Low-silica X Research Articles

Continuous flow synthesis of hierarchical low silica X zeolite

Zeolites, renowned for their versatile applications in catalysis, adsorption, and ion exchange, have long been synthesized using conventional batch processes. However, the inherent limitations of these methods, such as resource-intensive conditions and inconsistent product quality, underscore the need for a sustainable and efficient approach. In this study, a continuous flow synthesis process was established for the synthesis of industrially important low silica X (LSX) zeolite using a tubular reactor. The synthesis gel was subjected to aging for 5 days at room temperature to facilitate nucleation and crystal growth combined with the fast-heating rate in a tubular reactor at 363 K & 1.1 atm., which in turn produces LSX after 40 min. The synthesized product was confirmed by the XRD, FE-SEM, EDS, XRF, TEM, and N2 adsorption-desorption; the data was compared with the LSX sample synthesized by batch process. The result implies that LSX prepared by continuous flow has a pure phase of LSX with the hierarchical structure, which provides better adsorption capacity of CO2 at 298 K up to 20 bar. Due to continuous flow synthesis, the crystallization time was reduced and faster kinetics which may be helpful for scale-up the process for LSX synthesis.

Hyperfine couplings between the paramagnetic moment and nuclei in the metallic phase of low silica X zeolite loaded with potassium.

Temperature dependences of NMR spectra have been observed for 23Na and 27Al in the metallic phase of Na-K form low silica X (LSX) zeolite loaded with potassium, where the condition of saturation is achieved with a loading level of 9.0 atoms per supercage and the paramagnetic moment contributes to the magnetism of the system beyond simple isolated spin. Two separated peaks have been recognized for 23Na, where the shift values show a quite linear relationship with susceptibility, and the so-called K-χ plot works quite well to give values of 0.32 kOe μB-1 and 0.40 kOe μB-1 for hyperfine coupling constants. Although no separated peak is seen in the 27Al NMR spectrum, the spectral centroid deviates to the positive side. The shoulder of the spectrum scales to susceptibility and the K-χ plot also works well to give a value of 0.15 kOe μB-1 for the hyperfine coupling constant. The orbital of potassium-originated electrons confined in the cage of LSX is understood as seeping out over the framework of zeolite, which is wider than that of the sodium-originated case.

Crucial role of alkali metal ions and Si/Al ratio in selective adsorption of 1-octene using faujasite zeolites

Linear α-olefins (LAOs) are conventionally purified from paraffins via energy-intensive superfractionation. Adsorptive separation with zeolite-based adsorbents is a promising alternative to distillation for olefin/paraffin purification. However, very few zeolites with different Si/Al ratios and metal ion types have been tested to separate LAOs in the liquid phase. In this study, we investigated the ability of various alkali metal ion-exchanged faujasites with different Si/Al ratios to separate 1-octene/n-octane mixtures. We prepared low-silica X (LSX), X, and Y zeolites loaded with Li+, Na+, K+, and Rb+ via ion exchange in an aqueous solution. The 1-octene adsorption capacities and selectivities were analyzed via liquid-phase batch adsorption experiments. Among LSX, X, and Y exchanged with the Na+ and Li+, LSX which had the lowest Si/Al ratios exhibited the highest selectivity. The 1-octene selectivities for LSX were in the following order: Rb+ ≈ K+ < Na+ < Li+. LiLSX demonstrated the greatest separation efficiency among the zeolites owing to the presence of the largest number of cation sites and the highest charge density of Li+. The affinity constants calculated from the Langmuir-type adsorption isotherms and enthalpies of adsorption suggest that cation–π interactions between the C = C bond in olefins and metal ions influence selective adsorption. Density functional theory calculations support this theory of intermolecular interactions. Furthermore, a series of adsorption and desorption breakthrough experiments using a column packed with LiLSX validated its applicability for separating 1-octene and n-octane. We believe these adsorbents can be modified further and widely applied in the purification of higher olefins from chemical and biochemical products.

