Cations In Zeolites Research Articles

Removal of lead from aqueous solutions by using the natural and Fe(III)-modified zeolite

In the present study, the sorption of lead by the natural and Fe(III)-modified zeolite (clinoptilolite) is described. The characterization of the natural zeolite-rich rock and the Fe(III)-modified form was performed by chemical analysis, point of the zero charge (pHpzc), X-ray powder diffraction, applying the Rietveld/RIR method for the quantitative phase analysis, and scanning electron microscopy. The effects of sorbents dose and the initial lead concentrations on its sorption by two sorbents were investigated. For both sorbents, it was determined that at lower initial concentrations of lead, ion exchange of inorganic cations in zeolites with lead, together with uptake of hydrogen dominated, while at higher initial lead concentrations beside these processes, chemisorption of lead occurred. Significantly higher sorption of lead was achieved with Fe(III)-modified zeolite. From sorption isotherms, maximum sorbed amounts of lead, under the applied experimental conditions, were 66mg/g for the natural and 133mg/g for Fe(III)-modified zeolite. The best fit of experimental data was achieved with the Freundlich model (R2≥0.94).

Carbon monoxide adsorption on alkali-metal exchanged BEA zeolite: IR and thermodynamics study

Interaction of carbon monoxide with extra-framework Na + and K + cations of the BEA zeolite was investigated by FTIR spectroscopy carried out under various temperatures, which allowed to obtain the corresponding IR spectra and to determine isosteric enthalpies of individual adsorption complexes. Three distinct bands were observed in the spectra: (i) a red-shifted band at 2112 and 2120 cm −1 for Na- and K-BEA, respectively, assigned to O-down (isocarbonylic) complexes (ii) a blue-shifted band at 2180 and 2167 cm −1 for Na- and K-BEA, respectively, corresponding to C-down monocarbonyl complexes, and (iii) a band at the medium frequencies (2161 and 2149 cm −1 for Na- and K-BEA, respectively) ascribing to linearly bridged carbonyl complexes on dual cationic sites, where CO molecule interacts with two nearby cations via both carbon and oxygen atoms. Adsorption enthalpies of CO adsorbed on Na- and K-BEA zeolite were estimated by means of temperature and pressure dependent FTIR spectroscopy. The bridged carbonyl complexes on Na- and K-BEA were found to be about 4.2 and 2.7 kJ/mol, respectively, more stable than the appropriate C-down monocarbonyl complexes.

Grand canonical Monte Carlo simulation and volumetric equilibrium studies for adsorption of nitrogen, oxygen, and argon in cadmium (II) exchanged zeolite A

The adsorption of nitrogen, oxygen and argon has been studied in cadmium (II) ions exchanged zeolite A at 288.0 and 303.0 K. Experimentally measured adsorption isotherms are compared with theoretically calculated data using grand canonical Monte Carlo (GCMC) simulation. Nitrogen showed higher adsorption capacity and selectivity than oxygen and argon in these zeolite samples. The cadmium exchanged zeolite A showed increased adsorption capacity for nitrogen, oxygen, and argon with increase in cadmium (II) exchange levels. Isosteric heat of adsorption data showed stronger interactions of nitrogen molecules with cadmium (II) cations in zeolite samples. These observations have been explained in terms of higher electrostatic interaction of nitrogen with extra framework zeolite cations. The selectivity of oxygen over argon is explained in terms of its higher interaction of oxygen with cadmium exchanged zeolites than argon molecules. Heats of adsorption and adsorption isotherms were also calculated using grand canonical Monte Carlo simulation algorithm. Simulation studies expectedly show the proximity of nitrogen molecules to the locations of extra framework sodium and cadmium cations.

Oxide clusters as source of the third oxygen atom for the formation of carbonates in alkaline earth dehydrated zeolites

In our paper, we show that carbonates can be formed with almost no energetic barrier from CO 2 and metal-oxide binuclear MO X M species (M = Mg, Ca, Sr, Ba, with X = 1–4, depending on the cation) in alkaline earth zeolites, mordenite (MOR) and phillipsite (PHI), on the basis of quantum mechanical density functional theory (DFT) calculations at both isolated cluster and 3D periodic levels. The participation of MO X M species ( X = 1 and 3) explains the source of the third O atom in CO 3 species in dehydrated zeolites, on the basis of a good agreement between the calculated and experimental positions of the asymmetric and symmetric CO 3 vibration bands, of the ratio of their intensities, and of the weak dependence versus the cation and framework type. The reaction of formation of dimethylcarbonate from CaCO 3Ca in the 8-membered (8R) ring of MOR and methanol has also been considered, suggesting the carbonate activity as the source of CO 2 at elevated temperatures.

