Zeolite Cavities Research Articles

Incorporation of silver (I) ions within zeolite cavities and their photocatalytic reactivity for the decomposition of N2O into N2 and O2

Ag+/ZSM‐5 catalysts were prepared by an ion‐exchange method. UV‐irradiation of the Ag+/ZSM‐5 catalysts in the presence of N2O led to the photocatalytic decomposition of N2O into N2 and O2 at 298 K. Investigations of the effective wavelength of the irradiated UV‐light for the reaction as well as the in‐situ characterization of the catalysts by means of UV‐Vis, photoluminescence and FT‐IR spectroscopies revealed that the photoexcitation of the Ag+ ‐ N2O complexes formed between gaseous N2O and the isolated Ag+ ions exchanged within the zeolite cavities plays a significant role in the reaction.

Controlling chemistry with cations: photochemistry within zeolites.

The alkali ions present in the supercages of zeolites X and Y interact with included guest molecules through quadrupolar (cation-pi), and dipolar (cation-carbonyl) interactions. The presence of such interactions can be inferred through solid-state NMR spectra of the guest molecules. Alkali ions, as illustrated in this article, can be exploited to control the photochemical and photophysical behaviors of the guest molecules. For example, molecules that rarely phosphoresce can be induced to do so within heavy cation-exchanged zeolites. The nature (electronic configuration) of the lowest triplet state of carbonyl compounds can be altered with the help of light alkali metal ions. This state switch (n pi*-pi pi*) helps to bring out reactivity that normally remains dormant. Selectivity obtained during the singlet oxygen oxidation of olefins within zeolites illustrates the remarkable control that can be exerted on photoreactions with the help of a confined medium that also has active sites. The reaction cavities of zeolites, like enzymes, are not only well-defined and confined, but also have active sites that closely guide the reactant molecule from start to finish. The examples provided here illustrate that zeolites are far more useful than simple shape-selective catalysts.

Determination of the oxidation and coordination state of copper on different Cu-based catalysts by XANES spectroscopy in situ or in operando conditions

The use of XANES spectroscopy, both in classical and in dispersive geometries, is illustrated for the study of copper-based catalysts under in situ or in operando conditions. As case studies, copper-exchanged MFI zeolites and CuCl2/γ-Al2O3 systems are considered. In the former case, in situ XANES spectroscopy was used to characterise well defined complexes (Cu+N2, Cu+(CO)3, Cu+(NH3)(CO) and Cu+(NO)2) formed on copper ions inside the zeolite cavities under controlled conditions. From these results, useful information concerning the symmetry of the formed complexes can readily be gained. The latter case shows how the use of dispersive XANES spectroscopy allows to follow, in real time, the evolution of a system in working conditions. The simultaneous determination of the catalyst activity and of the average oxidation state of copper in the catalyst allows the evolution of a system in working conditions to be followed in real time. The criteria used for the quantification of the Cu(I) and Cu(II) fraction from XANES spectra are discussed in detail.

Co Cation Effects on Activity and Stability of Isolated Pd(II) Cations in Zeolite Matrices for Selective Catalytic Reduction of Nitric Oxide with Methane

Palladium ion-exchanged zeolites (ZSM-5, mordenite, ferrierite, Y, beta) were tested to evaluate catalytic activity for selective reduction of nitric oxide with methane (CH4-SCR) and stability in the presence of 10% water vapor at a relatively high temperature of 500°C. Deactivation of the catalytic activity was inevitable on Pd/HZSM-5 with a Si/Al ratio of 19.8 under the reaction conditions. When the Si/Al ratio decreased, the durability was slightly enhanced. Among the zeolites tested, palladium supported on mordenite showed stable NO conversions. The addition of cobalt effectively stabilized the activity of Pd–H-zeolites, particularly with the addition of 3.3 wt% cobalt onto Pd/HZSM-5, of which no deactivation was noticeable for at least 50 h. Isolated Pd2+ cations on HZSM-5 were the active and selective sites for CH4-SCR. The isolated Pd2+ cations could not be exchanged by Na+ cations in NaCl titration test after the addition of 3.3 wt% cobalt, while NH4NO3 titration revealed that the isolated Pd2+ existed in the sample. This means that Co2+ changes the ion-exchange property of the isolated Pd2+, leading to a much higher stability than Pd/HZSM-5 with regard to durability. The effect of Co2+ on the stability of the isolated Pd2+ cations and their sitings in zeolite cavities is discussed.

