Reactions In Heterogeneous Media Research Articles

Evaluation of Grafting Parameters of Styrene onto Alkaline Modified Sugarcane Bagasse using Fe2+/H2O2 and Fe2+/H2O2-N2H4 initiator systems and Characterization of the Hydrolyzed Grafted Polystyrene Chains

Cellulose the most abundant natural fibre can be modified, via grafting with vinyl monomers, in homogenous or heterogeneous reaction medium. Therefore, the objective of this paper was to study the grafting parameters of styrene onto alkaline modified sugarcane bagasse using Fe2+/H2O2 and Fe2+/H2O2- N2H4 initiator systems and characterized the hydrolyzed grafted polystyrene chains for their viscometry and average molecular weight distributions using standard appropriate methods. The grafting parameters increases within 0.10-0.25 M styrene concentration in both initiator systems and initiator concentration within the range 1.0x10-3 to 6.0x10-3 for Fe2+/H2O2- N2H4 and 1.0x10-3 to 9.0x10-3 for Fe2+/H2O2 systems, albeit with higher values in the former system. These parameters decrease at higher monomer and initiator concentrations in both systems. The viscometry of hydrolyzed polystyrene chain prepared at optimum initiator concentrations and 0.10-0.30 M styrene concentration in both initiator systems are in the range (1.6416-4.5327 cP), (0.6416-3.5327 cP) and (73.2277-202.1927 cP) for relative, specific and inherent viscosities respectively, consistent with increases in activation energy of flow (1.24-5.30 KJmol-1) and average viscosity molecular weights (307.92-565.79 gmol-1) , these parameters decreases at higher monomer concentration in both initiator systems. The number and weight-average molecular weight distributions and polydispersity indices indicates that grafted polystyrene chains in Fe2+/H2O2 system are broader and more polydispersed than those for Fe2+/H2O2- N2H4 initiator system. The results showed that the hydrazine enhanced initiator system was more effective for the copolymerization reaction with polystyrene polydispersity indices comparable to those of synthetic chain-growth polymers.

[formula omitted] -catalyzed trans-esterification of brassica carinata seed oil for biodiesel production

Brassica carinata seed is a non-edible oil containing crop grown for oil-based product development like biodiesel synthesis. However, recently technical challenges such as availability of feedstock, type of catalyst, cost, and quality of biodiesel hindered the feasibility and utilization of biodiesel. Thus, this study addressed those problems under the production of fatty acid methyl ester through trans-esterification reaction in the presence of heterogeneous catalyst (CaO), and methanol alcohol from Ethiopian brassica carinata seed oil. The synergetic and antagonistic effects of selected parameters (temperature, methanol to oil molar ratio, and amount of catalyst) on the yield of FAME were analyzed. Box-Behnken response surface methodology statistical analysis was applied to examine the parametric interaction effect, and optimization of reaction conditions. Accordingly, 90 % of fatty acid methyl ester (FAME) yield was achieved at the optimum value of 65 °C temperature, 14.85: 1 methanol to oil molar ratio, and 13.77 % catalyst load. The fuel properties of the resulted biodiesel were determined following standard procedures, and the results were within the standard limits (ASTM D6751). This implies that brassica carinata oil over heterogeneous catalyzed reaction medium under optimum reaction conditions provides higher biodiesel yield.

Integrated Biocatalytic Platform Based on Aqueous Biphasic Systems for the Sustainable Oligomerization of Rutin

Rutin is a known antioxidant compound that displays a broad range of biological activities and health-related benefits but presents a low water solubility that can be overcome by its polymerization. In this work, biocompatible aqueous biphasic systems composed of the ionic liquid cholinium dihydrogen phosphate ([CH][DHph]) and the polymer poly(ethylene glycol) 600 (PEG 600) were investigated as an efficient integrated reaction–separation platform for the laccase-catalyzed oligomerization of rutin. Two different approaches were studied to reuse laccase in several oligorutin production cycles, the main difference between them being the use of monophasic or biphasic regimes during the oligomerization reaction. The use of a biphasic regime in the second approach (heterogeneous reaction medium) allowed the successful reuse of the biocatalyst in three consecutive reaction–separation cycles while achieving noteworthy rutin oligomerization yields (95% in the first cycle, 91% in the second cycle, and 89% in the last cycle). These remarkable results were caused by the combination of the increased solubility of rutin in the PEG-rich phase together with the enhanced catalytic performance of laccase in the [Ch][DHph]-rich phase, alongside with the optimization of the pH of the reaction medium straightly linked to enzyme stability. Finally, a life-cycle assessment was performed to compare this integrated reaction–separation platform to three alternative processes, reinforcing its sustainability.

