A study of the rates of catalyzed Boudouard reaction

The rate of Boudouard (C-CO₂) reaction catalyzed by Li₂O and Li₂CO₃ was investigated in the temperature range 839°–1050°C. The rate data is reported in the form of Fractional burn-off (F) vs time (t) plots. The experimental rate constant (K) data deduced from the initial linear region of these plots is represented by $\text{log K = 2.901(±0.186) ?}\text{22,240(±1,020)}\text{2.303RT}$; K is in gm/gm min. The experimental rate constant data was corrected for pore-diffusion and film mass transfer effects. The intrinsic rate constant (K_v) was found to exhibit the following temperature-dependence: $\text{log K}{}_{\mathrm{v}}\text{= 5.471(±0.353) ?}\text{48,040(±1,920)}\text{2.303RT}$; K_v is in moles/cm³atm sec. An electrochemical mechanism was proposed to explain the catalytic activities of Li₂O and Li₂CO₃.     

Absorbance-time relationship at optically transparent electrode

In simultaneous spectral and electrochemical observation, absorbance-time relationship at an optically transparent electrode has been derived. This treats the cases that (i) the reaction product is completely stable, and (ii) the product is unstable, and homogeneous chemical reaction is involved. In case (i), the absorbance A is proportional to (time)^{$\text{1}\text{2}$} in the initial period of electrolysis, but after a long time of electrolysis A reaches a constant value. In case (ii), A-t curve shows a maximum depending on the ratio of homogeneous rate constants k_fto k_b.     

A kinetic study of ‘living’ coordination polymerization of propene with the soluble V(acac)3-Al(i-C4H9)2Cl system

The soluble V(acac)₃-Al(i-C₄H₉)₂Cl system initiated living polymerization of propene at ?78°C affording monodisperse polymers ($\text{M}\text{?}{}_{\mathrm{w}}\text{M}\text{?}{}_{\mathrm{n}}\text{= 1.15 ± 0.10}$). A kinetic study (of the living polymerization) was carried out to evaluate the rate coefficients for propagation. The equilibrium constant K_M for a propene monomer coordinated to an active vanadium and the rate constant k_p for a subsequent insertion of coordinated monomer into a living polymer chain were determined and compared with the values for the polymerization of propene with other soluble vanadium-based catalyst systems. The relation between K_M and k_p revealed that a strong interaction between vanadium and propene is unfavourable for the insertion of the coordinated propene into a living polymer chain. The mechanism of an initiation reaction involving alkylation and complexing of V(acac)₃ with Al(i-C₄H₉)₂Cl has been proposed.     

Linear sweep voltammetry of manganate(VII), manganate(VI), and manganate(V) in alkaline media

Electrochemical reactions were assigned to the voltammetric waves obtained in alkaline KMnO₄ and K₂MnO₄ solutions. Plots of i_p (AC°ν^{$\text{1}\text{2}$})^?1 and i_api_cp^?1 νs log ν indicate that at slow potential scan rates in MnO^?₄ solutions ranging from 0·07 to 0·19 F in NaOH the first step in MnO^?₄ reduction is a l-electron reversible charge transfer with no coupled chemical reaction; at fast scan rates (? V s^?1 the process becomes quasi-reversible. The standard rate constant and the activation energy for the MnO^?₄/MnO^2?₄ charge transfer step were estimated. Plots of i_p (AC°ν^{$\text{1}\text{2}$})^?1 and i_api_cp^?1 νs log ν indicate that in 4·O F KOH and at potential scan rates between 0·005 and 2·V s^?1 the reduction of MNO^2?₄ to MnO^3?₄ is a reversible charge transfer with no coupled chemical reaction. The diffusion coefficients of MnO^?₄ and MnO^2?₄ in alkaline solutions are reported.     

