The effects of substitution on the electrochemical reduction of naphthacenetetrone in N,N-Dimethylformamide

The polarographic reduction of 5,6,11,12-naphthacenetetrone (NT) and several of its chloromethyl, and nitro derivatives were investigated. The electrochemical behavior of NT was compared with 6, 11-dihydroxy-5, 12-naphthacencequinone (DHNQ) previously studied. Like DHNQ, NT exhibits two reduction waves in N,N-dimethylformamide. However, two additional post adsorption waves which were not present in DHNQ reduction were also observed. The first step reduction of NT, representing the formation of the radical anion NT^?, has a potential (0.01 V vs sce) much more anodic than that of its DHNQ counterpart ( ?0.75 V vs sce). The substituent effects of NTs were noted. A linear Hammett relationship similar to that of the DHNQ series was also found for the NTs. In both cases, the averaged values of σ_meta and σ_para were used as the substituent constants     

PtSiO2: I. Percentage exposed and its effect upon the reactivity of adsorbed oxygen

This paper reports the preparation of a set of platinum catalysts on wide-pore silica gel by impregnation with H₂PtCl₆ aq and by ion exchange with Pt(NH₃)₄²⁺. Some catalysts were reduced directly and others after preliminary calcination. Percentages exposed (platinum dispersion) measured at 25 °C by hydrogen chemisorption, hydrogen desorption, and hydrogen-oxygen titration vary from 6.3 to 81%. Hydrogen-oxygen titration gives values which are about 88% of those given by hydrogen chemisorption. Upon storage in air at 25 °C, the content in surface oxygen increases substantially beyond that resulting from the short exposure to oxygen employed in the hydrogen-oxygen titration. Exposure to oxygen at 300 °C results in still a further increase in the content in surface oxygen. The surface oxide resulting from the short exposure to oxygen employed in hydrogen-oxygen titration is rapidly reduced by hydrogen at 25 °C but coverages by oxygen much in excess of those resulting from the short exposures result in much reduced rates of reaction with hydrogen. The effect is structure sensitive: The degree of reduction in rate is much larger on the catalysts with a higher percentage exposed. The percentage exposed of the catalysts with the lowest percentage exposed appears to be larger when stored catalyst exposed to oxygen at 300 °C is reduced at 25 or 100 °C than when it is reduced at 300 °C and swept with argon at 450 °C.     

Simulation of reversible nylon-6,6 polymerization

The forward and reverse rate constants for nylon-6,6 polymer formation have been determined through curve fitting of experimental data by Ogata¹. The molecular weight distribution (MWD) in a batch reactor is simulated and determined numerically for up to eleven hours of reversible polymerization. The MWD obtained differs from Flory's distribution due to reversibility of the reaction. The deviation increases as the time of polymerization progresses. The polydispersity index increases asymptotically to a value of 1.8 in contrast with the value of 2 for irreversible polymerization. This asymptotic value is found to increase as the reaction temperature increases and reduce as the initial water content increases.     

Studies of the hydrogen held by solids: XXII. The surface chemistry of reduced molybdenaalumina catalysts

