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Results for “ZEOLITES”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2013

Large-Scale Screening of Zeolite Structures for CO2 Membrane Separations

We have conducted large-scale screening of zeolite materials for CO2/CH4 and CO2/N-2 membrane separation applications using the free energy landscape of the guest molecules inside these porous materials. We show how advanced molecular simulations can be integrated with the design of a simple separation process to arrive at a metric to rank performance of over 87 000 different zeolite structures, including the known IZA zeolite structures. Our novel, efficient algorithm using graphics processing units can accurately characterize both the adsorption and diffusion properties of a given structure in just a few seconds and accordingly find a set of optimal structures for different desired purity of separated gases from a large database of porous materials in reasonable wall time. Our analysis reveals that the optimal structures for separations usually consist of channels with adsorption sites spread relatively uniformly across the entire channel such that they feature well-balanced CO2 adsorption and diffusion properties. Our screening also shows that the top structures in the predicted zeolite database outperform the best known zeolite by a factor of 4-7. Finally, we have identified a completely different optimal set of zeolite structures that are suitable for an inverse process, in which the CO2 is retained while CH4 or N-2 is passed through a membrane.

Kim, JH↗
DOE OSTI2012

Zeolite with trimodal porosity by desilication of zeolite nanocrystals aggregate

Zeolite with trimodal porosity can be synthesized by desilication of zeolite nanocrystal aggregate. In the desilication process, the originally existed intercrystalline mesopores of zeolite nanocrystal aggregate were enlarged into large mesopore, and the new small intracrystalline mesopore channel was created, thus the Zeolite with trimodal porosity was formed. The structure of resulted zeolite, both on aggregate and mesopore level can be fine tuned by the desilication degree. - Graphical abstract: The Si from the edges and boundary of nanocrystals was first removed resulted the surface roughness and enlarges of the originally existed intercrystalline mesopores. As the degree of alkali-treatment increasing, the Si species inside zeolite nanocrystals was also removed, leading to further enlarges the intercrystalline mesopores and the formation of small intracrystalline mesopores. In case the alkali-treatment is serve enough to completely dissolve the bridges between zeolite nanocrystals, zeolite nanocrystals were exfoliated from the aggregate. Highlights: Black-Right-Pointing-Pointer Zeolite with trimodal porosity by desilication of zeolite nanocrystals aggregate. Black-Right-Pointing-Pointer The original intercrystalline mesopores were enlarged into large mesopore. Black-Right-Pointing-Pointer The new intracrystalline mesopores were created as the inside Si extracted out. Black-Right-Pointing-Pointer The aggregate structure, crystallinity and acidity of parent zeolite remained. Black-Right-Pointing-Pointer Desilication is start on the edges then in the inner part of zeolite.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

A quantitative study of the structure-activity relationship in hierarchical zeolites using liquid-phase reactions

Micro/meso/macroporous (hierarchical) zeolites show remarkable catalytic performance for reactions involving bulky reactants. However, quantitative assessment of the microstructural characteristics contributing to the observed performance remains elusive. Here, structure–activity relationships are established for a set of micro/mesoporous self-pillared pentasil (SPP) zeolites using two parallel liquid-phase reactions (benzyl alcohol alkylation and self-etherification) based on analysis of mass transport and reaction kinetics. A reaction–diffusion mathematical model is developed that quantitatively assigns the catalytic contributions of the external surface and micropores of SPP zeolites for these reactions. In addition, the effect of the zeolite external surface structure on the corresponding catalytic activity is quantitatively assessed by comparing SPP zeolites (with MFI structure) with MCM-22 (with MWW structure). This work demonstrates that reaction–diffusion modeling allows quantitative description of the catalytic performance of hierarchical zeolites and provides a model reaction to assess nm-sized characteristic diffusion lengths in MFI.

bio-inspired↗
DOE OSTI2021

Exfoliation of surfactant swollen layered MWW zeolites into two-dimensional zeolite nanosheets using telechelic liquid polybutadiene

