Assembly and Evolution of Amorphous Precursors in Zeolite L Crystallization

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 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. <i>Ex situ</i> SEM images reveal the progressive formation of crystal nuclei, which deviates from the classical LaMer process that posits a nearly <i>instantaneous</i> 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

Elucidating the Effects of Polyprotic Acid Speciation in Calcium Oxalate Crystallization

Polyprotic acids tend to be very effective modifiers of crystals in synthetic, natural, and biological systems. Examples include calcium biomineralization where proteins and organic acids decorated with carboxylic acids act as inhibitors of crystal growth. For crystals implicated in pathological diseases, large variations in the pH of the growth medium can alter the speciation of polyprotic acids. This is particularly true for calcium minerals comprised of polyprotic counterions wherein changes in solute speciation affect supersaturation, and thus the kinetics of crystal growth. Here, we explore the combined effects of solute and modifier speciation, selecting calcium oxalate monohydrate (COM) as a representative system for calcification. COM is a major constituent of human kidney stones where crystallization <i>in vivo</i> occurs over a broad range of pH spanning 5–8. Common modifiers of COM and its solute (oxalate) are polyprotic acids. Few studies report the effects of oxalate speciation on COM growth. Moreover, it remains to be determined how pH influences the efficacy of polyprotic molecules used to inhibit COM growth, such as citrate (CA) and its molecular analogue hydroxycitrate (HCA). Here, we show that there is a dramatic reduction in the rate of COM growth in the lower limit of physiological pH, commensurate with the loss of oxalate net negative charge. Our findings reveal that CA and HCA exhibit dual modes of action as promoters and inhibitors of COM crystallization at low and high pH, respectively. We also observe distinct differences in the efficacy of each modifier and discuss how local changes in pH near charged crystal interfaces can have a marked impact on local supersaturation and the speciation of adsorbed modifiers. On the basis of our observations of COM crystallization, it is reasonable to expect that changes in pH, or more specifically the speciation of solute and modifier(s), could similarly impact the growth (and growth inhibition) of other crystals employing polyprotic acids

A Facile Strategy To Design Zeolite L Crystals with Tunable Morphology and Surface Architecture

Tailoring the anisotropic growth rates of materials to achieve desired structural outcomes is a pervasive challenge in synthetic crystallization. Here we discuss a method to selectively control the growth of zeolite crystals, which are used extensively in a wide range of industrial applications. This facile method cooperatively tunes crystal properties, such as morphology and surface architecture, through the use of inexpensive, commercially available chemicals with specificity for binding to crystallographic surfaces and mediating anisotropic growth. We examined over 30 molecules as potential zeolite growth modifiers (ZGMs) of zeolite L (LTL type) crystallization. ZGM efficacy was quantified through a combination of macroscopic (bulk) and microscopic (surface) investigations that identified modifiers capable of dramatically altering the cylindrical morphology of LTL crystals. We demonstrate an ability to tailor properties critical to zeolite performance, such as external porous surface area, crystal shape, and pore length, which can enhance sorbate accessibility to LTL pores, tune the supramolecular organization of guest–host composites, and minimize the diffusion path length, respectively. We report that a synergistic combination of ZGMs and the judicious adjustment of synthesis parameters produce LTL crystals with unique surface features, and a range of length-to-diameter aspect ratios spanning 3 orders of magnitude. A systematic examination of different ZGM structures and molecular compositions (i.e., hydrophobicity and binding moieties) reveal interesting physicochemical properties governing their efficacy and specificity. Results of this study suggest this versatile strategy may prove applicable for a host of framework types to produce unrivaled materials that have eluded more conventional techniques

Sweep Flocculation and Adsorption of Viruses on Aluminum Flocs during Electrochemical Treatment Prior to Surface Water Microfiltration