Effects of solution viscosity on heterogeneous electron transfer across a liquid/liquid interface

Scanning electrochemical microscopy (SECM) is employed to investigate the effect of solution viscosity on the rate constants of electron transfer (ET) reaction between potassium ferricyanide in water and 7,7,8,8-tetracyanoquinodimethane (TCNQ) in 1,2-dichloroethane. Either tetrabutylammonium (TBA⁺) or ClO₄⁻ is chosen as the common ion in both phases to control the interfacial potential drop. The rate constant of heterogeneous ET reaction between TCNQ and ferrocyanide produced in-situ, k₁₂, is evaluated by SECM and is inversely proportional to the viscosity of the aqueous solution and directly proportional to the diffusion coefficient of K₄Fe(CN)₆ in water when the concentration of TCNQ in the DCE phase is in excess. The k₁₂ dependence on viscosity is explained in terms of the longitudinal relaxation time of the solution. The rate constant of the heterogeneous ET reaction between TCNQ⁻ and ferricyanide, k₂₁, is also obtained by SECM and these results cannot be explained by the same manner.     

Catalyst Screening through Quantum Chemical Calculations and Microkinetic Modeling: Hydrolysis of Carbon Dioxide

Quantum chemical calculations are emerging as an effective way to screen catalysts for particular applications. In this contribution, we demonstrate the power of density functional theory to study CO₂ hydrolysisby six carbonic anhydrase mimics, evaluating thermodynamic and kinetic parameters at the mechanistic level. A microkinetic model was then built based on the kinetics and thermodynamics calculated from first principles. The intrinsic reaction rate constant was calculated from the results of the microkinetic model and compared with experimental data. Overall, the rate constants were in good agreement with experimental values, except for zinc-tri and complex b, which were overestimated. This was ascribed to their ineffective complexation with Zn²⁺. How the reaction rate constants vary with time was also investigated. From 0 to 12 ms, the rate constants of complexes a and d decreased to 50 and 67% of their initial values, respectively; the rate constants of complexes b and f2 were almost invariant with time; the rate constant of complex f1 showed an unusual double sigmoidal shape. The pK_a values of these six carbonic anhydrase mimics as well as three additional mimics were calculated. A correlation between pK_a values and the binding free energy of OH-was obtained by fitting data from five zinc(II) aza-macrocyclic complexes. The reaction rate constants were found to increase linearly with the pK_a value, indicating CO₂ adsorption is the rate-limiting step.     

A new representation of viscosity data as a function of molecular weight

It is proposed to represent viscosity data by plotting $\text{1}\text{[η]}$versus$\text{1}\text{M}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$. This gives a straight line even when the exponent a in the Mark-Houwink-Sakurada plot is not constant. The inverse of the slope of the straight line is approximately equal to K_?. The intercept determined by extrapolating the straight line plot to infinite M increases with the quality of the solvent.     

Kinetic measurements on a stationary disk electrode in a uniformly rotating fluid (SDERF) — I. The limiting diffusion current in the Fe(CN)4−6-Fe(CN)3−6-system

The construction and operation of an electrochemical cell provided with a sde in a uniformly rotating fluid (SDERF-cell) are reported. The test of the SDERF-cell with respect to the diffusional kinetics is made with the Fe(CN)^4?₆-Fe(CN)^3?₆ system in 1 M solutions of NaOH, KCl and KNO₃, in potentiodynamic conditions. The linear relationship between the limiting diffusion current I_d and the square root of the angular velocity of the rotor, ω^{$\text{1}\text{2}$}_r holds, but the straight line does not intercept the coordinate axes in the origin. The square root of the angular velocity of fluid, ω^{$\text{1}\text{2}$}_f — calculated from I_d — depends only on the ω^{$\text{1}\text{2}$}_r — value irrespective of the concentration and the physical properties of the fluid. The diffusion coefficients and the activation energy of diffusion, E_a, in 1 M KNO₃ were measured; E_a = 3.15 kcal/mol for both Fe(CN)^4?₆ and Fe(CN)^3?₆-ions.     

