LTA Crystals Research Articles

Mixed matrix membrane based on DOCDA polyimide with twisted kink structure and zeolite 4A for energy-efficient oxygen purification

Membrane technology has advanced, offering high-quality polymeric membranes with great potential for commercial use. However, producing these advanced membranes is usually costly and involves complex manufacturing. To tackle this issue, a cost-effective alternative has emerged in the form of polyimide (PI) membranes, particularly those derived from DOCDA-ODA, a low-cost organic compound. However, the initial gas permeance of DOCDA-ODA membranes is lower than those of commercial PI membranes such as Matrimid® 5218 and 6FDA-DAM membranes. To address this limitation, this study was aimed at fabricating mixed matrix membranes (MMMs) by incorporating zeolite 4A into the DOCDA-ODA matrix. The MMMs demonstrate promising O2/N2 separation performance, surpassing that of Matrimid® 5218-based membranes. Furthermore, they exhibit an O2/N2 selectivity of 14.8 with an O2 permeability of 2.3 Barrer, surpassing recent benchmarks established by the Robeson upper bounds for polymeric membranes. This exceptional O2/N2 separation performance can be attributed to favorable interfacial interactions between the polymer chains and LTA crystals. Molecular dynamics simulations clarify the role of electronegative atoms, especially oxygen, within the kinked structure of DOCDA-ODA.. Thus, DOCDA-ODA-based MMMs hold potential for O2 purification applications, reducing energy consumption by an impressive 49.3% compared to equivalent commercial membrane processes.

Combining Phase Separation with Pseudomorphic Transformation for the Control of the Pore Architecture of Functional Materials: A Review

Chemical phase separation and pseudomorphic transformation are very powerful tools for fine tuning the porous architecture of inorganic oxides. In this short review the basic principles of the preparation of centimetric bodies made of zeolites is introduced. The synthesis of monoliths with homogeneous distributions of interconnected macropores with skeleton made of SOD, LTA and FAU crystals is described. Their unique hierarchical porous texture featuring the structural micropores, intercrystalline mesopores and flow-through macropores leads to remarkable hydrodynamic behavior in separation, catalysis and ion exchange processes operated in continuous-flow mode. In the base-catalyzed carbon-carbon bond formation, productivities twice those achievable with fixed-beds of packed particles are observed. In the capture of strontium present in radioactive effluents their efficiency is three orders of magnitude that of traditional powder in batch.

Selection between LTA or FAU phases by manipulating the synthesis gel composition, crystallization time and temperature

Abstract LTA and FAU zeolites were synthesized with the addition of cetyltrimethylammonium bromide (CTAB) to assess the effects of Si/Al, Na/Al and H2O/Al ratios, time and temperature on the yield of each structure. The significance of each variable was determined by means of experimental designs, whose response variables were the areas of specific XRD peaks. CTAB did not act as a structure-directing agent, but rather as a growth modifier, increasing the growth rate of LTA and FAU crystals. At low temperature, LTA was afforded at low Si/Al ratio, while FAU crystallized at higher ratios, indicating a competition between the phases. Increasing temperature accelerated the kinetics of recrystallization of the less dense and metastable LTA and FAU phases in denser and more stable phases, such as sodalite and cancrinite. Decreasing the concentration of Na+ and water reduced the crystallinity of both structures, while increasing the concentration of Na+ favored LTA over FAU.

Synthesis of highly dispersed cobalt oxide clusters encapsulated within LTA zeolites

