Microstructure and Hydrothermal Stability of Microporous Niobia-Silica Membranes: Effect of Niobium Doping Contents

Abstract

Methyl-modified niobium-doped silica (Nb/SiO₂) materials with various Nb/Si molar ratios (n_Nb) were fabricated using tetraethoxysilane and methyltriethoxysilane as the silica source and niobium pentachloride as the niobium source by the sol–gel method, and the Nb/SiO₂ membranes were prepared thereof by the dip-coating process under an N₂ calcining atmosphere. Their microstructures were characterized and gas permeances tested. The results showed that the niobium element existed in the formation of the Nb-O groups in the Nb/SiO₂ materials. When the niobium doping content and the calcining temperature were large enough, the Nb₂O₅ crystals could be formed in the SiO₂ frameworks. With the increase of n_Nb and calcination temperature, the formed particle sizes increased. The doping of Nb could enhance the H₂/CO₂ and H₂/N₂ permselectivities of SiO₂ membranes. When n_Nb was equal to 0.08, the Nb/SiO₂ membrane achieved a maximal H₂ permeance of 4.83 × 10⁻⁶ mol·m⁻²·Pa⁻¹·s⁻¹ and H₂/CO₂ permselectivity of 15.49 at 200 °C and 0.1 MPa, which also exhibited great hydrothermal stability and thermal reproducibility.

1. Introduction

Environmental protection and resource shortage are nowadays issues that need to be faced in the process of world development. The continuous growth of the world’s population has led to the increasing consumption of the earth’s resources. Some experts have suggested hydrogen as an alternative fuel because of its zero pollution. Nowadays, hydrogen is primarily produced from fossil fuels, such as natural gas and coal through steam reforming/gasification and water gas shift reactions [1]. In order to obtain high purity hydrogen from either syngas or the products of the water-gas shift reaction, separation of H₂ from other gases such as CO₂, CO, or CH₄ is necessary. Consequently, hydrogen purification from the above CO₂-containing reaction gas mixture is becoming an important issue. After long-term research and efforts by scientists, a large number of experimental results have shown that the membrane separation technology has shown great potential in gas separation. An inorganic membrane has the advantages of good resistance to high temperature and pressure, high mechanical strength, good chemical stability, long service life, and resistance to halite, which make it attractive in the field of gas separation [2]. In the last two decades, H₂-separation membranes have been developed using various materials, such as palladium and its alloys, silica, alumina, etc. In the research to date, inorganic silica membranes, especially those derived from the sol-gel technique, are some of the best among the various inorganic materials for the separation of hydrogen-containing gas mixtures.

However, in high temperature and humid air, pure SiO₂ membranes showed poor hydrothermal stability. The Si-O-Si bonds were broken and Si-OH bonds were formed when inter-played with water, resulting in the densification of the silica structure [3,4]. Many scientists have carried out a lot of research work to improve the hydrothermal stability of silica membrane materials. In recent years, the two primary methods have been the introduction of groups such as F^–, Cl^–, -C_nH_2n, -C_nH_2n+1, phenyl groups, etc. [5,6], and the doping of transition metals such as nickel [7,8,9], palladium [10,11], zirconium [12], niobium [13], magnesium [14], aluminum [15], cobalt [16,17], etc. Wei et al. [6] prepared a perfluorodecyl-modified silica membrane by the sol-gel method using tetraethylorthosilicate (TEOS) and 1H, 1H, 2H, and 2H-perflouorodecyltriethoxysilane (PFDTES) as precursors. The H₂ permeance of the as-prepared membrane was 9.71 × 10⁻⁷ mol·m⁻²·s⁻¹·Pa⁻¹, and the H₂/CO₂ permselectivity and binary gas separation factor were 7.19 and 12.11, respectively. Under humid conditions with a temperature of 250 °C and a water vapor molar ratio of 5%, the single H₂ permeance and H₂/CO₂ permselectivity remained almost constant for at least 200 h.

Among the transition metals, niobium doping has caught researchers’ eyes. Boffa et al. [18] prepared niobia-silica membranes using tetraethyl orthosilicate (TEOS) as the Si source and niobium penta (n-butoxide) as the Nb source. Their research results showed that the hydrothermal stability of the microporous niobia-silica membranes was better than that of the pure SiO₂ membrane because the incorporation of Nb ions into the silica matrix. Qi et al. [13] prepared a novel microporous hybrid silica membrane using 1,2-bis (triethoxysilyl) ethane (BTESE) and niobium penta (n-butoxide) as precursors for the permselectivity of CO₂. The result showed that the permselectivity of H₂/CO₂ for the Nb-BTESE membrane could be tuned by altering the calcination temperature. Lin et al. [19] investigated the influence of the sol particle size on the gas permselectivity of the niobium-doped hybrid silica membrane. The prepared Nb/SiO₂ membrane had an H₂ permeability of 8.36 × 10⁻⁸ mol·m⁻²·s⁻¹·Pa⁻¹ with a mean particle size of 5 nm. However, there have been few studies focusing on the effects of niobium doping content on the microstructures and gas permeances of niobium-doped SiO₂ membranes.

