NaA Crystals Research Articles

Synthesis of kaolin-based zeolite NaA and its crystallization mechanism

The zeolite NaA was successfully synthesized by hydrothermal method using kaolin as a raw material. In order to study the crystallization mechanism, the products and intermediates of different crystallization stages were systematically characterized using x-ray diffraction (XRD), Fourier transform infrared (FTIR), and scanning electron microscopy (SEM)-energy-dispersive spectrometry (EDS). The results show that the metakaolin is completely dissolved during the first hour of the crystallization process, forming a rich crystal nucleus. As the crystallization time increases, the nucleus grows rapidly to form zeolite NaA crystals. When the crystallization time reaches 3 h, the percentage crystallinity of zeolite NaA reaches a maximum. Therefore, it can be concluded that the optimum crystallization time of zeolite NaA is 3 h. Meanwhile, we can come to the conclusion that the synthesis of zeolite NaA from kaolin is a liquid phase transition mechanism. The microstructure of zeolite NaA synthesized at 3 h of crystallization was further investigated by transmission electron microscopy (TEM) technique. The SAED patterns indicate the polycrystallinity of the zeolite NaA structure. The electron diffraction pattern indicates that the crystal structure type of the zeolite NaA belongs to the body center cube (BCC). The textural properties of the zeolite NaA synthesized at 3 h of crystallization were studied by nitrogen adsorption technique. It can be concluded that the BET surface area of the zeolite NaA obtained by the BET method is 21.106 m2·g−1. Meanwhile, from the BJH method, it is found that the microporous surface area and micropore volume of the zeolite NaA are 13.929 m2·g−1 and 0.052 cm3·g−1, respectively.

Single‐step process without organic template for the formation of zeolite A from RHA

AbstractDice‐shaped zeolite A (NaA) was prepared via direct dissolution of rice husk ash (96.5% SiO2) in the presence of NaOH and sodium aluminate solution by autoclave process at 90°C/6 h without using any templating agent. The prepared particles were characterized by DTA/TG, XRD, FTIR, N2 adsorption‐desorption physisorption analysis, FESEM, and TEM. XRD results confirmed crystallization of pure NaA zeolite phase. FTIR study shows the characteristic bands at 554 cm−1 for double 4 membered ring (D4R) of NaA zeolite. The total BET surface area of the product was found to be 31.6 m2 g−1. FESEM and TEM images show dice shaped NaA particles of size around 1 μm which is formed via oriented crystalline aggregation of primary particles (30‐40 nm). A tentative mechanism was proposed for the formation of NaA crystals through direct dissolution of rice husk ash. The synthesized NaA zeolite could be used for purification of alcohol and separation of toxic and radioactive ions from waste water via a cost‐effective process.

Binderless zeolite NaX microspheres with enhanced CO2 adsorption selectivity

Abstract Zeolite NaX@NaA core-shell microspheres were prepared via a post-treatment secondary growth of zeolite NaA films on outer surface of binderless zeolite NaX microspheres. The obtained core-shell microspheres were composed of intergrown octahedral NaX particles inside, with particles size of ca. 500–750 nm, and continuous zeolite NaA films on the outer surface with the thickness of about 2 μm. Higher CO2 separation performance was observed for the core-shell microspheres comparing to the parental binderless zeolite NaX microspheres. The ideal separation factors of zeolite NaX@NaA core-shell microspheres for CO2/CH4 and CO2/N2 were 13 and 47, and the adsorption selectivities for the corresponding binary mixtures were 308 and 923, significantly higher than the binderless zeolite NaX microspheres of 9 and 19 as well as 264 and 735, respectively. After K+ ion exchanging, the core-shell zeolite microspheres have even higher adsorption selectivities of 326 and 1109 for CO2/CH4 and CO2/N2 binary mixtures. The crushing strength of the binderless zeolite NaX microspheres was increased from 0.46 MPa to 3.42 MPa after the secondary growth. In addition, the growth of zeolite A film was resultant from interzeolite conversion and the interzeolite conversion was investigated by the conversion of zeolite NaX to NaA crystals in NaA membrane synthesis gel.