Copper-Silver Bimetallic System on Natural Clinoptilolite: Thermal Reduction of Cu2+ and Ag+ Exchanged

Copper-silver bimetallic system supported on natural clinoptilolite from Tasajeras deposit (Cuba) was studied. Bimetallic samples were prepared by simultaneous ion exchange, and reduced in a wide temperature range in a hydrogen flow. The main goal of the work was analysis of the mutual influence of both metals on their reduction process and the properties of the resultant particles. Analysis was done by combined use of XRD and UV-Vis spectroscopy. The reduction of Cu2+ and Ag+ cations shows existence of notable inter-influence between both cations during this process. The Cu2+ reduction is favored by the presence of Ag+, which should be related with the synergetic influence of silver cations and/or clusters formed on the first stages of reduction on Cu(2+)-framework interaction, facilitating the Cu2+ reduction even at low temperature (25 and 50 degrees C). The aggregation of the reduced highly dispersed species both for copper and silver is limited in this bimetallic system. The introduction of Ag+ as the second cation in the copper-exchanged zeolites favors the copper reduction at lower temperatures (25 and 50 degrees C), and appears to be the efficient tool for the control of the size of the resultant reduced nanoparticles (it means their dispersion).

Comparative Study of Formation and Stabilization of Gold and Silver Clusters and Nanoparticles in Mordenites

Supporting silver and gold on mordenites by ion-exchange method with further reduction with H2 leads to formation of neutral and charged metal clusters inside zeolite channels as well as metal nanoparticles on external surface of mordenite. A portion of the cluster states of the metals and stability of the clusters depend strongly on acidity of zeolite (determined by SiO2/Al2O3 molar ratio) and nature of zeolite cation (H+, Na+, NH4+). The investigations of silver and gold samples after prolonged storage for 6 and 12 months revealed that silver clusters are comparatively stable while oxidation of gold clusters and nanoparticles by air is the probable cause of deactivation of gold catalysts. The comparison of the results for Au and Ag samples allow suggesting NaM15 and NaM24 mordenites for effective synthesis of complex Au-Ag clusters as active and stable species of catalytic reactions occurring at room temperature.

Structural Changes in Nanoporous MFI Zeolites Induced by Tetrachloroethene Adsorption: A Joint Experimental and Simulation Study

A joint experimental and molecular simulation study was performed to investigate the adsorption of tetrachloroethene on two MFI zeolites, ZSM-5 with a Si/Al ratio of 26.5, and silicalite-1, which is the pure silica form of ZSM-5. Adsorption isotherms and isosteric heats of adsorption of tetrachloroethene were measured and compared to molecular simulation results. Experimental curves for both MFI zeolites show a step at loading of 4 molecules·uc−1 that was interpreted in terms of a structural change of the host framework. A thermodynamic analysis based on the osmotic ensemble scheme allowed attributing this step to a symmetry change from ORTHO (orthorhombic form with Pnma symmetry) to PARA (orthorhombic form with P212121 symmetry) of both adsorbents upon micropore filling. The adsorption mechanism was also characterized. The first tetrachloroethene molecules adsorbs in the channel intersections of the MONO (monoclinic form with P21/n11 symmetry) and ORTHO structures of silicalite-1 and ZSM-5, respectively. Then, the above-mentioned structural change from ORTHO to PARA occurs, leading to a filling of sinusoidal channels until saturation was reached, i.e. ∼8 molecules.uc-1 for silicalite-1 and 6 molecules·uc−1 for ZSM-5. In addition, simulation results indicate that the sodium cations in the ZSM-5 zeolite are located at the channel intersections during the whole adsorption process.

Inception and Trapping of ZnO Nanoparticles within Desilicated Mordenite and ZSM‐5 Zeolites

AbstractZnO nanoparticle inception within mordenite and ZSM‐5 zeolites, of varied Si/Al ratios, was achieved via the exchange of zeolite cations with Zn2+, followed by NaOH leaching, and thermal treatment at 550 °C under O2. ZnO particles were observed by electron microscopy; further characterization included X‐ray diffraction, thermogravimetric analysis, and N2 sorption. NaOH addition caused a partial desilication of mordenite and ZSM‐5 zeolites as shown by a decreased Si/Al ratio. The bulk and surface Zn and Al concentrations were measured by ICP‐AE spectrometry and XPS, respectively. N2 sorption isotherms on ZnO/zeolite specimens indicated the existence of micropores and mesopores; the amounts of them in mordenite and ZSM‐5 zeolites were determined from adsorption data at low pressures. The progressive growth of ZnO nanoparticles induced some micropore widening as well as the inception of a slit‐like intrazeolite mesoporosity due to the desilication of the substrate during the NaOH leaching process. The final zeolite products are potentially useful as they are endowed with a twofold micro‐mesoporosity.

