b Institute of New Energy Technology, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Science, Ningbo 315201, China;
c Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Department of Chemistry, East China Normal University, Shanghai 200241, China
The development of renewable and clean energy has attracted much attention due to the problems of energy shortage and environmental pollution. Fuel ethanol, as a renewable and clean energy, has been extensively investigated. Ethanol dehydration is one of the most important steps of the producing fuel ethanol. There are various separation methods to get highly ethanolenriched mixtures, such as distillation, adsorption, and pervaporation [1, 2]. Pervaporation is the most promising method to obtain highly ethanol-enriched mixtures due to its outstanding advantages, such as high efficiency, low energy consumption, and easy process design.
Pervaporation, a membrane-based technology, contains three significant steps for the separation of liquid mixtures through a membrane [3-6]: (1) adsorption from the liquid phase, (2) diffusion through the membrane, (3) desorption into the vapor phase. The membrane for pervaporation separation of ethanol/water mixture should be high chemically and thermally stable. LTA zeolite membranes, with high hydrophilic feature, uniform pore size of about 0.4 nm, and chemical and thermal stability, becomes the most outstanding candidate for de-watering of bio-ethanol [2, 7-14]. Indeed, the Na-LTA zeolite membranes have been developed in industrial application to obtain anhydrous ethanol [2, 12]. Normally, Na-LTA zeolite membranes show high water selectivity with water/ethanol separation factor from hundreds to thousands. However, it is still high desired to increase the water flux of the Na-LTA zeolite membranes . It is well known that the pore size of the zeolite Na-LTA can be tuned by cation exchange, such as zeolite K-LTA (3A) with pore size of about 0.3 nm and zeolite Ca-LTA (5A) with pore size of about 0.5 nm [16-18]. Recently, we reported the preparation of Ag-LTA zeolite with enhanced gas separation performance by silver-exchange treatment of the as-synthesized zeolite Na-LTA membrane . Therefore, it can also be expected to improve the pervaporation performances of the zeolite Na-LTA membranes by tuning the pore size through ion exchange.
The ion size of copper ions (0.073 nm) is a bit smaller than that of sodium ions (0.102 nm), and one copper ion will exchange two sodium ions in order to keep the equilibrium of charge. Thus, the pore size of the copper-exchanged LTA (here after called Cu-LTA) zeolite membrane should be larger than that of the starting Na-LTA zeolite membrane [16, 20]. Therefore, it can be expected that CuLTA zeolite membranes will display a higher water flux in the pervaporation separation of ethanol/water mixture than Na-LTA zeolite membranes. As far as we know, there is no report on the preparation of Cu-LTA zeolite membranes. In this paper, we report the synthesis of phase-pure and well intergrown Cu-LTA zeolite membranes on the 3-aminopropyltriethoxysilane (APTES) modified α-Al2O3 tubes, as shown in Fig. 1. In the first step, a thin and well intergrown Na-LTA zeolite membrane is prepared on the APTES-modified α-Al2O3 tube . And then, a following copperexchange treatment is carried out to obtain Cu-LTA zeolite membrane.
|Fig. 1. Scheme of the synthesis of supported Cu-LTA zeolite membrane through copper ion exchange of Na-LTA zeolite membrane.|
The Na-LTA and Cu-LTA crystals as well as membranes were prepared by hydrothermal synthesis. The details of the synthesis of LTA zeolite crystals and membranes are shown in the Supporting information. Figs. 2a and b show the FESEM images of Na-LTA and Cu-LTA crystals, respectively. It can be seen that the crystals morphology of the Cu-LTA is completely same to that of the NaLTA, suggesting that sodium substitute by copper ions does not destroy the structure of the Na-LTA zeolite. The structure stability of the Na-LTA is further confirmed with XRD. As shown in Fig. S1 (Supporting information), the crystal structure of the Na-LTA zeolite keep constant after copper exchange
|Fig. 2. FESEM images of Na-LTA (a) and Cu-LTA (b) zeolite crystals, Na-LTA (c, d) and Cu-LTA (e, f) zeolite membrane. Top view (c, e) and cross-section (d, f).|
Before hydrothermal synthesis of the LTA zeolite membranes, the porous α-Al2O3 tubes were treated with 3-aminopropyltriethoxysilane (APTES, 0.45 mmol/L in 10 mL toluene) at 110 ℃ for 1 h [21, 22]. The Na-LTA zeolite membranes were prepared on the APTES-modified α-Al2O3 tubes through in-situ growth for 24 h at 60 ℃, by using a clear synthesis solution with molar ratio of 50Na2O:1Al2O3:5SiO2:1000H2O [21-25]. The FESEM images of the Na-LTA zeolite membrane are shown in Figs. 2c and d. it can be seen that a compact Na-LTA membrane covered by uniform and cubic-shaped Na-LTA crystals is formed on the surface of the α- Al2O3 tube, and there are no observable pinholes and other macroscopic defects can be found. From the cross-section view (Fig. 2d), the Na-LTA zeolite membrane is well intergrown with a thickness of about 3.0 μm. As reported in our previous report [21, 22], by simple modification with APTES, 3-aminopropylsilyl functional groups were introduced onto the support surface, which are helpful to promote the nucleation and growth of compact NaLTA zeolite membrane. The formation of a phase-pure Na-LTA zeolite membrane was confirmed by XRD (Fig. S2 in Supporting information). The XRD patterns of Na-LTA zeolite membrane indicates that all peaks match well with those of zeolite LTA crystals besides the α-Al2O3 signals from the support. After copper ion exchange, the morphology and thickness as well as structure of the Cu-LTA zeolite membrane is consistent with that of the Na-LTA zeolite membrane, and there are no any visible defects (Figs. 2e and f) and detectable impure phases (Fig. S2c) are introduced.
