Search

Abstract: Previous investigations have shown that an all-silica BEA-type (Si-BEA) material is difficult to be nucleated in basic media and that the fluorosilicic precursors formed in near neutral media play an important role in its formation. The present study reports on the synthesis of this microporous material under basic conditions by using TEA2SiF6 as an additional silica source that provides preformed fluorosilicate species. The effect of the amount of TEA2SiF6 on the purity of the final product, crystallization kinetics and morphological features of Si-BEA were systematically studied. The investigation showed that above a certain concentration corresponding to TEA2SiF6/SiO2 =0.07 pure Si-BEA was obtained and further increase in the concentration improved substantially the crystallization kinetics. The increase in TEA2SiF6 content led also to a gradual increase of the size of the pyramidal (h 0l) face coupled with the decrease of pinacoidal (001) one. Thus, the morphology of the zeolite crystals can be varied from well developed truncated bipyramids to plate-like crystals with dominating pinacoidal faces. A consequence of this morphological changes is the number the pore openings of the channel systems running along the a- and c-axes per total external surface. The framework defectness and water content in the synthesized materials were studied by 29Si NMR spectroscopy and TG analysis, respectively. This combined study revealed that materials with a high level of hydrophobicity, due to the specific effect of fluorine anions, have been synthesized under basic conditions. [Copyright &y& Elsevier]

The effect of room temperature aging on the crystallization rate and size of MFI-type crystals synthesized under high temperature was investigated. First, synthesis mixtures yielding MFI-type zeolite were aged at room temperature for periods of time between 1 and 60 days. This was followed by a rapid synthesis in a pre-heated at 230 °C conventional oven. The utilization of a pre-aged initial systems and high heating rates allowed very rapid synthesis of zeolite. The effect of aging time, crystallization temperature, duration of synthesis and aluminum content on the crystal size and yield was systematically studied. Zeolite nanoparticles with sizes ranging from 130 to 260 nm and relatively narrow particle size distributions were synthesized employing the pure-silica system. The obtained nanocrystals were characterized by X-ray diffraction, scanning electron microscopy, dynamic light scattering and N2 adsorption measurements. [Copyright &y& Elsevier]

FER-type zeolites were synthesized in the presence of Na-Ethylendiamine (Na–En), Na–Pyrrolidine (Na–Py) and Na–K. The crystallization times varied between 96 and 240 h. Ferrierite harvested from the Na–En synthesis was also used as seeds and reduced crystallization time from 240 to 30 h. The product of seeded synthesis was not uniform: nanosized with micronsized crystals as well as complex aggregates of different sizes were obtained. Hierarchical FER-type zeolites were prepared using seeded Na–En synthesized FER as a parent. Secondary pores in zeolite crystals were formed by etching at room temperature with HF-NH 4 F containing solutions, using the HF concentration and treatment time as variables. Crystal morphology and physicochemical characteristics of micron-, nano-sized and hierarchical FER-type crystals were studied by complementary methods including: X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetry (TG/dTG), 27 Al and 29 Si MAS NMR, IR spectroscopy of adsorbed probe molecules (Pyridine, Acetonitrile and Lutidine), nitrogen adsorption and chemical analysis. We demonstrate that FER-type zeolites with pre-determined properties can be easily prepared. [ABSTRACT FROM AUTHOR]

Abstract: Hierarchical porous composite with a core–shell structure possessing FAU-type zeolite core (zeolite X) and LTA-type zeolite shell (zeolite A) has been prepared. The intimate growth of zeolite shell is achieved by in situ crystallization of the LTA zeolite crystals with size of 1μm on large single FAU-type crystals with size of about 60–90μm. Direct visualization of the core–shell material is provided by X-ray microbeam measurements using synchrotron radiation on a single crystal complemented with field emission scanning electron microscopic study. The nitrogen sorption data confirmed that the access to large single FAU-type core is controlled by the defect-free LTA-type shell. Sorption measurements showed that about 98% of core crystals are covered by the LTA shells. Further the FAU–LTA core–shell material is used as a host for the stabilization of platinum clusters. The Pt location, mean size and distribution have been studied by CO chemisorption followed by FT-IR spectroscopy. High loading of platinum accessible to CO molecules in the core–shell material is achieved. The use of the FAU–LTA matrix allowed the stabilization of two different sizes of Pt clusters. Additional advantage of the core–shell material is the protection of the clusters from thermal sintering and poisoning, which is of significant importance for future catalysts. [Copyright &y& Elsevier]