Intensities of IR stretching bands as a criterion of polarization and initial chemical activation of adsorbed molecules in acid catalysis. Ethane adsorption and dehydrogenation by zinc ions in ZnZSM-5 zeolite

A DRIFT study of ethane adsorbed by zinc cations in ZnZSM-5 prepared by chemical reaction of the hydrogen form of the zeolite with zinc vapor at 770 K, or by wet ion exchange, reveals unusual spectra of adsorbed C2H6 species. In addition to the weakly perturbed narrow bands in the region of C-H stretching vibrations, these spectra exhibit a very intense broad IR band with a frequency that is more than 200 cm(-1) lower than those of the C-H stretching vibrations of gaseous or physically adsorbed ethane. The very high relative intensity of this band indicates a very strong polarizability of the corresponding vibrational mode. It is concluded that these strongly polarized vibrations are closely connected with the subsequent heterolytic dissociation of ethane at moderately elevated temperatures, resulting in the formation of acidic hydroxyl groups and zinc ethyl fragments. At higher temperatures, the zinc ethyl fragments decomposed, resulting in the formation of zinc hydrides and ethylene. The unusual DRIFT spectrum of the molecular form of ethane adsorption by zinc cations represents a first example of anisotropy of polarizability of adsorbed molecules, which may provide a new explanation for selectivity of the acid-catalyzed reactions. In this connection, it is suggested to use the relative intensities of IR bands of adsorbed molecules as a reactivity index that is directly connected with chemical activation of adsorbed molecules via their polarization by the active sites.     

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.     

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.     

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.     

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.     

Simple evaluation of the adsorption states of benzene molecule on the hydroxyl,H+ and Na+ sites of Y-zeolite surfaces by using UV absorption spectroscopy

Chemical properties of benzene molecules adsorbed on the hydroxyl, H⁺ and Na⁺ sites of Y-zeolite surfaces were investigated by using UV absorption and FT-IR spectroscopies. The analyses on the IR peaks assigned to a C–H out-of-plane vibration mode revealed two different adsorption states of benzene: (1) a benzene molecule located a little distance from the hydroxyl groups of zeolite inner walls, and (2) a benzene molecule positioned a short distance from the H⁺ or Na⁺ sites of zeolite walls. Furthermore, the electronic properties of benzene molecules adsorbed on these Y-zeolites were investigated by UV absorption measurements. The vibrational splitting in UV absorption spectra of benzene provided the information about IR-inactive skeletal vibrations of the benzene ring, such as C–C–C in-plane bending and breathing modes. The benzene molecules strongly interacting with H⁺ or Na⁺ sites of Y-zeolites showed smaller breathing vibration energy as compared to benzene in gas or liquid phases, clearly indicating the stabilization of the benzene ring. In contrast, the benzene molecules weakly interacting with hydroxyl groups of siliceous USY zeolite were barely stabilized.     

Study of the catalytic properties of pentasil-containing composites in transformation of hydrocarbons 4. Formation of products of condensation during oligomerization and aromatization of ethylene on the pentasil-aluminum oxide system

The process of formation of products of condensation (PC) during oligomerization (280°C) and aromatization (500°C) of ethylene in the presence of H-pentasil (HTsVM)--Al₂O₃ was studied by IR spectroscopy, DTA, DTG, and chemical analysis. The mutual effect of the components of the composite on the formation of PC in aromatization of C₂H₄ was detected. PC are formed with the participation of acid sites both inside the channels and on the outer surface of the zeolite. Three types of PC were found: linear polyene structures with C/H=0.8 (oligomerization), aromatic structures with C/H=1.5 (aromatization on HTsVM), and crosslinked polyene structures with C/H=2.45 (aromatization on Al₂O₃). The composition and structure of the PC formed on HTsVM during transformations of ethylene are a function of the molecular-sieve properties of the pentasil and the experimental conditions.For previous communication, see [1].Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 6, pp. 1244–1250, June, 1990.     

