Degenerate Branching Research Articles

Противодействие живых систем развитию возрастной катаракты. Физико-химическое объяснение

Simultaneous processes occur in living systems that lead to both an increase in entropy (chaos) and its decrease. A decrease in entropy is possible only in open systems, associated with a contin-uous flow of energy-intensive compounds into them, such as glucose, the oxidation of which is energetically associated with the synthesis of ATP, the universal carrier of chemical energy to en-sure functioning of living systems. When energy-intensive compounds are oxidized, entropy in-creases, and when ATP is synthesized, it decreases. In living systems, not only reactions associated with the synthesis of ATP and their functioning occur, but also aging does, namely, the oxidation of hydrophobic formations in cells, in particular, the membrane matrix, following the type of chain reactions with degenerate branching. The development of such chain reactions of oxidation in cells can be prevented by the penetration into them of various antioxidants, for example, resvera-trol, which is contained in grapes and other plants, and quenchers of molecular oxygen in the sin-glet state, for example, iodides, as well as by creating cyclic processes involving desaturases that prevent an increase in viscosity in the matrix of membranes up to certain limiting values, after which cell death becomes inevitable. In this regard, the processes leading to cataracts (clouding of the eye lenses) are indicative. In the absence of desaturases in the lens fibers of rats, exposure to UV light leads to cataracts in 2–4 months, while the presence of desaturases in human eye lens fi-bers makes it possible to avoid the occurrence of cataracts for many decades. The reaction of con-version of lipids with saturated fatty acids into unsaturated ones, catalyzed by desaturases, leads to counteracting the aging of the membrane matrix. The vital feature of this reaction is that it oc-curs only when the membrane matrix is cured. This reaction first leads to liquefaction of the ma-trix up to a certain state, after which it stops. The reasons leading either to revival of desaturase or to decrease in its activity are discussed in this article.

Experimental and computational investigation of the influence of iso-butanol on autoignition of n-decane and n-heptane in non-premixed flows

Experimental and computational investigation is carried out to elucidate the influence of iso-butanol on critical conditions of autoignition of n-decane and n-heptane. The counterflow configuration is employed. In this configuration an axisymmetric stream of air is directed over the surface of an evaporating pool of a liquid fuel. A stagnation plane is established. The temperature of the air is increased until autoignition is achieved in the mixing layer in the vicinity of the stagnation plane. The temperature of the air stream at autoignition, Tig, is measured at various values of the strain rate, which is defined as the axial gradient of the axial component of the flow velocity at the stagnation plane. Fuels tested are n-decane, n-heptane, iso-butanol and various mixtures of n-decane/iso-butanol and n-heptane/iso-butanol. Kinetic modeling is carried out using the recently updated version of the comprehensive CRECK chemical-kinetic mechanism. Critical conditions of autoignition are predicted for all fuels and fuel mixtures and the results are compared with the measurements. Computations show that low-temperature chemistry plays a significant role in promoting autoignition of n-decane and n-heptane, and the influence of low-temperature chemistry decreases with increasing strain rates because there is insufficient residence for the low-temperature reactions to take place. Experimental data and numerical simulations show that addition of even small amounts of iso-butanol to n-decane or n-heptane increases the value of Tig at low strain rates indicating that iso-butanol strongly inhibits the low-temperature chemistry of n-decane and n-heptane. Furthermore, the simulations show that when iso-butanol is present in the liquid fuel, only a small amount of n-decane is available in the gas phase and its low concentration cannot effectively sustain the low temperature degenerate branching oxidation process. Predicted flame structures show that the peak values of mole fraction of ketohydroperoxide are significantly reduced when iso-butanol is added to n-decane indicating that the kinetic pathway to low temperature ignition is blocked. This observation is confirmed from sensitivity analysis.

