Submicrometer-thick b-oriented Fe–silicalite-1 membranes: microwave-assisted fabrication and pervaporation performances

Abstract

A b-oriented seed layer was prepared by rubbing coffin-shaped Fe–silicalite-1 crystals after depositing a chitosan layer on glass, then the continuous, ca. 270 nm-thick and highly b-oriented Fe–silicalite-1 membrane was formed using secondary growth under microwave irradiation for 2 h. The same method has been further applied to fabricate the defect-free and ca. 1 μm-thick b-oriented membrane on an α-Al₂O₃ support, which exhibited a separation factor of 41 and a high total flux of 3.58 kg m⁻² h⁻¹ in separating 5 wt% ethanol/water mixtures at 50 °C.

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In recent years, zeolite films and membranes have been widely applied in the field of separation,^1–3 catalysis,⁴ chemical sensors,⁵ and corrosion protection.⁶ The MFI zeolite membranes have been extensively studied because of their good stabilities and straight channels (b-axis) of 0.53 nm × 0.56 nm, which are close to the kinetic diameter of many common organic molecules. The preferentially b-oriented MFI thin films on porous supports can drastically decrease the transport resistance through the membrane and show superior separation performances.^2,3,7 Therefore, the fabrication of thin MFI membrane and controllable assembly of constituent microcrystals into a highly orientated membrane are of great interest and challenging.

Oriented and thin zeolite membranes can be synthesized by in situ crystallization^8–10 and secondary growth method.^2,3,11–17 However, the former method always needs more synthesis time, which results in some a-oriented MFI twin crystals and increases the thickness and roughness of membrane. By taking the advantage of a b-oriented seed layer, the secondary growth method has shown to be more effective in controlling the thickness and b-oriented formation of MFI membrane by decoupling the nucleation and crystal growth process. The continuous and dense MFI zeolite membrane with preferential b-orientation can be fabricated using secondary growth method by conventional heating. Tsapatsis et al. used trimer-TPAOH as structure-directing agent (SDA) to prepare uniformly b-oriented MFI membranes with the thickness of 1 μm at 175 °C for 1 d.² Yoon et al. employed tetraethylammonium hydroxide (TEAOH) and (NH₄)₂SiF₆ as co-SDAs to obtain highly b-oriented MFI films at 165 °C for 7 d.³ Zhou et al. fabricated b-oriented MFI membranes in fluoride media on a graded alumina support at 100 °C for 2 d.¹³ It can be seen that the long synthesis time is needed to reach the target crystallization temperature due to the temperature gradient and low heating rate under conventional heating, which would increase the consumption of energy and the cost of membrane production.

Microwave technique is a good candidate to remarkably reduce the synthesis time and greatly improve the membrane synthesis efficiency. Meanwhile, the zeolite membranes microstructure (orientation, thickness etc.) could be controlled accurately due to the rapid heating and uniform temperature distribution under microwave irradiation. Wang et al. synthesized b-oriented silicalite-1 films with the thickness of 600 nm on the glass supported chitosan films.¹¹ However, the practical application couldn't be operated due to the use of non-porous substrates. The same phenomenons were also observed in Li's¹⁸ and Liu's¹⁹ reports.

The organic templates that occluded in zeolite pores should be removed before the application. Typically, the templates are often removed by conventional calcination method at 450–550 °C. However, the cracks and intercrystal boundary gaps would be easily formed because of the mismatch of thermal expansion coefficient between zeolite membrane and porous substrate at high temperatures.²⁰ Liu et al. developed a controllable Pd/SiO₂ catalytic hydro-cracking template removal method for MnMgAlPO-5 molecular sieve at a mild temperature (340 °C), which would not damage the crystal structure and generate new defects on zeolite polycrystalline structure.²¹ In addition, the incorporation of Fe atom into MFI zeolite membrane could enhance the catalytic activity,^22–24 so the difficulty of removing template could be decreased via the Pd/SiO₂ catalytic hydro-cracking method at mild temperature.

