Assembly and Evolution of Amorphous Precursors ... - ACS Publications

Subscriber access provided by NEW YORK UNIV

Article

Assembly and Evolution of Amorphous Precursors in Zeolite L Crystallization Manjesh Kumar, Rui Li, and Jeffrey D Rimer Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b04569 • Publication Date (Web): 20 Jan 2016 Downloaded from http://pubs.acs.org on February 3, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Chemistry of Materials is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 14

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Chemistry of Materials

Assembly and Evolution of Amorphous Precursors in Zeolite L Crystallization Manjesh Kumar†, Rui Li†, Jeffrey D. Rimer* University of Houston, Chemical and Biomolecular Engineering, Houston, TX 77204 ABSTRACT: The formation of amorphous bulk phases in zeolite synthesis is a common phenomenon, yet there are many questions pertaining to the physicochemical properties of these precursors and their putative role(s) in the growth of microporous materials. Here, we study the formation of zeolite L, which is a large-pore framework (LTL type) with properties that are well-suited for catalysis, separations, photonics, and drug delivery, among other applications. We investigate the structural and morphological evolution of aluminosilicate precursors during zeolite L crystallization using a variety of colloidal and microscopy techniques. Dynamic light scattering measurements of growth solutions and scanning electron microscopy (SEM) images of extracted solids collectively reveal that zeolite L precursors assemble through a series of steps, leading to branched worm-like particles (WLPs). Transmission electron microscopy and electron dispersion spectroscopy show that WLPs have a heterogeneous composition that predominantly consists of silica-rich domains. We demonstrate that static light scattering (SLS) measurements can be used to identify the approximate induction time and is a reliable method to quantitatively track the extent of crystallization. During the induction period, the average size of zeolite L precursors monotonically increases by the accretion of soluble species. Precursor growth continues until the onset of zeolite L nucleation when WLPs reach a maximum size. During zeolite L growth, the number density of precursors decreases in favor of a growing population of crystallites. Ex situ SEM images reveal the progressive formation of crystal nuclei, which deviates from the classical LaMer process that posits a nearly instantaneous generation (or burst) of nuclei. These findings provide evidence of zeolite L growth via a nonclassical pathway involving crystallization by particle attachment (CPA). Given the ubiquitous presence of WLP-like precursors in syntheses of numerous zeolites, CPA processes may prove to be broadly representative of growth mechanisms for other zeolite framework types and related materials.

INTRODUCTION Zeolites are microporous aluminosilicates that possess unique properties based on their diverse porous topologies, tunable acidity, and exceptional thermal stability1, 2. The commercial relevance of these materials has been demonstrated for a wide range of applications in areas spanning energy to medicine. Examples include their use in catalysis, adsorption, ion-exchange, selective separations, photonics, and drug delivery.3-9 More than 220 different framework types have been synthetically realized. The majority of zeolite syntheses require the use of an organic structuredirecting agent (OSDA), which is a molecule with a size and geometry that is commensurate with the channels and cages of zeolites. OSDAs facilitate crystallization and provide kinetic pathways to achieve metastable structures that otherwise are incapable of forming in their absence; however, due to economic and environmental considerations there is increased interest to develop synthetic methods that avoid the use of OSDAs. Approaches to either remove or reduce the quantity of OSDA in zeolite synthesis include the design of kinetic phase diagrams,10 the use of inorganic structuredirecting agents (e.g., alkali metals),11 crystal seeding techniques,12, 13 intercrystalline conversion,14, 15 and the development of recyclable OSDAs.16 OSDAs are typically expensive and become occluded within zeolite pores, thus requiring energy-intensive post-synthesis calcination to remove the organics. OSDA-free synthesis is preferred for commercial zeolite production; however, these syntheses are challenging to implement due to their propensity to generate crystal polymorphs (i.e., impurities). One of the most significant obstacles in the rational design of zeolites is the significant knowledge gap between the fun-

damental understandings of zeolite crystal growth mechanisms and the ability to use this information to predictively tailor the physicochemical properties of materials. Crystals grow by either classical or nonclassical pathways. The former proceeds via the addition of monomers (i.e., ions or molecules) and the latter refers to crystallization by particle attachment (CPA) where precursors vary from oligomers and multi-ion complexes to nanoparticles and bulk amorphous phases.17 There is mounting evidence that many biogenic, natural, and synthetic materials grow by CPA.18-25 The growth of zeolite silicalite-1 (MFI type) is a preeminent example. In a prior study26 we used in situ atomic force microscopy to show that silicalite-1 grows by a dynamic sequence of events involving the addition of primary particles (ca. 3 nm) to crystal surfaces, the structural rearrangement of particles post-attachment (i.e., disorder-to-order transition), and molecule attachment. Another zeolite that grows by CPA is beta (BEA), which forms via the aggregation of precursors into secondary particles that subsequently rearrange and fuse together to form the final crystalline phase.27 Similar mechanisms have been postulated for a variety of zeolite framework types such as zeolite A (LTA),28 zeolite X (FAU),29 TS-1 (Ti-MFI),30 analcime (ANA),31 and beta (BEA).32 In many cases it is believed that growth occurs through the addition of smaller precursors (soluble species) that range from (alumino)silicate monomers/oligomers to complex composite building units (CBUs) such as polytetrahedra33 and hydrated organic-inorganic networks.34 The formation of amorphous precursors is ubiquitous in zeolite synthesis; and their mere presence throughout crystallization suggests that nonclassical pathways play a role in zeolite growth. There are many unknown aspects of zeolite precur-

1

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

sors, such as their microstructure, mobility, interparticle interactions, and their fundamental role in crystallization. It is possible that precursors are metastable phases that serve as a reservoir of (alumino)silicate nutrient that dissolves to provide a constant supply of monomers, oligomers, and/or other soluble species for crystallization. Studies of several zeolite framework types29, 35-37 provide evidence that precursors (or aggregates of precursors) are involved in nucleation. The formation of the amorphous (or semi-ordered) phase presumably lowers the energetic barrier for heterogeneous nucleation.38 To this end, the formation of precursors may be analogous to the two-step nucleation mechanism proposed for proteins39 and other systems (e.g., calcium minerals)24 wherein the amorphous phase is a highly supersaturated region that promotes molecule ordering. Indeed, the concept of precursor (or gel) transformation in zeolite synthesis has been proposed for MFI,40, 41 LTA,36 and FAU29 frameworks. The schematic in Figure 1 highlights the possible pathways associated with zeolite crystallization. Growth solutions can be prepared using a variety of different silica and alumina sources. For the purpose of this study, we will focus on the use of colloidal silica and aluminum sulfate. The majority of colloidal silica remains suspended in solution at the early stages of growth, while a fraction of the silica source dissolves to produce low molecular weight (alumino)silicate molecules (path i in Figure 1). Growth solutions are prepared with high silica/alumina supersaturation, which leads to the formation of metastable aggregates through the selfassembly of amorphous primary particles, which can further aggregate into bulk amorphous phases (path ii in Figure 1). One attribute of zeolite synthesis that transcends many different framework types is the presence of bulk amorphous particles with similar shapes and sizes. For instance, growth solutions of framework types ZSM-22 (TON),42 zeolite L,43, 44 ZSM-541, and zeolite P (GIS)45 contain amorphous precursors that resemble worm-like particles (WLPs) – a term originated by Subotić and coworkers41 to describe the highlybranched configuration of these species. Once formed, WLPs may continue to grow with prolonged hydrothermal treatment via the addition of molecules (path iii in Figure 1) or through aggregation events. Zeolite nucleation and growth can proceed by nonclassical pathways involving the attachment of WLPs (path iv in Figure 1) and/or a classical route involving the addition of molecules (path v in Figure 1). It is likely that both of these pathways occur simultaneously, although direct evidence is difficult to obtain given that zeolite growth solutions are typically not amenable to characterization using in situ techniques. For instance, obtaining timeresolved in situ data is difficult owing to several factors: (i) slow kinetics of crystallization requires long sampling times, (ii) harsh synthesis conditions (e.g., high temperature, pressure, and alkalinity) are difficult to reproduce with many imaging techniques, and (iii) non-ideal physical properties of growth solutions (e.g., high viscosity and opaque suspensions) render these systems inoperable to characterize by common analytical techniques. Nevertheless, ex situ studies have provided valuable information about mechanisms of zeolite growth35, 46-49 and methods to tailor their physicochemical properties.50, 51 In this paper we characterize precursors that form during the crystallization of zeolite L (LTL type), which has onedimensional (1D) pores (ca. 0.7-nm pore aperture) and a

Page 2 of 14

Figure 1. Illustration of the potential pathways involved in zeolite crystallization using colloidal silica as a representative silica source. Colloidal silica can dissolve (path i) to generate soluble species (i.e., monomers and/or oligomers) that can directly add to crystals (path v) via a classical pathway of crystallization. Colloidal silica can also aggregate to form worm-like particle precursors (path ii) that grow via the addition of molecules (path iii). Zeolite growth by precursor attachment (path iv) represents a nonclassical pathway of crystallization.