The surface hydroxyl concentrations of a fresh molybdenaalumina catalyst (8% Mo) and of the alumina from which it was made have been determined as a function of the temperature of pretreatment. Similar data were obtained for catalysts reduced with H₂ or with CO. In all cases, the hydroxyl concentrations decreased with increasing pretreatment temperatures. The difference between the curves for the parent alumina and the molybdenaalumina preparation made from it provided a measure of the number of hydroxyl groups eliminated as the epitaxial monolayer of molybdena was grown onto the surface. The values obtained (1.7 ± 0.6 OH/Mo) showed that the surface hydroxyl groups of alumina are replaced by molybdate anions. When the catalyst was reduced with CO to about $\text{e}\text{Mo}\text{= 1.5}$ (average valence, Mo^+4.5), the curve obtained was almost identical with that for the unreduced catalyst, but when the catalyst was reduced with H₂, values for the retained hydrogen were higher than for the oxidized catalyst and approached those of the parent alumina as its evacuation temperature was increased to 550 °C. This increase in hydroxyl concentration was in agreement with earlier deductions.The hydroxyl region of the infrared spectra of similar preparations was recorded. Four distinct bands could be characterized for the parent alumina at 3780, 3740, 3705, and 3650 cm^?1 and a shoulder at 3795 cm^?1. The same bands were present on the oxidized catalyst, but with lower intensities and with altered intensity ratios; i.e., some bands were affected more than others as hydroxyl groups were replaced by molybdena species. Spectra from catalysts reduced with CO were indistinguishable in the OH region from those for the unreduced catalyst. No new bands appeared when the catalysts were reduced with H₂, but the intensities of bands attributable to alumina OH increased with the 3795 cm^?1 band strengthening noticeably more than the others. Thus, the new hydroxyl groups introduced on reduction are probably alumina OH rather than MoOH as previously supposed. A form of hydrogen which is chemisorbed but which can be removed from the catalyst as H₂ on evacuation at the reduction temperature also appeared in the OH region, mainly as a continuous contribution to the low frequency edge. From absorption coefficients derived from the present data, it was deduced that about twice as many hydrogen atoms were present in the H₂ formed than were supplied by these OH groups; i.e., the chemisorption appears heterolytic with half the atoms unseen by ir. A search was made for a band attributable to MoH, but without success. A brief study was made of this adsorption process, which was found to be slow but reversible, and to have a positive pressure dependence. When the catalyst was reduced with CO, rather than with H₂, a portion of the CO remained irreversibly chemisorbed in electronically comparable amounts. Infrared spectra of such samples contained a band at about 1585 cm^?1 attributable to a carbonate species. Data for the two reducing gases differed in that no reversibly chemisorbed CO was observed. At room temperature, CO was also chemisorbed as a linear species with the stretching frequency (2190 cm^?1) higher than that of the gaseous molecule (2143 cm^?1).     

Extraction of coals through dilute alkaline hydrolytic treatment at low temperature and ambient pressure

Assam and Talcher (bituminous) coals have been subjected to batch aqueous alkali treatment followed by extraction. It has been shown that alkali treatment increased extractability and reactivity of coal. Use of 1.4% aqueous sodium hydroxide was found to be capable of rendering about 40–50% coal extractable in ethylene diamine in two alkali treatments followed by extractions. Semibatch operation yielded about 26–33% extractable coal, and extraction data were found to be comparable with those obtained in batch degradation via a single alkali treatment. Of the solvents studied, polar and basic solvents were found to be the most effective.     

On the modelling of continuous emulsion polymerization using a population balance with instantaneous radical termination

An analytical solution of a population balance is used to mathematically describe continuous emulsion polymerization. Steady state performance is examined. The assumption of instantaneous free radical termination within particles is made. The desorption mechanism is included. Particle size distribution information is obtained, and the effect of the desorption mechanism is noted. A desorption rate constant is calculated when the model is fit to data found in the literature.     

Optimal conditions for obtaining polymeric membranes in binary systems: a numerical analysis

The mathematical model, describing solvent evaporation from a cast film, is used with the aim of formulating a general approach to the optimization of RO membrane production.The results were correlated in terms of a “productivity factor.” This productivity factor approaches unity when the casting film has a thin and compact layer with high asymmetric character on the evaporating surface.The model is tested for different values of the parameters contained in the concentration dependence law of the mutual diffusion coefficient in solvent-polymer binary system. Also different solvent rates at the evaporating surface (low, medium, high) are employed. The numerical results were expressed in terms of intrinsic properties of the binary system with the aid of the free volume theory.Testing the model indicates that increased productivities should be expected for solutions of low free-volume polymers in high cross-section solvents. However, in general, for a given polymer-solvent system, it is very important to adjust solvent evaporation rates by varying solvent concentration in the casting atmosphere.On the other hand it could be necessary to control the plasticizing action of the solvent on the polymer in order to influence the diffusion process.     

