No abstract

No abstract

Treatment of ZSM-5 in alkaline solution dramatically changes morphology of the ZSM-5, leading to the formation of mesopores whose size is almost uniform without destruction of microporous structure.

No abstract

No abstract

The structure of amorphous precursor species formed under hydrothermal conditions, prior to the onset of crystallization of microporous aluminosilicate zeolites, is determined employing high-energy X-ray diffraction (HEXRD). The investigation, combined with the use of reverse Monte Carlo modelling suggests that even numbered rings, especially 4R (R: ring) and 6R, which are the dominant aluminosilicate rings in zeolite A, have already been produced in the precursor. The model implies that the formation of double 4Rs occurs at the final step of the crystallization of zeolite A.

The entire sequence of crystallization processes, starting with the formation of precursor particles, proceeding through the nucleation stage, and finishing with complete transformation into Linde type A (LTA) zeolite nanocrystals and crystal growth, from a clear solution using tetramethylammonium cation with the composition 11.25:1.8:13.4:x:700 SiO2:Al2O3:(TMA)2O:NaOH:H2O (x = 0.6, 0.9, 1.2, or 1.5) has been monitored by simultaneous in situ small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS). Primary units with a size of ca. 0.5 nm and precursor particles with a size of ca. 4.5 nm are formed during the crystallization. The precursor particles with a size of 4.5 nm play an important role during the nucleation process. The influences of Na+ on the crystallization are studied by varying the concentration of Na+ cations in the synthesis solution. It is found that the Na+ cations affect not only the nucleation process but also crystal growth process. When x < 1.5, the number density of the precursor particles increases with the concentration of Na+, which is due to the structure-making influence and charge-compensating role of Na+. However, when the concentration of Na+ is relatively high (x = 1.5), the 6 nm sized particles in the synthesis solution are observed at the beginning of the synthesis, which is due to the salt-outing effect of Na+. On the other hand, the studies on the structure of the precursor particles suggest that they do not contain long-range order, but some medium-range order related to the crystalline LTA structure, which become ordered during the course of hydrothermal treatment.

Faujasite zeolite is synthesized hydrothermally after an aging process under various conditions. Hydrogel is formed during the aging, and the amount increases with increasing the duration. Aluminosilicate species included in the hydrogel phase during the aging is investigated for the changes in Si/Al ratios and Q units during the aging and the crystallizing processes by means of 29Si magic-angle spinning NMR spectroscopy. Aging for more than 1 day is necessary to obtain faujasite with high crystallinity. The distribution in Q4 units in the hydrogel is gradually changed, and the aluminosilicate species having mainly a Q4(4Al) unit is obtained after 7 days of aging, while 2 days of aging results in remanent silicate species derived from the starting silica source. The former gives faujasite crystallites with higher crystallinity, sharper particle size distribution, and a lower Si/Al ratio. On the basis of these findings, the crystallization mechanism is discussed.