Low-silica Zeolites Research Articles

Low-silica zeolite MTT: Synthesis and efficient catalysis in methanol dehydration

Zeolite MTT features a one-dimensional (1D) 10-membered ring (10 MR) straight channel system. This unique structure provides superior catalytic performance in petrochemical processes such as isomerization, dewaxing, and olefin oligomerization. The conventional synthesis of zeolite MTT typically requires large amounts of organic structure directing agents (OSDA), and the resulting zeolite MTT usually has a Si/Al ratio greater than 20. Herein, we present a facile strategy for synthesizing zeolite MTT possessing a low Si/Al ratio of 12.5 and a greatly improved specific surface area of 308.0 m2/g with the assistance of MTT seeds. This low-silica zeolite MTT demonstrates a higher methanol conversion (84 %) at a low temperature (170 °C) in the methanol dehydration reaction compared to conventionally synthesized zeolite MTT, which achieves less than 70 % conversion.

Preparation of MAZ-Type Zeolite with High Silica.

The Si/Al molar ratio of MAZ aluminosilicate zeolite prepared by the direct hydrothermal method is generally less than five, thus giving rise to poor thermal and hydrothermal stability for this low-silica zeolite. With the purpose of enhancing the Si/Al molar ratio of MAZ zeolite, post-synthesized methods including acetic acid treatment and steaming treatment, as well as interzeolite transformation from FAU zeolite, were employed to prepare MAZ zeolite with high silica. It was found that steaming treatment was more effective in increasing the Si/Al molar ratio in comparison with acetic acid treatment, affording a maximum Si/Al molar ratio of 16.9 along with a preserved crystallinity of approximately 75%. Additionally, high-silica MAZ zeolite with a Si/Al molar ratio of up to 7.3 was also capable of being directly hydrothermally synthesized using interzeolite transformation from FAU zeolite.

Synthesis of industrially appealing low-silica zeolites using aluminum scraps and sand

In this study, Carbon footprint (CF) and hydric footprint (HF) were estimated for the synthesis of zeolite A and X using aluminum cans and sand as sources of aluminum and silicon, respectively. The fuse reagent prepared from sand, at 250 and 500° C, and the filtrated of fuse reagent product were considered as input variables in the experimental design. Aluminum waste, the mixture of aluminum waste treated with sodium hydroxide, and the formed Al(OH)3 were also considered as input parameters for the aluminum sources. Results showed that for obtaining of the fuse reagent based on sand, a temperature as low as 250 °C was sufficient to obtain the desired zeolites A, X and P without increasing the CF or the HF. The type of aluminum source did not display a significant impact on the CF and FH for the synthesis of the targeted zeolites.

Properties of multiple Lewis acid sites in alkali metal-exchanged chabazites probed by CO adsorption

Carbon monoxide adsorption on alkali-metal exchanged chabazites (M-CHA, where M = Li, Na, K) was investigated across various Si/Al ratios. The study reveals significant insights into the adsorption behavior, including the persistence of cationic preferences with decreasing Si/Al ratios and the existence of multiple-center interactions involving alkali-metal cations and CO. Results show that for high-silica M-CHA zeolites, CO adsorption is effectively described by single and dual adsorption site models, with cation preferences varying by type. In low-silica zeolites, cation positions are primarily influenced by the aluminum distribution and Coulombic interactions. However, the propensity for single-site cation positions (Si/Al→∞) is preserved to a certain degree. The most noticeable example is the small difference between SIII’ occupancies (cations in 8-membered ring windows) in Na-CHA-2 and K-CHA-2 (0.80 vs. 0.85) that strongly influences the rate of diffusion of CO in the M-CHA-2 samples. While FT-IR spectra of high-silica zeolites can be accurately described using cation site stabilities, interaction energies, and CO stretching frequencies, predicting spectra of low-silica chabazites requires a statistical approach and/or molecular dynamics simulations at the DFT level. The findings demonstrate that the dynamical behavior of adsorbates changes dramatically between different alkali metal-exchanged chabazites, highlighting the complex nature of CO adsorption at multiple Lewis acid sites.