P6/mmm space group. Zeolite L is synthesized by OSDA– free methods and is used as a commercial catalyst for aromatization52 (e.g., Chevron Aromax® cyclization process)53, dehydrogenation,54 hydrogenation,55, 56 and cracking reactions57. Additional applications of zeolite L include its use as an adsorbent or host to accommodate guest molecules, such as dyes or drugs, for artificial antenna,4 photonic devices,58 and drug delivery scaffolds.9 Here, we explore the selfassembly and structural evolution of amorphous WLPs during zeolite L crystallization through a combination of experimental techniques. Our findings reveal that WLPs are highly-siliceous structures that grow in size during the early stages of zeolite L nucleation. We use a combination of light scattering and electron microscopy to examine the putative pathway(s) of zeolite L formation. Our findings provide evidence that growth occurs by complex processes involving both classical and nonclassical pathways.

EXPERIMENTAL SECTION Materials. The following chemicals for zeolite synthesis were purchased from Sigma Aldrich (St. Louis, MO): Ludox AS-40 (40 wt% suspension in water), Ludox SM-30 (30 wt% in water), potassium hydroxide (85% pellets), aluminum sulfate hydrate (98%, 14-18 H2O, calculated as 18H2O), and 1-butanol (99.4%). Deionized (DI) water used in all experiments was purified with an Aqua Solutions RODI-C-12A purification system (18.2 MΩ). All reagents were used as received without further purification. Zeolite Crystallization. Zeolite L crystals were synthesized in the absence of an organic using K+ as an inorganic structure-directing agent. Growth solutions were prepared with a molar ratio of 0.5 Al2O3:20 SiO2:10.2 K2O:1030 H2O.51 Potassium hydroxide (0.64g, 9.66 x10-3 mol) was first dissolved in water (7.78g), followed by the addition of aluminum sulfate (0.17g, 4.73 x10-4 mol). This solution was stirred until clear (ca. 5 min). The silica source was added and the resulting solution was left to stir overnight (ca. 21 h) at room temperature (the “aging” period). Several silica sources were tested in this study. Unless otherwise stated, the growth solution was prepared with Ludox AS-40 (1.42g, 9.47 x10-3 mol). The colloidal silica source was added to the growth solution dropwise while stirring. After aging was complete, the growth solution (ca. 10g) was placed in a Tef-


2

Page 3 of 14

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


lon-lined stainless steel acid digestion bomb (Parr Instruments) and was heated under static conditions (i.e., without mixing) in a ThermoFisher Precision Premium 3050 Series gravity oven at 180°C and autogenous pressure. Zeolite L growth solutions and the resulting crystals prepared by this procedure are referred to herein as the control. Aliquots of the growth solution were extracted for light scattering measurements. The pH of the growth solution was measured with a Thermo Scientific Orion 3 Star meter. The ionic conductivity of the solution was measured with a VWR international EC meter (Model 2052). For X-ray and microscopy analyses, the particulates in the growth solution (amorphous and/or crystalline) were isolated as a white powder (ca. 200 mg) by centrifugation at 13,000 rpm for 45 min per cycle. The solid was washed with DI water to remove the supernatant. The centrifuge-wash cycle was repeated twice and the resulting gel was dried at ambient conditions. During the preparation of microscopy samples, an aliquot of the supernatant was placed on a glass slide and dried overnight. Crystals on the glass slide were transferred to SEM sample holders (Ted Pella) by gently pressing the glass slide on carbon tape. Materials Characterization. Solids extracted from zeolite L growth solutions were characterized by powder X-ray diffraction (XRD) using a Siemens D5000 X-ray diffractometer with CuKα radiation (40 kV, 30mA, 1.54 Å). Scanning electron microscopy (SEM) was performed with a FEI 235 Dual-Beam (Focused Ion-beam) system operated at 15 kV and a 5-mm working distance. All SEM samples were coated with a thin layer of carbon (ca. 30 nm) prior to imaging. Transmission electron microscopy (TEM) was performed at the TAMU Microscopy & Imaging Center (College Station, TX) using a TECNAI F20 Super-Twin TEM fitted with a Schottky field emission gun, a Gatan CCD camera (2k x 2k), and an EDAX instruments ultrathin window energy dispersive spectroscopy (EDS) detector. TEM images and EDS spectra were collected at a 200 kV accelerating voltage. TEM samples were prepared by dispersing a small quantity of dry powder in DI water by sonication. An aliquot of this dilute, translucent solution was placed on a TEM grid (300 mesh lacey carbon on Cu) and dried at ambient conditions. Light scattering measurements were performed on a Brookhaven Instruments BI-200SM machine equipped with a TurboCorr Digital Correlator, a red HeNe laser diode (35mW, 637 nm), and a decalin bath that was filtered to remove dust. The liquid sample cell was regulated at 25°C with a Polyscience digital temperature controller. Samples were prepared by diluting zeolite growth solutions in prefiltered DI water (0.45-µm nylon syringe filter, Pall Life Sciences) to achieve reasonable scattering counts (>20,000 count-sec-1). Dynamic light scattering (DLS) was performed at a scattering angle of 90°. At least three measurements were performed per sample. Autocorrelation functions were collected over a 2 min timeframe and data were analyzed by two techniques: (i) the method of cumulants to obtain the hydrodynamic diameter DH, and (ii) CONTIN59 to assess particle size distribution. Viscosity measurements were conducted with a calibrated CUC-25 Cannon-Ubbelohde Viscometer (9721-K50, kinematic viscosity range 0.5 - 2 mm2/s). At least three measurements were performed for each sample. Dynamic viscosity of the sample was obtained by multiplying the measured kinetic viscosity value with the sample solution density. The average dynamic viscosity of the sample was compared with that of DI water at the same temperature. Textural analysis

of zeolite L crystals was performed with a Micromeritics ASAP 2020 instrument using N2 as a probe gas for physisorption with an incremental dosing rate of 3 m3/g STP and an analysis bath temperature of 77 K. Static light scattering (SLS) measurements were performed to assess the angular dependence of scattering intensity over a range of angles from 30° to 150° (with an angular resolution of 5°). A log-log plot of SLS data was generated for each sample with the intensity I(q) plotted as a function of the scattering vector, = 4 ∙ sin ⁄2, where n is the refractive index of the solvent (estimated as pure water), λ is the wavelength of the laser, and θ is the scattering angle. The scattering intensity is given by the relationship, = V ∆

(1)

where is the particle volume fraction, V is the particle volume, ρ is the scattering length density, ∆ρ is the contrast, P(q) is the form factor, and S(q) is the structure factor.60 A 2-dimensional fractal dimension df,2D of amorphous precursors in zeolite L growth solutions was calculated from SEM images using the conventional box counting technique (see Gamage et al.61 for a detailed procedure). In brief, high resolution scanning electron micrographs were subdivided into grids of uniform square box sizes of length L. This procedure was repeated multiple times using box sizes of L = 2, 3, 4, 6, 8, 12, 16, 32, and 64 pixels. The following equation was used to calculate the fractal dimension, , = lim"→$ −

&'() &'("

(2)

where L is the size (or resolution) of the square box and N is the number of boxes either fully or partially occupied by the particle. Values of N were obtained using the software package ImageJ, and the fractal dimension was calculated from the slope of a log-log plot of N versus L. This procedure was repeated for three particles in different SEM images to obtain an average df,2D. For this analysis, we selected particles that were representative of the batch. The maximum value of df,2D is 2.0 corresponding to non-fractal particles. RESULTS AND DISCUSSION Self-Assembly of Amorphous Precursors. The schematic in Figure 2A outlines the putative pathways of zeolite L precursor formation from growth solutions that are initially comprised of 25-nm colloidal silica suspension that is placed in contact with the alumina source. Soluble species in the growth solution include silica, alumina, and/or aluminosilicate molecules that are present as either monomers or oligomers. Prior studies have shown that zeolite precursor solutions may contain oligomers with diverse size and connectivity.62-64 SEM images of precursors extracted from growth solutions at periodic heating times reveal the initial formation of relatively monodisperse spheroidal particles with an average diameter of 70 ± 8 nm (Figure 2B). These particles form within the first hour of hydrothermal treatment, presumably through the aggregation of undissolved colloidal silica; however, the absence of distinct 25-nm features on the surfaces of these particles (Figure 2C), equal to the size of the initial colloidal silica, suggests that aggregation is accompanied by a coarsening (ripening) process. The schematic in Figure 2A depicts these species as a single particle representing an aggregate of “fused” colloidal silica. Definitive


3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

proof of this mechanism cannot be discerned from SEM images alone; however, studies of silica dissolution and TEM images (discussed in later sections) provide additional evidence for this proposed pathway of formation. After 2 h of heating, we observe the onset of worm-like particle (WLP) formation via the aggregation of spheroidal precursors (Figure 2D). Particle aggregation results in the formation of branched structures (Figure 2E) with domains that appear to be the ca. 75-nm spheroidal particles. WLPs exhibit fractal morphology (Figure 2F) wherein the degree of branching increases during the first 4 h of heating, then diminishes at later times as the frequency of spheroidal particle attachment to WLPs subsides.

Figure 2. (A) Proposed pathway(s) of worm-like particle (WLP) formation during the induction period of zeolite L crystallization. (B – G) Scanning electron micrographs capturing the evolution of amorphous bulk phases at periodic times during hydrothermal treatment: (B and C) 1 h, (D and E) 2 h, and (F and G) 4 h. SEM images on the left- and right-hand sides are low and high resolution depictions of amorphous precursors, respectively.

