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Naoki Toyama, Takehiro Nagashima, Kaori Wada, Kenzo Deguchi, [Shinobu Ohki](https://orcid.org/0000-0002-7357-3833), [Yuuki Mogami](https://orcid.org/0000-0002-9807-3165), [Masataka Tansho](https://orcid.org/0000-0001-7986-3199), [Atsushi Goto](https://orcid.org/0000-0002-9472-4098), Shigeki Furukawa

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[Synthesis of mesoporous SiO2–Al2O3 hollow spheres using ultrasonic irradiation and their activity for hydrolysis of ammonia borane](https://mdr.nims.go.jp/datasets/94885a4d-527e-409e-b57b-d4f24edb27e2)

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Synthesis of mesoporous SiO2–Al2O3 hollow spheres using ultrasonic irradiation and their activity for hydrolysis of ammonia boraneResearch articleSynthesis of mesoporous SiO2–Al2O3 hollow spheres using ultrasonic irradiation and their activity for hydrolysis of ammonia boraneNaoki Toyama a,* , Takehiro Nagashima a, Kaori Wada b, Kenzo Deguchi c, Shinobu Ohki c,  Yuuki Mogami c, Masataka Tansho c, Atsushi Goto c , Shigeki Furukawa aa Department of Sustainable Engineering, College of Industrial Technology, Nihon University, 1-2-1, Izuni-cho, Narashino, Chiba 275-8575, Japanb Department of Applied Chemistry, Institute of Science and Engineering, Chuo University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551, Japanc National Institute for Materials Science, 3-13, Sakura, Tsukuba, Ibaraki 305-0003, JapanA R T I C L E  I N F OKeywords:MesoporousHollow spheresSilica–aluminaAmmonia boraneUltrasonic irradiationA B S T R A C TIn this study, mesoporous silica–alumina (SiO2–Al2O3) hollow spheres (HSs) were synthesized through ultrasonic irradiation and evaluated in terms of their activity for the hydrolysis of ammonia borane (NH3BH3). The mesoporous SiO2–Al2O3 HSs were formed using a sol–gel method under ultrasonic irradiation using polystyrene particles as templates. We synthesized the samples at ultrasonic irradiation times for 0, 3, and 6 h. The morphologies of the samples were observed using transmission electron microscopy. The shell thickness of the samples increased with increasing ultrasonic irradiation time. In 27Al solid-state nuclear magnetic resonance spectra, the sample with ultrasonic irradiation for 6 h showed the highest proportion of 4-coordinated aluminum species attributed to aluminum in the silica network and the lowest proportion of 5-coordinated aluminum species attributed to amorphous aluminum. The activity for hydrolysis of NH3BH3 was determined at room temperature in the presence of the samples with ultrasonic irradiation for 0, 3, and 6 h. According to the results, 7.7-, 10.6-, and 12.3-mL hydrogen were finally generated in the presence of the samples with ultrasonic irradiation for 0, 3, and 6 h, respectively. These results indicate that the amount of hydrogen evolution increased with increasing ultrasonic irradiation time. Furthermore, the amount of hydrogen evolution depended on the number of acid sites of the samples measured using the neutralization titration method. Meanwhile, the reaction time to complete hydrogen generation increased as the ultrasonic irradiation time of the samples increased. Therefore, we measured the pore size distribution of the samples using N2 adsorption–desorption measurement. The samples with ultrasonic irradiation for 0, 3, and 6 h had peaks at approximately 2.4, 2.1, and a value smaller than 1.4 nm nm, respectively. From these results, the mesopore size of the samples decreased with increasing ultrasonic irradiation time. This result might be related to the number of mesopores formed on the shell of the HSs.1. IntroductionAmorphous silica–alumina (SiO2–Al2O3) is widely utilized solid acid catalyst for various chemical reactions, such as hydrocracking, isomerization, and alkylation in the petrochemical industry [1–6]. Amorphous SiO2–Al2O3 forms acid sites when aluminum (Al) substitutes for silicon (Si) in the SiO2 structure. The