Preparation and characterization of hydrophobic ceramic hollow fibre membrane

https://doi.org/10.1016/j.memsci.2006.12.039Get rights and content

Abstract

Alumina hollow fibre membranes were prepared by spinning alumina/binder suspensions to hollow fibre precursors, which were then sintered at elevated temperatures. The obtained membranes are hydrophilic in nature and their outer surfaces were then modified to be hydrophobic by grafting with 0.01 mol/l of 1H, 1H, 2H, 2H-perfluorooctylethoxysilane (FAS) solution. The results from membrane characterization experiments revealed that the membrane surfaces were successfully changed from hydrophilic to hydrophobic, as reflected by the contact angles, which were increased from less than 80° to more than 100°. The gas permeation results indicated a slight decrease in gas permeability of the membranes due to the additional resistance from a FAS layer on the membrane surface. The membranes were found to be thermally stable up to 250 °C and showed no changes in their hydrophobicity after in contact with hexane for 96 h.

Introduction

Hollow fibre membranes continue to attract a considerable interest due to their exceptionally high surface area per volume, enabling membrane units to have a much higher packing density compared to flat-sheet and tubular membranes [1]. They have been applied to many membrane processes such as membrane contactor, pervaporation, membrane reactor, nanofiltration, etc. [2], [3], [4], [5]. Currently, most of the commercially available hollow fibre membranes are made from polymeric materials such as polypropylene, polyethylene, polyimide, etc., most of which are susceptible to chemical and thermal stresses, leading to membrane swelling and morphological changes [6], [7]. These changes have deteriorating effects on the membrane performance, as reported in literatures [6], [7], [8]. Therefore, polymeric membranes are only limited to the applications with mild operating conditions, i.e. low acidity, alkalinity and low temperature.

Recently, a new method was developed in the production of ceramic hollow fibre membranes [9], [10], [11]. Owning to their better chemical and thermal stability, ceramic membranes are superior to polymeric membranes in harsh environment [10], [12]. Several studies have shown the success in preparation of hollow fibre membranes from ceramic materials such as aluminium oxide (Al2O3), SrCe0.95Yb0.05O3−α (SCYb), Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF), La1−xSrxCo1−yFe3−α (LSCF), etc. [10], [2], [13], [14], [15], [16]. Nevertheless, most ceramic membranes are made from metal oxides, which are hydrophilic in nature because of the presence of hydroxyl (OH–) groups on the surface. This property prevents them to be used in some applications which require only hydrophobic membranes, for instance, membrane contactor, non-polar solvent extraction, water ozonation and vapour permeation [17], [18], [19], [20], [21]. The development of hydrophobic ceramic hollow fibre membranes will definitely extend the current capability of the membranes to these areas.

The hydrophobicity of ceramic membranes can be promoted by surface modification. Several types of surface modifying agents can be used. For example, chloroalkylsilanes, fluoroalkylsilanes (FAS), alcohol, polydimethylsiloxane (PDMS), etc. [20], [22], [23], [24], [25], [26], [27], [28], [29]. However, in this study, we are interested in fluoroalkylsilanes only as they were reported to be effective in the surface modification of flat-sheet and tubular membranes [25], [27]. Fluoroalkylsilanes (FAS) are organosilanes which have hydrolysable groups and other hydrophobic ends in their structures. The hydrolysable groups are coupled with the hydroxyl groups (–OH) on the ceramic surface, forming a chemically bound hydrophobic layer, as illustrated in Fig. 1. Several factors including the number of functional groups in the FAS structure, the length of hydrophobic tails, grafting time, and grafting temperature play important roles in the surface grafting process [22], [24], [25], [26], [30].

The surface modification studies carried out so far involve only flat-sheet and tubular membranes [17], [22], [23], [24], [26], [27], [30], [31], [32]. However, the study on the surface modification of ceramic hollow fibre membranes has not been reported. Thus, this study focuses on the preparation and the characterization of hydrophobic ceramic hollow fibre membranes with surface modified by FAS grafting.

Section snippets

Preparation of alumina hollow fibre membranes

Hollow fibre membranes were prepared from starting suspensions containing alumina particles (Alfa Aesar), polyethersulfone (PESf, Goodfellow Cambridge), N-methyl-2-pyrrolidone (NMP, Riedel-de Haën) and polyvinylpyrrolidone (PVP, Acros Organics). The ratio between alumina and PESf was varied from 7 to 10, as shown in Table 1, to prepare membranes with different pore sizes and porosities. The alumina used was composed of two different average sizes: 0.3 and 1 μm in diameter. The ratio between

Membrane hydrophobicity

As can be seen in Fig. 3, the contact angles of the original membranes were all lower than 90°, indicating that they were hydrophilic and likely to absorb water. The contact angles of the membranes sintered higher temperature were obviously higher. For instance, the contact angles of the membranes sintered at 1500 °C were around 80°, much higher than those of the membranes sintered at 1200 °C which were around 50–60°. This could be explained by the change in the membrane surface chemistry. In the

Conclusions

In this study, hydrophobic ceramic hollow fibre membranes were prepared by the phase inversion/sintering method. The ratio between alumina and PESf in the spinning suspension and the sintering temperature were two important factors which influence the membrane pore radius and effective surface porosity. The pore radius and the effective surface porosity of the membranes were in the range of 25–78 nm and 2446–24,549 m−1, respectively. In general, higher alumina/PESf ratios and/or higher sintering

Acknowledgements

The authors gratefully acknowledge the research funding provided by EPSRC in the United Kingdom (grant no. GR/S87553). They would also like to thank Prof. Hua Chun ZENG, Department of Chemical and Biomolecular Engineering, Faculty of Engineering, National University of Singapore, for XPS and TGA analysis.

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