Elsevier

Journal of Membrane Science

Volume 207, Issue 1, 1 September 2002, Pages 73-89
Journal of Membrane Science

Alumina and titania multilayer membranes for nanofiltration: preparation, characterization and chemical stability

https://doi.org/10.1016/S0376-7388(02)00053-4Get rights and content

Abstract

The preparation and characterization of porous ceramic multilayer nanofiltration (NF) membranes is described. During preparation, special care was given to each sub-layer that forms a part of the multilayer configuration: the macroporous substrate, the membrane interlayers and the NF toplayers. High-quality macroporous supports are prepared from α-Al2O3. Three types of colloidal sol–gel derived mesoporous interlayers are considered: Al2O3, TiO2 and mixed Al2O3–TiO2. The active NF toplayer is a very thin and fine textured polymeric TiO2 toplayer.

Optimized α-Al2O3/γ-Al2O3/anatase and α-Al2O3/anatase/anatase multilayer configurations show high retentions for relatively small organic molecules (molecular weight cut-off <200). Different membrane layers have very different crystallographic properties resulting in a considerably different chemical stability. Corrosion measurements showed that application of a multilayer configuration including weakly crystallized γ-Al2O3 layers is restricted to mild aqueous media (pH 3–11) or non-aqueous media (organic solvents). For NF applications in aqueous media with a lower or higher pH, the multilayer membrane composed of anatase on a α-Al2O3 support is to be preferred.

Introduction

Nanofiltration (NF) membranes can be generally classified into two major groups according to their material properties: organic polymeric membranes and inorganic ceramic membranes. Polymeric NF-membranes are today commercially available in a large number of different polymeric materials (polysulfone, cellulose acetate, polyamide, …). Membrane filtration based on such membranes has found numerous applications in the area of process, waste and drinking water treatment. However, the major drawback is that the chemical stability of the conventional polymeric membranes is limited with respect to corrosive media like strong acids and organic solvents [1], [2]. Therefore, much research to date focuses on ceramic NF-membranes. Since it is generally recognized that inorganic materials are inherently more stable than polymers, they are expected to constitute a promising alternative for polymeric membranes [3].

Ceramic NF-membranes with high performance parameters such as low cut-off values or high fluxes can only be obtained in an asymmetric multilayer configuration. The development of such a multilayer configuration includes: shaping of an appropriate support material; formation of mesoporous interlayers; and synthesis of a microporous toplayer with a cut-off value below 1000. Alumina, titania, zirconia or silica are considered as the main ceramic materials for the formation of the multilayer structures [4], [5], [6].

The preparation route starts with the production of a high-quality support system, as the effectiveness of the developed NF membrane is greatly influenced by the structural properties of the membrane support. Based on earlier studies in our group, alumina supports produced by conventional slip-casting and reaction bonded Al2O3 (RBAO) manufacturing routes are used as substrate material for the development of a multilayer membrane [7], [8], [9].

The second part deals with the synthesis of several intermediate membrane layers. The aim of this modification is twofold: fine-grained mesoporous membrane layers are necessary in order to reduce the coarse pore structure of the support material and large surface irregularities or defects can only be covered properly by a multiple dip-coating procedure. In this study, colloidal sol–gel derived membrane layers consisting of alumina or titania are considered. Firstly, γ-Al2O3 and anatase/TiO2 membrane interlayers were prepared based on earlier studies of Burggraaf et al. [10], Zaspalis et al. [11], Kumar et al. [12] and Lin et al. [13]. Additionally, different combined Al2O3–TiO2 membrane interlayers were developed in order to combine the specific properties of both materials, namely the possibility of obtaining membrane layers with a high thickness and without defects on one hand and a good chemical stability, on the other hand.

