High-Performance FAU Zeolite Membranes Derived from Nano-Seeds for Gas Separation

1. Introduction

The high energy consumption of gas separation has become one of the important environmental issues with the seriousness of global climate change, because the increase in traditional energy consumption will lead to an increase in CO₂ emissions. Sholl et al. pointed out that the separation of alkenes from alkanes is one of the “seven chemical separations to change the world” [1]. The separation and purification of propylene and ethylene alone accounts for 0.3% of global energy use. Among them, propylene has long been one of the important chemical raw materials due to the wide application of its downstream products in the chemical and petroleum industries. Currently, propylene/propane separation is mainly performed by cryogenic distillation. However, it is a great challenge to separate propylene from propane since the boiling points of propylene (225.7 K) and propane (231 K) are close, and the distillation process requires a large tower with 150-300 stages and a high reflux ratio [2]. The significant energy requirements and capital costs of cryogenic distillation processes have prompted research into other methods to separate these gases. Membrane-based gas separation is an attractive and forward-looking technology because it can be performed under mild conditions without phase changes, and can be completed in one step, which can significantly reduce energy consumption by up to 80% with the related carbon footprint [3,4].

Various membrane materials, such as polymer membranes [5,6], zeolite membranes [3,7,8], metal-organic framework (MOF) membranes [9,10], mixed matrix membranes [11,12], and silicon carbide membranes [13,14,15], have been explored for separation. Among them, zeolites have become promising candidates for high-performance membranes in separation processes, catalytic membranes, and sensors, considering their well-defined pore size, molecular sieving performance, high thermal stability, and high mechanical strength [16]. To date, much effort has been devoted to developing fine zeolite membranes. The FAU-type zeolite (including NaX with Si/Al ratio ≤ 1.5, and NaY with Si/Al ratio >1.5) membranes with a pore diameter of 0.74 nm are suitable for separating large molecules that cannot be effectively handled by MFI (0.55 nm), LTA (0.42 nm) and CHA (0.38 nm) zeolite membranes. FAU membranes have been used in liquid separation (pervaporation), gas separation, and ion removal [17]. Combining the large pore size and affinity of FAU zeolites endows the membrane with high flux and selectivity [18].

So far, two typical methods for synthesizing supported FAU membranes have been developed [19]. The first method is in-situ growth; that is, the porous support is immersed in the synthesis solution, and then the required membrane layer is grown directly on the surface of the support by hydrothermal synthesis. Although this method is very simple, forming a pure phase and dense FAU membrane is difficult due to the poor heterogeneous nucleation of FAU crystal nuclei on the support surface during the in-situ growth process [20]. The second widely used strategy for FAU membrane synthesis is secondary growth. The secondary growth includes depositing seed crystals on the support surface via rub coating, dip coating, or vacuum suction, followed by secondary hydrothermal synthesis [21]. The secondary growth has many advantages compared to in-situ crystallization methods. It facilitates control of the membrane microstructure (e.g., thickness and orientation). In particular, the seed layers can weaken the impact of the support, reduce defects, and achieve high reproducibility [18].

Over the past decades, FAU membranes have received extensive attention from researchers [22]. Shrestha et al. synthesized a continuous FAU membrane on a porous polyethersulfone support by secondary growth; the membrane exhibited a selectivity of 2.4 ± 0.8 for C₃H₆/C₃H₈ with a C₃H₆ permeance of (4.05 ± 1.86) × 10^-7 mol/(m² s Pa). Gu et al. synthesized dense FAU membranes on porous α-alumina (α-Al₂O₃) supports using FAU seeds with 1-1.5 µm sizes [23]. The membrane had a thickness of 4 µm and exhibited CO₂ selectivity with a separation factor of 31.2 for the CO₂/N₂ dry gas mixture and a CO₂ permeance of 2.1 × 10^-8 mol/(m² s Pa). Zhou et al. prepared an FAU membrane with a thickness of ~2.3 µm on polydopamine (PDA)-modified α-Al₂O₃ support through in-situ growth [19]. The membrane showed H₂/CH₄ and H₂/C₃H₈ separation factors of 9.9 and 127.7, respectively, and exhibited H₂ permeance as high as 1.9× 10^-7 mol/(m² s Pa). However, the support modification process was relatively complex, and the expensive PDA further limited the large-area preparation of the membrane. Recently, Nazir et al. systematically investigated the effect of FAU seed size (0.75-5.5 µm) on the formation of FAU membranes on α-Al₂O₃ supports [22]. Their results showed that the growth of the membrane layer was enhanced, and the membrane defects were successfully reduced by using smaller seed particles of 750 nm, which is attributed to the minimal gaps between the seed particles. However, the size of the seeds (750 nm) was still large for obtaining high-quality, defect-free FAU membranes. Furthermore, an impurity phase NaP is easily formed in the zeolite layer during the FAU membrane synthesis, making it difficult to prepare a pure-phase FAU membrane.