In this work, we prepared a kind of new niobium-doped SiO₂ membrane using tetraethoxysilane (TEOS) and methyltriethoxysilane (MTES) as the Si sources and niobium pentachloride (NbCl₅) as the niobium source. Nb/SiO₂ membranes with different Nb/Si molar ratios (n_Nb) were prepared. The effects of n_Nb and calcination temperature on the microstructures of Nb/SiO₂ materials were studied in detail. Characterization and results were attained by Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), X-ray photo electron spectroscopy (XPS), N₂ adsorption/desorption measurements, and scanning electron microscopy (SEM). The gas permeation tests and hydrothermal stability of the Nb/SiO₂ membranes were performed and are discussed.

2. Experimental

2.1. Fabrication of Nb/SiO₂ Sols

The Nb/SiO₂ sols were prepared using tetraethoxysilane (TEOS) and methyltriethoxysilane (MTES) as the silica sources, niobium pentachloride (NbCl₅) as the niobium source, absolute ethanol (EtOH) as the solvent, and HCl as the catalyst. High purity solid NbCl₅ powder was dissolved in absolute ethanol to obtain a 0.43 M NbCl₅ solution. This process was carried out in a fume hood, stirring and dissolving with a glass rod until the HCl gas was released. The reaction equation is as follows:

NbCl₅ (s) + nC₂H₅OH (l) → NbCl_5-n(OC₂H₅)_n (l) + nHCl (g)

According to the mol ratio of TEOS:MTES:EtOH:H₂O:HCl:NbCl₅ = 1:0.8:16:7:0.085:n_Nb, the EtOH, TEOS, and MTES were first mixed with the solution of NbCl₅ solution in an ice bath and stirred magnetically for 40 min. After that, a mixture of HCl and water was carefully added drop-wise and then refluxed in a water bath at 60 °C for 3 h. In this way, the Nb-doped SiO₂ sols were obtained. The n_Nb is the molar ratio of Nb/TEOS, which was 0, 0.08, 0.2, and 1, respectively. The Nb-doped SiO₂ sol with n_Nb = 0 is also referred to as the SiO₂ sol. The Nb-doped SiO₂ sols were diluted three times using absolute ethanol to obtain the final Nb/SiO₂ sols.

2.2. Fabrication of Nb/SiO₂ Materials

The as-prepared Nb/SiO₂ sols were dried in a vacuum oven to prepare the dry gels. The obtained dry gels were then ground into fine powders and calcined under N₂ atmosphere in a temperature-controlled tubular furnace at various temperatures (200 °C, 400 °C, 600 °C, 800 °C) for 2 h with a ramping rate of 0.5 °C·min⁻¹. The final niobium-doped silica (Nd/SiO₂) materials were produced. The Nb/SiO₂ materials with n_Nb = 0 were also referred to as the SiO₂ materials.

2.3. Fabrication of Nb/SiO₂ Membranes

To obtain the Nb/SiO₂ membranes, part of the above Nb/SiO₂ sols were applied to the surface of porous α-Al₂O₃ composite discs (Hefei Shijie Membrane Engineering Co., Ltd., Hefei, China) by the dip-coating method. The discs each had a thickness of 4 mm, a diameter of 30 mm, a mean pore diameter of 100 nm, and a porosity of 40%. The dipping time was 6 s. Four-layer Nb/SiO₂ membranes were prepared, and each Nb/SiO₂ layer was individually calcined under N₂ atmospheres in a temperature-controlled furnace to 400 °C at a ramping rate of 0.5 °C·min⁻¹ and with a dwell time of 2 h. The prepared Nb/SiO₂ membranes were used to test the permeances of H₂, CO₂, and N₂; the preparation process is shown in Figure 1.

2.4. Steam-Treatment and Regeneration of Nb/SiO₂ Membranes

The steam stability of the membranes was tested by exposure to saturated steam at 25 °C for 10 d. The thermal regeneration of Nb/SiO₂ membranes after steam treatment was carried out at a calcination temperature of 350 °C by the same calcining procedure as described above. After the steam-treatment and regeneration, the gas permeances of Nb/SiO₂ membranes were tested, respectively.

