14 research outputs found
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 CaCO<sub>3</sub>). 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)<sub>3</sub> 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<sub><i>x</i></sub>). 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