Synthesis of zeolites using kaolin in concentrated sodium hydroxide-aluminate solutions

The synthesis of zeolites through the hydrothermal treatment of kaolin presents a cost-effective and sustainable method of production, however, the phases which remain stable in concentrated alkali solution with elevated NaAl(OH)4 are unclear. This study evaluated the equilibrium precipitation behaviour of zeolites from kaolin under a range of NaOH and NaAl(OH)4 concentrations at 523 K. The precipitates contained three types of low-silica (1:1 Si:Al) zeolites: zeolite JBW, two polymorphs of cancrinite (hydroxy- and aluminate-), and (hydroxy-)sodalite, as well as a novel sodium aluminate-intercalated nepheline-carnegieite phase at concentrated alkali-aluminate conditions. The construction of a phase diagram revealed distinct regions of stability and transition zones between phases through changing NaOH and NaAl(OH)4 concentration. The kaolin-to-zeolite phase transformation at early reaction time and Bayer pre-desilication conditions supported the phase transformation series: kaolinite → solution silicate species → aluminate-cancrinite → sodalite or cancrinite or a mixture. Substitution of caged aluminate [Al(OH)4−] for hydroxide [(OH)−] within zeolites was modelled, and revealed distinct deviation from ideal zeolite composition when starting molar Al2O3:SiO2 ≫ 1. As direct kaolin-to-zeolite technology emerges as a low-cost and low-energy alternative to conventional zeolite production, there will only be continued interest in its use in the future.

Effects of acid concentrations and acid treatment time on structural and textural characteristics of mesoporous zeolite 13X

Mesoporous zeolites containing mesopores with sizes ranging between 2 and 50 nm connected to micropores have gained increasing interests due to their potentials for various applications. Acid treatment has been shown to be a relatively simple method for modifying low-silica zeolite pore structure from micropores to mesopores. In this study, the effects of acid concentration (0.2, 0.3 and 0.4 M HCl) and treatment time (15, 30, 45, 60 and 120 min) on the structural and textural characteristics of commercial zeolite 13X were investigated. The data indicated that the faujasite structure of the zeolite still remained when treated with 0.2 M HCl. In contrast, the acid treatment with 0.3 M HCl and 0.4 M HCl caused the disruption of the crystalline structure. Regarding the treatment time, the faujasite structure of the commercial zeolite still remained when treated with 0.2 M HCl for the treatment time as long as 120 min. In addition, the N2 adsorption isotherms of the zeolite treated with 0.2 M HCl for 15 min and 120 min were not significantly different, indicating that the alteration of the pore size might occur during the first 15 min of the acid treatment. Finally, the results showed that the acid treatment caused the alteration from micropores to mostly small mesopores with the pore sizes of 3–4 nm.

Process optimisation of low silica zeolite synthesis from spodumene leachate residue

Lithium extraction from hard rock sources, such as spodumene, generates an aluminosilicate waste that can be converted into zeolite materials. Increased lithium demand in recent years highlights a growing need to develop a residue mitigation strategy. Zeolite synthesis from aluminosilicate waste is more complex than conventional synthetic zeolite production routes and typically involves an activation step, such as alkali fusion. In this context, a key challenge to increase product quality involved understanding the influence of factors such as aluminium supplement addition and alkali fusion processing options. Zeolite X crystalline content and particle morphology was compared in relation to various reaction gel compositions. Zeolite X was produced from alkali-fused SLR which was found to contain 60 wt% crystalline zeolite X after adjusting key synthesis parameters to 2.0 Si:Al ratio, 1.4 Na2O:SiO2 ratio, 30 H2O:Na2O. The product was highly agglomerated (D[4,3] = 51.1 µm) with small crystal sizes of< 2 µm. Holding this gel composition constant and introducing the aluminium supplementation during gel aging, the average particle agglomeration was reduced (D[4,3] = 14.1 µm). Further processing of the fused SLR qualitatively reduced agglomeration and increased zeolite X crystal size, with little impact on the crystalline zeolite X content; the latter strategy produced a zeolite X material with similar morphological characteristics to a commercially available analogue. This work demonstrated that controlling the processing and additions of activated aluminosilicate was a key consideration in terms of controlling zeolite crystal morphology.

