Two-step Kinetic Model Research Articles

Response surface optimized extraction of 1-deoxynojirimycin from mulberry leaves (Morus alba L.) and preparative separation with resins.

In the present study, the extraction technology and preparative separation of 1-deoxynojirimycin from mulberry leaves were systematically investigated. Four extraction parameters (ethanol concentration, extraction temperature, extraction time and ratio of solvent to sample) were explored by response surface methodology (RSM). The results indicated that the maximal yield of 1-deoxynojirimycin was achieved with an ethanol concentration of 55%, extraction temperature of 80 °C, extraction time of 1.2 h and ratio of solvent to sample of 12:1. The extraction yield under these optimum conditions was found to be 256 mg/100 g dry mulberry leaves. A column packed with a selected resin was used to perform dynamic adsorption and desorption tests to optimize the separation process. The results show that the preparative separation of 1-deoxynojirimycin from mulberry leaves can be easily and effectively done by adopting 732 resin. In conclusion, 732 resin is the most appropriate for the separation of 1-deoxynojirimycin from other components in mulberry leaves extracts, and its adsorption behavior can be described with Langmuir isotherms and a two-step adsorption kinetics model. The recovery and purity of 1-deoxynojirimycin in the final product were 90.51% and 15.3%, respectively.

Femtosecond Infrared Transient Absorption Dynamics of Benzimidazole-Based Ruthenium Complexes on TiO2 Films for Dye-Sensitized Solar Cells

By means of femtosecond infrared transient absorption spectra, we measured the interfacial electron-transfer dynamics for benzimidazole-based heteroleptic ruthenium dyes (RD5, RD12, RD15−RD18) sensitized on TiO2 thin films. For all measurements, the first singlet metal-to-ligand charge-transfer states ( 1 MLCT) of the ruthenium complexes were excited at 519 nm and the injected electrons in the conduction band of TiO2 were probed at 4.3 μm. All transient signals featured two rising components on a femtosecond−picosecond scale due to a two-step electron injection and an offset (N719, RD16−RD18) or a slow-decay (RD5, RD12, and RD15) component on a nanosecond−microsecond scale due to a back electron transfer. A complicated two-step kinetic model was derived analytically to interpret the observed two rising components for which the rapid (τ1 < 300 fs) and slow (τ2 =1 0−20 ps) electron injections arose from the singlet 1 MLCT and triplet 3 MLCT states, respectively. The amplitudes of the two electron-injection components (A1 and A2) were controlled by the rate coefficient of the 1 MLCT → 3 MLCT intersystem crossing; the variations of A1 and A2 are consistent with the trend of the corresponding Stokes shifts rationalized with a conventional energy-gap law for nonradiative transitions. Compared with the kinetics observed for the N719 dye, the involvement of a benzimidazole ligand in RD dyes had the effect of accelerating the two electron injections, thus improving the short circuit current of the device. RD dyes substituted with fluorine atoms and/or thiophene units in the benzimidazole ligands showed a retardation of 3 MLCT electron injection relative to that of the nonsubstituted RD5 dye. Acceleration of the BET process was observed for the RD5 dye (9 ns), and both fluoro-substituted dyes (14 ns for RD12 and 21 ns for RD15) and thiophene-substituted dyes (nonobservable for RD16−RD18) had significantly retarded BET kinetics. The observed kinetics of the 3 MLCT electron injection for all RD dyes is satisfactorily simulated with the Marcus theory.

Mechanistic Aspects of Multistep Bioelectrocatalytic Oxidation of Fuels

Quantitative models were developed for dihydroxyacetone (DHA) production from glycerol oxidation coupled with electrocatalysis. A two-step kinetic model was developed to describe the reactions in bioelectrocatalysis involving electrochemical NADH regeneration. Planar and porous bioelectronic interfaces are modeled to simulate, predict and evaluate kinetics and diffusion in the bioconversion (Figure 1). Key parameters involve diffusivity of cofactor/substrate, electrocatalytic activity and enzyme kinetics. Model-based analysis indicates that the bioreactor is controlled by glycerol dehydrogenase kinetics for enzyme loadings lower than 1 mM and shifts to mass transport and incipient NADH oxidation kinetic control at higher enzyme loadings.