Accelerated synthesis of zeolites via radicalized seeds

By means of milling and heating, the zeolite crystals can be radicalized forming surface non-bridging oxygen hole center (NBOHC, ≡Si–O·). The corresponding g values of 2.0043 and 2.0048 unambiguously confirm the existence of the ≡Si–O· radicals in the milled and heated crystals via electron paramagnetic resonance. The active ≡Si–O· radicals can react with water to form hydroxyl free radicals to accelerate the crystallization of zeolite along with seeds. Radicalized crystals are used as seeds that show obvious accelerating effect in the crystallization of Na-A and nanosized silicalite-1 than the non-radicalized seeds. At the same crystallization time, solid products with much better crystallinity and more yields could be obtained using radicalized seeds compared with non-radicalized seeds. A new seeding-synthesis-related strategy is developed to accelerate the crystallization of zeolites.

Understanding the effect of zeolite crystal expansion/contraction on separation performance of NaA zeolite membrane: A combined experimental and molecular simulation study

Experimental measurements and molecular simulations were used to investigate the effect of adsorption-induced changes in zeolite crystal size on the separation performance of NaA zeolite membrane for dehydration of alcohols. The vapor permeation (VP) separations of water/IPA showed a dramatic decrease in selectivity due to increase of IPA flux as the feed water concentration decreased. However, in the case of water/methanol and water/ethanol mixtures, the alcohol fluxes were almost independent of the feed water concentration. Permporosimetry measurements as well as molecular simulations show that at low loading of water, NaA crystals contract slightly, while they expand at higher loadings, which significantly affects the intercrystalline defects. In addition, at low water loadings intercrystalline defects are not blocked by water adsorbed at the defect sites. Both methanol and ethanol can enter the zeolite to reduce the crystal contraction. However, isopropanol cannot enter the NaA crystal and is thus unable to mitigate the effects of low water loadings. Based on this knowledge, the presence of methanol or ethanol in the water/isopropanol mixtures with low water content was expected to improve the dehydration performance of NaA zeolite membrane. This result was also observed for the dehydration of water/other large molecular mixture. Our studies here provide an improved understanding of the permeation and separations for NaA zeolite membrane.

Mechanism on solvent-free crystallization of NaA zeolite

It is of great importance to deeply understand the crystallization mechanism of zeolite in both fundamental research and industrial applications. Here we have systemically investigated the crystallization of zeolite NaA using hydrated silica (SiO2·3H2O) and sodium aluminate solids in the absence of water solvent, which is well characterized by X-ray diffraction (XRD), scanning electron micrography (SEM), UV-Raman, and 27Al MQMAS and 29Si MAS NMR techniques. The XRD patterns show that the complete crystallization requires for 5 h at 80 °C. The SEM images suggest that the nucleation mainly occurs on the surface of solid particles. The UV-Raman spectra exhibit that a large amount of the zeolite building units of four-membered rings (4R) formed in the starting solid particles, which even occurs before the crystallization, while crystal growth is always along with the formation of zeolite building units of six-membered rings (6R). The 27Al 2DMQMAS NMR spectra show that the D4R species are dominant in the precursors for crystallization of NaA. According to these results, we believe that the rearrangement of zeolite building units such as aluminosilicate D4R plays a critical role for the crystallization of zeolite NaA in the absence of water solvent.

