Two-Step to One-Step Nucleation of a Zeolite through a Metastable Gyroid Mesophase

Abstract

The importance of nonclassical nucleation pathways in the formation of complex crystals has become apparent in recent years. Nonclassical pathways were unraveled for, among others, the crystallization of proteins, colloids, and clathrates. In those cases, the formation of a metastable fluid with density close to the crystal decreases the crystallization barrier. Recent simulations indicate that mesophases can facilitate the nucleation of zeolites. Here, we use molecular simulations to investigate the role of a gyroid mesophase on the crystallization of a model zeolite from liquid. The nucleation pathway is always nonclassical. At warmer temperatures, the mechanism proceeds in two well-defined steps: nucleation of a metastable gyroid followed by its crystallization into a zeolite. At colder temperatures, the second barrier becomes negligible, and the crystallization occurs in one step. This second scenario is also nonclassical, as the critical nucleus for the crystallization has the structure of the gyroid and seamlessly transforms into a zeolite as it grows past its critical size. To our knowledge, this is the first report of a nonclassical mechanism of crystallization mediated by a mesophase.

Is Ice Nucleation by Organic Crystals Nonclassical? An Assessment of the Monolayer Hypothesis of Ice Nucleation

“…We now address the core prediction of the monolayer hypothesis: that the ordering of interfacial water results in a nonclassical mechanism of ice nucleation. A nonclassical mechanism − for ice nucleation could involve one or two steps. In the latter, there are two well-defined steps with distinct barriers and separated time scales.…”

“…The second possible nonclassical mechanism would be one in which there is a single barrier controlled by the formation of a fluctuation of an intermediate phase , an ordered interfacial patch in the case of ice nucleation by organicswhich then crosses over seamlessly into the final phase, ice, without an additional barrier. For this pathway to be nonclassical the transition state that controls the nucleation should precede the formation of ice on the surface.…”

Effects of Synthesis Parameters on the Crystallization Profile and Morphological Properties of SAPO-5 Templated by 1-Benzyl-2,3-Dimethylimidazolium Hydroxide

“…Furthermore, SAPO-5 may undergo different kinds of structural change via reconstructive phase transformation to a more metastable phase at high temperature. This is achieved by unit cell volume contraction through the removal of the water and/or [bzmIm] + cations from the pores [22,39]. Furthermore, the addition of SiO 2 as one of the primary building units also affects the entire crystallization process of SAPO-5.…”

Free energy surface of two-step nucleation

“…In the second step, the stable phase appears and grows from within the finite cluster of the intermediate phase. TSN has been identified and studied in a growing range of systems, both in experiments 11,13,16,[21][22][23][24][25][26][27][28] and simulations 16,[29][30][31][32][33][34][35][36][37][38][39][40][41] .…”