Small Co3O4 nanoparticles uniformly distributed in size were encapsulated within LTA zeolite crystals in a one-step process through hydrothermal self-assembly of crystalline frameworks around ligated Co2+ precursors. The use of bifunctional ligands containing a chelating bidentate amine functionality and an alkoxysilane moiety prevented the precipitation of Co2+ species as colloidal hydroxides in the highly alkaline synthesis gels, while also allowing the formation of linkages between precursors and the framework during the nucleation and growth of LTA crystals. Oxidative treatments of ligated compounds occluded within zeolite crystals removed ligand residues and formed small Co3O4 nanoparticles visible in transmission electron micrographs. These nanoparticles retained their small size (average diameter 1.5 nm) after oxidative treatment at 620–870 K, a reflection of their stabilization by confinement within zeolite voids. The infrared spectra of adsorbed CO on Co-LTA samples confirmed the absence of Co2+ as exchanged cations or aluminosilicates, indicating the presence of Co oxide clusters, with dynamics and stoichiometry of reduction in H2 corresponding to small Co3O4 clusters. Ethanol oxidation rates on Co-LTA samples, exchanged with K+ or Ca2+ cations to vary the diffusive properties of LTA crystals, indicated that more than 97% of the active surfaces on these Co3O4 clusters resided within zeolite crystals, where ethanol and O2 concentrations depend on the diffusive properties of the LTA framework. The Co3O4 clusters prepared by these methods, in contrast with Co2+ in exchanged or aluminosilicate forms, exhibit reactivity in CO and NO oxidation. Their turnover rates (per exposed Co atom), however, were lower than on bulk Co3O4 powders, because of the combined effects of diffusional constraints imposed by the confining framework and the small size of these clusters, which leads to lower intrinsic reactivities as a result of their more difficult reduction during catalytic redox cycles. These clusters would be attractive in catalytic applications requiring stability against sintering during reaction or regeneration, reactant or product shape selectivity, or protection from contact with large molecules that block active surfaces. Such oxide clusters cannot be formed by sequential ion exchange, detachment by reduction to Co0, and re-oxidation because the extremely high temperatures required for reduction destroy the aluminosilicate frameworks. The synthesis protocols and their mechanistic interpretations described herein represent a conceptual and practical platform for the encapsulation of nanoparticles of base elements within a broad range of confining crystalline environments through one-step hydrothermal self-assembly.

Influence of the nature of amino acids on the formation of mesoporous LTA-type zeolite

Hydrothermal crystallization of LTA zeolite has been conducted in the presence of amino acid in order to introduce mesoporosity. Eight different types of proteinogenic amino acids have been employed. The influence of their structure and physicochemical properties on the formation of hierarchically mesoporous LTA (MLTA) zeolite have been investigated. All eight amino acids bearing either basic, or acidic, or polar non-ionizable, or non-polar non-ionizable side chains were able to generate mesopores within LTA crystals. The isoelectric points of amino acids influenced the resulting MLTA's morphology, crystallization kinetics, and impurity content. Pure phase MLTA with controllable shape was obtained. Amino acid with a higher isoelectric point generated smaller and more spherical MLTA in less crystallization time, but induced slightly more impurity FAU phase. The mesopore size generally decreased with hydropathy index so that amino acids with non-polar, non-ionizable side groups generated 14–15 nm mesopores, 5–10 nm smaller than those generated by amino acids in other categories. The hydrophobic moieties in amino acids tended to shrink the mesopores, in contrast with their pore expansion effect in surfactant-mediated mesopore generation route, suggesting a different mesopore formation mechanism. Stability of amino acid during the hydrothermal zeolite synthesis and easy amino acid removal by washing with water was further confirmed. The findings discovered in this study paves a new way for tailoring hierarchical zeolite materials with controlled texture properties.

Mixed-matrix membranes containing inorganically surface-modified 5A zeolite for enhanced CO2/CH4 separation

LTA zeolite with highly roughened surfaces were prepared in an aqueous phase by the ion-exchange-induced growth of Mg(OH)2 nanostructures on the zeolite surfaces. The morphology of LTA crystals was tuned by the systematic modification of reaction parameters such as the pH of the reaction medium and the amount of magnesium loaded in the substrates. After converting surface-modified LTA to 5A, which has been known as a good candidate for selective CO2 uptake and transport, by replacing extra-framework cations remaining in LTA with Ca2+, a series of mixed-matrix membrane incorporating Matrimid® and 5A was fabricated. Owing to improved zeolite/polymer adhesion property along with the selective transport of CO2 by the fillers, mixed-matrix membranes containing surface modified 5A showed enhanced CO2/CH4 separation properties, which were measured by a binary mixture permeation testing. In light of that, a dramatic increase in CO2 permeability (ca. 120%) was observed for Matrimid®/20 wt% 5A membrane which also showed an improved CO2/CH4 selectivity. In contrast, untreated 5A decreased CO2/CH4 selectivity of Matrimid® membrane due to defects formed at zeolite/polymer interfaces.