As mentioned above, the synthesis of high-performance FAU zeolite membranes still faces challenges, such as the formation of intercrystalline defects, impure phases, and poor reproducibility. Many parameters could influence the formation of FAU membranes, such as gel composition, aging conditions, hydrothermal temperature and time, as well as easily overlooked support properties and seed size. Understanding the influence of these parameters is important to control the synthesis process and producing high-performance FAU membranes. This work used nano-sized FAU seeds to explore the effects of crucial parameters (i.e., hydrothermal temperature and time) on the formation of FAU membranes on α-Al₂O₃ and mullite supports, respectively. High-performance FAU membranes were prepared by carefully tuning synthesis parameters to obtain optimal formation conditions. The as-prepared membranes exhibited excellent separation performance for H₂/C₃H₈ and C₃H₆/C₃H₈.

2. Experimental

2.1. Materials

All procedures were performed under an ambient atmosphere unless otherwise stated. All reagents were obtained from commercial suppliers and used without further purification: Ludox HS-30 colloidal silica (SiO₂/Na₂O= 90, 30 wt% suspension in H₂O, Aldrich); tetramethylammonium hydroxide (TMAOH, 25 wt% in H₂O, Aldrich); aluminum isopropoxide (Al(O-i-Pr)₃, 98 wt%, Aldrich); tetramethylammonium bromide (TMABr, 99 wt%, Aladdin); sodium aluminate (AlNaO₂, Al/NaOH=0.79, Wako); sodium hydroxide (NaOH, 97 wt%, Aladdin); sodium silicate solution (reagent grade, SiO₂=26.5 wt%, Na₂O=10.6 wt%, Aldrich); porous α-Al₂O₃ (average pore diameter: 200 nm, outer diameter: 12 mm, length: 6 cm) and mullite (average pore diameter: 1.3 μm, outer diameter: 12 mm, length: 6 cm) tube supports were provided by Nanjing Tech University. The preparation procedure of nano-sized NaY seeds and Nay-type zeolite FAU membranes was summarized in Figure 1.

2.2. Synthesis of nano-NaY seeds

Nano-sized NaY seeds were synthesized according to the literature [24] with slight modifications. The silicon and aluminum sources were prepared separately before mixing. Typically, the molar composition of the silicon source was 0.048Na₂O: 4.35SiO₂: 0.96TMAOH: 48.46H₂O; that is, 26.2g Ludox HS-30 colloidal silica and 10.46g TMAOH were mixed and stirred at room temperature for 30 min. The molar composition of the alumina source was 1Al₂O₃: 3.84TMAOH: 2.4TMABr: 200.54H₂O. 76.6 g H₂O and 41.9 g TMAOH were mixed, and 12.5 g Al(O-i-Pr)₃ was dissolved in this mixture while stirring in a water bath at 343 K until the suspension became clear. After cooling to room temperature, 11.2 g TMABr was added to the mixture and stirred at room temperature for 15 min. Then, the silica source was slowly added to the aluminum source while stirring. The final molar composition of the nano-sized NaY synthesis solution was 0.048Na₂O: 4.80TMAOH: 2.4TMABr: 4.35SiO₂: 1.0Al₂O₃: 249H₂O. The clear solution was aged with stirring at room temperature for 3 days, then transferred into a Teflon-lined autoclave and heated in an oven at 373 K for 4 days. In the product mixture, nano-sized NaY particles were captured using an ultracentrifuge and washed with distilled water until pH 7. Finally, the seeds were dispersed in water to obtain a 1 wt% seed suspension.

