Heterogeneous asymmetric reactions. 23. Enantioselective hydrogenation of ethyl pyruvate over cinchonine- and α-isocinchonine-modified platinum catalysts

The enantioselective hydrogenation of ethyl pyruvate to (S)-ethyl lactate over cinchonine- and -isocinchonine-modified Pt/Al₂O₃ catalysts was studied as a function of modifier concentration and reaction temperature. The maximum enantioselectivities obtained under the applied mild conditions were 89% ee using cinchonine (0.014 mmoldm^–3, 1 bar H₂, 23°C, 6% AcOH in toluene), and 76% ee in the case of -isocinchonine (0.14 mmoldm^–3, 1 bar H₂, –10°C, 6% AcOH in toluene). Since -isocinchonine of rigid structure exists only in anti-open conformation these data provide additional experimental evidence to support the former suggestion concerning the dominating role of anti-open conformation in these cinchona-modified enantioselective hydrogenations.     

Molybdenum nitride for the direct decomposition of NO

The physico-chemical properties of passivated -Mo₂N have been investigated. The material showed high activities for NO direct decomposition: nearly 100% NO conversion and 95% N₂ selectivity were achieved at 450C. The amount of O₂ taken up by -Mo₂N increased with temperature rise and reached 3133.9 molg^–1 at 450C; we conclude that there formation of Mo₂O_xN_y occurred. This oxygen-saturated -Mo₂N material was catalytically active: NO conversion and N₂ selectivity were 89 and 92% at 450C. We found that by means of H₂ reduction at 450C, Mo₂O_xN_y could be reduced back to -Mo₂N and the oxidation/reduction cycle is repeatable; such a behaviour and the high oxygen capacity (3133.9 molg^–1) of -Mo₂N suggest that -Mo₂N is a promising catalytic material for automobile exhaust purification.     

Cooperative inclusion of sodium 1-pyrenesulfonate by γ-cyclodextrin

Summary The interaction of -cyclodextrin(-CD) with sodium 1-pyrenesulfonate(PS) was studied spectrophotometrically. -CD was found to cause much larger decrease in the absorption maxima of PS than -CD. The fluorescence spectra of PS in the presence of -CD showed excimer emission, while those of PS with -CD showed only monomer emission, indicating that -CD forms 12 (-CDPS) complexes in which two PS molecules are included in the -CD cavity in a face-to-face fashion. The binding isotherm showed a sigmoidal curve. The association constants were estimated by computer simulation of the binding curve. The 12 (CDPS) complex was found to be much more stable (K=10⁶ M^–1) than the 11 complex (K=1 M^–1). At high concentration of -CD another -CD cooperates in binding two PS molecules, resulting in the formation of a 22 complex.     

Lean NOx catalysis over alumina‐supported catalysts

aluminasupported catalysts show promise as lean NO_x catalysts. The role of alumina in influencing the structural and chemical properties of the active phase supported on it is discussed using some effective aluminabased lean NO_x catalysts. These include Ag/Al₂O₃, CoO_x/Al₂O₃ and SnO₂/Al₂O₃. Alumina plays an important role in stabilizing Ag in the oxidic phase and cobalt in the 2+ oxidation state. For SnO₂/Al₂O₃, alumina increases the SnO₂ surface area. On both Ag/Al₂O₃ and SnO₂/Al₂O₃, alumina also participates actively in the NO_x reduction reaction. An active organic intermediate is formed on Ag or Sn oxide which reacts with NO_x subsequently on alumina to form N₂.     

Temperature programmed oxidation of Cu/ZnO catalysts with N2O: an attempt for characterization of copper catalysts

For characterization of the surface structure of metallic copper formed on the support, temperature programmed oxidation (TPO) with N₂O was carried out over various Cu/ZnO catalysts. Four peaks of the N₂ formation (, , and ) were observed at 223, 400, 545 and 600 K in the TPO runs. The average copper crystallite size estimated from the sum of the amount of - and -peaks agreed fairly with those determined by X-ray diffraction and transmission electron microscopy. It was concluded that - and -peaks resulted from the oxidation of metallic copper atoms on the steps, corners and/ or defect sites, and on the flat sites of the surface of copper crystallites, respectively, while - and -peaks resulted from the bulk oxidation of copper.     

