Mass transfer with a chemical reaction of the first order. Effect of axial diffusion

The role of diffusion in the direction of convection transport, which is usually neglected in mass (heat) transfer problems, is systematically investigated. Analytical solutions and asymptotic expressions are derived for cases of mass transfer coupled with a homogeneous and heterogeneous irreversible chemical reaction of the first order in plug flow, in Couette flow and in Couette flow with a moving interface. Irrespective of the flow situation, the effect of axial diffusion is of importance for small transformed distances in the direction of the convective transport. Then, the local mass transfer co-efficient is a square of the coefficient which is evaluated without the axial diffusion term. As expected, with an increase in the transformed distance both mass transfer coefficients tend to the same value. In the case of heterogeneous (surface) chemical reactions of the first order, the diffusion in the direction of convective transport is of minor importance. The same is observed when the case of simultaneous homogeneous and heterogeneous chemical reactions of the first order in uniform flow is considered.     

Stochastic Simulation of Imperfect Mixing in Free Radical Polymerization

A modified algorithm for the stochastic simulation of chemical reactions subject to mass transfer limitation (imperfect mixing) is presented. This algorithm takes into account the mixing by diffusion of the reacting species between two consecutive reactions. The method is used to simulate the effect of mass transfer limitation in free-radical polymerization. Since this is a stiff reaction network, a hybrid stochastic-deterministic approach is considered. The hybrid stochastic algorithm under imperfect mixing (HSSA-IM) is applied to the bulk polymerization of methyl methacrylate up to high conversions. The accuracy of the algorithm relies on the precise determination of diffusion coefficients during the reaction.     

Importance of trimethylaluminum diffusion in three-step ABC molecular layer deposition using trimethylaluminum, ethanolamine, and maleic anhydride

Hybrid organic-inorganic films were grown by molecular layer deposition (MLD) with a three-step ABC reaction sequence using (A) trimethylaluminum (TMA), (B) ethanolamine (EA), and (C) maleic anhydride (MA) at 90 °C. Very large steady state mass gains of 1854-4220 ng/(cm(2) cycle) were measured depending on reaction conditions. These mass gains are much larger than typical mass gains for surface reactions. The quartz crystal microbalance (QCM) mass profiles during the TMA reaction were consistent with TMA diffusion into and out of the ABC films. The ABC mass gains per cycle also displayed a strong dependence on the TMA dose and purge times that was consistent with the effects of TMA diffusion. Multiple dose experiments conducted at 130 °C revealed that the ABC reactions were self-limiting for thin ABC films. For thicker ABC films, increased TMA diffusion into the ABC film led to non-self-limiting behavior. Numerical modeling assuming Fickian diffusion for TMA diffusing into and out of the ABC film could fit the QCM mass profiles. The results all indicate that TMA diffusion into the ABC MLD film plays a key role in the thin film growth. In addition, X-ray reflectivity (XRR) measurements revealed that the ABC films were exceptionally smooth.     

Understanding supported reactions in spherical compartments: a general algorithm to model and determine rate constants, diffusion coefficients, and spatial product distributions

A general algorithm allowing the numerical modeling of the time and space dependence of product formation in spherical reaction volumes is described. The algorithm is described by the complete set of mass balance equations. On the basis of these equations, the effects of the diffusion coefficient, reaction rate, bead size, reagent excess, and packing density of the resin beads on the overall reaction rates are determined for second-order reactions. Experimental data of reaction progress are employed to calculate reaction rates and diffusion coefficients in polymer-supported reactions. In addition, the conditions for shell-like product formation are determined, and various strategies for the radial patterning of resin beads are compared. The effect of diffusion on polymer-supported enzyme-catalyzed reactions of the Michaelis-Menten type is treated, as well. Finally, the effects of typical nonideal solid-phase phenomena, namely, the inhomogeneity of rate constants and the concentration dependence of diffusion coefficients, on overall rates are discussed.     

