Wen-Yan Pan,
Liang-Liang Peng,
Wen-Jing Wang,
Yuan-Yuan Li and
Xue-Ling Wei*
School of Chemical and Environmental Engineering, Anhui Polytechnic University, Wuhu, Anhui 241000, China. E-mail: xueling_wei@163.com
First published on 17th November 2021
Zeolite membranes with unique physical and chemical properties are emerging as attractive candidates for membrane separation. However, defects in the zeolite layer seriously affect their molecular sieving performance. In this study, a novel strategy for preparing compact zeolite membranes on rough supports with the assistance of a reticulated hydrotalcite layer was developed. The reticulated hydrotalcite layer was grown on the inner surface of a 170 mm length ceramic tube by an in situ hydrothermal method, and a NaA zeolite membrane was prepared on this reticulated layer by the microwave-heating method. The hydrotalcite interlayer could not only improve the smoothness and regularity of the surface of the support but also fix the Si/Al active ingredients using its reticulate structure, finally effectively improving the quality and stability of the zeolite layer. The optimal molar ratio of the synthesis solution for the synthesis of the zeolite membrane was 3Na2O:
2SiO2
:
Al2O3
:
200H2O. The permeance flux of H2 through the zeolite membrane synthesized under the optimal conditions was high as 0.47 × 10−6 mol m−2 s−1 Pa−1, and its permselectivity for H2 over N2 was 4.7, which was higher than the corresponding Knudsen diffusion coefficient. This study provides a new idea for the preparation of defect-free membranes on rough supports.
In order to eliminate defects and improve the quality of the membrane, various membrane preparation methods5–8 and post-treatment techniques9–12 have been developed to synthesize and/or modify zeolite membranes. Recently, Wei et al. eliminated the defects caused by bubbles by pre-degassing the synthesis system before the pressure-driven hydrothermal synthesis,13 and then modified the zeolite membranes with the help of the space network structure formed by methylcellulose at high temperature.14 Like other preparation methods and post-treatment techniques, those methods improved the density of the zeolite membrane to a certain extent, but it is still difficult to eliminate the defects completely so as to provide satisfactory permeation performance with excellent reproducibility in zeolite membranes. Reportedly, the rough and porous support results in uncontrolled infiltration of the active ingredients into the surface of the support, leading to the formation of defects in the zeolite layer and reducing the density of the zeolite membrane.15,16 Therefore, making the active ingredients evenly distributed on the rough surface of the support is the key to the preparation of compact zeolite membranes.
In recent years, researchers assembled organics, such as cationic polymers17,18 or covalent linkers,19,20 on the rough surface of the support and used the organics to capture the active ingredients in the synthesis solution, promoting the formation of well intergrown and compact zeolite membranes. While this method could provide appreciably high selectivity membranes, the membranes usually exhibited low permeability with stability issues owing to the organics contained in the membrane. Furthermore, the synthesis process lacks environmental benignity due to the employment of organic agents. Another common method to promote the compactness of the zeolite membrane is using smooth support to replace the rough support.21,22 The smooth support is composed of nanoparticles or coated with nanoparticles on the rough surface. While smooth support was helpful in preparing a high-quality zeolite membrane, it would increase the membrane preparation cost and thus limit its industrial application.23
In this work, a novel strategy for the preparation of compact zeolite membrane on rough tube support using a microwave heating method with the assistance of a reticulated hydrotalcite interlayer is reported. The grid-like reticulated hydrotalcite layer was in situ grown on the surface of the rough tube support and then NaA zeolite membrane was grown on this interlayer by the microwave heating method. The characterization results showed that the hydrotalcite interlayer can not only improve the flatness and integrity of the support, limiting the formation of defects but also can control the infiltration of the active ingredients onto the surface of support using its reticulated structure, improving the stability of the top layer with the bottom layer. This work is helpful for the preparation of defect-free membranes on rough support.