A short review on recent advances in porous adsorbents for separation of oxygen from atmospheric air

AbstractPure oxygen demand is continuously increasing worldwide due to wide applications including medical and industrial. Pressure swing adsorption (PSA) is one of the promising techniques to separate O2 from atmospheric air. The porous solid adsorbent is a key element in the PSA to trap nitrogen (N2) and release O2. Several adsorbents including low silica X (LSX), zeolite 5A, ion‐exchanged LSX (Li‐LSX, AgLi‐LSX, Ca‐LSX), Engelhard titanosilicate (Na‐ETS‐10, Ag‐ETS‐10), and SSZ‐13 have been investigated for adsorption of N2 from the air. This review article is a summarization of recent research work published on O2 separation using different porous adsorbents via PSA. This review also emphasizes on the best porous sorbent for purification of O2 by sorption of N2 and Ar. The adsorption capacities with experimental conditions are also rigorously discussed. The review also proposed future trends and prospects for O2 separation. This review will be helpful to choose remarkable adsorbent for the generation pure O2.

Electrochemical sensor based on low silica X zeolite modified carbon paste for carbaryl determination

A new and simple approach for carbaryl determination in natural sample was proposed using Low Silica X (LSX) zeolite modified carbon paste electrode. LSX zeolite with a porous structure was incorporated into carbon paste electrode in the appropriate portion. The prepared electrode was then characterized using scanning electron microscopy, cyclic voltammetry and electrochemical impedance spectroscopy. Various experimental parameters as the zeolite amounts, pH, accumulation time, and differential pulse voltammetric parameters were optimized. Under optimal conditions, a linear response was obtained in the range of 1–100µM of carbaryl using differential pulse voltammetry with detection limit of 0.3µM (S/N=3). The sensors showed good selectivity, stability, and reproducibility and has been successfully applied for detection of carbaryl in tomato samples with good recoveries.

Decomposition of the potassic rocks by sub‐molten salt method and synthesis of low silica X zeolite

AbstractIn this work, the potassic rock (PR), an ore rich in aluminosilicate and alkali metal sources, was decomposed to active amorphous material in mild condition for the first time via sub‐molten salt method. Low silica X (LSX) zeolite was successfully synthesized using the amorphous precursor from PRs as starting material. The effects of temperature, time, and mass concentration of alkali on the decomposition of PRs powder, and the effects of molar ratios of SiO2/Al2O3 and H2O/(Na2O + K2O) on the synthesis of LSX zeolite, were investigated. X‐ray powder diffraction, scanning electron microscopy, energy dispersive X‐ray spectrometer, and Fourier transform infrared spectroscopy were employed to characterize the precursors and as‐prepared zeolites. The results indicated that the highly active amorphous precursor can be obtained in (Na, K)OH sub‐molten salt medium with 80% of mass concentration of alkali at 190 °C for 3.0 h. The LSX zeolite can be synthesized in 5.85Na2O:1.65K2O:Al2O3:(2.1–2.3)SiO2:(113–125)H2O system using the slurry obtained from the decomposition of PRs. Copyright © 2016 Curtin University of Technology and John Wiley &amp; Sons, Ltd.

Ferromagnetism of Na–K Alloy Clusters Incorporated in Zeolite Low-Silica X

In a porous crystal of zeolite low-silica X (LSX), β-cages and supercages (cavities) are arrayed in a diamond structure, respectively. We loaded potassium metal into zeolite LSX which has a chemical formula of Na7.3K4.7Al12Si12O48 per β-cage (or supercage), and generated Na–K alloy clusters in β-cages and/or supercages. We have investigated the magnetic properties, the optical ones and the electrical resistivity at various values of K-loading density n per β-cage (or supercage) up to n = 9.7. Localized magnetic moments are observed at 8.2 < n < 9.7. Almost simultaneously, nearly pure ferromagnetism is observed at 8.4 < n < 9.7. The highest Curie temperature is ≈12 K at n ≈ 9. Optical reflection spectra for 8 < n have a new band at 2.8 eV which is assigned to the optical excitation of s-electrons of clusters generated at β-cages. The origin of the magnetic moments is assigned to these β-cage clusters because of the coincidence between the growths of the 2.8 eV band and localized magnetic moments. The origi...

An investigation into the crystallization of low-silica X zeolite

The crystallization of low-silica X (LSX) zeolite with FAU topology was examined under hydrothermal synthesis conditions. PXRD was employed to follow the evolution of the long-range ordering of the gel. Raman spectra provided information on various ring and cage species existing in the gel. 27Al and 29Si solid-state NMR spectroscopy was utilized to monitor the change in local environment of tetrahedral sites. The results indicate that an amorphous aluminosilicate phase was formed immediately upon mixing different reactive species. Hydrothermal treatment led to the formation of sodalite-cage like species and the species with larger cavities, joint four-member rings (4Rs) and branched 4Rs, which are the structural building units of the FAU framework. These units were assembled into the crystalline structure of LSX zeolite. 23Na and 39K solid-state NMR results show that the transformation process was accompanied by the changes of the local structure of hydrated Na+ and K+ ions. The two types of cations may work synergistically to template the crystallization of LSX zeolite.

