Assembly and Evolution of Amorphous Precursors in Zeolite L Crystallization

Abstract

The formation of amorphous bulk phases in zeolite synthesis is a common phenomenon, yet there are many questions pertaining to the physicochemical properties of these precursors and their putative role(s) in the growth of microporous materials. Here, we study the formation of zeolite L, which is a large-pore framework (LTL type) with properties that are well-suited for catalysis, separations, photonics, and drug delivery, among other applications. We investigate the structural and morphological evolution of aluminosilicate precursors during zeolite L crystallization using a variety of colloidal and microscopy techniques. Dynamic light scattering measurements of growth solutions and scanning electron microscopy (SEM) images of extracted solids collectively reveal that zeolite L precursors assemble through a series of steps, leading to branched worm-like particles (WLPs). Transmission electron microscopy and electron dispersion spectroscopy show that WLPs have a heterogeneous composition that predominantly consists of silica-rich domains. We demonstrate that static light scattering can be used to identify the approximate induction time and is a reliable method to quantitatively track the extent of crystallization. During the induction period, the average size of zeolite L precursors monotonically increases by the accretion of soluble species. Precursor growth continues until the onset of zeolite L nucleation when WLPs reach a maximum size. During zeolite L growth, the number density of precursors decreases in favor of a growing population of crystallites. Ex situ SEM images reveal the progressive formation of crystal nuclei, which deviates from the classical LaMer process that posits a nearly instantaneous generation (or burst) of nuclei. These findings provide evidence of zeolite L growth via a nonclassical pathway involving crystallization by particle attachment (CPA). Given the ubiquitous presence of WLP-like precursors in syntheses of numerous zeolites, CPA processes may prove to be broadly representative of growth mechanisms for other zeolite framework types and related materials.

Organic‐Free Interzeolite Transformation in the Absence of Common Building Units

“…CBUs or oligomers) that remains after room temperature aging, although qualitative evidence points to the fact that there is some memory of the FAU structure. SEM images of samples extracted during early heating time show what appears to be amorphous wormlike particles (Figure S5, Supporting Information), analogous to those reported for other zeolites . Conversely, SEM images of as‐received USY (Figure A) show the presence of approximately 800 nm particles with ill‐defined morphology.…”

Unraveling the Twin and Tunability of the Crystal Domain Sizes in the Medium‐Pore Zeolite ZSM‐57 by Electron Crystallography

“…Although distinct crystal growth mechanisms occur in both systems, similar initial intact pentagonal ZSM‐57 nanoplates/nanoprisms with smooth surfaces crystallize from the solution medium (liquid/solid, the classical pathway) after the induction period (as evidenced by the TEM results presented in Figures and ). This rare initial pathway for the crystallization process varies from the mechanisms previously reported for LTL ‐, CHA ‐, MTW ‐, and MFI ‐type zeolites, which involve the solid/solid‐based nonclassical pathway of growth from amorphous nanoparticles. Thereafter, the distinct surface terrain features ascribed to different crystallization pathways are also evidenced by AFM observations (see Figure S10 in the Supporting Information).…”

“…A small amount of ZSM‐57‐Na‐Inter with a smooth surface and around 200 nm in size starts to appear (as verified by PXRD and SEM, Figure a, 18 h) in this system. After 36 h of heating, the individual particles seem to merge and cross‐link together to form amorphous worm‐like particles (WLPs, Figure a, 36 h and Figure S7), which have been recognized as typical precursors in the heterogeneous medium of zeolite synthesis, for example, CHA ‐, MFI ‐, MTW ‐, TON ‐, LTL ‐, and GIS ‐type zeolites. In the sample after 54 h of heating, initial pentagonal nanoplates with a smooth surface (ca.…”

“…Besides determining novel structures through electron crystallography, TEM is also one of the effective approaches used to probe the mechanism of zeolite growth. During the crystallization of zeolites, two concerted pathways have been identified: 1) Soluble molecules accrete on the crystal surface (classical) and 2) amorphous nanoparticles attach to the crystal surface (nonclassical) . Crystal growth by the classical layer‐by‐layer mechanism exhibits a well‐defined crystal habit, whereas the nonclassical mechanism involving attachment and subsequent rearrangement of amorphous precursors leads to an irregular morphology with a lack of distinct facets and/or rough surface features composed of protrusions with sizes equivalent to the precursors .…”

Diagnosing the Internal Architecture of Zeolite Ferrierite

“…This is equivalent to the step height measured by AFM, and a schematic of this layer‐by‐layer growth mechanism is shown in Figure d. Growth in this dimension is much slower than growth in the directions of the 8‐ and 10‐MRs, which is reflected in the plate‐like morphology of the crystals. These regular terraces are consistent with a classical crystal growth mechanism where crystals grow via the addition of monomers in solution, which is an interesting finding as zeolites are known to grow by both classical and non‐classical mechanisms . However, the classical growth mechanism may only apply towards the end of zeolite crystallization as we did not probe the initial stages of crystallization when a non‐classical mechanism may apply.…”