Microwave hydrothermal synthesis of core-shell structured boehmite
Graphical abstract
Highlights
► Only using Al2(SO4)3, H2O and urea, we established an additive-free microwave hydrothermal route. ► Core-shell structured boehmite was firstly synthesized via microwave hydrothermal (M-H) route. ► The M-H reaction time for core-shell structured boehmite was reduced to only 40 min.
Introduction
In recent years, the design and synthesis of functional materials with controlled size and desired morphology have stimulated great research interest [1], [2], [3], [4]. The simpler method and more effective control was always the immutable target of morphology-control synthesis domain. Therefore, as an important target materials, core-shell structured ultra-fine boehmite powders with nanometer to micrometer dimensions generated great interest due to their special physical and chemical properties which were different from solid particles, owing to their low density, large surface area, special core-shell structure and nanostructured wall [5], [6], [7], [8], [9].
Although hydrothermal method is an efficient way to synthesize core-shell structured boehmite, some approaches tend to be rather complicated and environmentally incompatible, with the obvious drawbacks that these processes required the amphiphilic copolymer [8], [9] or inorganic salts(e.g. sodium tartrate [5], trisodium citrate dehydrate [6], [7]) as additive, and the relevant additives removal process might compromise the structural integrity of the final products or result in environmental pollution which set great limitations on their practical application. Obviously, to prevent waste was better than to treat it or clean it up after it has formed [10]. Thus, additive-free route is a preferable choice. The other main drawback of hydrothermal method is the slow kinetics at any given temperature. On account of the fact that less reaction time usually means less energy consumption or more eco-friendly design, it is still an unremitting pursuit to develop time-saving boehmite synthesis routes, which will greatly facilitate their future industrial applications.
As is known to all, microwave-assisted heating is generally faster, eco-friendly and most energy efficient. Such a combination is termed as the microwave hydrothermal (M-H) method [11], [12], [13], [14]. We successfully synthesized core-shell structured amorphous aluminum hydroxide [12] via 2 h M-H process at 150 °C. However, to transform into alumina from boehmite is more energy efficient than from amorphous aluminum hydroxide, so it still remains a great challenge to directly synthesize core-shell structured boehmite via a microwave hydrothermal route. Following this development trend, in this study, we firstly report the time-saving and additive-free microwave hydrothermal syntheses of core-shell structured boehmite.
Section snippets
Experimental procedure
In a typical M-H synthesis process, 10 mL of 0.1 mol L−1 Al2(SO4)3 aqueous solution and 30 mL distilled water were added to a double-walled Teflon-lined digestion vessel of ∼50 mL capacity. After 0.182 g urea (accounting for 100% theoretical dosage) was added, the vessel was sealed and placed on a turntable for uniform heating using a MDS-6 microwave hydrothermal system (Sineo, China) [11], [12], [13]. Temperature-controlled mode M-H treatments were conducted at 180 °C by non-pulsed heating type for
Results and discussion
As shown in Fig. 1(d), diffraction peaks corresponding to boehmite (PDF no 21-1307) have been found for the final product which was prepared at 180 °C with 40 min reaction time under full microwave power range of 0–1000 W. No obvious XRD peaks arising from other phases of alumina are found indicating pure γ-AlOOH phase of the microwave hydrothermal product.
Fig. 2(a) shows the typical TEM image of the core-shell structured γ-AlOOH. The distinct dark and light contradistinctions indicate the sample
Conclusions
To sum up, we conclude that it was the introduction of full microwave heating power and 180 °C reaction temperature that has enabled us to accomplish the morphology evolution of core-shell structured boehmite in only 30–40 min. In the meantime, the final product could also complete its crystal form transformation process from amorphous Al(OH)3 to boehmite completely. In addition, the appropriate dosage of urea we used also helped us to control the core-shell morphology transformation process
Acknowledgments
This work has been supported by the National Natural Science Foundation of China(Grant no. 21136008) and the Taishan Scholars Program of Shandong Province, China (ts20081119).
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