Co Cation Effects on Activity and Stability of Isolated Pd(II) Cations in Zeolite Matrices for Selective Catalytic Reduction of Nitric Oxide with Methane

Palladium ion-exchanged zeolites (ZSM-5, mordenite, ferrierite, Y, beta) were tested to evaluate catalytic activity for selective reduction of nitric oxide with methane (CH 4-SCR) and stability in the presence of 10% water vapor at a relatively high temperature of 500°C. Deactivation of the catalytic activity was inevitable on Pd/HZSM-5 with a Si/Al ratio of 19.8 under the reaction conditions. When the Si/Al ratio decreased, the durability was slightly enhanced. Among the zeolites tested, palladium supported on mordenite showed stable NO conversions. The addition of cobalt effectively stabilized the activity of Pd–H-zeolites, particularly with the addition of 3.3 wt% cobalt onto Pd/HZSM-5, of which no deactivation was noticeable for at least 50 h. Isolated Pd 2+ cations on HZSM-5 were the active and selective sites for CH 4-SCR. The isolated Pd 2+ cations could not be exchanged by Na + cations in NaCl titration test after the addition of 3.3 wt% cobalt, while NH 4NO 3 titration revealed that the isolated Pd 2+ existed in the sample. This means that Co 2+ changes the ion-exchange property of the isolated Pd 2+, leading to a much higher stability than Pd/HZSM-5 with regard to durability. The effect of Co 2+ on the stability of the isolated Pd 2+ cations and their sitings in zeolite cavities is discussed.

Photooxygenation of aromatic alkenes in zeolite nanocavities

Aromatic alkenes such as styrene (1), 1,1-diphenylethene (2), and cis- and trans-stilbenes (3), but not triphenylethene (4), formed their contact charge-transfer (CCT) complexes with O2 in solutions. In zeolite NaY, the CCT absorption band was observed only for 1 and 3. Irradiation of alkenes 1–4 included in the zeolite nanocavities under O2 produced benzaldehyde and benzophenone as the major oxygenation products. In particular, for 3 and 4, the photooxygenation competed with a photoelectrocyclic reaction, which subsequently yielded phenanthrenes as the exclusive photoproducts under O2 in solution. It is likely that the oxygenation products were produced through the alkene cation radicals and superoxide anion radical generated by excitation of the CCT complexes and/or photoinduced electron transfer from the excited alkenes to O2. YAG laser (266 nm) excitation of 1 included in the zeolite cavities under vacuum produced its alkene cation radical and the trapped electron, Na43+, both of which were quenched by O2. On the basis of the optimum structure for the guest molecules obtained by semi-empirical molecular orbital calculations (AM1), it is suggested that the photooxygenation reaction was regulated by the electrostatic interaction between the guest molecules and alkali-metal cations in the nanocavities as well as by the strong electrostatic field which stabilized the ion radical pairs generated.

Characterization of zinc oxide nanoparticles encapsulated into zeolite-Y: An in-situ combined X-ray diffraction, XAFS, and SAXS study

Characterization of zinc oxide containing zeolite Y was performed employing time-resolved in situ small angle X-ray scattering (SAXS), WAXS and EXAFS techniques. Extending procedures used previously for the preparation of CdO clusters in side zeolite cages, zinc oxide particles have been successfully prepared within the cavities of zeolite Y. The preparation entails ion-exchange of Zn(II) followed by treatment with an alkaline solution before thermal treatment to produce oxide particles. Experimental results give strong evidence for the formation of small zinc oxide clusters with sizes in the range of 10–20 Å. Particle sizes estimated from the analysis of EXAFS and SAXS data are consistent with the cavity size of zeolite Y.

