Yanju
Wang
a,
Xiaoqin
Zou
*a,
Lei
Sun
b,
Huazhen
Rong
a and
Guangshan
Zhu
*a
aFaculty of Chemistry, Northeast Normal University, Changchun 130024, P. R. China
bState Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, P. R. China. E-mail: zouxq100@nenu.edu.cn
First published on 24th January 2018
Membrane desalination has been proposed as a promising strategy to address the worldwide challenge of water scarcity. The development of membrane desalination is impeded by the limited availability of membrane materials, thus, selective membranes with high water fluxes are in high demand. Therefore, the synthesis of a zeolite-like aluminophosphate membrane for water desalination is reported herein. An AlPO4-18 membrane without visible cracks was prepared on stainless steel nets via a seeded approach. The specific adsorption and diffusion properties of the AlPO4-18 membrane toward water were simultaneously investigated by vapor adsorption measurements and molecular simulation. A large adsorption capacity indicates the excellent water affinity of the AlPO4-18 membrane, and water gathering around other water molecules suggests stronger water–water interactions than framework–water interactions. Meanwhile, water molecules transport rapidly through the pores, assisted by a built-in pathway and directed diffusion. The as-synthesized membranes are further evaluated for water desalination. The desalination tests of seawater containing both single and multi-component ions demonstrate that the AlPO4-18 membrane is very selective for water transport, evidenced by very high rejection degrees for cations (>99%). Moreover, the AlPO4-18 membrane exhibits unprecedented high water fluxes (2.14 kg m−2 h−1 in average). The superior performances of good selectivity, high water permeability and high stability ensure the potential application of supported aluminophosphate membranes in desalination.
Fig. 1 Schematic presentation of water desalination via membrane pervaporation process using the AlPO4-18 membrane. |
The membrane acts as a selective barrier, separating the feed and permeate into two phases. When saline water is fed in, water passes through a semi-permeable membrane, driven by a pressure difference, while ions are simultaneously repelled in the retentate. Vaporized water is collected on the permeate side. The semi-permeable membrane is the core of the membrane pervaporation process, which determines water flux and ion rejection. However, limited membrane materials impede the development of membranes for desalination. Current membranes suffer undesirable tradeoffs between flux and ion rejection, namely, a highly permeable membrane has a low rejection and vice versa. In this regard, research in alternative membrane materials could enable more opportunities to balance flux and rejection in a better manner towards applications in desalination.
Among emerging membrane materials, zeolites have intrinsic advantages such as high permeability and good selectivity due to their well-defined pore structures and high porosities.3 Zeolite membranes are polycrystalline thin films commonly synthesized on rigid and porous substrates by in situ crystallization and seeded secondary growth methods.4 Aluminophosphates (AlPO4) as zeolite analogues are crystalline microporous materials with interesting properties and specific functions which impact our everyday lives, namely as catalysts and adsorbents.5 Similar to aluminosilicate zeolites, aluminophosphates are built from adjacent TO4 (T = Al, P) tetrahedra via corner-sharing to form ordered pores of molecular dimensions. Zeolitic membranes have found wide applications, particularly in gas or liquid separation. In the past two decades, intensive investigations have been carried out on zeolite membranes in terms of material discovery, synthesis renewal, and engineering technique advancement to enhance the separation performance.6,7 In parallel, continuous research efforts have been devoted to expand the applications of zeolite membranes beyond the realm of gas separation. The earliest simulation study in 2001 predicted that zeolite materials were perfect candidates for membrane desalination due to their uniform pore sizes and shapes.8 Subsequently, many membranes based on classical zeolites, including LTA, FAU, SOD and MFI types, have been experimentally prepared for water desalination.9–30 Unfortunately, limited applications were found for aluminophosphate membranes because of the poor membrane-formation ability. In recent studies, several aluminophosphate membranes have been attempted for gas separation.31–33 Motivated by the need for new materials for advanced applications, we herein aim to achieve the synthesis of aluminophosphate membranes and to extend their application to the new arena of seawater desalination.
In the present study, AlPO4-18 was selected as the target aluminophosphate material.34–36 AlPO4-18 has an AEI-type topology, which is constructed from equimolar AlO4 and PO4 units in strict alternation (Fig. 1). Consequently, the Al/P ratio in AlPO4-18 is close to unity and thus leads to a neutral framework. Additionally, AlPO4-18 possesses a three-dimensional 8-ring channel system, giving rise to pore openings of 3.8 Å and a cage diameter of ∼8.0 Å at the channel intersections. Small-size windows can maximize the molecular sieving effect to allow water molecules to pass through while excluding solvated salt ions based on their size discrepancies. In addition, the three-dimensional porous structure favours the formation of highly interconnected pathways for fast water transport (Fig. 1). The seeded method is employed for the synthesis of AlPO4-18 membranes owing to the advantages of decoupled nucleation and secondary growth during membrane crystallization. From a structural consideration, AlPO4-18 membranes are appealing for the separation of small molecules.