Comparative study on the pyrolysis behaviors of corn stalk and pine sawdust using TG-MS

The pyrolysis behaviors of corn stalk (CS) and pine sawdust (PS) were investigated with thermogravimetry-mass spectroscopy (TG-MS). The peak temperature of PS was higher and the main decomposition region shifted to higher temperature compared with CS, which implied that the hemicellulose and cellulose of PS were more thermally stable than those of CS. However, the hemicellulose and cellulose of PS were more easily decomposed into gaseous products than those of CS during pyrolysis. The pyrolysis process of biomass can be described by a two-step independent first-order kinetic model. This fundamental study provides a basic insight into the biomass pyrolysis, which is beneficial for understanding the pyrolysis mechanism of biomass and developing an advanced thermal process for effective utilization of biomass.

Determining the kinetics of sunflower hulls using dilute acid pretreatment in the production of xylose and furfural

Pretreatment of sunflower hulls was conducted under varied dilute acid concentrations (0.5–2.0 wt%), reaction temperatures ranging between 140°C and 160°C and the reaction time up to 30 min. The conversion of xylan into xylose and furfural was investigated. The maximum xylose and furfural recoveries, 54.5±0.7 and 24.0±1.1 wt%, respectively, were obtained at different reaction times with 2.0 wt% acid concentration at 160°C. The experimental data were fitted into a two-step kinetic model based on irreversible pseudo-first-order kinetics at each step. The model was successfully validated using the F-test. Sunflower hulls showed a greater recalcitrance to acid pretreatment than other agricultural crops, such as kenaf, sorghum and sunn hemp. This feature was ascribed to the occurrence of a wax layer on the cell wall surface with a high lignin content, which may act as a barrier hindering the acid access to acetyl linkages in xylan.

A comparative PM-IRRAS and ellipsometry study of the adsorptive behaviour of bovine serum albumin on a gold surface

Polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) and ellipsometry were used ex situ to investigate adsorption of bovine serum albumin (BSA) on a gold surface, in terms of the adsorption equilibrium and kinetics. The aim of the work was to examine if the two different techniques give similar/complementary results under the same experimental conditions employing the same protein/surface system, and thus validate the use of the techniques for the investigation of protein/surface interactions under the applied experimental conditions, in general. It was found that the adsorption of BSA on gold follows Type I isotherm, which can be described by the Freundlich isotherm. The initial BSA adsorption kinetics was found to be very fast, and the results were modelled using a two-step kinetic model. The first step represents reversible BSA adsorption that yields BSA adsorbed in a native configuration (θ1) that is not thermodynamically stable. The second step represents the irreversible transformation of this protein configuration into a thermodynamically stable surface-adsorbed configuration (θ2). It was found that the θ2/θ1 ratio increased with time. Finally, the comparison of the results obtained by the two techniques showed a very good agreement.

Thermodynamics and kinetics of NAD+ adsorption on a glassy carbon electrode

Thermodynamics and kinetics of nicotinamide adenine dinucleotide (NAD+) adsorption on a glassy carbon (GC) electrode surface was investigated at various electrode potentials and NAD+ concentrations using differential capacitance (DC) and attenuated total reflection Fourier transform infrared (ATR-FTIR) techniques. Equilibrium adsorption measurements confirmed that NAD+ spontaneously and strongly adsorbs on the GC electrode surface. The affinity of NAD+ towards adsorption on the GC electrode surface was found to increase with an increase in electrode potential (charge) to more positive values; the corresponding apparent Gibbs free energy of adsorption was −32.80 ± 0.25, −35.61 ± 0.86, and −38.02 ± 0.40 kJ mol−1 on negatively, neutral, and positively charged electrode surfaces, respectively. The kinetics of NAD+ adsorption is also found to be highly dependent on the electrode surface potential (charge), and it increases with an increase in electrode potential (charge) to positive values. The adsorption process was modeled using a two-step kinetic model, in which the adsorption process involves the formation of two forms of NAD+ on the surface: the thermodynamically unstable (NAD+ads,rev) and stable (NAD+ads,stable) forms. ATR-FTIR further confirmed that NAD+, indeed, adsorbed on the GC electrode surface.

