Spatiotemporal dynamics of the Landolt reaction in an open spatial reactor with conical geometry

In a previous study, the iodate-sulfite proton autoactivated reaction (Landolt reaction) was shown to exhibit spatial bistability and spatiotemporal oscillations when operated in an open spatial reactor with fixed "thickness", i.e., feed boundary to core distance. Here, we show that the spatial reactors with conical geometry enable one to rapidly probe the sensitivity of the above phenomena over a large range of the "thickness" parameter. This often-neglected parameter in chemical pattern studies plays an important role on the selection and stability of states. We reveal that the quenching capacity of slow diffusing polyacrylate ions on the spatiotemporal oscillations depends on this "thickness". The presented results should be useful for further research on reaction diffusion patterns and chemomechanical structures.     

Catalytic spatiotemporal thermal patterns during CO oxidation on cylindrical surfaces: experiments and simulations

Dynamics of spatiotemporal thermal patterns during the catalytic CO oxidation over Pd supported on a glass-fiber catalytic cloth rolled into a tube of 20 mm diameter and 80 mm length has been studied in a continuous flow reactor by IR thermography. A specially designed aluminum mirror built in the reactor provided image of the entire surface of the horizontally held catalytic tube. With flow in the main axial direction and through the tube surface, we observed periodic motions of a pulse, which was born downstream and propagated upstream. The temperature pulse motion was accompanied by conversion oscillations of CO2. With flow in the main axial direction, parallel to the surface, we observed a stationary hot zone after an oscillatory transient. These patterns can be simulated with a plug-flow-reactor-like heterogeneous reactor model that incorporates previously determined kinetic and transport parameters.     

Pattern formation in the iodate-sulfite-thiosulfate reaction-diffusion system

Sodium polyacrylate-induced pH pattern formation and starch-induced iodine pattern formation were investigated in the iodate-sulfite-thiosulfate (IST) reaction in a one-side fed disc gel reactor (OSFR). As binding agents of the autocatalyst of hydrogen ions or iodide ions, different content of sodium polyacrylate or starch has induced various types of pattern formation. We observed pH pulses, striped patterns, mixed spots and stripes, and hexagonal spots upon increasing the content of sodium polyacrylate and observed iodine pulses, branched patterns, and labyrinthine patterns upon increasing the starch content in the system. Coexistence of a pH front and an iodine front was also studied in a batch IST reaction-diffusion system. Both pH and iodine front instabilities were observed in the presence of sodium polyacrylate, i.e., cellular fronts and transient Turing structures resulting from the decrease in diffusion coefficients of activators. The mechanism of multiple feedback may explain the different patterns in the IST reaction-diffusion system.     

Microdischarge-Induced Decomposition of Ammonia and Reduction of Silver Ions for Formation of Two-Dimensional Network Structure

Microdischarge-induced reaction processes working at atmospheric pressure create fractal-like network structure of metal nano-particles which shows variable electric and optical properties. Due to their smallness, microdischarges or microplasmas can be installed in a gas-tubing system, and they enable us to create a compact chemical reduction reactor which includes decomposers of molecules, gas flows, and aqueous solutions with metallic ions at atmospheric pressure. Ammonia (NH₃) gas is successfully decomposed in this reactor, and its products which include mainly hydrazine (N₂H₄) and flow in the downstream induce reduction reactions for AgNO₃ solution. Various parameters in the reactor trigger formation of functional patterns of silver nano-particles like partially transparent layers whose conductivity is variable. Optical properties of this equivalent films show some absorption spectra coming from structure resonances, which can be an optical metamaterials in this self-assembly process.     

