Facile Preparation and Promising Hydrothermal Stability of Spherical γ-Alumina Support with High Specific Surface Area

Abstract

It is of great importance to develop a spherical γ-alumina support with high hydrothermal stability to be used in platinum reforming catalyst processes. The porous pseudo-boehmite powder with a high surface area was first synthesized via a simple separate nucleation and aging steps method, and was then used as a precursor to produce a spherical γ-Al₂O₃ support via an oil–ammonia column method. The as-synthesized pseudo-boehmite has a substantially greater specific surface area of 336.0 m²·g⁻¹ in comparison with the commercial Sasol boehmite powder (293.0 m²·g⁻¹) from Sasol Chemicals. In addition, the as-prepared spherical γ-Al₂O₃ support derived from the as-synthesized pseudo-boehmite also possesses a higher specific surface area of 280.0 m²·g⁻¹ compared to the corresponding Sasol sample. Moreover, the as-prepared spherical γ-Al₂O₃ balls demonstrate a much higher specific surface area of 185.0 m²·g⁻¹ compared with the Sasol sample of 142.0 m²·g⁻¹ after hydrothermal tests at 600 °C, suggesting its promising application in the chemical industry.

1. Introduction

Petroleum-based fossil fuels are still the most widely used energy source, and heterogeneous catalysts are indispensable in the petrochemical industry, particularly in the platinum reforming catalyst process [1,2]. Because of its high specific surface area and distinctive pore structure, γ-Al₂O₃ is by far the most commonly utilized inorganic material as a catalyst or catalytic support for heterogeneous catalysis [3,4,5,6]. Spherical γ-Al₂O₃ balls have attracted significant attention due to their uniform morphology, large specific surface area, small bulk density, and excellent fluidity [7,8].

Unfortunately, γ-Al₂O₃ can easily adsorb water molecules and can be transformed into hydrated alumina in water steam at high temperatures, giving rise to the reduction in the specific surface area and destruction of the pore structure [9]. Many methods, such as element doping [10,11,12], and optimization of the synthesis process [13,14,15,16], have been adopted to improve the hydrothermal stability of γ-Al₂O₃. For example, López Pérez et al. [17] developed a condensation-enhanced self-assembly and pyrolysis crystallization method to produce transition alumina with enhanced hydrothermal stability. Moreover, Fujisaki et al. [18] employed a sol-gel method to prepare La₂O₃- and/or Ga₂O₃-doped γ-Al₂O₃, and revealed that La₂O₃ on the grain surface of Ga₂O₃-Al₂O₃ solid solution improved the hydrothermal stability of γ-Al₂O₃. Additionally, Gu et al. [19] adopted a chemical vapor deposition method to fabricate silica-doped alumina with high hydrothermal stability at 873 K. However, most of the reported methods involve complicated preparation processes or some impurities, which may cause a negative effect on the catalytic performance. Therefore, it is desirable to develop a simple method to prepare spherical γ-Al₂O₃ balls with high hydrothermal stability.

Herein, a simple process composed of the preparation of pseudo-boehmite (PB) with a high specific surface area and subsequent modeling of pseudo-boehmite into spherical γ-Al₂O₃ balls was proposed. At first, pseudo-boehmite powder with a specific surface area of 336.0 m²·g⁻¹ was synthesized via a simple separate nucleation and aging steps method developed in our laboratory [20,21,22], which can be used to prepare nanomaterials on a large scale, e.g., 3500 t/a layered double hydroxides. In this method, all the nuclei were formed in a few seconds, and then the nuclei were crystallized and grown at the same time. Subsequently, pseudo-boehmite sol was prepared and dropped into an oil-ammonia column to form pseudo-boehmite gel balls, which were then dried and calcined at 600 °C to produce spherical γ-Al₂O₃ balls. Promisingly, the obtained spherical γ-Al₂O₃ balls possess the advantages of a high specific surface area, and excellent hydrothermal stability, indicating their hopeful application in the platinum reforming catalyst process.

2. Results and Discussion

2.1. Morphologies and Structure of PB Powder

Figure 1 shows SEM and TEM images of PB and Sasol boehmite (SB) samples. PB powder (Figure 1a,c) consists of loosely stacked nanoparticles and an abundant pore structure, and some small particles assemble into nanofibers. In contrast, SB powder is composed of tightly integrated nanoparticles with less pores (Figure 1b,d).

Powder X-ray diffraction (XRD) pattern measurements were performed to investigate the crystal structure of PB and SB powder, and Figure 2a displays the powder XRD patterns of both samples. Here, there are six diffraction peaks that appear at 14.39, 28.13, 38.27, 48.93, 54.94 and 63.69°/2θ in the XRD pattern of SB powder, which are consistent with the (020), (201), (130), (002), (151) and (200) diffraction peaks of pseudo-boehmite [23]. One can observe that the XRD patterns of PB are similar to those of SB, suggesting that the as-synthesized PB has the same crystal structure as the SB powder. Fourier transform infrared spectroscopy (FT-IR) characterization was conducted to further investigate the composition of PB. As depicted in Figure 2b, the spectrum of PB is also similar to that of SB powder, indicating that PB and SB have the same characteristic functional groups. Specifically, the two broad bands at 3400 and 3093 cm⁻¹ can be assigned to the stretch vibration of the hydroxyl groups. The absorption band at 1078 cm⁻¹ is attributed to the symmetrical bending vibration of the Al-O-H group, and the two adsorption bands at 626 and 485 cm⁻¹ could be ascribed to the vibration of the Al-O bond.

