Zirconia-Doped Methylated Silica Membranes via Sol-Gel Process: Microstructure and Hydrogen Permselectivity

Abstract

In order to obtain a steam-stable hydrogen permselectivity membrane, with tetraethylorthosilicate (TEOS) as the silicon source, zirconium nitrate pentahydrate (Zr(NO₃)₄·5H₂O) as the zirconium source, and methyltriethoxysilane (MTES) as the hydrophobic modifier, the methyl-modified ZrO₂-SiO₂ (ZrO₂-MSiO₂) membranes were prepared via the sol-gel method. The microstructure and gas permeance of the ZrO₂-MSiO₂ membranes were studied. The physical-chemical properties of the membranes were characterized by Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscope (SEM), and N₂ adsorption–desorption analysis. The hydrogen permselectivity of ZrO₂-MSiO₂ membranes was evaluated with Zr content, temperature, pressure difference, drying control chemical additive (glycerol) content, and hydrothermal stability as the inferred factors. XRD and pore structure analysis revealed that, as n_Zr increased, the MSiO₂ peak gradually shifted to a higher 2θ value, and the intensity gradually decreased. The study found that the permeation mechanism of H₂ and other gases is mainly based on the activation–diffusion mechanism. The separation of H₂ is facilitated by an increase in temperature. The ZrO₂-MSiO₂ membrane with n_Zr = 0.15 has a better pore structure and a suitable ratio of micropores to mesopores, which improved the gas permselectivities. At 200 °C, the H₂ permeance of MSiO₂ and ZrO₂-MSiO₂ membranes was 3.66 × 10⁻⁶ and 6.46 × 10⁻⁶ mol·m⁻²·s⁻¹·Pa⁻¹, respectively. Compared with the MSiO₂ membrane, the H₂/CO₂ and H₂/N₂ permselectivities of the ZrO₂-MSiO₂ membrane were improved by 79.18% and 26.75%, respectively. The added amount of glycerol as the drying control chemical additive increased from 20% to 30%, the permeance of H₂ decreased by 11.55%, and the permselectivities of H₂/CO₂ and H₂/N₂ rose by 2.14% and 0.28%, respectively. The final results demonstrate that the ZrO₂-MSiO₂ membrane possesses excellent hydrothermal stability and regeneration capability.

1. Introduction

It is well-known that hydrogen is a clean energy source [1]. At present, there are many ways to obtain H₂, but the biggest problem preventing its commercialization is the purification and separation of H₂. The purification of H₂ can be achieved in three main ways: pressure swing adsorption, cryogenic distillation, and membrane separation [2,3]. Although pressure swing adsorption and cryogenic distillation can be operated commercially, the economic benefits are low. The main commercial application of membranes in gas separation is the separation of hydrogen from nitrogen, methane, and argon in an ammonia sweep gas stream. In the past few years, hundreds of new polymer materials have been reported, and only eight or nine polymer materials have been used to make gas separation membrane bases. Surprisingly few of them were used to make industrial membranes [4]. Membrane separation technology is also one of the most promising hydrogen purification methods. At high temperatures, the H₂ separation membrane has attracted much attention in the application of membrane reactors, for example, in the steam reforming of natural gas. The characteristics of lower energy consumption and investment cost, as well as simple operation, have made the membrane separation method widely concerned. Compared to other techniques of hydrogen purification, the membrane separation method offers more energy efficiency and environmental friendliness. The quality of the separation membrane directly affects the separation performance. Therefore, it is very necessary to choose suitable materials to prepare efficient and stable membrane materials [5].

In recent years, research on hydrogen separation membranes has mainly focused on molecular sieves, alloys, and microporous silica [6,7,8,9]. Prominently, silica membranes have received extensive attention for H₂ separation due to their advantages such as high permselectivity and considerable thermal stability [10]. Amorphous silica membranes derived from sol-gel and chemical vapor deposition (CVD) methods received an enormous amount of attention [11,12]. They have a stable chemical structure and molecular sieve mechanism, which can separate hydrogen across a broad temperature range [13]. However, silica membranes have demonstrated poor steam and thermal stability. Due to the hydrophilic nature of silica membranes, if they are frequently exposed to humid, low-temperature atmospheres, the flux and permselectivity of H₂ will largely be reduced [14,15]. The study found that the addition of hydrophobic groups can reduce the affinity of silica for water and improve gas permselectivity [15]. At this stage, methyl, vinyl, perfluorodecalin, etc., are often used in the hydrophobic modification of gas permeation separation in silica membrane, and the effect of increasing the hydrothermal stability is obvious [16]. Wei et al. [17] prepared perfluorodecyl hydrophobically modified silica membranes. The results demonstrated that the addition of perfluorodecyl made the modified silica membrane change from hydrophilic to hydrophobic. The membrane exhibited excellent hydrothermal stability at 250 °C and a water vapor molar ratio of 5%. Debarati et al. [18] hydrophobically modified the surface of ceramic membranes with polydimethylsiloxane to achieve a contact angle of 141°. It shows that the surface of the ceramic membrane with the methyl group is highly hydrophobic. Somayeh et al. [19] also demonstrated that vinyl-modified silica particles can improve the hydrophobic properties of the membrane.

