Preparation and Modification of Silicalite-2 Membranes

Abstract

Silicalite-2 membranes were successfully prepared on tubular α-Al₂O₃ supports by secondary hydrothermal synthesis, and the pervaporation performance of the membrane was evaluated by separation of a 5 wt% ethanol/H₂O mixture at 60 °C. The effects of templating agent content, water–silicon ratio and crystallization time on the separation performance of Silicalite-2 membranes were investigated. When the TBAOH/SiO₂ and H₂O/SiO₂ molar ratios of the precursor synthesis solution were 0.2 and 120, a dense Silicalite-2 membrane could be prepared on the surface of the tubular α-Al₂O₃ support after 72 h. The silane coupling agent was utilized to treat the Silicalite-2 membranes, and the effects of silane coupling agent dosage on their properties were also explored. The pervaporation performance of the Silicalite-2 membrane was greatly improved with a 5.7 wt% trimethylchlorosilane (TMCS) solution and the flux and separation factor of the membrane reached 1.75 kg·m⁻²·h⁻¹ and 22 for separation of 5 wt% EtOH/H₂O at 60 °C, respectively.

1. Introduction

Fuel ethanol, as a renewable energy source, has garnered significant attention due to the escalating concerns about environmental protection and the energy crisis [1,2,3]. At present, fermentation is key for producing fuel ethanol [4]. Improving efficiency and reducing the energy consumption of fermentation are important tasks in the production of ethanol. Kaseno et al. [5] found that continuously removing ethanol retained the high productivity of the fermentation process. Pervaporation (PV) is considered as one of the most promising energy-efficient processes [6,7]. In PV application, excellent membrane materials are important. Zeolite membranes have unique advantages, such as high temperature resistance, corrosion resistance, regular pore structure and excellent adsorption selection, which are promising alternative membranes for PV application [8,9,10]. Wu et al. [11] utilized hydrophobic MFI zeolite membranes to enrich butanol from an acetone–butanol–ethanol (ABE) solution, which is a promising process for producing biobutanol.

Silicalite membranes (Silicalite-1 and Silicalite-2), which are free of aluminium and have high hydrophobicity, play a significant role in the condensation of a low-concentration ethanol solution [12]. Lan et al. [13] prepared Silicalite-1 membranes for separating a 3 wt% ethanol/H₂O solution mixture. The separation factor and flux achieved were as high as 79 and 1.40 kg·m⁻²·h⁻¹, respectively. Li et al. [14] prepared silanol group-free Silicalite-1 membranes that showed a stable separation performance over 80 h.

Silicalite-2 is an MEL-type zeolite and has straight a-axis and b-axis channels, which are favorable for molecular diffusion and application of liquid separation [15]. Kosinov et al. [16] prepared thin and pure Silicalite-2 membranes on porous α-alumina hollow fiber supports. The separation factor and the flux were 17.2 and 3.60 kg·m⁻²·h⁻¹ for the 5 wt% ethanol/H₂O mixture at 60 °C. Gong et al. [17] prepared Silicalite-2 membranes using two seed crystals with different sizes. The optimal separation factor and flux were 6.7 and 7.61 kg·m⁻²·h⁻¹ for the 5 wt% ethanol/H₂O mixture at 60 °C. Wu et al. [18] prepared Silicalite-2 membranes on mesoporous α-Al₂O₃ supports by secondary hydrothermal synthesis; the separation factor and flux were 5.1 and 1.12 kg·m⁻²·h⁻¹ for a 5 wt% ethanol/H₂O mixture at 70 °C. In addition, Table 1 summarized the pervaporation (PV) performance of Silicalite-2 membranes for a 5 wt% ethanol/H₂O mixture, where the separation performance was poor.

However, the preparation of dense Silicalite-2 membranes is difficult and modification is an effective method to change the hydrophobic features and improve the separation performance of the zeolite membrane. Ren et al. modified Silicalite-1 membranes with silane coupling agents and improved the separation performance [21]. Fariba et al. [22] prepared and modified Silicalite-1 membranes with polydimethylsiloxane and a dual active layer (superhydrophobic (TALS) Silicalite-1/(PDMS) membranes) was formed by the spraying method. The flux and separation factor of the membrane were 1.88 kg·m⁻²·h⁻¹ and 32.34 for the 5 wt% ethanol/H₂O solution. Kosinov et al. [19] modified the surface of Silicalite-2 membranes with triethoxysilafluorosilane to improve the separation factor of the membrane.

