High-quality Zeolite Research Articles

Factorial design methodology for synthesizing zeolites: A critical review

AbstractThis review shows an outlook about several works which have used the factorial design methodology for synthesizing zeolites and mesoporous materials, included the use of wastes as sources of silicon and aluminum. In the bibliography several interesting works were identified with a complete development of the factorial design methodology. However, there are other works that showed a limited statistical analysis of the results. In spite of these issues, their contribution in the advance of synthesis of zeolites is important. Moreover, different ways to measure the crystallinity were detected being the X‐ray diffraction (XRD) the most used technique. This work also gives a proposal of the complete factorial design methodology as a guide in future works for synthesizing zeolites or zeo‐types materials. This work presents a detailed review of the methodology of designing experiments for the synthesis of zeolite or zeo‐type materials, where it has been shown and analyzed the type of design used, because the choice of a design involves a large number of experiments or few experiments thus facilitating the production of zeolite, in addition to the factors that influence the synthesis give information of the type to obtained zeolites, also includes a guide for applying this methodology. Experiment design methodology can help to produce high quality zeolites from virgin raw material or waste that impact the economy of the zeolite industry.

Large-area, high-permeance and acid-resistant zeolite SSZ-13 membranes for efficient pervaporative separation of water/acetic acid mixtures

The development of acid-stable zeolite membrane is still a great challenge for pervaporative dehydration of water/acetic acid mixtures. Herein, we reported the fabrication of high-quality zeolite SSZ-13 membranes on the 50-cm-long tubular alumina supports via vacuum seeding method. The uniform seed layer by vacuum seeding method resulted in a uniform and intergrown zeolite membrane with thickness of 8.0–9.0 μm. The typical membrane with membrane area of 180 cm2 and Si/Al ratio of 12 displayed extremely high water permeance of 2.4 × 10−6 mol m−2 s−1 Pa−1 and high selectivity of 7700 in a 40/60 wt% water/AcOH mixture at 363 K. The corresponding water flux and separation index were 7.0 kg m−2 h−1 and 1.95 mol m−2 s−1, respectively. The overall separation performance (for example separation index) of the large-area SSZ-13 membrane was the highest one among the reported membranes for water/acetic acid separation. The effects of feed concentration and temperature on membrane performances were investigated for the separation of H2O/AcOH mixtures. Water flux increased greatly but water permeance decreased a little with increasing water content in the mixture and temperature. Permeance is a driving force normalized form and reflects the inherent property of the membrane only. Stability of the membrane in aqueous AcOH solutions was also studied in the 10-d continuous test.

Ultrasonic-pretreated hydrothermal synthesis of less dense zeolite CHA from the transformation of zeolite T

Because of containing the same double 6-ring (D6R) building unit, the pure zeolite CHA with lower framework density (FDSi = 15.1 T/1000 Å3) has been transformed from zeolite T with higher framework density (FDSi = 16.1 T/1000 Å3) through ultrasonic-pretreated hydrothermal synthesis in MOH (KOH and NaOH) solution without adding organic template or seed crystals. Ultrasonic pretreatment facilitates the transformation rate and generates high-quality zeolite CHA. The ultrasound condition should be precisely controlled because that CHA phase is metastable, which is inclined to transform to other more stable phase. The ultrasonic conditions at 313 K and 333 K have been investigated in detail. In KOH solution, the ultrasonic treatment at 313 K can effectively restrain the generation of MER phase, however, it is hard to avoid the existence of MER phase when ultrasound temperature is 333 K. In NaOH solution, the samples with ultrasonic treatment of 313 K show the small particles size of about 1 μm, and the GIS framework topology starts to grow with the ultrasonic treatment of 333 K. The products prepared with the appropriate ultrasonic pretreatment represents smaller particles size, larger mesopore volume and higher CO2 adsorption capacity than the sample without the ultrasonic pretreatment. The structural evolution of interzeolite transformation has been explored by XRD, FT-IR and SEM observations. With the assistance of ultrasound, the parent zeolite T can quickly decompose into intermediate phase and then regenerate into CHA phase.