Fig. 2 X-ray diffraction patterns of the (a) simulated AEI-type structure, (b) AlPO4-18 seed layer, and (c) as-prepared AlPO4-18 membrane. |
The morphologies of the AlPO4-18 seed layer and membrane were inspected in detail by scanning electron microscopy (SEM), and the SEM pictures are shown in Fig. 3. As can be seen, the AlPO4-18 seeds display hexagonal plate-like shapes, and the sizes are estimated to be around 500 nm (Fig. 3a). Meanwhile, the net support is homogenously covered by a continuous layer of AlPO4-18 nanocrystals. The full coverage of the support by the seeds can promote uniform crystallization during the secondary-growth stage to facilitate the formation of a continuous membrane. The as-prepared AlPO4-18 membrane after hydrothermal treatment was also monitored by SEM. From the top view (Fig. 3b), a continuous AlPO4-18 membrane is obtained after an additional 24 h-treatment in the synthesis solution. The secondary crystallization of the AlPO4-18 seeds has two consequences: crystal enlargement and intergrowth.
As shown in a magnified SEM image in Fig. 3c, individual crystallites in the membrane are approximately 3.0 μm in length, which are significantly larger than the seeds (500 nm). Due to limited space on the support, the AlPO4-18 crystals undergo an intergrowth process during the secondary crystallization. This overgrowth causes healing of the grain boundaries between neighboring crystals. Both phenomena account for high continuity and good compactness, supported by the polycrystalline AlPO4-18 membrane without visible cracks (Fig. 3c). Fig. 3d shows the cross-sectional image of the membrane from which a thickness of approximately 15 μm is determined. With a close look, we can observe that plate-like AlPO4-18 crystals are inter-grown in a stack-by-stack mode.
More structural properties were further investigated by combined techniques, and the texture data is summarized in Table S1.† The Al/P ratio in the as-synthesized AlPO4-18 membrane is measured by ICP to be 1.02, confirming the equivalence of Al and P atoms in the framework. The surface area of the AlPO4-18 crystals collected from the membrane is calculated to be 754.0 m2 g−1. The pore size is estimated to be 8.23 Å, corresponding to the crystallographic value of the AEI-type cavity. A total and micropore volumes of 0.647 and 0.283 cm3 g−1 are also determined by nitrogen physical sorption.
To microscopically elucidate the sorption behavior, molecular simulations of water adsorption on AlPO4-18 were performed.38–42 The simulated isotherm of water adsorption in AlPO4-18 at 298 K is shown in Fig. 4b.
It can be seen that the simulation reproduces the experimental adsorption isotherm. At low pressures (below 250 Pa), water molecules are hardly adsorbed into the pores. For example, the average molecule loading excess at 100 Pa is only 0.008 molecules per unit cell (Fig. 5a). Clearly, the water–water interactions are stronger than the water–framework interactions. With increasing pressure, the isotherm shows a vertical jump in the water sorption around 300 Pa, at which the average molecule loading excess is 20.40 molecules per unit cell. This behavior is mainly due to the occurrence of condensation transition. The water molecules bind together through intermolecular hydrogen interactions between oxygen and hydrogen atoms as shown in Fig. 5b. With a further increase in pressure, the water uptake slowly increases. At 2000 Pa, the average molecule loading excess is 48.75 molecules per unit cell. Water molecules fill the large pores, and the vapor-like water gradually transforms into liquid-like water as shown in Fig. 5c. The simulation result reflects the cooperative effect of the water molecules, which has been described in previous works.43,44 When several water molecules adsorb in the pore, they behave as active sites where more water molecules assemble through hydrogen bonding, due to the stronger interactions between the water molecules. Based on the maximum equilibrium uptake of 16.5 mol kg−1, we determined the number of water molecules per cage to be 12 (assuming a homogeneous distribution of water molecules in AlPO4-18 cages, four cages per unit cell, Vcell = 3.19 nm3),34 which means that each AlPO4-18 cavity is occupied by a twelve-water cluster.37 The high occupancy allows frequent water transport from one cage to another in the membrane.
