Enhanced performance of a macroporous ceramic support for nanofiltration by using α-Al2O3 with narrow size distribution
Introduction
As a pressure-driven membrane process without phase transition, nanofiltration (NF) membrane processes are widely used in separation, refinement and condensation that are ubiquitously present in process industry like water-treatment, pigment, food, paper-making, pharmacy and chemical engineering [1]. This new technology has gained much more attention due to its simple process without heating, no chemical reaction and is environmental friendly. The membrane materials, used in NF processes can be categorized into polymeric membranes and ceramic ones, of which polymeric nanofiltration membranes are commercially available for many years [2], [3]. Nevertheless, polymeric NF membrane that can be applied under extreme conditions (e.g. lower or higher pH, aggressive organic chemicals) remains one of the main challenges in this field of membrane research [2]. In comparison with polymeric membranes, ceramic membranes have distinct advantages such as chemical, thermal and mechanical stability, which are suitable for application under harsh environments [4]. However, the fabrication of ceramic membranes is much more complicated than that of its organic counterpart [3], which severely limits implementation. Ceramic membranes are typically fabricated with an asymmetric layered structure consisting of support, intermediate layers and top layer, which fulfils the separation process [5]. At present, much more investigations are focused on the development of NF membrane materials, including γ-Al2O3, TiO2, ZrO2 and HfO2 [3], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], of which TiO2 is commercially available by Inopor, Germany. With respect to fabrication of ceramic NF membranes, porous alumina materials are normally used as support providing a pore size in the range of 1–10 μm, which require 2–5 subsequent intermediate layers to reduce support pore size and roughness, so as to facilitate NF membrane deposition [4]. Each layer must separately undergo a drying and sintering step, which inevitably hampers implementation of ceramic NF membranes for large amounts of potential applications.
To optimize the support structure in such a way that it is suitable for the formation of NF membranes in one subsequent step, a high quality alumina powder with narrow particle size distribution was used to fabricate supports possessing a homogeneous pore structure, with an average pore size of ∼100 nm [17]. Supports prepared from stabilized suspensions of Sumitomo AKP30 (with a mean particle size of ∼400 nm) exhibited ideal surface morphology (with an average roughness of ∼30 nm) and excellent mechanical strength (>200 MPa), but inadequate permeability for gases (e.g. the permeance for N2 was ∼5.15×10−7 mol m−2 s−1 Pa−1) [17], and liquids (e.g. the permeance for water was 3.3 L m−2 h−1 bar−1) [18]. Alumina with an average particle size of∼600 nm, was used to fabricate a more permeable support [17], [19]. However, liquid permeance was still not high enough (4.5 L m−2 h−1 bar−1). Although the NF membrane processes are generally carried out at higher pressure of 10–20 bar, those low permeances for ceramic NF membranes still cannot meet the requirements for industrial implementation. To improve the permeability of ceramic NF membranes, further optimization of the preparation procedures of NF separation layers (including hydrolysis and condensation of precursors, dip-coating, drying and calcination processes) is required. On the other hand, it is well known that high quality porous supports are essential for NF membranes. Therefore, optimizing support structures, reduction of the amount of intermediate layers, is another way to improve the permeability of NF membranes. With respect to the latter strategy to improve the permeance of ceramic NF membranes, research progress can be summarized as follows. First of all, coarse alumina with a mean size as large as 3 μm was adopted to fabricate supports [17], which showed much higher gas permeability in the order of∼×10−5 mol m−2 s−1 Pa−1. Secondly, supports with gradient structures were prepared through a single-step processing method, by using powders with relative broad size distributions [20], [21], [22]. Highly porous structure with a continuously increasing mean pore size from top to bottom was obtained by a controlled sedimentation technique. Results showed that those functionally gradient supports exhibited pure water permeance as high as 25.2 L m−2 h−1 bar−1, with top layer pore size less than 50 nm was achieved. However, neither liquid permeances of supports nor subsequent NF membrane fabrication was reported in the above-mentioned research.
In this paper, we report on the fabrication of a high performance macroporous support, starting from alumina powder with an average size of 1.1 μm. Such alumina particles would result in relative large pore size of the partially sintered porous structure and, hence, improve permeance of the macroporous support. On the other hand, the relatively narrow particle size distribution induces a surface morphology that enables the reduction of defects in subsequently deposited NF membrane layers. The effect of sintering temperature on properties of this promising support, as well as that properties in comparison with the data cited from references, was investigated in detail. Nanofiltration properties of γ-Al2O3 mesoporous membrane, which is deposited on this promising support, were also reported.
Section snippets
Preparation of macroporous alumina supports
α-Al2O3 powder (purity >99.5%, Nanjing, China) with a mean particle size of ca. 1.1 μm, were used as received. 75 g α-Al2O3 powder was introduced into 175 g deionized water under stirring and pH value of the suspension was adjusted to 1.5 with the addition of 1 mol L−1 HNO3. Subsequently, the suspension was poured into a dedicated glass bottle and ultrasonically dispersed for 20 min (VC 505, Sonics and Materials Inc.). After screening with a stainless-steel sieve (mesh size: 0.1 mm), 25 ml
Properties of macroporous alumina support as a function of sintering temperature
Fig. 3 shows the particle size distribution of α-Al2O3 powders used in this study. It was found that the alumina particle size was in the range of 0.7 –2 μm, with an average size of ∼1.1 μm. It should be noted that a small amount of particles, with size smaller than 0.5 μm, can also be observed in Fig. 3. The narrow particle size distribution is a prerequisite for the formation of a uniform microstructure, which is obtained through random packing of solid particles. Fig. 4 gives the linear
Conclusions
- (1)
A macroporous alumina support was fabricated through a colloidal filtration route, by using α-Al2O3 powders with an average particle size of 1.1 μm. At a sintering temperature of 1250 °C, support A1100 with integrated performances of permeability, mechanical strength and surface roughness was obtained. This support provided relative high permeances towards water (101 L h−1 m−2 bar−1) and nitrogen (∼2×10−6mol m−2 s−1 Pa−1), while maintained an average surface roughness of ∼175 nm and a sufficient
Acknowledgments
The authors would like to thank the financial support from the National Natural Science Foundation of China (20906047, 21276123), the National High Technology Research and Development Program of China (2012AA03A606), State Key Laboratory of Materials-Oriented Chemical Engineering (ZK201002), the Natural Science Research Plan of Jiangsu Universities (11KJB530006) and the “Summit of the Six Top Talents” Program of Jiangsu Province.
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