Preparation of high-performance zeolite membrane on a macroporous support by novel intermittent hydrothermal synthesis

Constructing high-quality zeolite membrane by regulating solely the kinetics of zeolite nucleation and growth is desired because of its simplicity and potential industrial viability. Herein, a high-performance mordenite membrane was prepared without using organic template successfully on a macroporous tubular α-alumina support using a novel and simple intermittent hydrothermal synthesis (IHS) method. We aimed at regulating membrane synthesis kinetics by adjusting the concentration of the synthetic gel by introducing an unheated period of the oven in the middle stage of the synthesis process. In the proposed IHS method, the power supply of the oven can be switched off and on with the aid of a smart socket without manual operation, ensuring its feasibility and convenience. A thin and dense membrane layer was formed via the appropriate adjustment of presynthesis time, unheated time, and resynthesis time. The effects of each duration on the morphologies, microstructures, and pervaporation performances of the prepared mordenite membranes were investigated. Under optimized conditions, the as-synthesized mordenite membrane showed high pervaporation performance with a permeation flux and separation factor of 5.57 kg m−2 h−1 and >10000, respectively, in the dehydration of 90 wt% isopropanol/water mixture at 75 °C. This work provides a novel, simple, and feasible method for the preparation of high-performance zeolite membrane.

Metal ion (Ca2+, Mg2+, Zn2+) catalyzed synthesis of high-quality zeolite templated carbon

Three metal ions (Ca 2+ , Mg 2+ , and Zn 2+ ) were found to be successful catalysts to produce high-quality zeolite templated carbon (ZTC), a periodic three-dimensional graphene-like carbon with ordered microporous structure. Their roles in high-quality ZTC formation were studied by characterizing chemical component, surface morphology, structure, and pore properties. Ca 2+ , Mg 2+ , and Zn 2+ minimize or eliminate the blockage of channels by carbon deposits and non-selective thermal deposition on the external surface of the zeolite particles. The growth temperature was lowered to only 450 °C. Moreover, the formation mechanism of ordered ZTC structure was revealed for the first time. • Ca 2+ , Mg 2+ , and Zn 2+ were found to catalyze high-quality ZTC synthesis. • The inner channel blockage and external surface deposition problems were solved. • The required carbon growth temperature was lowered to only 450 °C. • A single layered, curved, graphene-like carbon network, likely a carbon schwartzite was obtained. • The formation mechanism of ordered ZTC structure was revealed for the first time.

Synthesis of zeolites from circulated fluidized bed coal fly ash

High quality zeolite A was successfully synthesised from circulating fluidized bed combustion (CFBC) fly ash by alkali fusion and hydrothermal treatment.

Effect of coal fly ash treatments on synthesis of high-quality zeolite A as a potential additive for warm mix asphalt

An experimental investigation on the synthesis of zeolite A, based on the treatment of fly ash and controlled extraction of silica and alumina from coal fly ash, combined with the adjustment of the reaction mixtures aiming minimum amount of secondary phases, is presented herein. The precursor samples of fly ash and the obtained zeolites were widely characterized considering different techniques including powder X-ray diffraction, infrared vibrational spectroscopy, scanning electron microscopy, and thermogravimetric analysis. A new pore characterization technique that combines experimental CO₂ isotherms at 273 K and molecular simulation was specially developed for this system. The synthesized zeolites presented zeolite A as the major phase, with high crystallinity and low degree of defects in the supercages. The treated fly ashes affected the synthesis yield and the properties of the obtained zeolites, which proved to be efficient in decreasing the viscosity of the modified asphalt cement, during heating, contributing to improve its workability.