UV–Vis spectrum of simple hydrocarbons in a zeolite cavity. A supramolecular charge transfer

Experimental findings show that ethylene included in a zeolite cavity absorbs visible light to allow photochemical reactions. This evident supramolecular effect is explained in this Letter in terms of a charge transfer transition upon excitation. According a priori NDOL calculations the charge transfer of ethylene occurs between the HOMO, localized on the trapped molecule, and electronic states distributed on the zeolite network, together with a very perturbed intramolecular transition in ethylene. The cases of methane and ethane were also studied and a drop of about 4 eV is always observed in the first electronic transition energy when included in the zeolite cage with respect to the isolated molecules.

Polyelectrolytes confined to spherical cavities

The effects of confining a polyelectrolyte solution containing a polyion and its counterions in spherical cavities of different sizes have been investigated by Monte Carlo simulations using a simple model system. Polyions of three different linear charge densities and counterions of three different valences have been examined. Structure, energy, and free energy properties of all systems have been determined as a function of the sphere radius. In nearly all cases, the free energy of confining the polyelectrolyte solution increases as the sphere radius is decreased. The free energy cost decreases as the linear charge density of the polyion is reduced and as the counterion valence is increased, although for completely different reasons. A strong consistency among the variations of the different structural and thermodynamic results on the linear charge density and counterion valence was found. The study provides information relevant for experimental systems of polyions confined to, e.g., viruses, vesicles, and zeolite cavities.

Chemical Anchoring of Palladium by Fe Oxo Ions in Zeolite ZSM-5

Zeolite supported mono- and bimetallic catalysts Pd/ZSM-5 and PdFe/ZSM-5 were prepared by exchanging Pd from a dilute aqueous solution of [Pd(NH3)4](NO3)2 into either H/ZSM-5 or Fe/ZSM-5. The Fe/ZSM-5 was prepared by sublimation; it contains a major part of the Fe as dinuclear oxygen-bridged Fe3+ ions. Significant differences between Pd/ZSM-5 and PdFe/ZSM-5 are revealed after calcination: in Pd/ZSM-5 the Pd is present as large PdO particles, but in PdFe/ZSM-5 it is highly dispersed over the zeolite cavities and invisible in TEM. Evidently, the Fe ions and/or Fe oxo ions act as very effective chemical anchors. It is argued that Pd0 is formed by autoreduction of [Pd(NH3)4]2+ ions even under calcination conditions. The Fe ions promote nucleation and limit growth of Pd0n clusters, which are subsequently converted to PdO.

On the jump diffusion of molecules in zeolites measured by quasi-elastic neutron scattering

Molecular diffusion in the regularly spaced cavities of zeolites involves various jump types, within and between the cavities, which may occur at different time scales. These jump processes can be modelled by simulation techniques but they are extremely difficult to be measured experimentally. The quasi-elastic neutron scattering (QENS) technique is a unique probe for studying both the dynamical and geometrical aspects of such molecular motions. For benzene adsorbed in the supercages of NaY zeolite, Auerbach et al. have assumed that intracage and intercage jumps were uncorrelated, whereas one finds by QENS that they occur on the same time scale. On the other hand for n-butane–methane mixtures in silicalite-1, the convolution of fast jumps within a channel segment by slower jumps between channels is in agreement with previous MD simulations.

Structure of intrazeolite molybdenum oxide clusters and their catalysis of the oxidation of ethyl alcohol

Molybdenum oxide clusters confined in zeolite supercages were synthesized using a precursor Mo(CO)6 adsorbed in the pores of zeolite and characterized by XAFS, HREM, LRS and XPS. The structure of the intrazeolite Mo oxide clusters and their catalysis of the oxidation of ethyl alcohol to acetaldehyde were investigated. It was found by Mo K-edge XAFS that intrazeolite Mo(VI) oxide dimer species with an octahedral symmetry of Mo are formed by thermal oxidation of Mo(CO)6 entrapped in an NaY zeolite irrespective of the Mo loading up to two Mo atoms per supercage. HREM and XRD detected no degradation of the crystallinity of the host zeolite. The dimer catalyst showed a specific activity for the oxidation of ethyl alcohol 10 times higher than an impregnation catalyst, in which isolated tetrahedral Mo oxide species are formed. The Mo oxide dimer species were transformed to monomeric species by a heat treatment at 673 K. This led to a considerable decrease in catalytic activity. The Mo oxide clusters prepared by thermal oxidation of Mo(CO)6 encaged in the cavity of a high silica FAU zeolite (Si/Al = 630) showed a significantly higher specific activity than the Mo oxide clusters in NaY. From the Mo K-edge EXAFS analysis, it was found that Mo oxide clusters containing several Mo atoms are constructed in the pores of the high silica zeolite. It is concluded that the structure and size of Mo oxide clusters encaged in zeolite are controlled by the zeolite composition and heat treatment and that larger Mo oxide clusters exhibit a higher specific activity for the partial oxidation of ethyl alcohol.

