An in situ approach to synthesize pure phase FAU-type zeolite membranes: effect of aging and formation mechanism

FAU-type zeolite membranes were prepared on porous α-Al₂O₃ tubes by a two-stage in situ synthesis method. The synthesis was carried out in clear solutions without the aid of seeds or organic templates. The first stage synthesis was devoted to the in situ nucleation of FAU-type zeolite, aiming to induce an evenly covered zeolite layer on the support. In situ aging played an important role at this stage as it suppressed LTA phase and promoted the formation of a continuous zeolite layer. The second stage synthesis was performed in a dilute solution, where the pre-formed zeolite layer went on to grow and became compact. Scanning electron microscopy (SEM) observations show that the membranes were made up of well-shaped intergrown crystals, with the thickness around 7 μm. Pervaporation (a membrane-based separation process in which the liquid components diffuse through a membrane and are subsequently vaporized due to the low-pressure exerted on the permeate side of the membrane) measurements were carried out in a water/ethanol (10/90, w/w) mixture to evaluate the membrane performance. The structural and morphological evolutions of FAU-type zeolite membranes during synthesis were tracked down by X-ray diffraction and SEM. The formation mechanism was illuminated based on the experimental results.     

Seeded growth of beta zeolite membranes using zeolite structure-directing agent

In this work, beta zeolite membranes were prepared on seeded α-Al₂O₃ substrate tubes using zeolite structure-directing agent (ZSDA), instead of directly using organic agent as templates. This ZSDA method consists of two steps: preparation of beta ZSDA and growth of beta zeolite membranes with ZSDA by the secondary hydrothermal synthesis. The molar compositions of the synthesis solution of ZSDA and zeolite membrane are of 1SiO₂/0.012Al₂O₃/0.167(TEA)₂O/0.031Na₂O/9.4H₂O and 1SiO₂/0.03Al₂O₃/0.027(TEA)₂O/0.27Na₂O/25H₂O, respectively. Preparing the membranes only needed a little ZSDA, which was used to substitute for conventional templates in the secondary growth process of the membrane. The membranes were characterized by SEM, XRD and nitrogen permeance. It was found that the prepared membranes are continuous and defect-free. The mechanism of membrane growth with ZSDA was discussed simply.     

Crystallization of gallium analog of zeolite Nu-23/ferrierite

The synthesis of pure Ga-substituted Nu-23 zeolite in the presence of cetyltrimethylammonium bromide (CTMA-Br) as templating agent has been carried out using the gel composition 2 (CTMA)₂O:6.25 Na₂O:M₂O₃:21.75 SiO₂:800 H₂O at 453 K. The phase purity and % crystallinity of the zeolite product was estimated by XRD, sorption, i.r. and MAS n.m.r. measurements. Thermoanalytical curves (d.t.a./t.g.) indicated that the templating species were occluded in the pores of the Ga-Nu-23 zeolite phase and the same was decomposed around 973 K. The MAS n.m.r. spectra of the calcined sample indicated partial removal of Ga from the framework position. It is surprising to note that replacement of the source of Ga³⁺ by Al³⁺ in the above gel composition yields mordenite instead of Nu-23.     

Synthesis of zeolite Li-ABW from fly ash by fusion method

The zeolite Li-ABW was synthesized by fusion method using fly ash as raw material. It comprised alkaline fusion followed by hydrothermal treatment in LiOH·H₂O medium. Crystallinity of zeolite as high as 97.8% was attained under the following conditions: LiOH·H₂O concentration, 3 M; hydrothermal temperature, 180 °C; the corresponding aging time, 12 h. The content of Li-ABW increased at the expense of lithium aluminum silicate or quartz with an increase of LiOH·H₂O concentration. With increasing hydrothermal temperature and aging time, the soluble species re-crystallized and crystalline phase transformation between different zeolites was observed. Scanning electron microscopy (SEM) observation revealed that the obtained zeolite Li-ABW was a rod-like crystal. The pore size distribution curve indicated the presence of mesopores.     