Bench-scale experiments were performed to evaluate virus control by an integrated electrochemical–microfiltration (MF) process from turbid (15 NTU) surface water containing moderate amounts of dissolved organic carbon (DOC, 5 mg C/L) and calcium hardness (50 mg/L as CaCO₃). Higher reductions in MS2 bacteriophage concentrations were obtained by aluminum electrocoagulation and electroflotation compared with conventional aluminum sulfate coagulation. This was attributed to electrophoretic migration of viruses, which increased their concentrations in the microenvironment of the sacrificial anode where coagulant precursors are dissolved leading to better destabilization during electrolysis. In all cases, viruses were not inactivated implying measured reductions were solely due to their removal. Sweep flocculation was the primary virus destabilization mechanism. Direct evidence for virus enmeshment in flocs was provided by two independent methods: quantitative elution using beef extract at elevated pH and quantitating fluorescence from labeled viruses. Atomic force microscopy studies revealed a monotonically increasing adhesion force between viruses immobilized on AFM tips and floc surfaces with electrocoagulant dosage, which suggests secondary contributions to virus uptake on flocs from adsorption. Virus sorption mechanisms include charge neutralization and hydrophobic interactions with natural organic matter removed during coagulation. This also provided the basis for interpreting additional removal of viruses by the thick cake formed on the surface of the microfilter following electrocoagulation. Enhancements in virus removal as progressively more aluminum was electrolyzed therefore embodies contributions from (i) better encapsulation onto greater amounts of fresh Al­(OH)₃ precipitates, (ii) increased adsorption capacity associated with higher available coagulant surface area, (iii) greater virus-floc binding affinity due to effective charge neutralization and hydrophobic interactions, and/or (iv) additional removal by a dynamic membrane if a thick cake layer of flocs is deposited

Tuning Zeolite Precursor Interactions by Switching the Valence of Polyamine Modifiers

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

Computational Assessment of the Dominant Factors Governing the Mechanism of Methanol Dehydration over H‑ZSM‑5 with Heterogeneous Aluminum Distribution

A van der Waals (vdW) corrected density functional theory (DFT) study of the methanol-to-DME reaction on H-ZSM-5 is conducted for both the associative and dissociative pathways. Calculations are performed for four different active site locations corresponding to Al sitings in sinusoidal and straight channels, and their intersections in the MFI zeolite framework. The Gibbs free energy landscape along the reaction paths computed for a typical set of conditions shows that the associative route is preferred, regardless of Al siting, but a transition in the mechanism from associative to dissociative is observed at higher temperatures. The crossover temperature, however, is not identical for the various active site locations, resulting in a temperature range over which both mechanisms are active. This observation may explain why methoxy, which is the key intermediate along the dissociative pathway, has been observed spectroscopically, whereas kinetic analysis points to dominant contributions of the associative pathway under similar conditions. Pore confinement effects largely contribute to transition state stabilization and have a significant impact on the reaction mechanism. The effect of acidity on kinetic performance is also tested by the substitution of three different heteroatom dopants (Al, Ga, In) at the active sites, but only a minor transition-state energy variation was observed. The fundamental information obtained in this study contributes to a better understanding of the complex interplay between pore confinement, acidity, and reaction conditions, and their effect on pathway selectivity. This knowledge can be utilized to either optimize DME production from methanol or facilitate the production of desired hydrocarbons in the methanol-to-hydrocarbon (MTH) process, which requires DME formation to initiate the conversion

Controlling Crystal Polymorphism in Organic-Free Synthesis of Na-Zeolites

Controlling polymorphism is critical in areas such as pharmaceuticals, biomineralization, and catalysis. Notably, the formation of unwanted polymorphs is a ubiquitous problem in zeolite synthesis. In this study, we propose a new platform for controlling polymorphism in organic-free Na-zeolite synthesis that enables crystal composition and properties to be tailored without sacrificing crystal phase purity. Through systematic adjustment of multiple synthesis parameters, we identified ternary (kinetic) phase diagrams at specific compositions (i.e., Si, Al, and NaOH mole fractions) using colloidal silica and sodium aluminate. Our studies identify multiple stages of zeolite phase transformations involving the framework types FAU, LTA, EMT, GIS, SOD, ANA, CAN, and JBW. We report an initial amorphous-to-crystalline transition of core-shell particles (silica core and alumina shell) to low-density framework types and their subsequent transformation to more dense structures with increasing temperature and/or time. We show that reduced water content facilitates the formation of structures such as EMT that are challenging to synthesize in organic-free media and reduces the synthesis temperature required to achieve higher-density framework types. A hypothesis is proposed for the sequence of phase transformations that is consistent with the Ostwald rule of stages, wherein metastable structures dissolve and recrystallize into more thermodynamically stable structures. The ternary diagrams developed here are a broadly applicable platform for rational design that offers an alternative to time- and cost-intensive methods of <i>ad hoc</i> parameter selection without <i>a priori</i> knowledge of crystal phase behavior

SSZ-13 Crystallization by Particle Attachment and Deterministic Pathways to Crystal Size Control

