Membrane Dispersion Microstructure Reactor Preparation Of Nano-zinc Oxide And Mesoporous Zinc Oxide Microspheres

ZnO nanoparticles have been widely used as a rubber accelerator, as efficient absorbent, and have great potential for use in photocatalytic degradation and as gas sensors. Currently, the industrial preparation of ZnO nanoparticles utilizes the homogeneous precipitation method. Disadvantages of this method include the large diameters of the ZnO particles obtained, the wide distribution of particle diameter, as well as the expensive raw materials. A stirred tank is used in a typical precipitation operation process. According to theories about crystallization kinetics, the supersaturation ratio of the solution dominates the particle diameters. However, in a stirred tank, a high supersaturation ratio and good mixing performance are very difficult to achieve. The nanoparticles prepared by the stirred tank have the large diameters and the wide distribution of particle diameter. Therefore, developing a mixing equipment with high efficiency is the key to solve this problem.Accordingly, the main work done and main results are summarized as followings:1. In this work, ZnO nanoparticles were prepared using ZnSO4 and NH4HCO3 aqueous solutions as feedstocks in a membrane dispersion micro-structured reactor exhibiting efficient mixing. The effects of calcinations temperatures for the precursors, calcination time, feed concentrations, and feed flow rates on the crystal sizes of ZnO nanoparticles were investigated. The ZnO nanoparticles obtained were characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), and Brunauer-Emmet-Teller analysis (BET). The XRD results indicate that the prepared ZnO nanoparticles had pure hexagonal structures with an average crystal size of about 9.4 nm. The specific surface area could reach 74.2 m2/g; meanwhile, the TEM photographs show that the ZnO nanoparticles were of a pseudo-spherical shape with an average particle size of about 9.33 nm, which was very close to the XRD results. Additionally, the calcination temperatures were found to have a great effect on diameters and purities of ZnO nanoparticles. Advantages of this method include the small and narrow particle size distribution of prepared ZnO nanoparticles, the cheap raw materials, and the continuous operation mode.2. Mesoporous ZnO microspheres with a diameter of about 10μm were prepared through the combination of a membrane dispersion micro-structured reactor with a hydrothermal process. First, ZnSO4 reacted with NH4HCO3 in a membrane dispersion reactor to produce the Zn5(CO3)2(OH)6 precursor, and then the precursor was treated at the temperature of 80℃for 2h to form the Zn5(CO3)2(OH)6 microshperes aided by the surfactant CTAB or F127; after the calcinations, mesoporous ZnO microspheres were obtained. The experimental results showed that the morphology of ZnO microspheres obtained using the membrane dispersion micro-structured reactor to mix the raw materials was better than that of those obtained using stirred tanks. Without surfactants and the hydrothermal treatment of the precursors, uniform ZnO microspheres could not be obtained. The SEM images showed that the monodispersed ZnO microspheres were assembled by the plates, and deeper observations revealed that these plates were aggregated by the ZnO nanorods formed by ZnO nanoparticles with a diameter of 9.0nm. The adsorption-desorption isotherm curve indicated that the ZnO microspheres had mesoporous structures with an average pore diameter of 13.8 nm when using CTAB as the surfactant, and the specific surface area of the ZnO microspheres could reach 71.3 m2/g, much higher than that of ZnO nanoparticles.