2.3. Synthesis of FAU (NaY) membrane

FAU (NaY) zeolite membranes were hydrothermally synthesized on the outer surface of tube supports. As reported previously [3], the NaY seeds were coated on the outer surface of the tubular support via a dip-coating technique. Before seeding, both ends of the tubular support were sealed with silicone plugs to prevent the inner surface from coating with seeds. The dried tubular supports were immersed in the seed suspension for 60 s and subsequently dried at 373 K for 15 min.

The molar composition used for membrane growth was 80Na₂O: 1Al₂O₃: 19SiO₂: 5000H₂O, and the specific procedure was described below. AlNaO₂ was added to a mixed solution of deionized water and NaOH while stirring at room temperature for 0.5 h. Sodium silicate solution was added to the above solution while stirring for 12 h, then transferred into a Teflon-lined autoclave. The seeded tubular supports were placed vertically in an autoclave and fully immersed in the gel. Hydrothermal synthesis was done in an oven at 358-378 K for 3-10 h. After crystallization, the synthesized membranes were removed from the solution, washed with water, and dried at 333 K.

2.4. Characterization and gas permeation

The morphology and Si/Al ratio of seeds and membranes were observed using a Hitachi S-4800 scanning electron microscope (SEM) coupled with an energy-dispersive X-ray (EDX) analyzer. The crystalline phases of seeds and membranes were identified by X-ray diffraction (XRD, Ultima IV) using Cu Kα radiation in the 2θ range from 5 to 45°. Before gas permeation measurement, the membrane was placed in a vacuum oven at 373 K overnight to remove water from the zeolite pores. Single-gas permeation performance of FAU membranes was evaluated at room temperature using an experimental setup schematically shown elsewhere [25,26]. Single gases (H₂, 0.289 nm; CO₂, 0.33 nm; N₂, 0.364 nm; CH₄, 0.38 nm; C₃H₆, 0.47 nm; C₃H₈, 0.51 nm, and i-C₄H₁₀, 0.53 nm) with different kinetic diameters were fed to the outside of the cylindrical membrane. The feed gas was pressurized at ~0.4 MPa (except for i-C₄H₁₀, which was at 0.1 MPa), while the permeate stream was maintained at atmospheric pressure. The permeate flow rate was measured via a soap-film meter. The selectivity is defined as the permeance ratio of single gases [27,28].

3. Results and discussion

3.1. Characterization of FAU seed and seed layer

Figure 2 shows the XRD pattern and SEM image of the as-synthesized FAU seeds. The XRD pattern (Figure 2a) of the as-synthesized zeolite sample shows a series of characteristic peaks at 2θ= 6.2°, 10.1°, 15.58°, 18.62°, 23.54°, and 31.27°, which matched well with the standard FAU, indicating that the seed crystals were pure FAU. The SEM image (Figure 2b) shows that the zeolite particle morphology was roughly spherical, with an average particle size of approximately 50 nm. The Si/Al ratio of the seed crystals was 1.9 by the EDX analysis, which is within the Si/Al range of NaY zeolite. Therefore, nano-sized NaY seeds with pure phase were successfully synthesized.

Some seeding methods have been used to prepare zeolite membranes, among which the dip-coating method is simpler. Figure 3 shows the surface, and cross-sectional SEM images of the seeded α-Al₂O₃ and mullite supports via dip-coating. It was observed that the surface of the porous α-Al₂O₃ support was completely covered with a uniform, smooth, densely-packed FAU seed layer with a thickness of approximately 1.4 µm. The surface of the porous mullite support was also completely covered with a uniform FAU seed layer with a thickness of ~1.1 µm. However, the seed layer on mullite support exhibited obvious cracks, which could be attributed to the high roughness of the microporous (1.3 µm) mullite surface. In addition, it can be observed from the cross-sectional SEM images of the seeded supports that the seeds had not penetrated the internal pores of the α-Al₂O₃ support; on the contrary, the seeds penetrated the mullite support and filled the internal pores (highlighted in red line in Figure 3d), and there was no obvious boundary between the seed layer and the support. Different supports will impact the microstructure and properties of FAU membranes, as will be discussed later when describing the synthesis of the membranes.