It is well-known that the addition of metal oxides (such as TiO₂ [20,21], Al₂O₃ [22,23], Fe₂O₃ [24], CoO [25], and ZrO₂ [26,27]) can not only enhance the hydrothermal stability of the membrane, but also further improve the antifouling ability and performance of the membrane. This indicates that the incorporation of metal oxides can form mixed oxide network structures that are more stable than amorphous silica materials [28,29]. In particular, ZrO₂ is an excellent transition metal oxide, which is often utilized in studies of gas membrane separation. Li et al. [30] prepared a zirconia membrane via the polymeric sol-gel method, which possessed H₂ permeance of about 5 × 10⁻⁸ mol·m⁻²·s⁻¹·Pa⁻¹, H₂/CO₂ permselectivity of 14, and outstanding hydrothermal stability under a steam pressure of 100 KPa. Gu et al. [31] used the sol-gel method to prepared the microporous zirconia membrane. After being treated with 0.50 mol·L⁻¹ of H₂SO₄, the hydrogen permeance was (2.8 to 3.0) × 10⁻⁶ mol·Pa⁻¹·m⁻²·s⁻¹. The H₂ permselectivities of the equimolar binary system were 6 and 9, respectively. Doping ZrO₂ in the SiO₂ matrix can improved the hydrophobicity and hydrothermal stability of the membrane matrix, and further enhance the gas permeability. Numerous studies have been carried out on ZrO₂-doped silica materials/membranes. According to mesoporous stabilized zirconia intermediate layers, Gestel et al. [32] revealed a much better membrane setup. For various CO₂/N₂ combinations, the as-prepared membrane demonstrated permselectivities of 20–30 and CO₂ permeances of 1.5 to 4 m³/(m²∙h∙bar). Ahn et al. [33] prepared a silica-zirconia membrane with hydrogen permselectivity on a porous alumina support using tetraethyl orthosilicate (TEOS) and tert-butanol zirconium (IV) under 923 K conditions by chemical vapor deposition. The H₂ permeance of the obtained membrane was 3.8 × 10⁻⁷ mol·m⁻²·s⁻¹·Pa⁻¹, and the permselectivities for CO₂ and N₂ were 1100 and 1400, respectively. Hove et al. [34] compared the gas permeation properties of a hybrid silica (BTESE) membrane, Zr-doped BTESE membrane, and silica membrane before and after hydrothermal treatment under the same circumstances. At 100 °C, the hybrid silica membrane and Zr-doped BTESE membrane maintained good hydrothermal stability, while the silica membrane lost selectivity for all the studied gases. After hydrothermal treatment at 200 or 300 °C, the CO₂ permeance of the Zr-doped BTESE membrane decreased significantly, and the H₂/CO₂ permselectivity increased significantly, by 65.71%. So far, many scholars have demonstrated the effect of different conditions during preparation on the properties of zirconia-doped silica materials/membranes. The influence of the Zr/Si molar ratio on the microstructure of the membrane and the permeability of the gas is crucial. Unfortunately, there are few reports in this regard. Furthermore, the effects of methyl modification on the microstructure and steam stability of ZrO₂-SiO₂ membranes were rarely described in papers. Some scholars have found that adding a drying control chemical additive (DCCA) in the process of preparing the membrane via the sol-gel method can effectively reduce the uneven shrinkage of the membrane during the heating process and during the calcining process [35], and improve the gas permselectivity of the membrane.

In this paper, methyl-modified ZrO₂-SiO₂ (ZrO₂-MSiO₂) materials/membranes with various Zr/Si molar ratios (n_Zr) were fabricated. Glycerol was chosen to be the DCCA. The impact of n_Zr on the microstructures and H₂ permselectivities of ZrO₂-MSiO₂ membranes was thoroughly addressed. The water vapor stability of ZrO₂-MSiO₂ membranes was investigated further by comparing the gas permeability characteristics of the ZrO₂-MSiO₂ membranes before and after steam treatment. The heat regeneration performance of ZrO₂-MSiO₂ membranes was also investigated.

2. Materials and Methods

2.1. Preparation of MSiO₂ Sols

The MSiO₂ sols were prepared by tetraethylorthosilicate (TEOS, purchased from Xi’an chemical reagent Co., Ltd., Xi’an, China) as a silica source, methyltriethoxysilane (MTES, purchased from Hangzhou Guibao Chemical Co., Ltd., Hangzhou, China) as a hydrophobic modified agent, anhydrous ethanol (EtOH, purchased from Tianjin Branch Micro-Europe Chemical Reagent Co., Ltd., Tianjin, China) as a solvent, and nitric acid (HNO₃, purchased from Sichuan Xilong Reagent Co., Ltd., Chengdu, China) as a catalyst. To begin, TEOS, MTES, and EtOH were completely combined in a three-necked flask using a magnetic stirrer. The flask was correctly immersed in an ice-water combination. The solution was then agitated for 50 min using a magnetic stirrer to ensure thorough mixing. The H₂O and HNO₃ combination was then dropped into the mixture while it was still being stirred. The reaction mixture was then agitated in a three-necked flask at a constant temperature of 60 °C for 3 h to yield the MSiO₂ sol.

