On the Role of Aging, Washing, and Drying in the Synthesis of Polycrystalline Zinc Oxide by Precipitation: Combining Fas
- PDF / 646,143 Bytes
- 6 Pages / 595.276 x 790.866 pts Page_size
- 53 Downloads / 162 Views
On the Role of Aging, Washing, and Drying in the Synthesis of Polycrystalline Zinc Oxide by Precipitation: Combining Fast Continuous Mixing, Spray Drying and Freeze Drying to Unravel the Solid-State Transformations of the Precipitate Stefan Kaluza Æ Martin Muhler
Received: 21 August 2008 / Accepted: 5 December 2008 / Published online: 23 December 2008 Ó Springer Science+Business Media, LLC 2008
Abstract The continuous precipitation of zinc carbonates using aqueous solutions of zinc nitrate and sodium carbonate was quenched within a few seconds by combining a micromixer with a bench-top spray dryer. In this way, it was possible to monitor the slow phase transformation of the initially formed sodium zinc carbonate into zinc hydroxy carbonate during subsequent washing and drying. An increased stirring rate applied during washing was found to accelerate this phase transformation, thus influencing the specific surface areas and pore size distributions of the ZnO powder materials finally obtained after calcination. Keywords ZnO Continuous precipitation Micromixing Spray drying Freeze drying
1 Introduction Precipitation plays an important role in the field of bulk catalyst preparation [1]. Due to its high degree of flexibility, the generation of homogeneous component distributions and the possibility to create pure materials, it is one of the most frequently applied synthetic methods. Polycrystalline zinc oxide can be prepared by the precipitation of zinc salts with alkali carbonates or hydroxides followed by a controlled calcination process [2]. ZnO is catalytically active in hydrogenation reactions [3], or it is used as catalyst support [4, 5]. Other catalysts of industrial importance prepared by precipitation methods are the catalysts for Fischer–Tropsch synthesis [6], the V/P/O catalyst for the oxidation of n-butane S. Kaluza M. Muhler (&) Laboratory of Industrial Chemistry, Ruhr-University Bochum, Universita¨tsstr. 150, 44780 Bochum, Germany e-mail: [email protected]
to maleic anhydride [7, 8], and the Cu/ZnO catalyst for methanol synthesis [4, 5]. In the latter case, Bems et al. [9] showed that the properties and performance of the final catalyst are strongly influenced by a large set of experimental parameters applied during the precipitation process. They investigated the particle formation of binary catalyst precursors by varying the pH, the Cu–Zn ratio, and the aging time, and observed that the system is extremely sensitive to small changes of the synthesis conditions. Additionally, further parameters such as temperature, concentration, solvent, flow rates and stirrer speed have to be considered [1]. This huge set of parameters for one specific precipitation process requires an elaborate optimization. The degree of supersaturation that is created by bringing the corresponding solutions into contact is essential for the particle formation [10]. During a conventional precipitation in a batch reactor local supersaturation is created when a droplet of one solution hits the other solution. Due to inten
Data Loading...