Within the first 10 h of heating, WLPs grow to 0.5 – 1.0 µm in contour length (Figure 2G) and exhibit a broad distribution of branched motifs. Based on powder XRD analysis, WLPs extracted from growth solutions are amorphous (see Figure S1 of the Supporting Information, SI). As illustrated in Figure 2A, the formation of WLPs can be arbitrarily divided into an early stage when spheroidal particles first begin to aggregate and fuse, and a later stage when growth ceases and WLPs reach their final size and shape. We tracked the change in precursor size throughout the progression of events in Figure 2A using a combination of DLS at

Page 4 of 14

earlier times and SEM at later times. DLS provides measurements of the hydrodynamic diameter DH of particles, which was ideal for tracking the assembly and growth of spherical particles. As shown in Figure 3A, the spheroidal aggregates are formed almost immediately upon the mixing of reagents prior to room temperature aging. This observation is consistent with the aggregation pathway proposed in Figure 2A. Indeed, growth solutions with colloidal silica are initially clear, but become opaque with the addition of an alumina source (Figure S2). DLS measurements of the growth solution immediately after mixing the reagents reveal a relatively monodisperse distribution of particles (DH ≈ 75 nm). The particle size does not appreciably change over the course of room temperature aging; however, once the growth solutions are placed in the oven at 180°C, there is a monotonic increase in DH (Figure 3B) from 75 to 200 nm within 4 h that is attributed to a combination of spheroidal particle

Figure 3. (A) Dynamic light scattering analysis of the hydrodynamic diameter DH of amorphous precursors during room temperature aging of the growth solution. Inset: schematic highlighting the WLP dimensions. Data was collected at 25°C using the refractive index of water and a measured viscosity of 0.897 cP. (B) Ex situ DLS measurements of DH for samples extracted from a growth solution during hydrothermal treatment at 180°C. (C) The width of WLPs measured from SEM images of extracted solid over a 16-h period of heating. The shaded area corresponds to the induction period when WLPs first begin to form. DLS data are the average of at least three measurements (error bars are less than the size of the symbols). Each WLP width is an average of at least 50 measurements and the error bars equal two standard deviations. The solid line is a linear regression and the dashed lines are interpolations to help guide the eye.


4

Page 5 of 14

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


aggregation and growth. During the first hour of heating there is a slight decrease in spheroidal aggregate size due to particle dissolution. Moreover, it should be noted that the onset of growth is partially delayed due to heat transfer effects in the acid digestion bomb (synthesis vessel), i.e., there is a gradual increase in the growth solution temperature from ambient to the oven set point.65 DH values measured by DLS correspond to the hydrodynamic diameter of spheroidal aggregates. Once WLPs begin to form (ca. 2 h), their anisotropic shape gives rise to DH values that reflect a geometrically-averaged particle size (illustrated in the inset of Figure 3A). The branched geometry of WLPs is non-ideal for DLS measurements; therefore, we used SEM images as an alternative method to track the temporal evolution of WLP size at prolonged heating times, focusing on changes in WLP width. There is excellent agreement between DLS and SEM measurements of spherical aggregate size at early heating times (0 – 4 h). SEM images reveal that the width of WLPs monotonically increases with heating time (Figure 3C), reaching a plateau in size around 12 h. The plateau in WLP size occurs around the same time as zeolite L nucleation, as we will discuss in the following section. The incremental growth of WLPs is qualitatively consistent with a pathway dominated by the addition of soluble species from solution. Indeed, WLP growth by the alternative mechanism – the addition of spheroidal aggregates – would lead to stepwise changes in WLP width on the order of a single particle (ca. 75 nm), which is not observed in SEM images. Conversely, an increase in WLP length or branching is primarily attributed to the attachment of spheroidal aggregates. The appearance of a seemingly continuous, smooth linkage between each WLP domain in electron micrographs suggests post-attachment growth and/or rearrangement of each spheroidal aggregate that integrates into the WLP (possibly via a ripening process). It is not well understood what fundamentally drives particle attachment and

Figure 4. (A) Two-dimensional fractal dimension, df,2D, of WLPs during the early stage of synthesis (i.e., first 4 h of the induction period). (B) Representative profiles of WLPs after 2, 3, and 4 h of hydrothermal treatment. The gridlines illustrate the box counting technique for df,2D calculations. All scale bars equal 100 nm.

why WLPs reach a maximum size without continued growth and/or aggregation after 12 h of heating. The 2-dimensional (2D) fractal dimension df,2D of WLPs was assessed from SEM images using the conventional box counting technique (Eq. 2) to characterize the degree of branching that defines their bulk morphology. We began tracking the fractal dimension at the onset of WLP formation (ca. 2 h). As shown in Figure 4, WLPs that form during the early stage of heating are more spheroidal in geometry and less fractal (df,2D = 1.8, close to the maximum 2.0); however, there is a decrease in df,2D to values of 1.6 with increased heating time. This is consistent with a higher degree of branching (Figure 2G) during the temporal evolution of WLP morphology. Figure 4B provides representative examples of two-dimensional WLP profiles at periodic heating times that were used in box counting calculations. Tracking the Extent of Crystallization. The structural evolution from precursors to fully-grown crystals was monitored by static light scattering (SLS). At the very beginning of room temperature aging, the initial suspension of colloidal silica particles (ca. 25 nm) transitions into a population of larger spheroidal precursors (ca. 75 nm). The form factor P(q) for spherical particles with radius R is the following: = *

+,-. /0 /0 1', /0 /02

3

(3)

Simulated plots of P(q) for spheres with diameters 25 and 75 nm are provided in Figure 5A (black and red lines,

Figure 5. Simulated SLS scattering patterns using the form factors P(q) for monodisperse (A) spheres and (B) cylinders. In panel A, the plots of spheres with diameters of 25 nm (black line) and 75 nm (red line) represent colloidal silica and the spheroidal precursors in zeolite growth solutions, respectively. In panel B, the plots of cylinders correspond to the average size of WLPs (black line; dimensions R = 86 nm and L = 650 nm) and LTL crystals (red line; dimensions R = 882 nm and L = 3300 nm). The blue highlighted regions correspond to the q range of SLS measurements. Dashed lines with labels indicate the slopes.


5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 14

respectively). The q-range of the SLS measurements in the blue shaded region (0.0068 to 0.025 nm-1) is inside the Guinier region. In theory, the slope of this curve should be zero when plotting the data on a log-log scale; however, we measured a slope equal to –2.0, which can be attributed to structure factor S(q) effects related to interparticle interactions.66 The morphology of WLPs can be approximated as a cylinder with radius R, length L, and the form factor @/ 567 /"/9:5;

= 4$

/ 7 "/7 9:5 7 ;

?7 /056; / 7 07 567 ;

AB

(4)

where J1 is a first-order Bessel function.67 A simulated plot of P(q) for WLP cylinders is shown in Figure 5B (black line). The q-range of SLS measurements (blue shaded region) lies within the linear region with a slope equal to – 1.4.66 A simulated plot of P(q) for cylindrical LTL crystals (Figure 5B, red line) reveals a shift to lower q values, which is expected on the basis of their increased size relative to WLPs. The q-range for LTL crystals lies within the crosssection region where the slope is –3.2.66 Periodic oscillations at higher q are characteristic of monodisperse particles; however, the systems in this study are polydisperse, which results in linear plots. Representative SLS I(q) data for the spherical precursors, WLPs, and LTL crystals are provided in Figure S3 of the SI. Herein we refer to the slopes of log-log I(q) plots as β. In Figure 6A we show the monotonic change in β for growth solutions heated for periodic times. Solutions were quenched to room temperature and diluted in DI water prior to SLS analysis. The initial decrease in β is attributed to the transition from spherical to WLP precursors. Beginning around 6 h we observed a relatively rapid increase in β, which was then followed by a gradual increase with heating time until the maximum value of ca. 3.0 was reached at approximately 25 h. The initial increase in β is attributed to the onset of nucleation and its continued increase with time is due to LTL crystallization. The measured intensity in SLS data reflects the scattering from both WLPs and LTL crystals. As the population of WLPs decreases in favor of an increasing population of LTL crystals, the value of β monotonically increases. The maximum β is in good agreement with the value obtained from the simulated P(q) curve (Figure 5B). To provide additional evidence that the SLS pattern does indeed reflect the growth of LTL crystals, we isolated the WLP particles by filtering growth solutions with a 0.45-µm syringe filter to remove LTL crystals, which are approximately 3-times larger than WLPs. The remaining particulates in the filtered supernatant are primarily WLPs, but may also contain a minor fraction of zeolite L nuclei and/or small crystallites that would contribute to the scattering intensity. SLS analysis of the filtered growth solutions (Figure 6A, circles) revealed that the β value of WLPs is roughly constant with increased heating time over a 16-h period, consistent with SEM images at later stages of crystallization showing a constant WLP size (see Figure 3C). Light scattering measurements at times greater than 16 h were difficult to perform due to the insufficient scattering intensity attributed to the decreased population of precursors during zeolite L growth. Figure 6B contains powder XRD patterns of solids extracted over the course of zeolite L crystallization. When comparing XRD and SLS data, the latter is a more sensitive method of detecting the onset of crystal nucleation.

Figure 6. (A) The slope of I(q) curves, β, for zeolite L growth solutions as a function of heating time. Samples were extracted at periodic times from an oven operated at 180°C. Samples were cooled to room temperature and diluted with DI water prior to analysis. Ex situ static light scattering measurements were performed with samples that were either filtered using a 0.45-µm membrane (circles) or unfiltered (diamonds). Insert: SEM image of a crystal (arrow) formed after 7 h of heating, marking the approximate onset of nucleation (scale bar = 200 nm). (B) Powder XRD patterns of extracted solids from growth solutions after 8, 10, 12, and 24 h of heating.

