Distribution of the strength of acid sites in mildly steamed HZSM-5 studied by IR spectroscopy

Steaming resulted in modification of the distribution of acid strength of OH groups. The contribution of less acidic sites increased and new, very strongly acidic hydroxyls characterized by IR band at 3590 cm^-1 appeared.     

Influence of the chemical composition of the catalyst on the dehydrochlorination of 1,2-dichloroethane

The experiments have been performed by a microreactor pulse method at 623 K. The activity and selectivity of the catalysts, alumina/metal chloride and silica gel/metal chloride or oxide, have been found to depend on the donor-acceptor properties of their surfaces.

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Ammonia sorption by Dawson acid studied by IR spectroscopy and microbalance

Using in-situ microbalance and infrared spectroscopy techniques the double ammonia proton complexation was traced. The results confirm the formation of N₂H₇⁺ dimer in solid Dawson acid H₆P₂W₁₈O₆₂, previously reported only for N₂H₇I and for (N₂H₇)₄SiW₁₂O₄₀. The formation of such dimers was evidenced by the microbalance results, the molar ratio of ammonia to proton was measured as 2:1 at 10.7 kPa and 298 K. The formation of NH₄⁺ monomer (band at 1410 cm⁻¹) and N₂H₇⁺ dimer (1460 cm⁻¹) was revealed by IR spectroscopy. Enthalpy of ammonia sorption on Dawson structure was calculated to be −127.9 kJ mol⁻¹, indicating the lower acid strength of Dawson-type compared to that of the Keggin-type heteropolyacids, like H₄SiW₁₂O₄₀.     

Selective oxidation of methanol over supported vanadium oxide catalysts as studied by solid-state NMR spectroscopy

Methanol catalytic oxidation over VO_x/Al₂O₃, VO_x/ZrO₂ and VO_x/MgO catalysts has been studied by solid-state nuclear magnetic resonance (NMR) spectroscopy. It was found that stronger acid sites in VO_x/Al₂O₃ result in almost the same selectivities for dimethoxymethane, paraformaldehyde and formic acid, and weaker acid sites in VO_x/ZrO₂ favor the formation of paraformaldehyde, while the VO_x/MgO catalyst with the base support shows high selectivity for formate. Supporting VO_x species on γ-Al₂O₃ and ZrO₂ leads to the formation of Brønsted acid sites as revealed by the adsorption of probe molecules. The acid strength of Brønsted acid sites on the VO_x/Al₂O₃ catalyst is found to be stronger than that of the VO_x/ZrO₂ catalyst which has the acid strength similar to zeolite HZSM-5's. The proposed bridging hydroxyl models accounting for the Brønsted acid sites formation were also confirmed by quantum chemical calculation.     

Composition of plasma-chemical silicon dioxide films as studied by IR spectroscopy

The results of a comparative analysis of the compositions of silicon dioxide films prepared by the decomposition of tetraethoxysilane vapor in a glow discharge and the deposition of the products onto a substrate are presented. The compositions of films prepared in gas mixtures containing argon, oxygen, tetraethoxysilane vapor, and NaCl were compared using IR spectroscopy. The electric discharge was excited at a frequency of 19 kHz and in a radiofrequency range at 81.36 MHz. It was found that the additives of oxygen and sodium-containing vapors exerted a noticeable effect on the composition of the films. The compositions of the films prepared at the low-frequency and high-frequency discharge excitation were also different.     

Adsorption sites of an iron-aluminum catalyst for ammonia oxidation as studied by the IR spectroscopy of the adsorbed NO probe molecule