One-dimensional low silica zeolite supported TiO2 for the high-efficiency production of pure methane via Photocatalytic conversion of CO2 gas

One-dimensional low silica zeolite supported TiO2 for the high-efficiency production of pure methane via Photocatalytic conversion of CO2 gas

Hydrothermal synthesis of zeolites from green container glass

Landfilling and stockpiling unrecycled colored container glass represents a considerable failure in sustainability with respect to the conservation of energy and mineral resources. In this study, the single-step hydrothermal synthesis of low-silica zeolites from a mixture of waste green container glass and aluminum foil (Al:Si = 1) in 4 M NaOH(aq) at 125 °C was followed at 1, 3, 7 and 14 days. The principal phases, sodalite and cancrinite, appeared within 1 day accompanied by minor quantities of hydrogarnet and tobermorite arising from a stoichiometric excess of calcium ions in the parent glass. Products of 63, 67, 71 and 72% crystallinity were obtained at 1, 3, 7 and 14 days, respectively, with partial successive conversion of sodalite to cancrinite over time. Ion-exchange and catalytic applications of sodalite and cancrinite arise from the high anionic charge of the 1:1 ratio of alternating SiO44- and AlO45- units within their aluminosilicate frameworks. In this respect, the uptake capacity of the 14-day zeolitic product for Cu2+ and Cd2+ ions (1.58 meq g-1 and 1.66 meq g-1, respectively) was within the expected range for zeolites and compared favorably with those reported for other inorganic sorbents derived from industrial and municipal wastes. The 14-day product was also found to be an effective basic heterogeneous catalyst for the Knoevenagel condensation reaction.

Characterization of fly ash by ED-XRF and INAA for the synthesis of low silica zeolites

The elemental composition and phase analysis of fly ash collected from a thermal power plant in West Bengal, India was evaluated using X-ray diffraction (XRD), scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM–EDX), energy dispersive X-ray fluorescence (ED-XRF) spectrometry, neutron activation analysis (NAA). Further, the particle size and surface morphology of the particles were also examined. Zeolite-A was prepared by fusion followed by hydrothermal synthesis method and its formation was confirmed by XRD. Based on the composition analysis, the Si/Al ratio was found to be 1.51, which is a good proportion to synthesise low silica zeolites.

Comparative study of direct methylation of benzene with methane on cobalt-exchanged ZSM-5 and ZSM-11 zeolites

Methane exists in massive quantity on the Earth, and considerable efforts have been made to develop effective methods to convert chemically inert methane into value-added products. Recently, cobalt-exchanged ZSM-5 zeolite has been found to be a competent catalyst for non-oxidative methylation of benzene using methane as a methylation agent. Herein, cobalt-exchanged ZSM-11 zeolite, with close topological structure to that of ZSM-5 zeolite, was discovered to be capable of catalyzing non-oxidative methylation of benzene with methane for the first time. ZSM-5 and ZSM-11 zeolites both with Si/Al molar ratio of ca. 36 and 18 were synthesized. However, higher cobalt loadings were obtained on ZSM-11 zeolites, and cobalt-exchanged ZSM-11 catalysts displayed superior toluene production rates and turnover frequencies. In addition, low silica zeolite catalysts possessed higher methylation selectivity. Diffuse reflectance visible–near infrared spectra revealed that divalent cobalt cations on the ion exchange sites existed mainly in two different coordination states, and the cobalt cation on the α site, coordinating weakly to the framework oxygen atoms, was supposed to be the active site because of the positive correlation of its concentration and the methane activation performance. Since more α-type Co2+ could be created in ZSM-11 zeolites than in ZSM-5 zeolites with the same Si/Al molar ratio, it was concluded that cobalt-exchanged ZSM-11 catalyst was a promising candidate for the non-oxidative methylation of benzene with methane.