Chemistry of Combustion Waves in Substances with a Complicated Structure of Reagent Molecules. 1. Structure of the Front of Rich Isopentane Flame

Results on the structure of the lowtemperature relaxation zone of the front of a laminar Bunsen flame of isoC₅H₁₂ (2methylbutane) under atmospheric pressure are presented. The flame of a premixed mixture isoC₅H₁₂ + O₂ + Ar with a fueltoair equivalence ratio of 1.7 is examined. The mass fluxes, total rates of reactions of matter consumption and expenditure, balance of substances, and profiles of bulk heatrelease rates are calculated on the basis of the experimental concentration and temperature profiles. The results obtained indicate that there is a vast region of lowtemperature conversion of isopentane in the flame front. It is found that only part of the products sampled by the microprobe from different points of the flame front results from transformations in the lowtemperature region, namely, oxygen, isopentane, water, carbon monoxide, propane, methane, and methanol. Ethylene, propylene, hydrogen dioxide, and formaldehyde are present in the lowtemperature zone in insignificant amounts; they are secondary products of conversion of methyl and propyl radicals. It is assumed that the observed feature is a result of the competing interaction of two mechanisms of fuelmixture conversion: selfcatalysis and thermal selfacceleration. Based on the previously suggested mechanism of oxidation pyrolysis by the scheme of intramolecular quadratic destruction, experimentally observed fragmentation of the isopentane molecule is demonstrated. In contrast to npentane, formation of methyl alcohol has been found in isopentane convection products.     

Synthesis, Characterization and Optimum Reaction Conditions of Oligo-N, N′-bis (2-hydroxy-1-naphthalidene) Thiosemicarbazone

The oxidative polycondenzation reaction conditions of N, N-bis (2-hydroxy-1-naphthalidene) thiosemicarbazone (HNTSC) using air oxygen, H₂O₂ and NaOCl were studied in an aqueous alkaline medium between 50–90°C. Oligo-N, N-bis (2-hydroxy-1-naphthalidene) thiosemicarbazone was characterized by ¹H-NMR, FT-IR, UV-Vis, size exclusion chromatography (SEC) and elemental analysis techniques. Solubility testing of oligomer was investigated using organic solvents such as DMF, THF, DMSO, methanol, ethanol, CHCl₃, CCl₄, toluene acetonitrile, ethyl acetate, concentrated H₂SO₄ and an aqueous alkaline solution. Using NaOCl, H₂O₂ and air O₂ oxidants, conversion to oligo-N, N-bis (2-hydroxy-1-naphthalidene) thiosemicarbazone (OHNTSC) of N, N-bis (2-hydroxy-1-naphthalidene) thiosemicarbazone was found to be 85, 80 and 76%, respectively, in an aqueous alkaline medium. According to the SEC analyses, the number-average molecular weight, weight-average molecular weight and polydispersity index values of OHNTSC synthesized were found to be 1050 gmol^–1 1715 gmol^–1 and 1.63, using NaOCl, and 2137, 2957 gmol^–1 and 1.38, using air O₂ and 2155 gmol^–1 4164 gmol^–1 and 1.93, using air H₂O₂, respectively. Also, TG analysis was shown to be unstable of oligo-N, N-bis (2-hydroxy-1-naphthalidene) thiosemicarbazone against thermo-oxidative decomposition. The weight loss of OHNTSC was found to be 97.29% at 900°C.     