Herein, Silicalite-2 membranes were successfully prepared on α-Al₂O₃ supports by a secondary hydrothermal process. The effects of the TBAOH (tetrabutylammonium hydroxide)/SiO₂ ratio, H₂O/SiO₂ ratio of the precursor synthesis solution and crystallization time on the growth of Silicalite-2 membranes were investigated. In addition, PV performance of the membrane was modified and improved with a silane coupling agent (TMCS). The optimal TMCS concentration was also investigated in this work.

2. Experimental Section

2.1. Experimental Materials

Silicalite-2 membranes were prepared on α-Al₂O₃ tubular supports (OD. 12 mm, ID. 9 mm, length. 10 cm, Kailai Chemical, Yixing, China). The silica source and organic templates included tetraethyl orthosilicate (TEOS, 99%, Innochem, Pyeongtaek-si, Republic of Korea) and TBAOH (25%, Alaadin, Fukuoka, Japan). Ammonia (25%, Aladdin) and trimethylchlorosilane (TMCS, 98%, Aladdin) were used to improve the hydrophobicity of Silicalite-2 membranes.

2.2. Preparation of Silicalite-2 Zeolites

Silicalite-2 zeolites were prepared by in situ hydrothermal processes. The molar composition of the precursor synthesis solution was 1TEOS: 0.25TBAOH: 20H₂O. TBAOH and TEOS were mixed with water and hydrolyzed at room temperature for 24 h. Then, the mixture was transferred to a stainless-steel autoclave at 130 °C for 48 h. Finally, Silicalite-2 zeolites were obtained by washing, centrifugation, drying and calcination at 550 °C for 10 h.

2.3. Preparation of Silicalite-2 Membranes

Silicalite-2 membranes were prepared by secondary hydrothermal synthesis. The molar composition of the precursor solutions was 1TEOS: x TBAOH: yH₂O (x = 0.15~0.30, y = 120~240). The details of the synthesis conditions of Silicalite-2 membranes are summarized in Table 2. TBAOH, TEOS and de-ionized water were mixed and stirred continuously at room temperature to form a clarified solution. The Silicalite-2 seed crystals were attached to the outer surface of α-Al₂O₃ tubular supports by a rubbing coating. The seeded support and transparent synthesis solution were loaded into a stainless-steel autoclave, and crystallization continued at 150 °C for 48, 72, and 96 h, respectively. The membranes were washed with water after crystallization. Thereafter, calcination of the Silicalite-2 membranes was carried out at 500 °C to remove the organic templating agent.

2.4. Modification of Silicalite-2 Membrane

Modification of Silicalite-2 membrane was carried out as follows: TEOS, ammonia, ethanol and water were mixed and formed a silica solution at 50 °C for 2 h. Silicalite-2 membranes were sealed at both ends and submerged into the silica solution for 30 s, and then the membranes were dried and calcined at 450 °C for 1 h. Then, the Silicalite-2 membranes were placed into the mixture of TMCS and ethanol at 50 °C for 5 h and the TMCS concentration in the mixture ranged from 0 to 7.5 wt%.

2.5. Characterization and PV Performance of Silicalite-2 Membranes

X-ray diffraction (XRD) patterns of the samples were recorded on a Rigaku Ultima IV diffractometer using Cu Kα radiation (λ = 0.15405 nm, 40 kV, and 40 mA) at a scanning rate of 6°·minQ⁻¹ and the 2θ angle range was 5~50°. Morphology and crystal size of the Silicalite-2 membranes were determined by a field emission scanning electron microscope (FE-SEM, Hitachi SU8020, Tokyo, Japan). Hydrophobicity/hydrophilicity of the membrane was analyzed using contact angle equipment (HKCA-10, Haker, Nanjing, China).

The PV performance of Silicalite-2 membranes was tested using 5 wt% ethanol/H₂O solution at 60 °C. A schematic diagram of the PV device is presented in Figure 1. Permeation was collected at the fixed time intervals, and gas chromatography (GC-2014C, Shimadzu, Kyoto, Japan) was applied to analyze the composition of the permeation. Pervaporation performance was evaluated by two parameters: permeate flux Q (kg·m⁻²·h⁻¹) and separation factor (α). The flux and the separation factor were calculated by the following formula:

(1)$\mathrm{Q}=\frac{\mathrm{W}}{\mathrm{t}\times \mathrm{A}}$

(2)$\mathsf{\alpha }=\frac{y_a/y_b}{x_a/x_b}$

where W is the total mass of the permeate side (kg), A is the effective membrane area (m²), and t is the time interval (h). y_a, y_b, x_a and x_b are the contents of the permeation and feed mixture, respectively.