2.2. Pore Structure of PB Powder

The pore structure of PB and SB was evaluated by the low-temperature N₂ adsorption–desorption method. Figure 3a demonstrates that the PB sample has similar adsorption and desorption isotherms and a similar type H3 hysteresis loop to the SB sample, suggesting the presence of abundant mesopores in both PB and SB. The calculated Brunauer–Emmett Teller (BET) specific surface area is ca. 336.0 m²·g⁻¹ for the as-synthesized PB, which is larger than that of SB powder (293.0 m²·g⁻¹). Figure 3b shows the pore size distribution curves of PB and SB obtained according to the Barret–Joyner–Halenda (BJH) method. In comparison, the as-prepared PB sample has narrower pore size distribution, ranging from 3 to 8 nm, in comparison to the SB sample’s distribution from 2 to 10 nm, possibly resulting from its small and homogeneous particle size. The results suggest that the separate nucleation and aging steps method is useful to produce porous pseudo-boehmite with a large specific surface area and narrow pores.

2.3. Morphologies and Structure of Spherical γ-Al₂O₃ Balls

Spherical γ-Al₂O₃ balls, marked as Al₂O₃-I, were modeled using an oil–ammonia column method with the PB powder as the precursor, and calcined at 600 °C under air atmosphere for 4 h. Figure 4a,b show the photographs of Al₂O₃-I from the as-prepared PB sample and commercial γ-Al₂O₃-S balls from Sosal Chemicals, respectively. TheAl₂O₃-I balls possess uniform sizes with diameters of 2.0 + 0.1 mm, and smooth surfaces. SEM characterization was further carried out to investigate the microstructures of the obtained γ-Al₂O₃ balls. Figure 4c,d display the scanning electron microscope (SEM) images of Al₂O₃-S with smooth outside surfaces and porous inside structures, which are different from those of Al₂O₃-I, as shown in Figure 4e,f. One can observe that Al₂O₃-I consists of many particles, and its porous structure may be a result of the space among neighboring particles.

Figure 5 depicts the XRD patterns of the powder ground from both of the Al₂O₃-I and Al₂O₃-S samples, respectively. Evidently, the diffraction peaks of PB and SB disappear, while a series of new diffraction peaks ascribed to γ-Al₂O₃ appear, and the corresponding hkl values are marked on the graph, suggesting the successful transformation from pseudo-boehmite precursors into γ-Al₂O₃ after calcination at 600 °C.

2.4. Hydrothermal Stability of γ-Al₂O₃

The hydrothermal stability of γ-Al₂O₃ as a catalyst support is crucial for its application in the platinum reforming catalyst process [24,25]. During this catalytic process, the catalysts have worse catalytic activity and selectivity when the surface area is below 140 m²·g⁻¹. Figure 6 shows the low-temperature nitrogen adsorption–desorption isotherms (a, b), and the corresponding pore size distribution curves (c, d) for spherical Al₂O₃-I (a, c) and Al₂O₃-S (b, d) before and after hydrothermal treatment at 600 °C, under water vapor. One can observe that the adsorption isotherms of Al₂O₃-I and Al₂O₃-S after hydrothermal treatment are similar to those of Al₂O₃-I and Al₂O₃-S before hydrothermal treatment. However, the desorption isotherms are somewhat different. The beginning desorption relative pressures of the samples after hydrothermal tests are slightly higher than the corresponding initial samples, which implies that the pore size becomes larger after hydrothermal treatment. The pore size distribution curves in Figure 6c,d further confirm this result. The pore size of Al₂O₃-I increases from 7.2 nm to 10.80 nm after hydrothermal treatment for 48 h. Meanwhile, the pore size of Al₂O₃-S increases from 7.2 nm to 8.70 nm. Moreover, the pore size distribution curves of both compounds show minimal changes after the first cycle, indicating that the pore structures remain stable after 48 h of hydrothermal treatment.

