Raman spectroscopic evidence of formation of p-dimethoxybenzene cation on Cu- and Ru-montmorillonites

It has been demonstrated through Raman spectroscopy that p-dimethoxybenzene (DMOB) is stably adsorbed on Cu- and Ru-montmorillonites as a cation. DMOB adsorbed is oxidized by Cu²⁺ ion in a dry atmosphere and is reduced reversibly in the presence of water vapor, the process being represented by a reversible reaction: Cu²⁺(H₂O)_n + DMOB ? Cu⁺(H₂O)_n-m + DMOB⁺ + mH₂O.     

Spectroscopic identification of adsorption properties of Zn2+ ions at cationic positions of high-silica zeolites with distant placing of aluminium ions

For Zn²⁺ cations in ZnZSM-5 zeolite unusual type of cationic positions, formed by two distantly placed framework aluminium atoms, is considered. Some extent of structural destabilization of cations in these cationic positions in comparison with traditional localization should result in promoted Lewis activity and adsorption activity of these sites. The last ones are manifested in the significantly increased IR low frequency shifts for adsorbed molecules and in their ability for heterolytic dissociation at elevating temperature. DFT cluster quantum chemical modeling of light alkane adsorption on Zn²⁺ in ZnZSM-5 zeolites confirms these conjectures in full agreement with recent experiments. Similar to the previously considered dihydrogen and methane molecule adsorption, we present here the calculations of ethane molecular and dissociative adsorption on these sites. It is shown that the unusually large ethane IR frequency shift recently observed in ZnZSM-5 zeolite can result from adsorptive interaction of C₂H₆ with Zn²⁺ stabilized in a cationic position with distantly placed aluminium ions. The dissociative adsorption of ethane molecules with the formation of bridged hydroxyl group and Zn–C₂H₅ structure is considered and an activation energy of ethylene formation from the alkyl fragment is evaluated.     

Study on antimony oxide self-assembled inside HZSM-5

Sb/ZSM-5 was obtained by solid-state reaction with the mixture of Sb₂O₃ and zeolite HZSM-5 under a dry nitrogen flow at 773 K. Characterization of the treated zeolite was undertaken with XRD, ²⁷Al MAS NMR, BET, TGA and FT-IR. The results revealed that part of the antimony oxides migrated into the channels of zeolite, and decreased the Brönsted acid sites in Sb/ZSM-5 remarkably. The other part of antimony oxides together with the amorphous alumino-silicate in the products distributed on the external surface of zeolite ZSM-5 and modified it, while the framework of ZSM-5 in crystal phase was retained. The structure of occluded antimony oxide inside the channels of ZSM-5 was studied by XRD Rietveld method. The result showed that their structure can be described as a chain of non-perfect [Sb₅O₅(H₂O)₂]_n⁵ⁿ⁺, which is parallel to the straight channel of ZSM-5. There is about 0.6 [Sb₅O₅(H₂O)₂]⁵⁺ unit in every cell of the ZSM-5 on an average.     

Hydration of GABA and of the GABA·ZN2+ complex: A Monte Carlo simulation study

With empirical and theoretical atom–atom potentials the GABA·nH₂O, n = 25, 192 and GABA·Zn²⁺ · nH₂O, n = 25, 50, 100 complexes are simulated at 298.15 K by the Monte Carlo technique. The results show that the carboxyl group of GABA coordinates six water molecules. Two geometries of the GABA·Zn²⁺ complex, corresponding to the “direct” and “through-water” interaction of Zn²⁺ with the carboxyl group of GABA were found. For the latter interaction a GABA·Zn²⁺ · 6H₂O complex was found whereas the hydration of the former interaction leads to a GABA·Zn²⁺ · 5H₂O complex. Here the carboxyl group of GABA displaces only one water molecule in the first hydration shell of Zn²⁺. Energetically the “direct” and “through-water” geometries seem to be competitive, the former being slightly favored.     