Study of the Kinetics of Adsorption and Desorption of Water Vapor on Low-Silica Zeolites

Granular LTA and SOD zeolites have been synthesized, characterized, and further used to study the kinetics of water adsorption and desorption. It has been shown that LTA zeolite in a static mode provides adsorption of water vapor at the level of ~30 wt % for LTA and ~20 wt % for SOD. It has been found that during water adsorption from the gas phase on the LTA or SOD zeolite in a flow-through unit, the dew point reaches –50 or –9°С, respectively. The kinetics of adsorption of water vapor on zeolite granules has been studied; the values of apparent activation energy of the adsorption process have been calculated, which amount to about 50 kJ mol–1; and the rate order of the process has been determined (0.2 for LTA and 0.9 for SOD). The apparent activation energies for water desorption from the zeolites have been calculated by isoconversional analysis in terms of the Friedman, Ozawa–Flynn–Wall, and Kissinger–Akahira–Sunose models. It has been shown that all three models give similar results. It has been found that at given degree of conversion, the activation energies are not higher than 60 kJ mol–1, a value that corresponds to desorption of physically bound water. The increase in activation energy to 110 kJ mol–1 (for SOD) or 80 kJ mol–1 (for LTA) is associated with the removal of water from the α- and β-cages of zeolites.

Synthesis and Characterization of Zeolite-Y from natural clay of Wadi Hagul, Egypt

Zeolites were synthesized ‎from alternative cheap raw material as ‎clay minerals which used as a common source for Al2O3 and SiO2 for ‎‎synthesis zeolite-Y. Synthetic zeolite-Y has many characteristics, high ‎purity and larger cavities that distinguish it from its counterparts. This study concerned with evaluation of the viability of producing low-silica zeolite through the process of hydrothermal‎‎ calcination (metakaolinization) of raw material (kaolin) from Wadi Hagul, Egypt at 800oC for 6 h. Metakaolinite was synthesized under hydrothermal treatment with ‎‎ 1.0 M NaOH solution without additional any catalysts as silica source at 90oC/2d under atmospheric pressure. Zeolite Na-Y of molar ratio of 2.23 was ‎‎achieved. The Cation Exchange Capacity (CEC) test showed an impressive value of 401.09 meq/ 100 g clay for prepared zeolite-Y. The raw kaolin, metakaolin and zeolite were characterized by XRF, XRD and SEM. The results confirmed that Zeolite was successfully produced from the raw material extracted from Wadi Hagul with an ideal percentage which approximately equal to 100% of the raw material used. ‏The synthetic ‎zeolite-y can be applied for remediation of ‎wastewater ‎and polluted soil. ‎

Facile synthesis of low-silica zeolite erionite by ultrasonic-assisted method

Low-silica zeolite erionite (ERI) with a high purity/crystallinity was synthesized via ultrasonic-assisted aging for only 2 h with an initial dilute sol in the presence of offretite (OFF) seeds. Compared to the conventional approaches, ultrasound in the synthesis can enhance the crystallization rate and the ERI content because the ultrasonic aging facilitates the formation of ERI precursors. The resultant sample has hierarchical column-like aggregates with a great specific surface area (i.e., 415.7 m2/g), and abundant intracrystalline mesopores (i.e., 0.25 cm3/g), which are determined by X-ray diffraction, scanning electron microscopy and transmission electron microscopy and N2 adsorption-desorption. The aggregates exhibit a high CO2 adsorption capacity probably since a rich aluminum content in the framework preferentially adsorbs CO2.