For instance, the increase in β around 6 h heating time seemingly indicates the onset of zeolite L nucleation, which is consistent with SEM images of extracted solids that contain a small percentage of cylindrical particles (Figure 6A, inset) similar in size and habit to those of fully-grown zeolite L crystals. The first appearance of Bragg peaks in XRD patterns is detected around 10 h. It is well known that XRD is not an accurate method for assessing the induction period of crystallization because Bragg peaks are not visible in spectra until crystals have reached an appreciable number and/or size where they constitute approximately 3 – 4 wt% of the total sample.29 We propose that β can be used as a descriptor to track zeolite crystallization. To our knowledge, this is the first time SLS data has been used to characterize crystal nucleation and growth. The use of β to track crystallization may have broader implications as a generalized tool for assessing the synthesis of other zeolite framework types, particularly given the ubiquitous formation of amorphous precursors in zeolite growth solutions. To further validate the use of SLS data as a method to characterize zeolite L crystallization, we assessed the timeelapsed evolution of β at three different synthesis temperatures: 140, 160, and 180°C. It is well known that an increase in temperature results in a reduced induction period and more rapid crystallization. As shown Figure 7A, we observe similar temporal profiles of β at each synthesis temperature: there is an initial decrease due to the formation of WLPs, an inflection at the onset of nucleation followed by a monotonic increase due to crystal growth, and a plateau once crystallization is complete. The primary differences in β profiles are the times corresponding to the onset and completion of


6

Page 7 of 14

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Chemistry of Materials and wide-angle scattering data.80-83 Our results indicate that β from SLS data is an alternative metric to track the extent of zeolite crystallization. Comparison of the three curves in Figure 7A reveals a distinct trend in the minimum β, which slightly decreases with increased temperature owing to changes in WLP aspect ratio. Bulk crystallization studies also show that changes in temperature influence the size of zeolite L crystals (Figure S5), which can be attributed to changes in the rate of nucleation as a function of synthesis temperature. For instance, higher temperature generally leads to fewer nuclei, resulting in larger crystals.84, 85

Figure 7. (A) Temporal changes in the slope of I(q) data, β, from growth solutions heated at periodic times. A comparison is made for 3 different temperatures: 140°C (squares), 160°C (circles), and 180°C (diamonds). Each set of data includes measurements from at least 3 separately-prepared batches for statistical validation of the trends. Measurements of filtered samples containing WLPs are provided in Figure S6 of the SI. (B) Arrhenius plot of zeolite L crystal growth where the rate constant k using the Avrami-Erofeev relation (Eq. 5) was used to determine the extent of crystallization (see Figure S7). The solid line is linear regression (R2 = 0.94).

crystal growth. As expected, the induction period decreases with increased temperature as 25, 10, and 6 h for syntheses at 140, 160, and 180°C, respectively (this trend was also confirmed from time-resolved powder XRD patterns in Figure S4). The crystallization time also decreases with increased temperature as follows (from low to high temperature): 60, 40, and 30 h. A kinetic model of zeolite L growth was developed using the Avrami-Erofeev equation,68 ln1 − D = −EF6

(5)

where α is the fraction (or extent) of crystallization, which was modelled as β/β0 (where β0 = 3 corresponds to fullycrystalline zeolite L), k is the rate constant that is calculated using the Arrhenius expression k ∝ exp[–EA/RT], and n is the Avrami constant. The expression in Eq. 5 was originally developed for isothermal solid-solid transformations, but was later modified for non-isothermal conditions and has been applied to studies of different systems, such as polymer melts69, fatty acids70, zeolitic imidazolate frameworks (ZIFs)71, 72, and metal oxides.73, 74 Despite its inherent limitations, Eq. 5 has also been used to model the hydrothermal crystallization of zeolites.68, 75-78 Applying the equation to our study of zeolite L growth, we obtained a linear Arrhenius plot (Figure 7B) and an activation energy EA = 14 kcal/mol, which is in good agreement with previously reported values for zeolite L growth (e.g., 17 kcal/mol reported by Joshi et al.79). In prior studies, the extent of zeolite growth has been monitored by temporal changes in XRD patterns, IR spectra,

Heterogeneous Composition of WLPs. Zeolite growth solutions prepared with colloidal silica sources result in an initial dispersion of silicate particles that do not fully dissolve in basic solutions during room temperature aging.10 In the absence of alumina, we observe the partial dissolution of colloidal silica within the first 10 h of aging, after which the solution reaches equilibrium. We tracked silica dissolution by monitoring the temporal changes in solution pH and ionic conductivity. Dissolution of amorphous silica SiO2(s) releases silicic acid Si(OH)4 (Eq. 6), which in basic solutions can undergo a series of dissociation reactions (Eqs. 7 and 8) to produce negatively-charged silicates. For simplicity, we refer to the solution chemistry of monomers, realizing that zeolite growth solutions are more complex and contain a variety of oligomers.

≡ BI+ BIJ5 K3J IM NOOOP 3 ≡ BIJ5 K BIJ=,Q/ R BIJ=,Q/ NOOOP BIIJ +,Q/ K JQ/ R BIIJ +,Q/ NOOOP BI IJ

K JQ/ ,Q/

(6) (7) (8)

The release of a single silicic acid generates three Si-OH groups on the surface of the colloidal particles (we use the symbol ≡ to denote surface species). These freshly generated sites also dissociate in alkaline growth solutions,

R ≡ BIJ5 NOOOP ≡ BI5 K JQ/

(9)

The release of H+ ions during the dissolution of colloidal silica decreases solution pH, which was experimentally verified. Notably, we observed a decrease from pH 13.7 to 12.7 during 10 h of stirring at room temperature (Figure 8, closed circles). Changes in pH and silica speciation impact the ionic conductivity σ, S = ∑ U − ∑V K W U.Y K ∑ Z U Q

(10)

where Ci is the ion’s concentration, λi is the ion’s limiting molar conductivity, A and B are the Debye-Hückel-Onsager coefficients, and a and D are empirical fitting parameters.86 Competing effects of decreasing OH– concentration and increasing concentration of negatively-charged silicates lead to different trends in σ. Given the large difference in λi for OH– and SiO(OH)3– ions (λ = 198 and 31 S cm2 mole-1, respectively), the dissolution of colloidal silica results in a net decrease in σ (Figure 8, closed squares). Temporal changes in pH and σ are qualitatively consistent with DLS measurements of Al-free solutions that reveal a monotonic decrease in the hydrodynamic diameter of colloidal silica within the first 7 h of aging (see Figure S8).


7


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 8. Measurements of growth solution pH (circles; left axis) and conductivity σ (squares; right axis) during room temperature aging (i.e., prior to hydrothermal treatment). Control solutions containing both silica and alumina (open symbols) exhibit no change in their properties with aging time. For comparison, we analyzed the same solutions without alumina (closed symbols) and observed a decrease in both pH and σ within the first 10 h of continuous stirring due to silica dissolution.

Once the alumina source is introduced, the growth solution becomes a heterogeneous sol comprised of an aluminarich aqueous solution and a silica-rich colloidal dispersion. The deposition of alumina on the exterior surface of colloidal silica generates a core-shell particle, which we previously verified using energy filtered transmission electron microscopy.10 The alumina-rich shell inhibits dissolution of the silica-rich core, as evidenced by the constant pH and σ during room temperature aging (Figure 8, open symbols).87, 88 This agrees with DLS measurements (Figure 3A) showing that the size of precursors is approximately constant during aging. The hydrodynamic diameter of the spheroidal particles is nearly 3-times larger than that of the colloidal silica, which suggests alumina promotes the formation of 70-nm precursors. This is qualitatively consistent with the observation that growth solutions containing only silica are optically transparent, but instantaneously become opaque with the addition of alumina. As WLPs assemble and evolve during heating, spheroidal aggregates fuse together into branched particles. Transmission electron microscopy (TEM) images of representative precursors during the early (Figure 9A) and late (Figure 9B) stages of the induction period give no indication that precursor particles are fractal aggregates, which lends further evidence of the growth mechanism proposed in Figure 2A wherein particles fuse together during WLP formation. Indeed, nitrogen adsorption/desorption measurements of 4-h samples reveal a type II isotherm (Figure S9) and a low surface area of 19 m2/g, which is predominantly attributed to the exterior surface of WLP particles. The absence of micropores and mesopores in the adsorption isotherm suggests that precursors are dense aggregates (or that pores within the WLP interior are inaccessible to N2). The spatially-resolved elemental composition of precursors measured by electron dispersive spectroscopy (EDS) revealed that even after heating, WLPs exhibit a molar Si/Al ratio (SAR) that is greater than 100 (see Figure 9). This silica content is considerably higher than that of crystalline zeolite L,51 which has SAR = 3.3. Such a large disparity in elemental composition suggests that a single WLP cannot be converted into zeolite L without a solution-mediated process involving molecule exchange between the solid and liquid phases to extract Al from the growth solution. Another observation in this study was that WLPs do not uniformly

Page 8 of 14

Figure 9. Transmission electron micrographs of WLPs taken after (A) 1 h and (B) 4 h of heating at 180°C. Electron dispersive scattering (EDS) at various points within the sample was performed. The resulting silicon-to-aluminum molar ratio (Si/Al or SAR) is reported for each EDS position labeled in the respective images.