Seawater Reinforces Synthesis of Mesoporous and Microporous Zeolites from Egyptian Fly Ash for Removal Ions of Cadmium, Iron, Nickel, and Lead from Artificially Contaminated Water

This study focuses on the production of mesoporous and microporous zeolites via fusion pre-treating fly ash with NaOH followed by hydrothermal treatment utilizing seawater. Three pretreatments for fly ash were carried out utilizing 1:1, 1.2:1, and 1.4:1 of NaOH:fly ash ratio to compare the removal efficiency for cadmium Cd(II), iron Fe(II), nickel Ni(II), and lead Pb(II) from artificially contaminated water. The impacts of several variables including concentrations, weights, pH, and contact times were examined to acquire knowledge on the adsorption rate. X-Ray Diffraction (XRD), Scanning Electron Microscope (SEM), and Brunaauer-Emmett-Teller (BET) were utilized to investigate zeolite structures. N2 adsorption/desorption isotherms demonstrated that the synthetic zeolites were mesoporous and microporous materials with a higher specific area (347, 240, and 127 m2/g) than the values for raw fly ash (15m2/g). The X-ray diffraction outcome suggested that the synthetic products mainly belonged to phillipsite, carbonate cancrinite, and hydroxysodalite. These outcomes showed that fly ash and seawater from power plants are appropriate for synthesizing high-quality zeolites. For contaminated water treatment, the products are effective for removal Cd(II), Fe(II), Ni(II), and Pb(II) at pH 7, contact time 45 min, and dose 1 g/L. Zeolites recycling outcomes showed that the removal efficiency of investigated metal ions by Z1, Z2, and Z3 was reduced by an average of 7%, 5%, and 3% after regeneration.

Synthesis of high-quality zeolite LTA from alum sludge generated in drinking water treatment plants

This study demonstrated the conversion of alum sludge produced at water treatment plants to high quality zeolite LTA. Thus, problems with sludge landfilling costs and environmental contamination may be avoided. The methodology involved alkali fusion pre-treatment of alum sludge followed by hydrothermal synthesis to improve the quantity and quality of zeolite LTA made. Alum sludge activation using fusion facilitated formation of high-quality zeolite LTA (97 wt%) which was comparable to commercial material (98 wt%) and superior to samples made using hydrothermal synthesis without fusion pre-treatment (81 wt%). Notably, the alum sludge derived samples contained greater amounts of impurities such as iron, calcium, and magnesium. Preferred synthesis conditions using fusion-pre-treatment were: SiO2/Al2O3 = 2.5; Na2O/SiO2 = 3; H2O/Na2O = 40; reaction time =5 h: reaction temperature = 80 °C. The average zeolite LTA particle size ranged from 0.7 (hydrothermal) to 1.1 (fusion pre-treated) to 3.0 (commercial) microns. Kinetic studies indicated that the exchange of calcium ions with all zeolite samples reached equilibrium within 60 min. Equilibrium ion exchange data revealed the maximum loading of calcium ions was 106.6, 83.9, and 64.6 mg Ca2+/g for fusion pre-treated, hydrothermally treated, and commercial LTA, respectively. However, on a “dry basis” the cation capacity of the commercial and fusion pre-treated samples was similar.

Preparation of high performance ZSM-5 zeolite membranes for CO2/H2 separation

High-quality zeolite ZSM-5 zeolite membranes were successfully synthesized on the outer surface of tubular α-alumina substrates by manipulating the microstructure of the zeolite layer using a new different-sized seeding in combination with a variable-temperature/time methods. In this method the defects/microcracks in the bottom seed layer, which are filled by smaller seeds, are effectively eliminated using a designed temperature/time profile, resulting in a significant increase in selectivity without scarifying permeance. The effects of number and combination of seed layers, seed particle size, and the synthesis temperature-time profile on the microstructure, N2/SF6 gas permeation, and H2/CO2 separation performance of membranes are evaluated in detail. The synthesized ZSM-5 zeolite membranes show the microstructure of thin, fully inter-grown, and densified zeolite layer that is responsible for high selectivity without any negative effect on the permeance. The best synthesized defect-free ZSM-5 zeolite membranes on alumina supports exhibited a very low H2/CO2 selectivity of 0.1 with CO2 permeance of 3×10−5molm−2s−1Pa−1.

