#Neat image 7.6 full verification
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HSILs form upon hydrolysis of tetraethyl orthosilicate (TEOS) in stoichiometric, strongly alkaline aqueous solutions (>2.5 M MOH, M = alkali metal).

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As a result, even in zeolite crystallization from clear solutions, zeolite crystallization from true monophasic liquids, free of any amorphous aggregates, has not yet been captured by characterization, even though the literature suggests that zeolite crystallization is driven by the liquid medium. Research has shown that zeolite formation proceeds via these aggregates (2,7,33) in a nonclassical manner. (28−33) However, while the name “clear solution” suggests these synthesis liquids to be true solutions, they are optically clear suspensions of nanosized amorphous aggregates. This approach revealed a wealth of information about the crystallization of high-silicate zeolites such as silicalite-1 and zeolite beta. Among them, the so-called “clear solution” zeolite synthesis strategy uses dilute aqueous solutions of tetraalkylammonium hydroxide to nucleate zeolites under conditions more compatible with molecular characterization techniques. To gain more insights into zeolite formation, alternatives to (alumino)silicate gel synthesis were developed. Zeolite synthesis thus typically occurs in multiphasic media, obscuring experimental observation of early nucleation steps. (24−27) Under natural conditions, zeolite formation spans geological timescales, but increasing alkalinity, temperature, and concentration of framework elements allowed development of heterogeneous synthesis gels yielding zeolites within hours up to days under hydrothermal conditions. In the early years of zeolite synthesis, Barrer, Breck, and co-workers mimicked the natural conditions in which zeolites form.

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This work shows zeolites, a class of microporous materials with high societal relevance, (23) to offer unsuspected opportunities to advance molecular verification of the fundamentals of non-CNT.

(17,19,22) This depends on molecular-level characterization of crystal nucleation, a significant experimental challenge, even in model systems such as the aforementioned calcium carbonates and calcium phosphates. Recent advances start to reveal the role of molecular interactions such as ion-pairing in kinetic nucleation barriers, offering opportunities to also include kinetic aspects in the description of nonclassical pathways. (18) However, the specific entities involved, and their molecular-level interactions determining their condensation pathway, often remain subject to speculation. (17,18,20,21) Extending upon CNT, modern theory proposes selection between classical and nonclassical pathways. (19) These can directly condense into crystals or pass through an amorphous prephase, depending on the surface energy of the respective solid, relative to the energy of the fluid species. (11−18) Disputing the assumptions of CNT, nucleation of these inorganic phases has been shown to proceed via condensation of prenucleation clusters. (2) Metastable states, such as those arising from liquid–liquid demixing or supramolecular assemblies, not only play a role in the formation of porous crystals or biomolecular condensates (3−10) but also can initiate crystallization of dense inorganic salts, such as calcium carbonates and calcium phosphates, or oxides such as magnetite. (1) However, for processes involving a phase separation via metastable intermediates, nonclassical approaches are required. Within the classical model, nucleation rates can be derived for a given degree of supersaturation, assuming invariable interfacial and bulk free energies. In CNT, stochastic, thermally driven density fluctuations in supersaturated homogeneous liquids result in the assembly of viable nuclei from the growth medium. The formation of crystalline nanoporous materials with structurally ordered arrays of nanoscopic pores is a prime example, where CNT does not capture nucleation. Many solidification processes from a liquid cannot be adequately described by classical nucleation theory (CNT).