3. Results and Discussion

3.1. Effect of TBAOH/SiO₂ Molar Ratio

XRD patterns and FE-SEM images of Silicalite-2 zeolites are shown in Figure 2. The XRD pattern exhibited five typical diffraction peaks for MEL zeolites at 2θ values of 7.9°, 8.8°, 23.1°, 23.9°, and 45.2°. SEM images revealed that the Silicalite-2 zeolites had a walnut-like shape, and the zeolite crystal size was around 400–500 nm.

Organic templating agents played a crucial role in the nucleation and crystallization of silicalites or silicalite membranes [23]. In this study, the effects of the TBAOH/SiO₂ molar ratio on the growth of Silicalite-2 membranes (C-1, C-2, C-3, and C-4) were investigated. As summarized in Table 2, the TBAOH/SiO2 molar ratios of the precursor synthesis solution were 0.15, 0.20, 0.25, and 0.30. XRD patterns and SEM images of the membranes are shown in Figure 3 and Figure 4. Although the TBAOH/SiO₂ molar ratios varied from 0.15 to 0.30, all membranes exhibited five characteristic diffraction peaks of MEL zeolites (2θ = 7.9°, 8.8°, 23.1°, 23.9°, and 45.2°), confirming that the Silicalite-2 membranes were successfully prepared in this study [24].

Additionally, surface SEM images demonstrate that the TBAOH/SiO₂ molar ratio significantly influenced the growth of the Silicalite-2 membranes. When the TBAOH/SiO₂ molar ratio of the precursor synthesis solution was 0.15, few fine crystals were observed on the support surface, and a large amount of amorphous covered the support (Figure 4a,e), the amount of amorphous material decreased with high TBAOH/SiO₂ molar ratio, the fine Silicalite-2 crystals grew into a walnut-like shape, and formed a continuous zeolite layer on the support surface (Figure 4b–d,f–h). However, when the TBAOH content in the initial synthesis gel was excessive, cracks appeared on the surface of the Silicalite-2 membrane (C-4) (Figure 4d).

3.2. Effect of H₂O/SiO₂ Molar Ratio

The water content of the precursor synthesis solution played a crucial role in heat transfer and significantly influenced the growth of zeolite membranes [25]. In this study, Silicalite-2 membranes (C-5, C-6, and C-7) were prepared using different H₂O/SiO₂ molar ratios. H₂O/SiO₂ molar ratios of the precursor synthesis solution were 120, 200, and 240. XRD patterns and SEM images of the membranes are presented in Figure 5 and Figure 6.

All Silicalite-2 membranes exhibited the characteristic peaks of the MEL zeolite structure. However, crystallinity of the membranes gradually decreased with increasing water content in the precursor synthesis solution, which was consistent with the SEM images. When the H₂O/SiO₂ molar ratio of the initial synthesis solution was 120, closely arranged walnut-like crystals formed, resulting in a dense zeolite layer on the support surface (Figure 6a,d). As the H₂O/SiO₂ molar ratio increased to 240, a greater number of fine crystals and amorphous material appeared on the support surface, and the thickness of the zeolite layers decreased (Figure 6b,c,e,f). Hence, the highH₂O/SiO₂ ratios resulted in a thin zeolite layer and less defined crystal structures. Precise control of the water content was essential for optimizing the membrane’s structural integrity and performance.

3.3. Effect of Crystallization Time

Crystallization time was crucial for preparing a compact zeolite membrane and significantly affected the crystallinity of the zeolite membranes [26]. In this study, the effects of crystallization time (C-8, C-9, and C-10) on the growth of Silicalite-2 membranes were explored, with crystallization times of 48, 72, and 96 h. XRD patterns and SEM images of the membranes are presented in Figure 7 and Figure 8.

