Cation-exchanged Zeolites Research Articles

Thermodynamics of Hydrogen Adsorption on Metal‐Organic Frameworks

Interaction between adsorbed hydrogen and the coordinatively unsaturated Mg(2+) and Co(2+) cationic centres in Mg-MOF-74 and Co-MOF-74, respectively, was studied by means of variable-temperature infrared (VTIR) spectroscopy. Perturbation of the H(2) molecule by the cationic adsorbing centre renders the H--H stretching mode IR-active at 4088 and 4043 cm(-1) for Mg-MOF-74 and Co-MOF-74, respectively. Simultaneous measurement of integrated IR absorbance and hydrogen equilibrium pressure for spectra taken over the temperature range of 79-95 K allowed standard adsorption enthalpy and entropy to be determined. Mg-MOF-74 showed ΔH(0)=-9.4 kJ mol(-1) and ΔS(0)=-120 J mol(-1) K(-1), whereas for Co-MOF-74 the corresponding values of ΔH(0)=-11.2 kJ mol(-1) and ΔS(0)=-130 J mol(-1) K(-1) were obtained. The observed positive correlation between standard adsorption enthalpy and entropy is discussed in the broader context of corresponding data for hydrogen adsorption on cation-exchanged zeolites, with a focus on the resulting implications for hydrogen storage and delivering.

Selective Adsorption of Oxygen over Argon in Alkaline-Earth-Metal Cation-Exchanged Zeolite X

The separation of argon and oxygen from their gaseous mixture by adsorption is challenging because of the closeness of their molecular properties and, hence, adsorption behavior. In the present study, the potential of zeolite X with strontium as the extraframework cation as an oxygen-selective adsorbent for the separation and purification of argon is discussed. Equilibrium adsorption isotherms of oxygen and argon on calcium, strontium, and barium cation-exchanged zeolite X were measured at 288 and 303 K. The equilibrium adsorption capacity and selectivity for oxygen over argon were observed to be higher in zeolite X exchanged with alkaline-earth-metal cations than in NaX. Among these alkaline-earth-metal cation-exchanged zeolite X adsorbents, SrX showed the highest adsorption selectivity, in the range of 2.0 at 50 mmHg to 1.8 at 760 mmHg for oxygen over argon at 303 K. The increased selectivity for oxygen over argon in SrX is discussed in terms of the size, location, and effective partial charges on the e...

Human detoxification of perfluorinated compounds

There has been no proven method thus far to accelerate the clearance of potentially toxic perfluorinated compounds (PFCs) in humans. PFCs are a family of commonly used synthetic compounds with many applications, including repelling oil and stains on furniture, clothing, carpets and food packaging, as well as in the manufacturing of polytetrafluoroethylene - a non-stick surfacing often used in cookware (e.g. Teflon(r)). Some PFCs remain persistent within the environment due to their inherent chemical stability, and are very slowly eliminated from the human body due, in part, to enterohepatic recirculation. Exposure to PFCs is widespread and some subpopulations, living in proximity to or working in fluorochemical manufacturing plants, are highly contaminated. PFC bioaccumulation has become an increasing public health concern as emerging evidence suggests reproductive toxicity, neurotoxicity and hepatotoxicity, and some PFCs are considered to be likely human carcinogens. A case history is presented where an individual with high concentrations of PFCs in serum provided: (1) sweat samples after use of a sauna; and (2) stool samples before and after oral administration of each of two bile acid sequestrants - cholestyramine (CSM) and saponin compounds (SPCs). Stool samples before and after use of a cation-exchange zeolite compound were also examined. PFCs found in serum were not detected in substantial quantities in sweat or in stool prior to treatment. Minimal amounts of perfluorooctanoic acid, but no other PFCs, were detected in stool after SPC use; minimal amounts of perfluorooctanesulfonate, but no other PFCs, were detected in stool after zeolite use. All PFC congeners found in serum were detected in stool after CSM use. Serum levels of all PFCs subsequently declined after regular use of CSM. Further study is required but this report suggests that CSM therapy may facilitate gastrointestinal elimination of some PFCs from the human body.

