IR Spectroscopic Study of Cyclopropane Adsorption on Zeolite Y: I. Cyclopropane Adsorption and Transformation on Sodium- and Hydrogen-Exchanged Zeolite Y

IR spectroscopy is used to study the adsorption of cyclopropane on hydrogen- and sodium-exchanged zeolite Y. It is shown that cyclohexane weakly adsorbs on the sodium-exchanged zeolite, whereas it is grafted on the surface of the hydrogen-exchanged zeolite in the form of propyl groups. A possible scheme of cyclopropane transformation into propyl groups and the configuration of the corresponding transition state are considered. It is shown that the coordinate of this reaction corresponds to the complex composite vibrations of the C–C bond with the deformational vibrations of the CH₂ groups, which have the highest extinction coefficients in the IR spectrum of the adsorbed molecule.     

Vibrational analysis of propene adsorbed in zeolites Y

Normal coordinate analysis was performed for propene adsorbed in faujasites NaY, CaY and MgY applying a valence force model. The force fields obtained were compared with that of the free molecule. Only small shifts of the fundamentals are observed due to the weak interaction with the zeolites. Although in general the results are comparable to those of propene adsorbed in different zeolites A, some distinct features such as a stronger influence on the CC stretching vibration, a smaller hindrance of the CH bending vibration and a higher reduction of the CH (CH₃) bond strength are detectable, which must be attributed to both the different composition and structure of the zeolite framework. The frequency shifts and the corresponding changes of the force constants are related to the geometry of the sorption complex of propene on the zeolite surface.     

IR Spectroscopic Study of Ethylene Adsorption and Oligomerization on the Hydrogen Form of Mordenite

Diffuse-reflectance IR spectroscopy is used to study adsorption and oligomerization of ethylene on the hydrogen form of mordenite at room temperature. Ethylene adsorbs on bridging acid hydroxyl groups of the zeolite and forms -complexes with a firm hydrogen bond. The interaction with hydroxyl groups most strongly excites composite vibrations in adsorbed molecules. These vibrations are a combination of the stretching vibration of a double bond and the deformational vibrations of the CH₂ group. A conjecture is drawn that these composite vibrations correspond to the reaction coordinate of ethylene transformation to the ethoxy groups. Their further reactions with weakly adsorbed molecules result in ethylene oligomerization. A linear oligomer is formed, grafted on the zeolite surface and filling the pores of zeolites.     

FT-IR evidence for the participation of protonic sites in the heterolytic cleavage of C?H bonds of methane over HZSM-5 zeolite

It has been observed by FT-IR spectroscopy that both kinds of Brönsted acid protons present in HZSM-5 zeolite may be involved in the adsorption of methane at low temperature (173 K) and exchange with CH₄ or CD₄ at high temperature (>500°C). The sites which can adsorb methane at low temperature are the active sites for methane conversion at high temperature. Over HZSM-5 zeolite, the activation of methane is suggested to occur via a heterolytic cleavage of C–H bond with the assistance of protons.     

Interplay of bonding and geometry of the adsorption complexes of light alkanes within cationic faujasites. Combined spectroscopic and computational study

A FT-IR spectroscopic study of methane, ethane, and propane adsorption on magnesium and calcium forms of zeolite Y reveals different vibrational properties of the adsorbed molecules depending on the exchanged cation. This is attributed to different adsorption conformations of the hydrocarbons. Two-fold eta(2) coordination of light alkanes is realized for MgY, whereas in case of CaY zeolite quite different adsorption modes are found, involving more C-H bonds in the interaction with the cation. The topological analysis of the electron density distribution function of the adsorption complexes shows that when a hydrocarbon coordinates to the exchanged Mg(2+) ions, van der Waals bonds between H atoms of the alkane and basic zeolitic oxygens significantly contribute to the overall adsorption energy, whereas in case of CaY zeolite such interactions play only an indirect role. It is found that, due to the much smaller ionic radius of the Mg(2+) ion as compared to that of Ca(2+), the former ions are significantly shielded with the surrounding oxygens of the zeolitic cation site. This results in a small electrostatic contribution to the stabilization of the adsorbed molecules. In contrast, for CaY zeolite the stabilization of alkanes in the electrostatic field of the partially shielded Ca(2+) cation significantly contributes to the adsorption energy. This is in agreement with the experimentally observed lower overall absorption of C-H stretching vibrations of alkanes loaded to MgY as compared to those for CaY zeolite. The preferred conformation of the adsorbed alkanes is controlled by the bonding within the adsorption complexes that, in turn, strongly depends on the size and location of the cations in the zeolite cavity.     