Slow stable crack growth in high density polyethylenes

The phenomenon of slow stable crack growth in polyethylene is investigated using notched specimens subject to constant load and the concepts of fracture mechanics. The effect of specimen geometry and dimension, the loading and the mode of loading on the applied stress intensity factor versus crack speed ($\text{K}{}_{\mathrm{c}}\text{-}\text{a}\text{?}$) curves has been studied to demonstrate that K_c is the controlling stress parameter for crack growth under suitable conditions. $\text{K}{}_{\mathrm{c}}\text{-}\text{a}\text{?}$ curves are obtained for a high density polyethylene homopolymer in distilled water and in a diluted detergent solution at four different temperatures. Results are also obtained for a much tougher medium density polyethylene copolymer whenever possible. Several mechanisms can be identified from the form of the $\text{K}{}_{\mathrm{c}}\text{-}\text{a}\text{?}$ curves. Two, in particular, have been observed but not explained before: (i) crack growth with a time dependence of 0.25, and (ii) the high $\text{K}{}_{\mathrm{c}}\text{-}\text{a}\text{?}$ slopes for crack growth in a tough copolymer. With the help of scanning electron microscopic studies of the fracture surfaces, (i) is postulated to be due to diffusion controlled void growth process and (ii) is considered to be the result of crack tip blunting effects. From the temperature dependence of crack growth, the activation energy of the diffusion controlled crack growth process is found to coincide with that of the x-relaxation process in polyethylene implying that diffusion controlled crack growth may be related to the motion of main chains in the polymer.     

Effect of carboxylic acids on the cathodic reduction of benzophenone in N,N-dimethyl-formamide. Polarographic and CNDO/2 studies

Effect of eight carboxylic acids on the polarographic reduction of benzophenone in N,N-dimethylformamide has been studied. In the presence of these acids, a new cathodic wave (“pre-wave”) appears at more positive potentials than the original wave of benzophenone. Linear correlation has been found between ΔE_{$\text{1}\text{2}$} and the pK^DMF_a values of acids, where ΔE_{$\text{1}\text{2}$}  (E^pre-wave_{$\text{1}\text{2}$}  E^{original wave}_{$\text{1}\text{2}$}). It is suggested that in all the cases investigated the reduction proceeds via a hydrogen-bonded complex formed between the carbonyl compound and the acid in the double-layer. The structural aspects of the benzophenoneformic acid adduct have been solved using CNDO/2 method.     

Synthesis of electroreactive polymers from poly (m,p-chloromethylstyrene) and copoly (m,p-chloromethylstyrene-styrene)

Polymerization, and copolymerization with styrene, of m,p-chloromethylstyrene have been carried out at 75°C, in chlorobenzene and in the presence of AIBN ($\text{[}\text{AIBN}\text{] ? 6 × 10}{}^{\mathrm{?2}}$, and $\text{12 × 10}{}^{\mathrm{?2}}\text{m}$, respectively). The polymer molecular weights, determined by g.p.c., are: $\text{M}\text{?}{}_{\mathrm{w}}\text{= 8670}$, $\text{M}\text{?}{}_{\mathrm{n}}\text{= 5860}$, and $\text{M}\text{?}{}_{\mathrm{w}}\text{/-M}{}_{\mathrm{n}}\text{= 1.48}$ for the homopolymer, poly(m,p-chloromethylstyrene), (1a); and $\text{M}\text{?}{}_{\mathrm{w}}\text{= 8805}$, $\text{M}\text{?}{}_{\mathrm{n}}\text{= 5144}$, and $\text{M}\text{?}{}_{\mathrm{w}}\text{/-M}{}_{\mathrm{n}}\text{= 1.71}$ for the copolymer, copoly(m,p-chloromethylstyrene-styrene), (2a). A series of phosphine derivatives of both 1a and 2a are prepared by the reaction of the polymers with either chlorodiphenylphosphine/lithium, or diphenylphosphine/potassium tert. butoxide. A number of other potentially electroreactive derivatives of 2a are obtained by reacting the polymer with 2-aminoanthraquinone, 3-N-methylamino-propionitrile, or 2-(2-aminoethyl) pyridine. The phosphinated polymers are reacted with bis-benzonitrilepalladium-(II) chloride to obtain a series of polymer-palladium(II) complexes containing 8.5–12.9% palladium. Similarly, reaction of the last-named bidentate polymeric ligand with cupric acetylacetonate, or cupric sulphate pentahydrate, produces polymer-copper(II) complexes having 5.8, or 3.3% copper, respectively. The inter/intra-chain nature of some of the side reactions during the derivatization of the chloromethylated polymers, and that of the complex formation between transition metal centres and macromolecular ligands, are briefly discussed in view of the experimental results.     