Structure and thermal stability of zinc–nickel electrodeposits

ZnNi alloys were electrodeposited from a chloride bath on steel substrates. The effect of nickel bath concentration on chemical composition, structure and microstructure of the deposits is demonstrated. From 0 to 13 nickel, the phases obtained do not correspond to that reported on the thermodynamic phase diagram. It is shown that the substitution of zinc by nickel is responsible for the formation of distorted _d and _d phases corresponding to the supersaturated hexagonal phase of zinc and to the unsaturated cubic phase of Zn–Ni alloy, respectively. Differential scanning calorimetry indicates that the thermal instability of the alloys containing up to 13 wt of nickel, results from the crystallization of the phase from the _d and _d phases at around 200 °C and 250 °C, respectively.     

Porous mullite ceramics through diphasic gels of large boehmite and small silica particles

Mullite formation process has been studied in stoichiometric mullite (3Al₂O₃·2SiO₂) diphasic gel containing large boehmite (1 m) and small silica (10 nm) particles. It has been found that initial mullitization did not take place inside the silica phase (cristobalite), but took place in the defect -alumina phase. -alumina was stabilized by silica when the temperature was below 1350°C. At temperatures above 1350°C, mullite crystallized directly. It was suggested that silica diffused into the pores (<1.8 nm) of -alumina and prevented the collapse of -alumina pore structure. On the other hand, when silica was not present, the pore structure of -alumina collapsed and -alumina crystallized at 1100°C. Porous mullite ceramics were obtained by using special diphasic gels containing large boehmite and small silica particles.     

Determination of optical purity of substituted β-lactones

An octakis (3-O-butyryl-2,6-di-O-pentyl)--cyclodextrin capillary gas chromatographic column was used to determine the optical purity of seven different -lactones, having substituents in - or -position, and DL-lactide. The resolving power varied from one lactone to another. The optical purity values determined by GC were in agreement with those obtained from NMR spectroscopy or optical rotation.     

Electrochromic properties of alkylviologen-cyclodextrin systems

Electrochromic properties of methyl, ethyl, n-hexyl and n-heptyl viologens, and -, -, -cyclodextrin systems were studied using cyclic voltammetry and absorption spectroscopy. Colourations of red-violet ( _max = 530 or 560 nm), blue ( _max = 600nm) and light purple ( _max = 560 and 600 nm) could be obtained by changing the length of the alkyl chain of viologens or the radius of cyclodextrin. The intermolecular interaction between viologen radicals is weakened when the radicals form inclusion complexes so the blue colour inherent in the monomer of viologen radical appears. The red-violet colour is ascribed to the dimer of the reduced viologen. With the addition of -cyclodextrin, viologens were found to be included as dimer. The light purple colour is ascribed to the mixed colour of the monomer and the dimer. Furthermore, since the ratio of monomer and dimer depends upon the concentration of cyclodextrin, any colour between the red-violet of the dimer and the blue of the monomer can be obtained by adjusting the concentration of cyclodextrin. The redox reaction of an inclusion complex was found to depend upon the length of the alkyl viologen and the size of the cavity in cyclodextrin.     

Interfacial behaviour of various liquid electrode/β″-alumina systems

Three different liquid electrode/sodium-alumina interfaces were investigated. The NaNO₃/-alumina interface displays ohmic behaviour although the resistance is somewhat larger than expected. The sodium/-alumina interface exhibits asymmetric resistance with respect to d.c. current flow. Cyclic voltammetry results suggest that a Na₂O film is being modified electrochemically during the sweeps. When a sodium amalgam electrode is utilized, the film, if it is present, has very little effect. The asymmetric resistance is not observed and there is good agreement between d.c. and a.c. resistances after only a small amount of sodium deposition.     

Calorimetric,X-ray and infra-red investigations on poly(hexamethylene adipamide)

Summary In dependence on crystallization conditions three ranges with different crystal structure and heat of fusion were found by DSC,WAXS,and IR for unoriented PA 6.6 samples of densities between 1.10 and 1.17gcm^–3: Range I:_I triclinic, _c ^I =1.225 gcm^–3,H _M ^I = 235 Jg^–1. Range II:_II triclinic, _c ^II =1.165 gcm^–3, H _M ^II =185 Jg^–1. Range III:Continuous variation from _c ^I ,H _M ^I to _c ^II , H _M ^II . _a=1.095 gcm^–3 is independent of crystallization. conditions. The transition between _I and _II is probably due to changes of the chain conformation.     