Magnetic resonance imaging of flow-distributed oscillations

The formation of stationary concentration patterns in a packed-bed reactor (PBR), using a manganese-catalyzed Belousov-Zhabotinsky (BZ) reaction in a mixed sulfuric-phosphoric acid medium, was studied using magnetic resonance imaging (MRI). The PBR was composed of a column filled with glass beads, which was fed by a continuous stirred tank reactor (CSTR). As the reactor is optically opaque, investigation of the three-dimensional (3D) structure of these reaction-diffusion-advection waves is not possible using conventional image capture techniques. MRI has been used to probe this system and the formation, 3D structure, and development of these waves has been studied. At reactor startup, traveling waves were observed. After this initial period the waves stabilized and became stationary. Once fixed, they were found to be remarkably stable. There was significant heterogeneity of the reaction fronts, which were not flat, as would be expected from a plug-flow reactor. Instead, the reaction wave fronts were observed to be conical in shape due to the local hydrodynamics of the bed and specifically the higher velocities and therefore lower residence times close to the wall of the reactor.     

Asymmetric catalysis in a micro reactor—Ce, Yb and Lu catalysed enantioselective addition of trimethylsilyl cyanide to benzaldehyde

A T-shaped micro reactor was used for the optimisation of reaction conditions for the enantioselective silylcyanation of benzaldehyde catalysed by lanthanide-pybox complexes. Compared to a conventional batch procedure, higher conversion was observed within shorter reaction time. The micro reactor process involving Lu(III) afforded essentially the same enantioselectivity as the batch process (73 vs 76% ee), whereas the enantioselectivity was lower in the micro reactor for catalysts containing Yb(III) (53 compared to 72%). Ce(III) provided very low selectivity in both types of processes (1 and 11% ee, respectively). A study of the effect of additives showed that the enantioselectivity in the Yb catalysed reaction performed in the micro reactor could be increased to 66%, whereas only a minor improvement, to 78% ee, was observed in the reaction with Lu.     

Pressurized CO<Subscript>2</Subscript>-enhanced gasification of coal

Carbon dioxide was considered as a co-gasifying agent in a coal gasification reactor. The work presented herein describes the simulation results for the process and the experimental data on coal char gasification with CO₂ addition as the rate-controlling step for the entire process. To study the potentially beneficial effect of the introduction of CO₂ into the gasification system, several simulations were conducted using the commercial process simulation software ChemCAD 6.3^®. The results of a Gibbs equilibrium reactor were evaluated. The Boudouard reaction is a critical path for the development of this process, and the kinetics were studied experimentally. Four chars derived from the pyrolysis of Polish coals of different origins were selected for the experiments. The kinetic characteristics of this system were examined using a custom-designed pressurized fixed-bed reactor. To determine the effect of pressure on the gasification rate, several preliminary studies on the gasification of coal chars were performed isothermally at the temperature of 950 °C and pressures of 1, 10, and 20 bars. In contrast to the thermodynamic calculations, the experimental data revealed that increasing the CO₂ pressure leads to a higher reaction rate for medium-rank coal chars and low-rank lignite coal char, resulting in higher efficiency for carbon monoxide production. The pressure influences the reactivity more strongly when varied from 1 to 10 bars; a further increase in pressure affects the rate almost insignificantly. The observed behavior representing the changes in carbon conversion degree during gasification is satisfactorily described by the grain model.     

Patterns of the ferrocyanide-iodate-sulfite reaction revisited: the role of immobilized carboxylic functions

We experimentally demonstrate that the standing lamella reaction-diffusion patterns initially observed 17 years ago in a gel-filled open spatial reactor operated with the ferrocyanide-iodate-sulfite bistable reaction requires an upper critical concentration of low-mobility species with weak acid functional groups, a parameter that was overlooked at the time and had made observations difficult to reproduce. The present approach enables the control of the space scale separation between activatory and inhibitory processes. It makes the wealth of exotic pattern dynamics observed earlier easier to reproduce and understand. This contribution should considerably revive the interest in this reaction and boost the search for the control of reaction-diffusion patterns in other bistable systems.     

