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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.

Developing structure-performance relationships with the underlying goal of optimizing known zeolite catalysts involves the manipulation of their physicochemical properties. Here, we systematically assessed the impact of mesoscopic gradients in acid site concentration, which has received little attention in the design of zeolite catalysts for hydrocarbon upgrading. A series of core-shell MEL-type zeolites were synthesized with catalytically active ZSM-11 cores and passivated silicalite-2 shells of varying thickness. Our findings revealed that ZSM-11@silicalite-2 particles with ultrathin shells (

There is a widespread need to understand and improve the aging characteristics and properties of catalysts that support essential chemical reactions, including methanol-to-hydrocarbons (MTH) conver...

Long-term catheterised individuals are at significant risk of developing catheter-associated urinary tract infections (CAUTIs), with up to 50% of patients experiencing recurrent episodes of catheter encrustation and blockage. Catheter blockage is a result of accumulation of carbonate apatite and struvite formed upon precipitation of ions within urine due to an infection-induced rise in pH. The aim of this study was to investigate the antimicrobial and anti-encrustation activities of tetrasodium ethylenediaminetetraacetic acid (tEDTA) to evaluate its potential efficacy in preventing CAUTIs and catheter blockages. The antimicrobial activity of tEDTA against uropathogens was assessed using time kill assays performed in artificial urine (AU). Crystallisation studies and in vitro bladder model assays were conducted to investigate the effect of tEDTA on struvite crystallisation and catheter blockage. tEDTA displayed bacteriostatic activity against Proteus mirabilis and prevented precipitation of ions in the AU. Crystallisation studies confirmed tEDTA inhibits struvite nucleation and growth via Mg2+ chelation with 7.63 mM tEDTA, equimolar to the concentration of divalent cations in AU, preventing the formation of crystalline deposits and blockage of Foley catheters for $168 h. The promising chelating abilities of low tEDTA concentrations could be exploited to inhibit encrustation and blockage of indwelling catheters. The fundamental research presented will inform our future development of an effective tEDTA-eluting catheter coating aimed at preventing catheter encrustation.

Zeolite crystallization occurs by complex processes involving a variety of possible mechanisms. The sol gel media used to prepare zeolites leads to heterogeneous mixtures of solution and solid states with diverse solute species. At later stages of zeolite synthesis when growth occurs predominantly from solution, classical two-dimensional nucleation and spreading of layers on crystal surfaces via the addition of soluble species is the dominant pathway. At earlier stages, these processes occur in parallel with nonclassical pathways involving crystallization by particle attachment (CPA). The relative roles of solution- and solid-state species in zeolite crystallization have been a subject of debate. Here, we investigate the growth mechanism of a commercially relevant zeolite, faujasite (FAU). In situ atomic force microscopy (AFM) measurements reveal that supernatant solutions extracted from a conventional FAU synthesis at various times do not result in growth, indicating that FAU growth predominantly occurs from the solid state through a disorder-to-order transition of amorphous precursors. Elemental analysis shows that supernatant solutions are significantly more siliceous than both the original growth mixture and the FAU zeolite product; however, in situ AFM studies using a dilute clear solution with a lower Si/Al ratio revealed three-dimensional growth of surfaces that is distinct from layer-by-layer and CPA pathways. This unique mechanism of growth differs from those observed in studies of other zeolites. Given that relatively few zeolite frameworks have been the subject of mechanistic investigation by in situ techniques, these observations of FAU crystallization raise the question whether its growth pathway is characteristic of other zeolite structures.

Given the strong influence of surface structure on the reactivity of heterogeneous catalysts, understanding the mechanisms that control crystal morphology is an important component of designing catalytic materials with targeted shape and functionality. Herein, we employ density functional theory to examine the impact of growth media on NiO crystal faceting in line with experimental findings, showing that molten-salt synthesis in alkali chlorides (KCl, LiCl, and NaCl) imposes shape selectivity on NiO particles. We find that the production of NiO octahedra is attributed to the dissociative adsorption of H2 O, whereas the formation of trapezohedral particles is associated with the control of the growth kinetics exerted by ordered salt structures on high-index facets. To our knowledge, this is the first observation that growth inhibition of metal-oxide facets occurs by a localized ordering of molten salts at the crystal-solvent interface. These findings provide new molecular-level insight on kinetics and thermodynamics of molten-salt synthesis as a predictive route to shape-engineer metal-oxide crystals.