2.2. Preparation of ZrO₂ Sols

In a three-necked flask, 0.6 M zirconium nitrate pentahydrate (Zr(NO₃)₄·5H₂O, purchased from Tianjin Fuchen Chemical Reagent Co., Ltd., Tianjin, China) and 0.2 M oxalic acid (C₂H₂O₄·2H₂O, purchased from Tianjin HedongHongyan Chemical Reagent Co., Ltd., Tianjin, China) solutions were combined at a molar ratio of 4.5:1.0 and agitated. The aforementioned mixture was then treated with 35% (v/v) glycerol (GL, purchased from Tianjin Kemiou Chemical Reagent Co., Ltd., Tianjin, China), and stirring was maintained in a water bath at 50 °C for 3 h to yield the ZrO₂ sols.

2.3. Preparation of ZrO₂-MSiO₂ Sols

The ZrO₂ sols were aged for 12 h at 25 °C. The ZrO₂ sols and EtOH were then added to the MSiO₂ sols and stirred for 60 min to create the required ZrO₂-MSiO₂ sols. The n_Zr ratio was 0, 0.08, 0.15, 0.3, and 0.05. The ZrO₂-MSiO₂ sols were diluted three times with ethanol after 12 h. GL was used as a drying control agent at 0%, 10%, 20%, and 30% (DCCA). After 60 min of stirring, ZrO₂-MSiO₂ sols with varied GL contents were obtained.

2.4. Preparation of ZrO₂-MSiO₂ Materials

The ZrO₂-MSiO₂ sols were then placed individually in petri plates for gelation at 30 °C. The gel materials were ground and pulverized with a mortar, and then calcined at a heating rate of 0.5 °C·min⁻¹ at 400 °C for 2 h under nitrogen atmosphere protection, and then cooled down naturally. The ZrO₂-MSiO₂ materials with different n_Zr were prepared. The ZrO₂-MSiO₂ materials with n_Zr = 0 are also referred to as “MSiO₂” materials.

2.5. Preparation of ZrO₂-MSiO₂ Membranes

The ZrO₂-MSiO₂ membranes were coated on top of composite interlayers supported by porous α-alumina discs. The discs are 5 mm-thick and 30 mm in diameter, with a porosity of 40% and an average pore size of 100 nm. ZrO₂-MSiO₂ membranes were effectively prepared by dip-coating the substrates in three-fold ethanol-diluted silica sol for 7 s, then drying and calcining them. Each sample was dried at 30 °C for 3 h before being calcined at 400 °C in a temperature-controlled furnace in a N₂ environment with a ramping rate of 0.5 °C·min⁻¹ and a dwell period of 2 h. The dip-coating-drying-calcining process was repeated three times. Figure 1 demonstrates the preparation process of the ZrO₂-MSiO₂ materials/membranes. The ZrO₂-MSiO₂ membranes with n_Zr = 0 are also referred to as “MSiO₂” membranes.

2.6. Steam Treatment and Regeneration of ZrO₂-MSiO₂ Membranes

The ZrO₂-MSiO₂ membranes were subjected to a 7-day steam stability test in which they were placed into saturated steam at 25 °C. After steam treatment, for thermal regeneration of ZrO₂-MSiO₂ membranes, they were processed at a calcination temperature of 350 °C, with the same calcination technique as before. The gas permeances of ZrO₂-MSiO₂ membranes were investigated after steam treatment and regeneration, respectively.

2.7. Characterizations

Using Fourier transform infrared spectroscopy, the functional groups of ZrO₂-MSiO₂ materials were characterized (FTIR, Spotlight 400 and Frontier, PerkinElmer Corporation, Waltham, MA, USA), and the wavelength measuring range was 400 to 4000 cm⁻¹ using the KBr compression technique. Using a Rigaku D/max-2550pc X-ray diffractometer (XRD, Rigaku D/max-2550pc, Hitachi, Tokyo, Japan) with CuKα radiation at 40 kV and 40 mA, the ZrO₂-MSiO₂ materials’ phase structure was found. The X-ray photoelectron spectra (XPS) were acquired on a K-Alpha X-ray photoelectron spectroscope from Thermo Fisher Scientific with AlKα excitation and were calibrated regarding the signal of adventitious carbon (XPS, ESCALAB250xi, Thermo Scientific, Waltham, MA, USA). The binding energy estimates were derived using the C (1s) line at 284.6 eV as the reference point. Transmission electron microscopy (TEM, JEM 2100F, JEOL, Tokyo, Japan) was utilized to investigate the ZrO₂-MSiO₂ powders’ crystallization. Operating at 5 kV, scanning electron microscopy (SEM, JEOL JSM-6300, Hitachi, Tokyo, Japan) was utilized to study the surface morphologies of the ZrO₂-MSiO₂ membranes. N₂ adsorption–desorption measurements were conducted using an automated Micromeritics, ASAP2020 analyzer (ASAP 2020, Micromeritics, Norcross, GA, USA). The ZrO₂-MSiO₂ materials’ BET surface area, pore volume, and pore size distribution were determined.

Figure 2 is a schematic of the experimental setup used to evaluate the performance of single gas permeation. Prior to the experiment, the pressure and temperature were set to the desired values for thirty minutes to allow the gas permeation to stabilize. The permeation properties of MSiO₂ and ZrO₂-MSiO₂ membranes were evaluated using H₂, CO₂, and N₂. The gas permeability was determined based on the outlet gas flow. The gas permselectivity values (ideal permselectivities) were calculated by the permeance ratio between two gases.