Luminescent Silver Sulfide Clusters

Silver sulfide clusters are synthesized in the cavities of sodium and calcium zeolite A microcrystals by exposing the activated Ag+-loaded zeolite to H2S. The growth of the silver sulfide clusters during rehydration of the samples is investigated by means of diffuse reflectance spectroscopy. Monomers of Ag2S are formed at low Ag+-loading through reaction of AgSH molecules, yielding colorless composites, which exhibit a characteristic blue-green photoluminescence. Yellow colored samples showing an orange-red luminescence with an average decay time of 81 μs (−160 °C) are obtained at medium silver sulfide content. Further increasing the silver sulfide loading and therefore the cluster size causes a bathochromic shift of this emission which is accompanied by a shortening of the luminescence lifetime. The samples generally exhibit large Stokes-shifts, which can be attributed to HOMO−LUMO transitions with small oscillator strengths. The Ag2S−zeolite host−guest system constitutes a three-dimensional array of sil...

Adsorption energy surfaces in faujasite type zeolites

A perturbational approach is used to study the adsorption behavior of small molecules in faujasite type zeolites. From the evaluated interaction energies, iso-interaction energy plots for N 2, O 2, CO 2 and C 2H 4 in LSX faujasite exchanged with Na + are calculated. From these plots the preferential adsorption sites in the zeolite cavity were located. The interaction energies on the preferential sites were found to predict rough trends for the separation behavior of the considered zeolites. A study of the different cation sites, based on an ab initio calculation of the atomic charges at the HF/3-21G level showed a good agreement with existing experimental results and previous theoretical calculations.

Reorientational and translational dynamics of benzene in zeolite NaY as studied by one- and two-dimensional exchange spectroscopy and static-field-gradient nuclear magnetic resonance

One- and two-dimensional H2- and C-NMR13 (nuclear magnetic resonance) echo spectroscopy and H1 static field gradient NMR self-diffusion experiments have been used to study the reorientational and translational dynamics of benzene molecules adsorbed on zeolite NaY as a function of loading. Comparison of the data with model calculations establish that the elementary motional process of the guest molecules is consistently identified as a jump process among well defined adsorption sites inside a supercage and/or a jump between nearby supercages. In cases where the zeolite cavities contain high loadings of guest molecules, each molecular jump is accompanied by concomitant relaxation of the local environment. Molecular jump events between adsorption sites correspond to the elementary processes from which long range translational diffusion evolves.

Increasing the life expectancy of carbanions by zeolite inclusion.

Carbanions have been detected for the first time in zeolite cavities where their lifetime is more than ten times longer than in solution; included carbanions show Grignard-like behaviour.

Homogeneous and encapsulated within the cavities of zeolite Y chiral manganese and copper complexes with C 2-multidentate ligands as catalysts for the selective oxidation of sulphides to sulfoxides or sulfones

A series of manganese and copper complexes with tetradentate C 2-symmetry ligands, [Mn(C 2-ligand)Cl(H 2O)]PF 6 and [Cu(C 2-ligand)]ClO 4, have been synthesised and characterised as homogeneous and encapsulated into the supercages of a large pore sized USY zeolite. These Mn and Cu complexes are excellent catalysts for the selective oxidation of organic sulphides to sulfoxides or sulfones with high selectivity, and moderate to low enantioselectivity. The chelation of zeolite-exchanged Mn 2+ by C 2–N-containing ligands gives rise to a whole class of heterogeneous liquid phase oxidation, which exhibits similar catalytic performances than the homogeneous ones. Moreover, those heterogenised catalysts can be recycled in successive runs, by a simple filtration, without a significant loss of activity and selectivity.