Experimental and numerical investigations of an atmospheric diffusion oxy-combustion flame in a gas turbine model combustor

An atmospheric diffusion oxy-combustion flame in a gas turbine model combustor has been investigated experimentally and numerically. Oxy-combustion and emission characterization, flame stabilization and oxy-combustion model validation analyses are the main goals of the present research work. The combustor is fuelled with CH4 and a mixture of CO2 and O2 as oxidizer. A modified two-step oxy-combustion reaction kinetics model for methane–oxygen combustion has been used in order to predict accurately the oxy-combustion characteristics. The conducted experimental results were used to validate the numerical model. Wide ranges of different operating parameters have been considered including equivalence ratio, percentage of O2/CO2 in the oxidizer mixture and fuel volume flow rate. The stability of the oxy-combustion diffusion flame is also investigated both experimentally and numerically. The experimental and numerical results showed that the stability of the oxy-combustion flame is affected when the operating percentage of oxygen in the oxidizer mixture is reduced below 25%. In all cases, flame was extinct for conditions of less than 21% oxygen in the oxidizer mixture. Flame visualization over a wide range of operating parameters has been carried out experimentally and comparisons with the numerical results have been conducted. The flames have been characterized in detail by measuring the exhaust gas temperatures and emissions and comparing them with those from the numerical model. The combustion was found to be improved with increasing the percentage of O2 at inlet however there is a limitation in temperature. Both experimental and numerical results are in good agreement. The modified two step reaction kinetics model was found to be capable of capturing the trends of temperature and the overall flame shape of the experimental data. Flame zone is also characterized in details by plotting the axial and radial temperatures, species concentrations and flow velocities using the numerical model.

Numerical investigation of oxygen permeation and methane oxy-combustion in a stagnation flow ion transport membrane reactor

In this work, a two-step oxy-combustion reaction kinetics model for methane-oxygen combustion is used to predict the oxy-combustion characteristics in the permeate side of the membrane. More accurate permeation rate characteristics inside this simple symmetric design ITM reactor is also expected using this model. New oxygen permeation model is introduced in this work for an LSCF-1991 ion transport membrane. The simulation of the oxygen permeation process across the membrane has been performed through series of visual C++ user defined function compiled and incorporated to FLUENT. The analysis of the permeation process has been conducted for separation only process (no reactions) using an inert gas (argon) as a sweep gas and a comparison has been done with cases of using CH4 plus CO2 as sweep gases. The effect of reactivity using the same sweep gases (CH4 plus CO2) is investigated by comparing the same cases with and without reactions in the permeate side. It was found that there are important parameters affecting the operation of ITM reactors like the inlet gases temperature, percentage of CH4 in the sweep gases mixture and the reactor geometry. Also, there are less important parameters like, feed and sweep volume flow rates, oxygen partial pressure in the feed side.

Investigations of Oxy-Fuel Combustion Characteristics and Oxygen Permeation Process in a Stagnation Flow ITM Reactor

In this work, a modified two-step reaction kinetics model for methane-oxygen combustion is used to predict oxy-combustion characteristics and permeation rate characteristics inside a stagnation flow simple symmetric ITM reactor. New coefficients oxygen permeation equation model is introduced herein this work by fitting the experimental data available in the literature for a LSCF-1991 ion transport membrane. Using CH4 as fuel plus CO2 as sweep gas, the effects of reactivity is analyzed using the same sweep gases (CH4 plus CO2) is investigated here by comparing the same cases with and without considering reactions in the permeate side. Also the influences of the percentage of CH4 in the sweep gases mixture on the permeation and combustion processes are included in this work. It was found that the oxygen permeation flux increases with activating chemical reactions in the permeate side of the membrane and this is due to increased partial pressure driving force across the membrane surface as a result of disappearance of oxygen molecules in the permeate side because of combustion. Another reason for this is the increase in the membrane temperature which affects the activation energy of the membrane and so the permeation rate. More details about the models used and the oxy-fuel combustion characteristics in the permeate side of the membrane are included in the present study.