3. Results

3.1. Chemical Structure Analysis

FTIR spectra were used to investigate the functional groups of ZrO₂-MSiO₂ materials. The FTIR spectra of ZrO₂-MSiO₂ materials containing various n_Zr contents are displayed in Figure 3. The absorption peak at around 3448 cm⁻¹ was assigned to the stretching and bending vibration of the -OH group from the absorbed water. The absorption peak at 1630 cm⁻¹ corresponds to Si-OH and Zr-OH on the surface of ZrO₂-MSiO₂ materials [36]. The antisymmetric stretching vibration absorption peak -CH₃ at 2985 cm⁻¹ was mainly from unhydrolyzed TEOS and MTES. The absorption peak at 1278 cm⁻¹ was attributed to the Si-CH₃ group. It is also the main hydrophobic functional group of the membrane. The absorption peak observed at 1050 cm⁻¹ was attributed to the Si-O-Si bond [37]. Compared with the materials with n_Zr = 0, the materials with n_Zr = 0.08–0.5 all showed a new absorption peak at the wavenumber of 448 cm⁻¹. This was related to the formation of Zr-O bonds [38]. Meanwhile, with the increase of n_Zr, the peak at 1050 cm⁻¹ shifted to around 1100 cm⁻¹. This may be ascribed to the fact that partial substitution of Zr atoms for Si atoms in the Si-O-Si network to form Zr-O-Si bonds occurred [39], breaking the symmetry of SiO₂ and leading to the shift of peak positions. However, there was no obvious Zr-O-Si bond in the FTIR spectrum of ZrO₂-MSiO₂ materials due to the overlap of the Zr-O-Si bond with Si-O-Si [40]. Furthermore, the decrease in the intensity of the silanol band at 779 and 835 cm⁻¹ with increasing n_Zr could be attributed to the substitution of Si-OH bonds by Zr-O-Si bonds [41]. It demonstrates the formation of Zr-O-Si bonds in the produced materials.

3.2. Phase Structure Analysis

The XRD patterns of the ZrO₂-MSiO₂ materials with varied n_Zr are presented in Figure 4. The peaks of amorphous SiO₂ were concentrated at 2θ = 23.1° [42]. The SiO₂ peak moved progressively towards higher 2θ values as n_Zr rose, and it slowly dropped in intensity. This is attributable to the replacement of the portion of silicon atoms by the inserted Zr atoms, producing Zr-O-Si bonds, resulting in a drop in the SiO₂ concentration. The peaks corresponding to a crystalline tetragonal structure of zirconia are clearly apparent in the ZrO₂-MSiO₂ materials with n_Zr = 0.15–0.5. The (101), (112), and (202) reflection planes of the body-centered ZrO₂ (t-ZrO₂) tetragonal phase were ascribed to the large diffraction peaks occurring at 60.2°, 50.7°, and 30.2°, respectively (JCPDS No. 79-1771). XRD analysis demonstrated that with the growth of the Zr concentration, the peak intensity corresponding to t-ZrO₂ progressively increased. In other words, the content of t-ZrO₂ increased with the growth in Zr content. Combined with the FTIR analysis, the Zr element in ZrO₂-MSiO₂ materials may exist in the form of Zr-O-Si bonds and t-ZrO₂.

To further investigate the presence of Zr and Si species in the ZrO₂-MSiO₂ materials, the XPS measurement was conducted. The survey XPS spectrum of ZrO₂-MSiO₂ material with n_Zr = 0.15 is shown in Figure 5. Figure 5 demonstrates that C, O, Si, and Zr elements are present in the ZrO₂-MSiO₂ material, which indicates the successful incorporation of Zr into the silica frameworks. Figure 6 presents the Si 2p and Zr 3d XPS spectra of the ZrO₂-MSiO₂ sample with n_Zr = 0.15. In Figure 6a, the peaks at the binding energies of 102.8 and 104.7 eV correspond to Si-C and Si-O bonds, respectively. In Figure 6b, the peaks at 186.6 and 183.3 eV correspond to the Zr-O 3d_3/2 and Zr-O 3d_5/2 peaks, respectively.

3.3. TEM Analysis

The TEM micrographs of the ZrO₂-MSiO₂ material with n_Zr = 0 and 0.15 at 400 °C under nitrogen atmosphere are illustrated in Figure 7. Figure 7a depicts that the silica particles in MSiO₂ materials are amorphous, while in Figure 7b, a small amount of particles with darker color appear and are mixed in the silica skeleton, which may be due to the presence of t-ZrO₂. Overall, the ZrO₂-MSiO₂ materials with n_Zr = 0.15 still maintained the amorphous state.