Adsorptive desulfurization of propylene derived from bio-ethanol

We studied adsorption characteristics of a series of LTA zeolite as an adsorbent for desulfurization of propylene, that was produced from bioethanol by ethanol-to-olefin (ETO) conversion. A breakthrough curve (BTC) for adsorption of methanethiol, as one of the sulfur impurities of propylene produced from bioethanol, in the presence of propylene was measured using a fixed-bed column packed with the LTA zeolite. The BTC revealed that the effect of the competitive adsorption of propylene on the LTA zeolite strongly depended on a cation species exchanged in the micropores of the zeolite. Among the cation species examined in this study, bivalent cation of zinc (Zn²⁺) was proved to be the most effective one to increase the amount of methanethiol adsorbed on the LTA zeolite under the presence of propylene. The specific interaction of methanethiol with the LTA zeolite exchanged with Zn²⁺ was confirmed by the measurement of a temperature-programmed desorption (TPD) spectrum of methanethiol.     

Heat of adsorption of carbon dioxyde and ethene on zeolites a exchanged with Li+, Ni2+ and Cu2+

Various contents of Li⁺, Ni²⁺ or Cu²⁺ were introduced in zeolite NaA by conventional cation exchange. Crystal damages are observed on samples having suffered the lowerpH. The heat of adsorption of CO₂ and C₂H₄ was determined by isothermal calorimetry. Very high initial heats (100–120 kJ mol^?1) are found in NaA as well as in Li⁺ exchanged samples, perhaps due to chemisorption on alkaline cations; they vanish when Ni²⁺ or Cu²⁺ replaces more than 20% of Na⁺, in like manner with Co²⁺ or ZnI²⁺. For the adsorption of C₂H₄, high initial heats are absent in the case of NaA, but gradually appear when divalent cations are introduced. Apart from these strong initial values, the heats of adsorption present a plateauvs. the adsorbed amount. Abnormal low values at the plateau are indicative of crystal damages.     

Thermochemistry study on H2O/NaY zeolite adsorption system

By using an automatic adiabatic calorimeter the heat capacity measurements in the temperature range of 220—320K for H₂O/NaY zeolite adsorption system with various amounts of adsorbed water have been made. In c_p-T curves obtained, there is no peak for solid-liquid phase transition of the adsorbed water. But for H₂O-NaY zeolite system which consists of a saturated H₂O/NaY adsorption system mixed mechanically with a certain amount of water, there are distinct peaks in their c_p-T curves. The peak in the c_p-T curve disappeared as soon as the mixed water in the latter system was evacuated. The facts mentioned above have been discussed from the point of view of the structure of the adsorbed layer and the pore size of zeolite.     

Studies on the equilibrated thermodesorption of <Emphasis Type="Italic">n</Emphasis>-hexane from ZSM-5 zeolite

Equilibrated thermodesorption (TPED) and quasi-equilibrated temperature programmed desorption and adsorption (QE-TPDA) were employed as methods for studying the influence of different extraframework cations (Na⁺, K⁺, Li⁺, Cu²⁺, Zn²⁺, or Mg²⁺) on adsorption of n-hexane on ZSM-5 zeolite with high Al content (Si/Al = 15). Considerable influence of the cations on both initial adsorption in the micropores and ordering of the adsorbed molecules, occurring at high coverages, has been observed. This influence is reflected by the values of the adsorption enthalpy and entropy, determined by fitting the dual site Langmuir (DSL) adsorption function to the equilibrated thermodesorption profiles. However, no clear correlation between the determined parameters and properties of the extraframework cations could be found.     