Interaction of Dimethylmethylphosphonate with Zeolite Y: Impedance-Based Sensor for Detecting Nerve Agent Simulants

Dimethylmethylphosphonate (DMMP) is a simulant for the highly toxic organophosphate nerve agent Sarin (GB). The influence of DMMP on the ionic conductivity of zeolite Y is investigated by impedance spectroscopy. In the presence of 20−100 ppm of DMMP, the ionic conductivity of the sodium exchanged form of the zeolite showed an increase. The interaction between DMMP and the zeolite was elucidated by examining different cation-exchanged zeolites as well as external surface modification with ceria to deactivate acidic groups. A mechanism involving the binding of the sodium cation with the phosphonate group of DMMP that results in facilitated inter cage motion of the cation is proposed. The change in impedance measured at a single frequency of 3000 Hz allows for the use of Na+-exchanged zeolite Y as a sensor for detecting ppm level of DMMP. Sensing data were obtained over a temperature range of 300−350 °C, with best results at 320 °C. This device exhibited minimal response to CO, NH3, methane, and propane, pos...

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.

Catalytic conversion of chloromethane to methanol and dimethyl ether over metal-exchanged zeolite Y

The catalytic hydrolysis of chloromethane to methanol and dimethyl ether was studied over metal-exchanged zeolite Y (NaY, KY, CsY, MgY, CuY and FeY). All catalysts showed higher conversions than the blank reaction in the temperature range between 453 and 673 K. Methanol was the major product on all catalysts, with selectivity ranging from 76 to 95%. Dimethyl ether (DME) was the other major product formed with selectivity of 5–7% over alkaline cation-exchanged zeolite Y and 10–24% over the zeolites exchanged with polyvalent cations. The higher selectivity to DME over MgY, CuY and FeY is associated with the presence of stronger acid sites with respect to NaY. Theoretical calculations indicated that a mechanistic pathway involving methanolysis of an adsorbed methoxy group may account for the formation of dimethyl ether on alkaline cation-exchanged zeolites.

Molecular recognition in cation‐exchanged zeolites

AbstractThe concepts of confinement‐ and molecular recognition‐driven chemical reactivity of cation‐exchanged zeolites is illustrated by our recent results from periodic and cluster density functional theory (DFT) calculations. The reactivity of alkali‐earth‐ and alkaline‐exchanged low‐silica zeolites for selective photo‐oxidation of alkenes with molecular oxygen and for N2O4 disproportionation is shown to be mainly due to the specific arrangement and the size of the cations in the zeolite cage. An attempt is made to separate the effects of basicity of the framework, the Lewis acidity of the extra‐framework cations and the electrostatic field in the zeolite cage as well as its geometrical properties for the respective reactions. The importance of the favorable adsorption fashion of the reagents controlled by noncovalent interactions with the microporous matrix is shown. The role of the weak interactions with the zeolite walls and the factors, which determine the preference for a particular adsorption mode, are discussed by the example of light alkanes adsorption to Mg‐ and Ca‐exchanged faujasites. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010

Cryoadsorption of hydrogen on divalent cation-exchanged X-zeolites

The cryoadsorption of hydrogen on divalent cation-exchanged zeolites and its relation with exchanged ions is investigated by volume measurements performed at near critical temperatures of 140–35 K and pressures up to 7 MPa. The hydrogen uptakes of the zeolites follow an order of NaX > MgX > ZnX > NiX, as same as that of the stability of the ions, but opposite to the hydration tendency of the ions and the isosteric heat of the adsorption of the zeolites, which indicate that the redox potential of the ions in the zeolite is a dominant factor in hydrogen adsorption. The drop of excess adsorption to negative amount and the following recovery at near critical temperature and pressure are explained by a reversed effect of capillary condensation and the bias of the compressibility factor from the assumed equation of state.

