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Formation Thermodynamics of zeolite oligomers.

Crystallization in media comprised of amorphous precursors is becoming a more common phenomenon for numerous synthetic, biological, and natural materials that grow by a combination of classical and nonclassical pathways. Amorphous phases can exhibit a wide range of physicochemical properties that may evolve during the course of nucleation and crystal growth. This creates challenges for establishing causal relationships between amorphous precursor properties and their effect(s) on the selection of mechanistic pathways of crystallization and ultimately the properties of the crystalline product. In this study, we examine ways to manipulate the composition and colloidal stability of amorphous (alumino)silicate precursors that are prevalent in nanoporous zeolite syntheses. Changes in the amorphous precursor properties are evaluated on the basis of their ability to enhance rates of crystal formation. Here, we use fumed silica as the primary silicon source and examine the effects of infusing the source or growth medium with additional alkali metal, which serves as an inorganic structure-directing agent to facilitate the formation of porous crystal structures. We also assess the impact of adding a polymer additive, which reduces the colloidal stability of precursors, wherein we posit that the confined pockets of solution within the interstitial spaces of the precursor aggregates play an important role in regulating the rate of zeolite crystallization. Three commercially relevant zeolites (mordenite, SSZ-13, and ZSM-5) were selected for this study based on their diverse frameworks and methods of preparation. Our findings reveal that alkali infusion significantly reduces the crystallization times for mordenite and SSZ-13, but has little impact on ZSM-5 synthesis. Conversely, we find that polymer addition markedly enhanced the rates of crystallization among all three zeolites, suggesting that this method may be a general approach to reduce zeolite synthesis times. Given the relatively high costs associated with commercial zeolite production, identifying new methods to improve the efficiency of hydrothermal syntheses can have significant practical implications beyond the fundamental benefits of developing new routes to tailor nonclassical crystallization.

Molecular modifiers can display a wide range of interactions with crystal interfaces to impede their growth. In this work we evaluate the efficacy of a naturally derived phosphorous-containing molecule, phytate, as an inhibitor of barite crystallization compared to the performance of a commercial organophosphorus standard. We show that both compounds inhibit barite nucleation and growth, with phytate demonstrating enhanced potency over the benchmark compound. Our findings reveal that phytate operates by a distinct mode of action on multiple crystal facets, imparting exceptional efficacy, which combined with its biocompatibility and widespread availability make phytate a potentially viable environmentally-friendly alternative to current barite scale treatments.

Few (macro)molecular inhibitors of inorganic scale can suppress both nucleation and crystal growth. In this study, we examine a series of potential inhibitors of barium sulfate (barite), which is a common scale that poses systemic problems owing to its low solubility. We show that alginate (an acidic polysaccharide) is an anomaly among a diverse set of carboxylate-based modifiers of barite crystallization based on its ability to completely suppress both nucleation and crystal growth. Bulk crystallization assays reveal that alginate completely suppresses barite nucleation. Experiments to quantify barite crystal growth kinetics at the macroscopic level under different flow conditions revealed that alginate is also a potent inhibitor of crystal growth, with full suppression of crystallization occurring at a modifier concentration of 60 nM. Time-resolved microfluidics experiments revealed alginate's affinity to interact with all principal crystallographic faces of barite, which is uncommon among inhibitors of various inorganic crystals reported in literature. In situ atomic force microscopy experiments to probe the interactions between alginate and barite crystal surfaces revealed a transition from step bunching to step pinning modes of action at low and high alginate concentrations, respectively. The findings in this study highlight the dual roles and exceptional performance of alginate as a barite scale inhibitor. Owing to its natural abundance in brown algae, alginate is a promising and green alternative to current scale treatments.

Despite bulk metal oxide non-stoichiometry often being recognized as a key determinant of catalytic performance, clarifying its catalytic function has remained elusive due in part to the highly complex nature of many catalyst surfaces. In this study, we demonstrate that the non-stoichiometric oxygen (NSO) density for thermally stable nickel oxide cubes can be manipulated without measurable changes in degree of crystallinity, particle size, and morphology. UV-vis spectroscopy, temperature programmed desorption, and H2-temperature programmed reduction analyses all evidence monotonic decreases in NSO density upon thermal treatment; crucially, these decreases persist upon exposure to ethane and oxygen under oxidative dehydrogenation of ethane (ODHE) reaction conditions. Such control over NSO density under reaction conditions is used to tune ODHE ethene selectivity; independent of reaction conditions, at isoconversion, ethene selectivity tracks qualitatively with NSO density. These trends in selectivity, however, do not extend to conditions resulting in higher fractional hydroxyl coverages, for example, in the presence of water co-feeds, where primary ODHE selectivities vary only at the highest treatment temperature used in our study. Overall, this study points to (i) the need for well-defined oxide materials that enable (ideally) the exclusive variation of a single physicochemical property, (ii) the importance of using multiple characterization techniques for quantifying various properties, and (iii) the sensitivity of selectivity trends to surface coverages of reaction intermediates prevalent during their measurement. For the purpose of elucidating structure–catalytic property relationships, the approach reported herein involving the use of well-defined thermally stable faceted oxide crystals has the potential to be broadly applicable within the field of bulk oxide catalysis.

