Investigation of the Physicochemical Changes Preceding Zeolite Nucleation in a Sodium-Rich Aluminosilicate Gel

Abstract

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 %.

Internal structure of a gel leading to NBA-ZSM-5 single crystals

“…3g. This confirms that the gel mass reached equilibrium at 2 h. The morphology of the gel walls resembled the open dendritic gel structure and the worm-like particles (WLP) reported by Valtchev [16] and Subotić [17], respectively.…”

“…In particular, two similar porous morphologies have been reported recently for the gel aggregates in systems with moderate concentration leading to ZSM-5. An open dendritic gel structure was first reported by Valtchev and colleagues [16] and a closely related structure depicted as consisting of worm-like particles (WLP) was later reported by Subotić and colleagues [17]. Both groups have clearly evidenced that lower silica/alumina ratios favor thicker structures with larger particles [16,17].…”

“…On the one hand, the open dendritic gel structure was shown to present a glassy aspect with the absence of pores in the case of gels leading to ZSM-5, whereas pores have been observed in gels resulting in other zeolites [16]. On the other hand, the formation of WLPs in gels resulting in ZSM-5 was proposed to stem from the aggregation of primary particles of approximately 3 nm as well as the coalescence and disaggregation of different intermediates, with aluminum-containing precursor species playing a major role in the aggregation process [18].…”

In situsolid-state NMR for heterogeneous catalysis: a joint experimental and theoretical approach

“…Laserhyperpolarized xenon is achieved by spin-exchange optical pumping to tremendously enhance the signal intensity by 4 to 5 orders of magnitude compared to conventional 129 Xe NMR. [53][54][55] It has been shown that the laser-hyperpolarized 129 Xe NMR technique is powerful for studying the porosity of porous materials, [56][57][58][59][60][61] or even diffusion and combustion process. 62,63 As shown in Fig.…”

129Xenon NMR: Review of recent insights into porous materials

“…spectrometry, X-ray diffraction, infrared spectroscopy, scanning electron microscopy and chemical analyses to study the changes in composition and structure of sodium hydroxide rich aluminosilicate gel during the synthesis of zeolite A [90]. Many years ago, it has been shown that xenon adsorbs in NaA zeolite only under high pressure (several atmospheres) and at high temperature.…”