Investigations of oxy-fuel combustion and oxygen permeation in an ITM reactor using a two-step oxy-combustion reaction kinetics model

In this work, a modified two-step oxy-combustion reaction kinetics model for methane–oxygen combustion is used to predict the oxy-combustion characteristics inside an ITM reactor. The membrane reactor has a symmetric design allowing the reduction of the number of coordinates to 2D without reducing the accuracy of the calculations. A detailed study is presented in order to understand the performance of an ITM reactor under the oxy-combustion conditions in the permeate side of the membrane using CH4 as a fuel and CO2 as sweep gas. Coefficients of the oxygen permeation equation used in this work have been calculated for a LSCF 1991 ion transport membrane by fitting of the experimental data in the literature. It was found that parameters such as the inlet gases temperature (feed and sweep), percentage of CH4 in the sweep gas mixture and the reactor geometry can have great effects on the operation of ITM reactors. In contrast, there are some other parameters that are less important such as feed and sweep gas volume flow rates and oxygen partial pressure in the feed side. The effect of reactivity is investigated through the comparison of cases that include and exclude reactivity. It was found that activation of the chemical reactions in the permeate side of the membrane results in increase in the oxygen permeation flux. This was attributed to the increase in the partial pressure driving force across the membrane surface as a result of depletion of oxygen molecules in the permeate side as a result of combustion. It was attributed to the rise of the membrane surface temperature which leads to reduction of the surface resistance of the membrane to oxygen permeation.

Phosphate sorption from seawater solutions: Particle concentration effect

Phosphate sorption experiments were performed with beach sand artificially coated with iron(III) (hydr)oxides and with a natural intertidal sandy sediment. Initial fast sorption was followed by a much slower exchange of phosphate between sorbent and seawater solution. A two-step kinetic model was developed to reproduce the time series sorption data. Because in many cases equilibrium was not reached at the end of the two-week experiments, the model was extrapolated to a nominal duration of one year to construct (near-) equilibrium isotherms. The results for both the iron(III) coated sand and the intertidal sediment showed a clear particle concentration effect, with higher sorption densities and steeper sorption isotherms at lower solid–solution ratios. Thus, while the commonly used Langmuir and Freundlich isotherms successfully reproduced individual experimental data series, the corresponding parameter values were dependent on the particle concentration. The particle concentration effect was accounted for by modifying the Freundlich isotherm according to the Metastable Equilibrium Adsorption (MEA) theory of Pan and Liss (1998a). The MEA-modified isotherm accounted well for the mixed suspensions of iron(III) coated sand, however, over-predicted phosphate sorption to the iron(III)-coated sand obtained under stagnant conditions. Although a mechanistic interpretation of the sorption process cannot be inferred from a fit to the MEA-modified Freundlich isotherm, the resulting parameter values facilitate the comparison of sorption studies carried out at variable solid–solution ratios.

Desorption and photodegradation of methylene blue from modified sugarcane bagasse surface by acid TiO2 hydrosol

Waste sugarcane bagasse (SCB) was modified by pyromellitic dianhydride to improve its adsorption capacity for cationic dyes. Results showed that the adsorption capacity of the modified SCB for methylene blue was 564mgg−1, which was about 12 times than that obtained on the unmodified SCB. Methylene blue loaded modified SCB was regenerated by a self-clean eluent: TiO2 hydrosol with pH ranged from 1 to 4, and HNO3 solution with the same pH range was tested at the same time for comparison. Results showed that desorption kinetics of methylene blue in the hydrosol systems fit two-step kinetic model and controlled mainly by the slow step. As a self-clean eluent, acid hydrosol could firstly desorb and then photodegrade methylene blue under sunlight irradiation. After five desorption–photodegradation cycles, 78.3% of the absorbed dyes could be desorbed by using hydrosol (pH 2) as eluent. The hydrosol could be continuously used in desorption and photodegradation process, which would economize large volume of the eluent and moreover it would not bring secondary pollution.