We report two-dimensional (2D) zeolites have the potential to enhance mass transport by reducing diffusion lengths and thus find applications in catalysis and separation. High aspect ratio 2D zeolite nanosheets are required for fabrication of thin, high-flux zeolite membranes. Preparation of 2D zeolite nanosheets can be achieved by exfoliation of layered zeolite precursors, which usually requires multiple steps including swelling with cationic surfactants, exfoliation by applying a mechanical force such as shearing or sonication, purification and removal of the surfactants. The exfoliation of surfactant swollen layered zeolite precursors has been studied using molten polystyrene and telechelic liquid polybutadiene, but still requires extensive shearing force. Recently, a novel one-step exfoliation method has been developed for nonswollen MWW zeolite precursors using tetrabutylammonium hydroxide (TBAOH) solution. In this work, we study the exfoliation of surfactant swollen MWW layered precursors (with and without aluminum) in commercially available liquid hydroxyl-terminated polybutadiene (HTPB) with the aim to provide fundamental understanding of the interaction between the liquid polybutadiene and the surfactant swollen layered zeolite precursors. It was found that surfactant swollen ITQ-1, (ITQ-1(S)), a pure silica layered precursor with MWW framework after being swollen with a cationic surfactant, can be exfoliated in liquid HTPB at room temperature without applying any additional shearing force. The aluminum containing surfactant swollen MWW precursors can be also exfoliated in HTPB at room temperature, but require additional shearing forces. The exfoliation process was monitored using rheology experiments and studied by tuning the composition of the zeolite precursors. The interaction between the layered zeolite precursor and the hydroxyl group of the liquid polybutadiene allows the polymer to intercalate the precursor and exfoliate it into nanosheets. The presence of framework aluminum and the nature of the organic structure directing agents (OSDAs) are key parameters affecting the exfoliation process. Framework Al is unfavorable for the exfoliation possibly due to the surface charges introduced by the aluminum. Bulky OSDA, trimethyladamantammonium (TMAda + ), in the interlayer spacing may facilitate the exfoliation.

2D nanosheets↗
DOE OSTI2022

Role of Zeolite Structural Properties toward Iodine Capture: A Head-to-head Evaluation of Framework Type and Chemical Composition

This study evaluated zeolite-based sorbents for iodine gas [I2(g)] capture. Based on the framework structures and porosities, five zeolites, including two faujasite (FAU), one ZSM-5 (MFI), one mesoMFI, one ZSM-22 (TON), as well as two mesoporous materials, were evaluated for I2(g) capture at room temperature and 150 °C in an iodine-saturated environment. From these preliminary studies, the three best-performing zeolites were ion-exchanged with Ag+ and evaluated for I2(g) capture under similar conditions. Energy-dispersive X-ray spectroscopy data suggest that Ag-FAU frameworks were the materials with the highest capacity for I2(g) in this study, showing ~3× higher adsorption compared to Ag-mordenite (Ag-MOR) at room temperature, but X-ray diffraction measurements show that the faujasite structure collapsed during the adsorption studies because of dealumination. The Ag-MFI zeolites are decent sorbents in real-life applications, showing both good sorption capacities and higher stability. In-depth analyses and characterizations, including synchrotron X-ray absorption spectroscopy, revealed the influence of structural and chemical properties of zeolites on the performance for iodine adsorption from the gas phase.

aluminosilicate zeolites, iodine, porosity, acidit↗
DOE OSTI2016

Synthesis Strategies for Ultrastable Zeolite GIS Polymorphs as Sorbents for Selective Separations

Abstract Designing zeolites with tunable physicochemical properties can substantially impact their performance in commercial applications, such as adsorption, separations, catalysis, and drug delivery. Zeolite synthesis typically requires an organic structure‐directing agent to produce crystals with specific pore topology. Attempts to remove organics from syntheses to achieve commercially viable methods of preparing zeolites often lead to the formation of impurities. Herein, we present organic‐free syntheses of two polymorphs of the small‐pore zeolite P (GIS), P1 and P2. Using a combination of adsorption measurements and density functional theory calculations, we show that GIS polymorphs are selective adsorbents for H 2 O relative to other light gases (e.g., H 2 , N 2 , CO 2 ). Our findings refute prior theoretical studies postulating that GIS‐type zeolites are excellent materials for CO 2 separation/sequestration. We also show that P2 is significantly more thermally stable than P1, which broadens the operating conditions for GIS‐type zeolites in commercial applications and opens new avenues for exploring their potential use in processes such as catalysis.

Oleksiak, Matthew D.↗
DOE OSTI2017

Developing a Descriptor-Based Approach for CO and NO Adsorption Strength to Transition Metal Sites in Zeolites

The discovery of new materials tailored for a given application typically requires the screening of a large number of compounds, and this process can be significantly accelerated by computational analysis. In such an approach the performance of a compound is correlated to a materials property, a so-called descriptor. Here we develop a descriptor-based approach for the adsorption of CO and NO to Cu, Ni, Co, and Fe sites in zeolites. Here we start out by discussing a possible design strategy for zeolite catalysts, define the studied test set of sites in the zeolites SSZ-13 and mordenite, and define a set of appropriate descriptors. In a subsequent step we use these descriptors in single-parameter, two-parameter, and multiparameter regression analysis and finally use a machine-learning genetic algorithm to reduce the number of variables. We find that one or two descriptors are not sufficient to accurately capture the interactions between molecules and metal centers in zeolites, and indeed a multiparameter approach is necessary. Even though many of the descriptors are directly correlated, we identify the position of the s orbital and the number of valence electrons of the active site as well as the HOMO–LUMO gap of the adsorbate as most important descriptors. Furthermore, the reconstruction of the active sites upon adsorption plays a crucial role, and when it is explicitly included in the analysis, correlations improve significantly. In the future we expect that the fundamental methodology developed here will be adapted and transferred to selected problems in adsorption and catalysis and will assist the rational design of materials for the given application.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Influence of Hydronium Ions in Zeolites on Sorption