Synthesis of MFI-type zeolite membranes on porous α-alumina supports by wet gel crystallization in the vapor phase

A new method has been developed for synthesis of MFI-type zeolite membranes on porous -alumina supports. Using this method, a thin layer of wet-gel precursor precoated from a synthesis sol containing SiO₂, tetrapropylammonium hydroxide (TPAOH), NaOH, and H₂O was converted to an MFI zeolite film by vapor-phase treatment at elevated temperatures. MFI zeolite films were formed on the -alumina substrates, respectively, in the vapor phases of TPAOH solution and an ethylenediamine(EDA)/triethylamine (TEA)/water mixture, but an amorphous material was obtained in the vapor phase of pure water. Scanning electron microscopy and helium permeation examinations before calcination showed that the MFI membrane obtained in the vapor phase of the TPAOH solution was of higher quality than that synthesized in the vapor phase of the EDA/TEA/water mixture in terms of zeolite film integrity and compactness. It was also found that the existence of the template TPA⁺ in the parental synthesis sol was critical to the formation of MFI zeolite under the investigated conditions. When precursor layers coated from a template-free colloidal silica system were used, the vapor-phase treatment resulted in formation of Na-P1 and ANA zeolite films, respectively, in the vapors of TPAOH solution and the EDA/TEA/water mixture. The method developed in this work has the advantages of improved controllability, minimal waste generation, and reduced chemical consumption that are desirable for large-scale production of zeolite membranes.     

Preparation of zeolite-coated cordierite honeycombs prepared by an in situ crystallization method

Zeolite thin films were prepared on cordierite (Mg₂Al₄Si₅O₁₈) honeycomb substrates by in situ crystallization using soft hydrothermal conditions. The synthesized zeolite films (zeolite A and ZSM-5) were characterized by XRD, FTIR, SEM, NH₃-TPD, and Ns₂ and propane gas adsorption. Zeolite-A films were prepared by dip-coating on a cordierite substrate in a precursor of molar composition of Na₂O:Al₂O_3: SiO_2: H₂O=4:1:2:100 and heating in an autoclave at 80 °C for 6–10 h. The resultant zeolite-A films consisted of cubic crystals about 2–3 μm in size, achieving a thickness of 20 μm after re-coating. ZSM-5 films were similarly formed using a microporous silica precursor obtained by selective leaching of metakaolinite with tetrapropylammonium hydroxide (TPAOH) as the templating agent. The molar composition of the precursor was NaOH:microporous silica:TPAOH:H₂O=1:10:1:200. The dipped substrate covered with the wet precursor gel was heated in an autoclave at 150 °C for 24 h. The resultant films were composed of short prismatic <1 μm crystals achieving thickness of several to 10 μm after re-coating. The microstructure and porous properties of the ZSM-5 films were found to change according to the chemical composition and surface treatment of the cordierite substrates. The presence of a SiO₂-rich interfacial layer between the substrate and zeolite film increased the amount of zeolite formed and the physical adsorption but decreased the solid acidity and amount of chemisorption.     

Preparation of zeolite-coated cordierite honeycombs prepared by an in situ crystallization method

Zeolite thin films were prepared on cordierite (Mg₂Al₄Si₅O₁₈) honeycomb substrates by in situ crystallization using soft hydrothermal conditions. The synthesized zeolite films (zeolite A and ZSM-5) were characterized by XRD, FTIR, SEM, NH₃-TPD, and Ns₂ and propane gas adsorption. Zeolite-A films were prepared by dip-coating on a cordierite substrate in a precursor of molar composition of Na₂O:Al₂O_3: SiO_2: H₂O = 4:1:2:100 and heating in an autoclave at 80 °C for 6–10 h. The resultant zeolite-A films consisted of cubic crystals about 2–3 μm in size, achieving a thickness of 20 μm after re-coating. ZSM-5 films were similarly formed using a microporous silica precursor obtained by selective leaching of metakaolinite with tetrapropylammonium hydroxide (TPAOH) as the templating agent. The molar composition of the precursor was NaOH:microporous silica:TPAOH:H₂O = 1:10:1:200. The dipped substrate covered with the wet precursor gel was heated in an autoclave at 150 °C for 24 h. The resultant films were composed of short prismatic <1 μm crystals achieving thickness of several to 10 μm after re-coating. The microstructure and porous properties of the ZSM-5 films were found to change according to the chemical composition and surface treatment of the cordierite substrates. The presence of a SiO₂-rich interfacial layer between the substrate and zeolite film increased the amount of zeolite formed and the physical adsorption but decreased the solid acidity and amount of chemisorption.     

An approach to prepare defect-free PES/MFI-type zeolite mixed matrix membranes for CO2/N2 separation

Polyethersulfone (PES) and MFI-type zeolite mixed matrix membranes (MMMs) were successfully fabricated by a novel particle suspension impregnating and solution-casting method. The effects of two different membrane preparation methods on the membrane morphology were investigated by scanning electron microscopy; the particle crystal type and distribution were explored by X-ray diffraction and fourier transform attenuated total reflectance infrared spectroscopy. The results showed that the MFI-zeolite particles were successfully synthesized, with particle diameter around 250 nm. Distribution and dispersion of MFI-zeolite particles suspended in the polymer matrix are homogeneous and uniform. The thermal analysis indicates that the glass transition temperature (T _g) of MMMs is around 230 °C and the initial degradation temperature is up to 460 °C in air. The increment of T _g compared to the neat PES confirms that the polymer chain rigidification is induced by zeolite particles. The MMMs have been evaluated by the CO₂/N₂ separation factor and permeability measured as a function of permeation temperature, the maximum separation factor of the PES/10 % MFI-silica zeolite for the pure gas reaches 35 at 25 °C.     