selectivity and reactivity of catalytic activities depend on the strength and number of acid sites of amorphous SiO2–Al2O3 [6]. On this basis, the development of technologies that can design and synthesize various structures of acid sites is promising. For instance, amorphous SiO2–Al2O3 has a random structure, producing various acid sites [6].Our group previously applied amorphous SiO2–Al2O3 for the hydrolysis of ammonia borane (NH3BH3) [7,8]. NH3BH3 has attracted attention as a chemical hydrogen storage material because of its high hydrogen storage density (19.6 mass%), nontoxicity, and chemical stability [9–11]. Hydrogen gas was obtained through a hydrolysis reaction from NH3BH3 at room temperature using catalysts [12–16]. Recently, it has been reported that catalysts with hollow and yolk-shell structures show high activity for this reaction [17–20]. We found that amorphous SiO2–Al2O3 hollow spheres (HSs) show much higher activity than SiO2–Al2O3 fine particles and spherical SiO2–Al2O3 particles [21]. * Corresponding author.E-mail address: toyama.naoki@nihon-u.ac.jp (N. Toyama). Contents lists available at ScienceDirectNext Materialsjournal homepage: www.sciencedirect.com/journal/next-materialshttps://doi.org/10.1016/j.nxmate.2025.100777Received 1 February 2025; Received in revised form 9 April 2025; Accepted 27 May 2025  Next Materials 8 (2025) 100777 Available online 4 June 2025 2949-8228/© 2025 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ ). https://orcid.org/0000-0002-7925-3240https://orcid.org/0000-0002-7925-3240https://orcid.org/0000-0002-9472-4098https://orcid.org/0000-0002-9472-4098mailto:toyama.naoki@nihon-u.ac.jpwww.sciencedirect.com/science/journal/29498228https://www.sciencedirect.com/journal/next-materialshttps://doi.org/10.1016/j.nxmate.2025.100777https://doi.org/10.1016/j.nxmate.2025.100777http://crossmark.crossref.org/dialog/?doi=10.1016/j.nxmate.2025.100777&domain=pdfhttp://creativecommons.org/licenses/by/4.0/Additionally, we revealed that the hydrolysis of NH3BH3 depends on the acid sites of samples [22,23]. From the viewpoint of synthetic material, it is important to determine the acid sites structure of the SiO2–Al2O3 HSs, but this problem remains a difficult challenge because of the amorphous phases. Thus, the observation of atomic structures is a key factor in this respect.Solid-state nuclear magnetic resonance (NMR) spectroscopy is a useful characterization tool because it allows the nondestructive assessment of information about the chemical environment of chosen probe nuclei. Moreover, contrary to X-ray diffraction (XRD) and other diffraction-based techniques, solid-state NMR spectra can be successfully recorded for crystalline, nanocrystalline, or amorphous phases, allowing for the characterization of atomic environments even in structurally disordered and heterogeneous materials [24–26]. In particular, solid-state 27Al NMR has been used in this way for many years in studies of SiO2–Al2O3 system zeolite alumina and related materials [27–29]. In general, amorphous SiO2–Al2O3 reportedly has 4-, 5-, and 6-coordinated Al [4,6], and the 4-coordinated Al is related to the acid sites of SiO2–Al2O3 because it indicates Al substituted into the SiO2 structure.In this study, we synthesized mesoporous SiO2–Al2O3 HSs using the sol–gel method under ultrasonic irradiation. Recently, various inorganic materials have been synthesized using ultrasonic irradiation [30,31]. In particular, metal oxide nanoparticles with good crystallinity have been synthesized using ultrasonic irradiation because the ultrasonic cavitation produced by ultrasonic irradiation provides a special reaction field [32–35]. In addition, we had problems with the dispersion of Al species. We tried to improve the dispersion of Al species of the SiO2–Al2O3 HSs by changing the solvent and the starting material of Al2O3 precursors. However, there is still further improvement. Ultrasonic irradiation can improve the dispersion of the particles