dissolve over the course of crystallization, but rather deplete in number. This suggests that the transport of solute from WLPs to growing zeolite L crystals may occur through more direct routes, such as WLP attachment to the crystal and/or dissolution in close proximity to the crystal surface. For instance, SEM images at periodic heating times will often capture what appears to be an intermediate stage of growth wherein spheroidal protrusions that are potentially the remnants of dissolving WLPs are detected on the surface of zeolite L crystals (see Figure S10). Effect of Colloidal Silica Size. We examined the influence of colloidal silica size on zeolite L precursor evolution and crystal growth using two different sources, Ludox AS-40 and Ludox SM-30, with average particle sizes of 25 and 8 nm, respectively. The nominal source used for experiments in this paper was the 25-nm silica particles. When the larger silica was replaced with particles less than half their size, we observed that the concentration of alumina in the growth solution was insufficient to suppress silica dissolution. As shown in Figure 10, there is a slight decrease in solution pH (Figure 10A) and a more significant reduction in conductivity (Figure 10B) during room temperature aging (time t = 0 – 21 h). These changes in solution properties are attributed to silica dissolution (Eqs. 6 – 9). Conversely, growth solutions containing larger silica particles exhibit constant pH and conductivity during room temperature aging. The disparity between silica sources is most likely due to the higher specific surface area of 8-nm silica particles, which can lead to thinner alumina shells and/or incomplete coverage of alumina on the exterior surfaces of colloidal silica. Once the growth solution is heated, there is a rapid (nearly step-wise) decrease in pH and conductivity for both samples, which is then followed by a gradual decrease in their values with prolonged heating time (Figure 10; t = 21 – 50 h). Net reductions in pH and σ are larger for growth solutions prepared with 25 nm colloidal silica.


8

Page 9 of 14

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 10. Time-resolved changes in the (A) pH and (B) ionic conductivity σ of growth solutions aged at room temperature (t = 0 – 21 h) and during heating at 180°C (t = 21 – 50 h). Zeolite L growth solutions were prepared with two different colloidal silica sources: 8-nm Ludox SM-30 (diamond symbols) and 25-nm Ludox AS-40 (square symbols). The molar compositions of the growth solutions are identical, but the SM-30 and AS-40 reagents contain small concentrations of stabilizing Na+ and NH4+ ions, respectively. The inset of B is a magnified view of the data to highlight the differences between the two samples during heating.

In Figure 11, we compare time-resolved SLS data for growth solutions prepared with smaller colloidal silica (SM30) and larger colloidal silica (AS-40). We observe nearly identical β values for WLPs formed from both colloidal silica sources, which suggests a similar degree of branching from the aggregation of spheroidal precursors. However, there are two differences in the temporal evolution of these samples. First, there is a shorter induction period for growth solutions prepared with the smaller SM-30 particles (ca. 4 h) compared to the control sample prepared with AS-40 particles (ca. 6 h). The rate of crystal growth, quantified using the Avrami–Erofeev relation (Eq. 5), is markedly higher for growth solutions prepared with SM-30 particles (Figure S11). For instance, SM-30 solutions reach full crystallinity within 8 h of heating, whereas growth solutions prepared with larger AS-40 particles require 25 h to completely crystallize. The WLPs generated from each silica source are initially similar in size, but the difference between WLP width diverges with increased heating time, as shown in Figure 11B. Thermal treatment of growth solutions prepared with the smaller SM-30 silica source results in smaller WLPs and also leads to the formation of zeolite L crystals with much smaller size than those formed with the AS-40 source. For instance, when examining SEM images of solids removed from the growth solution at different heating times (see Figure 12), there are clear differences between samples prepared with SM-30 (Figure 12A–D) and AS-40 (Figure 12E–H). The presence of zeolite L crystals in ex situ SEM images can be distinguished from amorphous WLPs on the basis of their morphology, i.e., cylindrical particles versus branched structures, respectively. Zeolite L crystals in SM-30 samples are

Figure 11. (A) Temporal change in β from SLS measurements of particulates in zeolite L growth solutions that were prepared with two different colloidal silica sources: 8-nm Ludox SM-30 (diamond symbols) and 25-nm Ludox AS-40 (square symbols). (B) Comparison of the WLP width for each colloidal silica source measured from SEM images of extracted solids during the first 4 h of heating. Dashed lines are interpolated for visual clarity. Error bars equal two standard deviations.

observed at shorter times (5 h), and within 8 h the number of WLPs in electron micrographs is negligible. Conversely, the AS-40 sample contains a significant fraction of WLPs after 12 h of heating. Putative role(s) of WLPs in zeolite L crystallization. The first crystals observed in SEM images, such as those in Figure 12G, have sizes and shapes that are comparable to fully-crystalline zeolite L after 72 h of heating (e.g., Figure S12). This seems to suggest that crystals reach some threshold size beyond which the rate of growth is substantially slower than the rate at which newly generated crystals grow. The emergence of crystals in electron micrographs coincides with a decrease in the WLP population, as well as the plateau in WLP width (see Figure 3C) and the increase in β (see Figure 6A). Moreover, zeolite L nucleation occurs by a process that is distinctly different from the LaMer mechanism. Classical nucleation theory (CNT)89 predicts that once a particle reaches a critical size, nucleation occurs and the initial “burst” of crystals leads to a relatively narrow particle size distribution that progressively shifts to larger crystallite size with time. Here, we observe the gradual emergence of zeolite L crystals with increased heating time, counter to the LaMer mechanism.90 Interestingly, control samples exhibit a crystal size distribution that is less polydisperse (average size = 3.3 ± 0.4 µm) than anticipated for a process where nuclei are generated over a long period of time (as opposed to the instantaneous formation of nuclei).


9


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 14

Figure 12. Time-resolved SEM images of solids extracted from zeolite L growth solutions at periodic times (1 – 12 h) during hydrothermal treatment. Here, we compare the evolution of WLPs and crystals in growth solutions prepared with different colloidal silica sources: (A – D) 8-nm Ludox SM-30 and (E – H) 25-nm Ludox AS-40. Electron micrographs show that WLPs are consumed more rapidly in the SM-30 sample, whereas the AS-40 sample contains residual amorphous precursors for much longer heating time.

A pervasive question in zeolite synthesis is the role of WLPs in crystallization. Numerous ex situ studies reveal that WLPs are present throughout crystallization, and are progressively consumed with the increased extent of crystal growth. It is unclear whether zeolite L growth predominantly occurs by the direct attachment of WLPs (path iv in Figure 1), or through molecule addition wherein the precursors are a reservoir of nutrient that dissolve (path iii in Figure 1) to produce soluble species that attach to crystal surfaces (path v in Figure 1). It is important to emphasize that these two processes are not mutually exclusive. For example, MFI-type zeolite grows by the addition of silica molecules (classical pathway) and the attachment of 3-nm primary particles (nonclassical pathway).26 The mechanism of zeolite L growth may similarly involve concerted events that encompass multiple pathways illustrated in Figure 1. It is difficult to rationalize a growth mechanism that does not include molecule addition on the basis of two observations: (i) zeolite L crystals are macroscopically smooth in SEM images relative to the rough crystals that tend to form by nonclassical pathways;26, 91 and (ii) there must be significant bond breakage and formation in order for the Si-rich domains within the WLPs to form a more aluminous zeolite product. Crystal morphology is a poor indicator of growth pathways. For example, nonclassical growth can be obscured by ripening processes that render crystals smooth and faceted, while unique shapes and features (e.g., intergrowths, twinning, and defects) that arise from classical pathways can be mistakenly attributed to CPA processes. To this end, ex situ micrographs should be interpreted with caution. We must also account for the possibility that the extraction of solids from their native growth environment can potentially alter the product. In the case of zeolite L, intermediate stages of growth are difficult to capture in ex situ images due to the fast rate of crystallization. In order to slow the kinetics of crystallization, we included the crystal growth modifier 1butanol in the synthesis solution (see Lupulescu et al.51 for details). We previously demonstrated that alcohols preferentially bind to LTL (001) surfaces, thereby reducing the rate

of crystal growth with concomitant changes in crystal aspect ratio. SEM images in Figure 13A and B appear to show WLPs directly attached to zeolite L crystals. While we cannot exclude the possibility that these features are formed during the drying procedure when solids are extracted from the growth solution, similar features are not observed in Figure 12, indicating that this phenomenon is not uniquely correlated to sample drying. Images in Figure 13 seemingly show WLPs directly “feeding” nutrient to the basal surfaces of zeolite L crystals, which were identified on the basis of their cylindrical disk-like morphology that mimics the fullygrown zeolite L crystal in Figure 13C. Moreover, electron micrographs reveal the presence of layers on the crystal surface (e.g., arrows in Figure 13A and B). It is unlikely that these features are single layers with heights corresponding to the [001] unit cell parameter (i.e., on the order of 1 nm), which would be indicative of 2D layer generation and spreading (a common feature of classical crystal growth via molecule addition). The visibility of layers in SEM images

Figure 13. (A and B) Scanning electron micrographs capturing an intermediate stage of zeolite L crystallization in the presence of 1-butanol modifier. The thin cylindrical disks are believed to be zeolite L due to their similarity in both size and shape to fully-grown crystals. It appears as though WLPs in each image are “fused” to the cylinder, directly supplying nutrient to the crystal surface. These images also reveal the presence of layers on the crystal surface (arrows) with unknown step height(s). (C) SEM image of a fully-grown zeolite L crystal with disklike morphology after heating a growth solution containing 10.6 wt% 1butanol for 3 days at 180°C. All scale bars equal 500 nm.