Cyclopropane Adsorption on Zeolites Studied by IR Spectroscopy: II. Adsorption and Conversion of Cyclopropane on Na- and Ca-Exchanged Zeolite Y

The adsorption and isomerization of cyclopropane on the calcium- and sodium-exchanged zeolite Y are studied by IR spectroscopy. Cyclopropane is adsorbed on CaY in two different forms. Weaker adsorption is related to residual sodium ions, which were not completely removed from the zeolite by ion exchange. This form can be removed from the surface after evacuation of the samples at room temperature. Stronger adsorption is attributed to the Ca ions. It is stable up to 100°C. The corresponding diffuse-reflectance spectrum indicates the C _2v symmetry of the cyclopropane complex with Ca ions. At 200°C cyclopropane adsorbed on the calcium-exchanged zeolite converts to propylene. On the sodium-exchanged zeolite, this reaction only occurs at 400°C. The reaction coordinate of cyclopropane isomerization on CaY is related to the simultaneous cleavage of the C–C bond in the cyclopropane ring and hydrogen atom transfer from one of the CH₂ fragments to another. The reaction coordinate corresponds to a combination of the stretching vibration of the C–C bond with the fan vibration of the CH₂ group and the stretching vibration of the CH bond. These composite vibrations result in the strong polarization of the C–C and C–H bonds and, hence, exhibit anomalously high molar absorption coefficients in the IR spectrum.     

Crystal Structures and Photoluminescence of Two Inorganic–Organic Hybrids of Anderson Anions

Two supramolecular compounds (Hbipy)₂[Cr(OH)₆Mo₆O₁₈H](bipy) (1) and (Hbipy)₃[Al(OH)₆Mo₆O₁₈]·3H₂O (2) were synthesized and their crystal structures were analyzed with x-ray diffraction technique. In 1 the Anderson anion with six hydroxyl groups forms six hydrogen bonds with bipy molecules, forming a supramolecular layer, the layers are linked by hydrogen bonds between anions. In 2 the Anderson anion with three hydroxyl groups and terminal/bridging oxygen atoms forms six hydrogen bonds with bipy molecules, and lattice water molecules and the anions form also hydrogen bonds, constructing a supramolecular architecture. The intensive emission in 650–740 nm of 1 is attributed to R-lines of Cr³⁺ and the high intensity may be caused by energy transfer of bipy molecules through hydrogen bonds. In contrast, 2 gives only the π* → π emission of bipy molecules at 460 nm.     

Quantum-chemical simulation of the adsorption and catalytic processes in a pore of the NaX-type zeolite

In terms of density functional theory, we perform quantum-chemical calculations of variations in the energetic and structural characteristics of methanol, benzene, and propane molecules as they are adsorbed in a pore of the NaX-type zeolite and on a fragment of its cavity. The position of benzene, alcohol, or propane molecules near the Na cation is the most energetically favorable; in addition, the alcohol molecule forms a hydrogen bond with the oxygen bridge of the zeolite. If two alcohol molecules are adsorbed, they also are localized near the Na cation and form an intermolecular hydrogen bond. A comparative simulation of the catalysis of the propane dehydration reaction in a cavity of NaX-type zeolite and on its fragment is performed; the effect of the steric factors on the simulation results is analyzed.     

IR Spectroscopic Study of Alkane Interaction with the Brønsted Acid Sites of Hydrogen-Exchanged Zeolites

Diffuse-reflectance IR spectroscopy is used to study the interaction of C₃ and C₆ alkanes (propane, n-hexane, 3-methylpentane, and cyclohexane) with the Brønsted acid sites of hydrogen-exchanged mordenite, ferrierite, ZSM-5, and faujasite. It is found that a shift of the absorption band of the stretching vibrations of acidic Si(OH)Al groups toward lower frequencies (OH) due to the formation of a hydrogen bond with adsorbed alkanes increases in the following series: OH(propane) < OH(n-hexane) = OH(3-methylpentane). The accessibility of Si(OH)Al groups to alkane molecules is determined by the dimension of rings through which molecules enter zeolite channels and cavities. It follows from the measured OH values that the strength of Brønsted acid sites decreases in the following series: HZSM-5 > H-mordenite H-ferrierite HY. The difference between the three high-silica zeolites is not great. The results obtained are compared with the published IR data on Si(OH)Al groups of zeolites with adsorbed alkanes and other weak bases.     

