Presence Of Diffusion Limitations Research Articles

Two-dimensional model in a laminar flow slit photoreactor for the determination of surface kinetic constants considering the presence of diffusion limitations

Narrow slit reactors are widely used in laboratory testing of photocatalytic materials, as well as for the determination of kinetic coefficients in surface photocatalyzed reactions. In this work, the behavior of a continuous laminar flow slit reactor was described by a two-dimensional model, addressing the advection–diffusion problem in the fluid phase, coupled with a first-order chemical reaction on the reactor walls. The concentration profile of the reactant, in the transverse and longitudinal directions of the reactor, depends on two parameters that characterize the system; the Péclet number and the Damköhler number, in addition to the geometry of the reactor. Once a short distance from the reactor’s entrance has been surpassed, the relationship between the transversally averaged reactant concentration and the concentration existing on the catalytic surface becomes essentially constant, thus defining an (external) effectiveness factor. The simulations carried out for a wide range of values of the parameters characterizing the model showed that the effectiveness factor only depends on the Damköhler number, being independent of the value of the Péclet number and of the relationship between the reactor length and the distance separating the plates. An analytical expression for the effectiveness factor as a function of the Damköhler number was proposed, which allows to determine the value of the kinetic coefficient for the surface reaction (ks), even in the presence of important limitations to mass transfer. The methodology proposed was illustrated performing experiments of photocatalytic oxidation of toluene in air (with Péclet numbers ranging from 20 to 40), employing a paint formulated with carbon-doped TiO2 in a continuous laboratory photoreactor of flat parallel plates. The observed toluene conversions were in the range of 13 to 26 %, consequently, the calculated Damköhler number was Da = 0.0307 and the surface kinetic coefficient thus determined (for 50 % of relative humidity and 42.3 W/m2 of irradiance flux) was ks = 1.262 10-4 m/s. The significance of this work lies in the development of a comprehensive model that can be used to know the surface kinetics in photocatalytic reactions, applicable to scenarios where there are considerable mass transfer limitations or not.

Enzymatic Δ1-Dehydrogenation of 3-Ketosteroids—Reconciliation of Kinetic Isotope Effects with the Reaction Mechanism

Δ1-Dehydrogenation of 3-ketosteroids catalyzed by flavin adenine dinucleotide (FAD)-dependent 3-ketosteroid dehydrogenases (Δ1-KSTD) is a crucial step in steroid degradation and synthesis of several steroid drugs. The catalytic mechanism assumes the formation of a double bond in two steps, proton abstraction by tyrosyl ion, and a rate-limiting hydride transfer to FAD. This hypothesis was never verified by quantum-mechanical studies despite contradictory results from the kinetic isotope effect (KIE) reported in 1960 by Jerussi and Ringold [Biochemistry 1965, 4 (10)]. In this paper, we present results that reconcile the mechanistic hypothesis with experimental evidence. Quantum mechanics/molecular mechanics molecular dynamics simulations show that the proposed mechanism is indeed the most probable, but barriers associated with substrate activation (13.4–16.3 kcal·mol–1) and hydride transfer (15.5–18.0 kcal·mol–1) are very close (1.7–2.1 kcal·mol–1), which explains normal KIE values for steroids labeled either at C1 or C2 atoms. We confirm that tyrosyl ion acting as the catalytic base is indeed necessary for efficient activation of the steroid. We explain the lower value of the observed KIE (1.5–3.5) by the nature of the free energy surface, the presence of diffusion limitation, and to a smaller extent, conformational changes of the enzyme upon substrate binding. Finally, we confirm the Ping-Pong bi–bi kinetics of the whole Δ1-dehydrogenation and demonstrate that substrate binding, steroid dehydrogenation, and enzyme reoxidation proceed at comparable rates.

Investigation of Electrochemical Dissolving the Amorphous and Nanocrystal Alloys by a Rotating Disk Electrode

The electrochemical processing characterized by the absence of force and heat effect on the processed material provides favorable conditions for maintaining unique properties of amorphous and nanocrystal alloys during processing. Understanding the peculiarities of the electrochemical dissolving of the material is possible in studying anode polarization characteristics. The investigations of the anode dissolving of alloys 5ВDSR and 82K3HSR conducted before showed the presence of diffusion limitations connected with the absence of the reaction products removal from the processing zone and supply a new portion of reacting substances. To eliminate the limitations, it is necessary to conduct investigations of the anode behavior by the moving electrolyte which is provided by the rotating disk electrode. It allows controlling the process of ion transfer due to the changes in the anode rotation speed and providing the accessibility of the working sample surface for the electrochemical action. The paper presents the results of polarization investigations of the anode dissolving by using the method of the rotating disk electrode in 10 % neutral solutions of salts NaNO3, Na2SO4 and NaCl. It is established that the dissolving process in the stationary and movable electrolytes is characterized by the presence of zones of slowdown of the anode dissolving process. After providing the electrolyte motion in the processing zone, it became possible to increase the value of the current density.