Catalyst optimization strategy: selective oxidation of o-xylene to phthalic anhydride

The oxidation of o-xylene and/or naphthalene to phthalic anhydride is one of the important industrial processes based on catalytic selective oxidation reactions. Vanadia--titania catalysts have been used in the industrial phthalic anyhdride process for the last 50 years. The operation parameters like the temperature range of operation, reactor inlet pressures, contact times, o-xylene loadings, etc. were constantly improved during this period of continuous process optimization so as to optimize catalyst performance and increase its life time. However, a fundamental understanding of the mutual interaction of the rather complex reaction network and the catalyst formulation is still missing. Recently, a detailed study of by-product formation as function of process conditions allowed us to develop a novel, improved reaction scheme for the catalytic oxidation of o-xylene. Based on this understanding, a detailed investigation was conducted for the first time of the by-product formation under varying operation conditions and as a function of the active mass variation exploiting high-throughput, as well as bench scales reactors. This high-throughput testing allowed us to relate reaction kinetics to novel catalyst formulations.     

Gas-phase proton transfer reactions involving multiply charged cytochrome c ions and water under thermal conditions

Investigations of gas-phase proton transfer reactions have been performed on protein molecular ions generated by electrospray ionization (ESI). Their reactions were studied in a heated capillary inlet/reactor prior to expansion into a quadrupole mass spectrometer. Results from investigations involving protonated horse heart cytochrome c and H, O suggest that Coulombit effects can lower reaction barriers as well as aid in entropically driven reactions. For example, the charge state distribution observed by a quadrupole mass spectrometer for multiply protonated cytochrome c without the addition of any reactive gas ranges from 9+ to 19+ , with the [M + 15H]¹⁵⁺ ion being the most intense peak. With the addition of H₂O (proton affinity approximately 170.3±2 kcal/mol) to the capillary reactor at 120°C, the charge state distribution shifts to a lower charge, ranging from 13+ to less than 9+. Under the same conditions with argon (proton affinity approximately 100 kcal/mol) as the reactive gas, no shift in the charge state distribution is observed. The results demonstrate that proton transfer to water can occur for highly protonated molecular ions, a process that would be expected to be highly endothermic for singly protonated molecules (for which Coulombic destabilization is not significant). The results imply that the charge state distribution from ESI is somewhat dependent upon the mechanism and speed of the droplet evaporation/ion desolvation process, which may vary substantially with the ESI/mass spectrometry interface design.     

Spatiotemporal chaos arising from standing waves in a reaction-diffusion system with cross-diffusion

We show that quasi-standing wave patterns appear in the two-variable Oregonator model of the Belousov-Zhabotinsky reaction when a cross-diffusion term is added, no wave instability is required in this case. These standing waves have a frequency that is half the frequency of bulk oscillations displayed in the absence of diffusive coupling. The standing wave patterns show a dependence on the systems size. Regular standing waves can be observed for small systems, when the system size is an integer multiple of half the wavelength. For intermediate sizes, irregular patterns are observed. For large sizes, the system shows an irregular state of spatiotemporal chaos, where standing waves drift, merge, and split, and also phase slips may occur.     

Influence of Reaction Pressure on Semibatch Esterification Process of Poly(ethylene terephthalate) Synthesis

Summary: Influence of esterification pressure on oligomeric properties was studied by using a semibatch reactor. Esterification model for semibatch process was further improved by considering EG reflux in the column. It was observed that increasing the reaction pressure decreases EG/water ratio in the column while increasing the EG/TPA feed ratio increases EG/water ratio in the column. By controlling the EG reflux in a semibatch reactor, it is possible to generate oligomers with similar oligomeric properties observed at different stages of continuous process.     