Probing Substrate/Catalyst Effects Using QSPR Analysis on Friedel-Crafts Acylation Reactions over Hierarchical BEA Zeolites.

Attempts to optimize heterogeneous catalysis often lack quantitative comparative analysis. The use of kinetic modelling leads to rate (k) and relative sorption equilibrium constants (K), which can be further rationalized using Quantitative Structure-Property Relationships (QSPR) based on Multiple Linear Regressions (MLR). Friedel-Crafts acylation using commercial and hierarchical BEA zeolites as heterogeneous catalysts, acetic anhydride as the acylating agent, and a set of seven substrates with different sizes and chemical functionalities were herein studied. Catalytic results were correlated with the physicochemical properties of substrates and catalysts. From this analysis, a robust set of equations was obtained allowing inferences about the dominant factors governing the processes. Not entirely surprising, the rate and sorption equilibrium constants were found to be explained in part by common factors but of opposite signs: higher and stronger adsorption forces increase reaction rates, but they also make the zeolite active sites less accessible to new reactant molecules. The most relevant parameters are related to the substrates’ molecular size, which can be associated with different reaction steps, namely accessibility to micropores, diffusion capacity, and polarizability of molecules. The relatively large set of substrates used here reinforces previous findings and brings further insights into the factors that hamper/speed up Friedel-Crafts reactions in heterogeneous media.

Structural Model of Two-Component Particulate Mixtures with Maximum Heterogeneous Contacts

Heterodiffusion processes or chemical reactions in heterogeneous media are initiated at contact points between dissimilar particles of the reagents. Therefore, the number of heterogeneous contacts in the starting mixture is assumed to determine the rate of these processes: in particular, the rate of mass transfer in self-propagating high-temperature synthesis. The conditions for obtaining structures with maximum heterogeneous contacts have not been clearly defined so far in the literature. The paper determines for the first time the optimal ratio of particle sizes for a two-component mixture. This ratio contributes to the starting mixture composition that involves the maximum number of dissimilar contacts at a given weight or volume content of the reagents. A model of two-component densely packed disordered biparticulate granular media is proposed. According to the model, the starting mixture is considered a random packing of spherical particles of two sizes. The particle size ratio has been established so that the number of heterogeneous contacts between the particles of different phases is the highest. The concept of an ideal reaction area, which is a biparticulate structure consisting of the maximum number of dissimilar particles, has been developed and the role of the starting particle size composition in the formation of such structure has been established. An algorithm is presented for calculating the characteristics of the starting mixture to promote the structure with ideal reaction areas.

Study on hybrid combustion of aero-suspensions of boron-aluminum powders in a quiescent reaction medium

The present research deals with a hybrid combustion of aluminum/boron dust particles in a heterogeneous quiescent reaction medium with spatially discrete heat sources. A developed thermal model is employed to estimate flame propagation speed in a reaction medium. The burning velocity and minimum ignition energy are studied parametrically as a function of dust concentration and particle diameter for different percentages of boron powder in a hybrid mixture of aluminum/boron dust cloud. The model shows that the addition of boron powder as a component of the mixture decreases the burning rate and causes a higher amount of minimum ignition energy needed for ignition, owing to the role of boron as a heat sink. Comparison of the simulation results with the available experimental data shows that the model captures the flame propagation speed as a function of particle concentration, except at very low concentrations.

