Particle size effect of layered double hydroxide on the porosity of calcined metal oxide

The structure, porosity, and adsorption properties of mixed metal oxides (MMO) were investigated according to the physical property of the starting materials. Layered double hydroxides (LDH), the pristine MMO, were prepared to have the same composition of Mg₂Al(OH)₆(CO₃)_0.5, but with different particle size. The prepared LDH were calcined at 400 °C to obtain MMO named CMO, HMO, and UMO corresponding to the pristine MMO synthesized by coprecipitation, hydrothermal treatment, and urea hydrolysis method, respectively. The particle sizes of CMO, HMO, and UMO were comparable to those of the pristine MMO in the order UMO > HMO > CMO; on the other hand, the crystallite sizes were almost the same as each other, regardless of the pristine MMO. According to the high-resolution transmission electron microscopy and X-ray diffraction, we could suggest that a single particle of MMO was composed of smaller domains of metal oxide crystallite. Consequently, small MMO particle tended to have interparticulate macropore, while large MMO particle had more possibility of containing intraparticulate mesopores, which was cross-confirmed by the N₂ adsorption–desorption isotherms. Toluene and water vapor adsorption tests on three different MMO showed that the intraparticulate mesopores were advantageous to adsorb a variety of adsorbate. It was therefore concluded that MMO obtained by calcination of LDH would have high adsorption property when the pristine LDH has large particle dimension, thanks to the large portions of intraparticulate mesopores.