Kinetic parameters of the formation of oxygen-containing compounds in the vacuum gas oil oxycracking process

The kinetics of the formation of oxygen-containing compounds during oxycracking of vacuum gas oil was experimentally studied. The kinetic parameters of the process of formation of oxygen-containing compounds in five target fractions under oxycracking conditions are established: reaction rates, rate constants, reaction orders, activation energies. The average reaction rate of the formation of oxygen-containing compounds in oxycracking products in the first 600 s is limited by the rate of oxidation of the C10–C12 fraction, further from 600 up to 900 s by the oxidation of the C5–C9 fraction (during this period, the direction/mechanism of the formation of OCC in this fraction changes), and after 900 s—the fractions C1–C4. The presence of the induction period indicates that the formation of oxygen-containing compounds in the C1–C4 fraction occurs according to the unbranched chain mechanism, and C5–C9 and C22–C30—through the chain with degenerate branching. The combination of experimental and calculated data indicates a joint homogeneous heterogeneous oxycracking mechanism of vacuum gas oil, which excludes the single-valued contribution of only the volume or only the surface of the catalyst. It is shown that under oxycracking conditions the process proceeds both in the kinetic and in the internal transition region, depending on the temperature.

Kinetic Analysis of the Oxidative Conversion of Methane in Slow Combustion. I. Key Steps of the Chemical Mechanism

This paper presents a kinetic analysis of a chemical model for the oxidative conversion of methane which includes all possible elementary reactions that can occur in this complex radical chain process. It is found that the model fully reflects the kinetic features of degenerate chain branching reactions. The key steps of the process responsible for the formation of the main reaction products and the development of the process as a whole are identified. The nonlinear reactions of free peroxide radicals play a decisive role in these steps. The results of the kinetic analysis of the model confirm the previous direct experimental data on free radicals and their kinetic behavior during methane oxidation.

Liquid-phase oxidation of cyclohexane. Cyclohexyl hydroperoxide, cyclohexanol, and cyclohexanone, mechanisms of formation and transformation

The mechanisms of formation and transformation of cyclohexyl hydroperoxide, cyclohexanol, and cyclohexanone in the liquid-phase oxidation of cyclohexane are reviewed. Cyclohexyl hydroperoxide is formed in chain propagation reactions and decomposes via the degenerate branching reactions under the action of alkoxyl, alkyl, and peroxyl radicals, as well as via nonradical channels to form alcohol and ketone. The radicals of two types, $${\rm{H}}{{\rm{O}}_2}^ \cdot $$ radical and α-hydroxycyclohexylperoxyl radical, are involved in chain propagation in the course of cyclohexanol oxidation. In the chain propagation reactions, $${\rm{H}}{{\rm{O}}_2}^ \cdot $$ radicals are more reactive than α-hydroxycyclohexylperoxyl radicals. The dynamic equilibrium was established in the dissociation of 1-hydroxycyclohexylperoxyl radical to cyclohexanone and radical $${\rm{H}}{{\rm{O}}_2}^ \cdot $$ . During oxidation of cyclohexane, cyclohexanone is formed from peroxyl radicals, as well as by the decomposition of cyclohexyl hydroperoxide and chain-radical oxidation of cyclohexanol. The oxidation of cyclohexanone in positions 2 and 6 affords 2-hydroperoxycyclohexanone, which predominantly decomposes to form 2-hydroxycyclohexanone. The oxidation of 2-hydroxy-cyclohexanone proceeds via the main channels of destruction of the carbon chain of cyclohexane leading to adipic acid and adipic anhydride. The oxidation products of C—H bonds of all types in alcohol and ketone are accumulated simultaneously with the oxidation products of C(1)—H bonds in cyclohexanol and of C(2)—H and C(6)—H bonds in cyclohexanone.