Characterization of vacant coordination sites of cations on the surfaces of oxides and zeolites using infrared spectroscopy of adsorbed probe molecules

A critical review is given of the application of IR spectroscopy of small adsorbed probe molecules such as CO and N 2 for the determination of the coordination state of surface cations on oxides and zeolites. Special emphasis is devoted to the different kinds of geminal species. So-called complex-specified geminal species are formed as a result of stabilization of the structure by different factors (stable electron configuration, strong ligand–ligand interaction, etc.). During the decomposition of these species linear intermediates are not produced. When complex-specified geminal species are formed, direct conclusions on the original coordination state of the coordinatively unsaturated adsorption site can not be drawn. Site-specified geminal species are produced as a result of low coordination of the adsorption site. These species loose their ligands stepwise and show a tendency to form mixed-ligand species. Site-specified geminal complexes are typically formed with cation-exchanged zeolites, particularly with bulky cations having large radii. Analysis of adsorption isotherms of surface complexes bearing either one or more adsorbate molecules is performed and practical hints for their use are proposed. Finally, possible future directions of the research on this topic are summarized.

Acetoxylation and hydration of limonene and α-pinene using cation-exchanged zeolite beta

Hydration and acetoxylation of limonene and α-pinene into terpineol and terpinyl acetate in the liquid phase have been studied using transition metal and rare earth ion-exchanged beta zeolite. These catalysts under optimized reaction conditions showed higher activity and selectivity compared to conventionally used acid catalysts such as H 2SO 4 and amberlyst-15. Conversions of 9–26% and 58–82% were obtained for limonene the in presence of glacial and aqueous acetic acid, respectively, and the selectivity for major products α-terpinyl acetate and terpineol were up to 54% and 65%, respectively. Conversion values in the range of 62–100% and 72–100% were obtained for α-pinene in the presence of glacial and aqueous acetic acid, respectively. Virtually no oligomerisation of monoterpenes occurred under studied conditions. From the measured acidity data of these zeolites, it is observed that both hydration and acetoxylation are Brönsted acid-catalysed reactions.

Density functional studies on Lewis acidity of alkaline earth metal exchanged faujasite zeolite

Lewis acidity of alkaline earth metal exchanged faujasite zeolite is investigated using density functional based reactivity descriptors with a 6T ring cluster which represents active site of the zeolite. We calculated global reactivity descriptors such as chemical hardness, chemical softness, electrophilicity index of the optimised cluster and local reactivity descriptors namely fukui function and relative electrophilicity of the exchanged cations at PWC/DNP (Double numerical with polarisation) and VWN-BP/DNP levels of theory. The electrophilicity index of the modelled clusters and relative electrophilicity values of the exchanged cations show that Lewis acidity of alkaline earth metal cation-exchanged zeolites decreases in the order: Mg2+>Ca2+>Sr2+>Ba2+, which is in agreement with the available experimental results.

Sorption of N2, O2, and Ar in Mn(II)-Exchanged Zeolites A and X Using Volumetric Measurements and Grand Canonical Monte Carlo Simulation

The adsorption of nitrogen, oxygen, and argon has been studied in Mn(II)-cation-exchanged zeolite A and X powders and pellets at 288.2 and 303 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. Mn(II)-exchanged zeolite A (αO2/Ar = 1.2−1.4) and X (αO2/Ar = 1.4−1.8) showed small selectivity for oxygen over argon. In zeolite A, the adsorption capacity for nitrogen, oxygen, and argon increases with the increase in Mn(II) exchange levels. In the case of zeolite X, the adsorption capacity for nitrogen increases at lower adsorption pressures upon Mn(II) cation exchange, but at higher equilibrium pressures, the nitrogen adsorption capacity is slightly lower than that of NaX in the granular form. However, Mn(II)-exchanged zeolite X powder showed increased adsorption capacity for nitrogen, oxygen, and argon with i...