Many synthetic and natural crystalline materials are either known or postulated to grow via nonclassical pathways involving the initial self-assembly of precursors that serve as putative growth units for crystallization. Elucidating the pathway(s) by which precursors attach to crystal surfaces and structurally rearrange (postattachment) to incorporate into the underlying crystalline lattice is an active and expanding area of research comprising many unanswered fundamental questions. Here, we examine the crystallization of SSZ-13, which is an aluminosilicate zeolite that possesses exceptional physicochemical properties for applications in separations and catalysis (e.g., methanol upgrading to chemicals and the environmental remediation of NO_<i>x</i>). We show that SSZ-13 grows by two concerted mechanisms: nonclassical growth involving the attachment of amorphous aluminosilicate particles to crystal surfaces and classical layer-by-layer growth via the incorporation of molecules to advancing steps on the crystal surface. A facile, commercially viable method of tailoring SSZ-13 crystal size and morphology is introduced wherein growth modifiers are used to mediate precursor aggregation and attachment to crystal surfaces. We demonstrate that small quantities of polymers can be used to tune crystal size over 3 orders of magnitude (0.1–20 μm), alter crystal shape, and introduce mesoporosity. Given the ubiquitous presence of amorphous precursors in a wide variety of microporous crystals, insight of the SSZ-13 growth mechanism may prove to be broadly applicable to other materials. Moreover, the ability to selectively tailor the physical properties of SSZ-13 crystals through molecular design offers new routes to optimize their performance in a wide range of commercial applications

Biomimetic Assay for Hematin Crystallization Inhibitors: A New Platform To Screen Antimalarial Drugs

Molecular inhibitors are commonly employed in natural, synthetic, and biological crystallization as a means of regulating crystal size and habit. In pathological crystallization, growth inhibitors are commonly employed as therapies to decrease the rate of crystal growth, thereby reducing the incidence rate of diseases. When designing inhibitors for such purposes, it is often unknown a priori what properties (e.g., structure, functionality) are critical for drug efficacy and specificity. Identification of lead candidates is often accomplished through brute force screening without knowledge of the molecular-level driving force(s) governing favorable inhibitor–crystal interactions. Here, we present a biomimetic assay to characterize the crystallization of hematin, a critical component of <i>Plasmodium</i> parasite survival in malaria. Many antimalarial drugs have been shown to function as inhibitors that suppress hematin crystal growth. The increasing resistance of malaria parasites to current antimalarials has created an impetus to design alternative drugs; however, current approaches to identify lead candidates rely on combinatorial methods employing parasite assays to screen compounds, which are incapable of elucidating the effect(s) of inhibitors on hematin crystallization. In this study, we use citric-buffer saturated octanol (CBSO) as a physiologically relevant growth medium to develop a facile, robust method of screening compounds that inhibit hematin crystal growth. As benchmarks, we selected antimalarials and antibiotics that are commonly used in combination therapies. We assessed drug solubility in CBSO and designed a biomimetic assay to quantify the relative efficiency of hematin growth inhibitors. We demonstrate that this assay can be used as a high-throughput platform to screen libraries of compounds as a more streamlined approach to identify inhibitors of pathological crystallization

Epitaxial Growth of ZSM-5@Silicalite-1: A Core–Shell Zeolite Designed with Passivated Surface Acidity

The design of materials with spatially controlled chemical composition has potential advantages for wide-reaching applications that span energy to medicine. Here, we present a method for preparing a core–shell aluminosilicate zeolite with continuous translational symmetry of nanopores and an epitaxial shell of tunable thickness that passivates Brønsted acid sites associated with framework Al on exterior surfaces. For this study, we selected the commercially relevant MFI framework type and prepared core–shell particles consisting of an aluminosilicate core (ZSM-5) and a siliceous shell (silicalite-1). Transmission electron microscopy and gas adsorption studies confirmed that silicalite-1 forms an epitaxial layer on ZSM-5 crystals without blocking pore openings. Scanning electron microscopy and dynamic light scattering were used in combination to confirm that the shell thickness can be tailored with nanometer resolution (<i>e.g.</i>, 5–20 nm). X-ray photoelectron spectroscopy and temperature-programmed desorption measurements revealed the presence of a siliceous shell, while probe reactions using molecules that were either too large or adequately sized to access MFI pores confirmed the uniform shell coverage. The synthesis of ZSM-5@silicalite-1 offers a pathway for tailoring the physicochemical properties of MFI-type materials, notably in the area of catalysis, where surface passivation can enhance product selectivity without sacrificing catalyst activity. The method described herein may prove to be a general platform for zeolite core–shell design with potentially broader applicability to other porous materials