The behaviour of an electrochemical detector used in liquid chromatography and continuous flow voltammetry: Part III. Potential dependent current

The potential dependence of the steady-state current at a channel electrode has been calculated approximately. Six regions of behavior can be described from a consideration of the characteristic times of diffusion, fluid flow and electrochemical reaction. Wave shapes and half-wave potential shifts are as expected for amperometric cells under all values of the heterogeneous reaction rate constant, and for reversible electrochemical reactions in a coulometric cell. The wave adopts a peculiar shape for irreversible reactions at a coulometric electrode. This is caused by the inapplicability of the concept of mass transfer control in these systems.     

The investigation of substituent effects on the fragmentation pathways of pentacoordinated phenoxyspirophosphoranes by ESI‐MSn

The fragmentation pathways of pentacoordinated phenoxyspirophosphoranes were investigated in the positive mode by electrospray ionization multistage mass spectrometry. The results demonstrate that the sodium adducts of the title compounds undergo two competitive fragmentation pathways, and the fragmentation patterns are heavily dependent on the various substituent patterns at the phenolic group. An electron‐withdrawing substituent at the ortho‐position always results in the removal of a corresponding phenol analogue, while cleavage by spiroring opening becomes the predominant fragmentation pathway if an electron‐donating substituent is at the phenolic group. The substituent effects on the competitive fragmentation pathways were further elucidated by theoretical calculations, single crystal structure analysis, and high‐resolution mass spectrometry. The results contribute to the understanding of the gas‐phase fragmentation reactions and the structure identification of spirophosphorane analogues by electrospray ionization multistage mass spectrometry.     

Synthesis of multi-unit protein hetero-complexes in the gas phase via ion-ion chemistry

The synthesis of protein hetero-complex ions via ion-ion reactions in the gas phase is demonstrated in a quadrupole ion trap. Bovine cytochrome c cations and bovine ubiquitin anions are used as reactant species in the stepwise construction of complexes containing as many as six protein sub-units. For any set of reactants, a series of competitive and consecutive reactions is possible. The yield of complex ions for any given sequence of reactions is primarily limited by the presence of competitive reactions. Proton transfer represents the most important competitive reaction that adversely affects protein complex synthesis. In the present data, proton transfer takes place most extensively in the first step of complex synthesis, when single protein sub-units are subjected to reaction with one another. Proton transfer is found to be less extensive when one of the reactants is a protein complex. The generation of hexameric hetero-complexes containing two cytochrome c molecules and four ubiquitin molecules is demonstrated with two different synthesis approaches. The first involved the initial reaction of several charge states of cytochrome c and several charges states of ubiquitin. The sequence of reactions in this example illustrates the array of possible competitive and consecutive reactions associated with even a relatively simple set of multiply charged reactants. The second approach involved the initial reaction of the 9(+) charge state of cytochrome c and the 5(-) charge state of ubiquitin. The latter approach highlights the utility of the multi-stage mass spectrometric (MS(n)) capabilities of the ion trap in defining reactant ion identities (i.e. charge states and polarities) so that synthesis reactions can be directed along a particular set of pathways.     

活性氯在316L不锈钢电极表面的阴极行为研究

利用电化学方法研究了活性氯在3.5%NaCl溶液中316L不锈钢电极表面的阴极还原反应及其反应机制.实验表明,随着溶液中活性氯浓度的增加,316L不锈钢的自腐蚀电位正移,阴极反应的极限扩散电流明显增大,说明活性氯对316L不锈钢电极表面的两个阴极反应,即HClO还原和ClO-还原具有明显的促进作用.本文的研究确定了产生这两个还原反应的电位范围,并进一步探索了该还原反应速度的控制步骤.     

The formation of dead zones in nonisothermal porous catalyst with temperature-dependent diffusion coefficient

This paper considers the formation of dead zones in the porous catalyst pellets due to the chemical reaction and diffusion. We established and investigated the model with nonisothermal reaction of fractional order and activated temperature-dependent diffusivity. The effects of process parameters, catalyst shape, and reaction and diffusion parameters on the formation of the dead zone are studied numerically and characterized by the critical Thiele modulus. The lower bounds for the critical Thiele modulus are derived analytically in terms of process parameters for exothermic and endothermic reactions and verified numerically. The critical Thiele modulus increases with increasing Arrhenius number for diffusion and decreasing Arrhenius number for reaction in the case of exothermic reactions, whereas the opposite trends hold for the endothermic reactions. The critical Thiele modulus also increases with increasing fractional reaction order as well as with decreasing energy generation function, and increasing Biot numbers for heat and mass transfer. Moreover, the critical Thiele modulus is the highest for spherical pellets and the lowest for pellets with planar shape.     