and its related precursors [36, 37]. We can expect to increase the number of Al species into the SiO2 structure.Herein, we reported the effect of ultrasonic irradiation time on the morphology and physicochemical properties of mesoporous SiO2–Al2O3 HSs and their activity for the hydrolysis of NH3BH3. Solid acids containing the mesoporous SiO2–Al2O3 HSs are inexpensive compared to metal catalysts. However, the amount and rate of hydrogen generation from hydrolysis of NH3BH3 in the presence of solid acids exhibited low activity [38]. The mesoporous SiO2–Al2O3 HSs with ultrasonic irradiation can be exhibited the high activity for hydrolysis of NH3BH3 because of the improved dispersion of Al leading to increase the number of acid sites. These studies may be expected to be used as a portable hydrogen source for fuel cells in the future.2. Experiment2.1. Synthesis of mesoporous SiO2–Al2O3 HSsMonodisperse polystyrene (PS) template particles were prepared via emulsifier-free emulsion polymerization. Specifically, styrene (27 mL, Kanto Chem. Co., >99.0 %), poly(vinyl pyrrolidone) K30 (4.50 g, Fluka, MW ≈ 40,000), cationic initiator 2,2′-azodiisobutyramidine dihydrochloride (0.78 g, Wako Pure Chemical, >97.0 %), and ion-exchanged water (300 mL) were charged into a 500-mL three-necked flask equipped with a mechanical stirrer, a nitrogen (N2) inlet, and a Graham condenser. The reaction solution was deoxygenated by bubbling N2 gas through the solution at room temperature for 1 h. The solution in the three-necked flask was stirred at 343 K for 24 h at 200 rpm in an oil bath with a temperature controller. The obtained white suspension was added to ion-exchanged water (180 mL) and ethanol (120 mL). This mixture suspension (40 mL) was added in centrifuge tubes and centrifuged at 3800 rpm for 10 min. Then, the centrifuged suspension was discarded and added ethanol (10 mL, Kanto Chem. Co., >99.5 %). The centrifuge tubes were shaken to disperse collected PS contents in ethanol. In addition, the centrifuge tubes were centrifuged same condition. Finally, the centrifuged suspension was discarded and the collected white content were shaken the addition of ethanol (15 mL) to obtain PS suspension. Meanwhile, cetyltrimethylammonium bromide (2.56 g, CTAB, Sigma-Aldrich, >98 %) was added to ion-exchanged water (100 mL). The solution was stirred at room temperature for 10 min to obtain a 2.5mass% aqueous CTAB solution. The obtained PS suspension (20 g), ethanol (40 mL), ion-exchanged water (100 mL), 2.5 wt% aqueous CTAB solution (5 mL), and aqueous NH3 solution (1 mL, Kanto Chem. Co., >28.0 %–30.0 %) were stirred at room temperature for 30 min. Aluminum tri-sec-butoxide (11.0 μL, Kanto Chem. Co., >95.0 %) and tetraethoxysilane (389.7 μL, TEOS, Kanto Chem. Co., >99.9 %) were added to the mixed solution. The mixed suspension was stirred at 298–303 K for 3 or 6 h under ultrasonic irradiation (50 W, 40 μm) using water bath. After the suspension was dried in a desiccator overnight, mesoporous SiO2–Al2O3 HSs were obtained through calcination in air at 873 K at a heating rate of 1 K min− 1, and these samples were cooled immediately after the designated temperature was reached (holding time for 0 h). Mesoporous SiO2–Al2O3 HSs without ultrasonic irradiation were also used for comparison.2.2. CharacterizationThe morphology of the samples was observed using a JEOL JEM2010F transmission electron microscope (TEM) operating at an accelerating voltage of 200 kV. The crystalline phases of the samples were determined through powder XRD using a Rigaku MultiFlex X-ray diffractometer equipped with a Ni filter and a CuKα radiation source (36 kV and 16 mA). Solid-state 27Al magic angle spinning (MAS) NMR spectra were recorded on a JEOL 800-MHz wide-bore solid-state NMR system JNM-ECZ800R spectrometer (18.79 T). The relaxation delay time was 2 s, and the composites were spun at 20 kHz using a 3.2-mm ZrO2 rotor. The chemical shift was referenced to a 1.0-M aqueous Al chloride (Wako