Kinetics of Low-Temperature Oxidation of Enzymatic Lignin from Pine Wood (Pinus silvestris) in an Aqueous Alkaline Medium

The kinetics of oxidation of the enzymatic lignin from pine wood (brown rotted wood) by oxygen in an aqueous alkaline medium at 90–160°C was investigated. It was established that the vanillin yield increased gradually with the temperature increase in this range from 3.4 to 5.6 wt % with respect to lignin. The observed activation energy of the process of oxygen consumption varies in the range 6–19 kJ/mol depending on the process conditions. The process order with respect to oxygen pressure calculated from the rates of O2 consumption during the first two hours of oxidation at 90–120°C is 1.02 ± 0.05. The low value of activation energy of the process as well as the first order in oxygen indicates a diffusion-controlled process under these conditions. The kinetic data demonstrate that the role of degenerate chain branching processes increases with the increase in the degree of oxidation as a result of decomposition of the hydroperoxides formed. The hydroperoxide decomposition during oxidation in quantity of more than 16 mol % with respect to the initial lignin was registered using a volumetric method based on the oxygen release. The main causes and features determining the monotonically increasing function of the dependence of the selectivity of lignin oxidation to aromatic aldehydes on temperature were discussed.

Three stage cool flame droplet burning behavior of n-alkane droplets at elevated pressure conditions: Hot, warm and cool flame

Transient, isolated n-alkane droplet combustion is simulated at elevated pressure for helium-diluent substituted-air mixtures. We report the presence of unique quasi-steady, three-stage burning behavior of large sphero-symmetric n-alkane droplets at these elevated pressures and helium substituted ambient fractions. Upon initiation of reaction, hot-flame diffusive burning of large droplets is initiated that radiatively extinguishes to establish cool flame burning conditions in nitrogen/oxygen “air” at atmospheric and elevated pressures. However, at elevated pressure and moderate helium substitution for nitrogen (XHe > 20%), the initiated cool flame burning proceeds through two distinct, quasi-steady-state, cool flame burning conditions. The classical “Hot flame” (∼1500 K) radiatively extinguishes into a “Warm flame” burning mode at a moderate maximum reaction zone temperature (∼ 970 K), followed by a transition to a lower temperature (∼765 K), quasi-steady “Cool flame” burning condition. The reaction zone (“flame”) temperatures are associated with distinctly different yields in intermediate reaction products within the reaction zones and surrounding near-field, and the flame-standoff ratios characterizing each burning mode progressively decrease. The presence of all three stages first appears with helium substitution near 20%, and the duration of each stage is observed to be strongly dependent on helium substitutions level between 20–60%. For helium substitution greater than 60%, the hot flame extinction is followed by only the lower temperature cool flame burning mode. In addition to the strong coupling between the diffusive loss of both energy and species and the slowly evolving degenerate branching in the low and negative temperature coefficient (NTC) kinetic regimes, the competition between the low-temperature chain branching and intermediate-temperature chain termination reactions control the “Warm” and “Cool” flame quasi-steady conditions and transitioning dynamics. Experiments onboard the International Space Station with n-dodecane droplets confirm the existence of these combustion characteristics and predictions agree favorably with these observations.

Liquid-phase oxidation of cyclohexane. Elementary steps in the developed process, reactivity, catalysis, and problems of conversion and selectivity

The literature data concerning features of the kinetics and mechanisms of elementary steps of liquid-phase oxidation of cyclohexane and its oxygen derivatives are considered and analyzed. A comparison of rates of intermolecular and intramolecular reactions of cyclohexylperoxyl radicals under the industrial conditions indicated a necessity to take into account intramolecular interactions. The occurrence of cross recombination of hydroperoxyl and α-hydroxyperoxyl radicals without chain termination in the course of cyclohexanol and 2-hydroxycyclohexanol oxidation was proved. A significance of degenerate branching reactions involving cyclohexyl hydroperoxide in the industrial process of cyclohexane oxidation at 423 K was evaluated. The influence of the electron-withdrawing functional groups on the reactivity of carbon–hydrogen bonds of organic compounds in the reactions with electrophilic peroxyl radicals was studied. The low conversion of a substrate in the industrial process are mainly caused by the radicalchain oxidation of cyclohexanone leading only to by-products. The catalysts of cyclohexane oxidation, viz., compounds of variable valence metals, affect the reaction rate and ratio of the yields of the target products (cyclohexyl hydroperoxide, cyclohexanol, and cyclohexanone) but exert no effect on their relative reactivity. The use of the catalytic additives increasing the yield of cyclohexanone in the step of cyclohexane oxidation in the production of caprolactam is revealed to be inexpedient.