In presence of sufficient concentrations of water, stable hydrated hydronium ions are formed in inorganic solid acids such as zeolites. In their presence larger organic molecules can only adsorb in the pore volume not occupied by hydronium ions. In consequence, the available pore volume decreases proportionally to their concentrations. The higher charge density (the increasing ionic strength) induced by increasing hydronium ion concentration in zeolite crystalline induces higher activity coefficients of adsorbed substrates, weakening the interactions between the organic part of the molecules, favoring instead interactions with polar groups. The insight allows linking a collective property such as hydrophilicity and hydrophobicity of zeolites to specific interactions on the molecular level.

Eckstein, Sebastian↗
DOE OSTI2020

Mechanistic understanding of catalytic conversion of ethanol to 1-butene over 2D-pillared MFI zeolite

Ethanol is an important C2 platform molecule for producing value-added chemicals and distillate hydrocarbon fuels (e.g., jet and diesel). Among these, catalytic upgrading of ethanol to butenes can generate valuable commodity chemicals (e.g., 1-butene) and provide C4 olefin intermediates that can be further upgraded to jet/diesel fuels. 2D zeolites offer hierarchical mesoporous structures, leading to improved mass transport and reduced diffusion length, which can help to address the coking challenges faced by ethanol conversion to hydrocarbons over 3D zeolites. In this study, we investigate the acid-catalyzed conversion of ethanol to 1-butene over the Brønsted acid sites (BAS) in 2D-pillared MFI zeolite (2D-PMFI) using ab initio molecular dynamics (AIMD) simulations, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and calorimetric measurements. A detailed thermodynamic analysis, using quasi-harmonic approximation (QHA), on the Gibbs free energy pathway of ethanol conversion shows that the consideration of entropy is critical to accurately capture the detailed thermodynamic profiles. The formation of bulky ethoxy-ethene complex is found to be a potential rate-determining step (RDS), proceeding via stepwise mechanism. The reactivity of 2D-PMFI can be further tuned by manipulating RDS through a careful control of number of BAS and operating temperatures. The accuracy of computational model is confirmed through the comparison of calculated vibrational density of states (VDOS) against the experimental DRIFTS measurements. Overall, our study provides mechanistic insights into ethanol upgrading over the 2D-PMFI and shows the importance of evaluating entropic effects in such confined system.

Ethanol conversion, 2D-pillared zeolite, MFI, Anha↗
DOE OSTI2017

Sub-Micrometer Zeolite Films on Gold-Coated Silicon Wafers with Single-Crystal-Like Dielectric Constant and Elastic Modulus

A low-temperature synthesis coupled with mild activation produces zeolite films exhibiting low dielectric constant (low-k) matching the theoretically predicted and experimentally measured values for single crystals. This synthesis and activation method allows for the fabrication of a device consisting of a b-oriented film of the pure-silica zeolite MFI (silicalite-1) supported on a gold-coated silicon wafer. The zeolite seeds are assembled by a manual assembly process and subjected to optimized secondary growth conditions that do not cause corrosion of the gold underlayer, while strongly promoting in-plane growth. The traditional calcination process is replaced with a non-thermal photochemical activation to ensure preservation of an intact gold layer. The dielectric constant (k), obtained through measurement of electrical capacitance in a metal-insulator-metal configuration, highlights the ultralow k approximate to 1.7 of the synthetized films, which is among the lowest values reported for an MFI film. There is large improvement in elastic modulus of the film (E approximate to 54 GPa) over previous reports, potentially allowing for integration into silicon wafer processing technology.

Tiriolo, Raffaele↗
DOE OSTI2016

Nanoscale Control of Homoepitaxial Growth on a Two-Dimensional Zeolite

Nanoscale crystal growth control is crucial for tailoring two-dimensional (2D) zeolites (crystallites with thickness less than two unit cells) and thicker zeolite nanosheets for applications in separation membranes and as hierarchical catalysts. However, methods to control zeolite crystal growth with nanometer precision are still in their infancy. Herein, we report solution-based growth conditions leading to anisotropic epitaxial growth of 2D zeolites with rates as low as few nanometers per day. Contributions from misoriented surface nucleation and rotational intergrowths are eliminated. Growth monitoring at the single-unit-cell level reveals novel nanoscale crystal-growth phenomena associated with the lateral size and surface curvature of 2D zeolites.

Shete, Meera↗