Synthesis of Small Crystals of LTA Zeolites without used Organic Compounds
Advancing the chemical engineering fundamentals
Crystallization (T2-9P)
Keywords: Synthesis, Small crystal, Zeolite LTA, Hydrothermal, Aging time
Zeolites are crystalline alumino-silicate or silicate materials, which have a highly regular and open microporous structure formed by a three-dimensional network of SiO4 and AlO4 tetrahedra. The tetrahedra are linked together to give cages connected by pore openings of defined size; depending on the structural type, the pore sizes range from 0.3 to 1 nm approximately [1]. These exceptional properties have led to numerous technical applications as bulk powder form (catalysts, adsorbents and ion exchangers [2-4]. The synthesis of small or nanocrystal like zeolites is crucial to prepare new materials [5-8] as ultra-thin zeolite films in membranes among other examples [9-11]. In addition, the reduction of the crystal size to the nanometer scale is the basis for unique applications in microelectronics devices [12]. The synthesis of zeolite nanocrystals in the form of stable colloidal suspensions has been one of the most important research lines in zeolite science during the past decade. Several microporous materials, among others MFI-, LTA-, FAU-, BEA-type materials, have been prepared in the form of colloidal suspensions with narrow particle size distribution [13]. In general, very diluted systems containing abundant amounts of tetraalkylammonium hydroxides have been used in the synthesis of zeolite nanocrystals with narrow particle size distribution. The alkaline cation concentration in such systems has to be very limited in order to avoid the flocculation of gel particles. All these factors together with the careful choice of the silica source allow the stabilization of ‘‘clear’’ starting solutions where only discrete gel particles are present. Another important factor that influences zeolite crystal size is the temperature. The crystallization of zeolite nanoparticles has been usually performed at the lowest possible temperature for a particular zeolite, which results in a relatively long synthesis [15]. Therefore, a reduction of the crystallization time and optimization of the conditions yielding zeolite nanoparticles is highly desirable. Several authors have already reported on the synthesis of nanocrystalline zeolite A [16-20]. However, the drawback of these synthesis procedures is generally that large amounts of the comparatively expensive organic template tetramethylammonium hydroxide (TMAOH) has to be employed in order to promote nucleation and, thus, the formation of nanocrystals [13].
In this work, Small crystals of zeolites LTA were obtained in absence of organic templates using a crystallization temperature of 373 K. The as-synthesized samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and Fourier-Transform Infrared (FT-IR) spectroscopy. The aging time was found to be a crucial factor for the control of the crystal size. Samples aged between 72 to 144 h presented an average crystal size of ca. 200 to 500 nm.
Presented Wednesday 19, 13:30 to 15:00, in session Crystallization (T2-9P).