All C-8, C-9, and C-10 membranes exhibited the typical diffraction characteristic peaks of MEL zeolites, indicating successful synthesis. The crystallinity of the zeolite membranes increased with long crystallization times. When the synthesis time was 48 h, only fine crystals were observed on the support (Figure 8a,d). When the crystallization time was extended to 72 h, inter-grown walnut-like Silicalite-2 zeolites formed a dense zeolite layer on the support surface (Figure 8b,e). There large zeolite crystals formed a thick zeolite layer on the surface after 96 h (Figure 8c,f). A thicker zeolite layer may affect the permeability and performance of the membrane. Therefore, the crystallization time should be carefully controlled to achieve the optimal PV performance.

3.4. Influences of Trimethylchlorosilane Content

Generally, the hydrophobicity of the zeolite membranes can be modified and improved by a silane coupling agent, which attaches to the zeolite’s surface during a silicon hydroxyl reaction [27]. As shown in Table 3, the Silicalite-2 membranes (C-11) had poor PV performance for separation of the 5 wt% EtOH/H₂O mixture at 60 °C and the separation factor of the membrane was only 3, which was attributed to the presence of numerous inter-crystalline gaps within the membrane layer. The effects of TMCS content (3.9%, 5.7% and 7.5%) on the PV performance and morphology of the membranes (C-12, C-13 and C-14) were investigated in this work. As shown in Figure 9, XRD patterns of the modified membranes showed the typical diffraction peaks of MEL zeolites. SEM images of the modified membranes presented demonstrated that various spherical pellets were formed and covered the surface of Silicalite-2 membranes, and the amount of spherical pellets increased with TMCS content (Figure 10). In addition, Figure 11 shows the water contact angles of modified Silicalite-2 membranes with different TMCS contents. The water contact angle of the modified zeolite membrane was significantly larger than that of the original Silicalite-2 membrane, and the contact angle increased with increasing TMCS content, indicating that TMCS modification could improve the hydrophobicity of the Silicalite-2 membranes in this work. The excessive TMCS molecules may cause steric hindrance and lead to a less ordered arrangement of -OSi(CH3)3 groups on the membrane surface, as shown in Table 3. When the TMCS content increased to 7.5%, the water contact angle decreased from 121.1° to 108° and the surface became less uniformly covered with hydrophobic groups, reducing the overall hydrophobicity of the membrane. The PV performance of the modified membranes, especially the separation factor, was greatly improved by TMCS modification. For example, the separation factor and total flux of the membrane C-13 were 22 and 1.75 kg·m⁻²·h⁻¹, respectively [28].

Figure 12 presents a schematic diagram of Silicalite-2 membranes with TMCS modification. Several SiO₂ pellets were formed on the surface of Silicalite-2 membranes after treatment with TEOS solution, which increased the number of silica hydroxyl groups on the membrane surface and provided loading sites for TMCS. Thereafter, TMCS reacted with the silica hydroxyl groups and formed -Si-O-Si(CH₃)₃ groups on the surface of the membrane, which increased the hydrophobicity of the Silicalite-2 membrane.

4. Conclusions

When TBAOH/SiO₂ and H₂O/SiO₂ molar ratios of the precursor synthesis solution were 0.2 and 120, Silicalite-2 membranes were successfully prepared on tubular α-Al₂O₃ supports at 150 °C for 72 h by secondary hydrothermal synthesis. The PV performance of membranes could be improved by TMCS modification with medium concentrations. The separation factor and flux of the modified Silicalite-2 membrane were recorded as 22 and 1.75 kg·m⁻²·h⁻¹ for 5 wt.% ethanol/H₂O solution at 60 °C.

Author Contributions

Conceptualization, Y.Y.; methodology, J.L.; software, L.Z.; validation, L.Z.; formal analysis, Y.Y.; investigation, J.L.; resources, J.L.; W.Y.; data curation, Y.Y., Q.Z. and S.S.; writing—original draft preparation, Y.Y. and J.L.; writing—review and editing, M.Z.; visualization, H.K.; supervision, X.C.; project administration, M.Z.; funding acquisition, M.Z. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding authors.

Conflicts of Interest

The authors declare no conflicts of interest.

Footnotes

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Figures and Tables

Figure 1. Schematic diagram of PV device. 1: Vacuum pump; 2: safety bottle; 3: liquid nitrogen bottle; 4: collecting bottle; 5: Silicalite-2 membrane (OD. 12 mm, ID. 9 mm, Length. 10 cm); 6: reactor; 7: magnet; 8: bath oil pots; 9: vacuum gauge; 10: feed mixture.