Porous α-A12O3 tubes (outside diameter of 12 mm, inside diameter of 8 mm, length of 170 mm, the porosity of 50%, and average pore size of 1 μm) were purchased from Ningbo Damo Advanced Materials Technology Co., Ltd. and were used as support. The inner surface of the support was polished with 400 and 1000 grit-sand paper before using as support. Briefly, the sandpaper was cut into strips with a width of about 1 cm and wrapped around the glass rod fixing with double-sided tape. The glass rod was inserted into the ceramic tube and rotated in the same direction. When the surface of the support becomes smooth, the rotation of the glass rod could be stopped.
After polishing with sandpaper, the tubes were soaked in 12 mol L−1 sodium hydroxide solution for 12 h and then washed with deionized water repeatedly in an ultrasonic cleaner to remove the impurities after pretreatment. Finally, tubes were dried at 110 °C before further treatment.
The hydrotalcite layer synthesis solution was prepared as follows: a salt solution was first prepared by dissolving certain quantities of magnesium chloride hexahydrate and aluminum nitrate in deionized water under vigorous stirring, and then the solution was mixed with sodium carbonate aqueous solution under agitation. The pH of the solution was adjusted with 0.1 mol L−1 sodium hydroxide. The synthesis solution was vigorously stirred for 6 h before using it for the synthesis of the hydrotalcite layer.
The support was vertically placed into the autoclave, which had been charged with the above-prepared hydrotalcite synthesis solution. Then, the reactor was sealed and placed into the drying oven, which has been heated to 90 °C, and kept for 24 h. After that, the tube was taken out from the hydrotalcite synthesis solution when the reactor was cooled to room temperature, and repeatedly washed with deionized water until the pH of the washed water was neutral. Finally, the obtained tube was dried at 110 °C for 12 h before the subsequent synthesis of NaA zeolite membrane.
NaA zeolite membranes were synthesized by the microwave heating method. Firstly, the supports were vertically placed into the Teflon reactor, which had been charged with the above-prepared zeolite membrane synthesis solution. After that, the reactor was placed into a microwave oven, which was connected with a reflux device. The synthesis solution was raised to 90 °C in 5 min and maintained for 30 min under ambient pressure using microwave power. After the synthesis solution was cooled to room temperature, the tube was taken out from the zeolite membrane synthesis solution and washed with deionized water repeatedly until the pH of the washed water was neutral. Finally, the thus obtained tube was dried at 110 °C for 12 h before subsequent characterization.
The detailed information of the synthesis condition is shown in Table 1.
Sample | Hydrotalcite layer | Reaction composition of the zeolite membrane |
---|---|---|
M0 | Yes | None |
M1 | None | 2Na2O![]() ![]() ![]() ![]() ![]() ![]() |
M2 | Yes | 2Na2O![]() ![]() ![]() ![]() ![]() ![]() |
M3 | Yes | 2Na2O![]() ![]() ![]() ![]() ![]() ![]() |
M4 | Yes | 2Na2O![]() ![]() ![]() ![]() ![]() ![]() |
M5 | Yes | 2Na2O![]() ![]() ![]() ![]() ![]() ![]() |
M6 | Yes | 1Na2O![]() ![]() ![]() ![]() ![]() ![]() |
M7 | Yes | 3Na2O![]() ![]() ![]() ![]() ![]() ![]() |
M8 | Yes | 4Na2O![]() ![]() ![]() ![]() ![]() ![]() |
Scanning electron microscopy (SEM) was performed using an S4800 scanning electron microscope and the elemental content of the membranes was analyzed using energy-dispersive X-ray spectroscopy (EDX). The tube fragment with the outer smooth surface was selected for scanning electron microscopy studies.
Atomic Force Microscopy (AFM) was performed using a MultiMode 8 atomic force microscope. The tube fragment with the outer smooth surface was selected for atomic force microscopy studies.
