Effect of porous YxFeyZr1−x−yO2 coated TiO2 solid superacid nanoparticles on polyvinylidene fluoride membranes properties

doi:10.1016/j.memsci.2016.07.041

Journal of Membrane Science

Volume 520, 15 December 2016, Pages 54-65

https://doi.org/10.1016/j.memsci.2016.07.041 Get rights and content

Highlights

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The tetragonal ZrO₂ system in SYFZr-Tis is stabilized owing to doping Yttrium.
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SYFZr-Tis reach a bigger acidity value (H₀=−16.352) owing to doping Yttrium.
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SYFZr-Tis own numerous Lewis acid sites and OH groups due to doping Ferrum.
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Polymer hybrid membranes perform anti-fouling and hydrophilic properties due to numerous MRLs.

Abstract

In order to improve the anti-fouling and hydrophilic properties of polyvinylidene fluoride (PVDF) membranes, porous YxFeyZr_1−x−yO₂ coated TiO₂ solid superacid nanoparticles (SYFZr-Tis) were built through hydrolysis, coating, etching, doping, sulfation, etc., and then were doped into PVDF membranes to prepare SYFZr-Tis/PVDF hybrid membranes. The optimum preparation conditions of SYFZr-Tis were investigated and confirmed according to the acidity of SYFZr-Tis; SYFZr-Tis were characterized by scanning electron microscope (SEM), transmission electron microscope (TEM), Fourier transform infrared spectroscopy (FT-IR) and Brunauer–Emmett–Teller (BET). Alternatively, the optimum doping contents of SYFZr-Tis were studied and determined according to the properties of the SYFZr-Tis/PVDF hybrid membranes. The results show that SYFZr-Tis prepared under the optimum synthesis condition reach a bigger acidity value (H₀=−16.352), the diameter of SYFZr-Tis is around 400 nm and the thickness of porous Y_xFe_yZr_1−x−yO₂ shell is about 50 nm, and the surface of SYFZr-Tis has numerous hydroxyl groups. Compared with PVDF membranes, hydrophilicity, tensile strength, porosity and anti-compaction of SYFZr-Tis/PVDF hybrid membranes are improved. Meanwhile, SYFZr-Tis/PVDF hybrid membranes with the optimum content of SYFZr-Tis perform attractive anti-fouling and hydrophilic properties. Therefore, SYFZr-Tis are desirable as suitable fillers of PVDF hybrid membranes.

Graphical abstract

Porous Y_xFe_yZr_1−x−yO₂ coated TiO₂ solid superacid nanoparticles (SYFZr-Tis) were built successfully, and then were doped into PVDF membranes to prepare SYFZr-Tis/PVDF hybrid membranes. Firstly, the surface of TiO₂ particles was coated by using SiO₂. Then SiO₂ shell/ TiO₂ core particles were coated with ZrO₂ doped Yttrium (Y) and Ferrum (Fe). Finally, SYFZr-Tis were obtained by removing SiO₂ layer and sulfating, and then were doped into PVDF membranes to prepare SYFZr-Tis/PVDF hybrid membranes. The tetragonal ZrO₂ system in SYFZr-Tis is stabilized and SYFZr-Tis reach a bigger acidity (H₀=−16.352) owing to doping Y. Meanwhile, the surface of SYFZr-Tis has numerous hydroxyl groups and Lewis acid sites due to doping Fe. TiO₂ core can decompose organic pollutants and microbes by photocatalytic reaction. Moreover, Y_xFe_yZr_1−x−yO₂ solid superacid shell can react with inorganic pollutants such as metal oxides or restrain their formation inside channels and surface of membranes. Afterwards, SYFZr-Tis/PVDF hybrid membranes with micro reaction locations (MRLs) perform attractive anti-fouling and hydrophilic properties. Therefore, SYFZr-Tis are desirable as suitable fillers of PVDF hybrid membranes.