Herein, a b-oriented Fe–silicalite-1 seed layer was prepared both on glass and porous α-Al₂O₃ supported chitosan film by previously reported method.^11,25 Then the submicrometer-thick and preferential b-oriented Fe–silicalite-1 zeolite membrane was fabricated on the b-oriented seed layer under microwave irradiation. The experimental techniques such as SEM, XRD, UV-Vis, FT-IR, permporometry, N₂ permeation test, contact-angle measurement, and pervaporation separation of 5 wt% ethanol/water mixture were used to characterize the synthesized thin b-oriented Fe–silicalite-1 membrane. To the best of our knowledge, this is the first report on demonstrating the pervaporation performance of thin b-oriented Fe–silicalite-1 membrane on α-Al₂O₃ that is synthesized by microwave-assisted secondary growth.

There were some reports that the seed size has an important effect on the fabrication of zeolite membranes during secondary growth because the quality of the seed layer was directly affected by the seeds.^26,27 Xia et al. believed that the gap between the small seeds is much smaller, so the space limitation suppression is high for them, which allows the smaller seed crystals mainly grow along the direction perpendicular to the support surface under the suitable synthesis condition.²⁷

By comparison, the much smaller coffin-shaped Fe–silicalite-1 crystals with the thickness of ca. 60 nm in the b-direction were synthesized as shown in Fig. 1a and b. The corresponding XRD pattern (Fig. 2a) revealed the typical characteristic peaks of MFI-type zeolites. The uniform and closely-packed Fe–silicalite-1 seed layer could be obtained by manual assembly on glass plate supported chitosan layer (Fig. 1c and d). During the assembly process, chitosan served as the intermediate linker due to a large amount of hydroxyl and amino groups in its units, leading to much enhanced hydrogen binding between zeolite crystals and the supports.²⁵ All crystal grains were b-oriented with their largest faces parallel to the support surface. Based on visual observation, the seed layer covered on the chitosan surface was partly multi-layered. On the one hand, the seed grains were so thin along b-axis that the takeoff of the ones on the top was difficult by rubbing. On the other hand, the seed grains possessed very high surface area per unit of mass (m² kg⁻¹), and consequently higher concentration of hydroxyl groups per unit of mass, which facilitated formation of hydrogen bonds between seed grains and the chitosan surface, as well as between the flat [010] facets of two MFI crystals.²⁸

Fig. 1 SEM images of Fe–silicalite-1 crystal grains (a and b), top-view (c) and cross-sectional view (d) of Fe–silicalite-1 seed layer, top-view (e) and cross-sectional view (f) of Fe–silicalite-1 membrane on glass plate.

Fig. 2 XRD patterns of Fe–silicalite-1 crystals (a), Fe–silicalite-1 seed layer (b) and membrane on glass plate (c).

As shown in Fig. 1e, the seed grains were heavily intergrown after microwave-assisted hydrothermal secondary growth for 2 h, leading to a smooth and continuous membrane surface without any cracks and grain boundaries. Meanwhile, the as-synthesized Fe–silicalite-1 membrane was highly b-oriented without the existence of barely a-oriented twin crystals. Based on Fig. 1f, the thickness of the Fe–silicalite-1 membrane was only ca. 270 nm, which was much thinner than the b-oriented MFI zeolite film (600 nm) in our previous report.¹¹ The corresponding XRD patterns of the seed layer and Fe–silicalite-1 MFI film shown in Fig. 2b and c displayed the distinct peaks at 8.82°, 17.74°, 26.78°, 36.00°, and 45.48°, which were attributed to (020), (040), (060), (080), and (0100) reflections, indicating that the seed layer and membrane were purely b-oriented. Obviously, the ultrathin (ca. 270 nm) and solely b-oriented Fe–silicalite-1 membrane could be prepared on the chitosan supported b-oriented Fe–silicalite-1 seed layer under microwave irradiation secondary growth for 2 h. It was the small seed grains, especially in b-direction, that resulted in the thin seed layer and helped the formation of this thinner Fe–silicalite-1 membrane with satisfactory smoothness and uniformity.