Diffuse-Reflectance IR Spectra of Methane Adsorbed on NaZSM-5 and HZSM-5 Zeolites

Diffuse-reflectance IR spectra of methane adsorbed on high-silica NaZSM-5 and HZSM-5 zeolites point to a stronger adsorption of methane on sodium cations than on protons. For the asymmetric stretching vibration ₃, this form of adsorption is characterized by a doublet with band maxima at 2980 and 3010 cm^–1. For the fully symmetric stretching vibration ₁, it is characterized by a singlet with a maximum at 2880 cm^–1. Methane is also adsorbed on NaZSM-5 in a weaker form, which is characterized by absorption bands with maxima at 3002 (₃) and 2887 (₁) cm^–1. The weaker form of methane adsorption on acidic bridging hydroxy groups of HZSM-5 is characterized by absorption bands at 3001 and 2887 cm^–1 (₃ and ₁, respectively). A difference between this form of adsorption and weak adsorption on sodium-exchanged zeolite reveals itself in the somewhat higher intensity of the band at 2887 cm^–1. For methane adsorbed on NaZSM-5, the frequencies of deformational vibrations and a spectrum in the near IR region are obtained for the first time. It was found that the perturbance of adsorbed methane molecules is seen in the spectrum as in the low-frequency shifts of most of the bands that appear due to composite vibrations and overtones and as new adsorption bands that were not observed for gaseous methane.     

Blue-shifted hydrogen-bonded complexes. II. CH3F(HF)1 n 3 and CH2F2(HF)1 n 3

The equilibrium structures, binding energies, and vibrational spectra of the clusters CH₃F(HF)_{1 n 3} and CH₂F₂(HF)_{1 n 3} have been investigated with the aid of large-scale ab initio calculations performed at the Møller–Plesset second-order level. In all complexes, a strong C–FH–F halogen–hydrogen bond is formed. For the cases n = 2 and n = 3, blue-shifting C–HF–H hydrogen bonds are formed additionally. Blue shifts are, however, encountered for all C–H stretching vibrations of the fluoromethanes in all complexes, whether they take part in a hydrogen bond or not, in particular also for n = 1. For the case n = 3, blue shifts of the ν(C–H) stretching vibrational modes larger than 50 cm⁻¹ are predicted. As with the previously treated case of CHF₃(HF)_{1 n 3} complexes (A. Karpfen, E. S. Kryachko, J. Phys. Chem. A 107 (2003) 9724), the typical blue-shifting properties are to a large degree determined by the presence of a strong C–FH–F halogen–hydrogen bond. Therefore, the term blue-shifted appears more appropriate for this class of complexes. Stretching the C–F bond of a fluoromethane by forming a halogen–hydrogen bond causes a shortening of all C–H bonds. The shortening of the C–H bonds is proportional to the stretching of the C–F bond.     