Introduction

Nowadays, small sized functional materials are broadly applied in many domains such as membrane separation, energy storage, catalysts, etc [1], [2], [3]. Meanwhile, many nano-inorganic functional materials [4], [5] have been investigated and developed to improve anti-fouling and hydrophilic properties of polymer hybrid membranes such as ZnO, SiO₂, etc. [6], [7]. However, some deficiencies of these methods restrict further improvement of the integrated properties of polymer hybrid membranes due to few hydroxide radicals and Lewis acid sites of above mentioned stoichiometric monocomponent inorganic oxide materials. To solve the problems above, Zhang et al. manufactured phosphorylated Zr-doped hybrid silica (SZP) [8] and Ce-Doped nonstoichiometric nanosilica [9] to prepare PSF hybrid membranes respectively by taking the advantages of nonstoichiometric inorganic oxide materials with various point defects and numerous exposed hydroxide radicals, evidently improving the anti-fouling and hydrophilic properties of PSF membranes. Although the anti-fouling and hydrophilic properties of these polymer hybrid membranes can be improved by using the above methods, further enhancement of the anti-fouling and hydrophilic properties is limited. This is because these strategies just dope functional materials with small size into polymer membranes to enhance their anti-fouling and hydrophilic properties by physical interaction on the interface between aqueous solution and polymer hybrid membranes, but without any chemical reaction. Therefore, if the anti-fouling and hydrophilic properties of these membranes are further enhanced, the membranes should be modified by doping novel functionalized materials, making membranes have micro reaction locations (MRLs) inside channels and on the surface of membranes which perform good integrative capabilities. Based on the analysis of membrane pollution [10], pollutants which cause membrane fouling in the wastewater are mainly divided into three categories such as organic pollutants (hydrocarbon and soluble oil etc.), microbes and inorganic pollutants (metal oxides etc.) [11], [12]. It is reported that TiO₂ nanoparticles with high photocatalytic activity [13], [14] can effectively degrade organic pollutants and microbes, delaying or eliminating the formation of gel layer on the surface of membranes; alternatively, SO₄²⁻/ZrO₂ solid superacid nanoparticle is a sort of typical oxide with 10,000 times Hammett acidity of 100% pure H₂SO₄ [15], [16], which can decompose inorganic pollutants such as metal oxides or restrain their formation inside channels and surface of membranes, extending the membrane lifespan [17]. Functionalized fillers for preparing polymer hybrid membranes could be prepared by combining TiO₂ nanoparticles and SO₄²⁻/ZrO₂ solid superacid nanoparticles, making hybrid membranes have MRLs. Therefore, the strategy of building polymer hybrid membranes with MRLs could further enhance the anti-fouling and hydrophilic capabilities of polymer hybrid membranes. Zhang et al. [18] firstly prepared ZrO₂ solid superacid porous shell/void/TiO₂ core particles (ZVT) and then doped them to PVDF to prepare a novel polymer hybrid membrane which had good anti-fouling and anti-compaction properties, performing good performance when treating oily wastewater.

However, ZrO₂ with tetragonal phase is normally unstable and only when it is the tetragonal phase that can be sulfated to form SO₄²⁻/ZrO₂ solid superacid. Thus some strategies should be adopted to make ZrO₂ have the steady tetragonal phase structure. Recently, researchers found that ZrO₂ with tetragonal phase can be formed by doping some other elements to ZrO₂ lattice. For example, Kogler et al. [19] doped Y into ZrO₂ matrix to prepare nonstoichiometric solid superacid and the results show that doping Y can not only stabilize the tetragonal phase of ZrO₂, but also form the nonstoichiometric solid superacid with stronger acidity. Moreover, Wang et al. prepared a series of micro- and nanosulfated zirconia loaded on Fe₃O₄ and found that ZrO₂ with stronger activity can be formed by doping Fe to ZrO₂ lattice [20].

Based on the above analysis, in order to further enhance the integrative properties of PVDF hybrid membranes, porous Y_xFe_yZr_1−x−yO₂ coated TiO₂ solid superacid nanoparticles (SYFZr-Tis) are built and then were doped into PVDF membranes to prepare SYFZr-Tis/PVDF hybrid membranes through combining the advantages as below. First, TiO₂ nanomaterials with high photocatalytic activity [13], [14] can effectively degrade organic pollutants and microbes, delaying or eliminating the formation of gel layer on the surface of membranes; second, doping Y into ZrO₂ can stabilize the tetragonal phase of ZrO₂ and ensure good activity and superacid characteristic of SO₄²⁻/ZrO₂ [16], [21]; finally, doping Fe into ZrO₂ can make Zr atom alter to Zr⁴⁺ which has stronger Lewis acid centers due to Fe with high electronegativity value (1.83), leading electron cloud of Zr–O migrate to that of O–Fe, increasing the activity and hydrophilic property of ZrO₂ shell [22].