Complex and chaotic oscillations in a model for the catalytic hydrogen peroxide decomposition under open reactor conditions

Numerous periodic and aperiodic dynamic states obtained in a model for hydrogen peroxide decomposition in the presence of iodate and hydrogen ions (the Bray-Liebhafsky reaction) realized in an open reactor (CSTR), where the flow rate was the control parameter, have been investigated numerically. Between two Hopf bifurcation points, different simple and complex oscillations and different routes to chaos were observed. In the region of the mixed-mode evolution of the system, the transitions between two successive mixed-mode simple states are realized by period-doubling of the initial state leading to a chaotic window in which the next dynamic state emerges mixed with the initial one. It appears in increasing proportions in concatenated patterns until total domination. Thus, with increasing flow rate the period-doubling route to chaos was obtained, whereas with decreasing flow rate the peak-adding route to chaos was obtained. Moreover, in very narrow regions of flow rates, chaotic mixtures of mixed-mode patterns were observed. This evolution of patterns repeats until the end of the mixed-mode region at high flow rates that corresponds to chaotic mixtures of one large and many small amplitude oscillations. Starting from the reverse Hopf bifurcation point and decreasing the flow rate, simple small amplitude sinusoidal oscillations were encountered and then the period-doubling route to chaos. With a further decreasing flow rate, the mixed-mode oscillations emerge inside the chaotic window.     

Oscillatory reactions involving hydrogen peroxide and thiosulfate-kinetics of the oxidation of tetrathionate by hydrogen peroxide

The reaction between tetrathionate and hydrogen peroxide forms an important part of several pH oscillators based on the oxidation of thiosulfate. The kinetics of this reaction were examined in a batch reactor by measurement of the initial pH values in the range from 8 to 10.5. Experimental data were evaluated by the method of initial reaction rates combined with the assumption of instantaneously equilibrated acid-base reactions. The rate-determining step was found to be of the first order with respect to tetrathionate, hydrogen peroxide, and OH- ions with the value of rate constant k = (1.50 +/- 0.03) x 10(2) (M2 s)(-1) at 25 degrees C. In the alkaline solution, no distinct catalytic effect of Cu2+ was observed in contrast to earlier assumptions. The waveform of measured pH over the course of the reaction indicates that thiosulfate is probably an intermediate because characteristics of the curves are very similar to those for the oxidation of thiosulfate. We also measured the time evolution of concentrations of major components by the attenuated total internal reflectance infrared spectroscopy to further elucidate the underlying reaction mechanism. These measurements confirm the suspected role of thiosulfate as an intermediate in the studied reaction and provide valuable detailed information on the time evolution of thiosulfate, sulfite, sulfate, tetrathionate, and trithionate. These experimental observations are included in a simple mechanism that accurately simulates the reaction course in an alkaline solution. The results provide considerable new insights into the nature of autocatalysis in the hydrogen peroxide-thiosulfate-Cu2+ reaction and suggest that a new role for Cu2+ in the oscillatory dynamics observed in a flow-through reactor needs to be found.     

Quantitative investigation of spatiotemporal chaos in the ferroin-catalyzed Belousov-Zhabotinskii reaction in a batch reactor

We have analyzed experimental data on the time dependence of the potentials of 16 platinum point-contact electrodes for spatiotemporal chaos in the Belousov-Zhabotinskii reaction. We show that the largest Lyapunov exponent is a convenient characteristic for spatiotemporal chaos. We found that in going from temporal to spatiotemporal chaos, the embedding dimension of the system increases by a factor of three. We have observed that as the largest Lyapunov exponent increases, we observe a decrease in the degree of spatial correlation of the process.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 31, No. 1, pp. 34–39, January–February, 1995.We would like to thank the Foundation for Basic Research of the State Science and Technology Committee of Ukraine for financial support of this work (Project 3.3/87).     