3.4. Pore Structure Analysis

The N₂ adsorption-desorption isotherm of the ZrO₂-MSiO₂ materials with various n_Zr are shown in Figure 8a. According to the Brunauer-Deming-Deming-Teller (BDDT) classification, the ZrO₂-MSiO₂ materials showed a type I adsorption isotherm, while n_Zr = 0 indicated the formation of microporous structures. The isotherms for the four samples (n_Zr = 0.08–0.5) all showed a similar trend, which could be categorized as type IV isotherms. However, the shapes of the hysteresis loops for the four samples were different, implying the variation of pore structures. A significant proportion of adsorption occurred in the range of low relative pressure, P/P₀ < 0.1, indicating that the materials contain a large quantity of micropores. The shape of the hysteresis loop of the ZrO₂-MSiO₂ materials with n_Zr = 0.5 was altered, indicating the presence of larger mesopores or macropores. In addition, the distributions of pore size for all samples are depicted in Figure 8b. It is found that the samples with n_Zr = 0.08–0.5 showed a broader pore size distribution and a larger mean pore size than the samples with n_Zr = 0. The conclusion was also confirmed by the pore structure parameters of ZrO₂-MSiO₂ materials with various n_Zr in

Table 1. Pore structure parameters of the ZrO₂-MSiO₂ materials with various n_Zr.

nZr	BET Surface Area (m2·g−1)	Average Pore Size (nm)	Vtotal (STP) (cm3·g−1)	Vmicro (STP) (cm3·g−1)	Vmicro/Vtotal (%)
0	389.38	1.75	0.23	0.15	65.22
0.08	579.96	2.08	0.37	0.13	35.14
0.15	616.77	2.19	0.43	0.12	27.91
0.3	606.35	2.35	0.41	0.09	21.95
0.5	545.32	3.58	0.38	0.08	21.05

. The average pore size, BET specific surface area, and total pore volume of the ZrO₂-MSiO₂ materials gradually increased with n_Zr = 0.08 and 0.15, and the pore size distribution became wider. However, the total pore volume of the ZrO₂-MSiO₂ materials with n_Zr = 0.3 and 0.5 decreased instead. The fact is that the bond length of Zr-O (1.78 Å) was slightly longer than that of Si-O (1.64 Å) [43]. Figure 9 shows the molecular structure models of MSiO₂, ZrO₂-MSiO₂, and t-ZrO₂ crystallites, respectively. Hence, the formation of Zr-O-Si bonds contributes to the formation of pores. With the increase of Zr content, more and more t-ZrO₂ crystallites were formed and distributed in the framework of MSiO₂ materials, and the internal pore structure of the ZrO₂-MSiO₂ materials was hindered from shrinking and pore collapse, resulting in the decrease of the BET surface area and pore volume. From Table 1, it can be seen that the ZrO₂-MSiO₂ materials with n_Zr = 0.15 had the maximal total pore volume (0.43 cm³·g⁻¹), BET surface area (616.77 m²·g⁻¹), and the minimum mean pore size (2.19 nm).

3.5. Gas Permselectivity Analysis

3.5.1. The Influence of n_Zr

Figure 10 depicts the influence of n_Zr on the gas permeabilities and H₂ permselectivities of ZrO₂-MSiO₂ membranes with varying n_Zr and 0% DCCA addition at 25 °C and 0.1 MPa. In Figure 10a, with the increase of n_Zr, the H₂, CO₂, and N₂ permeances of the samples increased until n_Zr = 0.15, and then decreased. Compared with the MSiO₂ membranes (n_Zr = 0), the H₂, CO₂, and N₂ permeance of the ZrO₂-MSiO₂ membranes with n_Zr = 0.15 increased by 50.95%, 26.74%, and 36.36%, respectively. From the pore structure analysis (Table 1), the overall pore volumes of the ZrO₂-MSiO₂ membranes grew somewhat with increasing n_Zr until n_Zr = 0.15, and then decreased, which can explain why the ZrO₂-MSiO₂ membranes with n_Zr = 0.15 had the highest permeance to each gas. For the same membrane, the order of gas molecular permeance is H₂ > CO₂ > N₂. Gas permeance decreased with increasing d_k (0.289, 0.33, and 0.364 nm, respectively), indicating that all membranes exhibited molecular sieve properties. However, when n_Zr ≥ 0.15, the permeance of CO₂ decreased more closely to that of N₂. This behavior was related to the fact that following heat treatment at 400 °C, the Zr-O-Si bonds and t-ZrO₂ crystallites generated in the ZrO₂-SiO₂ membranes will generate a significant number of Brønsted acid sites. High acidity leads to a reduction in the affinity of the membranes for CO₂, hence lowering the CO₂ permeance [44].

It can be observed from Figure 10b that compared with MSiO₂ membranes, the H₂/CO₂ and H₂/N₂ permselectivities of ZrO₂-MSiO₂ membranes with n_Zr = 0.15 increased by 22.93% and 33.04%, respectively. Combined with the previous characterization test, it was found that the ZrO₂-MSiO₂ membranes with n_Zr = 0.15 had a good pore structure, which is beneficial to improve the permselectivity of gas. In addition, the acidic sites formed by the ZrO₂-MSiO₂ membranes reduced the affinity of the membranes for CO₂ and helped to separate it from H₂. However, the permselectivities of ZrO₂-MSiO₂ membranes after n_Zr = 0.15 showed a decreasing trend. Compared with the ZrO₂-MSiO₂ membranes with n_Zr = 0.15, the H₂/CO₂ and H₂/N₂ permselectivities of the membranes with n_Zr = 0.5 decreased by 9.35% and 20.15%, respectively. As a result, just because the n_Zr concentration is larger, it does not indicate that the separation effect is better. Since the Zr-O and Si-O bonds in zirconium-substituted siloxane rings are longer than in pure siloxane rings, for the ZrO₂-MSiO₂ membranes with n_Zr = 0.5, the number of siloxane rings containing Zr increased, and the pore size of the membranes became larger. Meanwhile, a large number of t-ZrO₂ crystals were produced, which led to the shrinkage of the pore structure inside the membranes and the collapse of the pores, resulting in the decrease of the permselectivities of the membranes.