Figure 2. XRD patterns (a) and SEM image (b) of Silicalite-2 zeolites.

Figure 3. XRD patterns of Silicalite-2 membranes with different TBAOH/SiO2 ratios: C-1 TBAOH/SiO2 = 0.15 (a), C-2 TBAOH/SiO2 = 0.20 (b), C-3 TBAOH/SiO2 = 0.25 (c) and C-4 TBAOH/SiO2 = 0.30 (d).

Figure 4. Surface and cross sectional SEM images of Silicalite-2 membranes with different TBAOH/SiO2 ratios: C-1 TBAOH/SiO2= 0.15 (a,e), C-2 TBAOH/SiO2 = 0.2 (b,f), C-3 TBAOH/SiO2 = 0.25 (c,g) and C-4 TBAOH/SiO2 = 0.3 (d,h).

Figure 5. XRD patterns of Silicalite-2 membranes with different H2O/SiO2 molar ratios: C-5, H2O/SiO2 = 120 (a), C-6, H2O/SiO2 = 200 (b) and C-7, H2O/SiO2 = 240 (c).

Figure 6. Surface and cross sectional SEM images of Silicalite-2 membranes with different H2O/SiO2 ratios: C-5, H2O/SiO2 = 120 (a,d), C-6, H2O/SiO2 = 200 (b,e) and C-7, H2O/SiO2 = 240 (c,f).

Figure 7. XRD patterns of Silicalite-2 membranes with different crystallization times: (a) C-8, 48 h, (b) C-9, 72 h and (c) C-10, 96 h.

Figure 8. Surface and cross sectional SEM images of Silicalite-2 membranes with different crystallization times: C-8, 48 h (a,d); C-9, 72 h (b,e); and C-10 6 h (c,f).

Figure 9. XRD patterns of the modified Silicalite-2 membranes with different TMCS contents: C-11, 0 wt% TMCS (a); C-12, 3.9 wt% TMCS (b); C-13, 5.7 wt% TMCS (c); C-14, 7.5 wt% TMCS (d).

Figure 10. Surface and cross sectional SEM images of Silicalite-2 membranes and modified Silicalite-2 membranes with different TMCS contents: C-11, 0 wt% TMCS (a,e); C-12, 3.9 wt% TMCS (b,f); C-13, 5.7 wt% TMCS (c,g); C-14, 7.5 wt% TMCS (d,h).

Figure 11. Water contact angles of Silicalite-2 membrane and modified Silicalite-2 membrane with different TMCS contents: C-11, 0 wt% TMCS (a); C-12, 3.9 wt% TMCS (b); C-13, 5.7 wt% TMCS (c) and C-14, 7.5 wt% TMCS (d).

Figure 12. Schematic diagram of Silicalite-2 membrane with TMCS modification.

Table 1

Comparison of Silicalite-2 membranes for the separation performance of 5 wt% ethanol/H₂O mixtures by pervaporation.

Support	PervaporationTemperature (°C)	Separation Factor (αEtOH/H2O)	Q (kg·m−2·h−1)	References
α-Al2O3 hollow fiber	60	17	3.60	[16]
α-Al2O3 support	60	7	7.61	[17]
α-Al2O3 support	70	5	1.12	[18]
α-Al2O3 hollow fiber	60	7	3.30	[19]
α-Al2O3 supports	70	6	1.28	[20]

Table 2

Preparation condition of Silicalite-2 membranes.

No.	Molar Composition of Synthesis Solution			Crystallization Time (h)	Crystallization Temperature (°C)
	TBAOH/SiO2	H2O/SiO2	TMCS Concentration (%)
C-1	0.15	120	---	72 h	150
C-2	0.20		---
C-3	0.25		---
C-4	0.30		---
C-5	0.20	120	---	72 h	150
C-6		200	---
C-7		240	---
C-8	0.20	120	---	48 h	150
C-9			---	72 h
C-10			---	96 h
C-11	0.20	120	0	72 h	150
C-12			3.9%
C-13			5.7%
C-14			7.5%

Table 3

PV performance of Silicalite-2 membrane and modified Silicalite-2 membranes for 5 wt % EtOH/H₂O mixture at 60 °C.

Membrane	TMCS Content (wt%)	αEtOH/H2O	Q (kg·m−2·h−1)
C-11	0	3	11.29
C-12	3.9%	19	2.73
C-13	5.7%	22	1.75
C-14	7.5%	17	2.00

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