Pure Chemical, >98.0 %) solution. The number of acid sites was measured through neutralization titration using n-butylamine (Kanto Chem. Co., >98.0 %).The mesoporous SiO2–Al2O3 HSs (50 mg) were dispersed in ethanol (20 mL) under sonication. The suspension was titrated with 0.1 M n- butylamine using methyl red (Kanto Chem. Co.) as an indicator. The number of acid sites in the samples was calculated from moles of n- butylamine per used sample. The N2 adsorption isotherms of the samples were measured at 77 K using a Belsorp-18 automatic physical adsorption apparatus. Before the measurements, the samples were degassed in a vacuum at 433 K for at least 10 h. The specific surface areas of the samples were calculated using the Brunauer–Emmett–Teller (BET) method using the adsorption data. Additionally, the pore size distributions of the samples were calculated using the Barrett–Joyner–Halenda method.2.3. Hydrolysis of NH3BH3The mesoporous SiO2–Al2O3 HSs (0.3 g) were placed in a two-necked round-bottom flask in air at room temperature; one neck was connected to a gas burette, and the other was connected to an addition funnel. NH3BH3 powder (0.0546 g, Sigma-Aldrich, 95 %) was dissolved in ion- exchanged water (40 mL) to obtain an aqueous NH3BH3 solution (0.044 M). The reaction was started by stirring a mixture of the sample and an aqueous NH3BH3 solution (5 mL). The gas evolution was monitored using the gas burette. This reaction was carried out three times in each sample.3. Results and discussionFirst, we investigated the morphology of mesoporous SiO2–Al2O3 HSs using TEM measurements. Herein, mesoporous SiO2–Al2O3 HSs without ultrasonic irradiation were noted under ultrasonic irradiation for 0 h. Fig. 1 shows TEM images of mesoporous SiO2–Al2O3 HSs with N. Toyama et al.                                                                                                                                                                                                                                Next Materials 8 (2025) 100777 2 various ultrasonic irradiation times. Fig. 1(a)–(c) shows that uniform HSs were observed in all samples with ultrasonic irradiation for 0, 3, and 6 h, respectively. Additionally, Fig. 1(d)–(f) demonstrates that the thicknesses of the samples with ultrasonic irradiation for 0, 3, and 6 h were approximately 100, 100, and 150 nm, respectively. According to these results, the shell thickness of the samples increased with increasing ultrasonic irradiation times. This might have been due to the presence of unreacted precursors under ultrasonic irradiation for 0 and 3 h. In the sample with ultrasonic irradiation for 6 h, the sol-gel reaction proceeds to increase shell thickness. Meanwhile, the shell thickness of the samples with ultrasonic irradiation for 0 h (stirring for 16 h at room temperature) was 100 nm, which is almost the same values as the samples with ultrasonic irradiation for 3 h. We speculated that the ultrasonic irradiation can promote the sol-gel reaction rate.The crystalline phases of the mesoporous SiO2–Al2O3 HSs were confirmed by XRD, as shown in Fig. 2. All samples showed only a broad hump at 2θ = 15◦–30◦, attributed to amorphous phases [39]. This suggests that the crystalline nature was not influenced by ultrasonic irradiation. Furthermore, we found that the crystalline nature did not change with increasing ultrasonic irradiation time.We investigated the influence of ultrasonic irradiation on the crystalline natures of the mesoporous SiO2–Al2O3 HSs using solid-state 27Al MAS NMR with various ultrasonic irradiation times, as shown in Fig. 3. Accordingly, three peaks were observed at around 62, 35, and 5 ppm in the spectra of all the HSs, corresponding to 4-, 5-, and 6-coordinated Al species, respectively [40,41]. The 4-coordinated Al species were assigned to Al species in the silica matrix, whereas the 6-coordinated Al species were assigned to Al species outside the silica matrix [42,43]. As shown in Table 1, the ratios of the peak area of the 4-coordinated Al species in the spectra of the mesoporous SiO2–Al2O3 HSs gradually increased with increasing ultrasonic irradiation time. The 4-coordinated Al species denotes Al species in the SiO2 structure, indicating that the dispersion of Al species improved. In other words, the dispersion of Al Fig. 1. TEM images of mesoporous SiO2–Al2O3 hollow spheres with ultrasonic irradiation for (a) 0, (b) 3, and (c) 6 h.Fig. 2. XRD patterns of mesoporous SiO2–Al2O3 hollow spheres with ultrasonic irradiation for (a) 0, (b) 3, and (c) 6 h.Fig. 3. 