Surface-gel-conversion synthesis of submicron-thick MFI zeolite membranes to expedite shape-selective separation of hexane isomers

Ultrathin zeolite membranes are of paramount importance in accelerating gas transport during membrane separation, and lowering down their membrane thicknesses to submicron scale is deemed to be very challenging. Herein, we develop an advanced approach of surface gel conversion for synthesis of submicron-thick pure silica MFI (silicalite-1) zeolite membranes. Viscous gel is prepared by finely adjusting the precursor composition, enabling its reduced wettability. The unfavorable wetting of the support surface can effectively prevent gel penetration into alumina support voids. Aided by the seeds, the surface gel is directly and fully crystallized into an MFI zeolite membrane with minimal water steam. A membrane with a thickness of 500 nm is successfully acquired and it is free of visible cracks. Additionally, the as-synthesized membranes exhibit rapid and selective separation of hexane isomers by virtue of unprecedentedly high n-hexane permeance of 24.5×10−7 mol m−2 s−1 Pa−1 and impressive separation factors of 13.3-22.6 for n-hexane over its isomers. This developed approach is of practical interest for sustainable synthesis of high-quality zeolite membranes.

Gel-Modulated Growth of High-Quality Zeolite Membranes

Trade-off between thickness (and thus gas permeance) and quality (and thus selectivity) of zeolite membranes significantly restricts their wide application. It is challenging to maintain the membrane thickness while minimizing nonselective defects in the selective membrane layer. Currently, continuous change of the synthesis gel concentration during membrane synthesis, instead of high gel concentration for membrane "skeleton" growth and low gel concentration for slow crystal growth to merge "skeleton" crystals, usually leads to thick membranes to compensate for the low selectivity. In this work, we report a gel-modulated synthesis approach to engineer the zeolite membrane synthesis. The gel concentration was suddenly reduced by a quenching process in the middle of the membrane synthesis, leading to reduced nuclei formation and crystal growth rate in the following synthesis process. Membrane quality was significantly improved without increasing membrane thickness, leading to a great increase in membrane selectivity but without sacrifice of gas permeance. Moreover, as an example, highly reproducible SAPO-34 membranes were successfully prepared by the gel-modulated growth method. We expect this novel synthesis strategy might be a viable and economic way of growing thin and high-quality zeolite membranes.

Formation of NaY zeolite membrane: influence of intermediate layer and its characterization

In the synthesis process of zeolite membranes, the zeolites crystals are crystallized and grow during crystallization period to form polycrystalline film.Secondary growth method is an effective method to synthesis high quality zeolite membranes which involves two main steps which are coating and hydrothermal treatment (HT) synthesis. There are several problems associated with the coating process in secondary growth method such as lack of good adherence of the seeds, rough surface of the support, and thermal mismatch between the zeolite seeds and supports during hydrothermal process. In order to solve these issues, an intermediate layer (buffer layer) could be applied between the support and the seeds layer hence to reduce the defects forms during the hydrothermal treatment (HT) synthesis process. In current study, two different types of intermediate layer were studied and results were compared in terms of zeolite seeds coverage on the surface of the support and zeolite layer formed during subsequent hydrothermal treatment (HT) to form zeolite membranes. The formed zeolite membranes were characterized with X-Ray diffraction (XRD) and scanning electron microscopy (SEM) and energy dispersive X-Ray spectroscopy (EDX).

An Hetero-Epitaxially Grown Zeolite Membrane.

The secondary growth methodology to form zeolite membranes has stringent requirements for homogeneous epitaxial intergrowth of the seed layer and limits the number of accessible high-quality zeolite membranes. Despite previous reports on hetero-epitaxial growth, high-performance zeolite membranes have yet to be reported using this approach. Here, the successful hetero-epitaxial growth of highly siliceous ZSM-58 (DDR-type zeolite) films from a SSZ-13 (CHA-type zeolite) seed layer is reported. The resulting membranes show excellent CO2 perm-selectivities, having maximum CO2 /N2 and CO2 /CH4 separation factors (SFs) as high as about 17 and 279, respectively, at 30 °C. Furthermore, the hybrid membrane maintains the CO2 perm-selectivity in the presence of water vapor (the third main component in both cases), that is, CO2 /N2 SF of about 14 and CO2 /CH4 SF of about 78, respectively, at 50 °C (a representative temperature of both CO2 -containing streams).