3.2. Membrane synthesis under different conditions

The preparation of zeolite membranes is affected by many factors, among which reasonable control of temperature and crystallization time is crucial to prepare pure FAU zeolite membranes and avoid the formation of impurity phases. In this study, FAU membranes were prepared by hydrothermal synthesis at 358-378 K for 3-10 h by immersing the seeded supports in an ultra-dilute solution with a molar composition of 80Na₂O: 1Al₂O₃: 19SiO₂: 5000H₂O. Compared with previous reports [19,23,29], the dilute clear reaction solution had a high water content, which plausibly reduced the nucleation effect in the bulk of the reaction solution and instead promoted crystal growth on the seed layer [21,30].

3.2.1. Effect of synthesis time on membrane formation on α-Al₂O₃ support

Figure 4 shows the XRD patterns of FAU membranes synthesized at 368 K for 3-10 h. It can be observed that the membrane synthesized for 3 h showed weak characteristic peaks of FAU zeolite at 2θ values of 6.2°, 10.1°, 15.58°, 23.54°, and 31.27°, indicating that the crystallinity of the zeolite layer was low. The peak intensity of as-synthesized FAU membranes increased with synthesis time, indicating the growth of the zeolite layer. However, when the synthesis time was prolonged to 10 h, some weak peaks were observed at 2θ= 12.5°, 17.7°, 21.7°, 28.2°, and 33.4°, indicating that NaP zeolite existed in the membrane as an impurity phase [31]. The formation of NaP impurity in preparing FAU zeolite membrane was similar to that of T-type zeolite in preparing LTA zeolite for a long synthesis time [32].

Figure 5 shows the SEM images of FAU membranes synthesized at 368 K for different synthesis times. It can be observed that the membrane synthesized for a crystallization period of 3 h has a loose membrane layer (Figure 5a), and the membrane thickness increased from a 1.4 µm thick seed layer to ~1.9 µm (Figure 5b). Notably, the original seed crystals with a size of 50 nm in the seed layer have grown larger (0.5-1 µm). Still, the crystal morphology was incomplete, indicating that it is not completely crystallized, which is consistent with the XRD results in Figure 4. When increasing the synthesis time to 6 h (Figure 5c,d), the facets (or outlines) of the crystals were slightly clear, but they were loosely connected to each other, and the thickness of the membrane was approximately 2.3 μm. When the synthesis time was extended to 8 h (Figure 5e,f), a continuous, uniform, and dense zeolite layer was formed on the support, and the thickness of the zeolite membrane was about 2.7 µm. When the crystallization time was prolonged to 10 h (Figure 5g,h), a large number of walnut-like crystals appeared on the surface of the zeolite layer. The walnut-like crystals were NaP zeolite according to the XRD pattern (Figure 4). Therefore, the membrane was roughly divided into two layers: the bottom layer was an FAU zeolite layer with a thickness of ~4.8 μm, and the top layer was a NaP layer with a thickness of ~4.1 μm. The presence of NaP crystals in the membrane layer could reduce the membrane permeability due to the smaller pore size of P zeolite (~0.29 nm) and reduce the thermal stability of the membrane due to the cubic-to-tetragonal phase transformation of NaP even at low temperatures [23,33]. The Si/Al ratio of the membrane synthesized at 368 K for 8 h measured by EDX was ~2.1, indicating that the as-synthesized FAU membrane belongs to the NaY type. To avoid impurity phases, the following investigations were conducted with a synthesis time of 8 h.

3.2.2. Effect of synthesis temperature on membrane formation on α-Al₂O₃ support

Crystallization temperature is an important parameter for forming a specific type of zeolite phase. The hydrothermal synthesis temperature affects the nucleation and crystallization of zeolites, with higher synthesis temperatures leading to higher energy, which is beneficial to the crystallization process [21].

Figure 6 and Figure 7 show the XRD patterns and SEM images of the membranes synthesized at different temperatures for 8 h, respectively. As shown in Figure 6, the intensity of characteristic peaks of the synthesized membranes increased with the synthesis temperature. The membrane synthesized at 358 K for 8 h showed weak FAU characteristic peaks, indicating the formation of FAU zeolite on the support. However, loosely packed small crystals with incomplete shapes formed the membrane-like layer with a thickness of 2.4 µm on the α-Al₂O₃ support (Figure 7). The intensity of characteristic peaks of the membranes synthesized at 368 K was significantly enhanced. And a pure, fully covered, dense FAU layer was obtained on the α-Al₂O₃ support (as mentioned before). When the synthesis temperature increased to 378 K, the characteristic peaks of NaP appeared in the XRD pattern of the synthesized membrane. The membrane layer included an FAU layer with a thickness of ~4.6 μm and a NaP layer of ~4.2 μm. Those results suggest that the crystallization rate increases with increasing the synthesis temperature; however, the phase transition from the FAU-type zeolite structure to the NaP impurity zeolite structure becomes more significant [34].

