Anal. Chem. 2001, 73, 686-688
Synthesis of Porous Titania Microspheres for HPLC Packings by Polymerization-Induced Colloid Aggregation (PICA) Zi-Tao Jiang† and Yu-Min Zuo*
Department of Chemistry, Nankai University, Tianjin 300071, People’s Republic of China
Porous titania microspheres with a very narrow particle size distribution (PSD) were synthesized by polymerization-induced colloid aggregation (PICA). After being sintered, the titania microspheres that are obtained have an average diameter of 3.5 µm, a surface area of 36.7 m2/g, an average pore volume of 0.30 mL/g, and an average pore diameter of 32.2 nm. The microspheres possess enough rigidity to withstand high packing pressure and are very useful as a new kind of chromatographic packing material for high performance liquid chromatography (HPLC). Titania has recently attracted interest as an alternate support material to silica for column packing in high performance liquid chromatography (HPLC) because of its high chemical stability, enough rigidity, and amphoteric ion-exchange properties.1-20 In the works that have been published,1-5 the titania particles synthesized by both the sol-gel and oil emulsion (OEM) * Corresponding author. Fax: 86-22-23502458. E-mail: ymzuo@yahoo. com. † Current address: Department of Food Science and Engineering, Tianjin University of Commerce, Tianjin 300400, P.R.C. (1) Tru ¨ dinger, U.; Mu ¨ ller, G.; Unger, K. K. J. Chromatogr. A 1990, 535, 111125. (2) Tani, K.; Suzuki, Y. Chromatographia 1994, 38, 291-294. (3) Tani, K.; Suzuki, Y. J. Chromatogr. A 1996, 722, 129-134. (4) Yoshida, A.; Takahashi, K. Chemitopia 1994, 15, 18-29. (5) Pesek, J. J.; Matyska, M. T.; Ramakrishnan, J. Chromatographia 1997, 44, 538-544. (6) Gru ¨ n, M.; Kurganov, A. A.; Schacht, S.; Schu ¨ th, F. Unger, K. K. J. Chromatogr. A 1996, 740, 1-9. (7) Kurganov, A.; Tru ¨ dinger, U.; Isaeva, T.; Unger, K. K. Chromatographia 1996, 42, 217-222. (8) Zaharescu, M.; Cserhati, T.; Forga´cs, E. J. Liq. Chrom. Relat. Technol. 1997, 20, 2997-3007. (9) Kawahara, M.; Nakamura, H.; Nakajima, T. J. Chromatogr. A 1990, 515, 149-158. (10) Murayama, K.; Nakamura, H.; Nakajima, T.; Takahashi, K.; Yoshida, A. Microchem. J. 1994, 49, 362-367. (11) Takahashi, K.; Yoshida, A. Japan Patent 7-294505, 1995, 11, 10. (12) Tani, K.; Miymoto, E. J. Liq. Chrom. Relat. Technol. 1999, 22, 857-871. (13) Ellwanger, A.; Matyska, M. T.; Albert, K.; Pesek, J. J. Chromatographia 1999, 49, 424-430. (14) Tani, K.; Suzuki, Y. J. Liq. Chrom. Relat. Technol. 1996, 19, 3037-3408. (15) Murayama, K.; Nakamura, H.; Nakajima, K.; Takahashi, K.; Yoshida, A. Anal. Sci. 1994, 10, 49. (16) Murayama, K.; Nakamura, H.; Nakajima, K.; Takahashi, K.; Yoshida, A. Anal. Sci. 1994, 10, 815-816. (17) Tani, K.; Suzuki,Y. Chromatographia 1997, 46, 623-627. (18) Tani, K.; Suzuki,Y. Chromatographia 1998, 47, 655-658. (19) Tani, K.; Ozawa, M. J. Liq. Chrom. Relat. Technol. 1999, 22, 843-856. (20) Jiang, Z. T.; Zhang, D. Y.; Zuo, Y. M. J. Liq. Chrom. Relat. Technol. 2000, 23, 1159-1169.