Purification of Hydrogen from CO with Cu/ZSM-5 Adsorbents

The transition to a hydrogen economy requires the development of cost-effective methods for purifying hydrogen from CO. In this study, we explore the possibilities of Cu/ZSM-5 as an adsorbent for this purpose. Samples obtained by cation exchange from aqueous solution (AE) and solid-state exchange with CuCl (SE) were characterized by in situ EPR and FTIR, H₂-TPR, CO-TPD, etc. The AE samples possess mainly isolated Cu²⁺ cations not adsorbing CO. Reduction generates Cu⁺ sites demonstrating different affinity to CO, with the strongest centres desorbing CO at about 350 °C. The SE samples have about twice higher Cu/Al ratios, as one H⁺ is exchanged with one Cu⁺ cation. Although some of the introduced Cu⁺ sites are oxidized to Cu²⁺ upon contact with air, they easily recover their original oxidation state after thermal treatment in vacuum or under inert gas stream. In addition, these Cu⁺ centres regenerate at relatively low temperatures. It is important that water does not block the CO adsorption sites because of the formation of Cu⁺(CO)(H₂O)_x complexes. Dynamic adsorption studies show that Cu/ZSM-5 selectively adsorbs CO in the presence of hydrogen. The results indicate that the SE samples are very perspective materials for purification of H₂ from CO.     

IR Spectra of Ethane Adsorbed on the Hydrogen,Sodium, and Zinc Forms of a Y-Type Zeolite: Interpretation Using <Emphasis Type="Italic">ab initio</Emphasis> Quantum Chemical Calculations

Ethane adsorption on the hydrogen, sodium, and zinc forms of faujasite brings about polarization anisotropy of the C-H stretching vibrations. This anisotropy shows itself most clearly as perturbation of the vibrational mode analogous to the breathing C-H mode ν₁ of free ethane. The relative intensity and the low-frequency shift of the absorption band due to the distorted ν₁ mode increase with increasing perturbation caused by the C₂H₆-adsorption site interaction. The polarizing power of adsorption sites increases in the order H⁺ < Na⁺ < Zn²⁺. The C-H stretching vibrations in ethane adsorbed on the cationic forms of the Y zeolite are not symmetry-forbidden; accordingly, adsorbed ethane gives more absorption bands than gaseous ethane. The interaction between ethane and zinc cations in the Y zeolite structure eliminates not only the symmetry forbiddenness but also the twofold degeneracy of the C-H stretches.__________Translated from Kinetika i Kataliz, Vol. 46, No. 3, 2005, pp. 434–440.Original Russian Text Copyright © 2005 by Pidko, Kazanskii.     

How Water Accelerates Bivalent Ion Diffusion at the Electrolyte/Electrode Interface

The effect of H₂O in electrolytes and in electrode lattices on the thermodynamics and kinetics of reversible multivalent‐ion intercalation chemistry based on a model platform of layered VOPO₄ has been investigated. The presence of H₂O at the electrolyte/electrode interface plays a key role in assisting Zn²⁺ diffusion from electrolyte to the surface, while H₂O in the lattice structure alters the working potential. More importantly, a dynamic equilibrium between bulk electrode and electrolyte is eventually reached for H₂O transport during the charge/discharge cycles, with the water activity serving as the key parameter determining the direction of water movement and the cycling stability.     

An infrared study of adsorption of N2O on ZnO

The adsorption of N₂O on finely divided ZnO at room temperature shows two principal infrared absorption bands at 2237 cm⁻¹ (strong) and 1255 cm⁻¹ (weak), corresponding to the reversible adsorption of an N₂O surface species. The N₂O is postulated to be coordinated to Zn²⁺ cations by the oxygen atom. Water pre-treatment of the ZnO surface gives only weak bands from adsorbed N₂O, indicating that the latter's adsorption site is taken up by adsorbed water. Spectroscopic experiments on ‘reduced’ surfaces of ZnO at 200°C show that limited reaction of N₂O with the surface has occurred, presumably through decomposition to nitrogen and adsorbed oxygen. New adsorptions on the ZnO surface itself, and a reduced amount of reversibly adsorbed N₂O, implied a reduction in pressure of the adsorbate. Such effects were not observed appreciably over ‘oxidised’ ZnO.     

