Influence of Morphology of Silica-Alumina Composites on Their Activity for Hydrolytic Dehydrogenation of Ammonia Borane

Abstract

In this work, we investigate the influence of the morphology of silica-alumina composites on their activity for hydrolytic dehydrogenation of ammonia borane. Three type of composites, hollow spheres, fine particles, and spherical particles are prepared by sol-gel method. The hollow spheres were prepared by using polystyrene particles as templates. The morphology of composites were observed by transmission electron microscopy. The activity of each type of composite for hydrolytic dehydrogenation of ammonia borane was compared. In the presence of the hollow spheres, the fine particles, and the spherical particles, 10, 2.5, and 1.5 mL of hydrogen was released with the completion times of the reaction being 12, 2, and 1 min, respectively. The amount of hydrogen evolution from the hollow spheres was much higher as compared to those from the fine particles and the spherical particles.Temperature-programmed desorption of ammonia suggested that the hollow spheres possess both weak and strong Brønsted acid sites, while the fine particles and the spherical particles possess only the weak Brønsted acid sites.These results indicate that the morphology of the silica-alumina composites influences their acidic properties, and the strong Brønsted acid sites are more effective for hydrolytic dehydrogenation of ammonia borane, as compared to the weak Brønsted acid sites. 本研究では，球状中空シリカ -アルミナ，シリカ -アルミナ微粒子および球状シリカ -アルミナをそれぞれ調製し，シリカ -アルミナ の形態の違いがアンモニアボラン加水分解活性に与える影響について検討を行った。すべての試料はゾル -ゲル法によって調製し， 球状中空シリカ -アルミナはポリスチレン粒子をテンプレー トとする方法で調製した。透過型電子顕微鏡写真の結果から， 球状中空体， 微粒子および球状体が確認できた。これらの試料を用いてアンモニアボラン加水分解活性を行った。球状中空体，微粒子および 球状体の存在下では，10，2.5，および 1.5 mL の水素発生が 12，2 および 1 min にそれぞれ完了した。この結果から，球状中空 体の活性が微粒子および球状体と比較して高活性であった。 またアンモニア昇温脱離測定の結果から， 球状中空体は弱ブレンステッ ド酸点および強ブレンステッ ド酸点を有しており，微粒子および球状体は弱ブレンステッ ド酸点のみ有していた。以上の結果から，シ リカ -アルミナの形態は酸性質に影響があり，強ブレンステッ ド酸点は，弱ブレンステッ ド酸点よりもアンモニアボラン加水分解に有効 であることが示唆された。

Role of Interparticle Space in Hollow Spheres of Silica-Based Solid Acids on Their Acidic Properties and Activity for Hydrolytic Dehydrogenation of Ammonia Borane

“…In order to evaluate the effect of the morphology and the dispersion of active aluminum species in the hollow spheres, the relation between the ratios of four-coordinated aluminum species and the activity for hydrogen evolution amount from aqueous solutions in the presence of various silica-aluminum composite particles. The relation calculated from some of our previous studies [52,61,62] was shown in Figure 5. From this figure, the hollow spheres showed unexpected high activity as compared with the fine particles, although the ratio of four-coordinated aluminum species in the hollow spheres was not significantly higher than the ratio of the fine particles.…”

“…The specific surface areas of the hollow spheres and the fine particles were found to be 393 and 295 m 2 g −1 , respectively, indicating that the specific surface areas do not significantly differ from each other, and the primary particles including the hollow spheres were slightly small compared with the particle size of the fine particles. On the other hand, both the hollow spheres and the fine particles consisted of a typical amorphous silica-alumina from the results of powder XRD measurements [51,52].…”

“…The hollow silica-alumina composite spheres show unexpected high activity for hydrolytic dehydrogenation of ammonia borane compared with the silica-alumina composite Figure 1. TEM images of (a) hollow silica-alumina composite spheres and (b) silica-alumina composite fine particles [52].…”

“…Figure 2. NH 3 -TPD profiles of (a) hollow silica-alumina composite spheres and (b) silica-alumina composite fine particles [52].…”

Influence of morphology of hollow silica–alumina composite spheres on their activity for hydrolytic dehydrogenation of ammonia borane

“…Additionally, NH 3 BH 3 releases hydrogen via a hydrolysis reaction in the presence of suitable acids and catalysts at room temperature (Eq. (1)) [13][14][15][16][17][18][19][20][21][22][23][24][25][26][27]: NH 3 BH 3 + 2H 2 O → NH 4 + + BO 2  + 3H 2 (1) It has been reported that solid acids such as H-type zeolites (H-BEA and H-MOR) show high activity for the hydrolytic dehydrogenation of NH 3 BH 3 [13]. We have reported that hollow silica-alumina composite spheres show higher activity compared with fine particles [14].…”