Simulation of the kinetics of complex heterogeneous catalytic reactions under diffusion limitations

A method is suggested for determining kinetic parameters of arbitrary complex heterogeneous catalytic reactions under conditions such that the reaction rate is significantly affected by heat and mass transfer to and in the catalyst pellets. Linear relationships are established between the concentrations of the key and dependent reactants (diffusion stoichiometry relationships) and between temperature and the concentrations of the key components. A solution procedure is suggested for the problems involved in the determination of kinetic parameters for reactions proceeding under diffusion limitations. The procedure is illustrated by analysis of data earlier reported for steam methane reforming over a nickel catalyst.     

A distributed parameter diffusion-reaction model for the alcoholic fermentation process

A distributed parameter model is developed for the yeast floc in the alcoholic fermentation process. The model takes into consideration the external mass transfer resistances, the mass transfer resistance through the cellular membrane, and the diffusion resistances inside the floc. The two-point boundary value differential equations for the membrane are manipulated analytically, whereas the nonlinear twopoint boundary value differential equations of diffusion and reaction inside the floc have been approximated using the orthogonal collocation technique. The evaluation of the necessary diffusion coefficients have involved a relatively large number of assumptions because of the present limited knowledge regarding the complex process of diffusion and biochemical reactions in these systems.     

乙酰苯胺和对氨基苯乙酮单分子离子分解机理

从实验和理论两方面对乙酰苯胶和对氨基苯乙酮单分子离子分解反应机理进行研究．在实验方面，采用质谱仪器的磁场／静电场联动扫描检测了两个化合物的亚稳峰，得到化合物在质谱中的分解途径．在理论方面，根据分子轨道理论，用3－21基组从头计算研究分解反应中的能量变化．获得了分解产物的总能量与前体离子的总能量之差△E，按照产物的稳定性和能量有利的原则，在竞争的分解反应中，△E最小的分解反应占优势．计算的结果与质谱实际分析的结果一致．     

Influence of the mass and heating rate in the kinetic parameters in solid=solid+gas reactions

A study about the influence of the mass and heating rate on the kinetic parameters in solid = solid +gas reactions shows that the low dispersion of the values for activation energy and order parameter support the idea that the use of a small mass is consistent with the theoretical suppositions. This should be taken into account in the diffusion phenomena and temperature gradient involved in the experimental process.     

Rapidly measuring reactivities of carboxylic acids to generate equireactive building block mixtures: a spectrometric assay

The relative reactivity of building blocks is critical for a successful preparation of combinatorial libraries. Here, we present a method for measuring the reactivity of carboxylic acid building blocks in amide-forming reactions. The method involves competitive reactions between a reference and test acid and a tetraphenylporphyrin reaction partner with four reactive sites. Relative reactivities are calculated on the basis of the distribution of substituted porphyrins found in MALDI-TOF mass spectra. Reactivities thus determined were used to prepare reactivity-adjusted building block mixtures. These were reacted with amino-terminal oligonucleotide and peptide scaffolds on solid support, generating small libraries suitable for spectrometrically monitored selection experiments (SMOSE). The rate of building block "drop outs" that fail to couple as expected was not substantially lowered by acquiring spectra from two reactions, performed with different ratios of building blocks, where the effect of a given substituent on the desorption/ionization yield of the porphyrin can be eliminated. Instead, coupling building blocks of similar size together or employing N-hydroxysuccinimide esters rather than activating with a "uronium salt" were found to improve the quality of libraries generated via competitive reactions.     