2.5. Characterization

The sol densities were determined using a pycnometer. The solid contents of sols were determined by the weighing method. The particle size distributions of Nb/SiO₂ sols were measured using a Malvern Nano ZS size analysis instrument (Malvern Instruments Ltd., Malvern, UK) The functional groups of samples were characterized by Fourier transform infrared spectroscopy (FTIR, Nicolet 5700, Thermo Nicolet Corporation, Fitchburg, WI, USA), and the wavelength measurement range was 400–4000 cm⁻¹ as determined by the KBr compression method. The material phase structure was determined by a Rigaku D/max-2550 pc X-ray diffractometer (XRD, Rigaku D/max-2550, Hitachi, Tokyo, Japan) with CuKα radiation under the conditions of 40 kV and 40 mA. The chemical components of Nb/SiO₂ samples were performed by an X-ray photoelectron spectrometer (XPS, ESCALAB250xi, Thermo Scientific, Waltham, MA, USA) with AlKα excitation. The morphologies of surfaces and cross-sections for the membranes were observed by scanning electron microscopy (SEM, JEOL JSM-6300, Hitachi, Tokyo, Japan) under 5 kV acceleration voltage. Before the SEM tests, the samples were treated with gold spraying. The BET surface area and pore volume of the samples were measured by N₂ sorption/desorption isotherms with a specific surface area and pore and pore-size analyzer (ASAP 2020, Micromeritics, Norcross, GA, USA).

Single gas permeances of Nb/SiO₂ membranes were measured under a transmembrane pressure of 0.1-0.4 MPa using the schematic diagram of the experimental setup shown in Figure 2. Prior to the gas permeation measurement, the membranes were mounted in a stainless-steel module with a cylindrical geometry and placed inside the furnace in the gas permeation measurement rig at a temperature range of 25 to 200 °C. The membranes were tested using single gases with different kinetic diameters (H₂, CO₂, and N₂), and permeance was measured by using a soap film flow meter. The gas permselectivities, also known as ideal selectivities, were calculated by the permeance ratio between two gases. The steam stability of the membranes was tested by exposure to saturated steam at 25 °C for 10 d. The thermal regeneration of Nb/SiO₂ membranes after steam stability testing was carried out at a calcination temperature of 350 °C by the same calcining procedure as described above. It should also be noted that permeate gas flow was only recorded after a steady state was attained.

3. Results and Discussion

3.1. Analysis of Nb/SiO₂ Sol Performance

The influence of n_Nb on the pH value, density, and solid content of Nb/SiO₂ sol is shown in

Table 1. The influence of n_Nb on the pH value, density, and solid content of Nb/SiO₂ sol.

nNb	pH	Density/g·cm−3	Solid Content/%
0	3.41 ± 0.04	0.8418 ± 0.0007	22.31 ± 0.04
0.08	2.93 ± 0.03	0.8529 ± 0.0006	22.58 ± 0.05
0.2	2.64 ± 0.02	0.8710 ± 0.0008	22.80 ± 0.06
1	1.02 ± 0.02	0.9130 ± 0.0006	24.46 ± 0.07

. The particle size distributions of Nb/SiO₂ sols with various n_Nb are depicted in Figure 3. As seen in Table 1, with increasing n_Nb, the sol pH value decreased while the density and solid content increased. As seen in Figure 3, with the increase of n_Nb, the mean particle sizes of Nb/SiO₂ sols increased, and their particle size distributions became wider. The particle size distributions of Nb/SiO₂ sols with n_Nb = 0, 0.08, and 0.2 were narrow, and their mean particle sizes were small. However, when n_Nb was increased to 1.0, the mean particle size of Nb/SiO₂ sol increased greatly. In the experiment, the prepared NbCl₅ solution was acidic. The increase of n_Nb in Nb/SiO₂ sol made the acidity of the sol increase, which sped up the hydrolysis–polycondensation reaction. As a result, the sol density, solid content, and particle size increased. On the other hand, with the occurrence of the hydrolysis-polycondensation reaction, Nb-O-Si bonds were formed gradually. The radius of the niobium atom (2.08 Å) is larger than that of the Si atom (1.46 Å), and the bond length of Nb-O is longer than that of Si-O (the bond lengths of Nb-O and Si-O are about 2.13 and 1.64 Å, respectively), which contributed to the increase of sol particles.

3.2. Chemical Structure Analysis

The phase-chemical structure of Nb/SiO₂ materials may be influenced by the introduced Nb element, so that the effect may have been more evident in the samples containing higher Nb contents. In order to study the effect of calcination temperature on Nb/SiO₂ materials, the Nb/SiO₂ samples with n_Nb = 1 were used for the measurement analysis. Figure 4 shows the FTIR spectra of Nb/SiO₂ materials with n_Nb = 1 at different calcination temperatures. In Figure 4, the absorption peaks located at 1055 and 788 cm⁻¹ were associated with the Si-O-Si bonds. The peaks at 2985 cm⁻¹ and 1630 cm⁻¹ were assigned to the mode of -CH₃ groups and Si-OH bonds, respectively [11]. The band at 1278 cm⁻¹ was designated as Si-CH₃ groups. The intensity of the absorption peaks at 2985 cm⁻¹ and 1278 cm⁻¹ decreased obviously with increases in the calcination temperatures and disappeared as the calcination temperature reached 600 °C, indicating that the -CH₃ groups had been broken down at this temperature. The absorption peak observed at 619 cm⁻¹ corresponded to the Nb-O bonds, which enhanced in intensity with the increase of calcination temperatures.