Synthesis of a pentasil‐type zeolite with ultra‐small few‐unit‐cell crystalline domains, which we call FDP (few‐unit‐cell crystalline domain pentasil), is reported. FDP is made using bis‐1,5(tributyl ammonium) pentamethylene cations as structure directing agent (SDA). This di‐quaternary ammonium SDA combines butyl ammonium, in place of the one commonly used for MFI synthesis, propyl ammonium, and a five‐carbon nitrogen‐connecting chain, in place of the six‐carbon connecting chain SDAs that are known to fit well within the MFI pores. X‐ray diffraction analysis and electron microscopy imaging of FDP indicate ca. 10 nm crystalline domains organized in hierarchical micro‐/meso‐porous aggregates exhibiting mesoscopic order with an aggregate particle size up to ca. 5 μm. Al and Sn can be incorporated into the FDP zeolite framework to produce active and selective methanol‐to‐hydrocarbon and glucose isomerization catalysts, respectively., A simple di‐quaternary ammonium cation was used as structure‐directing agent to prepare few‐unit‐cell (ca. 10 nm) MFI type zeolite catalysts and adsorbents with a wide range of composition (pure silica, aluminosilicate and stannosilicate). The SDA design was selected based on simple heuristic arguments regarding contributions of its structural elements to early crystal growth characteristics of MFI.

The synthesis of zeolites with nano-sized dimensions is often limited to a narrow design space that conventionally relies upon the design of organics to direct hierarchical materials. Here, it is demonstrated that the addition of an inorganic modifier, germanium oxide (GeO

We report a method to prepare core-shell zeolite beta (*BEA) with an aluminous core and an epitaxial Si-rich shell. This method capitalizes on the inherent defects in *BEA crystals to simultaneously passivate acid sites on external surfaces and increase intracrystalline mesoporosity through facile post-hydrothermal synthesis modification in alkaline media. This process creates more hydrophobic materials by reducing silanol defects and enriching the shell in silica via a combination of dealumination and the relocation of silica from the core to the shell during intracrystalline mesopore formation. The catalytic consequences of *BEA core-shells relative to conventional analogues were tested using the biomass conversion of levulinic acid and n-butanol to n-butyl levulinate as a benchmark reaction. Our findings reveal that siliceous shells and intracrystalline mesopores synergistically enhance the performance of *BEA catalysts. This work was supported by BASF Corporation. Additional support for JDR was provided by The Welch Foundation (Award E-1794). ToF-SIMS analysis was carried out with support provided by the National Science Foundation CBET-1626418. This work was conducted in part using resources of the Shared Equipment Authority at Rice University. JGM received funding for this project from the European Union’s Horizon 2020 research and innovation program under grant agreement No 872102. JGM and NL thank the Spanish Ministry of Science and Innovation and AEI/FEDER, UE through the project ref. RTI2018-099504-B-C21. NL acknowledges additional support from the University of Alicante (UATALENTO17-05).

Heterogeneous NiO–MgO materials and (Ni,Mg)O solid solutions have found application in catalysis owing to the combined redox properties of NiO and basic properties of MgO. Examples include the use ...

The preparation of metastable zeolites is often restricted to a limited range of synthesis conditions, which is exemplified in commercial syntheses lacking organics to stabilize the crystal structure. In the absence of an organic structure-directing agent, interzeolite transformation is a common phenomenon that can lead to undesirable products or impurities. Many studies have investigated the substitution of Si and Al in zeolite frameworks with alternative elements (heteroatoms) as a means of tailoring the properties of zeolites; however, relatively few studies have systematically explored the impact of heteroatoms on interzeolite transformations and their concomitant effects on zeolite crystallization. In this study, we examine methods to prepare isostructures of faujasite (FAU), which is one of the most commercially relevant zeolites and also a thermodynamically metastable structure. A survey of multivalent elements revealed that zinc is capable of stabilizing FAU at high temperatures and inhibiting its frequent transformation to zeolite gismondine (GIS). Using combined experimental and computational studies, we show that zinc alters the chemical nature of growth mixtures by sequestering silicates. Zinc heteroatoms incorporate in the FAU framework with a loading-dependent coordination. Our collective findings provide an improved understanding of driving forces for the FAU-to-GIS interzeolite transformation where we observe that heteroatoms (e.g., zinc) can stabilize zeolite FAU over a broad range of synthesis conditions. Given the growing interest in heteroatom-substituted zeolites, this approach to preparing zinc-containing FAU may prove applicable to a broader range of zeolite structures.

