Unusual Pathway of Crystallization of Zeolite ZSM-5 in a Heterogeneous System: Phenomenology and Starting Considerations

Abstract

Crystallization of zeolite ZSM-5 from a diluted heterogeneous system (12.5Na 2 O−Al 2 O 3 −8TPABr−60SiO 2 − 4000H 2 O) was investigated by various experimental methods such as X-ray diffraction (XRD), electron diffraction (ED), infrared spectroscopy (FTIR), X-ray fluorescence (XRF), scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), thermo-gravimetric analysis (TGA), particle size analysis (PSA), pH measurement, inductive coupling plasma (ICP) emission spectrometry, and dynamic light scattering (DLS). The crystallization process is characterized by a very long "induction period" (95% of the entire reaction time) and very fast transformation (5% of the entire reaction time) of amorphous to crystalline phase (zeolite ZSM-5) at the end of the crystallization process. Analysis of the obtained results has shown that the crystallization process takes place by a chain of processes: (i) formation of "primary" amorphous aluminosilicate precursor (gel) at room temperature, (ii) formation of "secondary" amorphous aluminosilicate precursor ("worm-like" particles, WLPs) at increased temperature (170 °C), (iii) formation of "tertiary" amorphous aluminosilicate precursor (condensed aggregates, CAs) by aggregation of the WLPs and densification (condensation) of aggregates, and (iv) formation of nuclei and their growth in the matrixes of CAs; these processes result in the formation of fully crystalline zeolite ZSM-5 in the form of polycrystalline aggregates.

First synthesis of SAPO molecular sieve with LTL-type structure by hydrothermal conversion of SAPO-37 with FAU-type structure

“…This is consistent with the finding that the amorphous phases obtained by ball-milling of zeolites A and X contain residual nano-sized quasicrystalline particles that cannot be detected by XRD, but can be identified by IR [21]. The nanoparts then act as nuclei for crystallization of LTLtype SAPO which can grow either in the amorphous matrix [22][23][24] or in the liquid phase after their releasing from the dissolved parts of the amorphous phase [25][26][27]. However, since the product (LTL-type SAPO) appear as discrete particles (crystals; Fig.…”

“…However, since the product (LTL-type SAPO) appear as discrete particles (crystals; Fig. 6) rather than polycrystalline aggregates [22][23][24], it is reasonable to assume that the nanoparts grow into LTL-type SAPO in the liquid phase, i.e., after their release from the amorphous matrix [25][26][27]. Fig.…”

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].…”

Nucleation of FAU and LTA Zeolites from Heterogeneous Aluminosilicate Precursors

“…These processes are hypothesized to involve the breakage and formation of Si-O-Si and Si-O-Al bonds. 31 The concept of "gel transformation" has been proposed for zeolites such as ZSM-5 (MFI), [32][33][34] zeolite A (LTA), 35 and zeolite X/Y (FAU). 36,37 For LTA, it was suggested that nucleation occurs within the interior of the gel, followed by crystal growth (from the gel interior to its exterior) by consuming nutrient within the gel particle.…”