Note of the estimation of the Θ temperature and Ψ parameter from the critical temperature data

Summary Critical values of the polymer volume fraction _2,c and the interaction parameter _c have been computed for the case that the equation for the chemical potential of solvent contains terms _{c 2} ³ and _{c 2} ⁴ in addition to ₂ ² . For 0 _c 1/3, the limits for infinite chain length are _2,c = 0 and _c = 0.5. Quite different results are obtained for _c > 1/3, _2,c being finite and _c lower than 1/2. Conclusions for the estimation of the temperature and the entropy-of-dilution parameter are discussed.     

Crystallization and melting of <Emphasis Type="Italic">α</Emphasis> and <Emphasis Type="Italic">β</Emphasis> phases in bulk syndiotactic polystyrene as monitored in situ via high-temperature wide-angle X-ray diffraction

Direct and non-intrusive observations of crystallization and melting behavior of and polymorphs in bulk syndiotactic polystyrene were made by means of temperature-programmed x-ray diffraction. Results indicated that the highest sustainable temperature identifiable via wide-angle x-ray diffraction using stepwise annealing at increasingly higher temperatures (T _a) for the perfected (with the initial crystallization temperature T _c = 245 °C, followed by annealing at stepwise increased T _a above 250 °C) phase may be at least 286 °C. In a similar manner, the highest sustainable temperature of the perfected (with T _c = 265 °C, followed by annealing at stepwise increased T _a above 275 °C) phase may be at least 280 °C. These observations suggest complete melting should occur only above the respective sustainable temperatures. It thus follows that equilibrium melting of the and the phases should occur at temperatures higher than 286 and 280 °C, respectively. Perfection of the less ordered form into the better ordered form within the family is observed to occur in the vicinity of 270 °C; no evidence of transformation between and phases is identified.     

Flow-through and flow-by porous electrodes of nickel foam Part IV: experimental electrode potential distributions in the flow-through and in the flow-by configurations

Experimental distributions of the solution potential in flow-through and flow-by porous electrodes of nickel foam operating in limiting current conditions are presented. These are in good agreement with the corresponding theoretical distributions. In the case of a flow-by configuration used in a two-compartment cell, the experiments confirm the validity of the models, presented in Part III, which take into account the presence of a separator (ceramic porous diaphragm or ion exchange membrane).Nomenclature a _e specific surface area per unit volume of electrode - C ₀ entrance ferricyanide concentration (y=0) - D molecular diffusion coefficient of ferricyanide - E _e cathode potential - F Faraday number - mean (and local) mass transfer coefficient - L electrode thickness - L _s-L separator thickness - m number of sheets of foam in a stack - n number of terms in Fourier series - Q volumetric flow-rate - r _s ohmic specific resistance of the separator - mean flow velocity based on empty channel - V constant potential - X conversion - x coordinate for the electrode thickness - y coordinate for the electrode length - y ₀ length of the porous electrode - z number of electrons in the electrochemical reaction Greek symbols parameter - parameter - ionic electrolyte conductivity - _sc solution potential in the pores of the cathode - _M matrix potential ( _sc = constant) - parameter [=n/y ₀] - electrolyte density - mean porosity - kinematic viscosity - E _c potential drop in the porous cathode - potential drop defined in Fig. 5 Indices c cathodic - o electrolyte alone - s separator     

Combustion and the transition to detonation in a gas mixture in a closed space

The possibility of going from deflagration to detonation in mixtures of CH₄+2(O₂+N₂) and 2H₂+O₂ ±N₂ (03.76) was investigated experimentally in a space enclosed by various combinations of three thin-walled metal concentric turbulator-sphere (diameters in a ratio 124) with large numbers of openings (permeability 0.1–0.4). Transition from deflagration to detonation was observed for 1 in the first mixture and for 3.2 in the second.Balashikha. Translated from Fizika Goreniya i Vzryva, Vol. 29, No. 3, pp. 171–174, May–June, 1993.     

