Zeolite NaA Crystals Research Articles

Study on the Synthesis and Structural Properties of Zeolite A-MgO Composite for Defluoridation of Water

Zeolite A-MgO composite was synthesized in presence of zeolite A particles, MgCl2 and urea solution under hydrothermal reaction at 150oC for 5 h followed by calcination at 600°C/2 h. XRD and FTIR results showed the crystallization of MgO along with NaA zeolite crystals. Nano-sheet like MgO particles (50-70 nm) were grown onto the surface of NaA zeolite crystals (1 μm). BET surface area and pore size of the sample were found to be 69 m2.g−1 and 3.9 nm, respectively. XPS analysis showed the binding energies of O1s, Si2p, Na1s, Al2p and Mg2p as 531.6, 102.4, 1072.2, 74.2 and 49.9 eV, respectively. The synthesized product (1 g.L−1 dose) showed 72% adsorption of fluoride (7.24 mg.L−1) within 5 min and reached up to 94% for 90 min at pH 6.8. The adsorption capacity of the sample was calculated as 107.64 mg.g−1 from Langmuir isotherm. The sample could be regenerated up to 5th cycle. A tentative mechanism for the adsorption of fluoride from aqueous solution by the synthesized zeolite A-MgO composite was proposed.

Meso-scale simulation on mechanism of Na+-gated water-conducting nanochannels in zeolite NaA

Na+-gated water-conducting nanochannels was created in previous work by assembling NaA zeolite crystals into a specialized membrane for sieving water from small gas molecules such as H2 and CO2. The quantum mechanics calculation provides only the reaction or adsorption information which are insufficient to understand the mechanism for the entire separation process. Herein, in addition to the density functional theory calculations, we illustrated the molecular sieving mechanism of Na+-gated water-conducting nanochannels in zeolite NaA by molecular simulations (grand canonical Monte Carlo and molecular dynamics simulations). The result shows that Na+ in water channel plays an important role in rejecting other gas molecules (H2 and CO2), and water molecule can pass through the channel easily due to its strong adsorption with zeolite NaA and low energy barrier for penetration. Moreover, simulation result shows that the diffusion path of H2O molecule in the zeolite NaA is not directly passing through the α-cages, but jumping between adjacent 8-oxygen rings. The proposed mechanism of water-conducting nanochannel advises the application of porous materials in water-related reactions via modulation of microenvironment.

Gamma-Ray Irradiation to Accelerate Crystallization of Mesoporous Zeolites.

Gamma-ray (γ-ray) irradiation was introduced into zeolite synthesis. The crystallization process of zeolite NaA, NaY, Silicalite-1, and ZSM-5 were greatly accelerated. The crystallization time of NaA zeolite was significantly decreased to 18 h under γ-ray irradiation at 20 °C, while more than 102 h was needed for the conventional process. Unexpectedly, more mesopores were created during this process, and thus the adsorption capacity of CO2 increased by 6-fold compared to the NaA prepared without γ-ray irradiation. Solid experimental evidence and density function theory (DFT) calculations demonstrated that hydroxyl free radicals (OH*) generated by γ-rays accelerated the crystallization of zeolite NaA. Besides NaA, mesoporous ZSM-5 with MFI topology was also successfully synthesized under γ-ray irradiation, which possessed excellent catalytic performance for methanol conversion, suggesting the universality of this new synthetic strategy for various zeolites.

Gamma‐Ray Irradiation to Accelerate Crystallization of Mesoporous Zeolites

AbstractGamma‐ray (γ‐ray) irradiation was introduced into zeolite synthesis. The crystallization process of zeolite NaA, NaY, Silicalite‐1, and ZSM‐5 were greatly accelerated. The crystallization time of NaA zeolite was significantly decreased to 18 h under γ‐ray irradiation at 20 °C, while more than 102 h was needed for the conventional process. Unexpectedly, more mesopores were created during this process, and thus the adsorption capacity of CO2 increased by 6‐fold compared to the NaA prepared without γ‐ray irradiation. Solid experimental evidence and density function theory (DFT) calculations demonstrated that hydroxyl free radicals (OH*) generated by γ‐rays accelerated the crystallization of zeolite NaA. Besides NaA, mesoporous ZSM‐5 with MFI topology was also successfully synthesized under γ‐ray irradiation, which possessed excellent catalytic performance for methanol conversion, suggesting the universality of this new synthetic strategy for various zeolites.