Difference in the catalytic activity of transition metals and their cations loaded in zeolite Y for ethanol steam reforming

The catalytic activity for ethanol steam reforming of Na-Y zeolite supporting transition metals was compared with that of zeolite Y encapsulating transition metal cations. It was found that zeolite Y supporting Co or Ni, prepared by the incipient wetness impregnation process, produced mainly hydrogen at 300 °C. Acetaldehyde products indicated that hydrogen production was due to the dehydrogenation of ethanol molecules. On the other hand, for zeolite Y encapsulating cations such as Co 2+ or Ni 2+, prepared by an ion exchange process, the reforming reaction resulted in mainly ethylene production, which was considered to be due to the dehydration of ethanol molecules. The results indicate that the presence of cations in zeolite Y has a significant influence on the ethanol reforming reaction.

ChemInform Abstract: Side-Chain Alkylation of p-Xylene with Methanol on Alkali Cation Zeolites.

Abstract The side-chain alkylation of p -xylene with methanol has been studied on alkali cation exchanged and impregnated zeolites. The results show that the activities and the selectivities for p -ethyltoluene and p -methylstyrene formation are correlated with the acid—base properties of the catalysts which are characterized by poisoning with phenol and infrared (IR) spectra of adsorbed acetic acid or pyridine. IR spectra show that there are absorption bands of acetate on all used alkali cation zeolites and pyridine adsorbed sites on LiX and NaX zeolites. It is also shown that the activities of the reaction increase when the catalysts are modified by copper or silver oxide. ESCA (electron spectroscopy for chemical analysis) results indicate that, under the reaction conditions, copper or silver oxide is present in its reduced state which can catalyze the dehydrogenation of methanol to formaldehyde, and give a promoting role for the side-chain alkylation.

Locating Extra-Framework Cations in Low-Silica Zeolites by a Combinatorial Approach of the Direct Space Method and Rietveld Refinement: Application to Ni2+ and Co2+ Enriched Clinoptilolite

The location of extra-framework cations in low-silica zeolites is determined from in-house X-ray powder diffraction pattern by a successful implementation of a newly developed methodology. The method combines reciprocal and direct space methods plus a cost function that accounts for simultaneous fit of the chemical composition and the X-ray diffractogram. We demonstrated that the iterative combination of relaxation methods (Monte Carlo exploration and lattice energy minimization) helps to improve the structural refinement. The methodology is successfully applied to the study of natural clinoptilolite samples enriched with Ni and Co; in both cases, two different cation sites were found octahedrally coordinated to water molecules. The most populated site is located in the center of the A channel, while the second one is found in the window of the B channel. This result was validated using XANES and EXAFS spectroscopies.

Quantification of cation-exchanged zeolites by XPS and EDS: A comparative study

Iron(II) and cesium are intercalated into zeolite L by wet impregnation. The elemental compositions of the cation-exchanged zeolite L were quantified for the first time by two different methods. X-ray Photoelectron Spectroscopy (XPS) and Energy Dispersive X-ray Spectroscopy (EDS) techniques were found to be suitable for the quantification, giving reliable results. Comparing the two different methods we found a small systematic difference in the amount of extraframework cations in zeolite L. These strategies could be applied for an easy and fast quantification of cations inside porous materials.

Sorption of nitrogen, oxygen, and argon in Cd (II) exchanged zeolite X: volumetric equilibrium adsorption and grand canonical Monte Carlo study

The adsorption of nitrogen, oxygen and argon has been studied in cadmium (II) cations exchanged zeolite X at 288.2 and 303.2 K. Experimentally measured adsorption isotherms are compared with theoretically calculated data using grand canonical Monte Carlo (GCMC) simulation. Nitrogen showed higher adsorption capacity and selectivity than oxygen and argon in these zeolite samples. The cadmium exchanged zeolite X was showed that increased adsorption capacity for nitrogen, oxygen, and argon with increase in Cd (II)-exchange levels, indicating as charge density increases adsorption capacity also increase. Isosteric heat of adsorption data showed stronger interactions of nitrogen molecules with cadmium cations in zeolite samples. These observations have been explained in terms of higher electrostatic interaction of nitrogen with extra framework zeolite cations. The selectivity of oxygen over argon is explained in terms of its higher interaction with Cd (II)-exchanged zeolites than argon molecules. The selectivity of N2/O2 of cadmium-exchanged zeolite X is better than only sodium containing zeolite-X. Heats of adsorption and adsorption isotherms were also calculated using GCMC simulation algorithm. Simulation studies expectedly show the proximity of nitrogen molecules to the locations of extra framework sodium and cadmium cations.