Synthesis of stable monodisperse AuPd, AuPt, and PdPt bimetallic clusters encapsulated within LTA-zeolites

AuPd, AuPt, and PdPt bimetallic clusters uniform in size and composition were prepared using hydrothermal assembly of LTA crystals around cationic precursors stabilized by protecting mercaptosilane ligands. The sulfur moiety in these bifunctional ligands forms adducts that prevent premature reduction or precipitation of metal precursors during crystallization. The silane groups can form bridges with silicate oligomers as they form, thus enforcing homogeneous distributions of precursors throughout crystals and ensuring that subsequent reductive treatments lead to the two elements residing within small and nearly monodisperse clusters. Their confinement within LTA crystals, evident from microscopy and titrations with large poisons, renders them stable against sintering during thermal treatments at high temperatures (820–870K). Infrared spectra of chemisorbed CO show that bimetallic surfaces are free of synthetic debris after thermal treatments; these spectra also indicate that intracluster segregation occurs upon CO chemisorption, a demonstration of the presence of the two elements within the same clusters. The number and type of atoms coordinated to a given absorber atom, determined from the fine structure in X-ray absorption spectra, are consistent with bimetallic structures of uniform composition. The rates of ethanol oxidative dehydrogenation on these bimetallic clusters were essentially unaffected by exposure to dibenzothiophene, a large poison that suppresses rates on unconfined clusters, indicating that bimetallic clusters are protected within the confines of LTA crystals. These synthetic protocols seem generally applicable to other bimetallic compositions and zeolites, for which the monometallic counterparts have been successfully encapsulated within several microporous frameworks using ligand-stabilized precursors and hydrothermal crystallization methods.

Structural and morphological evolutions of spent FCC catalyst pellets toward NaA zeolite

Spherically shaped pellets, usually discarded after use in fluidized-bed catalytic cracking units (FCCs), were converted to zeolite A-surface-enriched pellets. The final pellets were obtained through a two-step treatment consisting of: (1) alkaline thermal activation followed by (2) a hydrothermal crystallization in selected reaction conditions. The alkaline thermal activation provided pellets mainly constituted by silico-aluminate compounds. When the pellets are heated at 800 °C in contact with sodium carbonate, a structural rearrangement occurs which includes nepheline, crystalline aluminosilicate, and an amorphous fraction expected to be silicon-enriched, preserving the pellet original geometry. Afterward, reaction mixtures were prepared by adding sodium hydroxide solution to the heat-treated product. Then, commercial sodium aluminate was added. During the hydrothermal synthesis at 85 ± 3 °C, zeolite A was formed from the calcined product and NaAlSiO4 turned out to be an intermediary crystalline compound. The LTA crystals were already observed for a reaction time of 0.5 h, but the highest conversion to pure zeolite A was reached after 6 h. Those final round solid pellets showed an external surface fully covered by well anchored zeolite A cubic crystals about 1–2 μm in size. Such materials may be very useful in ion exchange, molecular sieving, and adsorption processes. The crystalline ordering was followed by XRD, SEM, NMR, and water adsorption.

Synthesis of Zeolite Nanomolecular Sieves of Different Si/Al Ratios

Nanosized zeolite molecular sieves of different Si/Al ratios have been prepared using microwave hydrothermal reactor (MHR) for their greater application in separation and catalytic science. The as‐synthesized molecular sieves belong to four different type zeolite families: MFI (infinite and high silica), FAU (moderate silica), LTA (low silica and high alumina), and AFI (alumina rich and silica‐free). The phase purity of molecular sieves has been assessed by X‐ray diffraction (XRD) analysis and morphological evaluation done by electron microscopy. Broad XRD peaks reveal that each zeolite molecular sieve sample is composed of nanocrystallites. Scanning electron microscopic images feature the notion that the incorporation of aluminum to MFI zeolite synthesis results in morphological change. The crystals of pure silica MFI zeolite (silicalite‐1) have hexagon lump/disk‐like shape, whereas MFI zeolite particles with Si/Al molar ratios 250 and 100 have distorted hexagonal lump/disk and pseudo spherical shapes, respectively. Furthermore, phase pure zeolite nanocrystals of octahedron (FAU), cubic (LTA), and rod (AFI) shape have been synthesized. The average sizes of MFI, FAU, LTA, and AFI zeolite crystals are 250, 150, 50, and 3000 nm, respectively. Although the length of AFI zeolite rods is in micron scale, the thickness and width are of a few nanometers.