Crystallization of mineral scale components ubiquitously plagues industrial systems for water treatment, energy production, and manufacturing. Chemical scale inhibitors and/or dissolvers are often employed to control scale formation, but their efficacy in flow conditions remains incompletely understood. We present a microfluidic platform to elucidate the time-resolved processes controlling crystallization and dissolution of barite, a highly insoluble and chemically resistant component of inorganic scale, in the presence of flow. In a growth environment, increasing the flow rate leads to a crossover from a transport-limited to a reaction-limited kinetic regime. In situ optical microscopy reveals that addition of diethylenetriaminepentaacetic acid (DTPA), a common dissolution agent, alters the morphology of barite crystals grown under flow. In a dissolution environment (i.e. alkaline solutions without barium sulfate), increasing the flux of DTPA, whether by increasing the flow rate or DTPA concentration, enhances the rate of dissolution of barite. Trends in the rate of barite dissolution with DTPA concentration and flow rate indicate an optimal combination of these parameters. The combined use of microfluidics and optical microscopy provides a robust and broadly-useful platform for capturing crystallization kinetics and morphological transformation under dynamic flow conditions.

Zeolite crystallization occurs in the presence of inorganic and/or organic structure-directing agents (SDAs) that facilitate the formation of microporous crystals with various pore geometries. One of the most common zeolites is ZSM-5 (MFI type), which is used as a catalyst and sorbent in a wide range of industrial applications. One of the challenges with ZSM-5 synthesis is to identify new and inexpensive organic SDAs that can tailor the physicochemical properties of the final product. The most frequently used SDA in ZSM-5 synthesis is tetrapropylammonium (TPA); however, recent studies have shown that the surfactant cetyltrimethylammonium (CTA) can be used as an alternative SDA. Here, we examine the effects of dual structure-directing agents using CTA and TPA as organic SDAs in combination with a variety of alkali metals as inorganic SDAs. Our findings reveal that the selection of SDA combinations has a significant impact on the kinetics of ZSM-5 crystallization, as well as the properties of the resulting crystals. Notably, we show that TPA/Na and CTA/K are optimal combinations of SDAs that can markedly alter the size, morphology, and aluminum distribution in ZSM-5. Using a combination of experiments and molecular modeling, we explore the use of CTA as an alternative organic SDA for zeolite MFI and show that we can achieve smaller crystals (ca. 600 nm) in similar time (

Zeolite HOU-2 (LTA type) is prepared with the highest silica content (Si/Al = 2.1) reported for Na-LTA zeolites without the use of an organic structure-directing agent. The rational design of Si-rich zeolites has the potential to improve their thermal stability for applications in catalysis, gas storage, and selective separations.

Hematin crystallization is the primary heme detoxification mechanism of malaria parasites infecting human erythrocytes and the target of currently applied antimalarial medications. The composition of the crystallization medium within the parasite's digestive vacuole (DV), the aqueous or lipid sub-phases, has been the subject of intense debate. Here we show that a blend of lipids, designed to mimic the lipid sub-phase in the parasite DV, contains significant amounts of soluble water that facilitates hematin crystal formation. We show that the hematin solubility in citric buffer-saturated n-octanol (CBSO), a model for the DV lipid sub-phase, is 100000-fold greater than in a biomimetic aqueous solution, indicating that organic-based crystallization provides an environment for faster hematin crystallization and more efficient heme detoxification. We demonstrate that hematin crystals grow with physiologically-relevant rates from CBSO and do not grow from our biomimetic aqueous solvents. Our findings suggest that hematin crystallization most likely requires the participation of lipid structures. We propose a mechanism of hematin crystallization in the parasite DV that reconciles this conclusion with data on hemoglobin transport and hematin generation and crystallization in vivo. The proposed mechanism suggests that hematin becomes incorporated into crystals via a layer of neutral lipids or possibly after penetrating the phospholipid bilayer of the DV. We specify guidelines for tests of the proposed mechanism and highlight its clinical and pharmacological implications.

The physicochemical properties of zeolite catalysts, such as crystal topology, composition, size, and morphology, can have a marked effect on their performance for a broad range of reactions, notably in catalyst activity, hydrothermal stability, shape selectivity, and/or lifetime. There are relatively few zeolite framework types employed as commercial catalysts. Contributing factors include the high cost of synthesis and the difficulty of tailoring crystal nucleation and growth to achieve the desired properties. There is an increasing amount of structure performance data in the literature and patents that can be used to guide the identification of effective zeolite “formulations”; however, the challenges for realizing these materials are generally twofold: (i) growth mechanisms are not well understood, which often prohibits the control of zeolite crystallization, and (ii) the impracticality of most design schemes hinders their economic feasibility and their potential for facile implementation. These aspects are often overlooked in the design of zeolite catalysts, yet they are essential for any plans aimed at eventual commercialization. In this review, we summarize our recent findings in the area of zeolite synthesis and characterization, focusing specifically on practical routes to control zeolite crystallization in the absence of costly organics, tailoring crystal habit through the use of versatile and recyclable zeolite growth modifiers, and pioneering techniques in zeolite surface science as a platform to expand our knowledge of crystal growth mechanisms. These concerted efforts in rational design bridge fundamental and applied research towards the development of zeolites with improved catalytic performance.