Conversion of high silicon fly ash to Na-P1 zeolite: Alkaline fusion followed by hydrothermal crystallization

In this research, we converted high silicon fly ash to a high ion-exchange capacity zeolite using a two stages conversion process. Alkaline fusion was applied to collapse the fly ash crystalline phases and release Si content. Then Si/Al ratio of the synthesis sol adjusted with appropriate amount of industrial grade materials. A synthesis solution with the molar ratio of 2.2 SiO₂:Al₂O₃:5.28 Na₂O:106 H₂O was hydrothermally crystallized to Na-P1 zeolite at 120 °C for 4 h. The as-synthesized zeolite characterized by means of X-ray diffraction, infrared spectroscopy, scanning electron microscopy and thermal analysis. Cation exchange capacity of the zeolite was determined using ammonium acetate method. The zeolitization remarkably improved the cation exchange capacity of the final product (e.g. 3.23 meq/g in comparison to the raw fly ash ～0.005–0.02 meq/g).Due to the high CEC and sufficient whiteness of the final product, we suggest that the as-synthesized zeolitic powder is a potential candidate as a detergent builder.     

On the crystallization mechanism of zeolite ZSM-5: Part 1. Kinetic compensation effect for the synthesis with some diamines

Zeolite ZSM-5 was synthesized from the initial gel: 33 Na₂O: 44R: Al₂O₃: 100 SiO₂: 4000 H₂O: 25 H₂SO₄, where R is tetrapropylammonium bromide (TPA) and C₂, C₃, C₄, and C₆ diamines are in the temperature range 423–453 K. The kinetic analysis of the crystallization of ZSM-5 permits us to follow the role of the used templates during the consecutive steps of the process. The diminishing of the crystallization rate with the increasing of the linear dimension of the molecule is proved. A kinetic compensation effect (KCE) was established between activation energy and the preexponential factor. The obtained KCE is proof for the uniform mechanism of crystallization of zeolite with C₂, C₃, C₄, and C₆ diamines and for a different one for the synthesis with TPA.     

Synthesis of LTA zeolite beads using alum sludge and silica rich wastes

This study successfully recycled alum sludge with either lithium slag or bottle glass to make high purity zeolite LTA beads. The basic synthesis approach was to fuse the waste material with sodium hydroxide then complete hydrothermal crystallization. Separate fusion of individual wastes promoted zeolite LTA formation compared to combined fusion. Preferred synthesis conditions were: 10 Na₂O:Al₂O₃:2.5 SiO₂:300 H₂O; hydrothermal temperature = 80 °C; time = 5 h. Zeolite LTA purity was between 80 and 85 wt% from the two waste combinations. Use of pseudoboehmite and carboxymethyl cellulose favoured granulation/extrusion/spheronization of zeolite LTA powder. However, the beading process decreased calcium exchange capacity (CEC) from 99.5 to 37 mg Ca²⁺/g for alum sludge + lithium slag and from 64.5 to 37 mg Ca²⁺/g for alum sludge + waste glass. Additionally, when using 20 wt% pseudo-boehmite as a binder the ion-exchange equilibrium time increased from 60 to 300 min. The importance of not only making zeolite powder from waste materials but also to extend studies to shaped forms was evident. It is recommended that future studies should also focus on making extrudates or beads as these are the forms used commercially.     

Effect of amine structure on CO2 capture by polymeric membranes

Abstract

Poly(amidoamine)s (PAMAMs) incorporated into a cross-linked poly(ethylene glycol) exhibited excellent CO₂ separation properties over H₂. However, the CO₂ permeability should be increased for practical applications. Monoethanolamine (MEA) used as a CO₂ determining agent in the current CO₂ capture technology at demonstration scale was readily immobilized in poly(vinyl alcohol) (PVA) matrix by solvent casting of aqueous mixture of PVA and the amine. The resulting polymeric membranes can be self-standing with the thickness above 3 μm and the amine fraction less than 80 wt%. The gas permeation properties were examined at 40 °C and under 80% relative humidity. The CO₂ separation performance increased with increase of the amine content in the polymeric membranes. When the amine fraction was 80 wt%, the CO₂ permeability coefficient of MEA containing membrane was 604 barrer with CO₂ selectivity of 58.5 over H₂, which was much higher than the PAMAM membrane (83.7 barrer and 51.8, respectively) under the same operation conditions. On the other hand, ethylamine (EA) was also incorporated into PVA matrix to form a thin membrane. However, the resulting polymeric membranes exhibited slight CO₂-selective gas permeation properties. The hydroxyl group of MEA was crucial for high CO₂ separation performance.     

