Tuning Zeolite Precursor Interactions by Switching the Valence of Polyamine Modifiers

Abstract

Nonclassical mechanisms of crystal growth often involve the formation of amorphous precursors that play a direct role in what is generally referred to as crystallization by particle attachment (or CPA). One of the most studied CPA systems in the literature is zeolite MFI, which is a microporous crystal with siliceous (silicalite-1) and aluminosilicate (ZSM-5) isostructures. The self-assembly, microstructural evolution, and mechanistic role of nanoparticle precursors (1-6 nm) during silicalite-1 crystallization have been the subjects of prior investigation by combined experimental and modeling techniques. Here we investigate for the first time the effects of zeolite growth modifiers (ZGMs) on MFI precursors. ZGMs are organic molecules that alter the anisotropic rate(s) of crystal growth as a means of tailoring crystal size and/or habit. We show that most ZGMs have little effect on precursor assembly and evolution during the prenucleation stages of silicalite-1 and ZSM-5 synthesis; however, studies at varying alkalinity reveal that pH can be used as a "switch" to tune ZGM speciation and concurrently the colloidal stability of precursors. This has been proven effective for various polyamine compounds, such as spermine, that exhibit divalent (positive) charge near negatively charged nanoparticle surfaces. Our finding is consistent with colloidal models that predict a higher concentration of divalent modifiers within the diffuse double layer surrounding the surfaces of (alumino)silicate precursors. Multivalent polyamines seemingly promote precursor-precursor aggregation at elevated temperature, which is consistent with a proposed hypothesis that modifiers with two or more sufficiently spaced cationic functional moieties are capable of bridging neighboring precursor surfaces, thus overcoming an electrostatic repulsive force that contributes to their colloidal stability. Given the importance of precursor-precursor and precursor-crystal interactions in zeolite nucleation and growth, respectively, our observations provide additional insight into the role of organics in zeolite crystallization.

Regulating Nonclassical Pathways of Silicalite‐1 Crystallization through Controlled Evolution of Amorphous Precursors

“…There is a growing interest to characterize growth species (i.e. soluble silicates and particulates) and elucidate their impact, in combination with organics, on zeolite crystal growth. In this study, we show that a switch from growth by particle attachment to one involving monomer addition is achieved by restricting the disorder‐to‐order transition of precursors during synthesis.…”

Regulating Nonclassical Pathways of Silicalite‐1 Crystallization through Controlled Evolution of Amorphous Precursors

“…Indeed, Rimer et al showed that as-synthesized silica nanoparticles have ar ate of dissolution that lies between amorphous silica and silicalite-1;a nd the disorder-to-order transition during nanoparticle evolution leads to amonotonic reduction in the rate of dissolution. Time-resolved SAXS patterns (Figure 2A) show am onotonic reduction in the scattering intensity at room temperature.Using areported protocol, [17] the average size of nanoparticles was extracted from SAXS data for all solutions aged at room temperature (0 h, as synthesized) and those heated at 60 8 8Cfor 5hto assess the difference between as-synthesized and evolved nanoparticles,r espectively. Time-resolved SAXS patterns (Figure 2A) show am onotonic reduction in the scattering intensity at room temperature.Using areported protocol, [17] the average size of nanoparticles was extracted from SAXS data for all solutions aged at room temperature (0 h, as synthesized) and those heated at 60 8 8Cfor 5hto assess the difference between as-synthesized and evolved nanoparticles,r espectively.…”

“…[22] Using dissolution rate as as urrogate for precursor structure,w et racked the temporal reduction in nanoparticle size with small-angle X-ray scattering (SAXS) by adding excess TAAOHp rior to the start of the experiment to increase solution pH, thereby initiating nanoparticle dissolution ( Figure S6). Time-resolved SAXS patterns (Figure 2A) show am onotonic reduction in the scattering intensity at room temperature.Using areported protocol, [17] the average size of nanoparticles was extracted from SAXS data for all solutions aged at room temperature (0 h, as synthesized) and those heated at 60 8 8Cfor 5hto assess the difference between as-synthesized and evolved nanoparticles,r espectively.…”

“…[12a] Shete et al [15] recently showed that changes in the OSDAcan suppress nonclassical growth by inhibiting nanoparticle attachment. the impact of organic additives (OSDAs and ZGMs) on the extent of precursor evolution, and the degree to which structural changes influence precursor-precursor and precursor-crystal interactions.T here is ag rowing interest to characterize growth species (i.e.s oluble silicates and particulates) and elucidate their impact, [17] in combination with organics, [18] on zeolite crystal growth. [8c, 16] Herein we assess the degree to which nanoparticle structure influences nonclassical pathways.W ei nvestigate Scheme 1.…”

“…the impact of organic additives (OSDAs and ZGMs) on the extent of precursor evolution, and the degree to which structural changes influence precursor-precursor and precursor-crystal interactions.T here is ag rowing interest to characterize growth species (i.e.s oluble silicates and particulates) and elucidate their impact, [17] in combination with organics, [18] on zeolite crystal growth. In this study,w es how that as witch from growth by particle attachment to one involving monomer addition is achieved by restricting the disorder-to-order transition of precursors during synthesis.…”