The final step in the multilayer preparation route involves the synthesis of a thin separation toplayer. Formation of such a layer with very small pores requires an appropriate sol consisting of very small nanometer-sized particles, which is typically obtained by the so-called ‘polymeric’ sol–gel technique. In this work, TiO2 NF toplayers are formed by a similar sol–gel procedure as previously described by de Lange et al. [14] and Keizer et al. [15]. They synthesized microporous amorphous SiO2 toplayers for the modification of mesoporous γ-Al2O3 membrane layers in order to obtain a multilayer gas separation membrane. Recently, Voigt et al. [16], [17] prepared also microporous toplayers, consisting of amorphous TiO2, on anatase/TiO2 membrane layers and proved their feasibility for NF applications (MWCO <500).

All the authors mentioned mainly considered on the structural properties of the developed membrane materials. Therefore, only limited data on the chemical stability of meso- or microporous membranes are available. Züter et al. [18] reported acid corrosion tests on mesoporous γ-Al2O3, anatase/TiO2 and ZrO2 membranes. In our group, Schaep et al. [19] tested the corrosion properties of mesoporous γ-Al2O3 membranes. These contributions have however mainly a descriptive character and the membrane stability was not investigated systematically by taking into account important parameters like the structural characteristics of the ceramic material (phase behaviour, crystallinity, surface area). To our knowledge, only Hollstein et al. [20] addressed in detail the corrosion properties of ceramic materials in aqueous media, but did not describe the stability of porous materials like sol–gel membrane materials.

The aim of this paper is to provide results on the chemical stability of different sol–gel derived membrane materials. The corrosion behaviour of mesoporous interlayers and microporous toplayers, along with the membrane characterization in terms of pore size, surface area, pore volume and crystallite size is described. For the determination of the corrosion rate, preference was given to simple static corrosion experiments whereby the amount of dissolved membrane material was determined in acid or alkaline solutions with a pH ranging from 1 to 13. With the experimental information, materials could be selected specifically for each membrane layer in order to develop an optimal multilayer configuration for particular process conditions (non-aqueous solvent, acid, alkali). The performance characteristics of the multilayer membranes are also described to indicate possible application fields of the developed NF-membranes.

Section snippets

Preparation of support materials

For the preparation of the membrane support, two routes were followed. In the first route, Al2O3 supports were prepared by a slip-casting technique, starting from an aqueous suspension of commercially available Al2O3 powder (AKP-30, Sumitomo) with organic additives (Vanisperse CNH, Borregaard) [7]. In the second route, Al2O3 supports were prepared by the reaction bonded Al2O3 (RBAO) manufacturing process [8]. In this process Al2O3 powder (HC Starck) and Al particles (Baudier) are milled in a

Support materials

For the development of high-quality supports the following properties are of major importance: pore size distribution; porosity; surface quality with the absence of large defects or large pores; mechanical and chemical stability.

Structural characteristics of the final Al2O3 membrane supports prepared according to the two production routes are summarized in Table 1. Basically, the support material consisted of α-Al2O3. The pore size depended on the preparation method: the fine-grained AKP-30

Dynamic characterization of the multilayer membranes

The retention behaviour of supported multilayer membranes was expressed by the MWCO, the molecular weight of the component from the PEG mixture that was retained for 90%. Membrane permeability was determined according to the Hagen–Poisseuille equation from flux measurements. As shown in Fig. 12, flux measurements as a function of pressure showed a linear dependency of flux on transmembrane pressure.

Table 7 summarizes the MWCO and the membrane permeability determined for membrane support systems

Conclusions

The membranes obtained consist of a macroporous α-Al2O3 support, two mesoporous membrane layers with an overall thickness ranging from 1 μm for titania to 4 μm for alumina, and a final polymeric titania toplayer with a thickness of ca. 100 nm. Depending on the thermal treatment, γ-AlOOH, γ-Al2O3, anatase, γ-AlOOH/anatase, amorphous mixed and α-Al2O3/anatase mixed membrane layers were formed with narrow pore size distributions ranging from the higher microporous region (<2 nm) to the lower

Acknowledgements

Herman Cooreman is gratefully acknowledged for performing the ICP-Mass Spectroscopy measurements and Frans Servaes for his kind help and technical support.

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