Integrated Stefan–Maxwell, Mean Field, and Single-Event Microkinetic Methodology for Simultaneous Diffusion and Reaction inside Microporous Materials

The assessment of intrinsic reaction kinetics in the presence of diffusion limitations within a porous material remains one of the key challenges within the field of catalysis. The model-guided design of medium-pore zeolite catalysts which typically give rise to mass transport limitations would offer a feasible alternative to conventional trial-and-error procedures. Intracrystalline diffusion limitations during n-hexane hydroconversion on Pt/H-ZSM5 were assessed using an integrated Stefan–Maxwell, mean field, and Single-Event MicroKinetic (SEMK) methodology. The former theory quantifies multicomponent diffusion through a microporous substituent from pure component properties, while framework parameters inherent to the ZSM5 topology are incorporated via a mean field approximation. The complex chemistry involved in n-hexane hydroconversion was described by an SEMK model which is based upon the reaction family concept. Model regression against experimental data resulted in excellent agreement between the mod...

Investigations of Reaction Kinetics for Immobilized Enzymes—Identification of Parameters in the Presence of Diffusion Limitation

A method is proposed for identification of kinetic parameters when diffusion of substrates is limiting in reactions catalyzed by immobilized enzymes. This method overcomes conventional sequential procedures, which assume immobilization does not affect the conformation of the enzyme and, thus, consider intrinsic and inherent kinetics to be the same. The coupled equations describing intraparticle mass transport are solved simultaneously using numerical methods and are used for direct estimation of kinetic parameters by fitting modeling results to time-course measurements in a stirred tank reactor. While most traditional procedures were based on Michaelis-Menten kinetics, the method presented here is applicable to more complex kinetic mechanisms involving multiple state variables, such as ping-pong bi-bi. The method is applied to the kinetic resolution of (R/S)-1-methoxy-2-propanol with vinyl acetate catalyzed by Candida antarctica lipase B. A mathematical model is developed consisting of irreversible ping-pong bi-bi kinetics, including competitive inhibition of both enantiomers. The kinetic model, which fits to experimental data over a wide range of both substrates (5-95%) and temperatures (5-56 degrees C), is used for simulations to study typical behavior of immobilized enzyme systems.

Titanium Anodes with Active Coatings Based on Iridium Oxides: The Corrosion Resistance and Electrochemical Behavior of Anodes Coated by Mixed Iridium, Ruthenium, and Titanium Oxides

A study of the corrosion resistance and electrochemical behavior of titanium anodes with active coatings prepared from mixed oxides iridium, ruthenium, and titanium (OIRTA) is continued. The dependence of the catalytic activity, selectivity, and corrosion resistance of these anodes with x mol % RuO2 + (30 − x ) mol % IrO2 + 70 mol % TiO2 is studied in conditions of chlorine electrolysis on the ratio of concentrations of IrO2 and RuO2 in them at a constant loading of iridium in the coatings. It is established that the maximum corrosion resistance and selectivity is inherent in OIRTA with the RuO2 concentration close to 4 mol %. Partial curves, which describe the dependence of the rates of dissolution of iridium out of OIRTA and the evolution of chlorine and oxygen in them on the electrode potential, are obtained. The dependence of the rates of these processes on the solution pH, the concentration of NaCl in it, and the thickness of the active layer is studied. It is shown that the rate of dissolution of iridium out of OIRTA and the concentration of oxygen in chlorine at a constant potential increase approximately proportionally to the coating thickness, from whence it follows that the said processes proceed over the entire depth of the coating. An assumption is put forth that the chlorine evolution on OIRTA of the optimum composition, with a loading of iridium equal to 2.5 g m−2, at high anodic currents occurs in an outer-kinetics regime in the presence of diffusion limitations on the removal of chlorine out of the coating's depth.