Oxygen Basicity in Alkaline Cation-Exchanged Zeolite and the Effect of Isomorphous Substitution. Use of Hard Descriptors

Density function theory calculations were carried out on 14T clusters wherein one of the 14 Si atoms was substituted by either B, Al, or Ga. The clusters were neutralized by one alkaline cation. Two accessible oxygen atoms, O1 and O4, bonded to the substituted atom are considered for their basic properties. The computed proton affinities and NO+ binding energies show the same trends as experimentally observed. These data point out that the O4 oxygen is the most basic for the B substitution and O1 is the most basic for the Al and Ga substitutions. In view of the hard character of the oxygen atom, hard descriptors are computed. Among them, the charge neither reproduces correctly the effect of the alkaline cation nor the effect of the isomorphous substitution. The local hardness reproduces well the isomorphous substitution but fails to predict the most basic site or the effect of the alkaline cation. The molecular electrostatic potential appears to be the best hard descriptor. It predicts well the effect of the monovalent cation as well as that of the isomorphous substitution. It is also able to predict which site is the most basic for a given substitution: O4 for B substitution and O1 for Al and Ga substitutions. In comparison with previous studies using soft indices to describe the oxygen basicity in zeolites, the molecular electrostatic potential is found to be a reliable and rigorous descriptor that can be used to predict both Lewis basicity and Brønsted−Lowry basicity.

Formation of surface magnetite nanoparticles from iron-exchanged zeolite using microwave radiation

Microwave radiation simplifies synthesis methods by reducing reaction times, requiring fewer materials, and also controlling reaction processes. We have successfully synthesized nanoparticles of iron oxide and zinc oxide coated on zeolite A using microwaves. The radiation assisted in displacing either ferrous or zinc ions from the pre-loaded zeolite network and increasing reaction speed with solution at the interface. Products were characterized by TEM, XRD, VSM, ICP-AES, and fluorescence. We demonstrate the ability of using cation-exchanged zeolites as microreactors to bias reactions onto the zeolite surface. Efficient structure-directed surface reactions are a potential route to making unique supported nanomaterials for applications such as sensors, environmental remediation, and chemical catalysis.

Catalytic epoxidation of α-pinene with molecular oxygen using cobalt(II)-exchanged zeolite Y-based heterogeneous catalysts

Cobalt cation-exchanged zeolite Y-based catalysts were studied for epoxidation of α-pinene with molecular oxygen in 20–100 psi pressure range using N, N-dimethylformamide (DMF) as a solvent at 373 K. The catalysts characterization with X-ray diffraction (XRD), diffuse reflectance spectroscopy (DRS) suggests that cobalt introduced into faujasite zeolite exists in Co(II) state and catalyzes the epoxidation of α-pinene with molecular oxygen. The alkali and alkaline earth metal co-exchanged with Co(II) gives higher conversion than Co(II) alone. Effect of temperature and pressure was also studied to determine the optimum conditions for the reaction. The best results were obtained using NaCsCoY20 with 47% α-pinene conversion and 61% epoxide selectivity at 80 psi pressure and 373 K. Solvent was observed to play important role in epoxidation of α-pinene, and best results were observed in N, N-dimethylformamide as a solvent. Free radical mechanism has been proposed for the epoxidation of α-pinene.

Generation and Reactivity of Simple Chloro(aryl)carbenes within the Cavities of Nonacidic Zeolites

A number of para-substituted chloro(aryl)carbenes are generated within the cavities of a series of dry alkali metal cation-exchanged zeolites (LiY, NaY, KY, RbY, and CsY) upon laser flash photolysis of the corresponding diazirine precursor. The absolute reactivity of the chloro(aryl)carbene is found to be strongly dependent on both the nature of the electron-donating and -withdrawing properties of the aryl substituent and the nature of the zeolite charge-balancing cations. The results strongly suggest that two opposing mechanisms for capture of the carbene can occur depending on whether the zeolite framework behaves as a nucleophilic reagent or an electrophilic reagent in its reaction with the carbene center. Hammett relationships for the decay of the carbene as a function of aryl substituent and zeolite counterion versus the sigma+ substituent parameter support a change in mechanism as the carbene center toggles between being electron poor and electron rich. For the electron-poor chloro(4-nitrophenyl)carbene, a framework adduct is proposed upon reaction of the nucleophilic [Si-O-Al]- bridge with the carbene center, and for the electron-rich chloro(4-methoxyphenyl)carbene, an adduct with the tight Li+ cation is proposed.