Energetics of C–C Bond Scission in Ethane Hydrogenolysis: A Theoretical Study of Possible Intermediates and Reaction Pathways

C–C bond scission steps, which are often considered as rate-determining in ethane hydrogenolysis, are studied by the Unity Bond Index–Quadratic Exponential UBI–QEP method. The binding energies of atomic carbon with Group VIII and IB metal surfaces Ni(111), Pd(111), Pt(111), Rh(111), Ru(001), Ir(111), Fe(110), Cu(111), and Au(111) are estimated using experimental data on the adsorption of various species on these surfaces. These estimates are corrected using data from density functional theory (DFT) on the adsorption heats of the CH _x species. Metal surfaces are arranged in the following series according to the binding strength of a carbon atom: Cu(111) < Au(111) < Pd(111) < Ru(001) Pt(111) < Ni(111) Rh(111) < Ir(111) < Fe(110). The values of chemisorption heats range from 121 kcal/mol for Au(111) to 193 kcal/mol for Fe(110). The activity of these surfaces toward C–C bond scission increases in the same series. The results of this work suggest that the most probable C–C bond scission precursors are ethyl, ethylidyne, adsorbed acetylene, CH₂CH, CH₂C, and CHC. Theoretical data obtained by different methods are compared and found to agree well with each other. An overview of experimental data on ethane hydrogenolysis mechanisms is given.     

Adsorption properties of low-silica zeolite ZK-5

Water vapor, methanol, and argon adsorption isotherms, as well as the heat of adsorption of ammonia, methanol, and carbon dioxide have been determined on zeolite types A, ZK-4, and ZK-5. The results are explained by the distribution and the bond lengths of cations in the zeolite crystal lattice. It was shown that the exchange of 10–15% of the Na⁺ ions with Li⁺ in zeolite ZK-5 leads to a deformation of the 8-membered ring impeding the diffusion of adsorbed molecules. Cations contained in the 8-membered ZK-5 zeolite ring are bound more strongly to the framework, than in zeolite NaA, resulting in their smaller adsorption capacity.I. V. Grebenshchikov Institute of Silicate Chemistry, Russian Academy of Sciences, 199164 St. Petersburg, Russia. Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya, No. 11, pp. 2494–2500, November, 1992.     

Diffuse Reflectance IR Spectra of Molecular Hydrogen and Deuterium Adsorbed on Zinc Oxide

Diffuse reflectance IR spectroscopy is used to study hydrogen and deuterium adsorption on zinc oxide at room temperature and 77 K. At room temperature, H₂ and D₂ molecules are dissociatively adsorbed with the formation of hydrides and hydroxy groups of three types. At 77 K, diffuse reflectance spectra reveal the bands from molecular hydrogen and deuterium in addition to the dissociatively adsorbed forms. The presence of several bands of stretching H–H and D–D vibrations points to the nonuniformity of adsorption sites. This nonuniformity is also confirmed by the fact that, after heating zinc oxide from 77 K to room temperature in an atmosphere of hydrogen, only an insignificant portion of adsorbed molecular hydrogen dissociates. Most of dissociatively adsorbed hydrogen is formed without a molecular precursor. The dissociation of H₂ and D₂ most likely occurs on very active adsorption species so rapidly that the molecular precursor is not observed. The bond energy in molecular deuterium precursors of dissociation estimated from the fundamental vibration frequency and the overtone of D–D vibrations suggests moderate excitation of the bond. This agrees well with the conclusion that the dissociative adsorption of hydrogen and deuterium occurs without a molecular precursor.     

C-H stretching frequencies and C-H bond strengths of CH3X compounds

The influence of the X-group in CH₃X type molecules on the frequency of the C-H symmetrical stretching vibration has been investigated by means of normal coordinate analysis and CNDO/2 partial force constant and localized orbital calculations. The frequency of the C—H symmetrical stretching vibration was found to be dependent on the HCH angle α and the C—H symmetrical stretching force constant F_CH. By studying the partial force constants for a number of CH₃X type molecules a distinct relation between F_CH and the C—H bond length could be shown. With the help of localized orbital calculations a relation was found between the C—H bond length and the percentage ionic character of the C—H bond.     