3.2.3. Effect of synthesis time on membrane formation on mullite support

Based on the details discussed thus far, employing FAU seeds with a size of 50 nm and an ultra-dilute reaction solution, the effects of synthesis temperature and time on the formation of FAU membranes on α-Al₂O₃ supports were investigated, and the optimized conditions for membrane synthesis were determined to be 368 K for 8 h. In this section, we set the synthesis temperature at 368 K and tried to prepare satisfactory FAU membranes on cheap mullite supports by controlling the crystallization time despite the different pore sizes of the mullite and α-Al₂O₃ supports.

Figure 8 and Figure 9 show the XRD patterns, and SEM images of the membranes synthesized on mullite supports at 368 K for different crystallization times, respectively. As shown in Figure 8, the intensity of characteristic peaks of the synthesized membranes increased with the synthesis time. When the synthesis time increased from 3 to 4.5 h, the intensity of the characteristic peaks of FAU was slightly enhanced, the crystals on the membrane surface were incomplete, and the membrane thickness increased from 1.1 µm (the seed layer) to 5.2 µm (Figure 9a–d). This indicates that the growth of seeds from the seed layer and support pores occurred within 4.5 h of synthesis time.

When the synthesis time increased from 4.5 to 6 h, the intensity of the characteristic peaks of FAU was significantly increased, while that increased slightly from 6 to 8 h. Figure 9e,f show that the membrane layer with a thickness of 7.9 μm was continuous and dense without obvious cracks and holes after 6 h of synthesis. The surface morphology of the membrane synthesized for 8 h was similar to that of the membrane synthesized for 6 h. The main difference was that after 8 h of synthesis, the membrane layer began to grow into the support, increasing the membrane thickness to ~9.2 μm (Figure 9g,h). However, a thick membrane layer increases mass transfer resistance and reduces permeability [35,36]. When the synthesis time was prolonged to 10 h, the characteristic peaks of the NaP impurity phase appeared. Figure 9i,j show that there were many walnut-like NaP crystals on the membrane surface, and the growth of the membrane layer towards the interior of the support became more severe, resulting in the membrane thickness increasing to ~22 μm. The Si/Al ratio of the membrane synthesized on the mullite support at 368 K for 6 h measured by EDX was ~2.9 (NaY type), and this membrane was selected as a promising FAU membrane for gas separation.

It is worth noting that membranes supported by mullite were thicker and had higher Si/Al ratios than that of membranes supported by α-Al₂O₃, which was plausibly attributed to the silicon element contained in mullite providing silicon-nutrient for the growth of FAU membranes.

3.3. Gas separation performance

Based on the characteristics described thus far, two types of FAU (NaY) membranes with continuous, uniform, dense zeolite layers, that is, α-Al₂O₃-supported membranes (hereafter referred to as MA-x) synthesized at 368K for 8 h and mullite-supported membranes (hereafter referred to as MM-x) synthesized at 368K for 6 h, were selected as promising FAU membranes for gas separation.