Selective Oxidation of para-Xylene to Terephthalic Acid by μ3-Oxo-Bridged Co/Mn Cluster Complexes Encapsulated in Zeolite–Y

Novel, solid catalysts of μ3-oxo-bridged Co/Mn cluster complexes, viz., [Co3(O)(CH3COO)6(pyridine)3]+, [Mn3(O) (CH3COO)6(pyridine)3]+, and CoMn2(O)(CH3COO)6(pyridine)3 (denoted Co3(O), Mn3(O), and CoMn2(O), respectively), encapsulated in zeolite-Y oxidize selectively, para-xylene to terephthalic acid with dioxygen. The catalysts were prepared by the “flexible ligand synthesis” method and characterized by X-ray diffraction, thermal analysis, cyclic voltammetry, Fourier transform infrared, diffuse reflectance ultraviolet–visible, X-ray photoelectron spectroscopy, and electron paramagnetic resonance spectroscopic techniques. The various physicochemical measurements confirm the presence and structural integrity of the μ3-oxo-bridged cluster complexes in zeolite cavities. The activity and selectivity of both the “neat” and encapsulated cluster complexes followed the order CoMn2(O)>Mn3(O)>Co3(O), revealing the superiority of the heteronuclear complexes. Under optimal conditions, both neat and encapsulated cluster catalysts exhibit 100% para-xylene conversion with >98% selectivity for terephthalic acid. It is important that the key impurity, 4-carboxybenzaldehyde, is significantly lower in abundance (than the current commercial catalysts) with one of the zeolite-encapsulated catalysts, CoMn2(O)–Y. Leaching of metal ions from the solid catalyst during reaction is minimal and the catalyst could be recycled without significant loss of activity. A more facile redox behavior of Co between +2 and +3 oxidation states in CoMn2(O) (confirmed by cyclic voltammetry) is perhaps responsible for high catalytic activity.

Photogeneration and Migration of Electrons and Holes in Zeolite NaY

Electron transfer and charge migration between electron donors and acceptors encapsulated within dry NaY zeolites are explored using nanosecond laser flash photolysis. The role of the zeolite in these redox processes is examined in order to characterize the intrazeolite mobility of electrons and holes. Electron migration is initiated by photoexciting trans-anethole in NaY containing coadsorbed 1,4-dicyanobenzene as an electron acceptor, while hole migration is initiated by photoexciting chloranil in NaY containing coadsorbed 4,4‘-dimethoxybicumene as an electron donor. The experimental results demonstrate that ultrafast redox reactions (>108 s-1) take place, leading to long-lived charge separated species within the zeolite cavities. The efficiency of these redox processes is examined as a function of donor−acceptor concentration and the presence of nitrous oxide as an electron trap. Interpretation of the experimental data with two independent models provides evidence that the redox chemistry occurring wit...

Removal of tetramethylammonium cations from zeolites

Zeolite α (high-silica LTA), a potential shape-selective catalyst, is synthesized in the presence of tetramethylammonium (TMA) ions. Since TMA + ions are incapable of forming olefins at low temperature, temperatures in excess of 500 °C are required to thermally decompose them and burn off the carbonaceous deposits, frequently causing damage to the structure. In this paper, the thermal decomposition of zeolitic TMA + ions is investigated. This work led to a less severe method for removing TMA + ions by stepwise reaction with ammonia at low temperatures. TMA + ions located in the supercage can easily be removed at a temperature as low as 250 °C, generating mono- and dimethylamine. Sodalite cage TMA + ions require a temperature of not more than 400 °C to be degraded. Although this treatment raises the Si/Al ratio somewhat, damage to the structure is minimal. Since the size of the zeolitic pores defines the type of molecules capable of escaping from the zeolite cavities, decomposition of TMA + ions in NaTMA-Y and NaTMA-high-silica sodalite have been included for comparison.