A continuous flow reactor setup as a tool for rapid synthesis of micron sized NaA zeolite

Slow crystallization kinetics and a limited thermodynamical stability of the target crystal phase are characteristic to zeolite formation, representing some of the key obstructions for fast zeolite synthesis. In this paper, the possibility of accelerating NaA zeolite synthesis in a continuous flow reactor (CFR) is studied. The CFR reduces the thermal lag by increasing surface to volume ratio, expediting heat transfer and mass transfer. The properties of the CFR and the reference batch synthesized particles were similar as confirmed by X-ray diffraction, scanning electron microscopy, particle size measurement using laser scattering and water adsorption equilibria. The reduced residence time and reduction in thermal lag provided an ideal synthesis environment for NaA zeolite, without side products, yielding 160 g/h per liter reactor volume of dry NaA crystals synthesized in 16 min and 2–3 μm particles for a single CFR. In comparison, the batch process produces 33 g/h per liter reactor volume The effects of diluting with NaOH-solution and temperature were studied in the CFR, allowing to determine the optimal conditions. With the enhanced reaction kinetics gained from the increased temperature and molar composition, NaA synthesis is performed 10 times faster than in the optimal batch synthesis. The optimal conditions for the synthesis of NaA in the CFR were determined as: a gel composition of Na2O:4.75 – SiO2:1.93 – Al2O3:1.0 – H2O:192, at a synthesis temperature of 150 °C during 16 min without, aging of the gel mixture. This paper shows that the bottlenecks in NaA zeolite synthesis can be widened resulting in faster synthesis in a CFR, making it a feasible pathway for more controllable zeolite synthesis at higher mass production rates (160 g/(h l-reactor)) while reducing the risk of blockage in a continuous flow reactor.

A novel processing technique for CNTs growth on Co-supported molecular sieve coated porous ceramics

Carbon Nanotubes (CNTs) on cobalt ions supported zeolites coated porous ceramics were grown by a novel process merging three individual synthesis techniques. The direct foaming technique was used to prepare the porous alumina-silica ceramics having an average pore size of ~10µm. Zeolite NaA crystals (~18nm) were synthesized and coated simultaneously on the porous ceramics by an in situ hydrothermal reaction followed by a simple ion exchange process. CNTs were synthesized on Co-supported zeolite-ceramics by catalytic chemical vapor deposition (CCVD) process for 60min. The effect of different reaction temperatures on the structural morphology and yield of CNTs were investigated.

Fabrication of zeolite NaA membranes on hollow fibers using nano-sized seeds exfoliated from mesoporous zeolite crystals

Micro-sized crystals of mesoporous zeolite NaA (meso-NaA) are prepared and their mechanical strength is compared with conventional NaA crystals. Crystals are characterized by nitrogen adsorption, X-ray diffraction, scanning electronic microscopy and dynamic light scattering. It is found that micro-sized meso-NaA crystals composed by numerous domains of interconnected nanocrystals can be easily smashed by hand-grinding, showing lower mechanical strength than conventional crystals. The suspension of zeolite NaA nanocrystals (∼110 nm) can be easily obtained after removing micro-sized crystals by centrifuging the suspension of the grinded crystals at 1000 rpm and can be used to seed hollow fiber supports. Zeolite membranes with high separation performance are obtained on the seeded hollow fiber supports by seeded growth. This method may open a new way to prepare zeolite nanocrystals for many applications.

Preparation of amphiphilic nano-sized NaA/glass films and powders using layer-by-layer in-situ sol–gel method

Nano-sized NaA/glass films were prepared by a layer-by-layer in-situ sol–gel method combined with the steam-assisted conversion technique. Consequently, nano-sized zeolite NaA powders were obtained by scraping the films and treating the resultant powders with a 0.6% acetic acid solution. The films and the powders were characterized by XRD, FT-IR, SEM, TEM, and AFM, and their hydrophilicity and antimicrobial activity after Ag+-exchange were examined. The preparation conditions such as the reaction time and the amount of water prefilled in the autoclave were examined. The NaA crystals on the films are about 50–100 nm in size and they are aggregated together. Their particle sizes can be further reduced to 40–80 nm and they can be changed from aggregated to highly dispersed particles after treated in the acid solution. The amount of water during the synthesis is quite important for successful synthesis of well-crystallized nano-sized zeolite NaA on the glass. And dispersed 40–50 nm zeolite NaA particles can be obtained when suitable amount of water is used. The nano-sized NaA/glass film exhibits superhydrophilicity and oleophilicity. After ion-exchange with Ag+, the NaA/glass film shows high antimicrobial activity, better than the nano-sized NaA powders and other NaA films or coatings reported in literature.