Highly crystalline LSX zeolite derived from biosilica for copper adsorption: the green synthesis for environmental treatment

Template-free Low Silica X (LSX) zeolite is known as environmentally-friendly zeolite in the application of wastewater treatment. It is one of the most open structures of 12-membered ring pores and contains a high number of cationic exchanged sites. The synthesis of LSX using agricultural byproducts even leads to a green and sustainable approach. This work suggests the optimized preparation method of LSX using the silica source solely from rice husk. Highly crystalline LSX zeolite (nSi/nAl = 1.03) is obtained in pure phase with high specific surface area of 798 m2 g−1 and total pore volume of 0.29 cm3 g−1. The green and cost-effective as-synthesized LSX shows a good potential adsorbent for heavy metal of copper(II) ions adsorption in aqueous solution with outstanding uptake capacity of 2.82 mmol g−1 or 179 mg g−1.

Crystal Structures of Heavily Na-Loaded Low-Silica X (LSX) Zeolites in Insulating and Metallic States

Crystal structures of sodium clusters incorporated in LSX zeolite [Na12Al12Si12O48 (Z = 8)] with FAU-type topology are analyzed by powder X-ray diffraction. The two different Na-loaded samples, Na9.4/Na12-LSX and Na16.7/Na12-LSX, show insulating and metallic states, respectively. The insulating state is changed to the metallic state upon increasing the loading density of Na atoms. The two samples have the space group Fd3̅ attributed to cubic symmetry and lattice constants of a = 2.49646(3) nm for Na9.4/Na12-LSX and a = 2.495060(11) nm for Na16.7/Na12-LSX. In both samples, three cation sites at the centers of two types of 6-ring of the sod-cage and the centers of the d6r prism are fully occupied by Na ions. In addition, a large number of Na cations forming Na clusters are distributed in the cavity (supercage), and Na distribution inside the cavity is disorderly, which is represented by several independent sites with low site occupancy. Electron-density distribution analysis clearly reveals the covalent bond of Si(Al)–O and disordered Na distribution in the cavity. In addition, electron densities based on the orbital hybridization between a Na cation and framework oxygen atom are observed around the 12-ring. In Na16.7/Na12-LSX, the formation of the chain-like Na cation distribution, which connects between the second neighboring cavities, is represented. However, this Na–Na connectivity is not formed in Na9.4/Na12-LSX. It was identified that the total number of Na cations in the cavity in Na16.7/Na12-LSX is much larger than that in Na9.4/Na12-LSX. The cation sites can be classified into two types of character: (a) close to the inner wall of cavity and (b) around the center of cavity and/or at the center of 12-ring pore window. In the domain (b), the number of cations remarkably increases with the increase in n. This change must be strongly related to the insulator-to-metal transition.

In situ synthesis of low silica X zeolite on ceramic honeycombs for adsorption of heavy metals

In this study, a two-step method involving low-temperature aging and high-temperature crystallization was employed for the first time in the in situ hydrothermal synthesis of low silica X (LSX) zeolite on a ceramic honeycomb. The synthesized samples were characterized by X-ray diffraction, scanning electron microscopy and Fourier transform-infrared spectroscopy. The LSX-zeolite-ceramic composites exhibited uniform morphology and had a low silica-to–alumina (Si/Al) ratio of 1.05. The samples were crystallized twice in situ by hydrothermal treatment and the resulting composites exhibited excellent uptake of the heavy metal ion, Cd2+, from aqueous solution. Moreover, the composite material could be reused several times after regeneration, and after five cycles, the removal rate of heavy metal ions was still as high as 99 %.