Characterisation of a novel support for biocatalysis in supercritical carbon dioxide

The work presents a characterisation study of Accurel EP100 (polypropylene based hydrophobic granules) as support material for lipase (Lypozyme 10,000l, from native Rhizomucor miehei) operating as biocatalyst in supercritical CO2 as solvent. The study involved assay of biocatalytic activity and operational stability as functions of system pressure and temperature. Furthermore, the presence of diffusion limitations was tested, by varying the bed diameter and support particle size. In addition, SEM and Gas Absorption were employed to test the mechanical stability. Results were compared with the commercially available biocatalyst Lipozyme™ IM60.Pressure did not have a significant effect on the activity or the stability, while temperature had a positive effect on the activity and negative effect on the stability. As expected, an ‘optimum’ value of system water content gave maximum catalytic activity for each biocatalyst. External- and internal-diffusion limitations were both found negligible. The mechanical stability analysis demonstrated little (if any) effect of supercritical carbon dioxide (scCO2) on the structural integrity of Accurel EP100, although subtle increases in pore volume and surface area were observed.

A Mathematical Model for the Performance of Raney Metal Gas Diffusion Electrodes

A mathematical model is developed to examine the effect of intragrain diffusion on the operation of Teflon‐bonded Raney metal gas diffusion electrodes. The model is based on the concept of “electrolyte flooded spherical catalyst grains” while the gas is filling the space between grains. Analytical equations are derived to evaluate the performance of a single catalyst grain in the presence of diffusion limitations. By incorporating the current generation expression for each grain into the differential equation of potential distribution, complete electrode performance under the combined influence of diffusion, activation, and ohmic overpotentials is predicted. This model is used to predict the polarization behavior of electrodes in terms of physically measurable properties such as exchange current density (intrinsic activity of catalyst), grain size, macroporosity and microporosity, specific surface area, catalyst loading (per unit surface area) and electrode thickness. Model equations are applied to a typical alkaline oxygen electrode, and the results are discussed in combination with the assumptions made in the model.

The influence of immobilization and reduced water activity on gaseous‐alkene oxidation by Mycobacterium PY1 and Xanthobacter PY2 in a gas–solid bioreactor

Immobilization of Mycobacterium PY1 and Xanthobacter PY2 in alginate and in or on hydroculture has a minor influence on the maximum rate of oxidation of propene and ethane. The apparent K(m) values of the immobilized cells are slightly higher than those of the free cells, indicating the presence of diffusion limitation in the immobilized systems. Both bacterial strains rapidly lose their alkene-oxidizing activity when the water activity is decreased. This decrease in activity is so rapid that most of the activity is lost already when the pores of the pertinent supports are still filled with water. Therefore, it is not possible with this system to study the transition of mass transfer of substrates entirely in the water phase to mass transfer to substrates entirely through the gas phase.

Photoimmobilization of acetylcholinesterase on 5-azido-1-naphthalenesulfonyl substituted glass beads and kinetic studies on the immobilized enzyme

1. 1. Reaction of 5-azido-1-naphthalenesulfonyl chloride with aminoaryl glass beads gave an 5-azido-1-naphthalenesulfonyl-substituted glass beads. 2. 2. Ultraviolet irradiation of a suspension of these glass beads in a solution of acetylcholinesterase resulted in covalent attachment and thus immobilization of the enzyme onto these glass beads. 3. 3. Kinetic studies on this immobilized enzyme indicated that the reaction was diffusion-controlled. Temperature studies further substantiates the existence of diffusion control. 4. 4. The pH optimum of the immobilized enzyme assayed at high substrate concentration was shifted by approximately 1 pH unit towards a more alkaline region which is consistent with the presence of diffusion limitation in the removal of the hydrolytic products from the immobilized enzyme beads.

Diffusion effects in catalytic gas oil cracking

AbstractWojciechowski's four parameter model of catalyst decay is applied to conversion data for gas oil cracking over a diffusion limited catalyst. The parameters thus obtained are compared with those obtained previously for a diffusion free form of the same catalyst, cracking the same feed stock under identical experimental conditions. The comparison shows that the presence of diffusion affects not only the rate of catalytic cracking but also the rate of aging. It is also shown that the introduction of diffusion phenomena has no effect on the mechanism of aging. The presence of diffusion limitations however has the effect of making the feed stock appear more homogeneous in reactivity