10

Page 11 of 14

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


suggests these layers are macrosteps with heights that are multiples of the unit cell parameter, which is indicative of 3D layer generation that occurs by CPA processes. Without time-resolved in situ data, we lack direct evidence of the growth pathway. The mere presence of WLPs in growth solutions suggests that nonclassical routes are viable; however, there are many unanswered questions regarding WLP formation and the events associated with WLP–crystal attachment, which include (but are not limited to) the effects of solvent, WLPcrystal interfacial forces, and the influence of precursor microstructure. If WLPs serve as growth units, as our findings suggest, there must be a difference between precursorprecursor and precursor-crystal interactions in order to rationalize why WLPs do not continually aggregate during crystallization. The maximum size of WLPs suggests there are kinetic factors preventing further aggregation and growth by molecule accretion, while at the same time regulating their rate of attachment to crystal surfaces. Further studies are clearly required to conclusively identify the role of precursors in zeolite L crystallization. These may include techniques of freeze drying (e.g., cryo-TEM) to capture intermediate stages of growth, as demonstrated in prior studies of metal oxide nanotubes,92, 93 as well as quantitative kinetic models25,35 that can account for crystal growth by precursor attachment.

CONCLUSION In summary, we propose a pathway for the formation of zeolite L precursors into worm-like particles. Our findings reveal that WLPs monotonically increase in size during zeolite L nucleation, reach a maximum size near the end of the induction period, and are then progressively depleted from the growth solution during zeolite crystallization. Electron micrographs indicate that WLP precursors are heterogeneous in composition with Si-rich domains (Si/Al >> 10), which significantly differs from the composition of zeolite L crystals (Si/Al ≈ 3.3) determined form elemental analysis. Collectively, these findings indicate that crystallization involves the structural reorganization of precursors (i.e., substantial bond breakage and formation) in concert with the addition of alumina species from solution. We demonstrated for the first time that static light scattering is an effective technique for tracking zeolite L crystallization. Notably, temporal changes in β (slope of log-log I(q) plots) can be used to estimate the induction time, quantify the kinetics of crystallization, and determine the approximate synthesis time. Comparison of growth solutions prepared with disparate-sized colloidal silica sources reveal that smaller particles markedly reduce the final crystal size, shorten the induction period, and reduce the timeframe of crystal growth. A physical explanation of these observations is difficult to surmise from this study; however, it is evident that smaller colloidal silica particles have the ability to dissolve more rapidly, thereby releasing silica into the growth solution. Higher specific surface area of smaller colloidal silica can lead to reduced coverage of alumina on their exterior surfaces, which facilitates the rate of silica dissolution. Therefore, it appears that the shift in growth solution composition from an initially Al-rich medium to one that more closely resembles the composition of the crystalline product offers a potential explanation for the reduced induction time. In turn, more rapid nucleation can lead to a greater number of nuclei, which is qualitatively consistent with the for-

mation of smaller zeolite L crystals. The increased rate of crystallization with smaller colloidal silica also seems to suggest that solution-mediated processes (e.g., molecule addition) play a role in zeolite L growth, though the relative contributions of WLP and molecule addition is unknown. Time-resolved ex-situ analyses of extracted solids from growth solutions reveal that WLPs do not appreciably dissolve with increased heating time, but rather decrease in number density. Electron micrographs seemingly show that WLPs supply nutrient directly to growing zeolite L crystals, which is consistent with the hypothesis that WLPs play a direct role in crystallization. Conclusive evidence that zeolite L grows by CPA, however, remains elusive. Moreover, details of its growth mechanism at a molecular level are not fully understood. Indeed, there are many remaining questions that pertain to the microstructure of the zeolite precursor, its role (if any) in nucleation, and the exact mechanism by which WLPs supply (alumino)silicate nutrient to growing crystals. It is reasonable to speculate that the phenomenon observed for zeolite L represents a generalized pathway for zeolite formation. For instance, the macroscopic properties of precursors that form in growth solutions of various zeolite structures are strikingly similar. An in-depth understanding of crystal growth pathways could provide answers to prevalent questions in zeolite synthesis that include the factors governing polymorphism (i.e., the formation of impurities) as well as the synthesis compositions and/or conditions that can be selected a priori to tailor the physicochemical properties of the crystalline product.

ASSOCIATED CONTENT SUPPORTING INFORMATION Details of materials characterization are provided, including XRD patterns, kinetic analysis, additional SEM images, DLS and SLS data, and a N2 adsorption/desorption isotherm of WLPs. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] † Authors contributed equally

Notes The authors declare no competing financial interests.

Author Contributions All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENTS We are sincerely grateful for the feedback received from Boris Subotić, who previewed an early draft of this manuscript. This work was supported by funding from the National Science Foundation (Award No. 1151098), the American Chemical Society Petroleum Research Fund (Award No. 52422-DNI5), and The Welch Foundation (Award No. E-1794).

REFERENCES (1) Davis, M. E.; Lobo, R. F., Zeolite and Molecular-sieve Synthesis Chem. Mater. 1992, 4, 756-768.

(2) Corma, A., State of the Art and Future Challenges of Zeolites as Catalysts. J. Catal. 2003, 216, 298-312.


11


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(3) Roman-Leshkov, Y.; Moliner, M.; Labinger, J. A.; Davis, M. E., Mechanism of Glucose Isomerization Using a Solid Lewis Acid Catalyst in Water. Angew. Chem.-Int. Edit. 2010, 49, 8954-8957. (4) Calzaferri, G.; Huber, S.; Maas, H.; Minkowski, C., Host-guest Antenna Materials. Angew. Chem.-Int. Edit. 2003, 42, 3732-3758. (5) Elangovan, S. P.; Ogura, M.; Davis, M. E.; Okubo, T., SSZ-33: A Promising Material for Use as a Hydrocarbon Trap. J. Phys. Chem. B 2004, 108, 13059-13061. (6) Cejka, J.; Mintova, S., Perspectives of Micro/mesoporous Composites in Catalysis. Catal. Rev.-Sci. Eng. 2007, 49, 457-509. (7) Teixeira, A. R.; Qi, X.; Conner, W. C.; Mountziaris, T. J.; Fan, W.; Dauenhauer, P. J., 2D Surface Structures in Small Zeolite MFI Crystals. Chem. Mater. 2015, 27, 4650-4660. (8) Jentoft, R. E.; Tsapatsis, M.; Davis, M. E.; Gates, B. C., Platinum Clusters Supported in Zeolite LTL: Influence of Catalyst Morphology on Performance in n-Hexane Reforming. J. Catal. 1998, 179, 565-580. (9) Lülf, H.; Bertucci, A.; Septiadi, D.; Corradini, R.; De Cola, L., Multifunctional Inorganic Nanocontainers for DNA and Drug Delivery into Living Cells. Chem. Eur. J. 2014, 20, 10900-10904. (10) Maldonado, M.; Oleksiak, M. D.; Chinta, S.; Rimer, J. D., Controlling Crystal Polymorphism in Organic-Free Synthesis of NaZeolites. J. Am. Chem. Soc. 2013, 135, 2641-2652. (11) Van Tendeloo, L.; Gobechiya, E.; Breynaert, E.; Martens, J. A.; Kirschhock, C. E. A., Alkaline Cations Directing the Transformation of FAU Zeolites into Five Different Framework Types. Chem. Commun. 2013, 49, 11737-11739. (12) Itabashi, K.; Kamimura, Y.; Iyoki, K.; Shimojima, A.; Okubo, T., A Working Hypothesis for Broadening Framework Types of Zeolites in Seed-Assisted Synthesis without Organic Structure-Directing Agent. J. Am. Chem. Soc. 2012, 134, 11542-11549. (13) Wu, Q.; Wang, X.; Meng, X.; Yang, C.; Liu, Y.; Jin, Y.; Yang, Q.; Xiao, F.-S., Organotemplate-free, Seed-directed, and Rapid Synthesis of Al-rich Zeolite MTT with Improved Catalytic Performance in Isomerization of m-Xylene. Micropor. Mesopor. Mater. 2014, 186, 106112. (14) Goel, S.; Zones, S. I.; Iglesia, E., Synthesis of Zeolites via Interzeolite Transformations without Organic Structure-Directing Agents. Chem. Mater. 2015, 27, 2056-2066. (15) Sano, T.; Itakura, M.; Sadakane, M., High Potential of Interzeolite Conversion Method for Zeolite Synthesis. J. Jpn. Pet. Inst 2013, 56, 183-197. (16) Lee, H.; Zones, S. I.; Davis, M. E., A Combustion-free Methodology for Synthesizing Zeolites and Zeolite-like Materials. Nature 2003, 425, 385-388. (17) De Yoreo, J. J.; Gilbert, P. U. P. A.; Sommerdijk, N. A. J. M.; Penn, R. L.; Whitelam, S.; Joester, D.; Zhang, H.; Rimer, J. D.; Navrotsky, A.; Banfield, J. F.; Wallace, A. F.; Michel, F. M.; Meldrum, F. C.; Cölfen, H.; Dove, P. M., Crystallization by Particle Attachment in Synthetic, Biogenic, and Geologic Environments. Science 2015, 349. (18) Liao, H. G.; Cui, L. K.; Whitelam, S.; Zheng, H. M., Real-time Imaging of Pt3Fe Nanorod Growth in Solution. Science 2012, 336, 1011-1014. (19) Zheng, H. M.; Smith, R. K.; Jun, Y. W.; Kisielowski, C.; Dahmen, U.; Alivisatos, A. P., Observation of Single Colloidal Platinum Nanocrystal Growth Trajectories. Science 2009, 324, 1309-1312. (20) Li, D. S.; Soberanis, F.; Fu, J.; Hou, W. T.; Wu, J. Z.; Kisailus, D., Growth Mechanism of Highly Nranched Titanium Dioxide Nanowires via Oriented Attachment. Cryst. Growth Des. 2013, 13, 422-428. (21) Banfield, J. F.; Welch, S. A.; Zhang, H. Z.; Ebert, T. T.; Penn, R. L., Aggregation-based Crystal Growth and Microstructure Development in Natural Iron Oxyhydroxide Biomineralization Products. Science 2000, 289, 751-754. (22) Baumgartner, J.; Dey, A.; Bomans, P. H. H.; Le Coadou, C.; Fratzl, P.; Sommerdijk, N.; Faivre, D., Nucleation and Growth of Magnetite from Solution. Nat. Mater. 2013, 12, 310-314. (23) Van Driessche, A. E. S.; Benning, L. G.; Rodriguez-Blanco, J. D.; Ossorio, M.; Bots, P.; Garcia-Ruiz, J. M., The Role and Implications of Bassanite as a Stable Precursor Phase to Gypsum Precipitation. Science 2012, 336, 69-72. (24) Nielsen, M. H.; Aloni, S.; De Yoreo, J. J., In situ TEM Imaging of CaCO3 Nucleation Reveals Coexistence of Direct and Indirect Pathways. Science 2014, 345, 1158-1162. (25) Yucelen, G. I.; Kang, D. Y.; Schmidt-Krey, I.; Beckham, H. W.; Nair, S., A Generalized Kinetic Model for the Formation and Growth of