Nature of Chemisorbed CO2 in Zeolite A

Formation of CO32– and HCO3– species without the participation of the framework-bridging oxygen atoms (−O−) upon chemisorption of CO2 in zeolite |Na12|-A is revealed. The transfer of O and H atoms is very likely to have proceeded via the involvement of residual H2O or −OH groups. A combined study by the solid-state 1H and 13C MAS NMR, quantum chemical calculations, and in situ infrared spectroscopy showed that the chemisorption mainly occurred by the formation of HCO3–. However, at a low surface coverage of physisorbed and acidic CO2, a significant fraction of HCO3– was deprotonated and transformed into CO32–. We expect that a similar chemisorption of CO2 would occur for low-silica zeolites and other basic silicates of interest for capture of CO2 from the gas mixtures.

High Water Tolerance of a Core-Shell-Structured Zeolite for CO2 Adsorptive Separation under Wet Conditions.

Dehumidification in CO2 adsorptive separation processes is an important issue, owing to its high energy consumption. However, available adsorbents such as low-silica zeolites show a significant decrease in CO2 adsorption capacity when water vapor is present. A core-shell-structured MFI-type zeolite with a hydrophilic ZSM-5 coated with a hydrophobic silicalite-1 shell layer was applied in CO2 adsorptive separation under wet conditions. This hybrid material demonstrated remarkably high water tolerance with stable CO2 adsorption performance without additional thermal treatment for regeneration, whereas a significant decrease in the CO2 adsorption amount because of water vapor was observed on the parent ZSM-5. The core-shell structure of zeolites with high pore volumes, such as LTA or CHA, could also be suitable candidates for high CO2 adsorption capacity and high water tolerance for practical applications.

Zeolite ‘adsorption’ capacities in aqueous acidic media; The role of acid choice and quantification method on ciprofloxacin removal

Zeolites prepared from coal fly ash were used for the ‘adsorptive’ removal of the antibiotic, ciprofloxacin from aqueous solutions. Synthesised zeolites and other standard zeolites were characterised using XRD, FTIR and SEM analysis, to determine their formation, crystalline phase, morphology and elemental composition. The effect of acid type used for pH adjustment on the adsorption behaviour of ciprofloxacin on zeolites was investigated. Residual ciprofloxacin solutions after contact with zeolites were comparatively monitored using UV–Vis spectrometric and LC-MS TOF/DAD chromatographic methods. Results obtained, indicated formation of pure phase low silica zeolites. Removal efficiency by the synthesised zeolites for ciprofloxacin was significantly higher in acetic acid (92–94%) compared to HCl (27–61%) at pH 3 emphasising a clear dependence of results on experimental conditions. At experimental pH, the zeolites underwent chemical changes that were linked to structural changes. The presence of organic substrate during the chemical changes may probably explain side reactions, which led to by-products of organic origin reported in this contribution. FTIR and SEM images are presented after adsorption and clearly showed changes in zeolite structures. A discuss on the use of conventional UV–Vis spectroscopic, and LC-MS-TOF/DAD chromatographic method for quantification of aqueous residual ciprofloxacin after the ‘adsorption’ process is also presented and highlights the shortcomings of the former method compared to the latter. The zeolites showed promise for aqueous ciprofloxacin remediation reducing chemical oxygen demand (COD) levels, with process mechanisms supporting chemical interactions with substrate in a manner similar to degradation reactions.

Pore Fluid Evolution Influenced by Volcanic Activities and Related Diagenetic Processes in a Rift Basin: Evidence from the Paleogene Medium-Deep Reservoirs of Huanghekou Sag, Bohai Bay Basin, China