Adsorption of carbon dioxide and nitrogen on zeolite rho prepared by hydrothermal synthesis using 18-crown-6 ether

Zeolite rho was prepared by hydrothermal synthesis using an 18-crown-6 ether (18C6) as a structure-directing agent, and the effects of the calcination temperature for removal of 18C6 on the physicochemical properties and CO₂-adsorption properties were investigated. CO₂ adsorption on zeolite rho calcined at 150 °C was lower than that on samples calcined at temperatures above 300 °C. For samples calcined above 300 °C, CO₂ adsorption increased with increasing calcination temperature up to 400 °C. It is thought that the pore volume for adsorption of CO₂ increased as a result of 18C6 removal, resulting in increasing CO₂ adsorption. A decrease in CO₂ adsorption for calcination from 400 °C to 500 °C was observed. The particle size of zeolite rho increased with increasing 18C6 molar ratio. Particle sizes of 1.0-2.1 μm and 1.4-2.6 μm were found by field-emission scanning electron microscopy and dynamic light-scattering, respectively. The particle size is controlled in these regions by adjusting the 18C6 molar ratio. XRD showed that zeolite rho samples with 18C6 molar ratios of 0.25-1.5 had high crystallinity. The adsorbed amount of CO₂ is almost constant, at 3.4 mmol-CO₂ g⁻¹, regardless of the 18C6 molar ratio. However, CO₂ selectivity, which is the CO₂/N₂ adsorption ratio, decreased. The amount of CO₂ adsorbed on zeolite rho is lower than that on zeolite NaX, but higher than that on SAPO-34. The CO₂/N₂ adsorption ratio for zeolite rho was higher than those for SAPO-34 and zeolite NaX.     

Adsorption change of cyclohexyl acetylene on gold nanoparticle surfaces

The conformational changes of cyclohexyl acetylene (CHAL) on gold nanoparticle surfaces were investigated by means of concentration- and temperature-dependent surface-enhanced Raman scattering (SERS). Depending on concentrations and temperatures, the spectral changes of the acetylene ν(C≡C) stretching vibration on gold nanoparticles appeared to be more conspicuous than those of cyclohexyl ring modes. The density functional theory (DFT) calculation was performed at the level of B3LYP/6-31G++(d,p) to compare the energetic stability and vibrational frequencies of the various conformers of cyclohexanethiol (CHT) and CHAL. The DFT calculations were also carried out at the level of B3LYP/LACVP** on the CHAL molecule adsorbed on Au clusters at several sites to explain the spectral changes of the acetylene ν(C≡C) stretching vibration.     

Static and Kinetic Studies on the Adsorption Behavior of Sulfadiazene

To investigate the nature of interactive forces between sulfadiazene molecules and alumina surface the experiments were performed for the adsorption of sulfadiazene (SD) from its aqueous sulution onto the alumina surfaces at 25 ± 0.2°C and the influence of factors such as increasing concentration of SD (4.0–20.0 × 10^–3 mol cm^–3), the time required for adsorption equilibrium, pH (2.0–12.0) and temperature (5–45°C) of the adsorption medium, the presence of ions like Cl^–, SO^2– ₄ and PO^3– ₄ (0.01–0.30 M) and organic solvents (5% v/v) were observed on the course of adsorption of SD. Various adsorption and kinetic parameters such as adsorption coefficient, the rate constants for adsorption and desorption were also evaluated. The results of the above cited studies facilitated to formulate the mechanisms of interaction between SD and alumina surfaces. From application view point the present work may be a potential tool for an effective chromatographic separation of sulfa drugs from industrial effluents.     

The Mechanism of Transformation of Triethylsilane by Collision-Free IR Multiphoton Excitation of Its Molecules

The mechanism of IR multiphoton dissociation of triethylsilane (TES) in the collision-free mode was studied. The principal unimolecular reactions are the C–Si and C–C bond rupture. The reactions of molecular elimination of CH₄, C₂H₄, and C₃H₆do not play a substantial part in the mechanism of chemical transformations of TES. The spontaneous fragmentation of C₂H₅and (C₂H₅)₂Si(CH₂)H radicals produced by C–Si and C–C bond rupture, respectively, was shown to be feasible.     