Page 12 of 14

Single-Walled Metal Oxide Nanotubes. Chem. Eng. Sci. 2013, 90, 200212. (26) Lupulescu, A. I.; Rimer, J. D., In Situ Imaging of Silicalite-1 Surface Growth Reveals the Mechanism of Crystallization. Science 2014, 344, 729-732. (27) Hould, N. D.; Kumar, S.; Tsapatsis, M.; Nikolakis, V.; Lobo, R. F., Structure and Colloidal Stability of Nanosized Zeolite Beta Precursors. Langmuir 2010, 26, 1260-1270. (28) Greer, H.; Wheatley, P. S.; Ashbrook, S. E.; Morris, R. E.; Zhou, W. Z., Early Stage Reversed Crystal Growth of Zeolite A and Its Phase Transformation to Sodalite. J. Am. Chem. Soc. 2009, 131, 17986-17992. (29) Valtchev, V. P.; Bozhilov, K. N., Transmission Electron Microscopy Study of the Formation of FAU-type Zeolite at Room Temperature. J. Phys. Chem. B 2004, 108, 15587-15598. (30) Fan, W. B.; Duan, R. G.; Yokoi, T.; Wu, P.; Kubota, Y.; Tatsumi, T., Synthesis, Crystallization Mechanism, and Catalytic Properties of Titanium-rich TS-1 Free of Extraframework Titanium Species. J. Am. Chem. Soc. 2008, 130, 10150-10164. (31) Wang, Y.; Li, X.; Xue, Z.; Dai, L.; Xie, S.; Li, Q., Preparation of Zeolite ANA Crystal from Zeolite Y by in situ Solid Phase Iso-structure Transformation. J. Phys. Chem. B 2010, 114, 5747-5754. (32) Chen, J.; Hua, W. M.; Xiao, Y.; Huo, Q. S.; Zhu, K. K.; Zhou, X. G., Tailoring the Structure of Hierarchically Porous Zeolite Beta through Modified Orientated Attachment Growth in a Dry Gel System. Chem.-Eur. J. 2014, 20, 14744-14755. (33) Breck, D. W., Crystalline Molecular Sieves. J. Chem. Educ. 1964, 41, 678-689. (34) Burkett, S. L.; Davis, M. E., Mechanisms of Structure Direction in the Synthesis of Pure-silica Zeolites. 1. Synthesis of TPA/Si-ZSM-5. Chem. Mater. 1995, 7, 920-928. (35) Davis, T. M.; Drews, T. O.; Ramanan, H.; He, C.; Dong, J.; Schnablegger, H.; Katsoulakis, M. A.; Kokkoli, E.; McCormick, A. V.; Penn, R. L.; Tsapatsis, M., Mechanistic Principles of Nanoparticle Evolution to Zeolite Crystals. Nat Mater 2006, 5, 400-408. (36) Mintova, S.; Olson, N. H.; Valtchev, V.; Bein, T., Mechanism of Zeolite A Nanocrystal Growth from Colloids at Room Temperature. Science 1999, 283, 958-960. (37) Valtchev, V. P.; Bozhilov, K. N., Evidences for Zeolite Nucleation at the Solid−Liquid Interface of Gel Cavities. J. Am. Chem. Soc. 2005, 127, 16171-16177. (38) Auerbach, S. M.; Ford, M. H.; Monson, P. A., New Insights into Zeolite Formation from Molecular Modeling. Curr. Opin. Colloid Interface Sci. 2005, 10, 220-225. (39) Vekilov, P. G., Dense Liquid Precursor for the Nucleation of Ordered Solid Phases from Solution. Cryst. Growth Des. 2004, 4, 671685. (40) Chang, C. D.; Bell, A. T., Studies on the Mechanism of ZSM-5 Formation. Catal. Lett. 1991, 8, 305-316. (41) Ren, N.; Subotic, B.; Bronic, J.; Tang, Y.; Sikiric, M. D.; Misic, T.; Svetlicic, V.; Bosnar, S.; Jelic, T. A., Unusual Pathway of Crystallization of Zeolite ZSM-5 in a Heterogeneous System: Phenomenology and Starting Considerations. Chem. Mater. 2012, 24, 1726-1737. (42) Direnzo, F.; Remoue, F.; Massiani, P.; Fajula, F.; Figueras, F.; Descourieres, T., Crystallization Kinetics of Zeolite TON. Zeolites 1991, 11, 539-548. (43) Larlus, O.; Valtchev, V. P., Crystal Morphology Control of LTLtype Zeolite Crystals. Chem. Mater. 2004, 16, 3381-3389. (44) Lee, Y. J.; Lee, J. S.; Yoon, K. B., Synthesis of Long Zeolite-L Crystals with Flat Facets. Micropor. Mesopor. Mater. 2005, 80, 237246. (45) Sharma, P.; Yeo, J.-g.; Han, M. H.; Cho, C. H., Knobby Surfaced, Mesoporous, Single-phase GIS-NaP1 Zeolite Microsphere Synthesis and Characterization for H2 Gas Adsorption. J. Mater. Chem. A 2013, 1, 2602-2612. (46) Brent, R.; Anderson, M. W., Fundamental Crystal Growth Mechanism in Zeolite L Revealed by Atomic Force Microscopy. Angew. Chem.-Int. Edit. 2008, 47, 5327-5330. (47) Brent, R.; Cubillas, P.; Stevens, S. M.; Jelfs, K. E.; Umemura, A.; Gebbie, J. T.; Slater, B.; Terasaki, O.; Holden, M. A.; Anderson, M. W., Unstitching the Nanoscopic Mystery of Zeolite Crystal Formation. J. Am. Chem. Soc. 2010, 132, 13858-13868.