Influence of temperature on the reactivity of limestone particles with sulfur dioxide

A modified version of the grain theory was used to analyse the effect of temperature on the sulfation reaction. The theory combines the influence of temperature on the grain size, its effect on the rate of transport of SO₂ and the chemical reaction. An efficient numerical procedure is presented for solving the model equations which take into account the radial gradient of the effective diffusivity in a particle.     

“Inverted” peaks and current oscillations in cyclic voltammetry

Inverted peaks and strongly increased peaks have been reported and different explanations of these phenomena have been suggested[1,2]. The present communication discusses the peak shape distortions (“inverted” peaks, current oscillations) in terms of tangential movements of the mercury electrode surface. These distortions are explained similarly to the maxima in classical polarography: inhomogeneous polarization and inhomogeneous adsorption are considered as the two major forces causing the mercury surface movement and as a result, leading to the peak shape distortions. Experimental examples of the phenomena in aprotic and protic media and for organic and inorganic redox couples are given and explained in accordance with the proposed theory.     

Isothermal oscillations in surface reactions with coverage independent parameters

A procedure for obtaining necessary and sufficient conditions for the existence of periodic solutions in surface reactions with constant temperature and coverage independent parameters is described. An analytic method for the analysis of bifurcation to periodic solutions is developed. This formulation, based on the fundamental matrix of a system of ordinary differential equations, provides a simple means of examining the behavior of system in the neighborhood of a singular point. This method is applied into a reaction model which can yield oscillatory solutions with both single and multiple steady states. When a unique steady state becomes unstable, a limit cycle develops. With three steady states, a state may develop into a stable or unstable limit cycle or may exhibit only periodic solutions without a limit cycle.     

Modelling of resole type phenol formaldehyde polymerization

The kinetics of polymerization of phenol formaldehyde has been modelled to account for the formation of highly-branched resole molecules. The conversions of phenol and formaldehyde, number-average chain length, degree of branching and the concentrations of internal and external ortho and para-H atoms are obtained as functions of time, for several values of the various rate constants and the initial phenol to formaldehyde ratios. The conversions of phenol and formaldehyde are found to be most sensitive to the rate constant associated with the reaction between the external para-H atom with ---CH₂OH groups. The condensation between two ---CH₂OH groups prevents complete depletion of formaldehyde and leads to highly-branched, high molecular weight final products at large reaction times. Our studies show that at low values of the initial phenol to formaldehyde ratio, small amounts of lightly-branched, low molecular weight polymer with a high degree of substitution by ---CH₂OH groups is obtained. At some intermediate initial phenol to formaldehyde ratio, highly-branched, high molecular weight product is obtained and at high values of this feed ratio, larger amounts of lightly-branched, lower average molecular-weight product is obtained.     

Optimal temperature profiles for heterogeneous catalytic parallel reactions

The problem of determination of the optimal temperature profiles in a fixed-bed catalytic reactor for the catalytic parallel reactions including transport phenomena (external or internal diffusion) has been worked out.Special attention was given to predict the shapes of the optimal temperature profiles. The general considerations are illustrated by some numerical examples. It has been found that for every studied case the optimal temperature profile need not to end by the isotherm T = T_max.     

Effects of temperature on the flow and tensile strengths of powders

Measurements have been made of the flow rates and tensile strengths of a variety of powders — magnesia, lactose, fatty acids, organic drugs — over a range of temperatures from ?20° to 200 °C. The results have been explained in terms of the effects of temperature on the hardness and elasticities of the materials concerned, and the distances between the particles.Under compression, the asperities on their surfaces deform plastically and may melt, if the temperature is raised above about 0.9 of the conventional melting point in K, to form welded bonds.The activation energy of bonding for the different materials is between about 8 and 11 kJ mole^?1.     

The effect of catalyst temperature zoning on hydrotreating a coal-derived liquid

The effect of catalyst temperature zoning on hydrotreating reactions, coke formation, and changes in catalyst surface area and pore volume was examined. A coal liquid was hydrotreated with the top zone operated in the range of 400–500° C (752–932° F) while the lower zone was maintained at 400° C (752° F). Increasing the top zone from 400° C (752° F) to 450° C (842° F) had beneficial effects on hydrogenation, HDN, and HDM at no cost of increased catalyst coke content. A further temperature increase was detrimental because of excessive coking and possible thermodynamic equilibrium limitations.     