Exotic magnetism of s-electron cluster arrays: Ferromagnetism, ferrimagnetism and antiferromagnetism

Alkali metal nanoclusters can be stabilized in the regular cages of zeolite crystals by the loading of guest alkali metals. Cages are connected by the sharing of windows of the framework, and arrayed in simple cubic, diamond and body centered cubic structures in zeolites A, X and sodalite, respectively. The s-electrons have the localized nature of nanoclusters with magnetic moments, and have mutual interactions through the windows of cages. They show exotic magnetism depending on the structure type of zeolites, the kind of alkali metals and the average loading density of alkali atoms per cage. In zeolite A, potassium clusters are formed in α-cages that have an inside diameter of 11 A. They exhibit ferromagnetic properties explained by the canted antiferromagnetism of the Mott insulator, where the 1p-like degenerate orbitals of clusters play an essential role in the magnetic properties. Na-K alloy clusters generated at supercages and β-cages of low-silica X (LSX) zeolite exhibit Neel’s N-type ferrimagnetism at specific loading densities of alkali metals. Alkali metal clusters in sodalite show the ideal Heisenberg antiferromagnetism of the Mott insulator.

Insulator-to-metal transition and magnetism of potassium metals loaded into regular cages of zeolite LSX

Zeolite LSX (low-silica X) crystals have an aluminosilicate framework with regular supercages and β-cages. They are arrayed in a double diamond structure. The loading density of guest K atoms per supercage (or β cage), n, can be controlled from 0 to ≈ 9. At n 6. These properties are explained by the polaron effect including the electron correlation. Ferrimagnetic properties are observed at n ≈ 9. A remarkable increase in the resistivity is observed at very low temperatures at n ≈ 9, and is discussed in terms of the hypothesis of a Kondo insulator.

NMR study of thermally activated paramagnetism in metallic low-silica X zeolite filled with sodium atoms

We report a ${}^{23}$Na and ${}^{27}$Al nuclear magnetic resonance (NMR) investigation of low-silica X (LSX) zeolite with chemical formula Na${}_{12}$Al${}_{12}$Si${}_{12}$O${}_{48}$ (Na${}_{12}$-LSX) loaded with $n$ additional guest sodium atoms. Na${}_{n}$/Na${}_{12}$-LSX exhibits an insulator-to-metal transition around $n=11.6$, which is accompanied by a significant enhancement of bulk magnetic susceptibility. Paramagnetic moments are in the metallic Na${}_{12}$/Na${}_{12}$-LSX thermally activated with an activation energy of around 0.1 eV. Simultaneously a new shifted component appears in the ${}^{23}$Na NMR, whose large and positive NMR shift scales with bulk magnetic susceptibility. Its spin-lattice relaxation rate $1/{T}_{1}$ is governed by the local-field fluctuations characterized by the same activation energy as obtained from the bulk magnetic susceptibility data. The time scale of these fluctuations is typical for atomic motions, which suggest strong electron-phonon coupling, a hallmark of polaron states. The insulator-to-metal transition in Na${}_{n}$/Na${}_{12}$-LSX is thus discussed within a polaron model.

Insulating state and metallic phase transition of heavily sodium-doped low-silica X (LSX) zeolites

Electrical resistivity was measured for Na-doped low-silica X (LSX) zeolite at various Na-loading densities. Many s-electrons are introduced into the regular array of zeolite nanocages by the loading of guest Na atoms. The average Na-loading density per unite cage, n, was changed from 0 to ≈12. Nonmagnetic and insulating properties are observed for n≲11. A drastic decrease in the resistivity is observed at n≈12, indicating that the insulating phase is changed to metallic one by the addition of less than one Na-atom per unite cage. At the same time, a remarkable increase in paramagnetic susceptibility is observed at room temperature. Insulating states are explained by spin-singlet clusters which are assigned to the self-trapped s-electron pairs (small bipolarons) due to the strong electron–phonon interaction against the Coulomb repulsive interaction between two s-electrons. Pairs of s-electrons occupy successively the quantum states of clusters in zeolite cages with increasing the Na-loading density. The metallic phase transition is explained by the formation of large polarons which are mobile over many cages through the zeolite windows. The increase in the paramagnetic susceptibility is explained by the thermal excitation of small polarons expected in the adiabatic potential.

Magnetic properties of Na-K clusters in low-silica X zeolite doped by pressure loading

Low-silica X (LSX) zeolite has the FAU-type framework structure constructed of supercages and β-cages with effective inside diameters of 13 and 7 Å, respectively. Respective cages are arrayed in a diamond structure. Na-K clusters are formed by the loading of guest potassium atoms nK into LSX, where the unloaded LSX has 4Na and 8K zeolite-cations per supercage (or β-cage) as given by Na4K8Al12Si12O48. N-type ferrimagnetism has been observed at ambient loading densities of K atoms, 6.5 < n < 8.5. Ferrimagnetism was not observed at the maximum loading density at ambient pressure, n ≈ 9. In the present study, we extended the loading density for 9 < n by the pressure-loading technique. Spontaneous magnetization is observed again below 10.5 and 9.1 K at the loading pressures ≈ 400 and ≈ 520 MPa, respectively. Respective Weiss temperatures are −23 and −32 K. These results indicate that the ferrimagnetism appears again at higher loading densities.