SEM images of surface topography for MSiO₂ and ZrO₂-MSiO₂ (n_Zr = 0.15) membranes calcined at 400 °C are shown in Figure 11. Compared to the MSiO₂ membranes, the particle size and distribution of the ZrO₂-MSiO₂ membranes were more uniform, the membranes’ surfaces had no obvious defects, and the surface was uniform and smooth. The particle size of MSiO₂ membranes was between 1.1 and 5.8 nm, while the particle size of ZrO₂-MSiO₂ membranes was between 1.3 and 8.9 nm. The formed ZrO₂-MSiO₂ membranes with a smooth surface and uniform membrane pores were more conducive to the gas permselectivitiy.

3.5.2. The Influence of Temperature

The permeances and permselectivities of the MSiO₂ and ZrO₂-MSiO₂ (n_Zr = 0.15) membranes with 0% DCCA addition at a pressure difference of 0.1 MPa and temperature changing from 25 to 200 °C are shown in Figure 12. Obviously, with the increasing temperature, the permeance of H₂ of MSiO₂ and ZrO₂-MSiO₂ membranes increased gradually, as seen in Figure 12a. From 25 to 200 °C, the H₂ permeance of MSiO₂ and ZrO₂-MSiO₂ membranes rose by 19.66% and 7.12%, respectively, demonstrating that the H₂ permeation behavior in the two membranes followed the activated diffusion transport mechanism. In contrast, the permeabilities of CO₂ and N₂ were similar to the Knudsen diffusion trend, whereby both slightly decreased. The CO₂ permeance of MSiO₂ and ZrO₂-MSiO₂ membranes decreased by 31.46% and 30.20% from 25 to 200 °C, respectively, and the N₂ permeance decreased by 18.60% and 29.98%, respectively. The major explanations for the decrease in CO₂ and N₂ permeance were the violent movement of molecules and the rise in the mean free path as temperature increased.

In Figure 12b, it can be seen that the permselectivities in the membranes gradually increased with the increase of temperature. At 200 °C, compared with the MSiO₂ membranes, the H₂/CO₂ and H₂/N₂ permselectivities of the ZrO₂-MSiO₂ membranes increased by 21.11% and 23.34%, respectively. The above results show that the ZrO₂-MSiO₂ membranes had better permselectivity and permeance of H₂ than the MSiO₂ membranes under the same conditions.

Combined with the preceding studies, it was determined that a rise in temperature facilitated the separation of H₂ and that the separation process of H₂ from CO₂ and N₂ is dominated by activation diffusion, which is described by the Arrhenius equation [45]:

$$\begin{array}{l}P=P_0\exp (-\frac{E_{\mathrm{a}}}{RT})\\ \end{array}$$

(1)

In Formula (1), P is the permeation rate, E_a is the apparent activation energy, and P₀ is a constant, which depends on the pore wall–gas molecule interaction, gas selective layer thickness, and pore shape and tortuosity [46]. For linear fitting, 1000/RT was used as the abscissa and lnP as the ordinate, and the slope of the fitting equation might be used to obtain the apparent activation energy. Figure 13 depicts the Arrhenius fitting diagrams for the three gases.

Figure 13 demonstrates that the apparent activation energy of H₂ is positive, while that of various other gases is negative. This is related to the gas-activated transport behavior, whereby there are two parallel transmission channels for gas through the membrane: one is through selective micropores, with gas transport processed by a thermally activated surface diffusion mechanism in the micropore state [47], and the other is through larger pores [48]. The activation energies for CO₂ and N₂ are negative, indicating that there are permeation pathways large enough in these types of membranes to allow the diffusion of these larger gas. It is generally believed that E_a is composed of two parts [46], the adsorption heat, Q_st, of gas on the surface of the membrane and the activation energy, E_m, of gas flowing through the solid surface. The larger the E_m is, the harder it is for the gas to diffuse, E_a = E_m − Q_st. Arrhenius equation parameter values are shown in

Table 2. Arrhenius equation parameter values of MSiO₂ and ZrO₂-MSiO₂ (n_Zr = 0.15) membranes.

Membrane	Gases	Ea (kJ·mol −1)	Qst (kJ·mol−1)	Em (kJ·mol−1)
MSiO2	H2	2.32	6.00	8.32
	CO2	−1.53	24.00	22.47
	N2	−1.48	18.00	16.52
ZrO2-MSiO2	H2	2.10	6.00	8.10
	CO2	−1.64	24.00	22.36
	N2	−1.94	18.00	16.06

. Table 2 shows that the E_m values of the gases in the ZrO₂-MSiO₂ membrane are all less than those in the MSiO₂ membrane. It shows that the structure of ZrO₂-MSiO₂ membranes is not as dense as that of MSiO₂ membranes. This is in good accordance with the N₂ adsorption–desorption results. This finding also shows that ZrO₂ doping successfully diminishes the densification of the SiO₂ network. The higher porosity of the ZrO₂-MSiO₂ membranes allows the gas to cross the membrane pore barrier using their kinetic energy. Therefore, the gas (H₂, CO₂, and N₂) permeance of ZrO₂-MSiO₂ membranes is higher than that of MSiO₂ membranes.