Synthesis of least defect NaY membranes: Defining optimum heating and cooling rate of hydrothermal treatment (HT) and application of zeta potential measurement via different seeding methods

Zeolite membrane is one type of microporous inorganic membrane which has gained so much interests among researchers as an alternative especially for gas separation. Among major challenges to produce high quality zeolite membranes with less defects are to have a good seeding and hydrothermal treatment (HT) method. On this matter, a study on the colloidal stability of the zeolite NaY seeds in the seeds solution in order to obtain homogenous seeds layer on top of the alumina support via several seeding methods and pHs of the seeds solutions were investigated. Then, the seeded samples were taken into HT process in order to form zeolite NaY membranes. During this stage, heating and cooling rates ( o C/hr) were varied in order to study their effects towards formation of zeolite membrane with least defects. Characterizations were performed by using X-Ray Diffraction (XRD) in order to identify degree of crystallization and amount of impurities while Scanning Electron Microscopy (SEM) was used in order to study surface morphology of the samples. From this study, it was found that densification effect from different seeding methods played important factor in forming zeolite membranes with least defects. Accordingly, suitable heating and cooling rates were required in order to optimize the growths of zeolite NaY seeds to form zeolite layer on top of the alumina support. In this case, due to thermal mismatch between the substrate and zeolite material, it could form some cracks on top of the alumina support. Thus, by implementing optimum heating/cooling rate, a zeolite NaY membrane with least amount of defects i.e. cracks and pinholes could be achieved with less degree of impurities i.e. zeolite NaP during HT process.

The effects of metakaolinization and fused-metakaolinization on zeolites synthesized from quartz rich natural clays

Abstract A variety of zeolites have been synthesized by performing either metakaolinization or fused-metakaolinization of natural clays prior to the hydrothermal synthesis step. To understand the differences arising from performing fused metakaolinization rather than simple metakaolinization, the calcination conditions, gel composition, gelling and crystallization conditions were kept similar for both methods. The original clay material, a kaolinite group mineral with Si/Al molar ratio of 1, had a high content of quartz as impurity. The two treatment methods gave different zeolites of high crystallinity and relatively similar Si/Al molar ratio. Zeolite Na-A with 98% crystallinity was produced from the metakaolinization method within 8 h, while Na-X with 96% crystallinity was generated from the fused-metakaolinization (fusion) method within 48 h. In the fusion method, sodalite was formed at or beyond 48 h of hydrothermal reaction, while in the metakaolinite method, apart from a slight decrease in intensities of XRD peaks, no phase change was observed even after 168 h of hydrothermal reaction. Modification of the synthesis gel by increasing the Si/Al molar ratio via addition of fumed silica resulted in mixed zeolite products via the metakaolinite method, while Na–Y was the major product for the fusion method. Except for Na-A, all zeolite products had high surface area of up to 600 m2/g and micropore volume of 0.18 cm3/g. Our findings demonstrate the crucial role of the pre-treatment step in the synthesis of zeolites from clay minerals, and that a variety of high quality zeolites are achievable via choice of pre-treatment protocols.