686 Analytical Chemistry, Vol. 73, No. 3, February 1, 2001
Figure 1. SEM micrograph of porous titania microspheres by PICA.
technologies always have a broad particle size distribution (PSD) and smaller pore diameter. In this paper we report the synthesis of titania microspheres by means of polymerization-induced colloid aggregation (PICA) that was previously used to synthesize silica21,22 and zirconia.23-27 The PICA-synthesized titania was subjected to a step-by-step process of washing, drying, and calcinating. Finally, the resulting titania microspheres have a narrow PSD and excellent surface parameters and can be used directly for normal phase chromatography without time-consuming size classification. Furthermore, titania is also a better support for reversed-phase chromatography. EXPERIMENTAL SECTION Reagents. Titanium tetrachloride (Xingtai Chemical Factory, Xingtai, China), urea (Tianjin No.1 Chemical Reagent Factory, Tianjin, China), 38% formaldehyde solution (Jinan Organic Chemicals Factory, Jinan, China), isoamyl acetate (Tianjin No. 2 Chemical Reagent Factory, Tianjin, China), and acetylacetone (21) (22) (23) (24)
Kirkland, J. J. U.S. Patent 3,782,075, Jan 1, 1974. Unger, K. K. Porous Silica; Elsevier Press: New York, 1979; Chapter 2. Iler, R. K.; McQueston, H. J. U.S. Patent 4,010,242, 1997. Carr, P. W.; McCormick, A. V.; Annen, M. J.; Sun, L.; Brown, J. R. U.S. Patent 5,540,834, 1996. (25) Sun, L.; Annen, M. J.; Lorenzano-Porras, F.; Carr, P. W.; McCormick, A. V. J. Colloid Interface Sci. 1994, 163, 464-473. (26) Annen, M. J.; Kizhappali, R.; Carr, P. W.; McCormick, A. V. J. Mater. Sci. 1994, 29, 6123-6130. (27) Lorenzano-Porras, C. F.; Annen, M. J.; Flickinger, M. J.; Carr, P. W.; McCormick, A. V. J. Colloid Interface Sci. 1995, 170, 299-307. 10.1021/ac001008u CCC: $20.00
© 2001 American Chemical Society Published on Web 12/09/2000
Figure 2. Particle size distribution (PSD) of porous titania microspheres that were synthesized by PICA.
Figure 3. Plot of the pore size distribution, obtained by nitrogen adsorption, of porous titania microspheres that were synthesized by PICA.
(Beijing No. 3 Chemical Reagent Factory, Beijing, China) were analytical reagent grade. Octadecyltriethoxysilane (ACROS ORGANICS, New Jersey) was technical reagent grade. Apparatus. A Hitachi scanning electron microscope, model X-650 (Hitachi Corporation, Japan), was used to record the electron micrograph. A Micromeric model ASAP-2010 surface analysis instrument (Micromeric Corporation) was used to collect nitrogen adsorption/desorption isotherms at 77 K. A laser particle size analyzer, model POP IIIA (Omicron Instruments Limited Company, Zhuhai, China), was used to determine PSD. An x-ray diffractometer model D/max-2500 (Rigaku, Japan) was used to determine the crystal shape of the titania. Synthesis of Porous Titania Microspheres. 60 g of ice was added to a 500-mL round-bottom flask. Under conditions of an ice bath and stirring, 10 mL of titanium tetrachloride was added drop by drop. The hydrolysate, a yellow foamlike precipitate, was dissolved in cold water, after which titanium oxychloride sol was obtained as a starting material. 100 mL of water and 2 mL of acetylacetone were added successively. The pH of the solution was adjusted to 0.3-0.5 using 1 mol/L sodium hydroxide, and then the solution was diluted to 300 mL total volume using water. The solution was heated gradually to 60 °C and kept at that
Figure 4. Nitrogen adsorption/desorption isotherms of porous titania microspheres that were synthesized by PICA.