Table 3. E_a of H₂, pore diameter, H₂ permeance, and H₂ permselectivities for various membranes/films prepared by other researchers.

Type	Temperature/Pressure	Ea of H2 (kJ·mol−1)	Pore Diameter (nm)	H2 Permeance (mol·m−2·s−1·Pa−1)	H2 Permselectivities
					H2/CO2	H2/N2
SiO2 [49]	200 °C, 2 bar	-	0.30–0.54	4.62 × 10−7	3.7	10.5
ZIF-7-SiO2 [50]	200 °C	-	5	8 × 10−7	8.78	11.8
Pd-SiO2 [51]	200 °C, 0.3 MPa	-	0.57	7.26 × 10−7	4.3	14
ZrO2 [52]	350 °C	-	4.95	5.3 × 10−8	14.3	3.1
ZrO2-SiO2 [27]	550 °C	7.0	0.165	1.8 × 10−7	-	-
BTDA-DDS polyimide [53]	30 °C, 5 MPa	-	-	2.52 × 10−9	5.16	193.21
Cellulose acetate [54]	25 °C, 1 bar	-	-	3.55 × 10−9	-	30.3
ZrO2-MSiO2 *	200 °C, 0.1 MPa	2.10	2.19	6.46 × 10−6	11.64	13.88

* In this work.

displays the E_a of H₂, pore diameter, H₂ permeance, and H₂ permselectivities for several membranes/films prepared by other researchers. It is challenging to concurrently enhance the membranes’ permselectivity and gas permeability, as seen in Table 3. Generally, the larger the average pore size of the membrane, the higher the permeance to H₂, which is accompanied by a smaller E_a. Meanwhile, the E_a of H₂ is related to the interaction of H₂ with the membrane pore wall. It can be seen from Table 3 that the as-prepared ZrO₂-MSiO₂ membrane has a large H₂ permselectivity compared to other membranes.

3.5.3. The Influence of Pressure Difference

Figure 14 illustrates the effect of pressure difference on the gas permeances and H₂ permselectivities of MSiO₂ and ZrO₂-MSiO₂ (n_Zr = 0.15) membranes at 200 °C with 0% DCCA addition. In Figure 14a, it can be observed that the H₂ permeance of the ZrO₂-MSiO₂ membranes with n_Zr = 0.15 improved with the increase of the pressure difference, and the pressure dependence increased. However, the H₂ permeance of MSiO₂ membranes remained basically unchanged. The MSiO₂ and ZrO₂-MSiO₂ membranes increased their H₂ permeance by 2.16% and 19.96%, respectively, when the pressure was increased from 0.10 to 0.40 MPa. In Figure 14b, the H₂ permselectivities of MSiO₂ membranes did not change significantly, and the H₂/CO₂ and H₂/N₂ permselectivities decreased by 5.27% and 1.12%, respectively. However, the H₂ permselectivities of the ZrO₂-MSiO₂ membranes with n_Zr = 0.15 changed greatly, where the H₂/CO₂ and H₂/N₂ permselectivities decreased by 22.04% and 21.12%, respectively. Clearly, it can be seen that the permselectivity of the ZrO₂-MSiO₂ membranes was reduced more than that of the MSiO₂ membranes, that is, the pressure had a relatively small effect on the gas permeation of the MSiO₂ membranes. This phenomenon is attributed to the relatively dense MSiO₂ membranes with micropores as the main component. With the increase of the pressure difference between the two sides of the membranes, the power of the gas passing through the ZrO₂-MSiO₂ membranes increased, making it easier for the gas to pass through the mesopores or even the macropores, which has a greater impact on the permselectivity of the membranes. At the same time, it is shown that the H₂ diffusion mechanism of ZrO₂-MSiO₂ membranes was different from that of MSiO₂ membranes due to the influence of doping ZrO₂. The gas transport in the ZrO₂-MSiO₂ membranes follows the surface diffusion mechanism. In addition, when the pressure difference increased to 0.4 MPa, the permselectivities of H₂/CO₂ and H₂/N₂ were still higher than their respective Knudsen diffusion (4.69 and 3.74, respectively), indicating that they still have good gas permeation performance under high pressure.

3.5.4. The Influence of the DCCA

Comparing and analyzing the previous results, it was found that the addition of DCCA (glycerol) by the sol-gel method can effectively reduce the uneven shrinkage of the membrane when it is heated during firing. The gas permeances and H₂ permselectivities of ZrO₂-MSiO₂ membranes (n_Zr = 0.15) with various DCCA additions at 200 °C and 0.1 MPa are shown in Figure 15.