Optimization of crystal growth of hierarchical porosity ZSM-5 zeolite based on coal fly ash “waste utilization”

In this study, ZSM-5 zeolite with hierarchical porosity structure was synthesized by hydrothermal method using Al(OH)₃, SiO₂ extracted from coal fly ash as aluminum source and silicon source. The optimal synthesis parameters of ZSM-5 zeolite were determined via an orthogonal analysis table. The optimum synthesis conditions were: SiO₂/Al₂O₃/Na₂O/hexadecyltrimethylammonium bromide/tetrapropyl ammonium bromide/H₂O = 1/0.08/0.25/0.05/0.17/50, and the optimum crystallization temperature and crystallization time were 160 °C and 48 h, respectively. Scanning electron microscopy showed that the synthesized ZSM-5 zeolite exhibited a spherical shape. Nitrogen adsorption-desorption and Brunauer-Emmett-Teller measurement analysis displayed that the best sample ZSM-5 molecular sieve showed a typical type IV isotherm, and the micropores and mesopores coexisted in the sample, indicating the successful preparation of ZSM-5 zeolite with hierarchical porosity structure. The crystal growth of ZSM-5 under different hydrothermal conditions follows the “S” adjustment.     

Hetero-Polycrystalline Membranes with Narrow and Rigid Pores for Molecular Sieving

Molecular sieving membranes have great potential for energy-saving separations, but they suffer from permeability-selectivity trade-off limitation. In this report, simultaneous hetero-crystallization and hetero-linker coordination of metal–organic framework (MOF) hollow fiber membranes through one-pot synthesis for precise gas separation is reported. It is found that the hetero-polycrystalline membranes consist of 2D and 3D MOF phases and are defect-free and roughly orientated, hetero-linker exchange of 3D phase by larger geometric ones can narrow transport pathway, and framework rigidification occurs and thus fixes MOF channels. The prepared membranes are robust and reproducible, and exhibit substantially improved performance, with H₂/CO₂, H₂/N₂, and H₂/CH₄ selectivities up to 361, 482, and 541, respectively, accompanied by high H₂ permeance over 1100 gas permeation units, which can easily outclass trade-off upper bounds of state-of-the-art membranes.     

An EXAFS and XANES study of europium- and europium-nickel-exchanged Y zeolite and the effects of reduction

Eu³⁺ within the supercages of a ca. 10 wt% europium-exchanged Y zeolite of composition [Eu_7.8(NH₄)_30.5Na_0.1Si₁₃₉Al₅₃O₃₈₄]-_nH₂O can be partially reduced by hydrogen to Eu²⁺. XANES is shown to be a particularly sensitive means by which the reduction process can be monitored. The Eu³⁺ in similar sites in a ca. 3 wt% europium-1.5 wt% nickel coexchanged Y zeolite of composition [Eu_2.3Ni_3.0(NH₄)₄₁Na_0.1Si₁₄₁Al₅₁O₃₈₄]-_nH₂O is also amenable to partial hydrogen reduction to Eu²⁺. Some of the reduced species, probably located in the smaller cages as a result of migration during the reduction process, are resistant to reoxidation. The N²⁺ in the supercages of the coexchanged Y zeolite is more amenable to hydrogen reduction to metallic nickel than is Ni²⁺-exchanged Y zeolite in which the Ni²⁺ is located in the smaller cages.     

Influence of alkalinity on particle size distribution and crystalline structure in synthesis of zeolite beta

Influence of alkalinity (OH^-/SiO₂) on particle size distribution and crystalline structure in synthesis of zeolite beta at a short crystallization period (60 h) has been studied. The results indicate that the highest crystallinity of synthetic zeolite beta at alkalinity of 0.24, 0.35 occurs when Al₂O₃·H₂O and NaAlO₂ are respectively used as aluminium sources. At an alkalinity higher than 0.39, zeolite beta can not be obtained when Al₂O₃·H₂O is used as aluminium source. The difference of particle sizes between zeolite beta synthesized from gels of different alkalinity is 0.7–0.8 μm. The widest particle size distribution of zeolite beta synthesized occurs when the alkalinity is 0.23–0.24. Along with the increase of the alkalinity, zeolite beta synthesized changes as follows: vibration peak of 1071 cm⁻¹ by IR moves to a higher position, the framework Si/Al ratio determined by ²⁹Si MAS NMR first increases gradually and then decreases, but the change of OhAl/TdAl ratio determined by ²⁷Al MAS NMR is contrary to that of the framework of the Si/Al ratio. In addition, the Si(O⁻) site existing in the TEA-β is confirmed by NMR, and the mechanism of alkalinity influencing crystallization is discussed.     