Zeolite X with potassium diformate as a sustained-release antibacterial agent

A reaction of 800 mesh stellerite and a concentration of 15% hydrochloric acid in a solid to liquid ratio of 1:3 was carried out for 2 h at \(80^{\circ }\hbox {C}\). Most of its impurities were removed. It can be used as precursors for the preparation of high quality zeolite. Zeolite X which is octahedral crystal shape as cube, 96% of the degree of crystallinity has been synthesized hydrothermally from \(n(\hbox {SiO}_{2}/\hbox {Al}_{2}\hbox {O}_{3})=3, n(\hbox {Na}_{2}\hbox {O}/\hbox {SiO}_{2})=1.14\) and \(n(\hbox {H}_{2}\hbox {O/Na}_{2}\hbox {O})=37\) by controlling reaction temperature and time. The synthesis included zeolite X as carriers, chitosan as intermediates, a mass ratio of zeolite X:chitosan:potassium diformate \(=\) 3:1:2 in weight for 2 h at \(40^{\circ }\hbox {C}\). X-ray photoelectron spectroscopy analysis showed hydrogen bond formation by the chitosan amino hydrogen, potassium diformate oxygen and zeolite X oxygen or hydrogen bond formation by free hydroxyl group of zeolite X and oxygen of chitosan’s C–O, indicating effective grafting. Zeolite X sustained-release antibacterial agent was prepared with an inhibition rate of 78.16% by the antibacterial.

Solvent-Free Secondary Growth of Highly b-Oriented MFI Zeolite Films from Anhydrous Synthetic Powder.

Up to date, zeolite films have been mainly fabricated by in situ crystallization, secondary growth in a solution/hydrogel, or occasionally by vapor phase transformation of dry gel. Here we demonstrate for the first time a solvent-free secondary growth method for b-oriented silica MFI zeolite films using the synthetic powder from ground anhydrous raw solids in the presence of NH4F. Typically, precisely b-oriented MFI zeolite films are synthesized from seed layers of highly b-oriented MFI zeolite crystals in the synthetic powder of 1SiO2:0.035TPABr:0.05NH4F at 175 °C for 6 h. If needed, b-oriented MFI zeolite multilayer films can be acquired by changing the synthesis time or the amount of NH4F in the synthetic powder. Compared with the traditional hydrothermal synthesis, the approach developed here may provide a new avenue for fabricating high quality zeolite films/membranes.

Synthesis of high quality zeolites from coal fly ash: Mobility of hazardous elements and environmental applications

Converting fly ash into zeolite is an alternative way to reduce the waste landfills while also producing high value-added products. However, hydrothermal synthesis of zeolite from fly ash in alkaline media could also induce mobilization of toxic heavy metals, possibly causing environmental contamination. Systematic research into this subject is rare in the open literature and the mobility of heavy metals from fly ash derived zeolites is yet to be understood. In the present contribution, we investigated the migration of heavy metals and quantified their distribution from fly ash to product zeolites and waste water during the synthesis of high-quality type A zeolites (471 m2/g surface area). High conversions of major elements (98.2% aluminium and 96.5% silicon) were achieved, with zero secondary solid waste. Metalloid elements including arsenic and selenium, and those with strong amphoteric properties e.g. molybdenum were found highly mobile and mostly presented in the wastewater. In comparison, less than 20% of the heavy metals with weak amphoteric nature including copper, chromium and lead from the fly ash went to the wastewater; the rest along with almost all cadmium, iron and nickel were fixed into the product zeolites. Despite the existence of heavy metal elements in zeolites, there was no noticeable leaching under various pH conditions, hence deemed safe for applications. Furthermore, the effective removal of trace strontium and caesium cations from contaminated water using such zeolites was demonstrated.

Highly Oriented Thin Membrane Fabrication with Hierarchically Porous Zeolite Seed

Nanosized zeolite is widely used as seed for high quality zeolite membranes fabrication, while its complicated synthesis routine limits large-scale productions. In this work, a non-nanosized cubic hierarchically porous TS-1 zeolite (HTS-1), obtained by basic hydrothermal treatment of conventional ellipsoid solid TS-1, is used as seed to prepare highly oriented thin membranes. A capillary condensation phenomenon resulting from the unique hierarchically porous structure benefits gel attachment. Moreover, abundant ledges, kinks, and terraces on the HTS-1 surface promote epitaxial growth of the membrane. In contrast, the solid TS-1 seed induces intergrowth dominantly, which results in a thick TS-1 membrane. The HTS-1 membrane demonstrates superior CO2/N2 separation properties compared to the TS-1 one. It associates with thin oriented membrane morphology, leading to exposure of a high Miller index surface and less diffuse distance and tortuosity. The results suggest beneficial effects of a hierarchically porou...