Interaction of Hydrogen with Extraframework Cations in Zeolite Hosts Probed by Inelastic Neutron Scattering Spectroscopy

The hindered rotations of molecular hydrogen adsorbed at low loadings into a number of partially ion-exchanged zeolites A, Y and X have been studied at low temperatures with the use of inelastic neutron scattering (INS) techniques. The factors that determine the sorption sites and strength of the interaction with the host material are found to be a complex combination of the type, charge and size of the cations, their coordination to the host framework, and accessibility to the hydrogen molecule as well as the relative acidity of the framework, and lead to important criteria for the development of more effective hybrid materials for hydrogen storage. The highest barriers to rotation were found for the undercoordinated, exposed Li+ cations in LiA and in LiX. Interaction with the extra framework Cu2+ and Zn2+ cations in zeolite A is found to be noticeably stronger than with the neutral Zn- or Cu- containing clusters in metal-organic framework compounds. Our observation that binding of hydrogen in these charged frameworks is strongly enhanced relative to those that are neutral suggests an important approach to improvement of porous materials as ambient temperature hydrogen storage media.

Coligomerization of Styrene and a-Methylstyrene Catalyzed by y Zeolites

It is ascertained that during the interaction of styrene and a-methylstyrene in the presence of cation and cation-decationated forms of zeolite Y the activity of zeolite catalysts increases in the following order: NiNaY CaNaY <LaNaY 0.5 NY <La Y <NiHY 0.96 Y. The product of reaction in the presence of cation forms of zeolite (NiNaY, CaNaY, LaNaY) is a mixture of low-molecular (n = 2-4) and high-molecular oligomers (n = 14). Oligomers with the degree of oligomerization 2-8 are formed in the presence of other zeolite samples. Zeolites 0,96 Y and NiHY allow to receive gomo - and codimers with selectivity 68-80%. The main product of codimerization is cyclic dimer 1,1 – dimethyl – 3-phenylindane.

Silicoaluminophosphate Molecular Sieves STA-7 and STA-14 and Their Structure-Dependent Catalytic Performance in the Conversion of Methanol to Olefins

Details of the synthesis of the small-pore silicoaluminophosphate (SAPO) molecular sieves STA-7 (SAV) and STA-14 (KFI) prepared via a co-templating approach are described. STA-7 includes Si in the framework with Si/(Si+Al+P) ratios varying from 0.04 to 0.17, whereas STA-14 crystallizes from gels with a narrower compositional range (Si/(Si+Al+P) = 0.2−0.3). The main route to Si incorporation is by substitution for P, but in the presence of fluoride, aluminosilicate regions are formed in STA-7. 27Al 3Q MAS NMR studies enable resolution of tetrahedral Al in Al(OP)4 and Al(OP3,OSi) environments in both as-prepared and calcined materials. For STA-7 with low Si content the presence of five- and six-fold Al coordinated with water molecules or fluoride or hydroxide ions has also been confirmed. Upon calcination, all Al adopts tetrahedral coordination. High pressure CO2, CO, and CH4 adsorption at 100 °C indicates that the proton forms of the SAPO D6R zeotypes present effective polarities intermediate between cationic zeolites and pure silica chabazite. The methanol-to-olefin (MTO) performance of the SAPOs H-SAPO-34, H-STA-7, and H-STA-14 was found to depend on both crystallite size and cage connectivity and topology. H-STA-7 with a crystal size of 2−3 μm has MTO stability comparable to that observed for H-SAPO-34, whereas the same materials with larger crystal sizes (≥10 μm) deactivate rapidly. Ex situ GC-MS analyses of the SAPO catalysts after MTO reaction demonstrate that the uniformity in cage shape and size in cage-based, small-pore molecular sieves is a critical factor governing the type of the accumulated aromatic hydrocarbons and, hence, their MTO activity and deactivation behavior.