Conductometric studies of charge transfer complexes in solution

Conductometric study for formation of charge-transfer complexes, in the systems napthtalene: picric acid and biphenyl: picric acid in acetonitrile; and p-phenylendiamine:choloranil in acetone, methanol and acetonitrile yielded the stoichiometry of the napthalene:picric acid complex as 1:1 and that of the biphenyl:picric acid and p-phenylendediamine:chloranil complexes as 2:1. On dilution with inert solvent CCl₄μ_ν = A + Bm^{$\text{sol1}\text{2}$}_DA is found applicable but with positive slope. The positive values of B obtained are attributed to decreased ionization or/and enchanced ion pair formation with decrease in dielectric constant of the solvent medium.     

Polarographic reduction of benzoylacetanilides,mechanism and structure-reactivity relationships

The electroreduction of a series of substituted benzoylacetanilides (I) on a dme was investigated in ethanolic Britton—Robinson buffers, and the overall dissociation constants K* of I were evaluated spectrophotometrically under similar conditions. The polarographic curves showed in each case one, two electron cathodic wave, which corresponds to the reduction of the protonated keto form of I. From the pH dependence of the limiting current, the apparent polarographic K′ constants that depend on the protonation of I were determined. Values of the acid dissociation constants of the keto (K₁) and enol (K₂ tautomers of I were deduced. The values of E_{$\text{1}\text{2}$}, pK* and pK₁ were shown to be linear functions of Hammett substituent constants. The equations of the regression lines obtained are: E_{$\text{1}\text{2}$} = 0·284σ—1·379 (r = 0·990) at pH 8·0; pK* = 9·54–2·71σ, (r = 0·998); and pK₁ = 9·45–2·946σ, (r = 0·954), respectively. Correlation of pK₂ values with substituent constants showed a break at about σ = 0. Controlled potential electrolysis of benzoylacetanilide (Id) produces 3-phenyl-3-hydroxypropionanilide (IIId). A mechanism for the polarographic reduction of I is proposed.     

Electrochemical reduction of polymeric carbon

It was found that the electrochemical reduction of poly (tetrafluoroethylene) with lithium amalgam leading to the mixture of polymeric carbon and lithium fluoride as solid product is followed by a consecutive electrochemical reaction via a polymeric anion radical, [$\text{(}\text{?}$C_n$\text{)}\text{?}$^?]_m. The value of n was determined as 4.75 and was found to be independent of the reaction conditions. The charge of this radical is during the reaction neutralised with Li⁺ ions diffusing through the solid phase to form a chemical compound $\text{(}\text{?}$C_nLi$\text{)}\text{?}$_m whose CLi bonds have a localized character. Therefore, this compound does not evolve hydrogen in contact with water as the intercalation compounds C_nLi, but hydrolyses to a solid hydrocarbon $\text{(}\text{?}$C_nH$\text{)}\text{?}$_m.     

Incidence of the dehydration step on the electrochemical behaviour of 3-hydroxy 2-one benzodiazepine compounds (oxazepam and lorazepam) in aqueous acid media at the mercury electrode

The hypothesis of an e.c.e. mechanism may justify the electrochemical behaviour observed for the reduction at the mercury electrode of two benzodiazepinones: oxazepam $\text{1}$ and lorazepam $\text{1′}$.In aqueous acid medium, the first step $\text{2}$ would consist in the grossly irreversible reduction of the imine bond (2F mol^?1).The chemical rate limiting step of the overall e.c.e. mechanism (4 F mol^?1) would consist in an acid-catalysed dehydration of an intermediate $\text{2}$ yielding a species $\text{3}$ which undergoes a disproportionation reaction with the hydrated species $\text{2}$ generated in the first step. The following values of k′ (k_exp = k′.(H⁺)) were determined for the rate constant of the chemical rate limiting step: oxazepam $\text{1}$:k′ = (1.0 ± 0.1) 10⁶ s^?1; lorazépam $\text{1′}$ = (7.1 ± 0.5)10⁴s^?1.     

On the mechanism of promoting ammonia synthesis catalysts by an alkali

The mechanism of the promoting effect of an alkali addition in catalytic synthesis of ammonia is described on the basis of a dipole-dipole interaction model. The promoting effect (the increase of the specific rate constant K_sp) is considered as a consequence of a decrease in the energy of the dipole $\text{μ}{}^3$ ($\text{μ}{}^3$ is the dipole of the activated complex of the reaction limiting step) in the field of promoter dipoles μ. The dependence of the promoting effect magnitude on the concentration of the promoter activating particles N and on Δ? (the decrease of the iron electron work function resulting from promoting) are calculated. Cases of uniform and maximum nonuniform (two-patch surface) distribution of the promoter activating particles over the catalyst surface are considered. An appraisement of the limiting promoting effect is made.The possibility of a “chemical” promoting effect, consisting in the decrease of the valency orbital electronegativity of the surface Fe atoms, is considered qualitatively on the basis of the concept of group electronegativity.     