IRON-CONTAINING NANOCOMPOSITES BASED ON ZSM-5 ZEOLITE

Here we report synthesis and investigation of Fe /ZSM-5 magnetic nanocomposites. Intercalation of iron was performed by ion-exchange procedure and by impregnation of ZSM-5 zeolite in H + form by Fe ( CO )5. Magnetic nanocomposites were prepared by thermal treatment of impregnated samples in hydrogen flow at different temperatures. Chemical composition, structure and physical properties of the samples were characterized by chemical analysis, TGA, XRD, TEM, Mössbauer spectroscopy and SQUID magnetization measurements. Nanocomposites prepared by reduction of cation-exchanged zeolite indicate paramagnetic behavior even at 4 K. Repeatable ion-exchange/reduction procedure used to enrich ZSM-5 matrix with iron led to formation of bulk iron. Fe /ZSM-5 nanocomposites prepared by thermal decomposition of Fe ( CO )5 in the channels of ZSM matrix reveal their ferromagnetic behavior even at room temperature. The coercivity at 300 K for annealed samples attain 670 Oe for the crystallization temperature of 450°C. According XRD, TEM and Mössbauer data iron nanoparticles were formed in the cavities of zeolite.

Determination of the basic strength of solid catalysts in water by means of a kinetic tracer

The kinetic analysis of the isomerization of glucose into fructose in the presence of solid basic catalysts, cation-exchanged zeolites and anion-modified hydrotalcites, has been performed in order to evaluate the basicity of those solids in water as the solvent and under working reaction conditions. A standardization curve is established by plotting the logarithms of the rate constants of isomerization of glucose against pH for a series of sodium hydroxide solutions at different dilutions. It was then possible to calculate the corresponding pH value of solid basic catalysts from the measurement of their reaction rate constants and extrapolation on the previously established standardization plot. For 1 g of catalyst in 50 ml of water, pH values of 11 were calculated for NaA, Na- and Cs-exchanged X zeolites, and hydrotalcite in its carbonated form, whereas the pH value of meixnerite, the hydroxide form of the hydrotalcite precursor, was close to 12.4, thus confirming its higher basic properties. Moreover, from the determination of pH values after a 24 h-pre-equilibrium of solid catalysts in water, it was also possible to evaluate the percentage of the cationic species passed into solution which well agrees with the percentage obtained from elemental analyses.

Studies of Radio-Labelled Free Radicals Derived from a VOC (Volatile Organic Compound), Benzaldehyde, Adsorbed in Cation-Exchanged Zeolite X

Radio-labelled free radicals were formed by the addition of muonium-a radioactive hydrogen atom with a positive muon as its nucleus-to benzaldehyde, as adsorbed in cation-exchanged zeolite X. PhCHOMu• radicals were the major species formed and the activation parameters associated with their reorientational motion were measured using longitudinal-field muon spin relaxation (LF-MuSRx). Two distinct adsorbed fractions were detected in each of the samples containing Li+, Na+, K+, Ca2+, Sr2+ and Ba2+ cations, believed to be from (1) an effectively bulk benzaldehyde component with an almost common activation energy for molecular reorientation of ca 3kJ/mol, and (2) benzaldehyde adsorbed inside the zeolite supercages. MgX showed only the supercage fraction clearly.

Adsorption of Nitrogen, Oxygen, and Argon in Mono-, Di-, and Trivalent Cation-Exchanged Zeolite Mordenite

Adsorption of nitrogen, oxygen, and argon is studied in zeolite mordenite (Si/Al = 5.5) with mono-, di-, and trivalent Na+, Li+, K+, Cs+, Ca2+, Sr2+, Ba2+, La3+, and Ce3+ as extraframework cations....