Synthesis of nanosized and microporous zinc aluminate spinel by microwave assisted hydrothermal method (microwave–hydrothermal synthesis of ZnAl2O4)
Graphical abstract
Introduction
Oxide spinels comprise a very large group of structurally related compounds many of which are of considerable technological significance [1]. Among them, zinc aluminate with spinel structure ZnAl2O4 is of interest due to its combination of desirable properties such as high mechanical resistance, high thermal stability, low temperature sinterability, low surface acidity and better diffusion. Therefore, it is used as high temperature ceramic material [2], optical coating or host matrix [3], [4] but most importantly as catalytic material (catalyst or catalyst support). It is used in many catalytic reactions, such as dehydration [5], hydrogenation [6], dehydrogenation [7] and synthesis of fine chemicals [8], [9].
In general, there are many methods of preparation of zinc aluminate, for example, solid state-reaction [10] or wet chemical routs such as co-precipitation [11], sol–gel [12], [13] and hydrothermal method [14], [15]. Synthesis under hydrothermal conditions can provide a direct, one step route to prepare nanosized oxide spinels. However, this method usually requires pro-longed reaction time when low temperatures are applied. In recent years, conventional hydrothermal method is often substituted by the microwave assisted hydrothermal method because this method offers very rapid heating, faster kinetics, phase purity with better yield, homogeneity and high reproducibility [16], [17], [18]. Although microwave–hydrothermal method is used since 1992 [19] for the synthesis of various compounds and composite materials, the synthesis of nanosized zinc aluminate spinel under these conditions, to the best of our knowledge, has not been tried so far. Therefore, in the present study, an attempt is being made to synthesize single phase nanosized and microporous ZnAl2O4 with narrow particle and pore size distribution and high specific surface area by the microwave assisted hydrothermal method. We have also reported the preliminary results of the possibility of application of such obtained zinc aluminium oxide as convenient material for the preparation of thin films coatings with enhanced thermal stability on different planar substrates.
Section snippets
Samples
All the microwave assisted hydrothermal reactions were performed in a microwave accelerated reaction system MW Reactor Model 02-02 (ERTEC, Poland). The system operates at 2.45 GHz frequency with 0–100% of full power (1000 W). The system is controlled by a temperature (with accuracy of 10% in the range 20–300 °C) or by pressure (with accuracy of 2% up to 100 bar). The synthesis parameters such as temperature/pressure and hold time were varied systematically in order to estimate the optimum
Results and discussion
Several experiments were carried out under various conditions of microwave assisted hydrothermal synthesis to find the lowest parameters for the formation of nanosized zinc aluminate with spinel structure. These parameters were estimated as temperature 200 °C, pressure 20 bar and hold time 5 min (using full power of microwave heating) and were used for the preparation of samples described in this paper.
The XRD patterns of the powder samples, as-prepared and after further heating at 500 and
Conclusions
Microwave assisted hydrothermal method was successfully used to synthesize single-phase zinc aluminate of particles with an average size and narrow size distribution as was confirmed by powder X-ray diffraction and electron microscopy studies. Advisable reaction conditions were optimised as temperature 200 °C, pressure 20 bar and hold time 5 min. Liquid nitrogen adsorption–desorption measurements revealed that nanocrystalline ZnAl2O4 formed by this method of synthesis exhibits
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