Oscillatory cool flame combustion behavior of submillimeter sized n-alkane droplet under near limit conditions

This paper reports simulation results of oscillatory cool flame burning of an isolated, submillimeter sized n-heptane (n-C7H16) droplet in a selectively ozone (O3) seeded nitrogen-oxygen (N2-O2) environments at atmospheric pressure. An evolutionary one-dimensional droplet combustion code encompassing relevant physics and detailed chemistry was employed to explore the roles of low-temperature chemistry, O3 seeding, and dynamic flame structure on burning behaviors. For XO2 = 21% and a range of selective ozone seeding, near-quasi-steady cool flame burning is achieved directly (without requiring hot flame initiation and radiative extinction). Under low oxygen index conditions, but with significant O3 seeding (XO3 = 5%), a nearly quasi-steady cool flame is initially established that then transitions to a dynamically oscillating cool flame burning mode which continues until the droplet is completely consumed. It is found that the oscillation occurs as result of a initial depletion of fuel vapor-oxidizer layer evolving near the droplet surface and its dynamic re-establishment through liquid vaporization and vapor/oxidizer transport. A kinetic analysis indicates that the dynamic competition between the reaction classes- (a) degenerate chain branching and (b) chain termination/propagation - along with continuous fuel and oxygen leakage through the flame location contributes to an oscillatory burning phenomena of ever-increasing amplitude. Analysis based on single full-cycle of oscillatory burning shows that the reaction progression matrices (evolution of heat and species) for QOOH➔chain propagation/termination reactions (here, Q = C7H14-) directly scales with the gas phase temperature field. On the contrary, the QOOH➔degenerate branching reactions undergoes three distinct stages within the same oscillatory cycle. The coupled flame dynamics and kinetics suggest that in the oscillatory burning mode, kinetic processes dynamically cross through conditions characterizing the negative temperature coefficient (NTC) turnover temperature, separating low temperature and NTC kinetic regimes. In addition, a parametric study is conducted to determine the role of O3 seeding level on the observed oscillation phenomena.

KINETICS OF FIR WOOD OXIDATION BY OXYGEN IN AQUEOUS ALKALINE MEDIA

Kinetics of fir wood oxidation ( Abies sibirica ) in aqueous alkaline media were studied by measuring oxygen consumption rates. Oxidation of extracted wood conforms to the kinetics of non-branching chain mechanism. The same mechanism governs the initial stage of non-extracted wood oxidation, but a transition to degenerate branching mode soon occurs. This transition is probably caused by softwood's low hemicelluloses content and high content of propyl guaiacol lignin units that are prone to condensation involving phenolic hydroxyls. This leads to quick depletion of phenolic and uronic carboxylic groups; chain initiation by oxidizing the respective anions becomes impossible, and degenerate branching takes the lead. It is assumed that inert hydrophobic fir extractives physically shield hydroperoxides responsible for degenerate branching from alkali. This prevents alkali from causing heterolytic hydroperoxide cleavage that does result in chain branching. It is assumed that the catalyst accelerates the reaction of homolytic cleavage of the aforementioned hydroperoxides

ChemInform Abstract: Catalytic Formation of Monosaccharides: From the Formose Reaction Towards Selective Synthesis

AbstractReview: [110 refs.