Volcanic activities exert a significant influence on pore fluid property and related diagenetic processes that substantially controlled reservoirs quality. Analysis of Paleogene medium-deep sandstones on the Huanghekou Sag provides insight into relating the diagenetic processes to pore fluid property evolution influenced by volcanic activities. Three distinct types of pore fluids were identified on the basis of an integrated and systematic analysis including core and thin section observation, XRD, SEM, CL, and trace element. Alkaline aqueous medium environment occurred in E2s1+2 where volcanic activities have insignificant influence on pore fluids, evidenced by typical alkaline diagenetic events such as K-feldspar albitization, quartz dissolution, feldspar dissolution, and carbonate cementation. During the deposition of E3d3, influx of terrestrial freshwater and alteration of ferromagnesian-rich pore water result in the formation of mixing aqueous medium environment through volcanic eruption dormancy causing zeolite dissolution, clay mineral transformation, and K-feldspar albitization. Ferromagnesian-rich aqueous medium environment developed resulting from the intensive hydrolysis of the unstable ferromagnesian minerals formed due to intense volcanic activities during E3d1+2 and corresponding predominant diagenetic processes were characterized by the precipitation and dissolution of low-silica zeolites. Therefore, the differential properties of pore fluids caused various diagenetic processes controlling reservoir quality.

Synthesis of Zeolites from A Low-Quality Colombian Kaolin

AbstractAt present, no production of zeolites is ongoing in Colombia; thus, because of the high demand in the industrial sector, ~2500 tons is imported annually from other countries such as Cuba, Ecuador, Mexico, and the United States. In order minimize the need for these costly imports, the present study sought to evaluate the viability of producing low-silica zeolites through the hydrothermal synthesis of a Colombian kaolin, which contains quartz (40%) and iron-oxide impurities. The kaolin was subjected to a milling process to reduce the particle size to the order of 11 μm, and was heat treated to transform it to metakaolin. Optimization of the synthesis variables (Na2O/SiO2 and H2O/Al2O3 ratios, time, and temperature) was accomplished by applying an experimental design based on the ‘Response Surface Methodology’ technique. The degree of crystallinity and the cation exchange capacity (CEC) were used as response variables. The CEC was determined from the NTC 5167 standard. In addition, the mineralogical composition and the zeolite microstructure were evaluated using techniques such as scanning electron microscopy, X-ray diffraction, and solid state nuclear magnetic resonance spectroscopy. The results indicated that synthetic type A zeolites with a CEC value of 442 cmol(+)/kg can be obtained from the Colombian kaolin, with the following optimal processing conditions: Na2O/SiO2 molar ratio of 2.7, H2O/Al2O3 molar ratio of 150, temperature = 66°C, and processing time = 8 h. Note that this value (442 cmol(+)/kg) is greater than that reported for an imported commercial zeolite (408 cmol(+)/kg) of the same type, which is currently being used in industry in Colombia. The nationwide availability of the raw material and the quality of the final product present opportunities to make this material available to the Colombian market.

Brønsted sites and structural stabilization effect of acidic low-silica zeolite A prepared by partial ammonium exchange

It is demonstrated for low-silica zeolite 25NH4,Na-A (nSi/nAl = 1.0) with an ammonium exchange degree of 25% that the formation of acidic bridging OH groups (Si(OH)Al) after thermal deammoniation is accompanied by a weak dealumination of the framework. As the extra-framework aluminum species mainly exist predominantly in the form of Al3+ cations, they partially neutralize negative framework charges. The remaining framework charges are compensated by Na+ cations and hydroxyl protons of Si(OH)Al groups. Therefore, a lower number of Si(OH)Al groups is formed than expected by the stoichiometry of the ammonium exchange. For zeolite 25NH4,Na-A, only 17% of the introduced ammonium ions form Si(OH)Al groups after thermal deammoniation. A benefit of the cationic extra-framework aluminum species (Al3+) is their stabilizing effect exerted on the remaining framework aluminum atoms and the whole structure of the obtained zeolite 25H,Na-A. 1H MAS NMR signals of acidic OH groups were observed at 3.6 and 4.8 ppm and assigned to Si(OH)Al groups in the α- and β-cages, respectively. A low-field signal at 13.3 ppm is probably due to Si(OH)Al groups, however, involved in H-bondings (Si(OH⋯O)Al) with hydroxyl protons directed to very near oxygen atoms, such as expected in double 4-rings of the LTA structure.