Atomic Force Microscopy and High Resolution Scanning Electron Microscopy Investigation of Zeolite A Crystal Growth. Part 2: In Presence of Organic Additives

The nanoscopic details of the crystal growth of zeolite. A in the presence of the organic modifiers. diethanolmaine (DEA) and triethanolamine (TEA) has been determined using a combination of atomic force microscopy (AFM) and high-resolution scanning electron microscopy (HRSEM) coupled with Monte Carlo simulations. Crystallization of zeolite A in the presence of TEA was faster than when the growing solution contained DEA. In addition, the morphology of the final zeolite A crystals depended on the type of organic molecule, with TEA producing crystals bound only by {100} facets and DEA leading to the formation of relatively large {110} faces. These features can be explained in terms of the relative Si/Al in the growing medium and its control due to the different affinity of the organic molecules to Al. In addition, synthesis Si/Al in the growing medium and its contorl due to the different affinity of the organic molecules to Al. In additon synthesis performed at 90 degrees C showed the apperance of {211} facets Careful review of the HRSEM and AFM images, in addition to comparion with the MC simulations, reveals that these are in fact pseudofacets products of the slow dissolution of the metastable zeolite. A crsytals. This proves that the final habit of the LTA crystals can be governed by very small changes in saturation of the growing medium, and control of this parameter can prove advantageous when designing crystals for industrial applications.

Hydrothermal synthesis of LTA-encapsulated metal clusters and consequences for catalyst stability, reactivity, and selectivity

Noble metal clusters (Pt, Pd, Rh, Ir, Re, and Ag) are selectively encapsulated within LTA voids via hydrothermal synthesis using metal precursors with ligands (NH3 for Pt and Ir; ethylenediamine for Pd, Rh, Re and Ag) that prevent their premature precipitation as colloidal oxyhydroxides. Such stability appears to be necessary and sufficient for successful encapsulation of cationic precursors during nucleation and growth of zeolite frameworks. Mean cluster diameters measured by titration of exposed metal atoms (H2 on Pt, Pd, Rh, Ir and Re; O2 on Ag; 1.1–1.8nm) and by transmission electron microscopy (1.2–1.9nm) were similar, indicating that cluster surfaces were clean and accessible to molecules used as titrants or reactants. Metal clusters were narrowly distributed in size and stable against sintering and coalescence during oxidative thermal treatments (573–873K). Encapsulation selectivities were measured from turnover rates for reactions of small and large reactants, specifically hydrogenation of alkenes (ethene and isobutene) and oxidation of alkanols (methanol, ethanol, and isobutanol), which reflect the restricted access to encapsulated clusters by the larger molecules. These encapsulation selectivities, which reflect the ratio of metal surface areas within and outside LTA crystals ranged from 7.5 to 83 for all samples. Confinement within LTA crystals protects clusters from contact with thiophene and allows ethene hydrogenation to proceed at thiophene concentrations that fully suppressed reactivity for metal clusters dispersed on mesoporous SiO2. These protocols provide a general strategy for encapsulating clusters within small-pore zeolite voids, for which post-synthesis exchange is infeasible. Their successful encapsulation protects such clusters from coalescence and growth and allows them to select reactants and reject poisons based on their molecular size.

Growth morphologies and optical properties of LTA single crystal

Atomic force microscopy (AFM) has been used to study the growth morphologies of l-threonine acetate (abbreviated as LTA) crystal. Spiral growth hillocks and typical step patterns are described and discussed. Nuclei with various shapes often distribute at the larger step terraces. Eventually, in order to investigate microscopic second order nonlinear optical properties of LTA crystals, the molecular dipole moment (μ), polarizability (α), and first hyperpolarizability (β) were computed using a series of basis sets including polarized and diffuse functions at the framework of Hartree–Fock and density functional theory methods. The study is helpful to the further development of l-threonine analogs with improved nonlinear optical properties.

