Electrospinning Equipment: Cost-effective Fabrication of Mesoporous Alumina Nanofibers via Electrospinning

Mesoporous alumina nanofibers have attracted significant attention due to their wide applications in catalysis, refractories, pollutant absorption, composite reinforcement, and sound or thermal insulation. Alumina fibers play a crucial role in environmental protection, especially as catalyst supports in flameless combustion systems. However, traditional alumina fibers suffer from issues such as high brittleness, poor flexibility, and low specific surface area, which limit their application scope. Research has shown that reducing the fiber diameter and preventing the transformation of alumina into the stable α - Al₂O₃ crystalline phase can significantly improve the physical and chemical properties of the fibers. Additionally, silicon, as an additive, can delay the phase transformation of alumina to α - Al₂O₃, thereby stabilizing the metastable phase and further enhancing the fiber performance.

Electrospinning, a synthesis technique with controllable parameters via electrospinning machine, has great potential in the preparation of alumina fibers. However, compared with the preparation of pure polymer fibers, preparing ceramic fibers via electrospinning is more challenging. The key point for optimization lies in how to increase the ceramic/polymer precursor ratio to enhance the strength, flexibility, and continuity of the fibers. Currently, there are few studies using boehmite as a precursor. This study aims to prepare mesoporous alumina nanofibers via electrospinning with a high ceramic/polymer ratio by using boehmite sol and PVA, in order to reduce costs and improve fiber quality, while exploring its potential in practical applications.

In this study, boehmite sol and PVA were selected as the main precursors to prepare γ - Al₂O₃ nanofibers, which is cost - effective. Traditional preparation methods often use expensive and toxic precursors such as aluminum isopropoxide (AIP), which are not suitable for large - scale production. The innovation of this study lies in the use of inexpensive boehmite sol, which effectively reduces the production cost. In the experimental process, a spinning sol was successfully prepared by mixing 17% boehmite sol and 8% PVA solution at a boehmite/PVA weight ratio of 3. Then, the sol was loaded into a 5 mL plastic syringe and placed in a lab - scale electrospinning machine (made by Nanoazma Iranian Company) for electrospinning, and electrospinning was carried out smoothly (for experimental steps and parameter settings, see Figure 1 and Table 1). This process overcame the problem of low ceramic/polymer ratios in the synthesis of ceramic nanofibers via electrospinning, providing the possibility for large - scale production.

Figure 1. Schematic diagram of the synthesis steps of alumina nanofibers

Table 1. Experimental details of the spinning solutions and calcination temperatures

The nanofibers exhibited a high specific surface area of 204 m²/g and a controllable pore size distribution. Through BET analysis, the BS - 950 sample (containing silica sol and calcined at 950 °C) was tested, and its specific surface area was 204.19 m²/g (see Table 2). From the N₂ adsorption and desorption isotherms and BJH pore size curves in Figure 2, it can be seen that the pore size of the nanofibers is mainly distributed in the range of 1 - 10 nm, belonging to mesoporous fibers, and the pore size distribution is relatively narrow, indicating a controllable pore size distribution. These characteristics of high specific surface area and suitable pore size distribution endow the nanofibers with potential application value in fields such as catalyst supports.

Table 2. BET specifications

Figure 2. a,c,e) Nitrogen adsorption and desorption curves of BS fibers calcined at different temperatures and b,d,f) pore size distribution

A strong interaction between boehmite and PVA was found, which led to incomplete carbon removal even at a calcination temperature of 950 °C. STA analysis showed that the thermal behavior of sample B (boehmite/PVA) was significantly different from that of PVA. During the thermal decomposition of PVA, a broad endothermic peak appeared at 190 - 410 °C, corresponding to the melting and dehydration processes of PVA. In contrast, sample B had a sharp exothermic peak at 170 °C, which was due to the formation of Al - O - C bonds between boehmite particles and PVA (see Figure 3). The formation of these bonds and the hydrogen bonds between them changed the decomposition process of PVA, delaying its decomposition and resulting in incomplete carbon removal. FTIR analysis also provided evidence for this interaction. In the composite nanofibers (samples B and BS), the peaks of some functional groups of PVA and boehmite changed. For example, the peaks at 847 cm⁻¹ (long - chain rocking - bending vibration) and 1095 cm⁻¹ (alcohol C - O single bond stretching vibration) of PVA and 545 cm⁻¹ (AlO6 octahedral stretching vibration) of boehmite disappeared (see Figure 4), indicating that a chemical reaction occurred between them, forming new chemical bonds and thus affecting each other's thermal behavior.

Figure 3. STA diagrams of a) PVA, b) BS, c) B, d) BS - 950 nanofibers

Figure 4. FTIR spectra of uncalcined PVA, boehmite, boehmite/PVA, and boehmite/PVA/silica nanofibers before calcination

The coexistence of boehmite and PVA not only affected the thermal decomposition of PVA but also changed the thermal behavior of boehmite, delaying the release of hydroxyl groups in boehmite and postponing the phase transformation of alumina. XRD analysis showed that in the samples without silica, γ - AlOOH transformed into γ - Al₂O₃ at 550 °C. As the temperature increased to 950 °C, the change in the peak shape indicated an increase in crystallinity and a reduction in structural defects (see Figure 5). However, the XRD patterns of the silica - containing BS samples showed no significant changes in the range of 550 - 950 °C, indicating that the addition of silica sol inhibited the grain growth of alumina and stabilized the γ - Al₂O₃ phase. Combined with STA analysis, the peaks in the DTA curve of sample BS were wider and had lower intensities, indicating that the presence of silica created an energy barrier for the transformation from low - temperature phases to high - temperature phases, delaying the dehydration process of boehmite and the phase transformation of alumina, and enhancing the thermal stability of the γ - Al₂O₃ phase.

Figure 5. Field - emission scanning electron microscopy (FESEM) micrographs of a) B - 550 b) B - 750 c) B - 950 d) BS - 550 e) BS - 750 f) BS - 950 nanofibers (at 25 kx)

The preparation method of this study has significant advantages in obtaining and stabilizing the transitional alumina phase with a high specific surface area and small crystallite size, and can be used as a catalyst support with high thermal stability. γ - Al₂O₃ is widely used in heterogeneous catalytic reactions due to its defective (active) sites and high surface area. By controlling the preparation conditions and adding silica sol, the nanofibers prepared in this study have a high specific surface area and a stable γ - Al₂O₃ phase. When used as a catalyst support, they can provide a good surface for the dispersion and anchoring of active metal catalyst particles, improving the performance and stability of the catalyst and meeting the requirements of practical catalytic applications. In addition, compared with the expensive precursors (such as AIP) and pore - forming agents (such as P123 and P127) used in other studies, the precursors of this study are inexpensive and easily accessible. During the preparation process, a relatively high ceramic/polymer ratio (boehmite/PVA weight ratio of 3) can be achieved, improving production efficiency and product quality. Judging from the experimental results, the prepared nanofibers have excellent performance and the potential for large - scale production.

In general, certain experimental results have been achieved in the preparation of nanofibers. However, the calcination process still has an important impact on the microstructure and properties of the fibers. It is necessary to further optimize the calcination parameters, such as temperature and heating rate, to continuously improve the quality and performance of the fibers. At the same time, evaluating the practical application performance of the fibers in catalytic reactions, such as catalytic activity, selectivity, and stability, is crucial for determining their practical value in the industrial catalytic field, which provides a clearer direction for subsequent research and applications.

Electrospinning Nanofibers Article Source: https://doi.org/10.1016/j.ceramint.2025.01.550