Multistage oscillatory “Cool Flame” behavior for isolated alkane droplet combustion in elevated pressure microgravity condition

Recently, large diameter, isolated n-heptane droplet experiments under microgravity conditions (aboard the International Space Station) exhibited “Cool Flame” burning behavior, resulting from a heat loss mechanism that extinguishes hot combustion and a transition into a sustained, low temperature second stage combustion. In atmospheric pressure air, a single combustion mode transition to “Cool Flame” burning is followed by diffusive extinction. But with increasing pressure, multiple cycles of hot initiation followed by transition to “Cool Flame” burning are observed. This paper reports experimental observations that characterize the transition time histories of this multi-cycle, multi-stage behavior. Transient sphero-symmetric droplet combustion modeling that considers multi-stage detailed kinetics, multi-component diffusion, and spectral radiation is applied to analyze the experimental observations. The simulations indicate that as parameters change the chemical time scales dictating low temperature degenerate chain branching, multiple hot/cool flame burning transitions are induced by increasing the cool flame burning heat generation rate compared to the diffusive loss rate. The balance of these terms in the negative temperature coefficient kinetic regime defines whether reactions accelerate into re-ignition of a hot flame event, burn quasi-steadily in the cool flame mode, or diffusively extinguish. The rate of reactions controlling ketohydroperoxide formation and destruction are shown to be key re-ignition of hot combustion from the cool flame mode. Predictions are found to be in good agreement with the experimental measurements. Modeling is further applied to determine how these observations are dependent on initial experimental conditions, including pressure, and diluent species.

Catalytic formation of monosaccharides: from the formose reaction towards selective synthesis.

The formose reaction (FR) has been long the focus of intensive investigations as a simple method for synthesis of complex biologically important monosaccharides and other sugar-like molecules from the simplest organic substrate-formaldehyde. The fundamental importance of the FR is predominantly connected with the ascertainment of plausible scenarios of chemical evolution which could have occurred on the prebiotic Earth to produce the very first molecules of carbohydrates, amino- and nucleic acids, as well as other vitally important substances. The practical importance of studies on the FR is the elaboration of catalytic methods for the synthesis of rare and non-natural monosaccharides and polyols. This Minireview considers the FR from the point of view of chemists working in the field of catalysis with emphasis on the mechanisms of numerous parallel and consequent catalytic transformations that take place during the FR. Based on its kinetics, the FR may be considered as a non-radical chain process with degenerate branching. The Minireview also considers different approaches to the control of selectivity of carbohydrate synthesis from formaldehyde and lower monosaccharides.

Transport Limitations during Phase Transfer Catalyzed Ethyl-Benzene Oxidation: Facts and Fictions of “Halide Catalysis”

The mechanistic interpretation of the catalytic effect of phase transfer catalysts in the selective oxidation of ethyl benzene is hampered by mass transfer effects. We demonstrate that proper experiments lead to a more correct interpretation of the role of the quaternary ammonium salt (QAS) and its counterion. Specifically, experiments unequivocally show that the main action of the counterion is to enhance physical mass transfer processes, while its catalytic effect is limited to a shift in selectivity, not activity. The QAS as a whole accelerates the induction process in the ethyl benzene (EB) oxidation by degenerate branching of its hydroperoxide (EBHP). Proper mechanistic understanding of these phenomena in QAS catalysts is especially crucial under industrially relevant conditions

Fluctuation limits of strongly degenerate branching systems

Functional limit theorems for scaled fluctuations of occupation time processes of a sequence of critical branching particle systems in $\R^d$ with anisotropic space motions and strongly degenerated splitting abilities are proved in the cases of critical and intermediate dimensions. The results show that the limit processes are constant measure-valued Wienner processes with degenerated temporal and simple spatial structures.