Vibrational analysis of sodium α-, β- and γ-hydroxybutyrates. Inter- and intramolecular hydrogen bonds

The infrared and Raman spectra of sodium α-, β- and γ-hydroxybutyrates and their deuterated analogues are examined in the 4000-100 cm⁻¹ range and an assignment of the fundamental vibrations is given. Based on the localization of the asymmetric stretching vibrations ν_asOH and the out-of-plane vibration γOH, inter- and/or intramolecularly hydrogen-bonded forms are proposed: the low frequencies of ν_asOH (<3200 cm⁻¹) and high frequencies of γOH (≈800 cm⁻¹) argue in favour of the existence of intramolecular hydrogen bonding. Sodium α-hydroxybutyrate exhibits as a chelate ring with an intramolecular hydrogen bond between hydroxyl and carboxyl groups, whereas sodium, β-hydroxybutyrate has the two association forms with inter- and intramolecular hydrogen bonds. Sodium γ-hydroxybutyrate exists as a hydrogen-bonded polymer, with an intermolecular hydrogen bond between the hydroxyl groups and between the hydroxyl and carbonyl groups. At a crystallization temperature above 50°C, only the α- salt showed a structural change indicating the existence of intra- and intermolecular hydrogen bonds. This result is confirmed by differential scanning analysis.     

Intramolecular hydrogen bonding with the participation of the nitrogen atom of five- and six-membered heterocycles

A comparison of the frequencies of the valence vibrations of the OH group and of the chemical shifts of the protons of the hydroxyl groups in -naphthol derivatives containing the nitrogen atom of the condensed ring of pyridine, pyrazine, 1,2,5-selenadiazole, 1,2,5-thiadiazole, 1,2,5-oxadiazole, and imidazole in the peri position to the hydroxyl group is indicative of the decisive effect of the molecular geometry on intramolecular hydrogen bonding in systems with rigidly fixed configurations. All conditions being equal, the intramolecular hydrogen bond is considerably weaker when the nitrogen atom is part of a five-membered rather than a six-membered heterocycle. This is explained not only by an increase in the distance between the proton donor and acceptor (which may be the same in some cases), but also by the greater deviation of the orbital of the unshared electron pair of the nitrogen of the five-membered heterocycle from the O...N line and, thus, by its greater distance from the hydrogen atom. For the same favorable molecular geometry, the OH...N bond is stronger than the OH...O bond because of the high basicity of the nitrogen atom.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 238–244, February, 1971.     

Effect of Ethylene Glycol on the Molecular Organization of H2O in Comparison with Methanol and Glycerol: A Calorimetric Study

Excess partial molar enthalpies of ethylene glycol, H ^E _EG, in binary ethylene glycol–H₂O, and those of 1-propanol, H ^E _IP, in ternary 1-propanol–ethylene glycol (or methanol)–H₂O were determined at 25°C. From these data, the solute–solute interaction functions, H ^E _EG–EG = N(H ^E _EG/n _EG) and H ^E _1P–1P = N(H ^E _1P/n _1P), were calculated by graphical differentiation without resorting to curve fitting. Using these, together with the partial molar volume data, the effect of ethylene glycol on the molecular organization of H₂O was investigated in comparison with methanol and glycerol. We found that there are three concentration regions, in each of which the mixing scheme is qualitatively different from the other regions. Mixing scheme III operative in the solute-rich region is such that the solute molecules are in a similar situation as in the pure state, most likely in clusters of its own kind. Mixing scheme II, in the intermediate region, consists of two kinds of clusters each rich in solute and in H₂O, respectively. Thus, the bond percolation nature of the hydrogen bond network of liquid H₂O is lost. Mixing scheme I is a progressive modification of liquid H₂O by the solute, but the basic characteristics of liquid H₂O are still retained. In particular, the bond percolation of the hydrogen bond network is still intact. Similar to glycerol, ethylene glycol participates in the hydrogen bond network of H₂O via-OH groups, and reduces the global average of the hydrogen bond probability and the fluctuations inherent in liquid H₂O. In contrast to glycerol, there is also a sign of a weak hydrophobic effect caused by ethylene glycol. However, how these hydrophobic and hydrophilic effects of ethylene glycol work together in modifying the molecular organization of H₂O in mixing scheme I is yet to be elucidated.     

IR spectra and structure of poly(vinylamide) complexes with hydrogen peroxide

The IR spectra of anhydrous thin films of hydrogen peroxide complexes with cyclic and aliphatic poly(N-vinylamides) have been studied. Splitting of a band due to stretching vibrations of C=O groups in the IR spectra of the poly(vinylcaprolactam) complex is accounted for by the resonance interaction of v _C=O vibrations of two monomer units linked by a hydrogen peroxide molecule. The formation of a N-H···O=C intramolecular hydrogen bond between neighboring polymer units is responsible for the observed low absorption of hydrogen peroxide by N-vinylamide polymers and copolymers. The energy E _H of hydrogen bonds formed between hydrogen peroxide and polymer chain fragments has been estimated by quantum-mechanical calculations. Depending on the complex structure, the value of E _H varies from 13 to 29 kJ/mol.     

