Effect of firing temperature on the water permeability of SiO2–ZrO2 membranes for nanofiltration
Introduction
Membranes for water treatment can be categorized into inorganic and polymeric membranes. Inorganic membranes, which include ceramic membranes such as alumina (Al2O3), silica (SiO2), zirconia (ZrO2), etc., have received much attention because of their excellent stability in acidic and basic pHs, their thermal stability at high temperatures, and their high mechanical strength by comparison with polymeric membranes [1]. Ceramic membranes with pores that average several micrometers in diameter are categorized as microfiltration (MF), and are prepared by powder sintering methods, but the pore sizes are not small enough for use in molecular separation. Therefore, ceramic membranes with pore sizes in the nanometer range are prepared using ceramic microfiltration membranes as a substrate, followed by sol-coating, drying, and a firing process. Nanoporous membranes, particularly nanofiltration (NF) membranes, which have pore sizes of 1–2 nm and molecular weight cut-off (MWCO) in the range from 200–1000, have been prepared by the sol–gel process from a variety of metal oxides such as alumina, silica, zirconia, titania, etc. Van Gestel et al. [2] reported ceramic membranes with a γ-Al2O3 top layer that were prepared by a sol–gel procedure and were only chemically stable in non-aqueous or mild aqueous liquids (pH 3–11). For nanofiltration membranes in an aqueous solution that are intended for applications in strong acids or alkaline media, α-Al2O3 has become the preferred support with a top layer of either zirconia or titania, which has proven to be more stable than γ-alumina membranes in aqueous solutions. Van Gestel et al. [3] developed disc-type membranes that were comprised of either a microporous ZrO2 toplayer on γ-Al2O3 or a ZrO2 interlayer using α-Al2O3 supports, and they found ZrO2 membranes fired at 400 °C showed MWCOs in the range of 200–300 in the nanofiltration of aqueous polyethylene glycol solutions and high stability at low and high pH values. Voigt et al. [4] reported that TiO2 capillary membranes fired at 400 °C showed a MWCO of approximately 250. Recently, TiO2 membranes with nominal MWCOs of 200 were commercialized and evaluated for the rejection of organic chemicals [5].
In addition, silica is one of the most attractive membrane materials because the pore sizes can be controlled in a wide range from ultramicropores to mesopores. However, the hydrothermal stability of silica is a serious problem because it dissolves in water at room temperature, and its solubility increases with temperature. Doping silica with various types of metal ions such as zirconia (ZrO2), titania (TiO2), aluminum (Al), nickel (Ni) can improve its stability against water and can also provide novel separation characteristics [6]. We reported the use of SiO2–ZrO2 membranes for gas separation and pervaporation. Yoshida et al. [7] prepared silica–zirconia membranes with different zirconia content (Si/Zr: 9/1, 7/3 and 5/5) by coating the sol on cylindrical α-alumina porous tubes fired at 570 °C to test H2 separation performance under hydrothermal conditions. SiO2–ZrO2 membranes were hydrothermally treated at 500 °C in a gas mixture (H2, N2 and H2O). The permeance of H2 and N2 through Si/Zr (9/1) and Si/Zr (7/3) membranes was decreased drastically, which was likely due to the densification of the silica matrix by water vapor at high temperature during the first 60 min, then it decreased slightly from 60–600 min and approached a constant value. However, the H2 and N2 permeance of Si/Zr (5/5) membranes was unchanged. The addition of a zirconia content larger than 40 mol% is known to render silica–zirconia membranes quite stable for the pervaporation performance of aqueous solutions, while a zirconia content of less than 30 mol% is not stable in water/isopropyl alcohol (IPA) mixtures, because silica dissolves easily into water [8], [9].
A limited number of studies have investigated SiO2–ZrO2 membranes for nanofiltration in aqueous solutions. Tsuru et al. [10] prepared three silica–zirconia (Si/Zr molar ratio 9/1) membranes for nanofiltration by coating a silica–zirconia sol (colloidal diameter: 42 nm) on cylindrical α-alumina porous microfiltration membranes, followed by the coating of a final colloidal solution (colloidal diameter: 11, 13 and 16 nm) with firing at 570 °C. The membrane showed MWCOs of 200, 500 and 1000 Da, respectively, indicating the pore diameters of the silica–zirconia membranes were controlled by the colloidal diameters of the sol solutions in the final coating stage. The water permeabilities of silica–zirconia (9/1) membranes showed a steady flux after a gradual decrease during the initial two days. The decrease in water permeability was explained as silanol functional groups being generated by hydration of the silica–zirconia surface in the water and reducing the effective pore diameters for the permeation of aqueous solutions. Yazawa et al. [11] prepared SiO2–ZrO2 membranes with molar ratios of 9/1, 7/3, 5/5, and 3/7 by applying coatings of porous ceramic substrates for the desalination of aqueous NaCl solutions, but that study did not discuss pore size tuning in the range of nanofiltration.
The development of separation membranes with high flux, selectivity and stability is a highly anticipated industry goal. High flux can be achieved by using a thin and porous structure and appropriate interaction between the permeating molecules and the pore surface [8]. In the present study, the SiO2–ZrO2 (5/5) sols of controlled sizes were prepared by the concentration of alkoxides. The effect of the pore size of the supports on the pore size and water permeability of the SiO2–ZrO2 membranes was discussed. A particular focus of the present study was put on different firing temperatures of SiO2–ZrO2 membranes and the effect on water permeability and separation of aqueous solutions for nanofiltration.
Section snippets
Preparation of SiO2–ZrO2 (5/5) colloidal sols
SiO2–ZrO2 composite colloidal sols with a molar ratio of 5/5 and with total alkoxide concentrations of 1 and 2 wt% were prepared by hydrolysis and condensation reactions of tetraethoxysilane (TEOS) and zirconium-tetra-butoxide solutions (80% ZrBT in 1-butanol), as shown in Table 1. TEOS which is less reactive than ZrBT, was partially hydrolyzed in ethanol for 3 h, and then ZrBT was added by dropping into the solution with stirring for co-condensation reactions. After 12 h, a large amount of water
Characteristics of SiO2–ZrO2 sols and powders
SiO2–ZrO2 (5/5) colloidal sols with different colloidal sizes were prepared by hydrolysis and condensation. The sizes of SiO2–ZrO2 colloidal sols were measured using a dynamic laser light scattering technique, and are summarized in Table 1. It can be clearly seen that the 2 wt% SiO2–ZrO2 sol (35 nm) was larger size than the 1 wt% SiO2–ZrO2 sol (19 nm). The particle size was increased with an increase in the concentration of the SiO2–ZrO2 sol, which is consistent with our previous reports [6], [7],
Conclusions
SiO2–ZrO2 nanoporous membranes, which were prepared by the coating of SiO2–ZrO2 sols on different types of porous α-alumina supports (HU-A01; 2.1 μm, HU-A02; 2.9 μm and HU-A01-3; 3.6 μm), showed approximately the same average pore size, N2 permeance and water permeability irrespective of the pore sizes of the supports. Therefore, the use SiO2–ZrO2 sols with appropriate colloidal sol sizes as a selective layer can play an important role in determining the average pore size and water permeability of
Acknowledgments
This research was supported by the Royal Thai Government, Core Research for Evolutional Science and Technology (CREST), the Japan Science and Technology Agency (JST), and JSPS KAKENHI Grant Numbers 24246126, 15H02313.
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