9. Glucocorticoid receptor gene BclI polymorphism associates with immunomodulatory response to stress hormone in human PBMC

The functional consequences of signaling receptor endocytosis are determined by the endosomal sorting of receptors between degradation and recycling pathways. How receptors recycle efficiently, in a sequence-dependent manner that is distinct from bulk membrane recycling, is not known. Here, in live cells, we visualize the sorting of a prototypical sequence-dependent recycling receptor, the beta-2 adrenergic receptor, from bulk recycling proteins and the degrading delta-opioid receptor. Our results reveal a remarkable diversity in recycling routes at the level of individual endosomes, and indicate that sequence-dependent recycling is an active process mediated by distinct endosomal subdomains distinct from those mediating bulk recycling. We identify a specialized subset of tubular microdomains on endosomes, stabilized by a highly localized but dynamic actin machinery, that mediate this sorting, and provide evidence that these actin-stabilized domains provide the physical basis for a two-step kinetic and affinity-based model for protein sorting into the sequence-dependent recycling pathway.

Applicability of bi-pseudo component separated-stage model for decomposition of lignocellulosic materials in air at multiple heating rates

In our previous research (Liu et al., J Anal Appl Pyrol 63:303–325, 2002), the pseudo bi-component separated-stage model (PBSM) was suggested for the kinetic analysis on the decomposition of lignocellulosic materials in air at relatively lower heating rates. As a continuing work, this paper is intended to investigate the applicability of PBSM at different heating rates by experimental analyses. Decomposition of oil tea wood has been studied by means of non-isothermal thermogravimetric analysis in air atmosphere at 10–25 K min−1 heating rates. A two-step parallel reaction kinetic model is used to optimize the kinetic parameters of these materials in air. Meanwhile, an improved PBSM is developed to describe the thermal degradation process of oil tea wood. Furthermore, a comparison between the kinetic results of parallel model and PBSM reveals realistic applicability of PBSM. It is concluded that the PBSM has relatively high accuracy for the first decomposition step in the lower temperature range, while fails to predict the thermal decomposition behavior in the char oxidative process which occurs in the higher temperature range.

Sequence-Dependent Sorting of Recycling Proteins by Actin-Stabilized Endosomal Microdomains

The functional consequences of signaling receptor endocytosis are determined by the endosomal sorting of receptors between degradation and recycling pathways. How receptors recycle efficiently, in a sequence-dependent manner that is distinct from bulk membrane recycling, is not known. Here, in live cells, we visualize the sorting of a prototypical sequence-dependent recycling receptor, the beta-2 adrenergic receptor, from bulk recycling proteins and the degrading delta-opioid receptor. Our results reveal a remarkable diversity in recycling routes at the level of individual endosomes, and indicate that sequence-dependent recycling is an active process mediated by distinct endosomal subdomains distinct from those mediating bulk recycling. We identify a specialized subset of tubular microdomains on endosomes, stabilized by a highly localized but dynamic actin machinery, that mediate this sorting, and provide evidence that these actin-stabilized domains provide the physical basis for a two-step kinetic and affinity-based model for protein sorting into the sequence-dependent recycling pathway.