Occupation Time Fluctuations of Weakly Degenerate Branching Systems

We establish limit theorems for re-scaled occupation time fluctuations of a sequence of branching particle systems in $\R^d$ with anisotropic space motion and weakly degenerate splitting ability. In the case of large dimensions, our limit processes lead to a new class of operator-scaling Gaussian random fields with non-stationary increments. In the intermediate and critical dimensions, the limit processes have spatial structures analogous to (but more complicated than) those arising from the critical branching particle system without degeneration considered by Bojdecki et al.{\it Stochastic Process. Appl. 2006}. Due to the weakly degenerate branching ability, temporal structures of the limit processes in all three cases are different from those obtained by Bojdecki et al. .{\it Stochastic Process. Appl. 2006}.

The possibility of regime changing in chain reactions with degenerate branching under the influence of external magnetic field

A reaction of hydrocarbons oxidation in the liquid phase is treated theoretically. The reaction system under discussion is a flow reactor, to the inlet of which the hydrocarbon is constantly delivered in the mixture with an inhibitor under oxygen saturation conditions; the reaction mixture constantly flows from the chamber at the same rate. The reaction gives rise to radicals that can subsequently recombine. It is shown that under certain conditions in this reaction system, three steady states may arise, two of which are stable and the third state is unstable. By varying rate constants of radical reactions by means of an external magnetic field, one can disturb the steady state stability and transfer the system to another steady state, which will be accompanied by an abrupt change in the concentration of reacting substances.

Chain and photochain mechanisms of photooxidation of polymers

The mechanisms of photooxidation of the popular commercial polymers polystyrene (PS), polyethylene (PE), and an ethylene-carbon monoxide copolymer (polyketone, PK) differing in the polymer chain structure and the nature and concentration of chromophore groups are considered. In the case of the formation of photosensitive intermediates in polystyrene, taken as an example, the photochain oxidation mechanism was revealed and thoroughly studied, according to which the polymer “burns” into complete oxidation products (CO2, H2O) with a degree of conversion of ≥50% and a kinetic-chain length of l = 103–104 units. The hydroperoxide mechanism plays a minor role in the photooxidation of PS, it is a short-chain process (as in the case of thermal oxidation, l ∼ 10) and does not exceed 1.5% of the total amount of absorbed oxygen. Carbonyl groups, as weak photoinitiators, induce in PE and PK the conventional radical chain mechanism of photooxidation with degenerate branching of kinetic chains on hydroperoxide groups and other oxidations products.

Imperfection sensitivity of degenerate hilltop branching points

Imperfection sensitivity is investigated for a degenerate hilltop branching point, where a degenerate bifurcation point exists at a limit point. A degenerate hilltop branching point is important as it is a byproduct of optimization of shallow shell structures under non-linear buckling constraints. A systematic procedure is presented for asymptotic sensitivity analysis based on enumeration of vertices of a convex region defined by linear inequality constraints on the orders of the variables. The effectiveness of the proposed method is demonstrated by sensitivity analysis of degenerate hilltop branching points, considering minor and major imperfections, corresponding to an unstable-symmetric or asymmetric bifurcation point at the limit point. It is found that a hilltop branching point can be imperfection sensitive.

Autoacceleration of the free-radical chain luminescent oxidation of U(IV) with xenon trioxide in aqueous HClO4

Autoacceleration is observed in the chemiluminescent radical chain oxidation of U(IV) with xenon trioxide in aqueous perchloric acid in a Teflon (not conventional glass) reactor. The decay of chemiluminescence accompanying the reaction between U(IV) and XeO3 in the case of excess oxidizer obeys a first-order kinetic equation only at low U(IV) concentrations of 10−5–10−6 mol/l. Autoacceleration takes place at comparatively high reactant concentrations, when the contribution from the heterogeneous loss of radicals on the reactor walls into chain termination is comparatively small and the role of degenerate chain branching reactions is significant. It is inferred that a critical phenomenon rare for liquid-phase radical chain processes takes place in U(IV) oxidation with xenon trioxide: a comparative small increase in the reactant concentrations causes this chemiluminescent redox process to pass from the quasi-steady-state regime to autoacceleration.