Calculation of Temperature Dependences of the Viscosity and Volume Relaxation Time for Oxide Glass-Forming Melts from Chemical Composition and Dilatometric Glass Transition Temperature

The relaxation parameter K _sthat is equal to the ratio of the viscosity to the Kohlrausch volume relaxation time _s is analyzed. It is shown that this parameter can be evaluated from the temperature T ₁₃(corresponding to a viscosity of 10¹³P) and the glass transition temperature T ₈ ⁺determined from the dilatometric heating curve. The maximum error of the estimate with due regard for experimental errors is equal to ±(0.4–0.5)logK _sfor strong glasses and ±(0.6–0.8)logK _sfor fragile glasses, which, in both cases, corresponds to a change in the relaxation times with a change in the temperature by ±(8–10) K. It is revealed that the viscosity, the Kohlrausch volume relaxation time _s , and the shear modulus Gof glass-forming materials in silicate, borate, and germanate systems satisfy the relationship log( _s G/) 1. The procedure for calculating the temperature dependences of the viscosity and the relaxation times in the glass transition range from the chemical composition and the T ₈ ⁺temperature for glass-forming melts in the above systems is proposed. The root-mean-square deviations between the calculated and experimental temperatures T ₁₁and T ₁₃are equal to ±(6–8) K for all the studied (silicate, borate, germanate, and mixed) oxide glass-forming systems. The proposed relationships can be useful for evaluating the boundaries of the annealing range and changes in the properties and their temperature coefficients upon cooling of glass-forming melts.     

Crystal structure and morphology of electrodeposited zinc-iron binary alloys

Deposits of zinc-iron alloy have been prepared galvanostatically from a sulphate bath and the crystal structure has been determined by X-ray diffraction and transmission electron microscopy measurements. The electrodeposited zinc-iron alloys have metastable structures and the individual phases coexist over wide composition ranges. The phases are identified as (10073 at % zinc), (8748 at % zinc), ₁(7862 at % zinc) and (620 at % zinc). Thec andc/a in the h.c.p. lattice of the -phase decrease continuously with decrease of zinc concentrations, and the latter changes from 1.86 to 1.60 (a andc are the lattice constants of the -phase in the direction of thea- andc-axes, respectively). The -phase particles exhibit a hexagonal plate-like morphology which is thin in the direction of thec-axis. The morphology of the electrodeposits changes from plate-like to pyramidal shape when fine -phase particles (100 nm) start to form surrounding the -phase platelets, and then to lenticular or granular in the /₁ duplex region. The -phase forms in the low zinc concentration region and changes the electrodeposits to a fine cuboidal morphology.     

Comportement électrochimique des variétés alpha et beta PbO2 et influence de l'antimoine sur leur reactivité électrochimique en milieu H2SO4, 8N

Résumé L'expérience a montré qu'il est possible d'obtenir par l'oxydation anodique des variétés- et-PbO₂ parfaitement pures au point de vue cristallographique, et que la réduction de-PbO₂ se déroule à un potentiel plus élevé et plus constant que celui observé sur-PbO₂. La réactivité électrochimique de-PbO₂ est plus importante que celle de-PbO₂. L'introduction de Sb dans les réseaux cristallins de ces variétés diminue fortement leur cristallinité et dans le cas de-PbO₂ on obtient toujours simultanément- et-PbO₂. Du point de vue réactivité électrochimique, l'accroissement dû à la présence de Sb est de l'ordre de 33%.

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The results demonstrate the possibility of preparing through anodic oxidation rigorously pure, from the crystallographic point view,- and-PbO₂ phases, and that the reduction of-PbO₂ takes place at a potential which is more positive and more constant than the one obtained with-PbO₂. In a battery, the electrochemical reactivity of-PbO₂ is more important. The introduction of Sb into the lattice of these forms of PbO₂ decreases their crystallinity, and for the case of-PbO₂ we obtained simultaneously- and-PbO₂. Their electrochemical reactivity can increase by about 33%.