Similarities and differences in the electron ionization-induced fragmentation of structurally related N-alkoxymethyl lactams and sultams

Unimolecular fragmentation patterns of the molecular ions of selected lactams and sultams bearing alkoxymethyl group at the nitrogen atom were studied. The main common fragmentation reaction observed for all compounds studied in this work is the elimination of an aldehyde molecule. This reaction is considered to proceed via two different mechanisms. For lactams, hydrogen rearrangement within an alkoxymethyl group is observed, which leads to the appropriate N-methyl derivatives. For sultams, transfer of the methyl group to the nitrogen and oxygen atoms, proceeding through an ion-neutral complex, dominates. Another important fragmentation channel characteristic exclusively for lactams is the loss of an alkyl radical. This process takes place within the N-alkoxymethyl moiety, yielding the appropriate protonated ion of N-formyllactams. This process is accompanied by relatively high kinetic energy release.     

The nature of autocatalysis in the Butlerov reaction

The effect of the nature of an initiator on the kinetics of formaldehyde consumption and on product composition in the Butlerov reaction was studied in a stirred flow reactor and a batch reactor. It was found that, under flow conditions, the kinetics and the product composition of this reaction are independent of the nature of the initiator. The reaction schemes proposed previously for an autocatalytic process mechanism based on the formation of glycolaldehyde from two formaldehyde molecules are incorrect. A correlation between the initiating activities of various monosaccharides and the rates of their conversion into an enediol form was found with the use of a batch reactor. Solid enediol complexes with Ca²⁺ ions were isolated for glucose, fructose, ribose, and sorbose; the initiating activity of these complexes was found to be much higher than the initiating activity of pure monosaccharides. A self-consistent mechanism was proposed for Butlerov reaction initiation. The formation of the enediol forms of monosaccharides followed by degradation to lower carbohydrates plays a key role in this mechanism. In turn, the initiating activity depends on the position of the carbonyl group in the monosaccharide molecule. The condensation reactions of glycolaldehyde, glyceraldehyde, and dihydroxyacetone with each other were studied. Based on data on the condensation products of lower carbohydrates, a scheme was proposed for the Butlerov reaction. According to this reaction scheme, C₂ and C₃ carbohydrates mainly undergo an aldol condensation reaction with formaldehyde, whereas the formation of higher monosaccharides occurs by the aldol condensation of lower C₂–C₃ carbohydrates with each other.     

硫化物在铂电极上电氧化时空动力学的模型模拟和分析(英文)

根据铂电极上硫化物电催化氧化的反应机理,本文提取动力学模型并利用数值模拟研究了N型负微分阻抗(N-NDR)振荡区域的电极表面时空反应动力学.在均相体系模拟中观察到电流简单振荡和复杂振荡,其来源于双电层电势自催化与传质限制和毒化物种吸附负反馈的相互耦合.为了更接近于真实体系,在模型中考虑了平行和垂直于电极表面两个方向的传质过程.模拟结果发现了与实验现象具有相同演化行为的复杂斑图,如行波和闪烁波;同时在传质耦合体系模拟中观察到双电层电势双臂螺旋波.本研究工作促进对电化学体系时空斑图的理解和预测.     

Spatial desynchronization of glycolytic waves as revealed by Karhunen-Loeve analysis

The dynamics of glycolytic waves in a yeast extract have been investigated in an open spatial reactor. At low protein contents in the extract, we find a transition from inwardly moving target patterns at the beginning of the experiment to outwardly moving spiral- or circular-shaped waves at later stages. These two phases are separated by a transition phase of more complex spatiotemporal dynamics. We have analyzed the pattern dynamics in these three intervals at different spatial scales by means of a Karhunen-Loeve (KL) decomposition. During the initial phase of the experiment, the observed patterns are sufficiently described by the two dominant KL modes independently of the spatial scale. However, during the last stage of the experiment, at least 6 KL modes are needed to account for the observed patterns at spatial scales larger than 3 mm, while for smaller scales, 2 KL modes are still sufficient. This indicates that in the course of the experiment, the local glycolytic oscillators become desynchronized at spatial scales larger than 3 mm. Possible reasons for the desynchronization of the glycolytic waves are discussed.