27Al solid-state NMR spectra of mesoporous SiO2–Al2O3 hollow spheres with ultrasonic irradiation for (a) 0, (b) 3, and (c) 6 h.N. Toyama et al.                                                                                                                                                                                                                                Next Materials 8 (2025) 100777 3 precursor can be improved by ultrasonic irradiation. Meanwhile, the ratios of the peak area of the 5-coordinated Al species in the spectra of the mesoporous SiO2–Al2O3 HSs interestingly decreased with increasing ultrasonic irradiation time. Previous studies noted that 5-coordinated Al species could be attributed to amorphous phases [44,45]. This suggests that amorphous Al species can be crystallized by ultrasonic irradiation.Finally, hydrogen generation from the hydrolysis of NH3BH3 was evaluated in the presence of the mesoporous SiO2–Al2O3 HSs using a gas burette. Fig. 4 shows the activity for hydrolysis of NH3BH3 in the presence of the mesoporous SiO2–Al2O3 HSs with various ultrasonic irradiation times. Herein, 7.7-, 10.6-, and 12.3-mL hydrogen were finally generated in the presence of the samples with ultrasonic irradiation for 0, 3, and 6 h, respectively. The H2/NH3BH3 molar ratios of the samples with ultrasonic irradiation for 0, 3, and 6 h were calculated from the hydrogen generation as 1.4, 2.0, and 2.3, respectively. According to these results, the hydrogen evolution from the hydrolysis of NH3BH3 increased with increasing ultrasonic irradiation time. We speculated the reason that the H2/NH3BH3 molar ratio did not achieve 3.0. This result display that the fact was not sufficient amount of acid sites to completely allow the hydrolysis of NH3BH3. In fact, we found that the amount of hydrogen generation increases with increase the amount of solid acids used in the hydrolysis of NH3BH3[22,23]. We found that the amount of hydrogen generation was dependent on the number of acid sites. The acid sites, which promotes hydrolysis of NH3BH3, forms when Si species in the SiO2 structure is replaced by Al species. 4-coordinated Al species is significantly related to the acid sites because 4-coordinated Al species denote the Al species in the SiO2 structure. Therefore, the higher proportion of 4-coordinated Al species can be considered as higher number of acid sites, thereby increasing the amount of hydrogen generation from hydrolysis of NH3BH3. The number of acid sites was determined in the presence of the samples using the neutralization titration method. From these results, the samples with ultrasonic irradiation for 6 h showed a high number of acid sites, and the activity for the hydrolysis of NH3BH3 depended on the number of acid sites. We evaluated the reusability for hydrolysis of NH3BH3 in the presence of mesoporous SiO2–Al2O3 HSs synthesized with ultrasonic irradiation for 6 h. Fig. 5(a) shows the activity for hydrolysis of NH3BH3 in the presence of the reused samples with ultrasonic irradiation for 6 h. From this result, 0.1-mL hydrogen were finally generated in the presence of the reused samples with ultrasonic irradiation for 6 h (The H2/NH3BH3 molar ratios was 0.01). This result indicates that the activity for hydrolysis of NH3BH3 were not almost promoted in the presence of the reused samples with ultrasonic irradiation for 6 h. We reveal that the number of acid sites