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.

Zeolite crystallization often requires the presence of an organic structure-directing agent (SDA) to facilitate the formation of nanoporous cages and channels. Among the limited number of zeolites ...

This mini-review provides an overview of the current state of acid site control in zeolite catalysts, including methods of synthesis, advanced characterization, and measured effects of acid properties (speciation, concentration, proximity, siting, and spatial distribution) on a variety of commercially-relevant reactions. The diversity of aluminum species is described with respect to their location at specific sites in zeolite crystals as well as mesoscopic gradients in elemental composition that give rise to zoned or core-shell architectures. Challenges in the identification of acid siting are highlighted within the context of trial-and-error synthesis methods for a range of aluminosilicate frameworks, which hinder a priori design of catalysts, and limitations in techniques to suitably characterize active sites. Emphasis is also placed on knowledge gaps in zeolite catalysis wherein broad development of structure-performance relationships relies on future advancement of synthesis and analytical methods in parallel with atomistic modeling.

Bifunctional catalysis in zeolites possessing both Bronsted and Lewis acid sites offers unique opportunities to tailor shape selectivity and enhance catalyst performance. Here, we examine the impact of framework and extra-framework gallium species on enriched aromatics production in zeolite ZSM-5. In this study we compare three distinct methods of preparing Ga-ZSM-5 and reveal direct (single step) synthesis leads to optimal catalysts compared to post-synthesis methods. Using a combination of state-of-the-art characterization, catalyst testing, and density functional theory calculations, we show that Ga Lewis acid sites strongly favor aromatization. Our findings also suggest Ga(framework) - Ga(extra-framework) pairings, which can only be achieved in materials prepared by direct synthesis, are the most energetically favorable sites for reaction pathways leading to aromatics. Calculated acid site exchange energies between extra-framework Ga at framework sites comprised of either Al or Ga reveal a site-specific preference for stabilizing Lewis acids, which is qualitatively consistent with experimental measurements. These findings indicate the possibility of tailoring Lewis acid siting by the placement of Ga heteroatoms at distinct tetrahedral sites in the zeolite framework, which can have a marked impact on catalyst performance relative to conventional H-ZSM-5.

Metal oxides exposing high-index facets are potentially impactful in catalysis and adsorption processes owing to under-coordinated ions and polarities that alter their interfacial properties compared to low-index facets. Here, we report molten-salt syntheses of NiO particles exposing a variety of crystal facets. We show that for a given anion (nitrate or chloride), the alkali cation has a notable impact on the formation of crystals exposing {311}, {611}, {100}, and {111} faces. Based on a parametric analysis of synthesis conditions, we postulate that the crystallization mechanism is governed by the formation of growth units consisting of NiII complexes whose coordination numbers are determined by temperature and the selection of anion (associated to the coordination sphere) and alkali cation (associated with the outer coordination sphere). Notably, our findings reveal that high-index facets are particularly favored in chloride media and are stable under prolonged periods of catalysis and steaming.

The ammonia slip catalyst (ASC) is an essential final step in the emission control system and involves the selective oxidation of NH3 to N2. The state-of-the-art ASC has a dual-layer architecture c...

Tailoring processes of nucleation and growth to achieve desired material properties is a pervasive challenge in synthetic crystallization. In systems where crystals form via nonclassical pathways, engineering materials often requires the controlled assembly and structural evolution of colloidal precursors. In this study, we examine zeolite SSZ-13 crystallization and show that several polyquaternary amines function as efficient accelerants of nucleation, and, in select cases, tune crystal size by orders of magnitude. Among the additives tested, polydiallyldimethylammonium (PDDA) was found to have the most pronounced impact on the kinetics of SSZ-13 formation, leading to a 4-fold reduction in crystallization time. Our findings also reveal that enhanced nucleation occurs at an optimal PDDA concentration where a combination of light scattering techniques demonstrate these conditions lead to polymer-induced aggregation of amorphous precursors and the promotion of (alumino)silicate precipitation from growth solution. Here, we show that relatively low concentrations of polymer additives can be used in unique ways to dramatically enhance SSZ-13 crystallization rates, thereby improving the overall efficiency of zeolite synthesis. JDR received financial support primarily from BASF Corporation along with additional support from the Welch Foundation (Award E-1794). JGM received financial support from the European Commission through the H2020-MSCA-RISE-2019 program (Award ZEOBIOCHEM – 872102) and the Spanish MINECO and AEI/FEDER, UE through Award RTI2018-099504-B-C21. NL acknowledges the University of Alicante support (Award UATALENTO17-05).

