Metal Cation-exchanged Zeolites Research Articles

Confining platinum clusters in indium-modified ZSM-5 zeolite to promote propane dehydrogenation

Designing highly active and stable catalytic sites is often challenging due to the complex synthesis procedure and the agglomeration of active sites during high-temperature reactions. Here, we report a facile two-step method to synthesize Pt clusters confined by In-modified ZSM-5 zeolite. In-situ characterization confirms that In is located at the extra-framework position of ZSM-5 as In+, and the Pt clusters are stabilized by the In-ZSM-5 zeolite. The resulting Pt clusters confined in In-ZSM-5 show excellent propane conversion, propylene selectivity, and catalytic stability, outperforming monometallic Pt, In, and bimetallic PtIn alloys. The incorporation of In+ in ZSM-5 neutralizes Brønsted acid sites to inhibit side reactions, as well as tunes the electronic properties of Pt clusters to facilitate propane activation and propylene desorption. The strategy of combining precious metal clusters with metal cation-exchanged zeolites opens the avenue to develop stable heterogeneous catalysts for other reaction systems.

Metal cation-exchanged zeolites with the location, state, and size of metal species controlled.

Rh ion-exchanged MFI-type aluminosilicate zeolites with different Al distributions were prepared for controlling the location, state, and size of Rh species. The MFI-type aluminosilicate zeolite with the framework Al atoms predominantly located inside the channel intersections leads to the formation of relatively large Rh species, which were confirmed by ultraviolet-visible (UV-vis) and infrared (IR) spectroscopies. Moreover, this catalyst showed a high catalytic activity for the oxidative reforming reaction of methane.

On the use of metal cation-exchanged zeolites in sorption thermochemical storage: Some practical aspects in reference to the mechanism of water vapor adsorption

The sodium form of industrial X-zeolite (13X) was compared with two other samples obtained by a partial (about 70%) cation exchange with Mg2+ (Mg-X) or a complete exchange with Ca2+ (Ca-X) in view of their use as adsorbents for water vapor in energy storage systems by sorption. Various sorption methods based on the specific interactions between two probing molecules (N2 and NH3) and specific surface sites were employed to monitor the hydration state of the zeolite surface as a function of the temperature of sample drying performed under vacuum degassing or gas-flow conditions. X-ray diffraction, volumetric sorption technique, temperature-programmed desorption, and gas flow calorimetry were combined with Monte Carlo simulations of the mechanism of water vapor adsorption on model zeolites to demonstrate that the general tendency to operate under mild drying conditions would always result in a state of partial hydration of the zeolite surface. To increase the amount of heat released upon subsequent adsorption from a flow of helium at a relative pressure of water vapor of about 0.03, the use of Mg-X sample was demonstrated the best alternative if the activation procedure was to be performed at 200°C. Here, the heat effect was 4–5 times greater than that measured in the case of 13X. This result is of high importance for the industrial-scale implementation of the energy storage by sorption.

금속이온이 치환된 Y형 제올라이트에서 벤젠의 촉매연소반응

Catalytic combustion of benzene over various metal cation-exchanged zeolites has been investigated. Y(4.8)-type zeolite showed the highest activity among the used zeolites and Cu/Y(4.8) catalyst also showed the highest activity among metal cation/ Y(4.8) zeolites. The catalytic activity increased according to the amount of adsorbed oxygen acquired from O2 TPD results. The catalytic activity also increased with an increase of Cu cation concentration on Cu/Y(4.8) catalysts. The conversion of benzene on the combustion reaction depended on not benzene concentration but the oxygen concentration. In addition, the introduction of water into reactants decreased the catalytic activity.

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.

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.

Investigation of the Adsorption of Methanol on Alkali Metal Cation Exchanged Zeolite X by Inelastic Neutron Scattering

The adsorption of methanol on alkali metal cation exchanged zeolite X was studied by inelastic neutron scattering (INS) and Fourier transform infrared (FTIR) spectroscopy in the range 30−2000 and 1...

Absolute Reactivity of the 4-Methoxycumyl Cation in Non-Acid Zeolites

The reactivity of the 4-methoxycumyl cation in a series of alkali metal cation-exchanged zeolites (LiY, NaY, KY, RbY CsY, NaX, NaMor, and Naβ) in the absence and presence of coadsorbed alcohols and water is examined using nanosecond laser flash photolysis. In dry zeolites, the absolute reactivity of the carbocation is found to be strongly dependent on the nature of the alkali counterion, the Si/Al ratio, and the framework morphology, with the lifetime of the carbocation in Naβ being almost 10000-fold longer than in CsY. The results suggest a mechanism for carbocation decay involving direct participation of the zeolite framework as a nucleophile, leading to the generation of a framework-bound alkoxy species. Intrazeolite addition reactions of alcohols and water to the 4-methoxycumyl cation can be described in terms of both dynamic and static quenching involving molecular diffusion through the heterogeneous topology and rapid coupling between the alcohol and the carbocation encapsulated within the same cavity. The dynamics of the quenching reactions are different from similar reactions in homogeneous solution due to both the passive and active influences of the zeolite environment. In a passive sense, the zeolite decreases the reactivity of the nucleophilic quencher by hindering molecular diffusion. However, the zeolite actively promotes the efficiency of intracavity coupling by enhancing the deprotonation of the oxonium ion intermediate, allowing the reaction to go to completion.