Enhancing the oxygen permeability of Ba0.5Sr0.5Co0.8Fe0.2O5 + δ membranes by coating RBaCo2O5 + δ(R= Pr, Nd, Sm, Gd) layers

The effects of a thin RBaCo₂O_5 + δ (R= Pr, Nd, Sm, Gd) layer coating on the oxygen permeation flux through Ba_0.5Sr_0.5Co_0.8Fe_0.2O_5 + δ(BSCF) membrane were investigated. Due to the high oxygen adsorption and desorption rate constants k_a and k_d of the RBaCo₂O_5 + δ (R112) materials and the porous structure of the coating layers, the oxygen permeation flux through the BSCF membranes can be enhanced remarkably. It was found that the reaction between Pr112 and BSCF also has significant influence on the oxygen permeation flux. The reaction between Pr112 and BSCF forms impurities which may block oxygen permeation flux. However, Nd112, Sm112, and Gd112 do not react seriously with BSCF, therefore, coating layers of these materials can significantly enhance the oxygen permeation flux of BSCF membranes.     

Studies on fluoride adsorption of iron-impregnated granular ceramics from aqueous solution

This study evaluated the effectiveness of iron-impregnated granular ceramics in removing fluoride from aqueous solution. Kanuma mud, zeolite, starch and FeSO₄·7H₂O/Fe₂O₃ mixed to prepare granular ceramic at room temperature by granulation procedure with the mass ratio of 4:3:2:1. Both the adsorbents were characterized by SEM, EDS and BET surface area. The various experimental parameters investigated for fluoride adsorption from aqueous solution were: contact time, initial pH, initial fluoride concentration and temperature. The GC (FeSO₄·7H₂O) is more effective for fluoride removal than GC (Fe₂O₃) in adsorption capacity. The optimum pH for fluoride removal on GC (FeSO₄·7H₂O) and GC (Fe₂O₃) was 7.0 and 4.0, respectively. The experimental data fitted reasonably well to both Langmuir and Freundlich isotherm models for these two adsorbents. The two adsorption process followed pseudo-second-order kinetics with intra-particle diffusion as the rate determining step. The calculated thermodynamic parameters showed that both of the adsorption processes were thermodynamically favorable, spontaneous and endothermic in nature.     

Reutilization of Cr-Y zeolite obtained by biosorption in the catalytic oxidation of volatile organic compounds

This work aims at the reutilization of a Cr-loaded NaY zeolite obtained by biorecovery of chromium from water as catalyst in the oxidation of volatile organic compounds (VOC). Cr-NaY catalysts were obtained after biosorption of Cr(VI) using a bacterium, Arthrobacter viscosus, supported on the zeolite. The biosorption experiments were conducted at different pH values in the range 1-4. The catalysts were characterized by several techniques, namely ICP-AES, SEM-EDS, XRD, XPS, Raman, H₂-TPR and N₂ adsorption. The zeolite obtained at pH 4 has the highest content of chromium, 0.9%, and was selected as the best catalyst for the oxidation of different VOC, namely ethyl acetate, ethanol and toluene. For all VOC tested, the catalyst with chromium showed higher activity and selectivity to CO₂, in comparison with the starting zeolite NaY. The presence of chromium shifted also the reaction pathways. In terms of selectivity to CO₂, the following sequence was observed: ethyl acetate > toluene > ethanol.     

Synthesis of small crystal zeolite beta in a biphasic H2O-CTAB-alcohol system

Small crystal zeolite Beta with SiO₂/Al₂O₃ ratios ranging from 30 to 400 has been synthesized in a biphasic H₂O-CTAB-Alcohol system using colloidal precursor which contains secondary building units of zeolite Beta as the silica and aluminum sources. The experimental results evidenced that the synthesis system employed here resulted in readily the formation of small crystal size zeolite Beta, and the crystallite aggregates consisting of many small particles with size close to the range of nanometer were obtained. The obtained zeolite Beta samples synthesized in the biphasic system displayed higher catalytic activity for n-hexane hydrocracking than that synthesized in single aqueous system.