High performance zeolite NaA membranes synthesized on the inner surface of zeolite/PES–PI blend composite hollow fibers

Zeolite/polymer blend composite hollow fibers are prepared as a new type of zeolite membrane supports by the dry-wet phase inversion method. The spinning slurry is formed by mixing zeolite NaA crystals with miscible polyethersulfone–polyimide polymer blend. Zeolite NaA membranes are hydrothermally synthesized in the lumen side of the hollow fibers under dynamic conditions. Their properties are characterized by scanning electron microscopy, mechanical test, contact angle test and pervaporation. The effects of the N2 extrusion pressure and polyimide content on properties of the supports are investigated. Both of them limit the exchange rate between solvent and non-solvent during spinning process resulting in the supports with high mechanical properties. Zeolite membranes are thin and well intergrown with no macroscopic defects. The water flux is 10.6±0.4kgm−2h−1 with a separation factor of more than 10,000 in dehydration of a 90wt% ethanol aqueous solution at 75°C. The effect of hydrophobicity of the composite supports is discussed in the context of advantages of the supports.

Seed-free synthesis of highly permeable zeolite NaA membranes through deposition of APTES-functionalized alumina particles on macroporous supports

A new strategy through deposition of 3-aminopropyltriethoxysilane (APTES) functionalized Al2O3 particles on a coarse macrosporous tube with big defect holes was reported for seed-free synthesis of zeolite NaA membranes. Prior to the deposition of Al2O3 particles onto the macroporous support, APTES was covalently functionalized onto the surface of Al2O3 particles, aiming at providing heterogeneous sites for the growth of NaA zeolites on the support surface by serving as linker between the support and the silica and aluminum sources. Large APTES-Al2O3 particles were firstly deposited on the support surface to reduce the surface roughness and the effective pore sizes, subsequently the small APTES-Al2O3 particles were deposited on the support to provide plenty of heterogeneous nucleation sites for the growth of zeolite NaA crystals. In this way, a thin and continuous APTES-Al2O3 layer was formed on the support. After one single hydrothermal synthesis, a thin and compact zeolite NaA membrane with thickness of 2μm was achieved. The synthesized NaA membranes showed high fluxes of 3.7–3.9kgm−2h−1 with separation factors over 5000 in dehydrating the 90wt% ethanol/10wt% water mixtures at 75°C. The strategy presented in this study was expected to extend for preparation of other types of thin zeolite membranes on such coarse symmetric macroporous supports.

Studies on room-temperature synthesis of zeolite NaA

Zeolites NaA were prepared by hydrothermal synthesis under room-temperature conditions. The effects of crystallization time, Na2O/SiO2, H2O/SiO2, silicon and aluminum from different sources on the end products were studied. The results indicated that although 7 days product was fully crystalline according to the XRD investigation, further prolonging the crystallization time more, the crystals transferred from spherical to cubic in shape. The products synthesized at room temperature showed much smaller crystal sizes than those synthesized at high temperature, with the mean particle sizes of about 346 and 1374nm, respectively. Mild alkalinity circumstance was favorable for the crystallization of zeolite NaA. However, higher Na2O/SiO2 molar ratio resulted in the formation of sodalite. Furthermore, the silicon and aluminum sources determined whether or not zeolites NaA were formed. Well-developed zeolite NaA crystals were obtained with water glass, fumed silica as silicon sources and aluminum sulphate, aluminum isopropoxide, aluminum powder as aluminum sources.