Desorption behavior of methylene blue on pyromellitic dianhydride modified biosorbent by a novel eluent: Acid TiO 2 hydrosol

In this study, waste beer yeast powder was modified by pyromellitic dianhydride to improve its adsorption capacities for cationic dye: methylene blue (MB). According to the Langmuir equation, the maximum uptake capacities ( q m) of the modified biomass for MB was 830.8 mg g −1, which was about five times than that obtained on the unmodified biomass. Adsorption mechanism was investigated by FTIR. Desorption kinetics of methylene blue in six solvents: HCl (0.1 mol L −1), ethanol, mixtures of HCl (0.1 mol L −1) and ethanol with different volume ratio and a self-clean eluent: acid TiO 2 were studied in details. Results showed that desorption kinetics curve fit the two-step kinetic model, and methylene blue release process was distinctly divided into two steps: rapid and slow desorption steps. 52.2% of the methylene blue could be desorbed into TiO 2 hydrosol after 30 h desorption at the first desorption cycle, and the desorbed dye in TiO 2 hydrosol could be degrade completely under sunlight irradiation. After three desorption-photodegradation cycles, 80.0% of the absorbed dyes could be desorbed from the surface of the modified biomass. Although there was much work to do, the self-clean eluent: TiO 2 hydrosol had great potential in practical use.

Approximate critical conditions in thermal explosion theory for a two-step kinetic model

Approximate critical conditions for a thermal explosion problem is developed for a two-step reactions based on theories of Semenov and Frank-Kamenetskii. The aim is to examine the contributions of the radical termination step and the temperature dependent pre-exponential factor on the critical parameters within the framework of classical stationary and non-stationary theories. In the non-stationary case, a more general expression for the critical Semenov parameter (Ψ cr ) and critical temperature (θ cr ) were obtained by asymptotic procedure. In the stationary case, numerical estimates for the critical Frank-Kamenetskii parameter (δ cr ) and the critical temperature (θ cr ) were obtained by variational method technique. It was observed that the Semenov and Frank-Kamenetskii parameters are greatly influenced by the termination step and the variable pre-exponential factor. Apart from elucidating hitherto unknown features in the theory of thermal explosion, the results are more general as some known results are easily recovered.

Self-organized generation of transverse waves in diverging cylindrical detonations

Self-organized generation of transverse waves associated with the transverse wave instabilities at a diverging cylindrical detonation front was numerically studied by solving two-dimensional Euler equations implemented with an improved two-step chemical kinetic model. After solution validation, four mechanisms of the transverse wave generation were identified from numerical simulations, and referred to as the concave front focusing, the kinked front evolution, the wrinkled front evolution and the transverse wave merging, respectively. The propagation of the cylindrical detonation is maintained by the growth of the transverse waves that match the rate of increase in surface area of the detonation front to asymptotically approach a constant average number of transverse waves per unit length along the circumference of the detonation front. This cell bifurcation phenomenon of cellular detonations is discussed in detail to gain better understanding on detonation physics.

Electrochemical investigations of the interaction of C-reactive protein (CRP) with a CRP antibody chemically immobilized on a gold surface

A possibility of using a range of dc and ac electrochemical techniques to probe associative interactions of C-reactive protein (CRP) with CRP antibody (aCRP) immobilized on a gold electrode surface was investigated. It was demonstrated that the investigated electrochemical techniques can be used efficiently to probe these interactions over a wide CRP concentration range, from 1.15 × 10 −5 to 1.15 mg L −1. The measured sensitivity of the techniques is in the following decreasing order: differential pulse voltammetry, charge-transfer resistance obtained from electrochemical impedance spectroscopy (EIS), cyclic voltammetry, chronoamperometry, and double-layer capacitance deduced from EIS measurements which gave the poorest sensitivity. Measurements of kinetic parameters demonstrated that the associative interactions of CRP with the immobilized aCRP reached quasi-equilibrium after 20–30 min. The kinetics of these interactions was modeled successfully using a two-step kinetic model. In this model, the first step represents reversible CRP–aCRP associative–dissociative interactions, while the second step represents the irreversible transformation of the bound CRP into a thermodynamically stable configuration. It was demonstrated that the thermodynamically stable configuration of CRP starts prevailing after 7 min of interaction of CRP with the immobilized aCRP.