Carbon-13 kinetic isotope effects in the decarbonylation of formic acid of natural isotopic composition in phosphoric acid media

The¹³C kinetic isotope effect (K.I.E.) in the decarbonylation of formic acid of natural isotopic composition in 85% orthophosphoric acid, in 100% H₃PO₄, and in pyrophosphoric acid has been measured in different temperature intervals ranging from 19 to 133 °C. In 85% H₃PO₄ the carbon-13 K.I.E. is determined by the fractionation of carbon isotopes expected for C–O bond rupture (k ₁₂/k ₁₃=1.0531 at 70°C). In 100% H₃PO₄ the¹³C K.I.E. indicates that C–H bond rupture is the major component of the reaction coordinate motion (thek ₁₂/k ₁₃ lay in the range of 1.026–1.017 over the range 30–70 °C). In pyrophosphoric acid the fractionation factor for¹³C equals 1.010 at 19 °C. Activation parameters for the decarbonylation of H¹²COOH in phosphoric acid media have been determined also and suggestions concerning the intimate mechanisms of decarbonylation of formic acid in dilute and concentrated phosphoric acids are made.     

Catalytic Oxidative Deformylation of Polyethylene Glycols with the Participation of Molecular Oxygen

The kinetics of oxidation of diethylene glycol, triethylene glycol, and polyethylene glycols (PEGs) with molecular weights ranging from 400 to 2000 in the presence of Cu(II) ions and bases was studied. It was found that ethylene glycols can be oxidized by molecular oxygen in anhydrous media in a temperature range of 30–60°C at anomalouosly high rates which are higher than the rates of chain-radical PEG autoxidation by several orders of magnitude. Only terminal hydroxyl groups were subjected to oxidation. The reaction occurs with the cleavage of a C–C bond and results in the formation of formic acid and a PEG with the number of –(CH₂CH₂O)– groups lower than that in the parent compound by unity. The rate and selectivity of PEG oxidation were found to strongly depend on the molecular weight of the polymer; from diethylene glycol to PEG 2000, the specific rate of oxidation increased by a factor of 60 in terms of terminal hydroxyl groups. An oxidation mechanism was suggested, which involves the formation of ternary complexes [Cu²⁺···A^–···O₂], which undergo further degradation by a many-electron concerted mechanism to form formic acid and, probably, an unstable hemiacetal {RO–CH₂OH}. The rapid oxidative degradation of the latter leads to the formation of PEG with a lower molecular weight.     

Reductive cyclization of polychloroalkanes involving two dichloromethylene groups

Conclusions The reductive cyclization of CH₃CCl₂CH₂CCl₂CH₃ proceeds as 1,3-dechlorination to give 1,2-dichloro-1,2-dimethyl- and 1-chloro-1,2-dimethylcyclopropanes. It was postulated that the formation of the esters CH₂=C(CH₃)CH(CH₃)OC₂H₅ and CH₃CH=C(CH₃)CH₂OC₂H₅ as the main reaction products is due to isomerization with opening of the chloro-substituted cyclopropane ring and subsequent solvolysis of the isomerization product.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 8, pp. 1854–1856, August. 1980.     

Adsorption of mixtures of vapors on zeolites

Conclusions The adsorption of a binary mixture of vapors[CHCl₃-(C₂H₅)₂O]on zeolite NaX was studied at 60 and 72°. The thermodynamic properties of the adsorption solutions formed were described.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 7, pp. 1309–1311, July, 1964     

Molecular rearrangement induced by hydrogen co-adsorption: C2H4 on Pd(110)

The symmetry of low-coverage (0.1 ML) ethylene adsorbed on Pd(110) and the co-adsorption effect of hydrogen have been investigated by high-resolution electron energy-loss spectroscopy. Ethylene adsorbs with π-bond character intact on both clean and H-covered Pd(110) with characteristic C–C stretching vibrational energies at 178 and 186 meV, respectively. The symmetry of the adsorbed ethylene, however, drastically changes upon the co-adsorption of hydrogen: the C–C axis which is tilted to the clean Pd(110) surface (C₁ symmetry) is rearranged such that it becomes parallel to the H-covered Pd(110) surface (C₂ symmetry).