The state of surface adsorption sites in the IK-42-1 oxide catalyst for ammonia oxidation depending on catalyst preparation conditions (the nature of raw materials and the temperature of calcination) was studied in this work with the use of the diffuse reflectance IR spectroscopy of the adsorbed NO probe molecule. Hematite, which was prepared by a sulfate or chloride technology, was used as the starting raw material; Al₂O₃ binding agents were prepared by the reprecipitation or hydration of thermally activated gibbsite; and acetic or nitric acid was used as an electrolyte. The samples were calcined at 900–1000°C. It was found that mono- and dinitrosyl complexes with reduced coordinatively unsaturated Fe²⁺ cations and nitrite-nitrate complexes were formed upon the adsorption of NO on the catalyst surface (regardless of the catalyst preparation conditions). The samples differed in the amount and degree of coordinative unsaturation of adsorption sites depending on the preparation conditions. It was concluded that the most coordinatively unsaturated Fe²⁺ adsorption sites observed were formed on the surface of a solid solution of iron cations in aluminum oxide, which was formed in the course of catalyst preparation. It was found that an increase in the catalyst calcination temperature resulted in a decrease in the number of coordinatively unsaturated adsorption sites, which correlated with the observed decrease in the yield of NO. This correlation had the shape of a saturation curve, which can reflect the occurrence of a reaction in the diffusion mode at high degrees of conversion for the majority of catalysts.     

Models of active sites in supported Cu metal catalysts in 1,2-dichloroethane dechlorination. DFT analysis

It is suggested that a set of discrete Cu nanoclusters satisfying the conditions of structural and electron stability should be used as models of active sites on supported metal catalysts. The close-packed Cu₂₀ tetrahedral nanocluster that satisfies these two conditions was taken as a base model of active sites on supported copper catalysts. Theoretical analysis of two possible mechanisms of C-Cl bond dissociation of 1,2-dichloroethane on copper catalysts was performed by the density functional theory method. The first mechanism involves sequential splitting of C-Cl bonds in the molecule in three stages with further stabilization of chloroalkyl intermediates (stepwise mechanism). All these stages are activated. The limiting stage is the one that corresponds to dissociation of the first C-Cl bond with an activation energy of E# = 34.3 kcal/mol. The second mechanism corresponds to the simultaneous elimination of two chlorine atoms from 1,2-dichloroethane with liberation of ethylene in the gas phase; this is a one-stage process with an activation energy of E# = 26.1 kcal/mol (direct mechanism). A comparison of the two reaction routes shows that the direct mechanism is most probable on copper catalysts.     

Polymerization of 1,2-butylene oxide initiated by triphenylmethyl salts. I. Kinetics with 1,2-dichloroethane as solvent

1,2-Butylene oxide was polymerized in 1,2-dichloroethane in the temperature range ?20 to + 10°C. Triphenylmethyl salts of hexafluoroantimonate (Ph₃CSbF₆), hexafluoroarsenate (Ph₃CAsF₆), and hexafluorophosphate (Ph₃CPF₆) were employed as initiators. The reactions are characterized by a fast initiation process accompanied by rapid consumption of monomer. This initial stage is followed by a relatively slower polymerization during which the rate of monomer consumption is first-order in respect to monomer. Variation of the counteranion produces very little influence on the rate. The difference between the thermodynamic parameters found for the first and second stages of the reaction are explained on the basis of two different polymerization mechanisms. The linear dependence of rate on initiator concentration indicates that propagation takes place by ion-pairs only within the range investigated here.     

Structural relaxation in amorphous 1,2-dichloroethane studied by transformation between conformation isomers

Structural relaxation in amorphous 1,2-dichloroethane (DCE) samples prepared by vapor deposition on cold substrates were studied by Raman scattering. The gauche and trans molecules of DCE were found to coexist in amorphous states immediately after the deposition, and structural relaxation occurred with temperature elevation before crystallization. Mole fraction of the gauche isomer increased during this relaxation process, although trans is the stable isomer in gaseous and crystalline states. At the final amorphous stage immediately before crystallization, the gauche mole fraction was close to the mole fraction of the supercooled liquid state. It was also found that trans molecules located at positions with lower density were more easily transformed into the gauche conformation, while the distribution of the local structure around the resultant gauche molecules remained almost unchanged during the structural relaxation. Such behaviors of amorphous DCE are discussed from the viewpoint of the characteristic molecular structure of DCE.     