Solvothermal deposition and characterization of magnesium hydroxide nanostructures on zeolite crystals

A methodology to engineer roughened inorganic nanostructures on zeolites by deposition and growth of metal hydroxides is reported. Pure-silica (PS) MFI and (aluminosilicate) LTA zeolite crystals are treated by solvothermal methods to deposit inorganic hydroxide (e.g., Mg(OH) 2) coatings with nanoscale roughened features on the crystal surfaces in a controlled manner. Detailed characterization of the surface-modified MFI crystals by nitrogen physisorption allows the quantification of the surface roughness of the Mg(OH) 2 nanostructures. Pore volume characterization shows that the nanostructure deposition has only a marginal effect on the porosity of the MFI. The surface-roughened zeolites are used to fabricate zeolite/polymer mixed matrix films with considerably improved interfacial adhesion properties. Solvothermal deposition of inorganic nanostructures is also demonstrated on zeolite LTA crystals, which present aluminosilicate surfaces. In this case, the intracrystalline sodium ions in LTA are partially substituted with magnesium ions from the reagent solution during treatment. The solvothermal treatment is thus modified to deposit smaller Mg(OH) 2 nanostructures, resulting in more roughened zeolite LTA surfaces. Our detailed characterization reveals that the surface-treated LTA crystals may be promising materials for applications in mixed matrix membranes.

Unidirectional growth, structural, optical and mechanical properties of LTA crystal

Nonlinear optical amino acid single crystal of l-tartaric acid was successfully grown by unidirectional Sankaranarayanan–Ramasamy method under ambient conditions for the first time. The grown single crystal was subjected to different characterization analyses in order to find out its suitability for device fabrication. The crystal system and lattice parameters were determined from the powder X-ray diffraction analysis. The crystalline perfection was evaluated using high-resolution X-ray diffractometry. It is evident from the optical absorption study that crystal has excellent transmission in the entire visible region with its lower cut off wavelength around 220 nm. The mechanical properties of the grown crystals were studied using Vickers microhardness tester.

Ordered Mesoporous Aluminosilicates Assembled from Dissolved LTA Precursors

Aluminosilicate sols containing Si–O–Al bonds were obtained by dissolving zeolite LTA crystals in HCl solution. The solutions were used as single-source molecular precursors of silicon and aluminum. Highly ordered cage-like mesoporous aluminosilicates with Si/Al ratios ranging from 13 to 35 were synthesized using PEO106PPO70PEO106 triblock copolymer aqueous solution and LTA dissolved in HCl solution under mild acidic conditions.

Why is it so extremely difficult to prepare shape-selective Al-rich zeolite membranes like LTA and FAU for gas separation?

Two possible reasons for the difficulty to prepare shape-selective Al-rich zeolite membranes for gas separation are discussed. One reason is the strongly negative electric surface charge of Al-rich zeolites like LTA or FAU in aqueous media as found by zeta potential (ZP) measurements of suspended zeolite powder. This negative surface charge is expected to prevent the negatively charged silica species to enter the slits between the growing crystallites in a zeolite membrane layer. This effect can be avoided by adsorption of positively charged species called intergrowth supporting substance (ISS) neutralizing the electric surface charges of the growing zeolite membrane surface. Another problem for Al-rich membranes are extreme mismatches in the expansion coefficients between the zeolite layer and the ceramic or metal support upon drying the as synthesized membranes. The as synthesized FAU or LTA membrane layers contain water. We have simulated a membrane activation in the permeation cell by drying zeolite powder and recording in situ the XRD spectra. We found huge expansion and shrinking effects during the in situ de-watering of LTA and FAU powder samples. This observation is correlated with the experimental finding continuous damage of model membranes upon repeated re-activation cycles in vacuum at 150 °C. The model membranes consisted of a few large LTA and FAU crystals embedded into epoxy resin. After each de-watering step, (i) the fluxes of CO 2, CH 4 and N 2 increased and (ii) the ideal selectivities decreased which indicates a continuous membrane damage. However, these semi-damaged LTA and FAU membranes can still separate water from alcohols but do not show any gas separation.