The FTIR spectra of Nb/SiO₂ materials with various n_Nb calcined at 400 °C are provided in Figure 5. It can be seen from Figure 5 that the hydrophobic Si-CH₃ groups located at 1278 cm⁻¹ were all involved in the Nb/SiO₂ materials with various n_Nb. The absorption peak locations of Nb/SiO₂ materials with n_Nb = 0.08, 0.2, and 1 were roughly the same, but the intensities of individual peaks were different. The peak located at 619 cm⁻¹ corresponding to the Nb-O groups did not exist in the pure SiO₂ materials. This certified that the Nb had been successfully incorporated into the silica frameworks. At the same time, the peak intensity due to the Nb-O groups became strong continuously with the increase of niobium doping.

3.3. Phase Structure Analysis

The XRD patterns of Nb/SiO₂ materials with n_Nb = 1 calcined at various temperatures are given in Figure 6. The broad diffraction peak at the range of about 2θ = 20–30° was assigned to the amorphous SiO₂. An obvious crystallization peak of Nb₂O₅ was detected when the calcination temperature was increased to 600 °C. According to the conclusion from Kosutova, there will be two new crystalline phases formed sequentially when the sample is heated up to 450 °C [20]. The first crystalline phase was identified as hexagonal TT-Nb₂O₅ (JCPDS No-400-028-0317) [21]. The TT notation comes from the German Tief-Tief (low–low), referring to the temperature at which the structure was observed in the sequence of niobium oxides obtained at elevated temperatures, first used in Ref [22]. In Figure 6, the diffraction peaks at 2θ = 22.6°, 28.6°, 36.8°, 46.2°, 50.8°, and 55.2° were assigned to the (001), (180), (201), (002), (380), and (182) crystal planes, respectively, of hexagonal TT-Nb₂O₅, which indicated the formation of hexagonal TT-Nb₂O₅ (JCPDS No.00-030-0873). In Figure 6, another phase transition was observed at 800 °C, which was associated with the transition from hexagonal TT-Nb₂O₅ to orthorhombic T-Nb₂O₅ (JCPDS No. 404-007-0752) [23]. The transformation caused the splitting of the diffraction peaks [24]. It is evident that a high calcining temperature can improve the crystallinity of Nb₂O₅ species.

The XRD patterns of the Nb/SiO₂ materials with various n_Nb calcined at 400 °C are provided in Figure 7. From Figure 7, it can be seen that as the n_Nb ≤ 0.2, the XRD curves of the Nb/SiO₂ materials were similar, and only the diffraction peak between 20° and 30° corresponding to the amorphous SiO₂ could be observed. When the n_Nb was increased to 1, a new diffraction peak assigned to the hexagonal TT-Nb₂O₅ appeared besides for the amorphous SiO₂. This means there was no crystalline form of hexagonal TT-Nb₂O₅ when the Nb-doped content was low. When the niobium doping content and the calcining temperature were large enough, the Nb₂O₅ crystals could be formed in the SiO₂ frameworks.

In order to explore the crystal form of niobium, the Nb/SiO₂ materials with n_Nb = 1 calcined at various temperatures were characterized by XPS, which are shown in Figure 8. In Figure 8, the Nb^TT 3d and Nb^T 3d mean the Nb-O of the hexagonal crystal system TT-Nb₂O₅ and orthorhombic crystal system T-Nb₂O₅, respectively. It can be seen that the Nb^TT 3d_5/2 peak and the Nb^TT 3d_3/2 peak appeared at 206.4 eV and 209.1 eV, respectively, in the sample calcined at 400 °C. The data matched with the species of niobium oxide, which proved the formation of Nb₂O₅ crystals. The temperature related to the formation of the TT-Nb₂O₅ phase was reported to be approximately 500 °C or higher, according to the studies conducted on nanostructures or thin films [24,25]. Furthermore, the calcination temperature reached 600 °C, and the Nb^T 3d_5/2 and Nb^T 3d_3/2 peaks also appeared at 209.0 eV and 211.8 eV, respectively, which were the Nb 3d peaks of orthorhombic phase T-Nb₂O₅. The conclusion was as same as that of XRD analysis.

