384. Gallium oxide nanorods: Novel, template-free synthesis and high catalytic activity in epoxid…

Fig. 1 (A) Scheme of the formation of Ga2O3 nano rods (BuOH is 2-butanol). (B, C) TEM images of Ga2O3 nano rods with 40-80 nm length (B) and with 20-50 nm length (C). The size of the nanorods can be tuned by adjustments in the protocol of addition of H2O, without affecting the catalytic activity of the material. (D) HR-TEM picture of a selected nanorod (d105 =1.5 Å, d100 =2.5 Å, and angle of 58°). (E) Selected area electron diffraction pattern of Ga2O3 nano rod. (F) XRD pattern of: -Ga2O3 nano rod-lit (a) and Ga2O3 nano rod (b). The vertical lines correspond to reflections of the (100) and (105) planes of -Ga2O3. (G) 71Ga MAS NMR spectra of Ga2O3 nano rod (gray) and -Ga2O3 nano rod-lit (violet), showing Ga in tetrahedral (ca. 150 ppm) and octahedral (ca. 0 ppm) coordination.

Fig. 2 Proposed catalytic cycle for the epoxidation of alkenes with aqueous H2O2 over Ga2O3 nano rod.

Warunee Lueangchaichaweng, et al, Angew. Chem. Int. Ed. 253, 1585 (2014)
https://doi.org/10.1002/anie.201308384

(1) For a surface-related application such as heterogeneous catalysis, a key advantage of nanomaterials is provided by the increased surface-to-volume ratio that accompanies the decrease in the size of the catalyst particles.
(2) Gallium oxide nanorods were prepared using a precipitation method involving solvolysis of GaCl3 with 2-butanol, followed by hydrolysis and condensation of the formed species.
(3) The generation of nanorods is ascribed to an anisotropic structural feature of the intermediate species, most likely gallium oxyhydroxides, which are formed in the synthesis mixture.
(4) The conversion of gallium oxyhydroxide into oxide is expected to proceed through condensation steps followed by partial migration of Ga atoms from octahedral to tetrahedral sites.
(5) In the epoxidation of styrene, gallium oxide nanorods gave higher conversion and higher epoxide selectivity compared to titanium silica-lite 1 (TS-1).
(6) Lewis acid sites in gallium oxides are generally attributed to coordinatively unsaturated Ga3+ ions mainly located in tetrahedral sites on the surface, whereas surface Ga-OH groups are responsible for mild Brønsted acidity. These Brønsted acid sites can convert into Lewis acid sites by surface dehydration upon treatment at high temperature and tend to reconvert to Brønsted acid sites by rehydration at room temperature.
(7) The higher surface area is a consequence of the higher surface-to-volume ratio of nanosized materials, whereas the higher surface density of acid sites is attributed to the intrinsically defective nature of the nanorods and the consequent higher density of coordinatively unsaturated sites on their surface compared to materials consisting of larger particles.
(8) Both coordinatively unsaturated Ga atoms acting as Lewis acid sites and mainly tetrahedral Ga-OH groups acting as mild Brønsted acid sites located at the surface ofthe nanorods can be considered as catalytic sites for the epoxidation reaction.