Observing a Zeolite Nucleus (Subcrystal) with a Uniform Framework Structure and Its Oriented Attachment without Single‐Molecule Addition

Abstract

Multiple and complex crystallization process of zeolite including complementary single-molecule condensation and particle assembly,a nd alternately dominant nucleation and growth behavior,p layst he critical role in zeolite crystallization but meanwhile makes us hardt os tudy the respective effects.Herein, we strip nuclei from the synthetic solution and find that high-ordered nucleus (subcrystal) is the premise to ignite high-speed growth of zeolite crystal. The high-ordered subcrystals with the size of only 6-10 nm possess regular aperture structure and microporous area similar to zeolite nanocrystal. Interestingly,aunitary oriented aggregation process of the subcrystals towards nanosheets is well observed and characterized where single-molecule addition process is greatly repressed. If aw ider range of zeotype nuclei can be expanded, an ew synthetic strategy of zeotype materials with heterogeneous framework and active sites may be expected, which may novelizez eolite catalytic properties.

Coarse-Grained Model for the Hydrothermal Synthesis of Zeolites

“…We find that the polymerized NP has an irregular, non-spherical shape because it is glassy. The size and shape of the NPs polymerized in the simulations are similar to those reported in recent experiments of silicalite-1 synthesis at 403 K (Figure c) . The amorphous NPs have water molar fractions of 0.60 and 0.58 in the simulations with strengths of ε = 0.75 and 0.6 kcal mol –1 , respectively.…”

“…The size and shape of the NPs polymerized in the simulations are similar to those reported in recent experiments of silicalite-1 synthesis at 403 K (Figure 5c). 14 The amorphous NPs have water molar fractions of 0.60 and 0.58 in the simulations with strengths of ε = 0.75 and 0.6 kcal mol −1 , respectively. These agree well with the water mole fraction between 0.42 and 0.61 deduced from experiments.…”

“…In the next sections, we present the parameterization of the model and show that it reproduces the experimental speciation of the silicates a function of degree of condensation in the polymerization of silica 32 and is able to spontaneously nucleate and grow a zeolite from the hydrated amorphous phase, as observed in experiments. 14,24,36 The computational efficiency of the model presented here makes it appropriate to investigate the mechanisms involved in the multiple steps involved in the hydrothermal synthesis of zeolites: the formation of NPs, the evolution of these NPs to produce nanozeolites, and their growth by oriented attachment.…”

“…There are several effective routes for the preparation of zeolitic materials. − The most widely used is hydrothermal synthesis (HS). − In this route, tetraethylorthosilicate (TEOS) and an organic structure-directing agent (SDA), typically a tetraalkylammonium cation, are mixed in water under alkaline conditions in an autoclave at temperatures that typically range from 70 to 200 °C. ,,− HS of zeolites involves multiple processes: ,,− first, the polymerization of silica that leads to self-aggregation of silica and SDA species into amorphous aggregates; − second, the crystallization of the amorphous aggregates into zeolites; ,,,− and third, the growth of the zeolite crystals through a combination of oriented attachment of aggregates and the addition of monomers or oligomers to them. ,, Finally, the desorption of water from the pores of the zeolite with heat treatment. − Although different synthesis conditions and SDAs result in the formation of different crystal polymorphs, the processes of polymerization, crystal nucleation and growth, and water desorption are common to the HS of all zeolites.…”

“…The experimental characterization of these processes along the synthesis of zeolites is challenging. The early stages of silica polymerization are mainly followed by 29 Si NMR and, occasionally, by mass spectrometry. − Furthermore, recent advances in transmission electron microscopy (TEM), cryo-high-resolution transmission electron microscopy (HRTEM), and atomic force microscopy have resulted in unprecedented resolution of the size and shape of the condensed aggregates that result from the polymerization process. ,,− However, these ex situ studies cannot yet follow the evolution of individual amorphous nanoparticles (NPs) into zeolite crystals, and the high temperature and elevated pH of the HS of zeolites make it difficult to follow these processes in situ. Despite the enormous advances in the study of the synthesis of zeolites using advanced imaging and spectroscopies, ,,,− a spatially resolved, molecular-level understanding of the formation of the amorphous phase and its transformation into zeolite crystals is still lacking.…”