镧改性提高ZSM-5分子筛水热稳定性

基于12T 团簇模型, 利用密度泛函理论(DFT)研究了ZSM-5 分子筛的水解脱铝机理以及镧改性提高ZSM-5分子筛水热稳定性的机理. 对未改性分子筛水解脱铝机理的研究表明, 首先是第一个水分子吸附在分子筛表面的酸性位上, 对分子筛的Al—O键起弱化作用, 使Al—O键伸长; 接着第二个水分子吸附到分子筛表面,分别与第一个水分子和分子筛骨架形成氢键, 进一步弱化与其最邻近的Al—O键, 并引致该键断裂. 同样, 其它的三个Al—O键也被削弱并逐一断裂, 从而发生分子筛水解脱铝现象. 引入的镧物种与分子筛骨架的四个O原子成键, 将铝包埋, 增加了分子筛孔壁厚度, 增大了水分子攻击铝的空间位阻, 抑制了水分子对Al—O键的弱化, 从而延缓Al—O键的断裂, 提高分子筛的水热稳定性. 计算的水分子吸附能和水解能进一步证实镧的引入提高了ZSM-5分子筛的水热稳定性.     

Stoichiometry of Oxidation Reactions Involving α-Oxygen on FeZSM-5 Zeolite

The stoichiometry of the low-temperature reaction between surface -oxygen formed by decomposing N₂O over Fe-containing ZSM-5 zeolite and methane, hydrogen (deuterium), and carbon monoxide is studied. Methane and hydrogen react with -oxygen in stoichiometric ratios of 1 : 1.8 and 1 : 1.6, respectively. The observed stoichiometry is due to the mechanisms of the corresponding reactions. According to a mechanism proposed for the interaction of -oxygen with methane and hydrogen, this reaction is accompanied by the dissociation of CH₄and H₂molecules. For hydrogen, such a mechanism is supported by IR spectroscopic studies of resulting surface compounds, namely, of new hydroxyl groups that were formed on the zeolite surface in the course of the reaction. -Oxygen reacts with CO in the ratio of 1 : 1 to form CO₂in amounts equal to those of -oxygen on the surface.     

Effects of poly(ethylene oxide) adsorption on the dispersion of smectites

The adsorption of polymers on clay is important in many applications. However the mechanisms of poly(ethylene oxide) (PEO) adsorption on smectite and its effect on suspension rheology are not well elucidated at present. The aim of this study was to investigate the mechanisms responsible for PEO sorption on smectite and how this impacts on dispersive behavior of smectite suspension. The results indicated that the hydrophobic interaction between CH₂CH₂ groups and siloxane surface is the major driven force for PEO adsorption on smectite. In addition, PEO adsorbed preferentially on the surface of low-charge smectite and the delamination of low-charge smectite in water was enhanced due to PEO adsorption presumably due to the hydrophilic ether oxygen of adsorbed PEO.     

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.     

EPR study of the mechanism of interaction of NO and NO2 molecules with zeolite surfaces

The adsorption of the paramagnetic molecules of NO and NO₂ by zeolites in the alkali and alkaline earth cationic forms has been studied by EPR and reflectance spectroscopic methods. The change in the EPR spectra of adsorbed nitric oxide with increase in the degree of covering of the surface of the alkali cationic form of the zeolites, and also the nature of the change in the spectra when oxygen is adsorbed on zeolites on which NO has previously been adsorbed, indicate the existence of two types of adsorption center. At low degrees of covering of the surface, on the order of 10¹⁸ g^–1, as can be judged from the EPR spectra, the adsorbed NO molecule is strongly polarized and the unpaired electron is almost completely localized on the oxygen atom. At high degrees of covering, for an appreciable proportion of the NO molecules, the bond with the surface is weaker. In this case, the EPR spectra show a hyperfine structure (HFS) with a constant which changes with change in the cation in the order Li⁺ Na⁺ K⁺. The replacement of the singly charged Na⁺ by the doubly charged Ca²⁺ produces a marked change in the adsorption properties of the zeolite. The adsorption of NO on CaA leads not only to polarization of the adsorbed molecule but also to transfer of the electron from the nitrogen atom to the atoms of the adsorbent; this is recorded in the EPR spectrum in the form of an F-center. On further adsorption, the NO molecules are adsorbed both on the nitrogen atom and on the oxygen atom of the first molecule; thus, NO₂ and N₂O are formed.