Figure 5. X-ray diffraction of porous titania microspheres that were synthesized by PICA.
temperature for 2 h, then cooled to room temperature (∼25 °C). 21.6 g of urea was added. After the urea was dissolved, 21.2 mL Analytical Chemistry, Vol. 73, No. 3, February 1, 2001
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Figure 6. Chromatogram of the separation of aromatic hydrocarbons on ODT. Chromatograph: Varian 5060. Detector wavelength: 254 nm. Mobile phase: 80% methanol. Flow-rate: 1 mL/min. Column: 150 × 4.6 mm i.d. Injection volume: 10 µL. Solute: benzene (3.58), naphthalene (4.56), biphenyl (5.80), anthracene (7.65).
of 38% formaldehyde solution was added at approximately 15 °C and, the solution was mixed well. The urea and formaldehyde undergo acid-catalyzed polymerization, and the oligomer soformed adsorbs onto the surface of colloids, causing the colloids to aggregate. After 5 h of reaction, the mixture was diluted to about 400 mL with water and stirred for a few minutes. The aggregated microspheres were then separated by filtration and subjected to a series of washing processes using water and methanol and water. The drying process was carried out by azeotropic distillation using isoamyl acetate as an azeotrope former. The dehydrated microspheres were first heated in a vacuum oven at 120 °C for 12 h, then at 200 °C for 40 h. Finally, the microspheres were carbonized at 300 °C in a muffle furnace and the temperature was raised to 600 °C for 6 h to burn off completely the carbon and 900 °C for about 3 h to improve the mechanical strength of the titania. RESULTS AND DISCUSSION Titania particles are spherical and free from clustering. A SEM micrograph is shown in Figure 1. After being sintered, the average diameter of the microspheres, as determined by SEM, is 3.5 ( 0.5 µm. The PSD result is shown in Figure 2. It can be seen that the PSD of porous titania microspheres by PICA is narrower than those by OEM1 and sol-gel.2 The specific surface area of titania (as) is 36.7 m2/g, which was calculated using the standard Brunauer-Emmett-Teller (BET) method from adsorption data. The total pore volume of titania (vp) is 0.30 mL/g, which was evaluated by converting the volume of adsorbed nitrogen at relative pressure (p/po) of approximately 0.99 to the volume of liquid adsorbate. The average pore diameter (4V/A by BET) is 32.2 nm. The pore size distribution for titania was obtained from the adsorption data using the Barret-Joyner-Halenda (BJH) method, as shown in Figure 3. As is shown, the average pore diameter of the titania microspheres that were synthesized by PICA is somewhat larger than those synthesized by OEM1 and sol-gel.2 Sun et al.25 have stated that it is useful for HPLC separation of proteins that the packing particles have pores ranging from 20 to 50 nm in diameter. Figure 4 shows the plot of nitrogen adsorption/desorption isotherms of porous titania microspheres by PICA. The result of X-ray diffraction is shown in Figure 5. It was proved that the crystal shape of titania is anatase-type.
688 Analytical Chemistry, Vol. 73, No. 3, February 1, 2001
Table 1. Surface Parameters of Titania Particles Synthesized by PICA and Comparison with Those by Other Methods
TiO2 (OEM)1 TiO2 (sol-gel)2 TiO2 (PICA)
pd (µm)a
as (m2/g)b
vp (mL/g)c
dp (nm)d
4-7 3-6 3.5 ( 0.5
78 111 36.7
0.23 0.30 0.30
8 8.7 32.2
a p , particle diameter d diameter.
b
as, surface area
c
vp, pore volume
d
dp, pore
Table 1 presents the surface parameters of titania particles synthesized from a different process. A comparison of these data shows that the uniformity of particle size and the surface character of titania synthesized by PICA are the most excellent. The major factors that affect the formation of titania microspheres are the pH of the solution, the concentration of urea and formaldehyde, the reaction temperature, and time. On the other hand, the effect of sintering temperature on the particle diameter is very significant, with a decreasing of approximately 65% in diameter after the microspheres are sintered. The detailed work will be reported later. The titania that was obtained was converted into octadecyl titania (ODT) reversed-phase packings by making the titania react with octadecyltriethoxysilane. The chromatographic column (150 × 4.6 mm i.d.) was filled with ODT and a pressure of 55 MPa was applied by the use of a high-pressure pump, model 6752B100 (Beijing Analytical Instruments Technical Company, Beijing, China). Reversed-phase separation of aromatic hydrocarbons was performed on the ODT column using methanol solution (80%) as the mobile phase. The results are shown in Figure 5. As can be seen, titania that is synthesized by PICA is useful as a packing material for HPLC. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China, project no. 29575201.
Received for review August 22, 2000. Accepted October 17, 2000. AC001008U