Mechanism of carboxylic acid formation on vanadium-containing oxide catalysts

The surface intermediates in acrolein oxidation into acrylic acid on a V-Mo oxide catalyst, formaldehyde oxidation into formic acid on a V-Ti catalyst, and 3-pyridinecarboxaldehyde and -picoline oxidation into nicotinic acid on a V-Ti catalyst are identified by in situ IR spectroscopy. The acids are found to form by similar mechanisms. The intermediates in acid formation are saltlike surface compounds (formates, acrylates, and nicotinates) stabilized on vanadium ions. The role of vanadium in acid formation is discussed in terms of the mechanisms suggested.__________Translated from Kinetika i Kataliz, Vol. 46, No. 2, 2005, pp. 233–242.Original Russian Text Copyright © 2005 by Popova, Andrushkevich, Zakharov, Chesalov.     

Investigation of the surface of Ni-Pd catalysts by IR spectroscopy

The reaction of carbon monoxide with the surface of Ni, Pd, and Ni-Pd catalysts, deposited on -Al₂O₃, was investigated at 25°C by IR spectroscopy in conjunction with an adsorption volumetric technique. The IR spectra contained the following absorption bands (v, cm^–1): 2020–2100 (AB1) (linear and subcarbonyl forms of adsorbed CO); 1945–1985 (AB2) (bridging); 1920–1940 (AB3) with a shoulder at 1870–1885 (AB4) (bridging and many-center). In the spectrum of Ni the absorption band AB5 appears at 1770–1780 (CO-Ni⁺). It appears with surface coverage a 1.3 mole CO/mole M. The optical density (A) of AB1 for Ni₉₇Pd₃ is appreciably higher than for the other investigated samples. In the Ni-Pd catalysts the intensity of AB2 in relation to AB1 is higher than in nickel. The introduction of K⁺ ions into the support of the bimetallic sample reduces the optical density of AB1. In modified Ni-Pd-K the AB3 and AB4 bands disappear.N. D. Zelinskii Institute of Organic Chemistry, Russian Academy of Sciences, 117913 Moscow. Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya, No. 6, pp. 1276–1282, June, 1992.     

Acid sites and oxidation center in molybdena supported on tin oxide as studied by solid-state NMR spectroscopy and theoretical calculation

Solid-state NMR spectroscopy and density functional theory (DFT) calculations were employed to study the structure and properties, especially the solid acidity, of molybdenum oxide supported on tin oxide. As demonstrated by solid-state NMR experiments, Mo species are mainly dispersed on the surface of SnO(2) support rather than significantly dissolved into the SnO(2) structure and Br?nsted as well as Lewis acid sites are present on the MoO(3)/SnO(2) catalyst. Acid strength of the supported metal oxide is stronger than those of zeolites, e.g., HY and HZSM-5, though the concentration of acid sites is relatively lower. The DFT calculated (13)C chemical shift for acetone adsorbed on MoO(3)/SnO(2) is in good agreement with the experimental value, which confirms our proposed structure of -Mo-(OH)-Sn- for the Br?nsted acid site. Reducibility of the supported metal oxide is also demonstrated by solid-state NMR experiments and an active oxidation center of this catalyst is proposed as well.     

DFT analysis of the mechanism of 1,2-dichloroethane dechlorination on supported Cu-Pt bimetallic catalysts

The reaction routes of 1,2-dichloroethane dechlorination to ethylene on discrete nanoclusters that served as models of the active sites of supported Cu-Pt catalysts were calculated. Two reaction pathways were predicted. The first route corresponds to sequential elimination of the chlorine atoms from 1,2-dichloroethane; this is a three-stage reaction that occurs via two stable intermediates (stepwise mechanism). The limiting stage is the stage that corresponds to the dissociation of the first C-Cl bond. The second channel corresponds to a simultaneous one-stage elimination of two chlorine atoms (direct mechanism). Both reaction routes are thermodynamically possible, but the stepwise process is more probable, in contrast to the process on monometallic Cu catalysts. For the stepwise process, the vibrational spectra of stable intermediates were calculated for identification of the latter. A set of spectral data characteristic for the stepwise mechanism were determined. The three-step molecular mechanism suggested for 1,2-dichloroethane dechlorination to ethylene is compared with several kinetic schemes known from the literature. Possible modifications of the reaction route that forms ethane and monochloroethane are analyzed.     