In addition, Figure 7 further demonstrates the BET specific surface areas of Al₂O₃-I and Al₂O₃-S after hydrothermal treatment. After hydrothermal treatment (one cycle is48 h), the treated Al₂O₃ balls were calcinated at 600 °C for 4 h again for repeated hydrothermal treatment. The specific surface areas of Al₂O₃-I and Al₂O₃-S decrease from 280.0 m²·g⁻¹ to 206.0 m²·g⁻¹ and from 209.5 m²·g⁻¹ to 180.0 m²·g⁻¹, respectively, after the first cycle. One can observe that the specific surface areas of Al₂O₃-I and Al₂O₃-S decrease much more slowly in the second cycle in comparison with the first one, and then become more stable. Finally, the specific surface area of Al₂O₃-I remained at 185.0 m²·g⁻¹ after 4 cycles for 192 h, which is much higher than that of Al₂O₃-S for 142.0 m²·g⁻¹, indicating that Al₂O₃-I derived from the pseudo-boehmite prepared by the separate nucleation and aging steps method can retain a much larger specific surface area in comparison with the commercial powder from Sasol Chemicals. The larger specific surface area improves the catalytic performance of catalysts based on γ-Al₂O₃ supports, suggesting the promising application of Al₂O₃-I in the chemical industry.

3. Materials and Methods

3.1. Chemicals

Analytical grade aluminum sulfate (Al₂(SO₄)₃·18H₂O), sodium metaaluminate (NaAlO₂), ammonia, nitric acid and alcohol were used without further purification. The commercial SB powder and Al₂O₃-S balls were supplied by Sasol Chemicals.

3.2. Synthesis of Pseudo-Boehmite

Pseudo-boehmite (PB) was synthesized via a simple separate nucleation and aging steps method by using Al₂(SO₄)₃·18H₂O and NaAlO₂ as the reagents. First, 6.665 g (0.01 mol) of Al₂(SO₄)₃·18H₂O and 4.918 g (0.06 mol) of NaAlO₂ were dissolved in 100 mL of deionized water to form homogenous solutions, respectively. A heating mantle was used to heat Al₂(SO₄)₃ solution to 85 °C, and then the Al₂(SO₄)₃ and NaAlO₂ solutions were pumped into a rotating liquid membrane reactor at the same rate. The obtained mixture was transferred into a three-necked flask and stirred for 6 h at 100 °C. The PB precipitates were dried at 60 °C for 24 h after washing and centrifugating repeatedly with deionized water and alcohol.

3.3. Preparation of Spherical γ-Al₂O₃ Balls

Spherical γ-Al₂O₃ balls were prepared via an oil–ammonia column modeling method. Specifically, 12.0 g of the as-synthesized PB was dispersed in 28.0 mL of deionized water by magnetic stirring for 0.5 h, and the suspension was heated to 100 °C. After this, 21.0 g of dilute nitric acid solution (3 wt%) was added dropwise into the suspension under vigorous stirring to achieve a H⁺: Al³⁺ ratio of 0.05: 1. To obtain pseudo-boehmite sol, the resultant slurry was continuously stirred for 4 h at 100 °C. After cooling down to ambient temperature, the as-prepared sol was dropped into an oil–ammonia column by an injector, and the formed gel spheres were aged for 4 h to produce spherical pseudo-boehmite gel balls. The formed gel balls were washed with deionized water 8 times, followed by drying overnight at 90 °C. Finally, the as-prepared spherical pseudo-boehmite gel balls were calcined at 600 °C for 4 h to form spherical γ-Al₂O₃ balls, which were denoted as Al₂O₃-I.

3.4. Hydrothermal Stability Evaluation

The hydrothermal stability of spherical γ-Al₂O₃ balls was tested at 600 °C under a continuous water vapor steam in a tube furnace, and a portion of the balls were taken out for characterization every 8 h. After testing for 48 h, the spherical γ-Al₂O₃ balls were removed and then calcined at 600 °C for 4 h. The testing and calcination cycle was repeated three times.

3.5. Characterizations

Powder X-ray diffraction (XRD) patterns of the samples were recorded using an Ni-filtered Cu-Kα radiation (Rigaku D/max-Ultima III) X-ray diffractometer with a scan speed of 10° min⁻¹. Morphologies of the samples were investigated by a scanning electron microscope (SEM, Zeiss Supra 55) and high-resolution transmission electron microscopy (TME, JEOL JEM-2010 with an accelerating voltage of 200 kV). Fourier transform infrared spectroscopy (FT-IR) spectra of the samples were recorded on a FT-IR spectrometer (VECTOR 22, 4000–400 cm⁻¹). Pore structures were analyzed using the low-temperature nitrogen adsorption–desorption method and a Micromeritics ASAP 2390 volumetric adsorption analyzer.

4. Conclusions

In summary, pseudo-boehmite powder with a high specific surface area was successfully synthesized using the separate nucleation and aging steps method and spherical γ-Al₂O₃ balls with a high specific surface area were then obtained by an oil–ammonia column modeling method, using the obtained pseudo-boehmite as the precursor. The as-synthesized pseudo-boehmite has a high BET specific surface area of 336.0 m²·g⁻¹ and a pore size of 3.72 nm, giving rise to the high specific surface area of γ-Al₂O₃ (280.0 m²·g⁻¹). Meanwhile, the prepared γ-Al₂O₃ balls retain a specific surface area of 185.0 m²·g⁻¹ after four hydrothermal treatment cycles (192 h), which is much larger than that of the commercial powder from Sasol Chemicals (140.0 m²·g⁻¹). These results show the promising applications of Al₂O₃ as a catalyst support in the chemical industry.