Preparation of kapok–polyacrylonitrile core–shell composite microtube and its application as gold nanoparticles carrier
Highlights
► A new catalyst support kapok–PAN composite microtube was fabricated based on the natural kapok fiber by a cetyltrimethylammonium bromide (CTAB) assisted self-assembly method. ► Au nanoparticles (NPs) were immobilized on the surface of kapok–PAN microtubes via in situ reduction of chloroauric acid (HAuCl4) by sodium borohydride (NaBH4). ► The obtained kapok–PAN–Au composites exhibited excellent catalytic activity and reusability as demonstrated via the reduction of 4-nitrophenol to 4-aminophenol by NaBH4.
Introduction
Metal nanoparticles (NPs) have received continuous research interest due to their unusual physical and chemical properties compared to bulk metals and their wide applications in many areas, such as optics, electronics, sensor technology and catalysis [1], [2], [3], [4], [5], [6]. However, small metal NPs easily aggregate to minimize their surface area, resulting in a remarkable reduction in their catalytic activities. Although metal NPs stabilized by surfactant or polymer have high activity, stability and selectivity in homogeneous reactions [7], [8], [9], it is difficult for them to recover from the products and thus reuse. Since the availability of noble metal resources is limited, metal NPs, such as Au NPs as one of most promising catalysts are usually immobilized onto/into less expensive and larger scale supports, such as fiber, sphere, film, and nanotube [10], [11], [12], [13], [14], [15], [16].
With recent development of sustainable technology, supports from renewable resources have attracted much attention [17]. Kapok fibers are abundantly natural microtubes with round tube structure and have been widely utilized as stuffing, buoyancy material and oil-absorbing material [18], [19]. The outer diameter and the wall thickness of kapok microtubes are 15–25 μm and 0.5–2 μm, respectively, and the original length is in several centimeters, endowing kapok microtubes with relative large contact surface area and huge lumen volume [20], Such structure characters make the abundantly produced fibers to be a desirable candidate for catalyst carriers, which can allow the nanocatalysts on them to effectively contact with the reactants and catalyze the reactions. In addition, it is convenient to recover the catalysts due to the large scale of the supports. However, to the best of our knowledge, there is few report of using kapok microtubes as catalyst carriers for metallic NPs.
To exploit kapok microtubes as carrier for metal NPs, some original drawbacks should be overcome. The covered waxes on the surface endow kapok microtubes with hydrophobic and oleophilic feature. Even if the wax can be removed by solvent rinsing, wetting of kapok microtubes is yet difficult because the density of kapok containing lumen is as low as 0.28–0.32 g/cm3[20]. However, metal NPs are usually prepared by reduction of metallic salts in aqueous solution, and untreated kapok microtubes have low ability to adsorb metal ions [21], [22]. Therefore, modification of kapok microtubes is necessary to improve the chemical activity and the loading capacity. Herein, by virtue of the waxes, the surface of kapok microtubes was modified with polyacrylonitrile (PAN) coating to improve the hydrophilicity and adsorption ability via a cetyltrimethylammonium bromide (CTAB) assisted self-assembly method, which includes radical polymerization of PAN in aqueous solution and subsequent deposition by virtue of CTAB. PAN has good ability to adsorb many metal ions and can be further modified due to the polar cyano group [23], [24]. Compared with direct chemical modification by alkalization, acetylation and graft polymerization [25], [26], this method does not impair the structure of kapok and is convenient to operate and scale up. The influence of the concentration of AN and CTAB on the formation of kapok–PAN core–shell composite microtubes was studied and the formation mechanism was proposed. Furthermore, the obtained kapok–PAN composite microtubes were used as catalyst carriers for Au NPs, and the catalytic activity and reusability of kapok–PAN–Au was evaluated by the reduction reaction of 4-nitrophenol by NaBH4.
Section snippets
Materials
Natural kapok microtubes produced in Sichuan province, China were used. The microtubes were mechanically smashed to a length of 100–300 μm, washed thoroughly with acetone several times to remove ashes, and then dried at room temperature. Acrylonitrile (AN) was distilled under vacuum and stored at 4 °C. AN, Cetyltrimethylammonium bromide (CTAB), dichloro methane (DCM), sodium borohydride (NaBH4) and hydrochloroauric acid trihydrate (HAuCl4·3H2O) were purchased from Beijing Chemical Reagents Co.
Results and discussion
The assumed reaction mechanism is schematically shown in Fig. 1. When the natural hydrophobic kapok microtubes are dispersed in CTAB aqueous solution, CTAB will be adsorbed on the fibers due to the hydrophobic interaction. Some of the later injected AN will be adsorbed on kapok surface due to the electrostatic interaction between CTAB (positive) and AN (negative). When the temperature reaches 70 °C and KPS is introduced, polymerization of AN starts. The in situ prepared PAN on the kapok surface
Conclusion
In conclusion, taking advantage of waxes on the surface of kapok, kapok–PAN core–shell composite microtubes were prepared by a cetyltrimethylammonium bromide (CTAB) assisted self-assembly method. Surfactant CTAB played an important role in the deposition of the in situ polymerized PAN on the microtube surface. The PAN coating constructed by the aggregation of PAN NPs improved the specific surface area and hydrophilicity of the composite microtubes. When kapok–PAN microtubes were used as
Acknowledgement
The work was supported by NSFC (Nos. 50821062 and 51073166).
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