Preparation of halloysite nanotube-supported gold nanocomposite for solvent-free oxidation of benzyl alcohol
© Fu et al.; licensee Springer. 2014
Received: 14 April 2014
Accepted: 20 May 2014
Published: 2 June 2014
Gold nanoparticles supported on halloysite nanotubes (Au/HNTs) were prepared by a homogeneous deposition-precipitation method. The specific characteristics of the catalyst were characterized in detail, in relation to their performance for solvent-free oxidation of benzyl alcohol. The particular structure of the catalyst resulted in high catalytic activity and stability compared with other supported gold catalysts. The enhanced catalytic activity of the Au/HNTs catalyst was mainly attributed to the presence of a higher amount of oxidized gold species and the tubular structure of the HNTs.
Selective oxidation of alcohols to more valuable aldehydes, ketones, and carboxylic acids is of great importance to both the fine chemical industry and academia . Numerous stoichiometric oxidizing reagents have been involved to accomplish this transformation, such as dichromate and permanganate. However, these reagents have many drawbacks, such as being toxic, expensive, and un-recyclable. Thus, the developments of a heterogeneous solid catalyst that can use molecular oxygen as a primary oxidant have attracted much more attention. In this context, a series of noble metal supported catalysts for aerobic oxidation of alcohols have been exploited over the last decades.
Among the noble metal supported catalysts, gold supported catalysts have been paid more and more attention, owing to their unique catalytic properties under mild conditions, such as CO oxidation, hydrocarbon combustion, selective oxidation, and water gas shift reaction [2–5]. It is generally accepted that the catalytic performance of the gold catalysts strongly depended on not only the size of the gold particles but also the nature of the support material, the preparation method, and the activation procedure during the synthetic process . As supports, metal oxides have been employed, giving outstanding performance because of their facile activation of molecular oxygen [2, 7, 8]. At the same time, liquid-phase alcohol oxidation requires addition of soluble bases (metal carbonates, acetates, or borates), especially when inert supports such as silica, carbon, or polymers are used to disperse gold .
Halloysite nanotubes (HNTs) (Al2Si2O5(OH)4 · 2H2O), hydrated layered aluminosilicates of the kaolinite group, containing octahedral gibbsite Al(OH)3 and tetrahedral SiO4 sheets (i.e., halloysite nanotubes), possess a hollow cylinder formed by multiply rolled layers . Because of their structural features, they offer a potential application as support for catalytic composites and the additive for reinforcing polymers with remarkable, improved mechanical properties and dispersibility. Recently, Yang et al. reported Pd nanoparticles deposited on HNTs nanocomposite for hydrogenation of styrene with enhanced catalytic activity . They cast a new light on using HNTs as catalyst support. Herein, we reported the synthesis of Au/HNTs catalyst and the structure of the catalyst was characterized. The as-synthesized Au/HNTs catalyst showed high catalytic activity for solvent-free oxidation of benzyl alcohol.
In a typical procedure, 3.6 g urea was dissolved in 200 mL of 1.46 mmol L−1 HAuCl4 solution at room temperature. An amount of 0.55 g of HNTs support was then added to this clear solution (5% Au loading), and the temperature of the resulting slurry was increased gradually to 90°C. The temperature was maintained for 4 h, followed by filtering and washing several times with deionized water. The solid product was dried overnight before calcination at 300°C for 4 h in static air.
The crystalline phases were determined using a RIGAKU D/max-2550VB1 18-kW X-ray powder diffractometer (XRD; Shibuya-ku, Japan) with Cu Kα radiation (λ = 1.5418 Å). Transmission electron microscopy (TEM) images were obtained using a JEOL JEM-2010 F instrument (Akishima-shi, Japan) equipped with an energy-dispersive X-ray spectroscopy (EDS) at an accelerating voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) measurement was performed using PHI 5600 (Physical Electronics, Chanhassen, MN, USA) with a monochromated Al Kα radiation (hν = 1,486.6 eV), calibrated internally by the carbon deposit C 1 s (285.0 eV).
A reactor (50-mL round-bottle flask) was charged with 200 mg of catalyst and 100 mmol of benzyl alcohol. Molecular oxygen was bubbled through the reaction mixture (flow rate = 20 mL min−1). The resulting mixture was then heated at 383 K for 8 h and cooled to room temperature. The reaction products were analyzed by a Shimadzu QP5050 GC-MS (Kyoto, Japan).
Results and discussion
In conclusion, we have demonstrated that HNTs are an attractive support for gold nanoparticles, which results in an excellent catalytic activity in solvent-free oxidation of benzyl alcohol. The high catalytic activity is found to be related to the tubular structure of the HNTs and the oxidized gold species. This process is promising in the development of a truly heterogeneous catalyst for alcohol oxidation.
The authors would like to thank the supports from the National Natural Science Foundation of China (No. 21306061), Key Project of Educational Commission of Guangdong Province (No. 2012B091100296), and Project of Base of Production, Education and Research (No. cxzd1148).
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