Nucleation of Porous Crystals from Ion-Paired Prenucleation Clusters

Abstract

Current nucleation models propose manifold options for the formation of crystalline materials. Exploring and distinguishing between different crystallization pathways on the molecular level however remain a challenge, especially for complex porous materials. These usually consist of large unit cells with an ordered framework and pore components and often nucleate in complex, multiphasic synthesis media, restricting in-depth characterization. This work shows how aluminosilicate speciation during crystallization can be documented in detail in monophasic hydrated silicate ionic liquids (HSILs). The observations reveal that zeolites can form via supramolecular organization of ion-paired prenucleation clusters, consisting of aluminosilicate anions, ion-paired to alkali cations, and imply that zeolite crystallization from HSILs can be described within the spectrum of modern nucleation theory.

Ion-Pairs in Aluminosilicate-Alkali Synthesis Liquids Determine the Aluminum Content and Topology of Crystallizing Zeolites

“…Liquid-state 27 Al NMR measurements have shown that the aluminate speciation in HSIL synthesis liquids exclusively consists of small aluminosilicate oligomers, and zeolites crystallize from these liquids upon the supersaturation of specific aluminosilicate oligomers. 20 Therefore, it is possible to rationalize our observations by considerations of aluminosilicate solution chemistry. 1,69 Increasing chemical activity of hydroxide promotes depolymerization of aluminosilicates via deprotonation and hydrolysis, increasing the oligomer charge density and stabilizing oligomers of low nuclearity, including the Al-containing species.…”

“…At room temperature, upon the addition of aluminate, all mixtures with [SiO 2 ]/[MOH] < 0.5 and [H 2 O]/[MOH] < 15 form monophasic, optically clear liquids. 20 This region, characterized by high alkalinity and low water content, yields hypo-hydrated ionic liquids. 17 , 20 In these liquids, high charge density stabilizes aluminum in small aluminosilicate oligomers through intimate interactions with alkali cations, leading to aluminate solubility much higher than previously predicted by aluminosilicate speciation models, 12 , 20 − 23 similar as observed for the increased solubility of gibbsite in concentrated electrolytes.…”

“… 20 This region, characterized by high alkalinity and low water content, yields hypo-hydrated ionic liquids. 17 , 20 In these liquids, high charge density stabilizes aluminum in small aluminosilicate oligomers through intimate interactions with alkali cations, leading to aluminate solubility much higher than previously predicted by aluminosilicate speciation models, 12 , 20 − 23 similar as observed for the increased solubility of gibbsite in concentrated electrolytes. 24 With the increase in dilution, ionic stabilization of small aluminosilicate oligomers decreases, facilitating their condensation into larger aluminosilicate species with a lower solubility, therefore, triggering the formation of aggregated aluminosilicate species, manifesting as colloids 20 , 25 ( Figure 1 ).…”

“…Therefore, increasing ionic interactions aids the reduction of oligomer size. 20 This effect is most pronounced at high ion concentrations determined by the number density of the cation, that is, low values of [H 2 O]/[MOH] and [SiO 2 ]/[MOH], explaining both the flattening of the planes ( Figure 2 ) at low alkalinity and at high hydration.…”

“…These templating cations were often thought to attach to the growing crystal surface independently and induce reorganization of either an amorphous layer of framework species or small oligomers from solution into the most stable configuration. 1 , 12 Provided that ionic interactions between cations and aluminosilicate oligomers are already dominantly present before the onset of nucleation, 20 zeolite formation and transformation can instead be seen as a supramolecular organization, simultaneously incorporating framework and extra-framework species from solution in a concerted assembly. These oligomer–cation interactions are necessary to stabilize viable building units in the solution and also to facilitate their polycondensation on the growing crystal (templating effect).…”