In this paper, to enhance the anti-fouling and hydrophilic properties of PVDF membranes, (SYFZr-Tis) are built, and then are doped into PVDF membranes to prepare SYFZr-Tis/PVDF hybrid membranes. The optimum preparation conditions of SYFZr-Tis are investigated and confirmed according to the acidity of SYFZr-Tis; SYFZr-Tis are characterized by SEM, TEM, FT-IR and BET. Alternatively, the optimum doping contents of SYFZr-Tis are studied and determined according to the integrative properties of the SYFZr-Tis/PVDF hybrid membranes.

Section snippets

Materials

PVDF (1015) was supplied by Solvay Co., Ltd (USA) and its density was 1.78 g/cm³. Tetra-n-butyl Titanate (TBT, AR grade, 98.0% ), tetraethyl orthosilicate (TEOS, AR grade, the mass ratio of silica dioxide is 28%), zirconium oxychloride (ZrOCl₂·8H₂O, AR grade, 99.0%), yttrium nitrate (Y(NO₃)₃·6H₂O, AR grade, 98.0%), ammonium hydrogen carbonate (NH₄HCO_3, AR grade, 21.0%) and cetyltrimethylammonium bromide (CTAB, AR grade, 99.0% ) were purchased by Tianjin Guangfu Fine Chemical Institute. Ferric

The optimum preparation conditions of SYFZr-Tis

According to our previous suitable preparation conditions of ZVT, the molar ratio of Ti/Si is 1.72:1, the molar ratio of Ti/Zr is 2.95:1 and the sulfated calcinating temperature is 650 °C [18]. However, in the preparation stage of SYFZr-Tis, especially the coating and sulfating processes of porous Y_xFe_yZr_1−x−yO₂ shell have much influence on the structure and acidity of SYFZr-Tis. Therefore, in this paper, the effect of the molar ratio of NH₄HCO₃/NH₃·H₂O, the molar ratio of Y/Zr, the molar ratio

Conclusion

SYFZr-Tis were built and their properties were investigated. The optimum preparation conditions for SYFZr-Tis are: the molar ratio of NH₄HCO₃/NH₃·H₂O 1:3, the molar ratio of Y/Zr 4:100, the molar ratio of Fe/Zr 0.3:100 and the dipping time in sulphuric acid aqueous solution 3 h. The results of SEM, TEM, FT-IR and BET indicate that porous Y_xFe_yZr_1−x−yO₂ coated TiO₂ solid superacid nanoparticles (SYFZr-Tis) are successfully prepared, SYFZr-Tis reach a bigger acidity value (H₀=−16.352) and have

Acknowledgment

This project is supported by National Natural Science Foundation of China (No. 21076143), by the key technologies R & D program of Tianjin (15ZCZDSF00160), by the Basic Research of Tianjin Municipal Science and Technology Commission (13JCYBJC20100), by Tianjin Municipal Science and Technology Xinghai Program (No. KJXH2014-05), by State Key Laboratory of Chemical Engineering (No. SKL-ChE- 15B03).

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Effect of porous YxFeyZr1−x−yO2 coated TiO2 solid superacid nanoparticles on polyvinylidene fluoride membranes properties

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Materials

The optimum preparation conditions of SYFZr-Tis

Conclusion

Acknowledgment

J. Membr. Sci.

J. Membr. Sci.

Desalination

J. Membr. Sci.

J. Membr. Sci.

J. Hazard. Mater.

Appl. Catal. B

J. Hazard. Mater.

Chem. Eng. J.

J. Power Sources

Mater. Lett.

Chem. Eng. J.

Desalination

Spectrochim. Acta A

Effect of porous Y_xFe_yZr_1−x−yO₂ coated TiO₂ solid superacid nanoparticles on polyvinylidene fluoride membranes properties