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Fig. 1 XRD patterns of the deposited powders obtained under different treatments: (a) M0, (b) M1, and (c) M2 membranes. |
Fig. 2 shows the micrographs of the support and the hydrotalcite interlayer. The support is sintered with the heterogeneous α-Al2O3 particles, while the pores in the support are formed from the gaps between the sintered particles (Fig. 2a and c). The heterogeneous α-Al2O3 particles make the surface of the support full of bumps and holes (Fig. 2e), which may lead to the difficult uniform distribution of the zeolite crystal on the surface of the support. After the support is hydrothermally treated in the hydrotalcite synthesis solution, flaky hydrotalcite crystals with a diameter of several microns and thickness of several nanometers grow vertically on the surface of the support, and they cross each other to form a reticulated hydrotalcite layer (Fig. 2b). The cross-sectional view (Fig. 2d) shows that a reticulated hydrotalcite layer with thickness of 7.0 μm is grown over the support, and exposes a flat and uniform surface. The uneven surface of the support is filled with hydrotalcite, leading to no clear cracks existing between the hydrotalcite layer and the support. The surface becomes smooth and flat (Fig. 2f), indicating that the hydrotalcite layer greatly improves the smoothness and regularity of the surface.
Fig. 3 shows the SEM micrographs of the zeolite membranes. A layer of NaA zeolite with spherical morphology is formed on the surface of the support after the support is treated under microwave heating in the zeolite membrane synthesis solution (Fig. 3a and c), which agrees with the reports on microwave synthesis of zeolite membrane.26,27 The rough support makes the zeolite crystal unable to stack tightly and the zeolite layer uneven, resulting in large numbers of defects in the surface of the M1 membrane (Fig. 3a). In contrast, zeolite crystals are closely combined in the form of twins to form a defect-free zeolite layer in the M2 membrane (Fig. 3c) owing to the existence of the even reticulated hydrotalcite layer in the support, which agrees with the previous report.28 The cross-sectional view (Fig. 3b and d) shows that the zeolite layer with a thickness of 2.8 μm is grown over the support. However, some α-Al2O3 particles are exposed to the surface of the zeolite layer, leading to defects in the M1 membrane. The M2 membrane consists of support, hydrotalcite interlayer, and zeolite layer (Fig. 3d). The zeolite layer with a thickness of 3.5 μm is grown over the hydrotalcite layer and there is no clear crack existing between the zeolite layer and the hydrotalcite layer, indicating a strong interaction between them. The line EDS results show that the content of the elements is distinctly different between the different layers. There is a transition layer between each layer in the M2 membrane, indicating a strong combination between the layers. It may be due to the fact that the Si/Al active ingredients are deposited in the reticulate structure of the hydrotalcite layer and transferred into zeolite crystal, leading to the zeolite crystal growing in the reticulate structure and finally the reticulate structure effectively improves the stability of the zeolite layer. The partial dissolution of the hydrotalcite under strong alkali conditions results in a decrease in the thickness of the hydrotalcite layer.
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Fig. 3 SEM micrographs of the zeolite membranes: top-view of (a) M1 and (c) M2 membranes; cross-section of (b) M1 and (d) M2 membranes. |
A pencil hardness test was performed to further evaluate the mechanical strength of the zeolite membrane as reported elsewhere.29,30 The pencil is tilted to 45° on the zeolite membrane with a pressure force of 7.5 N. While some of the zeolite particles are broken when the 4H pencil streaks across the zeolite layer, the zeolite layer is not damaged, indicating that the zeolite layer can resist the 4H pencil hardness test. However, some scratches appear in the zeolite layer when the 5H pencil streaks across the zeolite layer, indicating that the hardness greater than that of the 4H pencils can damage the zeolite layer.
Accordingly, the hydrotalcite interlayer not only can improve the flatness and integrity of the support, limiting the formation of defects, but also can control the infiltration of the active ingredients into the surface of support using its reticulated structure, improving the stability of the top layer with the bottom layer.
