Abstract
Zeolite crystallization occurs by multifaceted processes involving molecule attachment and nonclassical pathways governed by the addition of amorphous precursors. Here, we use scanning probe microscopy to monitor zeolite LTA crystallization in situ with a spatiotemporal resolution that captures dynamic processes in real time. We report a distinctive pathway involving the formation of gel-like islands from supersaturated solutions comprised of (alumino)silicate molecules. Three-dimensional assembly and evolution of these islands constitutes a unique mode of growth that differs from classical theories. Time-resolved imaging also reveals that growth can occur by (nearly) oriented attachment. At later stages of crystallization, a progressive transition to lower supersaturation shifts growth to a layered mechanism involving two-dimensional nucleation and spreading of layers. Here, we show that LTA crystallization occurs by multiple pathways, thereby reconciling putative hypotheses of growth mechanisms while also highlighting new modes of nonclassical crystallization that may prove relevant to other zeolites and related materials.
Highlights
Zeolite crystallization occurs by multifaceted processes involving molecule attachment and nonclassical pathways governed by the addition of amorphous precursors
Hypotheses of LTA crystallization derived from spectroscopy[18,19], microscopy[20,21], and molecular simulations[22,23] postulate diverse pathways that include gel transformations[24], precursor agglomeration and densification[25], and the attachment of composite building units (CBUs)[26,27]
The growth media used for this study are the supernatants of a reported synthesis mixture[28] after preheating for periodic times to generate a range of supersaturation with respect to silica
Summary
Zeolite crystallization occurs by multifaceted processes involving molecule attachment and nonclassical pathways governed by the addition of amorphous precursors. In situ AFM measurements at two different temperatures (35 and 45 °C) and variable supersaturation reveal multiple modes of growth that range from classical molecule-by-molecule addition to nonclassical pathways that, to our knowledge, have not been previously reported for zeolite crystallization. Extraction and analysis of the supernatant after periodic heating times confirms that supersaturated solutions (Fig. 1c and Supplementary Table 1) have sufficient nutrients for the nucleation and growth of cubic LTA crystals (Fig. 1e and Supplementary Fig. 1).
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