Water-selective zeolite membranes

Catalysis The yield of many gas-phase industrial reactions is limited by the formation of water as a by-product. Li et al. harnessed the water-sieving properties of NaA zeolite crystals by forming them into continuous defect-free membranes within tube reactors (see the Perspective by Carreon). These membranes can let water pass but reject gases such as hydrogen, carbon monoxide, and carbon dioxide (CO2). When these membranes were used in CO2 hydrogenation to form methanol with water as a by-product, substantial increases were observed in both the CO2 conversion and methanol yield. Science , this issue p. [667][1]; see also p. [624][2] [1]: /lookup/doi/10.1126/science.aaz6053 [2]: /lookup/doi/10.1126/science.aba4997

Na+-gated water-conducting nanochannels for boosting CO2 conversion to liquid fuels.

Robust, gas-impeding water-conduction nanochannels that can sieve water from small gas molecules such as hydrogen (H2), particularly at high temperature and pressure, are desirable for boosting many important reactions severely restricted by water (the major by-product) both thermodynamically and kinetically. Identifying and constructing such nanochannels into large-area separation membranes without introducing extra defects is challenging. We found that sodium ion (Na+)-gated water-conduction nanochannels could be created by assembling NaA zeolite crystals into a continuous, defect-free separation membrane through a rationally designed method. Highly efficient in situ water removal through water-conduction nanochannels led to a substantial increase in carbon dioxide (CO2) conversion and methanol yield in CO2 hydrogenation for methanol production.

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.

Preparation of effective Ag-loaded zeolite antibacterial materials by solid phase ionic exchange method

Ag-loaded antibacterial zeolite materials were prepared by a solid phase ionic exchange approach with small NaA zeolite crystals synthesized by sol–gel method as carrier, NH4NO3 and AgNO3 as raw materials. The influence of AgNO3 amount, reaction temperature, and reaction time on silver load in the zeolite was investigated. The phase composition and morphology of the Ag-loaded zeolite were also characterized. In addition, the antibacterial activity, sustained-release performance, and anti-tarnish property of the Ag-loaded zeolite were studied. The results show that the Ag-loaded zeolite was synthesized under optimum conditions. The amount of silver load in the zeolite was up to 23.45% (mass ratio). The minimum inhibitory concentration value (MIC value) was 9.17 ppm, and the diameter of inhibition zone for Escherichia coli was 16.3 mm. The MIC value was 12.50 ppm, and the diameter of inhibition zone for Staphylococcus aureus was 14.5 mm. Furthermore, the Ag-loaded zeolite materials demonstrate good sustained-release performance and anti-tarnish property.

Making of Nanopore NaA Zeolite Membranes for Saline Wastewater Treatment

Zeolite membranes have been studied extensively for more than fifteen years, mainly focusing on gas separation and liquid pervaporation processes. Recently, molecular dynamic simulation has shown that zeolite membranes are theoretically suitable for ion removal from aqueous solutions. This work proposes to use zeolite membranes for desalination of saline recycled wastewater for the possibility of avoiding the costly treatment needed for saline wastewater by Conventional approaches. NaA zeolite membrane indicates that it may be possible to simultaneously separate ions and dissolved organic compounds from water by pervaporation (PV) processes. NaA zeolite membranes composed of a continuous intergrowth of NaA zeolite crystals have been prepared hydro thermally on the surface of porous tubular supports. In this research, synthesis of zeolite NaA membrane was investigated. SEM and XRD analysis have been used to confirm zeolite NaA membrane formation. Membrane performance has been evaluated using an experimental pervaporation setup. Effects of operation condition (temperature, rate and pressure) on the membrane structure and performance have been investigated for NaA zeolite membranes grown onto seeded mullite supports.

Preparation of a non-hydrothermal NaA zeolite membrane and defect elimination by vacuum-inhalation repair method

This work reports the preparation of a non-hydrothermal NaA zeolite and its stainless steel-supported membrane using geopolymer-gel-thermal-conversion (GGTC) combined with a dip-coating method, which does not require hydrothermal processing. The XRD and high-resolution transmission electron microscopy (HRTEM) analyses indicate that the 1.1Na2O–Al2O3–2SiO2–8H2O geopolymer gels cured at 25°C nucleate in the geopolymerization stage. After heat-treatment above 60°C, the crystal nuclei in the geopolymer gels continue to grow and transform into NaA zeolite crystals with good crystallinity. To improve the separation efficiency of the stainless steel-supported NaA zeolite membrane in the pervaporation (PV) process of an ethanol/water mixture, this paper reports a simple vacuum-inhalation repair method for non-zeolitic pores or other defects in the NaA zeolite membrane. After vacuum-inhalation repair of the non-hydrothermal NaA zeolite membrane with sodium alginate solutions and CaCl2 solutions, in that order, the repaired NaA zeolite membrane exhibited a separation factor that was enhanced approximately 7- to 14-fold over that of the unrepaired 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 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.

