Factors that control zeolite L crystal size.

Time-series hydrothermal syntheses from two organic-cation-free gels with different compositions were employed to study the factors that control the final size of zeolite L crystals. The first gel had a starting K/Al ratio of 10, whereas in the second one it was three times lower. The relatively simple chemical composition of the starting gels and the combination of complementary characterization methods allowed us to track down the different stages of transformation of the initial amorphous gels into zeolite crystals and the factors that control the nucleation and growth processes. The role of the starting mixture components in the formation of the primary amorphous particles was explored. It was found that the profoundly different reaction kinetics in the two systems are caused by the difference in diffusion rates, which in turn are controlled by the extent of the polymerization reactions at room temperature during mixing of the starting components prior to hydrothermal treatment. As a consequence, nucleation is fast and ubiquitous in the first system with higher water content and K/Al ratio, whereas it is slow and sporadic in the second system with lower water content and K/Al ratio. Ultimately, these differences in the kinetics lead to the formation of two distinctly different patterns of crystal-size distribution, with a large number of small nanocrystals in the first sample and fewer large crystals in the second sample. The new findings put zeolite crystal growth on a rational basis that would enable the control of zeolite crystal size in similar organic-template-free systems.

Micro- to macroscopic observations of MnAlPO-5 nanocrystal growth in ionic-liquid media.

Micro- and macroscopic studies of nucleation and growth processes of MnAlPO-5 nanosized crystals under ionothermal synthesis conditions are reported herein. The samples treated at 150 °C were extracted from the reaction mixture at various stages of crystallization, and characterized by XRD; SEM; thermogravimetric analysis (TGA); (31)P and (27)Al solid-state magic angle spinning (MAS) NMR, Raman, UV/Vis, and X-ray fluorescence spectroscopy (XRF). The starting raw materials (alumina, manganese, and phosphorous) were dissolved completely in the ionic liquid and transformed into an amorphous solid after 5 h of ionothermal treatment. This amorphous solid then undergoes structural changes over the following 5-25 h, which result in an intermediate phase that consists of octahedral Al species linked to the manganese and phosphate species. The first MnAlPO-5 nuclei on the surface of the intermediate can be observed after 50 h ionoheating. These nuclei further grow, as the surface of the intermediate is in full contact with the ionic liquid, to give crystalline MnAlPO-5 nanoparticles with a mean diameter of 80 nm. The crystals become fully detached from the intermediate and are then liberated as discrete particles after 90 h heating. The transformation process from amorphous to intermediate and then to the crystalline MnAlPO-5 nanoparticles shows that nucleation starts at the solid-liquid interface and continues through surface-to-core reversed-growth until the entire amorphous solid is transformed into discrete nanocrystals.

Investigation of the physicochemical changes preceding zeolite nucleation in a sodium-rich aluminosilicate gel.

All industrially available zeolites are obtained from hydrogel systems. Unfortunately the level of understanding of the events preceding zeolite crystallization is far from satisfactory. In this respect, revealing the nature of the processes taking place in the precursor gel is of paramount importance to understanding zeolite nucleation. The investigation of the gel structure, however, is a difficult task due to the complexity of the object in terms of both composition and topology. Therefore, a combination of hyperpolarized (HP) (129)Xe NMR-N(2) adsorption-high-resolution transmission electron microscopy-energy-dispersive spectrometry methods complemented by X-ray diffraction, infrared spectroscopy, scanning electron microscopy, and chemical analyses has been employed to study the changes in composition and structure of sodium hydroxide rich aluminosilicate gel yielding zeolite A. The role of each component in the system and the entire sequence of events during the induction, nucleation, and crystallization stages have been revealed. The high concentration of sodium hydroxide in the studied system has been found to control the size and structure of the gel particles in the beginning stage. During the initial polymerization of aluminosilicate species a significant part of the sodium hydroxide is expelled from the gel into the solution, which restricts extensive polymerization and leads to formation of small aluminosilicate particles with open pore structure. The induction period that follows is marked by incorporation of Na back in the bulk gel. The combined action of the Na ion as a structure-directing agent and the hydroxyl group as a mobilizer results in partial depolymerization of the gel and formation of voids with mesopore sizes. The nucleation maximum coincides temporally with development of pores with sizes in the range of 2-5 nm. The amorphous gel undergoes into crystalline zeolite only after these pores have disappeared and the chemistry of the gel has evolved to reach the stoichiometric zeolite composition. It was established unambiguously by high-resolution transmission electron microscopy and HP (129)Xe NMR that the nucleation of zeolite occurs in the solid part of the system and the succeeding crystallization commences only after the nuclei are released into the liquid, which is consistent with the autocatalytic mechanism. Also this investigation has demonstrated the unrivaled sensitivity of HP (129)Xe NMR that is capable of identifying presence of small amounts of crystalline zeolite material in amorphous medium with detection limit extending below 1 wt %.