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Fig. 4 XRD patterns of the deposited powders obtained under different Si/Al ratios from the synthesis solution: (a) M3 membrane, (b) M2 membrane, (c) M4 membrane, (d) M5 membrane. |
Fig. 5 shows SEM micrographs of the top view of supports treated with synthesis solutions of different Si/Al ratios. One can see that a continuous solid-particle layer was formed over all supports even though the morphologies of the solid particles were different. The spherical grains with uneven particle size are loosely aggregated over the surface of the support for the M3 membrane (Fig. 5a), causing the construction of the zeolite layer with many defects. The membrane becomes denser as the ratio of Si/Al increases, and almost no obvious defects can be identified in the M2 and M4 membranes (Fig. 5b and c). When the ratio of Si/Al in the synthesis solution far exceeds the standard Si/Al ratio of NaA zeolite (the Si/Al ratio of the standard NaA crystal is 1:
1), fine solid particles with different morphology is deposited on the surface of the support, forming a solid particle layer with many defects (Fig. 5d). The SEM results show that the ratio of Si/Al can significantly influence the formation of zeolite crystals and the Si/Al ratio of the synthesis solution for the M2 membrane can be considered as the best condition, which is similar to the XRD results.
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Fig. 5 SEM micrographs of the top view of the support treated with different Si/Al ratios from the synthesis solution: (a) M3 membrane, (b) M2 membrane, (c) M4 membrane, (d) M5 membrane. |
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Fig. 6 XRD patterns of the deposited powders obtained under different alkalinities in the synthesis solution: (a) M6 membrane, (b) M2 membrane, (c) M7 membrane, (d) M8 membrane. |
Fig. 7 shows SEM micrographs of the top view of supports treated under different alkalinities in the synthesis solution. One can see that a continuous solid particle layer has been formed over all supports even though the morphologies of the solid particles are different. The fine spherical grains are aggregated over the surface of the support for the M6 membrane (Fig. 7a). However, there are lots of defects in the zeolite layer due to insufficient contact between the spherical grains. The particle size of the spherical grains increases gradually with the increase in alkalinity, and they contact each other in the form of twins, leading to the quality of the membranes becoming denser with the increase in alkalinity. The surface of the M7 membrane (Fig. 7c) is the smoothest with almost no obvious defects, indicating that the zeolite layer is complete and compact. When the alkalinity is so high, pinhole-like defects are formed in the zeolite layer, which affects the compactness of the zeolite membrane. It can be due to the fact that the high alkalinity leads to the dissolution of some zeolite crystals, leaving holes to form defects. The SEM results show that the alkalinity in the synthesis solution can influence the quality of the zeolite membrane and the alkalinity of the synthesis solution for the M7 membrane can be considered the best condition, and the SEM data are similar to the XRD results.
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Fig. 7 SEM micrographs of the top view of the ceramic tube treated at different alkalinities in the synthesis solution: (a) M6 membrane, (b) M2 membrane, (c) M7 membrane, (d) M8 membrane. |
Membrane | Support/length | Synthesis method | H2 permeance flux (10−6 mol m−2 s−1 Pa−1) | α(H2/N2) | Reference | Knudsen diffusion coefficient |
---|---|---|---|---|---|---|
Support | Tube/170 mm | Microwave | 53.26 | 2.27 | This work | 3.74 |
M0 | Tube/170 mm | Microwave | 38.91 | 2.58 | This work | |
M1 | Tube/170 mm | Microwave | 14.33 | 3.18 | This work | |
M2 | Tube/170 mm | Microwave | 2.32 | 3.93 | This work | |
M7 | Tube/170 mm | Microwave | 0.47 | 4.70 | This work | |
NaA | Tube/70 mm | Microwave | 0.24 | 4.71 | 34 | |
NaA | Disks | Microwave | 2.36 | 4.14 | 35 | |
NaA | Disks | Microwave | 3.65 | 5.62 | 36 | |
NaA | Tube/— | Conventional | 0.528 | 6.66 | 17 | |
NaA | Disks | Conventional | 0.23 | 6.02 | 37 |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1ra05132f |
This journal is © The Royal Society of Chemistry 2021 |