3.3.1. Gas permeation

Figure 10 shows the single-gas permeance for H₂, CO₂, N₂, CH₄, C₃H₆, C₃H₈, and i-butane (i-C₄H₁₀) as a function of the kinetic diameter at room temperature through the MA-1 and MM-1 membranes. The permeances for the gases through both membranes decreased with the kinetic diameter of molecules except for CO₂ and N₂, and the permeance order was H₂ > CH₄ > C₃H₆ > C₃H₈ > i-C₄H₁₀. This is because the permeation of small-sized molecules (e.g., H₂, CO₂, N₂, and CH₄) through the FAU (pore size: 0.74 nm) membrane is mainly controlled by Knudsen diffusion, which is inversely proportional to the square root of the molecular weight ratio of the gases. However, the behavior of large-sized molecules permeating the membrane results from the combined effects of Knudsen diffusion, affinity (e.g., C₃H₆), and molecular sieving (e.g., i-C₄H₁₀). The H₂ permeance of the MA-1 membrane was as high as 5.34 × 10^-7 mol/(m² s Pa), which is 3.4 times that of the MM-1 membrane. In addition, the H₂/C₃H₈ selectivity of the MA-1 membrane was 183, 10.1 times that of the MM-1 membrane, and the H₂/i-C₄H₁₀ selectivity was as high as 315, 6.8 times that of the MM-1 membrane. The results demonstrate that compared to the MM-1 membrane, the MA-1 membrane layer was thinner without defects, reducing permeation resistance and maintaining high selectivity. It is worth noting that the C₃H₆/C₃H₈ selectivity of the MA-1 membrane was as high as 46, coupled with a remarkably high C₃H₆ permeance of 1.35× 10^-7 mol/(m² s Pa). In this work, both types of synthesized membranes had excellent selectivity for propylene/propane; that is, the permeance of propylene was higher than that of propane. This is because the propylene molecule has a smaller dynamic diameter, and there is a suitable electrostatic force between FAU (NaY) zeolite and the double bond of propylene, promoting propylene molecule permeation [37,38].

3.3.2. Membrane reproducibility

More than 3 FAU membranes were re-prepared for each type (i.e., MA and MM), and their gas separation performance was compared to confirm the reproducibility of the membranes, as listed in Table 1. Examination of all data, each type exhibited similar levels of permeability and selectivity. The relative standard deviations of permeance and selectivity were maintained within 14.8%, demonstrating that the membranes are reproducible [3].

3.3.3. Comparing gas separation performance with literature data

Substantial efforts have been made to develop high-performance zeolite membranes for gas separation. The H₂/C₃H₈ and C₃H₆/C₃H₈ separations are the most important and attractive processes in the petrochemical industry. For example, the products of direct dehydrogenation of propane to propylene (such as the Oleflex process) include abundant H₂, C₃H₆, and C₃H₈ [39]. The membrane needs to have good H₂/C₃H₈ and C₃H₆/C₃H₈ selectivity to achieve efficient separation and purification of the products. Figure 11 summarizes the separation performances of H₂/C₃H₈ and C₃H₆/C₃H₈ using FAU (MA-1) membrane synthesized on the α-Al₂O₃ support together with the membranes reported in recent years, which include polymer, silica, carbon, mixed matrix, and zeolite membranes (details are shown in Tables S1 and S2). Obviously, the MA-1 membrane has demonstrated attractive separation performance for H₂/C₃H₈ (H₂ permeance= 5.34 × 10^-7 mol (m² s Pa)^-1, selectivit = 183) and C₃H₆/C₃H₈ (C₃H₆ permeance= 1.35 × 10^-7 mol (m² s Pa)^-1, selectivity= 46). It is worth mentioning that compared with various types of membranes, the C₃H₆/C₃H₈ separation performance of the MA-1 membrane far surpasses their upper boundaries of the “tradeoff” line. The impressive gas separation capability was attributed to the thin membrane layer, defect-free, and high quality of the as-prepared FAU membrane (The optical photograph of the membrane is shown in Figure S1).

4. Conclusions

In this study, high-performance FAU (NaY type) zeolite membranes were successfully synthesized using small-sized seeds of 50 nm, and their gas separation performance was evaluated. The results illustrated that further extending the crystallization time or increasing the synthesis temperature led to the formation of a NaP impurity phase on the FAU membrane layer. The most promising FAU membrane with a thickness of 2.7 µm was synthesized on an α-Al₂O₃ support at 368 K for 8 h, and had good reproducibility. The H₂ permeance of the membrane was as high as 5.34×10^-7 mol/(m² s Pa), and the H₂/C₃H₈ and H₂/i-C₄H₁₀ selectivities were 183 and 315, respectively. The most attractive performance was that the C₃H₆/C₃H₈ selectivity of the membrane was as high as 46 with a remarkably high C₃H₆ permeance of 1.35× 10^-7 mol/(m² s Pa). The excellent separation performance of the membrane is mainly attributed to the thin, defect-free membrane layer and relatively wide pore size.