Effects of reaction order and convection around gas-bubbles in a gas-liquid reacting system

The mass transfer of gaseous reactant into liquid for chemical reaction is significantly affected by relative flow at the interface of gas and liquid. Two extreme cases are for a bubble behaving like a solid particle due to absorbed surfactant impurities and for a freely-internally circulating bubble with a relative interfacial velocity; the present calculations indicate a ratio of mass-transfer rates of a factor up to 1·4–2·45. The factor decreases with increasing reaction rate, becoming negligible for values of K > 2000 sec^?1. It is larger for a $\text{3}\text{2}$ order reaction than for a 1st order reaction.If there is internal circulation, the relative flow changes depending on whether the bubble is alone or in a rising bubble swarm. For small reaction rates the effect of this change in the mass transfer rate has been calculated to be 7–9% at typical bubble sizes of 0·1–0·2 cm radius. The mass transfer rate for a freely-circulating bubble is about 15% larger for a $\text{1}\text{2}$ order reaction than for a 1st order reaction at steady state. Transient and time averaged values of the Sherwood number were obtained. Shrinkage of bubbles from loss of reactant was also considered.     

The pressure beneath a growing rising bubble

On the basis of potential theory, the equations of motion and the associated pressure field are derived for an idealized growing spherical gas bubble rising in an inviscid liquid under the influence of gravity from a horizontal plate-orifice and from a free standing nozzle. The dimensionless pressure (p_N?p₀)/πga at the gas source N (where p₀ is the undisturbed ambient pressure, a the instantaneous bubble radius and π the liquid density) is found to rise to a maximum when the dimensionless bubble position h/a = 1·55 for bubbles formed at a plate orifice and (125/48)^{$\text{1}\text{3}$} ? 1·38 for bubbles formed at a free standing nozzle. These positions of maximum pressure correspond well to experimentally observed positions at which the gas supply to bubbles grown at constant flow rate in water is cut off by the collapse of the neck linking bubble and gas source. The volumes of bubbles at this instant are predicted theoretically and compare well with experimentally determined values.     

Fixed bed catalytic reactor modelling: The heat transfer problem

The issue of radial heat transport in fixed beds is discussed and discrepancies between observed and computed hot spots are attributed to neglect of axial dispersion of heat, which neglect also accounts for observed length-dependent radial conductivities.The Nusselt number for heat transport at the wall is inferred to be Nu_w = 5.73($\text{D}{}_{\mathrm{t}}\text{d}{}_{\mathrm{p}}$)^{$\text{1}\text{2}$}$\text{P}\text{r}$(0.11Re_p + 20.64)/Re_p^0.262 which is in accord with recently secured Sherwood numbers for mass transport at a packed wall.The recent success of unconventional reactor models is discussed.     

Effect of temperature and frequency in fatigue of polymers

The effects of frequency and temperature on fatigue crack propagation rate in poly(methyl methacrylate) and polycarbonate have been studied using centrally notched plate specimens cycled in tension between constant stress intensity limits. Crack growth was monitored at frequencies between 0.1 Hz and 100 Hz and at temperatures between ?60°C and 40°C. A linear relationship between the cyclic crack growth rate $\text{d}\text{(2a)}\text{d}\text{N}$ and appropriate levels of toughness, K, has been proposed: $\text{d}\text{(2a)}\text{d}\text{N}\text{= A?}{}^{\mathrm{\alpha }}$, where $\text{? =}\text{(λ ? λ}{}_{\mathrm{<mtext>th</mtext>}}\text{)}\text{(K}{}^2{}_{\mathrm{<mtext>1</mtext><mtext>C</mtext>}}\text{? K}{}^2{}_{\mathrm{<mtext>max</mtext>}}\text{)}$, λ = K²_max ? K²_min, λ_th is the threshold limit and A and α are constants. Also, the influence of mean stress intensity was briefly discussed.