Determination of basic sites on the surface of metal oxide catalysts by desorption of benzoic acid

A method has been developed for the determination of the concentration of basic sites on white and deeply colored solid porous materials, which is based on the displacement of adsorbed benzoic acid by acetic acid. The determination is rapid and gives results comparable with those obrained by the titration with benzoic acid using thymolphthalein as indicator.

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Polymorphism of phenylpyruvic acid studied by IR, Raman and solid state C NMR spectroscopy

Polymorphs I and II of phenylpyruvic acid are obtained as mixtures of both crystal forms or relatively pure crystals, from different solvents. Polymorph I is more stable than polymorph II at room temperature. Spectral characteristics of these polymorphs are discussed on the basis of IR, Raman and solid state ¹³C NMR spectra. Also, the assignment of the IR features observed in the 1600–1700 cm⁻¹ region is re-investigated by referring to the spectra of heavy-atom substituted derivatives. It is suggested that the C=O stretching band is split by the crystal field for both polymorphs.     

High-pressure induced conformational and phase transformations of 1,2-dichloroethane probed by Raman spectroscopy

1,2-Dichloroethane (DCE) was loaded into diamond anvil cells and compressed up to 30 GPa at room temperature. Pressure-induced transformations were probed using Raman spectroscopy. At pressures below 0.6 GPa, fluid DCE exists in two conformations, gauche and trans in equilibrium, which is shifted to gauche on compression. DCE transforms to a solid phase with exclusive trans conformation upon further compression. All the characteristic Raman shifts remain constant in fluid phase and move to higher frequencies in the solid phase with increasing pressure. At about 4-5 GPa, DCE transforms from a possible disordered phase into a crystalline phase as evidenced by the observation of several lattice modes and peak narrowing. At 8-9 GPa, dramatic changes in Raman patterns of DCE were observed. The splitting of the C-C-Cl bending mode at 325 cm-1, together with the observation of inactive internal mode at 684 cm-1 as well as new lattice modes indicates another pressure-induced phase transformation. All Raman modes exhibit significant changes in pressure dependence at the transformation pressure. The new phase remains crystalline, but likely with a lower symmetry. The observed transformations are reversible in the entire pressure region upon decompression.     

Effect of dioxane on the hydration of human serum albumin as studied by isothermal calorimetry and IR spectroscopy

A comparison of isothermal calorimetry data on the interaction of human serum albumin with water in the presence and absence of dioxane and of the isotherms of adsorption of vapors of water and dioxane on HSA as measured by IR spectroscopy made it possible to suggest an experimental method for isolating the contribution from the organic solvent to the thermodynamic and sorption characteristics of hydration of the protein over the entire range of water activities.     

Substituent effects in porphyrin dimer complexes studied by IR spectroscopy

A series of lanthanide porphyrin dimers have been synthesized and investigated with IR spectroscopic techniques. The spectra of the porphyrin dimers are compared not only with each other but also with those of their component monomer units. The experimental results exhibit that the IR spectra of the porphyrin dimers are closely related to those of their corresponding monomers. A detailed analysis of the IR spectra between the porphyrin dimers and monomers suggest that the dimer molecules can be treated as regular derivatives of metalloporphyrin monomers despite the symmetries of these two systems being different. The dimerization of the porphyrin rings only result in frequency shifts and intensity changes of the IR spectra. These shifts are attributed to the induced π–π interactions between these two macrocycles. The downshifts of the frequencies observed in Ce(OEP)₂ further indicate that the π–π interactions intrinsically decrease the bond strength of the entire molecule. Additionally, only the relative intensities instead of the frequencies of the ethyl vibrations in the region 2800–3000 cm⁻¹ are observed to be sensitive to the types and the positions of the substituent groups. These observations suggest that these ethyl vibrational modes of the OEP moiety can be used as characteristic bands to monitor subtle deformations of the porphyrin rings caused by the substituent groups in the dimer complexes.