Figure 15 demonstrates that the H₂ permeance of the ZrO₂-MSiO₂ membranes reduced by 21.30% as the DCCA addition increased from 0 to 30%. The permselectivities of H₂/CO₂ and H₂/N₂ increased by 21.77% and 14.07%, respectively. However, compared with the 20% membranes, the H₂/CO₂ and H₂/N₂ permselectivities of the membranes with the addition of 30% only increased by 2.14% and 0.28%, respectively. Figure 16 shows the relationship between DCCA (GL) contents and F_H2 × α_H2 (F_H2 is the H₂ permeance, α_H2 is the permselective of H₂). It was clearly observed that the addition of DCCA from 0 to 20% enhanced the F_H2 × α_H2 value, and after more than 20%, the F_H2 × α_H2 value showed a lower level. This is attributed to the fact that the addition of glycerol will gradually surround the sol particles, reduce the agglomeration caused by the collision of the colloid particles, accelerate the creation of the sol-gel network, and improve the stability of the sol structure [52]. In addition, the membrane layer collapsed and cracked easily during the drying and calcination processes, and the addition of glycerol can effectively reduce the liquid–gas surface tension to a certain extent, thereby protecting the gel skeleton from deformation [55]. However, the particles of the sol were surrounded by steric effects when too much GL was added, making the sol sticky and difficult to dry to form a membrane. The extension of the drying time makes the cross-linking of the sol more thorough and eventually leads to the densification of the membrane, which is not conducive to gas separation. From the above point of view, the membranes with 20% DCCA addition were the more worthy choice.

3.5.5. Steam Treatment and Regeneration Analysis

Figure 17 shows the effects of steam treatment and thermal regeneration on the gas permeances (H₂, CO₂, and N₂) and H₂ permselectivities of MSiO₂ and ZrO₂-MSiO₂ (n_Zr = 0.15) membranes with 0% DCCA addition at a pressure difference of 0.1 MPa and 25 °C. The permeances of H₂, CO₂, and N₂ for MSiO₂ and ZrO₂-MSiO₂ membranes appear to have reduced after steam treatment. After steam aging for 7 days, the permeance of H₂ for MSiO₂ and ZrO₂-MSiO₂ membranes dropped by 20.63% and 3.70%, respectively, as compared to untreated fresh samples, and the permselectivities of H₂/CO₂ and H₂/N₂ for MSiO₂ membranes decreased by 1.59% and 1.04%, respectively, whereas those of ZrO₂-MSiO₂ membranes increased by 0.09% and 0.43%, respectively.

The gas permeances (H₂, CO₂, and N₂), as well as the permselectivities of H₂/CO₂ and H₂/N₂ for two membranes, all exhibit an increased trend after regeneration by calcination at 350 °C. However, as compared to untreated fresh samples, the H₂ permeances of MSiO₂ and ZrO₂-MSiO₂ membranes after regeneration dropped by 9.96% and 1.65%, respectively, whereas the permselectivities of H₂/CO₂ and H₂/N₂ for MSiO₂ membranes improved by 1.12% and 4.71%, respectively, and those for ZrO₂-MSiO₂ membranes increased by 0.08% and 1.21%, respectively. The decrease in gas permeances in both membranes suggests that membrane pore shrinking occurs after calcination at 350 °C. Lower permeance and greater permselectivities are produced as a result of the smaller pores. This is attributed to the partial Zr atoms replacing the Si atoms in Si-O-Si to form more stable Zr-O-Si bonds, which further improves the hydrothermal stability of the membrane material. Therefore, the above results indicated that the ZrO₂-MSiO₂ membranes had a better hydrothermal stability and reproducibility than MSiO₂ membranes.

4. Conclusions

The ZrO₂-MSiO₂ membranes were manufactured to enhance the steam stability and H₂ permselectivity of SiO₂ membranes. It was found that with the increase of ZrO₂ content, the pore size distribution of the materials became wider and the average pore size increased, indicating that the doping of ZrO₂ had the effect of expanding the pores. The ZrO₂-MSiO₂ membranes with n_Zr = 0.15 had a good pore structure and suitable micropore/mesoporous ratio, which is beneficial to improve the permeance of natural gas. At 200 °C, the H₂/CO₂ and H₂/N₂ permselectivities of ZrO₂-MSiO₂ membranes were 79.18% and 26.75% greater than those of MSiO₂ membranes, respectively. Furthermore, when the pressure was increased to 0.4 MPa, the permselectivities were still higher than their respective Knudsen diffusion, indicating that they still had good gas permeance at high pressure. With the addition of DCCA from 20% to 30%, the H₂/CO₂ and H₂/N₂ permselectivities of ZrO₂-MSiO₂ membranes only increased by 2.14% and 0.28%, respectively, and the F_H2 × α_H2 value with 20% addition was the highest. In conclusion, it is worthwhile to choose 20% GL as the DCCA addition for ZrO₂-MSiO₂ membranes. Compared with the untreated fresh sample, after 7 days of water vapor aging, the permeance of ZrO₂-MSiO₂ membranes to H₂ decreased by only 3.70%, and the permselectivities of H₂/CO₂ and H₂/N₂ increased by only 0.09% and 0.43%, respectively. After regeneration at 350 °C, the H₂ permeance of the ZrO₂-MSiO₂ membranes decreased by 1.65%, and the permselectivities of H₂/CO₂ and H₂/N₂ increased by 0.08% and 1.21%. It is enough to show that the prepared ZrO₂-MSiO₂ membranes had a good hydrothermal stability and certain regeneration performance. In the future, the influence of high temperature (for example, ≥300 °C) and mixed gases on the gas permeances and permselectivies of ZrO₂-MSiO₂ membranes should be explored, which is important to the practical engineering applications.