3.4. SEM Analysis

Figure 9 shows the SEM images of Nb/SiO₂ materials with various n_Nb calcined at various temperatures. From Figure 9a–d, it can be seen that when calcined at 400 °C, the samples had the morphology of nanoparticles with dispersed amorphous structures, and the particle sizes increased with increasing n_Nb.

Comparing Figure 9d with Figure 9e,f, it can be clearly observed that the morphology and particle size of Nb/SiO₂ material with n_Nb = 1 underwent a large change after being calcined at 600 °C, and the materials had a tendency to develop from an amorphous state to a spherical shape. It can be seen from Figure 9f that when the calcination temperature reached 800 °C, the materials exhibited a spherical structure, the particle size increased, and there were more small-sized spherical particles formed around the large particles. In summary, with the increase of calcination temperature and Nb-doping, the particle sizes increased.

Moreover, the SEM images of the pure SiO₂ membrane and Nb/SiO₂ with n_Nb = 0.08 calcined at 400 °C are depicted in Figure 10. From Figure 10 it can be observed that there were no visible cracks or pinholes on the membrane surfaces, indicating that the membranes were well coated. It could be seen that niobium doping could make particle sizes increase.

3.5. Pore Structure Analysis

The pore properties of the as-prepared Nb/SiO₂ materials were investigated by N₂ adsorption/desorption to characterize the surface area, pore volume, and porosity. Figure 11 shows the N₂ adsorption/desorption isotherms of Nb/SiO₂ materials with various n_Nb calcined at 400 °C under an N₂ atmosphere. After calcining, all isotherms displayed similar tendencies, which were similar to a tape I isotherm. It could be proved that the Nb/SiO₂ materials all exhibited the adsorption isotherm characteristics of microporous materials. This material was subjected to a strong force due to the gas in the micropores, so it could quickly reach the absorption saturation state.

Figure 12 manifests the pore size distribution of Nb/SiO₂ materials with various n_Nb calcined at 400 °C. As shown in Figure 12, as n_Nb ≤ 0.2, with the Nb doping, the pore size of Nb/SiO₂ materials were widened, and the microporous structure was maintained. When n_Nb = 1, the Nb/SiO₂ material became dense.

The pore structure parameters of Nb/SiO₂ materials with various n_Nb calcined at 400 °C are shown in

Table 2. Pore structure parameters of Nb/SiO₂ materials with various n_Nb calcined at 400 °C.

nNb	BET/ m2·g−1	Vt/ cm3·g−1	Vmic/ cm3·g−1	Mean Pore Width/nm
0	386.4545	0.1716	0.1115	1.8759
0.08	778.7121	0.4901	0.0867	2.4549
0.2	535.4072	0.4632	0.0748	2.2591
1	86.1599	0.0762	0.0266	1.2176

. It can be observed that with the increases of n_Nb, the mean pore size, BET surface area, and total pore volume increased until n_Nb = 0.08, and then they began to decrease. This is because the bond length of Nb-O is longer than that of Si-O (the bond lengths of Nb-O and Si-O are about 2.13 and 1.64 Å, respectively). The formation of the Nb-O bond helped the formation of larger particles. Thus, with the increase of the Nb doping amount, the particle size increased, and the formed pore grew gradually. However, when n_Nb was >0.08, that changed. Some small hexagonal TT-Nb₂O₅ particles were formed and distributed in the SiO₂ network, which led to a decrease in the mean pore size, BET surface area, and total pore volume

3.6. Gas Permeation and Separation Property Analysis

Based on all the results of the above, the Nb-doping content showed a significant impact on the microstructures of Nb/SiO₂ materials. Compared with the Nb/SiO₂ membrane with n_Nb = 0.08 and n_Nb = 0.2, the pore volume of the Nb/SiO₂ membrane with n_Nb = 1 was too small, which is bad for gas separation. Therefore, the Nb/SiO₂ membrane with n_Nb = 1 was not considered here.

