Nucleation of FAU and LTA Zeolites from Heterogeneous Aluminosilicate Precursors

Abstract

The nucleation of many natural, biogenic, and synthetic crystals involves the initial formation of metastable precursors that provides a kinetic pathway for an amorphous-to-crystalline transformation. This nonclassical mechanism is believed to be the dominant crystallization pathway for microporous zeolites. Despite significant research on zeolite growth mechanisms, molecular level details regarding the assembly, physicochemical properties, and structural evolution of amorphous (alumino)silicate precursors remain elusive. Here we use a combination of diffraction, scattering, and microscopy techniques to characterize the amorphous precursors that assemble and evolve during the synthesis of zeolites FAU and LTA -two materials that are widely used in commercial applications such as catalysis, adsorption, separations, and ion-exchange. Nucleation occurs by a two-step mechanism involving the initial formation of aggregates that serve as heterogeneous sites for nucleation. Using colloidal silica as a reagent, we observe that precursors are comprised of heterogeneous silica and alumina domains due in part to the negligible dissolution of silica during room temperature aging. This indicates substantial Si-O-Si bond breakage must occur during hydrothermal treatment with concomitant exchange of soluble alumina species to achieve a final crystalline product with Si/Al ratio = 1.0 -2.5. All syntheses were performed with molar compositions of Si/Al ≥ 2.0, which favors the formation of FAU; however, we observe that certain growth conditions are capable of creating a "false" environment (i.e., Al-rich regions) that favors LTA nucleation, followed by intercrystalline transformation to FAU. Time-resolved ex situ transmission electron microscopy of extracted solids during zeolite crystallization indicates that nucleation occurs on the exterior surface of precursors. This observation is consistent with our proposed hypothesis that posits exterior surfaces are more energetically favorable sites for nucleation compared to the particle interior on the basis of confinement effects. Given that numerous zeolite syntheses involve the initial formation of metastable precursors with heterogeneous composition, the pathway for nucleation proposed in this study may prove to be generalizable to other zeolite structures and related materials.

Ultrasmall Zeolite L Crystals Prepared from Highly Interdispersed Alkali‐Silicate Precursors

“…In zeolite synthesis,t he addition of as ilicon source to alkaline media can result in little dissolution owing to low silica solubility and the deposition of alumina on exterior surfaces of silicates, [16] which decreases the rate of precursor dissolution. This leads to heterogeneous particles comprised of isolated silica and alumina domains where alkali metal distribution has not been sufficiently addressed.…”

“…All reagents yield sols with alumina on the exterior surfaces of silica particles (as reported in our previous study). [14,16] An exception is fumed silica, which initially yields av iscous gel that gradually decomposes to aliquid sol after rigorous stirring ( Figure S18). During room temperature aging,t ime-resolved changes in supernatant pH ( Figure 3D) and ionic conductivity ( Figure S19) reveal distinct differences among silicon sources,w hich likely reflect ad isparity of precursor dissolution.…”

“…Ther oles of precursors in zeolite nucleation, as well as their potential function as growth units during crystallization, have been postulated;h owever, these processes are generally not well understood for the vast majority of zeolites. [13b,14] Moreover, quantitative relationships between the physicochemical properties of source reagents [16] and their corresponding amorphous precursors [14] have yet to be established. It is reasonable to assume that the choice of silicon and aluminum sources can impact the pathways of zeolite crystallization, either through nonclassical routes involving growth by the attachment of amorphous precursors or via classical pathways governed by monomer addition.…”

Organic‐Free, ZnO‐Assisted Synthesis of Zeolite FAU with Tunable SiO2/Al2O3 Molar Ratio

“…[4] However,z eolite FAUi sn ormally synthesized with relatively low SiO 2 /Al 2 O 3 molar ratio and then, the interaction between CO 2 and the zeolitei st oo strong to desorbC O 2 .T o lessen the gas-solid interaction whilst maintain ag ood adsorption capacity,z eolite FAUw ith relatively high SiO 2 /Al 2 O 3 molar ratio is demanded. [20][21][22][23][24] Also, the tailor of SiO 2 /Al 2 O 3 molar ratio can be achieved through post synthesis modification (dealumination), however,i tw ill give rise to the appearanceo fm esoporosity and ar eduction of crystallinity, which is detrimental to the separation performance. [5] Additionally,z eolites with different SiO 2 /Al 2 O 3 molar ratios are preferable for the separation of CO 2 /CH 4 mixtures with different feed pressures.…”

“…The common organic-free synthesis methods include both with and without seed-directed strategies to obtain various zeolites. [20][21][22][23][24] Also, the tailor of SiO 2 /Al 2 O 3 molar ratio can be achieved through post synthesis modification (dealumination), however,i tw ill give rise to the appearanceo fm esoporosity and ar eduction of crystallinity, which is detrimental to the separation performance. [20][21][22][23][24] Also, the tailor of SiO 2 /Al 2 O 3 molar ratio can be achieved through post synthesis modification (dealumination), however,i tw ill give rise to the appearanceo fm esoporosity and ar eduction of crystallinity, which is detrimental to the separation performance.…”