Insulating state of Na clusters and their metallic transition in low-silica X zeolite

Optical and magnetic properties are investigated for low-silica X (LSX) zeolite loaded with Na metals at various loading densities. In LSX, β-cages with the inside diameter of ∼7 Å are arrayed in a diamond structure and the supercages with that of ∼13 Å are formed among them. When the average number of guest Na atoms per β cage, n, is smaller than ∼2, optical reflectance shows a peak at ∼2.5 eV. This corresponds to the 1s–1p transition of the cluster formed in β cage. At n>2, this peak disappears and several strong peaks are seen at 1–3 eV. The oscillator strength increases with n. They can be attributed to surface-plasmon-like excitations of the Na clusters formed in supercages. At n≤10, clear absorption tails with energy gaps are observed at near IR region indicating insulating states. However, when n is increased up to ∼12, a clear Drude reflection suddenly appears in the IR region, indicating that an insulator-to-metal transition abruptly occurs. This dramatic change of the electronic state may be caused by the successive connection between adjacent clusters in supercages due to some special arrangement of Na + ions as well as the delocalized wave function of electrons at high Na-loading density.

Strong dependence of ferrimagnetic properties on Na concentration in Na–K alloy clusters incorporated in low-silica X zeolite

Na concentration ( x) dependence of ferrimagnetic properties is investigated for Na–K alloy clusters incorporated in low-silica X (LSX) zeolite. In the LSX zeolite, β-cages of inner diameter ≈7 Å are arranged in a diamond structure, and supercages of inner diameter ≈13 Å are formed among them. The used LSX zeolite contains xNa + and (12− x)K + cations per β-cage or supercage. Guest nK atoms are loaded into the zeolite, namely the loading density is given by n per β-cage. The samples at x=4 have been reported to show Néel’s N-type ferrimagnetism in the specific region of n. This ferrimagnetism is explained by the model of antiferromagnetic coupling between two non-equivalent magnetic sublattices of clusters, the ones in β-cages and the others in supercages. In the present study, the value of x is changed from 4 to 0. Ferrimagnetic properties are found to show strong x-dependence. A systematic increase in loading densities of ferrimagnetic region is clearly observed with decreasing x. A remarkable change in temperature dependence of spontaneous magnetization is observed depending on x. Na + cations are known to be mainly distributed in β-cages. Hence, the decrease in Na concentration is proposed to change the electronic potential depth for clusters in β-cage, which leads to important differences in the interaction between electrons localized in β-cages and those in supercages.

Synthesis of LSX zeolite by microwave heating

Low silica X (LSX) zeolite was synthesised by traditional and microwave heating. In both syntheses the X zeolite showed high crystallinity and purity, low silicon/aluminium molar ratio and a potassium content per unit cell of around 7 wt.%. Microwave heating decreased the time of synthesis respect to the traditional heating under the same conditions. The reduction of the synthesis time was due to uniformity of the heating stage by microwaves. Microwave heating decreased the nucleation and crystallisation time. Basic catalytic properties were checked in the toluene alkylation with methanol. There were no differences in the catalytic activity for toluene alkylation between both LSX zeolites.

Structural studies of hydrated germanium X-type zeolite via Rietveld analysis of synchrotron powder X-ray diffraction data

An aluminogermanate analog of zeolite X has been synthesized by solution reaction, and characterized by Rietveld analysis of synchrotron powder X-ray diffraction data. The hydrated sample has a chemical formula Na 96Al 96Ge 96O 384· wH 2O, and the material possesses an ordered arrangement of Al and Ge in the framework. A cell parameter of 25.589(1) Å confirms the expected cell expansion over Low Silica X (LSX), although a smaller average framework T–O–T bond angle indicates greater relative cell collapse than for LSX. The distribution of sodium cations in the non-framework sites in NaGeX is significantly different from that in LSX, and appears to deviate from the proposed relationship between Al content and cation siting for hydrated aluminosilicate faujasite-type zeolites.