and the proportion of 4-coordinated Al play important role in the activity for hydrolysis of NH3BH3. Therefore, the number of the acid sites of the reused samples with ultrasonic irradiation for 6 h was measured by the neutralization titration method. From this result, the acid sites were not existed the reused samples with ultrasonic irradiation for 6 h. Furthermore, the coordination structure of the reused samples with ultrasonic irradiation for 6 h was measured using 27Al solid-state NMR. Fig. 5(b) shows 27Al solid-state NMR spectra of the reused samples with ultrasonic irradiation for 6 h. The proportion of 4-coordinated Al to total coordinated Al for these samples was calculated to be 0.62. There is no significant change in the coordination structure before and after the activity for hydrolysis of NH3BH3. On the basis of these results, it is possible that the protons (H+) present on the surface was consumed. The hydrolysis reaction proceeds to form ammonium ion (NH4+) and these NH4+ might be replaced by H+, it is inferred that the acid sites were not function.Meanwhile, the reaction times to complete the hydrogen generation were within 5, 30, and 35 min, indicating that the reaction time to final hydrogen evolution increased with increasing ultrasonic irradiation time. To investigate this difference, we measured the pore size distribution of the samples using N2 adsorption–desorption measurement. Fig. 6 shows the pore size distribution of the samples. The samples with ultrasonic irradiation for 0 and 3 h had peaks at 2.4 and 2.1 nm, suggesting that these samples had many mesopores. Meanwhile, the sample with ultrasonic irradiation for 6 h had a peak at a value smaller than approximately 1.4 nm. According to these results, the mesopore size of the samples decreased with increasing ultrasonic irradiation time. This result may be because mesopores were not formed by ultrasonic irradiation.Furthermore, Table 3 shows the BET specific surface area of the samples. The values for the samples with ultrasonic irradiation for 0 and 3 h were 961 and 938 m2 g− 1, indicating almost the same BET specific surface areas, whereas the sample with ultrasonic irradiation for 6 h showed a decrease to 688 m2 g− 1. These results may also be attributed to a decrease in the number of mesopores formed in the shells of the HSs. For the mesoporous SiO2-Al2O3 HSs, rod-shaped micelles were first formed on the surface of PS particles, and then the mesoporous SiO2- Al2O3 shells were formed by sol-gel method with ultrasound irradiation. Previous studies reported that the micelles were decomposed by cavitation formed by ultrasound irradiation [46,47]. We considered that the formation of uniform mesopores for the samples with ultrasonic irradiation was not possible due to the collapse of the micelles on the surface of PS particles. In fact, the reaction time to complete hydrogen generation of the samples with ultrasonic irradiation for 0 h were faster than ultrasonic irradiation for 3 and 6 h, probably because have the uniform pores from the results of pore size distribution. Meanwhile the samples with ultrasonic irradiation for 3 and 6 h may be formed the disordered and/or collapsed pores based on the result of pore size distribution. Table 1 Proportion of the peak area of mesoporous SiO2–Al2O3 hollow spheres with various ultrasonic irradiation times.Ultrasonic irradiation time Proportion of peak area4- / total 5- / total0 h 0.38 0.353 h 0.43 0.336 h 0.51 0.12Table 2 Number of acid sites of mesoporous SiO2–Al2O3 hollow spheres with various ultrasonic irradiation times.Ultrasonic irradiation time Acid sites / mmol g− 11st 2nd 3rd Ave0 h 0.46 0.42 0.45 0.443 h 0.66 0.63 0.63 0.646 h 0.78 0.79 0.77 0.78Fig. 4. Activity for hydrolysis of NH3BH3 in the presence of mesoporous SiO2–Al2O3 hollow spheres with various ultrasonic irradiation times at room temperature.N. Toyama et al.                                                                                                                                                                                                                                