As what would be demonstrated in the following, the established synthesis strategy could be applied to fabricate ultrathin and purely b-oriented Fe–silicalite-1 membranes over porous α-Al₂O₃ supports. The Fe–silicalite-1 seed grains were closely covered on the α-Al₂O₃ supported chitosan layer. Similar to the glass plate, the seed layer was also partly multi-layered over the α-Al₂O₃ support (Fig. 3a). Fig. 3b and c demonstrated the SEM images of the synthesized Fe–silicalite-1 membrane on α-Al₂O₃ support after secondary growth under microwave irradiation for 2 h. The crystal grains on the support surface were intergrown completely, resulting in the pure b-oriented Fe–silicalite-1 membrane with high degree of smoothness and uniformity. The cross-sectional SEM image shown in Fig. 3c indicated that the synthesized membrane was dense across the whole membrane, the thickness of which was less than 1.0 μm. The UV-Vis spectrum was used to check the coordination state and extent of aggregation of the iron species in the Fe–silicalite-1 membrane (Fig. 3d). It could be observed that a strong absorbance peak at 240 nm dominated by Fe³⁺ charge transfer appeared, indicating that Fe species incorporate into the zeolite framework. In the corresponding XRD pattern, the Fe–silicalite-1 seed layer proved b-oriented as evidenced by the peaks at 8.82°, 17.74°, 26.78°, 36.00°, and 45.48°, attributing to (020), (040), (060), (080), and (0100) reflections (Fig. 4b). As revealed in Fig. 4c, the peaks of (020), (040), (060), (080), and (0100) were greatly enhanced in a large degree while the characteristic peaks from the α-Al₂O₃ support were deeply weakened, suggesting that the Fe–silicalite-1 membrane had been intergrown well with pure and strong b-orientation, as well as high denseness. This was in good agreement with the SEM observation in Fig. 3.

Fig. 3 (a) Top-view SEM image of Fe–silicalite-1 seed layer after removal of the chitosan layer, (b and c) top- and cross-sectional-view SEM images of Fe–silicalite-1 membrane on α-Al₂O₃ support, (d) the UV-Vis spectrum of the Fe–silicalite-1 membrane.

Fig. 4 XRD patterns of α-Al₂O₃ support (a), Fe–silicalite-1 seed layer (b) and Fe–silicalite-1 membrane (c) on α-Al₂O₃ support. The asterisk (*) indicates XRD peaks of α-Al₂O₃ support.

The FT-IR spectra of the synthesized Fe–silicalite-1 membrane before and after calcination were presented in Fig. 5. The bands at 2979, 2882, and 1467 cm⁻¹ were attributed to TPAOH, which were greatly weakened after calcination at 350 °C in the hydrogen atmosphere. Generally, the separation performance of zeolite membranes would be reduced due to the presence of meso- and/or macropore defects, even at low defect concentrations. To check the contribution of meso- and/or macropore defects, the defect size distribution of the synthesized Fe–silicalite-1 membrane over the α-Al₂O₃ support was measured by permporometry. The maximal defect size was only ca. 1 nm, so there existed only micro-defects in the Fe–silicalite-1 membrane (not shown).

Fig. 5 FT-IR spectra of α-Al₂O₃ support (a), as-synthesized (b) and calcined (c) b-oriented Fe–silicalite-1 membrane.

The N₂ permeance under different pressure for the α-Al₂O₃ support, as-synthesized and calcined Fe–silicalite-1 membranes was measured and plotted in Fig. 6. Before template removal, the N₂ permeance was below 10⁻¹¹ mol m⁻² s⁻¹ Pa⁻¹, indicating that the pores in the as-synthesized membrane were blocked by template TPAOH. So the membrane was both dense and defect-free. After the zeolitic pores were opened by removing templates, the N₂ permeance of the membrane was in the range of 4.7–8.4 × 10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹, which was still lower than the bare α-Al₂O₃ support (1.2–2.0 × 10⁻⁶ mol m⁻² s⁻¹ Pa⁻¹). These results suggested that the b-oriented Fe–silicalite-1 membrane on the α-Al₂O₃ support prepared by microwave-assisted secondary growth with the low-temperature removal of templates was defect-free. At the same time, the contact angle of the membrane after template removal was 97° (Fig. S1†), which showed that the membrane was hydrophobic.