Isotopic Distribution Coefficient of Tritiated Water Adsorbed on Faujasite-Type Zeolite

The fixed-bed adsorption processusing zeolite has been studied as one of the promising processes for the recovery of tritiated water in the field of nuclear fusion. In H2O-HTO binary systems, the difference in adsorption rate must be considered to obtain the isotopic distribution coefficient of tritiated water adsorbed on zeolite. The static method is probably the best approach to measure the distribution coefficient considering the difference in adsorption rate. In this study, the influence of framework SiO2/Al2O3 ratio ranging from 2.0 to 10.0 and of cations (K+, Na+, Ca2+) on isotopic distribution coefficient of tritiated water adsorbed on faujasite-type zeolite was systematically examined. The isotopic distribution coefficient of tritiated water between water adsorbed on zeolite absorbent was 1.1˜1.4. The effect of framework SiO2 to Al2O3 ratio of the X/Y type zeolite on the isotopic distribution coefficient depended on the content of cation in zeolite. The framework SiO2 to Al2O3 ratio of the NaX/NaY zeolite had negligible effect on the isotopic distribution coefficient. The considerable effect of framework SiO2 to Al2O3 ratio was found in the case of KX/KY and CaX/CaY zeolite.

Femtosecond Dynamics Within Nanotubes and Nanocavities of Mesoporous and Zeolite Materials

The steady-state, picosecond and femtosecond time-resolved emission of Sudan I within MX (M = Na + , Li + Mg 2+ ), NaY zeolites, and MCM-41 mesoporous silica have been studied in the presence of solvents. The results show that the rate constant of the initial excited-state intramolecular proton-transfer (ESIPT) reactior in the trans-azoenol (trans-AZO) structure to give the trans-ketohydrazone (trans-HYZ) form of Sudan remains ultrafast (τ < 50 fs) as in pure solution. However, within these nanohosts, the time necessary for a trans—cis isomerization in the formed trans-HYZ tautomer to yield the cis-HYZ structure increases. The largest effect was obtained when the MX hosts contained polar and H-bonding solvents (water, methanol) The result is explained in terms of a stronger solvation of the metal cation enhancing its shielding and therefore lowering its interaction with the included dye. The use of different metal cations of MX zeolite shows thai nanosolvation of cations (M + ) increases the measured times for isomerization and relaxation of the excitec structures of Sudan I. Furthermore, the observation of similar times for NaX, NaY, and MCM-41 (hosts different in polarity and diameter of the pore/cavity) suggests that that the trans—cis isomerization occurs by an inversion mechanism. The slowing of the dynamics within the nanopores is reminiscent of that occurring in biological media, such as within proteins, and it should be relevant to photonic devices using this kind of nanomaterials.

Accurate gas – Zeolite interaction measurements by using high pressure gravimetric volumetric adsorption method

In this work, we discuss the purification of hydrogen by physical adsorption on zeolites Li–LSX exchanged 78%, 83% and 99%. A newly developed adsorption device is applied to the gas–solid adsorption measurements. Isotherms of hydrogen adsorption are gravimetric volumetrically measured at 293.15 K up to 5 MPa. The accuracy of this new device is compared to NIST gas density data's of hydrogen and nitrogen at 293.15 K. Further the real density of the zeolites is obtained by helium skeleton density measurements at high temperature (650 K). The paper provides an interpretation of hydrogen adsorption capacities according to the gas-surface interaction. Further the isosteric heat of adsorption is obtained for the studied materials and analysed in relation to the zeolite cations exchange rate. Moreover, we discuss the influence of the ratio of cation exchange on hydrogen gas adsorption.

Direct synthesis of N-acylpyrrolidines from tetrahydrofuran and nitriles of aliphatic and aromatic acids on zeolite catalysts under supercritical conditions

The tetrahydrofuran-nitrile (CH3CN, n-C4H9CN, C6H5CN)-zeolite (faujasites, mordenite, beta, pentasils) system and its transformations under supercritical conditions were studied. The feasibility of direct synthesis of the corresponding N-acylpyrrolidines from THF and nitriles of aliphatic and aromatic acids was shown for the first time. The dependence of the THF reaction with nitriles on the temperature, the reaction time, the nature of the exchanged cations in zeolite, the type of zeolite, and the catalyst pretreatment conditions was revealed. Acid zeolites were shown to exhibit a high catalytic activity. The yield of N-acylpyrrolidines at 350°C in the presence of the most active catalysts (HNaY and HCaLnY) was 10–40% at a selectivity of up to 80%. The key role of H2O was found, and a two-step scheme of the process, including the hydrolysis of nitrile to amide and the reaction of the latter with tetrahydrofuran resulting in the formation of N-acylpyrrolidine, was proposed.