Synthesis of zeolite NaA at room temperature: The effect of synthesis parameters on crystal size and its size distribution

Zeolite NaA crystals were prepared by hydrothermal synthesis under room-temperature conditions. The products were characterized by XRD, SEM, IR and particle size analysis. Some influence parameters such as crystallization time, aging time, stirring speed, different sources of silicon and aluminum on the crystalline end products were studied. The results showed that crystallization time was a crucial factor for the final products, well-shaped crystals could be obtained at the crystallization time of 72h. While further prolonging the crystallization time more, crystals continued to grow, along with the changes of crystal size distributions. The crystals obtained with the aging time of 1, 1.5, 2 and 3h show the mean particle sizes of 368, 356, 338 and 314nm, with the crystal size distribution ranges of 82–435, 70–441, 54–450 and 40–476nm, respectively. Longer aging time leads to the mean particle size of crystal decrease. Whilst, the stirring speed affects the particle size distribution only slightly. Moreover, the aluminum source has much more obviously influence on the crystal phase of final product than the silicon source does in this system.

SEM–EDX and XRD characterization of zeolite NaA, synthesized from rice husk and aluminium scrap by different procedures for preparation of the initial hydrogel

A number of investigations have demonstrated that zeolite NaA could be synthesized using Si, extracted from rice husk ash; however, experiments on direct extraction of Si from rice husk (RH) are scarce. The main objective of the present study was to explore the possibility to synthesize high-quality zeolite NaA from RH and waste aluminium cans (as a source of Al), applying different procedures for the preparation of initial hydrogel and a unified procedure for crystallization of zeolite NaA. Products were characterized by SEM–EDX and XRD analyses. The investigation demonstrated that Si could be extracted directly from RH, avoiding the process of RH burning. Practically complete dissolution of Si from RH was achieved by alkali treatment (with 10 % NaOH for 7 h) at boiling temperature and atmospheric pressure, i.e. using refluxing system instead of autoclave for the preparation of Si-gel. Zeolite NaA samples synthesized from such Si-gels were pure, highly crystalline and white. Furthermore, it was found that the direct dissolution of Al in Si-gel did not affect the quality of the final product. Although this investigation was not focused on the mechanism of zeolite NaA crystallization, the results obtained indicated clearly that the history of Si-gel preparation played an important role in the nucleation and growth of zeolite NaA crystals and influenced their yield, size, and shape. Therefore, the optimization of Si-gel preparation procedure has to be considered as essential not only for the economy of the synthesis of NaA from RH, but also for the quality of the final product.

Influence of zeolite crystal expansion/contraction on NaA zeolite membrane separations

In situ powder XRD measurements showed that adsorption causes the NaA zeolite unit cell to contract or expand, and these changes depend on adsorbate loading. Changes in zeolite crystal size correlated with permeation changes through NaA zeolite membranes. These membranes had high water/alcohol pervaporation selectivities, even though gas permeation was mainly through defects, as indicated by Knudsen selectivities for single gas permeation. At 300 K and a thermodynamic activity of 0.03, water contracted the NaA crystals by 0.22 vol%, and this contraction increased the helium flux through two NaA membranes by approximately 80%. Crystal contraction also increased the fluxes of i-butane during vapor permeation and i-propanol (IPA) during pervaporation (∼0.03 wt% water). At activities above 0.07, water expanded NaA crystals and correspondingly decreased the fluxes of helium, i-butane, and IPA through NaA zeolite membranes. Methanol also contracted NaA crystals, but by only 0.05 vol% at an activity of 0.02, and this contraction slightly increased the helium and i-butane fluxes through a NaA membrane. Above an activity of 0.06, methanol expanded the NaA crystals, and the fluxes of helium and i-butane through a NaA membrane decreased. These adsorbate-induced changes explain some pervaporation behavior reported by others, and they indicate that crystal expansion and contraction may increase or decrease zeolite NaA membrane selectivity by changing the defect sizes.