A transient test was conducted on the Nb/SiO₂ membranes with various n_Nb at 25 °C and a differential pressure of around 0.1 MPa, and the gas permeances and H₂ permselectivities are shown in Figure 13. In Figure 13a, the H₂ and N₂ permeances of Nb/SiO₂ membranes with n_Nb = 0.08 and 0.2 were greater than those of the pure SiO₂ membrane, while the change trends of CO₂ permeances were the contrary. Compared with the pure SiO₂ membrane, the H₂ permeances of Nb/SiO₂ membranes with n_Nb = 0.08 and 0.2 increased by 39.0% and 8.9%, respectively. In Figure 13b, the permselectivities of H₂/CO₂ and H₂/N₂ for the pure SiO₂ membrane were greater than the values based on Knudsen diffusion (4.69 and 3.74, respectively), which means the transport in pure SiO₂ membrane is controlled by molecular sieving plus Knudsen diffusion. The H₂/CO₂ and H₂/N₂ permselectivities for the Nb/SiO₂ membranes with n_Nb = 0.08 and 0.2, respectively, were obviously higher than those of the pure SiO₂ membrane. Compared with the pure SiO₂ membrane, the H₂/CO₂ permselectivity for the Nb/SiO₂ membranes with n_Nb = 0.08 and 0.2 increased by 59.5% and 6.3%, respectively. The larger mean pore size and higher pore volume of Nb/SiO₂ membranes with n_Nb = 0.08 and 0.2 than those of the SiO₂ membrane can explain the increase of gas permeance. The larger mean pore size of Nb/SiO₂ membrane indicated that the increase of H₂/CO₂ permselectivity was not due to molecular sieving. This suggests that the doping of Nb introduced another transport mechanism. Some researchers have proposed that doping transition metals into the microporous SiO₂ network will generate Lewis acids on the surface of the membrane materials and ultimately endow the Nb/SiO₂-derived microporous membrane with sufficiently high H₂/CO₂ permselectivity. The exceptionally low permeance of CO₂ is explained as a consequence of strong chemical interactions between CO₂ and the materials of the membrane pore surface, presumably Nb-bound hydroxy groups. The results indicate that the H₂/CO₂ separation was based on sorption rather than on the differences in molecular sizes [26]. In other words, the existence of acid sites on the surface of membranes may play a key role in reducing CO₂ permeance. The H₂/CO₂ and H₂/N₂ permselectivities of the Nb/SiO₂ membrane with n_Nb = 0.08 obtained the maximum. Furthermore, with the further increase of n_Nb, although there are still acid sites in the membrane materials, the formed Nb₂O₅ in the high-content Nb/SiO₂ materials agglomerates to form a non-selective interfacial gap, which will lead to the densification of the membrane materials [13]. Thereby the average pore size will become smaller, resulting in the decrease of gas permeances and H₂ permselectivities of the Nb/SiO₂ membrane.

Figure 14 displays the influence of temperature differences on the gas permeance of the Nb/SiO₂ membrane with various n_Nb at a pressure difference of 0.1 MPa. As shown in Figure 14, the H₂ permeance and H₂/CO₂ permselectivities in the Nb/SiO₂ membranes with different n_Nb revealed an upward trend with increasing temperature when maintaining a constant pressure difference of 0.1 MPa, which indicated that the transport of H₂ molecules through the membranes was activated.

The permeance and permselectivity of the Nb/SiO₂ membrane with n_Nb = 0.08 at a pressure difference of 0.1 MPa and temperature change from 25 °C to 200 °C are manifested in Figure 15. It can be clearly seen that H₂ permeance was significantly enhanced with increasing temperature, and the other permeances of CO₂ and N₂ were reduced slightly. The results were the same as the temperature dependency of permeance of several gases in the NS (Nb/SiO₂) membrane from Boffa [27]. Thus, this gives rise to the permselectivity of H₂/CO₂ and H₂/N₂ elevating with increasing temperature. With increasing temperatures, the H₂ permeance of the Nb/SiO₂ membrane increased gradually, which shows that the permeation behavior of H₂ in the membranes mainly follows an activation–diffusion mechanism. In the case of activated diffusion, molecules penetrate through the micropore while being subjected to a repulsive force from the pore walls, and the molecules with sufficient kinetic energy to overcome the repulsive force can penetrate the pores [16]. Conversely, the permeance of CO₂ and N₂ decreased slightly, similar to the trend of Knudsen diffusion, in which molecules collide with the pore walls more regularly than permeating molecules.

After the previous discussion, we believe that the permselectivity of Nb/SiO₂ membranes has a sorption separation mechanism. High temperature is conducive to the permselectivity of H₂, indicating that the permselectivity mechanism of H₂ to other gases is mainly dominated by activation diffusion, which follows the Arrhenius equation.

$$\mathrm{F}={\mathrm{A}}_0\exp \left(\frac{-{\mathrm{E}}_{\mathrm{a}}}{\mathrm{RT}}\right)$$

(1)

In the formula, F means permeance, A₀ means former factor, E_α means apparent activation energy, R means the ideal gas constant, T means the temperature, and Equation (1) can be described in another form:

$$\mathrm{lnF}={\mathrm{lnA}}_0-\frac{{\mathrm{E}}_{\mathrm{a}}}{\mathrm{RT}}=\mathrm{A}-\frac{{\mathrm{E}}_{\mathrm{a}}}{\mathrm{RT}}$$

(2)

The 1/T is used as the abscissa and lnF as the ordinate to draw the graph. It can be seen from the above formula that it is a straight line in theory. The apparent activation energy can be calculated from the slope of the straight line. Then the Arrhenius curves of the three gases are shown in Figure 16. Moreover, from the slope of the straight-line fitting in Figure 16, the apparent activation energy of the three gases can be obtained: E_m = E_a + Q_st [28]. The results are shown in Table 3.