Fig. 6 N₂ permeance of as-synthesized (a) and calcined b-oriented Fe–silicalite-1 membrane (b), and α-Al₂O₃ support (c).

The pervaporation performance at 50 °C of the b-oriented Fe–silicalite-1 membrane with the thickness of ca. 1 μm on the α-Al₂O₃ support was measured with respect to 5 wt% ethanol/water mixture. After the template was removed at the low-temperature, the membrane exhibited a very high total flux of ca. 3.5 kg m⁻² h⁻¹ with an acceptable separation factor of ca. 40 (Table 1). This simply suggested that the ultrathin MFI membrane with the pure b-orientation could indeed decrease the transport resistance through the membrane. The membrane fabrication was highly reproducible. The ethanol/water separation performances of other MFI membranes in the literature were also listed in Table 1.^29–34 The random silicalite-1 membrane with the thickness of 20–30 μm showed high separation factor of 106, whereas the flux was only 0.93 kg m⁻² h⁻¹.²⁹ Hedlund et al. prepared the silicalite-1 membrane on a graded α-Al₂O₃ disc, displaying a high flux of 10.7 kg m⁻² h⁻¹, but the separation factor was pretty low, only 4.2 at 60 °C.³⁰ For a preferentially oriented membrane, the separation performance was improved commonly because of the decreased mass transport resistance through the aligned zeolitic channels. Generally speaking, a porous membrane with a very high flux and an acceptable high separation factor should be more suitable for practical applications.³⁵ Compared to the listed oriented membranes, the ultrathin and b-oriented Fe–silicalite-1 membrane presented a very high flux with a moderate separation factor.

Table 1 Pervaporation performances of synthesized Fe–silicalite-1 membranes for dilute aqueous ethanol solution and comparison with literature data^a

Membrane	Orientation	Thickness (μm)	Temperature (°C)	EtOH Conc. in feed (wt%)	Total flux (kg m^−2 h^−1)	SF (EtOH Conc. in permeate, wt%)	Ref.
a Note: a, b, and c are three duplicated data using the same method mentioned in experiment section (see the ESI).
Silicalite-1	Random	20–30	60	5	0.93	106 (84.8)	29
Silicalite-1	Random	0.5	60	10	10.7	4.2 (31.8)	30
Fe–ZM-5	Random	3	50	5	0.71	43 (69.4)	31
Silicalite-1	h0h	12	60	3	2.9	66 (67.1)	32
Silicalite-1	c	12	60	5	1.51	39 (67.2)	33
Silicalite-1	b	1.5	60	5	2.1	85 (81.7)	34
Fe–silicalite-1^a	b	1	50	5	3.58	41 (68.6)	This work
Fe–silicalite-1^b	b	1	50	5	3.57	38 (67.0)	This work
Fe–silicalite-1^c	b	1	50	5	3.45	39 (67.9)	This work

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In conclusion, the well-intergrown and purely b-oriented Fe–silicalite-1 membranes were successfully fabricated on both glass plate (270 nm-thick) and porous α-Al₂O₃ (less than 1 μm-thick) supported chitosan layers by microwave-assisted secondary growth. The preferentially b-oriented MFI membranes could be fabricated with the deduction of a chitosan layer. The quality and defects of the b-oriented Fe–silicalite-1 membrane prepared on α-Al₂O₃ were characterized by FT-IR, permporometry, and N₂ permeation test, which proved that the membrane had no large defects after template removal at low-temperature. A very high flux and a moderate separation factor could be achieved for membrane separation of 5 wt% ethanol/water mixtures via pervaporation.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 21136008).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental and characterization details. See DOI: 10.1039/c6ra23327a

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