Morphology-controlled hydrothermal synthesis of boehmite via an anions competition method
Graphical abstract
In this study, Al2(SO4)3 was consciously and proportionally introduced as equally important concomitant raw materials into aluminum chloride/aluminum nitrate–urea hydrothermal process to form SO42 −–Cl− and SO42 −–NO3− competition systems. Boehmite from lamellar assemblies to hollow microsphere morphologies was facilely synthesized by succinctly altering the relative proportion of SO42 −:Cl− and SO42 −:NO3− via an anions competition method.
Highlights
► A morphology control concept named as anions competition method was proposed. ► To introduce another concomitant raw material is the key point. ► Boehmite from lamellar assemblies to hollow microspheres was hydrothermally synthesized.
Introduction
There are a large number of potential applications that might be realized by making novel morphologies of the materials [1], [2], [3]. In this respect, the preparation of inorganic particles with desired size and morphology is highly desired since these materials exhibit interesting properties for a number of applications [4], [5], [6]. Boehmite can be seen as partly dehydrated aluminum hydroxide which is an important precursor for the preparation of advanced catalysts, coatings, membranes, alumina and alumina-derived ceramics [5], [6], [7], [8]. Because it could transform into alumina after being heated between 400 and 700 °C without altering the morphology, many efforts have been made to control boehmite morphology [3], [4], [5], [6], [7], [8], [9], [10].
Although hydrothermal processes by utilizing inorganic salt as additives have showed effective control ability of morphology [11], [12], [13], these additives couldn't be transformed into the component of any kind of reaction product, thus causing low raw material efficiency and atom economy [14]. Obviously, to prevent waste is better than to treat it or clean it up after it has formed [15]. In a word, we appreciated the morphology control ability of anions given by the inorganic salt additives, but we couldn't accept its poor eco-friendly performance. In this study, we propose a mutually beneficial solution to overcome this contradiction.
Previous works manifested that in the simple aluminum chloride/aluminum nitrate–urea hydrothermal system, the morphology of boehmite was often bundles of nanostrips [6], nanoflake-like [11], needle-like [12] or spindle-like assemblies [13]. While in aluminum sulfate–urea hydrothermal system, the morphology of boehmite preferred to hollow microspheres [9], [11], [12], [16]. It was interesting to note that the above-mentioned significantly different morphologies of boehmite were acquired via extremely similar reaction conditions. After the same factors, such as cation (i.e. Al3 +), urea and hydrothermal conditions, were carefully eliminated, the only difference was the kind of anions. Thus, one may safely draw the conclusion that it is the difference of anions that plays a key role in the final determination of boehmite morphology.
Cai et al. [11] mentioned that Al2(SO4)3 could be used as additives to obtain boehmite hollow microspheres. And they laid particular stress on the general morphology control ability of SO42 − in their experimental design concept. Al2(SO4)3 was treated as one of the sources of SO42 − and could be indistinguishably replaced by other sulfate additives, such as: AlNH4(SO4)2·12H2O, KAl(SO4)2·12H2O, Na2SO4, (NH4)2SO4, and even MgSO4. Roh et al. [17] recently reported successful synthesis of monodispersed spherical aluminum hydrous oxide by using mixed Al(NO3)3 and Al2(SO4)3 under a forced hydrolysis method. Besides that, almost all of the previous boehmite synthesis literatures adopted hydrothermal route selected sole aluminum salt as raw materials to provide the full source of cations(i.e. Al3 +), while some elaborately selected inorganic salt additives were endowed with the morphology control mission, such as: sodium tetraborate [7], trisodium citrate dehydrate [8], Na2SO4, (NH4)2SO4, and MgSO4 [11]. In consideration of the cost and environment factors, these additives are not the optimal choice and far from raw material efficiency and atom economy [14]. Indeed, there was no difference between the same anion given by aluminum salt raw materials and inorganic salt additives. After reflecting on this question from a different perspective, we creatively decided to introduce another concomitant raw material which acts as a substitute for additives to not only share the total demand of cations but also provide the morphology control required competitive anions. And this could be seen as the key point of our morphology control concept named as anions competition method.
Section snippets
Experimental procedure
The amount of AlCl3 or Al(NO3)3 was fixed at 0.036 mol firstly, and the addition amounts of Al2(SO4)3 were as follows: 0.002, 0.004, 0.006, 0.008, 0.009, and 0.012 mol, thus the corresponding molar ratio of SO42 −:Cl− or SO42 −:NO3− were 1:18, 1:9, 1:6, 1:4.5, 1:4 and 1:3, respectively. The urea consumption was 1.875 times the total molar amount of aluminum ions. After distilled water was added to fill up to 80% of the total 50 ml capacity, the Teflon-lined stainless steel autoclave was sealed and
Phase structures
Fig. 1 shows the XRD patterns of two typical samples which were synthesized with different raw material combinations at 150 °C for 24 h. It is clear that the samples show the same phase structure and the main diffraction peaks can be assigned to crystalline boehmite (PDF no. 21-1307). No impurities such as Al(OH)3 or Al2O3 were detected, indicating the high purity of the products. More specifically, it can be observed that the intensity of the peak corresponding to the (020) crystal plane is
Conclusions
In this study, aluminum sulfate was consciously and proportionally introduced as another concomitant raw material into aluminum chloride/aluminum nitrate–urea hydrothermal process to form SO42 −–Cl− and SO42 −–NO3− competition systems. Boehmite from lamellar assemblies to hollow microsphere morphology was synthesized by succinctly altering the relative proportion of SO42 −:Cl− and SO42 −:NO3−. In conclusion, we named this morphology control concept as anions competition method. We think our simple
Acknowledgments
This work has been supported by the National Natural Science Foundation of China (grant no. 21136008) and Taishan Scholars Program of Shandong Province, China (ts20081119).
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