Effect of water content on the corrosion behaviour of Ni in methanol (MeOH) solutions

The effect of water content on the corrosion behaviour of nickel in methanolic solutions have been investigated tracing the potentiostatic curves at room temperature in a standard electrochemical cell.Preliminary results reported in this communication, shown that the electrochemical behaviour of Ni in MeOH is strongly influenced by the water content of the solution and that traces of water (of the order of 0.5%) are able to guarantee the passivity of nickel.     

Theoretical maximal yield of acetylene from coals by plasma pyrolysis

This paper attempts to correlate the yields of acetylene produced during the plasma pyrolysis of coals. Theoretical yields of acetylene produced directly from coals have been worked out. An attempt has been made to explain the existing wide gap between the experimental and the theoretical values of the yield of acetylene.     

Mechanism of the emulsion polymerization of methyl acrylate: 1. Initiation

The emulsion polymerization of methyl acrylate (MA) has been carried out under two different experimental conditions so as to establish the mode of chain initiation. In the first system, the initiator was Fenton's reagent (FeSO₄ and H₂O₂ redox pair) in acid solution at 20°C in the presence of a cationic detergent, cetyltrimethyl — ammonium bromide (CTAB), while in the second system the initiator was potassium persulphate (K₂S₂O₈) at 50°C in the presence of an anionic detergent, sodium lauryl sulphate (NaLS). The polymerization was followed by the conventional dilatometric and gravimetric methods. Initial rates were determined by keeping the conversion below 10% and by plotting the (yield/time) versus time, and extrapolating the straight line to zero time. It was found that in either case, Initial rate (v) ∝ (Initiator)^0.50 while the viscosity average molecular weight ($\text{M}{}_{\mathrm{v}}$) of the polymers up to 20% conversion was found to be approximately inversely proportional to the square root of the initiator concentration (I), i.e. $\text{(}\text{M}{}_{\mathrm{v}}\text{) ∝ (I)}{}^{\mathrm{?0.50}}$. The rate of polymerization up to 10% conversion was found to be approximately linear with the monomer concentration (from 0.5 to 5.0—, v/v) in the presence of detergents (above CMC) [critical monomer concentration] under the experimental conditions. Since the kinetics of the emulsion polymerization of MA at zero conversion are the same as that of the homogeneous polymerization of MA initiated by the free radicals, it is suggested that the initiation of the emulsion polymerization of MA takes place in the aqueous phase with little or no initiation in the detergent micelles containing monomer. If the polymerization is carried out within the solubility range of MA in the aqueous phase, then the initiation will take place entirely in the aqueous phase. Assuming homogeneous polymerization at zero conversion, the value of the termination rate constant (k_t) has been extracted from the experimental data at different initiator efficiencies.     

Small-angle neutron scattering from polymer solutions: 3. Semi-dilute solutions near the lower critical solution temperature

Measurements of the radius of gyration and the screening lengths have been made for semi-dilute solutions of polystyrene near the lower critical solution temperature. From these data two regions of behaviour are clearly discerned. By analogy with similar data near the upper critical solution temperature, a ‘phase diagram’ has been drawn from the results around the lower critical solution temperature.     

Effects of combined humidity and temperature on the friction of non-compacted iron,copper and hydroscopic powders

The effects of humidity and temperature on the interparticle friction of iron, copper, Portland cement and plaster of Paris powders have been investigated using a torsional apparatus. It is found that at ambient temperature (22 ± 2 °C), the interparticle friction of iron and copper powders, of a given particle shape, size distribution and size, first decreases then increases and again decreases with increasing humidity. At a constant relative humidity of ～30%, the friction of iron and copper powders is relatively insensitive to temperatures up to ～20 °C and then increases slowly but approximately linearly with increasing temperature. At 22 ± 2 °C, the interparticle friction of Portland cement and plaster of Paris powders increases gradually and then decreases with increasing humidity.