A core objective of synthesizing zeolites for widespread applications is to produce materials with properties and corresponding performances that exceed conventional counterparts. This places an impetus on elucidating and controlling processes of crystallization where one of the most critical design criteria is the ability to prepare zeolite crystals with ultrasmall dimensions to mitigate the deleterious effects of mass transport limitations. At the most fundamental level, this requires a comprehensive understanding of nucleation to address this ubiquitous materials gap. This Perspective highlights recent methodologies to alter zeolite nucleation by using seed-assisted protocols and the exploitation of interzeolite transformations to design advanced materials. Introduction of crystalline seeds in complex growth media used to synthesize zeolites can have wide-ranging effects on the physicochemical properties of the final product. Here we discuss the diverse pathways of zeolite nucleation, recent breakthroughs in seed-assisted syntheses of nanosized and hierarchical materials, and shortcomings for developing generalized guidelines to predict synthesis outcomes. We offer a critical analysis of state-of-the-art approaches to tailor zeolite crystallization wherein we conceptualize whether parallels between network theory and zeolite synthesis can be instrumental for translating key findings of individual discoveries across a broader set of zeolite crystal structures and/or synthesis conditions.

Designing zeolite catalysts with improved mass transport properties is crucial for restrictive networks of either one- or two-dimensional pore topologies. Here, we demonstrate the synthesis of finned ferrierite (FER), a commercial zeolite with two-dimensional pores, where protrusions on crystal surfaces behave as pseudo nanoparticles. Catalytic tests of 1-butene isomerization reveal a 3-fold enhancement of catalyst lifetime and an increase of 12 % selectivity to isobutene for finned samples compared to corresponding seeds. Electron tomography was used to confirm the identical crystallographic registry of fins and seeds. Time-resolved titration of Brønsted acid sites confirmed the improved mass transport properties of finned ferrierite compared to conventional analogues. These findings highlight the advantages of introducing fins through facile and tunable post-synthesis modification to impart material properties that are otherwise unattainable by conventional synthesis methods.

Among a wide variety of pathological crystalline materials in nephrolithiasis, uric acid and its salt form are the most abundant organic constituents. Even though rarely found in humans, ammonium urate (NH4HU) is the most prevalent kidney stone reported in managed bottlenose dolphins. In this study, we investigate the physicochemical properties of NH4HU crystals associated with dolphin kidney stones, which exhibit distinct hierarchical structures. We present a method of crystallizing NH4HU without common impurities, which allowed for the first measurement of NH4HU crystal solubility over a broad range of temperatures. Parametric evaluations of NH4HU crystallization are reported, wherein we assess the effects of supersaturation and solution alkalinity on crystal phase behavior, the kinetics of urate crystallization, and the size and morphology of synthetic NH4HU crystals. In vitro bulk crystallization assays from aqueous growth solutions containing supersaturated ionic species (NH4+ and urate1–) were perfo...

There is broad scientific interest in lamellar zeolitic materials for a large variety of technological applications. The traditional synthetic methods towards two-dimensional (2D) zeolitic precursors have made a great impact in the construction of families of related zeolites; however, the connection between structurally distinct 2D zeolitic precursors is much less investigated in comparison, thereby resulting in a synthetic obstacle that theoretically limits the types of zeolites that can be constructed from each layer. Herein, we report a Ge-recycling strategy for the topotactic conversion between different 2D zeolitic precursors through a three-dimensional (3D) germanosilicate. Specifically, the intermediate germanosilicate can be constructed within 150 min by taking advantage of its structural similarity with the parent lamellar precursor. This process enables the conversion of one 2D zeolite structure into another distinct structure, thus overcoming the synthetic obstacle between two families of zeolitic materials.

There has been significant interest in the area of zeolite synthesis, and more broadly in the development of microporous catalysts, to establish methods of reducing crystal size. A promising approa...