The ions exchange by copper was confirmed with XPS. As shown in Fig. 3, the peak intensity of sodium ions decreases after ion exchange, and a new peak of copper ion emerges, indicating that sodium ions in Na-LTA framework have been partially replaced by copper ions. Table S1 shows chemical compositions of the Na-LTA and Cu-LTA membrane. It can be seen that about 20% sodium ions is exchanged by copper ions in this study. In our previous report, about 82% sodium ions can be exchanged by silver ions . The relatively low exchange degree of the sodium ions in this study probably results from the fact that the copper ions are much more inactive than silver ions. We have ever tried to elevated exchange ratio with a high concentration of Cu(Ac)2 methanol solutions. However, the LTA zeolite membranes are easily damaged under condition (Fig. S3 in Supporting information). Further, the Si/Al ratio of the Cu-LTA membrane is about 1.09, which is close to the actual Si/Al ratio (1.10) and theoretical Si/Al ratio (1.00) of the NaLTA membrane, further confirming that the structure of the LTA membrane is unchanged after copper exchange.
|Fig. 3. X-ray photoelectron spectroscopy (XPS) wide scan of Na-LTA zeolite membrane (a), and Cu-LTA zeolite embrane (b).|
The separation performances of the Na-LTA and Cu-LTA zeolite membranes for ethanol dehydration were evaluated by pervaporation for dehydration of 90.0 wt% ethanol/10.0 wt% water mixtures at 30–75 ℃ in a home-made permeation module . Fig. 4 shows the effect of temperature on the separation performance of the Na-LTA and Cu-LTA membranes for pervaporation of 90.0 wt% ethanol/10.0 wt% water mixtures. As expected, for both Na-LTA and Cu-LTA membranes, the water fluxes increase rapidly with temperature, i.e., the water flux of the Na-LTA and CuLTA membrane is 0.32 and 0.59 kg m-2 h-1 at 30 ℃, and increase to 1.65 and 3.52 kg m-2 h-1 at 75 ℃. In this temperature range of 30– 75 ℃, all water fluxes through the Cu-LTA membrane are higher than those of the Na-LTA zeolite membrane. This experimental finding is caused by the increase of pore size after the exchange of sodium ions by copper ions. The enhancement of pore size of the Cu-LTA membrane will result in the reduction of the mass transfer resistance, thus leading to the increase of water flux. Further, at 75 ℃ the water/ethanol separation factors of the Na-LTA and CuLTA membranes are 4082 and 3591, respectively, suggesting that both Na-LTA and Cu-LTA membranes prepared on the APTESmodified α-Al2O3 tube displays a high pervaporation performance.
|Fig. 4. Water flux and water/ethanol separation factor of the Na(Cu)-LTA zeolite membrane as function of temperature for pervaporation of 90.0 wt% ethanol/ 10.0 wt% water mixtures.|
In conclusion, phase-pure and well-intergrown Cu-LTA membranes were developed through copper ions exchange of sodium ions in Na-LTA framework. Firstly, dense and phase-pure Na-LTA membranes were prepared on the APTES-modified α-Al2O3 tubes. Followed by copper ions exchange of the Na-LTA membranes, CuLTA membranes could form facilely, maintaining the cubic-shaped morphology and LTA structure of the Na-LTA membrane. Although only 20% sodium ions was exchanged by copper ions, the Cu-LTA membrane showed much higher water flux than Na-LTA membranes due to the enhancement of the pore size. For pervaporation of 90.0 wt% ethanol/10.0 wt% water mixtures through Cu-LTA zeolite membrane, the water/ethanol separation factor of 3591 and water flux of 3.52 kg m-2 h-1 at 75 ℃ could be obtained, which was promising for dehydrating ethanol. Further work is in progress for the production of the large-scale zeolite Cu-LTA membranes for the industrial application in dehydrating ethanol.Acknowledgments
This work was financially supported by theNational Natural Science Foundation of China (Nos. 21761132003, 21606246), and Ningbo Science and Technology Innovation Team (No. 2014B81004).Appendix A. Supplementary data
Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.cclet.2019.01.016.
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