12

Page 13 of 14

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(48) Hould, N. D.; Foster, A.; Lobo, R. F., Zeolite Beta Mechanisms of Nucleation and Growth. Micropor. Mesopor. Mater. 2011, 142, 104115. (49) Kumar, S.; Davis, T. M.; Ramanan, H.; Penn, R. L.; Tsapatsis, M., Aggregative Growth of Silicalite-1. J. Phys. Chem. B 2007, 111, 33983403. (50) Brent, R.; Lobo, A. J. W.; Lewis, D. W.; Anderson, M. W., Modifying the Crystal Habit of Zeolite L by Addition of an Organic Space Filler. J Phys Chem C 2010, 114, 18240-18246. (51) Lupulescu, A. I.; Kumar, M.; Rimer, J. D., A facile strategy to design zeolite L crystals with tunable morphology and surface architecture. J. Am. Chem. Soc. 2013, 135, 6608-6617. (52) Jacobs, G.; Alvarez, W. E.; Resasco, D. E., Study of Preparation Parameters of Powder and Pelletized Pt/KL Catalysts for n-Hexane Aromatization. Appl. Catal., A. 2001, 206, 267-282. (53) Tamm, P. W.; Mohr, D. H.; Wilson, C. R., Octane Enhancement By Selective Reforming of Light Paraffins. In Stud. Surf. Sci. Catal., Ward, J. W., Ed. Elsevier: 1988; Vol. Volume 38, pp 335-353. (54) Cortright, R. D.; Dumesic, J. A., L-zeolite-supported Platinum and Platinum/tin Catalysts for Isobutane Dehydrogenation. Appl. Catal., A. 1995, 129, 101-115. (55) Li, G.; Li, T.; Xu, Y., Pt/ML (M = Rb, Sr) as Highly Selective and Active Catalysts for Cinnamaldehyde Hydrogenation to Cinnamyl Alcohol. Chem. Commun. 1996, 497-498. (56) Álvarez-Rodríguez, J.; Guerrero-Ruiz, A.; Rodríguez-Ramos, I.; Arcoya-Martín, A., Modifications of the Citral Hydrogenation Selectivities over Ru/KL-zeolite Catalysts Induced by the Metal Precursors. Catal. Today. 2005, 107–108, 302-309. (57) Parra, C. F.; Ballivet, D.; Barthomeuf, D., Acidity, Catalytic Activity and Thermal Stability of Various L Zeolites. J. Catal. 1975, 40, 52-60. (58) Schulte, B.; Tsotsalas, M.; Becker, M.; Studer, A.; De Cola, L., Dynamic Microcrystal Assembly by Nitroxide Exchange Reactions. Angew. Chem. Int. Ed. 2010, 49, 6881-6884. (59) Li, Y.; Lubchenko, V.; Vekilov, P. G., The Use of Dynamic Light Scattering and Brownian Microscopy to Characterize Protein Aggregation. Rev. Sci. Instrum. 2011, 82. (60) Rimer, J. D.; Vlachos, D. G.; Lobo, R. F., Evolution of SelfAssembled Silica−Tetrapropylammonium Nanoparticles at Elevated Temperatures. J. Phys. Chem. B 2005, 109, 12762-12771. (61) Gamage, N. P.; Rimer, J. D.; Chellam, S., Improvements in Permeate Flux by Aluminum Electroflotation Pretreatment During Microfiltration of Surface Water. J. Membrane Sci. 2012, 411–412, 4553. (62) Knight, C. T. G.; Kinrade, S. D., Comment on "Identification of precursor species in the formation of MFI zeolite in the TPAOH-TEOSH2O system". J. Phys. Chem. B 2002, 106, 3329-3332. (63) Knight, C. T. G.; Balec, R. J.; Kinrade, S. D., The Structure of Silicate Anions in Aqueous Alkaline Solutions. Angew. Chem.-Int. Edit. 2007, 46, 8148-8152. (64) de Moor, P.; Beelen, T. P. M.; van Santen, R. A., In situ Observation of Nucleation and Crystal Growth in Zeolite Synthesis. A Small-angle X-ray Scattering Investigation on Si-TPA-MFI. J. Phys. Chem. B 1999, 103, 1639-1650. (65) Roux, B.; Louchart, O.; Terhmina, O., Hydrodynamic Aspect of Hydrothermal Synthesis of Quartz Bulk Flow Regimes. J. Phys. IV 1994, 4, 3-11. (66) Zemb, T.; Lindner, P., Neutrons, X-rays and Light: Scattering Methods Applied to Soft Condensed Matter. Elsevier: 2002. (67) Guinier, A.; Fournet, G., Small-angle Scattering of X-rays. Wiley: 1955. (68) Davies, A. T.; Sankar, G.; Catlow, C. R. A.; Clark, S. M., Following the Crystallization of Microporous Solids Using EDXRD Techniques. J. Phys. Chem. B 1997, 101, 10115-10120. (69) Miyata, T.; Masuko, T., Crystallization Behaviour of Poly(Llactide). Polymer 1998, 39, 5515-5521. (70) Toro-Vazquez, J. F.; Briceno-Montelongo, M.; Dibildox-Alvarado, E.; Charo-Alonso, M.; Reyes-Hernandez, J., Crystallization Kinetics of Palm Stearin in Blends with Sesame Seed Oil. J. Am. Oil Chem. Soc. 2000, 77, 297-310. (71) Moh, P. Y.; Brenda, M.; Anderson, M. W.; Attfield, M. P., Crystallisation of Solvothermally Synthesised ZIF-8 Investigated at the Bulk, Single Crystal and Surface Level. Crystengcomm 2013, 15, 96729678.

(72) Venna, S. R.; Jasinski, J. B.; Carreon, M. A., Structural Evolution of Zeolitic Imidazolate Framework-8. J. Am. Chem. Soc. 2010, 132, 18030-18033. (73) Kirsch, B. L.; Richman, E. K.; Riley, A. E.; Tolbert, S. H., In-situ X-ray Diffraction Study of the Crystallization Kinetics of Mesoporous Titania Films. J. Phys. Chem. B 2004, 108, 12698-12706. (74) Yoo, B.-K.; Kwon, O.-H.; Liu, H.; Tang, J.; Zewail, A. H., Observing in Space and Time the Ephemeral Nucleation of Liquid-tocrystal Phase Transitions. Nat Commun 2015, 6. (75) Walton, R. I.; Millange, F.; O'Hare, D.; Davies, A. T.; Sankar, G.; Catlow, C. R. A., An in situ Energy-dispersive X-ray Diffraction Study of the Hydrothermal Crystallization of Zeolite A. 1. Influence of Reaction Conditions and Transformation into Sodalite. J. Phys. Chem. B 2001, 105, 83-90. (76) Thompson, R. W., Analysis of Zeolite Crystallizations Using Avrami Transformation-Methods. Zeolites 1992, 12, 680-684. (77) Grizzetti, R.; Artioli, G., Kinetics of Nucleation and Growth of Zeolite LTA from Clear Solution by in situ and ex situ XRPD. Micropor. Mesopor. Mater. 2002, 54, 105-112. (78) Norby, P., Hydrothermal Conversion of Zeolites: An in situ Synchrotron X-ray Powder Diffraction Study. J. Am. Chem. Soc. 1997, 119, 5215-5221. (79) Joshi, P. N.; Kotasthane, A. N.; Shiralkar, V. P., Crystallization Kinetics of Zeolite- LTL Zeolites 1990, 10, 598-602. (80) Itani, L.; Liu, Y.; Zhang, W.; Bozhilov, K. N.; Delmotte, L.; Valtchev, V., Investigation of the Physicochemical Changes Preceding Zeolite Nucleation in a Sodium-rich Aluminosilicate Gel. J. Am. Chem. Soc. 2009, 131, 10127-10139. (81) Beale, A. M.; Weckhuysen, B. M., Understanding the Crystallisation Processes Leading to the Formation of Microporous Aluminophosphates. In From Zeolites to Porous Mof Materials: The 40th Anniversary of International Zeolite Conference, Proceedings of the 15th International Zeolite Conference, Xu, R.; Gao, Z.; Chen, J.; Yan, W., Eds. Elsevier Science Bv: Amsterdam, 2007; Vol. 170, pp 748-755. (82) Holmes, S. M.; Zholobenko, V. L.; Thursfield, A.; Plaisted, R. J.; Cundy, C. S.; Dwyer, J., In situ FTIR Study of the Formation of MCM41. J. Chem. Soc.-Faraday Trans. 1998, 94, 2025-2032. (83) O'Brien, M. G.; Beale, A. M.; Kuipers, B. W. M.; Erne, B. H.; Lewis, D. W.; Catlow, C. R. A., Role of Germanium on the Nucleation and Growth of Zeolite A from Clear Solutions As Studied by in Situ Small-Angle X-ray Scattering, Wide-Angle X-ray Scattering, and Dynamic Light Scattering. J Phys Chem C 2009, 113, 18614-18622. (84) Feoktistova, N. N.; Vtyurina, L. M., Temperature-Dependence of NAA Zeolite Nucleation in Silicoalumina Gels B ACAD SCI USSR CH+ 1988, 37, 617-619. (85) Li, Q.; Creaser, D.; Sterte, J., The Nucleation Period for TPAsilicalite-1 Crystallization Determined by a Two-stage Varyingtemperature Synthesis. Micropor. Mesopor. Mater. 1999, 31, 141-150. (86) Rimer, J. D.; Lobo, R. F.; Vlachos, D. G., Physical Basis for the Formation and Stability of Silica Nanoparticles in Basic Solutions of Monovalent Cations. Langmuir 2005, 21, 8960-8971. (87) Lindner, T.; Lechert, H., The Influence of Fluoride on the Crystallization Kinetics of Zeotie NAY. Zeolites 1994, 14, 582-587. (88) Dessau, R. M.; Valyocsik, E. W.; Goeke, N. H., Aluminum Zoning in ZSM-5 as Revealed by Selective Silica Removal. Zeolites 1992, 12, 776-779. (89) Auer, S.; Frenkel, D., Prediction of Absolute Crystal-nucleation Rate in Hard-sphere Colloids. Nature 2001, 409, 1020-1023. (90) LaMer, V. K.; Dinegar, R. H., Theory, Production and Mechanism of Formation of Monodispersed Hydrosols. J. Am. Chem. Soc. 1950, 72, 4847-4854. (91) Niederberger, M.; Colfen, H., Oriented Attachment and Mesocrystals: Non-classical Crystallization Mechanisms Based on Nanoparticle Assembly. Phys. Chem. Chem. Phys. 2006, 8, 3271-3287. (92) Mukherjee, S.; Kim, K.; Nair, S., Short, Highly Ordered, Singlewalled Mixed-oxide Nanotubes Assemble from Amorphous Nanoparticles. J. Am. Chem. Soc. 2007, 129, 6820-6826. (93) Yucelen, G. I.; Kang, D. Y.; Guerrero-Ferreira, R. C.; Wright, E. R.; Beckham, H. W.; Nair, S., Shaping Single-Walled Metal Oxide Nanotubes from Precursors of Controlled Curvature. Nano Lett. 2012, 12, 827-832.


13


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 14

Table of Contents Graphic


14

Assembly and Evolution of Amorphous Precursors ... - ACS Publications

Recommend Documents