Synthesis of Linde type A zeolite–goethite nanocomposite as an adsorbent for cationic and anionic pollutants

Linde type A zeolite (LTA)–goethite nanocomposite was synthesized by adding sodium orthosilicate solution to goethite, followed by addition of sodium aluminate and NaOH solutions at 100 °C. Optimum condition at the Si addition step required for nanocomposite formation was pH 10.0 and Si/Fe = 2.7. The final product composed mainly of LTA and goethite crystals. Formation of LTA–goethite nanocomposites in the final product was suggested by differences in IR spectra and SEM images between the final product and a mixture of LTA and goethite. The mixture separated into LTA and goethite components after washing with water, but the final product did not show such separation. Precipitation of silica on the surface of goethite and subsequent formation of Si–O–Fe bonds at the Si addition step contributed to formation of the LTA–goethite nanocomposite. The amount of adsorption of phosphate on the final product was more than 1.6 times the amount adsorbed on the mixture, indicating generation of synergistic effect in the LTA–goethite nanocomposite.

Hydrophilic and antimicrobial low-silica-zeolite LTA and high-silica-zeolite MFI hybrid coatings on aluminum alloys

A hydrophilic and antimicrobial two-layer zeolite composite coating was formed on aluminum alloys. The base layer is a high-silica-zeolite (HSZ) MFI coating formed directly on the aluminum alloy by in situ crystallization. The top layer is a crystalline zeolite hybrid coating of low-silica-zeolite (LSZ) LTA (zeolite A, or ZA) crystals imbedded within a HSZ-MFI matrix and is formed by a seeded growth method. The two-layer composite coatings are demonstrated by using ASTM D 3359B-02 method to have excellent adhesion to the aluminum alloy substrates. The hybrid layer is hydrophilic with contact angles below 5° and when they are silver-ion exchanged the hybrid coating is highly antimicrobial.

Zeolite LTA deposition on silicon wafer

The deposition of zeolite LTA layer on silicon wafer, from homogeneous synthesis systems, trough the seeding method, has been systematically investigated and high quality, oriented zeolite films are obtained. Basically the seeding method consists in various steps. In the first step the support surface is modified in order to promote the adhesion of pre-synthesized colloidal zeolite seeds, oppositely charged. The seeded support is then hydrothermally treated in the same synthesis solution to allow the growth of zeolite seeds into a dense film. In this work it was shown that the ageing of the synthesis solution before zeolite crystallization process, sensibly affects both size, amount and formation kinetics of the zeolite LTA crystals deposited on the seeded silicon surface.

Effects of silicon sources on the formation of nanosized LTA: An in situ small angle X-ray scattering and wide angle X-ray scattering study

The formation of LTA nanocrystals from clear solutions containing different silicon sources, tetramethylammonium (TMA) silicate, tetraethylorthosilicate (TEOS), and colloidal silica (Ludox SM 30) was studied by in situ small angle X-ray scattering and wide angle X-ray scattering. The size of the precursor particles formed at the initial stage of the reaction appears to strongly dependent on the type of silicon sources employed. When TMA silicate was used, 4.5 nm sized precursor particles were formed after heating for 10 min at 110 °C, which were consumed during the crystallization and played an important role in the nucleation process. 13 nm sized particles were formed from the beginning of reaction when TEOS was used as a silicon source. On the other hand, 17 nm sized particles were observed from the synthesis solution containing Ludox SM 30. The 13 and 17 nm sized particles formed in the synthesis solutions with TEOS and Ludox SM 30, respectively, may not be the precursor particles in the crystallization, but the reservoir of silicon source. 29Si NMR spectra suggest the formation of the D4R in the initial stage of crystallization depends on the silicon source, and similarly 27Al NMR spectra indicate that, as the crystallization proceeds, the distribution of Al sites changes progressively from small aluminosilicate species to cage-like species. The synthesized crystals were characterized by X-ray diffraction and scanning electron microscopy. The morphology of the LTA crystals synthesized from the three different synthesis solutions was found to be different depending on the silicon sources.