Fig. 5 Snapshots of water adsorption in AlPO4-18 at 298 K and (a) at 100 Pa, (b) at 300 Pa, and (c) at 2000 Pa water (the dotted line represents intermolecular hydrogen bonds). |
Feed solution | Concentration (mol L−1) | Water flux (kg m−2 h−1) | Cation rejection (%) |
---|---|---|---|
NaCl | 0.5 | 3.37 | 99.87 |
KCl | 0.5 | 2.23 | 99.96 |
MgCl2 | 0.5 | 1.82 | 99.97 |
CaCl2 | 0.5 | 1.84 | 100.00 |
Seawater (3.5% salts) | |||
Na+ | 0.5 | 2.14 | 99.70 |
K+ | 0.01 | 99.95 | |
Mg2+ | 0.05 | 100.00 | |
Ca2+ | 0.01 | 99.99 |
Notably, the water flux of the AlPO4-18 membrane herein is a new benchmark in polycrystalline zeolite membranes (Fig. 7),19–30 and its value (water flux of 2.14–3.37 kg m−2 h−1 bar−1 at rejection of >99.5%) also reaches the upper limit of currently available membranes for water desalination (2.5 L m−2 h−1 bar−1 at 100% rejection).45 The exceptionally high water flux can be rationalized using the descriptors of high solubility (S) and fast effective diffusivity of water (Deff). The high adsorption capacity improves the water solubility and thus leads to frequent water transport. The effective water diffusivity is controlled by its intrinsic diffusivity, diffusion length and transport direction. To prove this hypothesis, simulated studies were conducted.46,47 The dynamic trajectory of one water molecule in AlPO4-18 is shown in Fig. 8a. The points represent the MD trajectory. As can be seen, the water molecule can move freely among the large pores. It is also found that small pores (composite building units of d6r) in the framework are not filled (Fig. 8a). As seen from another direction (Fig. 8b), the d6r units consisting of 6-rings and 4-rings (Fig. 8c) are empty during the dynamic process. The smaller diameters of the 6-ring and the 4-ring prevent molecule transition. Therefore, the water molecules diffuse only through the bigger 8-rings in AlPO4-18, validating the molecular sieving effect of the membrane.
The self-diffusion coefficients (Ds) for m1, m8 and m396 are 12.06 × 10−9, 0.33 × 10−9 and 0.08 × 10−9 m2 s−1, respectively. With increasing molecule loading, the Ds of water decreases. Upon analyzing the dynamic process, we find that only one water molecule passes through the 8-ring at a time, which supplements the single molecular transport mechanism. The molecular diffusion is limited by the narrowness of the pore opening (8-ring), especially when more water molecules are introduced into the AlPO4-18 pores. However, an efficient water transport pathway is prone to be built on extensive water molecules in cavities as well as at pore mouths, which can be vividly observed in Fig. 8d. This pathway building can shorten the diffusion length of water from one position to another and consequently increase the water diffusion rate. To support this conclusion, the energy barrier for water permeation through the AlPO4-18 membrane was determined from the temperature-dependent fluxes (Fig. S1†). The activation energy is measured to be ∼20 kJ mol−1, which lies within the hydrogen-bond energies of 14–21 kJ mol−1 for vaporized water.48 This value is even lower than that of highly hydrophilic FAU zeolites.22 Moreover, the direction of water movement can be controlled by applying an external pressure across the membrane in the permeation experiments.
From both the computational and experimental studies, it can be concluded that the large adsorption capacity contributes to the high water solubility, and the synergistic factors of relatively fast intrinsic diffusivity, short diffusion length and directed water transport participate in increasing the effective diffusivity. The increase in solubility and diffusivity is an outstanding improvement in the overall water transport rate over the AlPO4-18 membrane.
To address the robustness and reproducibility of the membrane, water permeations for different time periods were evaluated. As shown in Fig. 9, the AlPO4-18 membrane retains the water flux (2.14 ± 0.1 kg m−2 h−1) and the rejection degrees (99.7% for Na+, 99.9% for K+, 100% for Mg2+, 100% for Ca2+) over a 90 h period. The membrane can be used repeatedly over a long period of time, showing the high stability of the supported membrane, which is applicable in seawater desalination. It is worth mentioning that the cation rejection degree (Fig. S2†) reaches a steady state (99–100%) in a short time (<6 h), which sheds light on the perfectly inter-grown membrane without grain boundaries and the neutral framework. This data is consistent with negligible concentrations of cations remaining in the AlPO4-18 membrane after the desalination test (Fig. S3†).
Fig. 9 Water flux and cation rejection degree of the AlPO4-18 membrane in simulated seawater as a function of test time (test conditions: 298 K, feed pressure of 1.0 atm). |
Footnote |
† Electronic supplementary information (ESI) available: Experimental details and additional figures and tables. See DOI: 10.1039/c7sc04974a |
This journal is © The Royal Society of Chemistry 2018 |