure around 40 MPa, and high back pressure may squash the chromatography packings. So it is important to measure the relationship between back pressure and flow rate for the column packed with the titania beads. Figure 10 exhibits the relationship between the back pressure and flow rate using pure water as the mobile phase. It is observed that the back pressure vs flow rate curve displayed excellent linearity up to a flow rate as high as 2160 cm h-1 (6 mL min-1). This result confirms that the titania-based packings had high mechanical stability at a back pressure up to 12 MPa. 3.7.2. Column Efficiency. High column efficiency is an important requirement for the isocratic adsorption chromatography and helps to translate even a small difference in the strength of interactions into well-resolved peaks.36 Column efficiency depends on different variables, such as particle size, morphology, porosity, and homogeneity of the packings and flow rate, etc., as described by the Van Deemter equation.37 In general, the Van Deemter curve is often used to illustrate the relationship between column efficiency and flow rate. The experimental data for the isocratic elution behavior of acetone and N,N-dimethylaniline was obtained. The direct comparison of column efficiencies for the two different solutes is recorded in Figure 11. As seen in Figure 11, by increasing the flow rate from 50 to 1000 cm h-1, the value of HETP for N,Ndimethylaniline increased from 0.21 to 0.58 mm, while for
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Figure 12. Separation of substituted aniline derivatives on the column packed with porous titania beads. Solutes: N,N-dimethylaniline (a); Nmethylaniline (b); aniline (c). Eluent: n-heptane and 2-propanol (99.5:0.5, v/v). Flow rate: 1 mL/min; detection, UV at 254 nm. Column was the same as in Figure 10.
and phase state of titania nanocrystals, specific surface area, pore volume, and porosity. By control of these factors, it was possible to fabricate porous titania beads of 7-150 µm in mean diameter. The use of larger-sized agarose gel could easily produce larger-sized titania beads. Finally, flow hydrodynamics, column efficiency, and separation performance were studied by using the column packed with the optimized titania beads of 15 µm. The results indicate that the porous titania beads had excellent mechanical stability, and the column exhibited high column efficiency and good separation performance of three aniline derivatives. Compared with other approaches to the fabrication of titania beads, the present strategy provides a readily controllable procedure for the preparation of beads of various sizes determined by the initial templates, so it may open a new way to the fabrication of well-defined inorganic beads with tailored properties for HPLC packings. Acknowledgment
acetone values of HETP ranging from 0.16 to 0.41 mm were obtained using the same column. In other words, the column efficiency for acetone was higher than that for N,N-dimethylaniline. This can be explained by the fact that there was larger interaction of the titania packings with N,N-dimethylaniline than acetone. So N,N-dimethylaniline had longer retention time and the kinetics of N,N-dimethylaniline partitioning between the two phases was also slower, which would compromise the column efficiency measured, according to the Van Deemter equation. The minimum value of HETP at about 50 cm h-1 was 0.16 mm for unretained acetone (plate number 6165 m-1) and 0.21 mm for N,N-dimethylaniline (plate number 4650 m-1). The values were similar to that of polymer packings in the similar range of particle size.35 3.7.3. Separation. The weakly acidic surface of titania-based packings allows the separation of basic compounds. The performance of the above-discussed column was examined by the separation of a mixture containing three different aniline derivatives (Figure 12). As can be seen, the retention of the solutes strongly depends on the methyl group in the molecules; the retention order is aniline (7.9 min) > N-methylaniline (2.1 min) > N,N-dimethylaniline (0.9 min). Clearly, the methyl group is the main influencing factor for the retention; more methyl groups lead to the decrease of the retention. In other words, the less sterically hindered solutes have been retarded longer. So, the retention mechanism on the titania column is size exclusion, as described elsewhere.13 Additionally, the column packed with titania beads can successfully separate the solutes, and all the peaks show good symmetry. The results indicate that the column was well-packed, and the titania beads prepared by the present method would be promising for application to HPLC. In addition, we have also checked the reproducibility of the preparation of titania beads for 13 batches. It confirms that the sol-gel templating method is reproducible and of high yield. The technique can readily be scaled up for greater production of the titania beads. 4. Conclusions In this work, agarose gel was successfully used as an effective shape and structure template for the controllable fabrication of well-defined titania beads by a sol-gel templating procedure. The impregnation time and repeated precipitation cycle played the important roles in the diffusion of the titanium precursors into templates and the maintenance of porous structure of the resultant titania beads. Calcination temperature affected the size
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ReceiVed for reView July 18, 2008 ReVised manuscript receiVed October 6, 2008 Accepted October 27, 2008 IE8011165