Application of the general rate model and the generalized Maxwell-Stefan equation to the study of the mass transfer kinetics of a pair of enantiomers

The general rate model of chromatography can be coupled with the generalized Maxwell-Stefan equation that describes the surface diffusion flux. The resulting model is useful to describe the behavior of two enantiomers during their separation on chiral phases, cases in which the mass transfer kinetics is known to be sluggish. A case in point is the modeling of the elution profiles of the racemic mixture of the two enantiomers of 1-phenyl-1-propanol on cellulose tribenzoate coated on silica, a popular chiral stationary phase. The competitive equilibrium isotherm behavior of the two enantiomers on the chiral stationary phase was described using the competitive Tóth isotherm model. An excellent agreement between the experimental and the calculated profiles was observed in the whole range of experimental conditions investigated, at low and high column loadings.     

PHOTOBINDING OF PSORALEN AND 8-METHOXYPSORALEN TO CALF THYMUS DNA

Abstract— The photobinding of psoralen and 8-methoxypsoralen to calf thymus DNA induced by 365 nm radiation has been measured for different concentrations of the furocoumarin and nucleotides. The results are consistent with the assumption that dark complexing of the furocoumarin to DNA is a pre-condition for photobinding, but do not exclude the possibility that the free furocoumarin participates in the reactions. The analysis with 'large target' diffusion theory indicates that photobinding should be inefficient for the free excited singlet state and competitive with reactions of the dark-complexed sensitizer for the free triplet state. The analysis indicates also that the diffusive reactions of singlet oxygen generated by the free furocoumarin can compete with photoadduct formation.     

Voltammetric and electrogravimetric study of manganese dioxide thin film electrodes. Part 1. Electrodeposited films

Voltammetry and electrochemical quartz crystal microbalance (EQCM) studies of MnO₂ films electrodeposited in acidic media at 85°C show the existence of matter exchange during repeated electrochemical reduction and oxidation in 1 M KOH solution. Usually, these reactions are written without mentioning the exchange of matter. The following are observed: a reversible exchange involving water between the crystal lattice and the solution within each successive sweep potential cycle; an irreversible mass loss caused by low dissolution and slow diffusion of Mn(III) and Mn(II) solubilized species; an irreversible large mass gain during the first cycle due to additional hydration of the material.     

Dynamics of the separation of amino acid and mineral salt in the stationary dialysis of solutions with an MK-40 profiled sulfo group cation exchange membrane

The conjugated diffusion transport of amino acid and mineral salt through a profiled sulfo group cation exchange membrane that simulates the extraction of amino acid from wash waters of microbiological production containing mineral components not used in synthesis is studied. The competitive nature of the conjugation of flows resulting in a decrease in the rate of the mass transfer of components and their separation factor is established from a comparative analysis of experimental data on the diffusion transfer of phenylalanine and sodium chloride in the form of hydrogen from individual and mixed solutions through a profiled sulfo group cation exchange membrane. The range of concentrations and the ratio of components in solution corresponding to the effective separation of phenylalanine and sodium chloride are determined.     

Triphase catalysis. I. Kinetics and mechanism of alkylation of phenylacetonitrile with alkyl halides catalyzed by aqueous naoh and polymer-supported benzyltriethylammonium chloride

The alkylation and cycloalkylation of phenylacetonitrile (PAN) with excess of 1-bromobutane and 1,4-dibromobutane respectively catalyzed by aqueous NaOH and insoluble polystyrene-bound benzyltriethylammonium chloride were studied and the rates depend upon several experimental parameters. The rate of cycloalkylation of PAN is five times faster than the alkylation and the former proceeds even in the absence of the solid catalyst. Both alkylation and cycloalkylation gave higher rates in reverse addition method than in direct addition. Deprotonation of PAN by hydroxide ion takes place without participation of the polymeric catalyst. The electrostatic force between the positive charge on the nitrogen atom of catalyst and carbanion is responsible for alkylation of active nitrile with polystyrene-bound phase-transfer catalyst. The rates of both the reactions increase with increased stirring speed, ring substitution, hydroxide ion concentration and catalyst amount and decrease with increased particle size and the degree of cross-linking of the polymer. Apparent activation energies for the reactions were calculated. The kinetic results are discussed in terms of mass transfer and a combination of intraparticle diffusion and intrinsic reactivity limitations of the rates.