Mesoporous aluminosilicates assembled from dissolved LTA zeolite and triblock copolymer in the presence of tetramethylammonium hydroxide

Zeolite Na-A crystals dissolved in a HCl solution were used as a single-source of silicon and aluminum for the synthesis of mesoporous aluminosilicates via a template-assisted method with an organic base tetramethylammonium hydroxide (TMAOH). Amphiphilic triblock copolymer Pluronic F127 (EO 106PO 70EO 106) was used as template. Increasing the amount of TMAOH in the synthetic solution resulted in an increase in the aluminum content of the products. On the other hand, mesostructural periodicity was deteriorated with higher content of aluminum incorporated into the mesoporous framework. The samples with low Si/Al ratios less than 5 have wormhole-like pore structure, while the samples with Si/Al ratios more than 7 possess highly ordered mesoporous structure, a body-centered Im 3 m symmetry, with single crystal like morphology. The samples with Si/Al ratio of 7, which prepared at TMAOH molar concentration of 25 mM in the templating solution, possess BET surface area of 470 m 2/g, pore size of 6.4 nm, and pore volume of 0.56 cm 3/g. Aluminum atoms have successfully been incorporated in a tetra-coordinated position and remained stable even after calcination at 600 °C.

Hollow zeolite structures formed by crystallization in crosslinked polyacrylamide hydrogels

Hollow zeolite structures including sodalite spheres and hollow zeolite NaA crystals were synthesized by introducing crosslinked polyacrylamide (C-PAM) hydrogels into zeolite synthesis gels. The synthesis gels with weight compositions of 0.8SiO2 : 1.0Al2O3 : 21.2Na2O : 86.0H2O : 7.4–28.7 acrylamide (AM) were used to produce hollow sodalite spheres. The synthesized hollow sodalite spheres had diameters of 1–5 μm and shell thicknesses of 0.5–1 μm, and the sphere diameters decreased from 5–10 μm to 1–3 μm as the amount of C-PAM increased. Hollow zeolite A crystals with sizes of 300–500 nm were grown from the synthesis gel with a weight ratio of 0.8SiO2 : 1.0Al2O3 : 2.6Na2O : 16.4H2O : 2.6–3.8AM. The experimental results suggest that the formation of hollow zeolite structures may involve a surface-to-core crystallization process induced by crosslinked polyacrylamide networks.

Hydrogen-permeable membranes composed of zeolite nano-blocks

A novel hydrogen permeable membrane was prepared from zeolite Na-A crystals, dissolved in a HCl solution and spin-coated on a Vycor glass plate. The membrane shows high-separation factors for H 2/CH 4 (>1200), H 2/N 2 (>1600) and H 2/CO (>660) binary gas mixtures at 298 K. Such a highly selective H 2 permeation suggests that the membrane composed of zeolite nano-blocks possesses no 8-membered oxygen rings as permeable pores. The membrane is characterized by X-ray diffraction, 27Al, 29Si NMR and Al, Si K-edge X-ray absorption near edge structure spectroscopy. A dealuminization from the zeolite framework occurs during the dissolution in acid solutions, but the zeolite nano-blocks still keep the local structure of zeolite although CO-permeable 8-membered oxygen rings disappear. Alumina-like nanoparticles formed by the dealuminization must be embedded in the aggregate of zeolite nano-blocks.

Functionalized zeolite A–nafion composite membranes for direct methanol fuel cells

A series of composite membranes based on zeolite A and Nafion 117 have been fabricated for direct methanol fuel cells. The external surface of zeolite A has been modified to enhance the interface bonding between inorganic zeolite crystals and Nafion ionomer. The modified zeolite samples have been characterised using a combination of thermal analysis and spectroscopic techniques. Methanol permeability of Nafion 117 could be reduced by as much as 86% by incorporating functionalised zeolite NaA crystals into the membrane. The effect of different functionalisation level is discussed.