Novel approaches towards the generation of excited triplets of organic guest molecules with zeolites

Alkali metal cation-exchanged zeolites (M+X or M+Y) can be used as ‘microreactors’ in which to carry out photochemical rearrangement reactions of organic guests.

The effect of ion-exchanged alkali metal cations on the excited states of xanthone and the photolysis of 2-pentanone included within zeolite cavities

In order to clarify the effect of the microenvironment of the host adsorbents on the photophysical and photochemical properties of the guest molecules in restricted void spaces, the characteristic properties of the phosphorescence of xanthone and photolysis of 2-pentanone included within alkali metal cation-exchanged zeolites were investigated. An enhancement of the phosphorescence yield of xanthone included within the zeolite cavities and a reduction in the phosphorescence lifetime were ovserved on changing the exchangeable alkali metal cation from Li + to Cs +. These results suggest an external heavy-atom effect derived from the alkali metal cation on the singlet-triplet transitions of xanthone molecules within the zeolite cavities. During the photolysis of 2-pentanone included within the zeolite cavities, the selectivity of the Norrish type II reaction was found to decrease remarkably due to the steric hindrance caused by the restricted void space of the zeolite cavities. IR and photoluminescence investigations indicated that 2-pentanone consists of two different adsorption types: one is adsorbed on the surface OH group by hydrogen bonding and the other is directly adsorbed onto the alkali metal cations. On changing the cation from Li + to Cs +, a decrease in the strength of the interaction between the alkali metal cation and 2-pentanone caused an increase in the yield of photolysis of 2-pentanone. Thus changes in the zeolite cavities, modified by the exchange of cations, have a significant effect on the excited states and chemical reactions of ketones.

Photophysical and photochemical processes in zeolite cavities: The effects of ion-exchanged alkalimetal cations on the excited states of xanthone and the photolysis of 2-pentanone

The characteristics properties of xanthone phosphorescence and of 2-pentanone photolysis in alkali metal cation-exchanged zeolites have been investigated to clarify the effect of the micro-environment of host-adsorbents on the photophysical and photochemical properties of guest-molecules in restricted void spaces. The enhancement of the phosphorescence yields of xanthone included in zeolites is observed by changing the exchangeablealkali metal cation from Li+ to Cs+. Simultaneously, the phosphorescence lifetimes were observed to continuously shorten by changing the cation from Li+ to Cs+. These results suggest that the external heavy-atom effect deriving from the alkali metal cations on the singlet-triplet transitions of xanthone molecules stabilized on alkali metal cations in the order of Li+, Na+, K+, Rb+, and Cs+. The yields for the photolysis of 2-pentanone included in zeolites increase with changing the alkali metal cation from Li+ to Cs+. IR investigations of the adsorption state of 2-pentanone indicate that strength of the interaction between the alkali metal cations and 2-pentanones decreases by changing the cation from Li+ to Cs+, which results in a longer lifetime of 2-pentanone. The selectivity of propylene formation is dramatically increased by changing the cation from Li+ to Cs+. The enhanced formation of propylene is asociated with the hydrogen absorption from propyl radicals by lattice oxygen, their basicity increasing by changing the cation from Li+ to Cs+. Thus, these changes in the zeolite cavities modified by exchanging cations caused significant effects not only on the excited state but also on the following chemical reactions of ketones.

Electronic structure of zeolites studied by X-Ray photoelectron spectroscopy

The electronic structure of zeolites was systematically investigated by utilizing XPS. Zeolites A, X, Y, and mordenites in their sodium and decationized forms as well as the series of alkali metal cation-exchanged X and Y zeolites were examined. The electronic structure of zeolites was demonstrated to depend strongly on both the composition and the cation involved. It is suggested that the basic strength of framework oxygen increases with increasing Al content regardless of the crystal structure and with the decreasing electronegativity of the counter cation. These results explain well certain catalytic properties of alkali metal cation-exchanged zeolites. On the basis of the XPS results, the nature of chemical bonding in zeolites is discussed.

Catalytic reactions of pyridines. VI. Heterogeneous vapor-phase ring alkylation of pyridines with alcohols over H+-, Li+-, and alkaline earth cation-exchanged zeolites.

The heterogeneous vapor-phase alkylation of pyridine with methanol was examined over metal cation-exchanged X-and Y-type zeolites in an atmosphere of nitrogen. β-Methylation was observed over H+-or Li+-exchanged zeolites, whereas α-and γ-methylation (the former occurred in preference) were observed over alkaline earth cation-exchanged zeolites. For example, at 400°C, the yields of α-, β-, and γ-picolines over the HY catalyst were 2.6, 12.2, and 2.6%, whereas those over the BaY catalyst were 22.7, 3.8, and 7.6%, respectively. This catalytic ring alkylation over metal cation-exchanged zeolites was also examined by the use of pyridine with ethanol and picolines with either methanol or ethanol.