Influence of a doubly charged cation nature on the formation and properties of mixed oxides MAlO x (M = Mg2+, Zn2+, Ni2+) obtained from the layered hydroxide precursors

Influence of the nature of a doubly charged cation in the layered double hydroxides (LDH) on the conditions of formation and properties of mixed oxide phase MAlO _x (M = Mg²⁺, Zn²⁺ and Ni²⁺), its ability to reconstruct the structure of the original precursor under contact with water has been studied. Hydrotalcite-like compounds and corresponding oxides with different M²⁺: M³⁺ ratio were investigated by XRD, TEM, TG-DTG-DTA, ²⁷Al NMR, N₂ adsorption, and differentiating dissolution. It has been found that the nature of the cation M²⁺ influences the conditions of LDH thermal decomposition, structural and textural characteristics of the formed mixed oxides. The obtained data can be used to synthesize the oxide supports with desired acid-base and adsorption properties.     

The Arrangement of First- and Second-shell Water Molecules Around Metal Ions: Effects of Charge and Size

Structural features of clusters involving a metal ion (Li⁺, Na⁺, Be²⁺, Mg²⁺, Zn²⁺, Al³⁺, or Ti⁴⁺) surrounded by a total of 18 water molecules arranged in two or more shells have been studied using density functional theory. Effects of the size and charge of each metal ion on the organization of the surrounding water molecules are compared to those found for a Mg[H₂O]₆²⁺• [H₂O]₁₂ cluster that has the lowest known energy on the Mg²⁺• [H₂O]₁₈ potential energy surface (Markham et al. in J Phys Chem B 106:5118–5134, 2002). The corresponding clusters with Zn²⁺ or Al³⁺ have similar structures. In contrast to this, clusters with a monovalent Li⁺ or Na⁺ ion, or with a very small Be²⁺ ion, differ in their hydrogen-bonding patterns and the coordination number can decrease to four. The tetravalent Ti⁴⁺ ionizes one inner-shell water molecule to a hydroxyl group leaving a Ti⁴⁺(H₂O)₅ (OH⁻) core, and an H₃O⁺• • • H₂O moiety dissociates from the second shell of water molecules. These observations highlight the influence of cation size and charge on the local structure of hydrated ions, the high-charge cations causing chemical changes and the low-charge cations being less efficient in maintaining the local order of water molecules. Electronic Supplementary Material: Supplementary material is available for this article at http://dx.doi.org/10.1007/S00214-005-0056-2.     

硅胶原位合成ZSM-5分子筛及其催化性能的研究

以正丁胺为模板剂,采用水热晶化法在固体硅胶小球上原位合成了ZSM-5分子筛。考察了晶化温度、晶化时间、初始凝胶硅铝比及H2O/ SiO2、BuNH2/ SiO2、OH-/SiO2对ZSM-5相对结晶度的影响,得到了ZSM 5的合成化学规律。采用XRD、SEM、FT IR及N2吸附手段对固体硅胶小球上原位合成的复合结构分子筛进行了表征。结果表明,这种材料具有介孔和微孔的复合结构特征,而且很好地保持了硅胶小球的形貌。以大庆VGO为原料,在重油微反装置上对该复合结构分子筛进行催化性能评价。结果表明,使用硅胶原位合成的ZSM-5分子筛可显著提高低碳烯烃选择性和收率。     

The impact of surface Cu2+ of ZnO/(Cu1−xZnx)O heterostructured nanowires on the adsorption and chemical transformation of carbonyl compounds