From

Table 4. E_a, Q_st, and E_m values of gases (H₂, CO₂, and N₂) calculated from the Arrhenius formula for the Nb/SiO₂ membrane with n_Nb = 0.08 at a pressure difference of 0.1 MPa.

Gases	Ea/ kJ·mol−1	Qst/ kJ·mol−1 [28]	Em/ kJ·mol−1
H2	2.53	6	8.53
CO2	−4.28	24	19.72
N2	−4.07	18	13.93

, Q_st means isosteric heat of adsorption, CO₂ exhibits a greater adsorption heat, and the gas with the lowest adsorption heat is H₂. The apparent activation energy (E_a) can be positive or negative, depending on their relative magnitudes. A negative value of E_a is generally interpreted as being caused by strong sorption of the molecule on the pore surface. Such a negative value suggests a high enthalpy of sorption [26]. The mobility energy (E_m) of gas molecules moving on the surface of the Nb/SiO₂ membrane was E_m (CO₂) > E_m (N₂) > E_m (H₂). The permselectivity of the Nb/SiO₂ membrane towards H₂/CO₂ increased rapidly as a function of temperature. This was probably a result of the high activation energy of the mobility of hydrogen and the high heat of sorption of carbon dioxide. This result has never been reported for pure silica [29,30,31,32]. Thus, the strong heat of adsorption should be related to the presence of Nb ions in the microporous framework. Table 3 shows the E_a value of H₂, H₂ permeance, H₂ permselectivities, and mean pore diameter of silica membranes by the sol–gel method from other researchers. From Table 3, it can be observed that it is difficult to improve the H₂ permeance and permselectivity at the same time. In addition, a higher E_a always corresponds to a smaller mean pore diameter and lower H₂ permeance. This means that the E_a value maybe have a link with the mean pore diameter and the interplay between the molecules of H₂ and the pore walls of the membrane. Therefore, compared with other research groups, the Nb-doping in this work may be the reason for the lower E_a value of H₂.

Table 3. E_a of H₂, H₂ permeances, H₂ permselectivities, and mean pore diameter for various SiO₂ membranes prepared by other researchers using the sol–gel process.

Membrane Type	Temperature and Pressure	Ea of H2 (kJ·mol−1)	H2 Permeance (mol·m−2·Pa−1·s−1)	H2 Permselectivities		Mean Pore Diameter (nm)	Ref.
				H2/CO2	H2/N2
SiO2	200 °C, 2 bar	-	4.62 × 10−7	3.7	10.5	0.30–0.54	[33]
SiO2(400)	200 °C, 1 bar	8	17.4 × 10−7	7.5	64	0.38–0.55	[28]
SiO2(600)	200 °C, 2 bar	7.6	4.03 × 10−7	66	-	0.36–0.38	[28]
Pd/SiO2	200 °C, 0.3 MPa	-	7.26 × 10−7	4.3	14	0.57	[34]
Co/SiO2	200 °C, 0.2 MPa	1.98	1.97 × 10−5	10.48	13.08	2.34	[35]
Nb/SiO2 *	200 °C, 0.1 MPa	2.53	4.83 × 10−6	15.49	9.54	2.4549

* In this work.

Table 3. E_a of H₂, H₂ permeances, H₂ permselectivities, and mean pore diameter for various SiO₂ membranes prepared by other researchers using the sol–gel process.

Membrane Type	Temperature and Pressure	Ea of H2 (kJ·mol−1)	H2 Permeance (mol·m−2·Pa−1·s−1)	H2 Permselectivities		Mean Pore Diameter (nm)	Ref.
				H2/CO2	H2/N2
SiO2	200 °C, 2 bar	-	4.62 × 10−7	3.7	10.5	0.30–0.54	[33]
SiO2(400)	200 °C, 1 bar	8	17.4 × 10−7	7.5	64	0.38–0.55	[28]
SiO2(600)	200 °C, 2 bar	7.6	4.03 × 10−7	66	-	0.36–0.38	[28]
Pd/SiO2	200 °C, 0.3 MPa	-	7.26 × 10−7	4.3	14	0.57	[34]
Co/SiO2	200 °C, 0.2 MPa	1.98	1.97 × 10−5	10.48	13.08	2.34	[35]
Nb/SiO2 *	200 °C, 0.1 MPa	2.53	4.83 × 10−6	15.49	9.54	2.4549

* In this work.

