Elsevier

Microporous and Mesoporous Materials

Volume 214, 15 September 2015, Pages 195-203
Microporous and Mesoporous Materials

Preparation of high-flux γ-alumina nanofiltration membranes by using a modified sol–gel method

https://doi.org/10.1016/j.micromeso.2015.04.027Get rights and content

Highlights

  • Colloidal boehmite sol with an average particle size of 16 nm was prepared.

  • Acetic acid was used as the peptizing agent instead of nitric acid.

  • The NF membranes showed a relatively high pure-water flux (>20 L/(m2 h bar)).

Abstract

Membrane technology has become a low-cost and high-efficiency separation technology for industrial processes over the past decennia. The colloidal route is an environmentally benign process for the preparation of ceramic membranes by using water as solvent. A stable, colloidal boehmite sol with an average particle size of 16 nm was prepared from aluminum tri-sec-butoxide using water as a solvent in the pH range of 3–4. Acetic acid was used as the peptizing agent instead of nitric acid. The stable sol was used to fabricate γ-alumina nanofiltration membranes on α-alumina microfiltration substrates by a dip-coating approach. The effects of the peptizing agents on the properties of the sols and the membranes were investigated. γ-alumina nanofiltration membranes with an average membrane thickness of 1.8 μm were obtained after sintered at 600 °C for 2 h. The molecular weight cut-off of the γ-alumina nanofiltration membranes, as obtained by filtration of an aqueous PEG solution, was approximately 1 kDa. This membrane also showed a typical retention for monovalent ions and divalent ions. Additionally, the nanofiltration membranes exhibited a relatively high pure-water flux (>20 L/(m2 h bar)), which is four times higher than that of the γ-alumina nanofiltration membranes reported in the literature. The contribution ratios of different microstructure factors on pure-water flux were calculated. The change in tortuosity with a contribution ratio of 75.6% was the main factor for the improvement of pure-water flux.

Introduction

Membrane technology has become a low-cost and high-efficiency separation technology for industrial processes over the past decennia. Nanofiltration (NF) is a new type of pressure-driven membrane process for the selective separation of solvents from solvent-solute mixture in the molecular weight range from 200 to 1000 Da, which ranges between ultrafiltration (UF) and reverse osmosis (RO) [1]. It plays an increasingly important role in many separation processes in the petrochemical, food, environmental, and other industries [2], [3], [4], [5], [6]. Nanofiltration membranes are generally classified into two major groups according to their materials properties: ceramic nanofiltration membranes and polymeric nanofiltration membranes. Ceramic nanofiltration membranes have a number of advantages compared to polymeric ones: they offer a higher mechanical strength, they are highly resistant to organic solvents, and some can be used over wide pH and temperature ranges [7], [8], [9], [10].

The sol–gel process is one of the most appropriate methods for the preparation of ceramic nanofiltration membranes [11], [12], [13]. Two sol–gel routes are generally described in the literature [14]. One is based on colloidal chemistry in aqueous media; the other utilizes the chemistry of metal-organic precursors in organic solvents. These two processes are termed the colloidal sol–gel route and the polymeric sol–gel route, respectively. The former, using water as the solvent instead of environmentally hazardous organic solvents, is much better suited for the large-scale production of ceramic nanofiltration membranes [14]. The normal method to obtain colloidal sols from oxide precursors is a two-step process. In the first step, a precipitate of a condensed, hydroxylated species is formed from hydrolyzed precursors. In the second step, this precipitate is transformed into a stable sol through a peptization reaction using basic or acidic electrolytes.

Mesoporous γ-alumina membranes were the first, and have subsequently become the most widely, investigated membranes to follow the colloidal preparation method [15]. These mesoporous γ-alumina membranes with an average pore size of 3–5 nm, corresponding to a molecular weight cut-off (MWCO) of 3000–10,000 Da, usually served as interlayers for ceramic nanofiltration membranes [16], [17], [18]. Although γ-alumina membranes with a mean pore size of approximately 1 nm, corresponding to an MWCO below 1000 Da, have also been reported [19], [20], [21], [22]. The permeabilities of these nanofiltration membranes were not high enough to meet the requirements of industrialization, and the pure water flux was generally less than 5 L/(m2 h bar).

In this paper, we obtained γ-alumina nanofiltration membranes with high permeability by an environmentally benign sol–gel process using water and acetic acid instead of an organic solvent and an inorganic acid. We systematically studied the effects of different peptizing agents on the properties of the Al-tri-sec-butoxide-derived sol, as well as the microstructure and performance of the nanofiltration membranes. The pure-water permeability as well as the retention for PEG and ions of the γ-alumina nanofiltration membranes were reported.

Section snippets

Preparation of boehmite sols

Al-tri-sec-butoxide was used as the precursor, which could produce relatively small pore sizes and high surface areas [23]. The sol was prepared by adding ethanol (SCRC, China) to the precursor Al-tri-sec-butoxide (ASB, Aladdin, China) in small quantities, and then mixing the solution with pure water to form γ-AlOOH (boehmite sol). The chemical reactions of the alumina sol showed hydrolysis and poly-condensation for ASB [24], which are given in Fig. 1. Acetic acid and nitric acid were used as

Support materials

The tubular microfiltration (MF) support has an average pore size of 0.2 μm and a porosity of 36%. The FESEM micrograph (Fig. 3) of the tubular microfiltration support shows a typical macroporous and asymmetric structure. In Fig. 3a, a flat and non-defective surface could be observed, which plays an important role in the subsequent dip-coating process to obtain a smooth and defect-free top layer. From the cross-sectional micrograph (Fig. 3b), an average membrane thickness of approximately 12 μm

Conclusions

The colloidal route was applied to prepare ceramic nanofiltration membranes using water as the solvent. It was demonstrated that acetic acid is a suitable peptizing agent that not only leads to a boehmite sol with a relatively small particle size and a narrow particle size distribution, but it also leads to a γ-alumina nanofiltration membrane with a relatively high porosity, narrow pore size distribution and small tortuosity. It was suggested that acetate ions served as a type of surfactant and

Acknowledgments

This work is financially supported by the National High Technical Research Program of China (2012AA03A606), the Project of Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), the “Peak Specialists in Six Industries” High-level Specialist Fund of Jiangsu Province (2012JNHB016) and the Major Program of the Nature Science for Higher Education Institutions of Jiangsu Province (12KJA530001).

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