The surface cation composition of nanoscale metal oxides critically determines the properties of various functional chemical processes including inhomogeneous catalysts and molecular sensors. Here we employ a gradual modulation of cation composition on a ZnO/(Cu_1−xZn_x)O heterostructured nanowire surface to study the effect of surface cation composition (Cu/Zn) on the adsorption and chemical transformation behaviors of volatile carbonyl compounds (nonanal: biomarker). Controlling cation diffusion at the ZnO(core)/CuO(shell) nanowire interface allows us to continuously manipulate the surface Cu/Zn ratio of ZnO/(Cu_1−xZn_x)O heterostructured nanowires, while keeping the nanowire morphology. We found that surface exposed copper significantly suppresses the adsorption of nonanal, which is not consistent with our initial expectation since the Lewis acidity of Cu²⁺ is strong enough and comparable to that of Zn²⁺. In addition, an increase of the Cu/Zn ratio on the nanowire surface suppresses the aldol condensation reaction of nonanal. Surface spectroscopic analysis and theoretical simulations reveal that the nonanal molecules adsorbed at surface Cu²⁺ sites are not activated, and a coordination-saturated in-plane square geometry of surface Cu²⁺ is responsible for the observed weak molecular adsorption behaviors. This inactive surface Cu²⁺ well explains the mechanism of suppressed surface aldol condensation reactions by preventing the neighboring of activated nonanal molecules. We apply this tailored cation composition surface for electrical molecular sensing of nonanal and successfully demonstrate the improvements of durability and recovery time as a consequence of controlled surface molecular behaviors.

Unexpected features of surface Cu²⁺ on ZnO/(Cu1−xZnx)O nanowires for molecular transformation and electrical sensing of carbonyl compounds were found.     

State and properties of ion-exchanged cations in zeolites: 1. IR spectra and chemical activation of adsorbed molecular hydrogen

Adsorption isotherms and of adsorbed molecular hydrogen indicate that H₂ is weakly adsorbed by alkali-metal forms of faujasites, mordenite, and high-silica zeolite ZSM-5. The alkaline-earth forms of the same zeolites adsorb hydrogen somewhat more strongly; nevertheless, the hydrogen molecules adsorbed by the barium form of mordenite are in the hindered rotation state. Molecular hydrogen is most strongly adsorbed by the zinc and cadmium forms of the high-silica zeolite. In this case, molecular hydrogen is strongly polarized and undergoes heterolytic dissociative adsorption, yielding acidic hydroxyl groups and cation-bound hydride ions.     

The appearance of adsorption ability of modified zeolites for sodium dodecylsulfate from its aqueous solution

Hydrophobic faujasite-type zeolites were prepared by the treatment of hydrophilic Na-Y_4.6 with silicon tetrachloride (SiCl₄) by way of dealumination-silicon exchange reaction. Hydrophilic-hydrophobic characters of these zeolite surfaces were evaluated by measurements of immersional heats into water orn-hexane. Adsorption rates of sodium dodecylsulfate (SDoS) on these zeolites from its aqueous solution and their adsorption characteristics were investigated, and compared with those on ZSM-5 with various Si/Al ratios.The adsorption ability of SDoS into zeolite micropores developed on the zeolite only when it was modified to some extent. The adsorption rates of SDoS were very slow and proved to be affected by molecular diffusion in zeolite pores. They depended on hydrophilic-hydrophobic character of zeolites as well as on pore structures. Adsorption isotherms of SDoS were of Langmuir type, which indicates that the adsorption occurred typically into zeolite pores. Adsorbed amount of SDoS depended on the balance of hydrophilic-hydrophobic character and number of cation site of zeolite surfaces, and had a maximum in case of ZSM-5 zeolites.     

A kinetic study of the Zn2+/Zn(Hg) system in dimethylsulfoxide and water + dimethylsulfoxide mixtures at various concentrations of NaClO4

The kinetics of the Zn²⁺/Zn(Hg) system have been studied at the hanging mercury drop electrode in dimethylsulfoxide (DMSO) and in H₂O + 50 % vol. DMSO mixtures at various concentrations of NaClO₄.The diffusion coefficients, the formal potentials, the standard rate constants and the parameters αn_α and βn_β have been determined.The influence of the concentration of electrolyte on Zn²⁺ reduction in DMSO has been explained by the Frumkin double-layer effect.The mechanism of the cathodic and anodic processes has been discussed.