The pressure dependence of various gas permeances and H₂/CO₂ permselectivities in the pure SiO₂ membrane and Nb/SiO₂ membrane with various n_Nb at 200 °C were further investigated in the pressure difference range from 0.10 MPa to 0.40 MPa, which is shown in Figure 17. It could be observed that H₂ permeance with various n_Nb increased with the pressure-dependence increase. This is because the pressure difference elevated, and the gas force increased, resulting in an elevation in the gas concentration in the membrane, and then leading to higher H₂ permeance. As seen in Figure 17b, the permselectivity of H₂/CO₂ changed slightly with increases in the pressure difference.

Figure 18 demonstrates the effect of pressure differences on the permeance and permselectivity of different gases for the Nb/SiO₂ membrane with n_Nb = 0.08 and a temperature at 200 °C. As seen in Figure 18a, as the pressure difference increased from 0.10 to 0.40 MPa, all of the gas permeances increased. The reason is that the increases in intake pressure resulted in an elevation of the gas concentration, thus yielding a higher permeance. Furthermore, the relationship between permselectivity and pressure differences is shown in Figure 18b. For example, with the pressure increasing from 0.10 MPa to 0.40 MPa, the permeance of N₂ and CO₂ slightly increased. This was due to the small influence of pressure on Knudsen diffusion, as previously reported in the literature [30]. Hence, no matter how high the pressure was, the change in permselectivity would not increase significantly.

Traditional SiO₂ membranes have poor hydrothermal stability due to a large amount of Si-OH groups on their surfaces, which easily absorb water vapor in the air. In the Nb/SiO₂ membrane, not only the Nb-doping but also the introduced hydrophobic groups can improve the vapor stability, which can reduce the hydroxyl groups on the pore surface and enhance the hydrophobicity. In order to test the hydrothermal stability of membranes, the Nb/SiO₂ membranes with n_Nb = 0 and 0.08 were chosen to investigate the gas permeance before and after steam treatment.

Figure 19 shows the permeances of various gases (H₂, CO₂, and N₂) and H₂ permselectivities of Nb/SiO₂ membranes with n_Nb = 0 and 0.08 at 25 °C and a pressure difference of 0.1 MPa before and after steam treatment and regeneration. As shown in Figure 19, compared with the fresh membranes, the H₂ permeances of SiO₂ and Nb/SiO₂ membranes after steam treatment decreased by 17.90% and 6.68%, respectively, while their H₂/CO₂ permselectivities decreased by 3.2% and increased by 1.6%, respectively. After regeneration by calcination at 350 °C, the gas permeances and the permselectivities of H₂/CO₂ and H₂/N₂ for the two membranes showed an upward trend. Compared with the fresh membranes, the H₂ permeances of SiO₂ and Nb/SiO₂ membranes after regeneration decreased by 10.95% and 3.21%, respectively, while their H₂/CO₂ permselectivities increased by 2.8% and 2.1%, respectively. The above results indicate that niobium doping improves the hydrothermal stability of SiO₂ membranes.

4. Conclusions

To sum up, using the sol-gel technique, Nb/SiO₂ materials and membranes with various n_Nb were successfully synthesized. Their microstructures and gas permeances were investigated. The results showed that the niobium element existed in the formation of the Nb-O groups in the Nb/SiO₂ materials. As the niobium doping content and the calcining temperature were high enough, the Nb₂O₅ crystals could be formed in the SiO₂ frameworks. With the increase of n_Nb, the formed particle sizes increased, and the mean pore size, BET surface area, and total pore volume also increased until n_Nb = 0.08, and then they began to decrease. The doping of Nb could enhance the H₂/CO₂ and H₂/N₂ permselectivities of the SiO₂ membrane. When n_Nb was equal to 0.08, the Nb/SiO₂ membrane achieved a maximal H₂ permeance of 4.83 × 10⁻⁶ mol·m⁻²·Pa⁻¹·s⁻¹ and H₂/CO₂ permselectivity of 15.49 at 200 °C and 0.1MPa, which increased by 36.7% and 155.47%, respectively, compared with that of the pure SiO₂ membrane. Compared with the fresh membranes, the H₂ permeances of SiO₂ and Nb/SiO₂ membranes after steam treatment decreased by 17.90% and 6.68%, respectively, while their H₂/CO₂ permselectivities decreased by 3.2% and increased by 1.6%, respectively. After regeneration, the gas permeances and the permselectivities of H₂/CO₂ and H₂/N₂ for the two membranes showed an upward trend. Compared with the fresh membrane, the H₂ permeances of SiO₂ and Nb/SiO₂ membranes after regeneration decreased by 10.95% and 3.21%, respectively, while their H₂/CO₂ permselectivities increased by 2.8% and 2.1%, respectively. Niobium doping improved the hydrothermal stability of the SiO₂ membrane. The Nb/SiO₂ membranes also exhibited great thermal reproducibility.