Pore Size Control of Macro Porous Carbonized Material by Pyrolysis of Resin/Carbon Composite: Synthesis of Na-A Zeolite Membrane

the porous carbon solid (mean pore size 0.1μm ; porosity 32%) was prepared by pyrolysis of composite paste of phenolic resin/AC with a mass ratio (1/1) and a mean grain size of AC of 2.28 μm. The porous agent (starch and PVB) is then added to the paste at 5% and 9% respectively to maintain a pore size of 0.1 microns and maximum porosity. A layer of pure zeolite Na-A is deposited on the carbon support after an oxidation by nitric acid and a seeding of the surface with pre-synthesized Na-A zeolite crystals.

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.

Preparation and Permeation Properties of NaA Zeolite Membrane by Vapor Phase Transformation Method

The NaA zeolite membranes with high permeance were synthesized by seeding and secondary growth both using vapor phase transformation method (VPT). The SEM image of seed layer on substrate indicated that the size of seeds was about 60nm. The XRD patterns showed that the formation of NaA zeolite membrane was after vapor phase transformation of 12h, and the pure NaA zeolite crystals formed on substrate. The SEM images showed that the NaA zeolite membrane consisted of compact polycrystalline crystals with an average size of ca. 2µm. The permeation results showed that the H2 permeance s of NaA membranes attained at the level of 10-6 mol/Pa·m2·s, and the maximum permselectivities of the H2/N2, H2/CO and H2/C3H8 were 4.15,4.78 and 10.1, respectively, which were higher than those of the corresponding Knudsen diffusion selectivities. However, the permeation of C3H8 suggested that there existed unwanted intercrystalline pores or defects in the membranes.

Influence of synthesis parameters on NaA zeolite crystals

Abstract The influences of the crystallization temperature, time, and alkalinity of the reaction system and aluminum sources (aluminum chloride and aluminum sulfate) on the synthesis of NaA zeolite have been systematically investigated using the dry-gel conversion method (DGC). The phase and morphology of the synthesis products were characterized using X-ray diffraction and SEM, respectively. This study showed that the alkalinity and temperature affected the NaA zeolite crystallization process and the size distribution of the crystalline end products. Furthermore, as the reaction time increased, the metastable phase transformed into a more stable phase. It was worth mentioning that the final NaA zeolite product contained a small quantity of NaX zeolite when aluminum chloride was used as the aluminum source while using aluminum sulfate as the aluminum source, the final product was mostly sodalite, indicating that the anions in the different aluminum sources have a significant effect on the crystallization process, most likely due to their electrostatic and steric interactions with the zeolite framework. To obtain NaA zeolite crystals with uniform sizes and high crystallinity, the optimal synthesis parameters for the two aluminum sources were determined.

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.

Effect of sol–gel-based silica buffer layer on the formation of NaA zeolite membrane

The support substrates were modified with sol–gel-based silica coating as intermediate buffer layer followed by NaA zeolite seed (prepared hydrothermally at 85 °C for 3 h) coating with 2 wt% aqueous dispersion. The secondary crystallization of NaA zeolite in the silica-seed-coated supports was carried out hydrothermally through crystal growth process at 65 °C for 4 and 8 h (single-stage each), (6 + 4) and (6 + 6) h (double-stage each), and (6 + 4 + 2) and (6 + 4 + 4) h (triple-stage each). The crystallization behaviours of the prepared NaA membranes were studied by X-ray diffraction, and the microstructures of the same films were observed by field emission scanning electron microscope. The efficiency of the membranes was tested with gas permeation study. The double-stage secondary crystallization at 65 °C for (6 + 4) h showed highly interlocked NaA grains in the membrane and it rendered the permeance values of 3.7 × 10−8 mol m−2 s−1 Pa−1 and 15.9 × 10−8 for single gas, nitrogen (N2) and hydrogen (H2) respectively with the ideal selectivity of 4.3.