Next Materials 8 (2025) 100777 4 Additionally, the shell thickness of the samples with ultrasonic irradiation for 6 h was approximately 150 nm from the result of TEM images, and the mesoporous SiO2-Al2O3 shells may be also formed on micelles. We considered that combination of these factors effect on the diffusion of NH3BH3 to the acid sites.4. ConclusionIn this study, mesoporous SiO2–Al2O3 HSs were synthesized using ultrasonic irradiation, and their samples were evaluated using the hydrolysis of NH3BH3. Uniform HSs were observed in the TEM images of samples with ultrasonic irradiation for 0, 3, and 6 h. The shell thickness of the samples increased with increasing ultrasonic irradiation time. 27Al solid-state NMR spectra showed that the proportion of 5-coordinated Al species attributed to amorphous Al species decreased and the proportion of 4-coordinated Al species attributed to Al species in the SiO2 structure increased as the ultrasonic irradiation time increased. These samples were evaluated in terms of their activity for the hydrolysis of NH3BH3. This result revealed that increasing the ultrasonic irradiation time increases the amount of hydrogen evolution from the hydrolysis of NH3BH3 due to the improved dispersion of Al species leading to increase the number of acid sites. On the other hand, the reaction time to complete hydrogen generation increased with increasing ultrasonic irradiation time due to the decrease the number of mesopores and thicker shell thickness. To improve this problem, the samples with thinner shell thickness may lead to faster hydrogen generation rate and development of practical applications.Declaration of Competing InterestThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.AcknowledgementsThis work was supported by NIMS Joint Research Hub Program (Grant number 2024–024) and "Advanced Research Infrastructure for Materials and Nanotechnology in Japan (ARIM)" of the Ministry of Education, Culture, Sports, Science and Technology (MEXT) (Grant Number JPMXP1224NM0107 and JPMXP1224UT0184). We are grateful to Mr. Oshikawa (Univ. of Tokyo) for using the TEM measurement.References[1] A. Corma, Inorganic solid acids and their use in acid-catalyzed hydrocarbon reactions, Chem. Rev. 95 (1995) 559–614, https://doi.org/10.1021/cr00035a006.[2] G. Busca, Acid catalysts in industrial hydrocarbon chemistry, Chem. 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Kojima, Fabrication of hollow silica-alumina composite spheres and their activity for hydrolytic dehydrogenation of ammonia borane, Int. J. Hydrog. Energy 39 (2014) 17136–17143, https://doi.org/10.1016/j. ijhydene.2014.08.057.[8] N. Toyama, T. Umegaki, Y. Kojima, Influence of Si/Al molar ratio of hollow silica- alumina composite spheres on their activity for hydrolytic dehydrogenation of ammonia borane, Int. J. Hydrog. Energy 40 (2015) 6151–6157, https://doi.org/ 10.1016/j.ijhydene.2015.03.021.[9] C.W. Hamilton, R.T. Baker, A. Staubitz, I. Manners, B–N compounds for chemical hydrogen storage, Chem. Soc. Rev. 38 (2009) 279–293, https://doi.org/10.1039/ b800312m.Fig. 5. (a) Activity for hydrolysis of NH3BH3 and (b) 27Al solid-state NMR spectra of the reused mesoporous SiO2–Al2O3 hollow spheres with ultrasonic irradiation for 6 h.Fig. 6. Pore size distributions of mesoporous SiO2–Al2O3 hollow spheres with various ultrasonic irradiation times.Table 3 BET specific surface area of mesoporous SiO2–Al2O3 hollow spheres with various ultrasonic irradiation times.Ultrasonic irradiation time BET specific surface area / m2 g− 10 h 9613 h 9316 h 688N. Toyama et al.                                                                                                                                                                                                                                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