Synthesis of waterborne polyurethane-urea dispersions with chain extension step in homogeneous and heterogeneous media

HypothesisThe possibility of tailoring the final properties of environmentally friendly waterborne polyurethane and polyurethane-urea dispersions and the films they produce makes them attractive for a wide range of applications. Both the reagents content and the synthesis route contribute to the observed final properties. ExperimentsA series of polyurethane-urea and polyurethane aqueous dispersions were synthesized using 1,2-ethanediamine and/or 1,4-butanediol as chain extenders. The diamine content was varied from 0 to 4.5wt%. Its addition was carried out either by the classical heterogeneous reaction medium (after phase inversion step), or else by the alternative homogeneous medium (prior to dispersion formation). Dispersions as well as films prepared from dispersions have been later extensively characterized. Findings1,2-Ethanediamine addition in heterogeneous medium leads to dispersions with high particle sizes and broad distributions whereas in homogeneous medium, lower particle sizes and narrow distributions were observed, thus leading to higher uniformity and cohesiveness among particles during film formation. Thereby, stress transfer is favored adding the diamine in a homogeneous medium; and thus the obtained films presented quite higher stress and modulus values. Furthermore, the higher uniformity of films tends to hinder water molecules transport through the film, resulting, in general, in a lower water absorption capacity.

Saponification reaction system: a detailed mass transfer coefficient determination

The saponification of an aromatic ester with an aqueous sodium hydroxide was studied within a heterogeneous reaction medium in order to determine the overall kinetics of the selected system. The extended thermo-kinetic model was developed compared to the previously used simple one. The reaction rate within a heterogeneous liquid-liquid system incorporates a chemical kinetics term as well as mass transfer between both phases. Chemical rate constant was obtained from experiments within a homogeneous medium, whilst the mass-transfer coefficient was determined separately. The measured thermal profiles were then the bases for determining the overall reaction-rate. This study presents the development of an extended kinetic model for considering mass transfer regarding the saponification of ethyl benzoate with sodium hydroxide within a heterogeneous reaction medium. The time-dependences are presented for the mass transfer coefficient and the interfacial areas at different heterogeneous stages and temperatures. The results indicated an important role of reliable kinetic model, as significant difference in k(L)a product was obtained with extended and simple approach.

Soluble non-cross-linked poly(norbornene) supports for peptide synthesis with minimal reagents.

Solid-phase peptide synthesis has been an attractive method for synthesizing peptides because it is quick and can be automated. The heterogeneous reaction medium in solid-phase peptide synthesis necessitates the use of large equivalents of reagents to drive the reactions to completion. Peptide synthesis using soluble, yet isolable, supports is an attractive alternative to solid-phase peptide synthesis. Reported herein is a soluble poly(norbornene)-derived support containing multiple attachment sites for high loading capacities and solubilizing oligoether/alkyl groups. The Ala-attached support has been used to synthesize tri- to octapeptides in 28 to 97% yields using only 1.2 equiv of amino acids and coupling reagents. The acyclic hexapeptide precursor to natural product segatalin A was synthesized in 41% yield on the support using one-eighth of the equivalents of coupling reagents compared to that in reported procedures. The support could be recovered in up to 98% yield after peptide synthesis, and the recovered support was utilized to synthesize tri- and tetrapeptides that contain amino acids other than Ala at the C-terminus in ca. 80% yields.

Unravelling the Impact of Obstacles in Diffusion and Kinetics of an Enzyme Catalysed Reaction

Lattice gas automata simulations of diffusion-limited reactions in heterogeneous media exhibit fractal-like kinetics, which is a generalised mass action kinetics with time-dependent rate constants.1,2,3 We develop a two dimensional lattice gas automata simulation of the Michaelis-Menten mechanism in diffusion-limited conditions to investigate the effect of density and size of obstacles on reactant diffusion and rate coefficients. In order to simulate more physicochemical realistic conditions, reactants rotate and interact according to their specific orientation. We also model weak interaction forces between reactants and obstacles. Our results show that obstacle density and size affect diffusion, first- and second- order rates. We also find that particle rotations and weak force interactions among particles lead to a significant decay in the fractal-like kinetic exponent h. These results suggest that the effects of fractal-like kinetics disappear under less restricted conditions than previously believed in lattice based simulations. [1] S. Schnell and T. E. Turner, Prog Biophys Mol Biol, 2004, 85, 235-260. [2] R. Kopelman, J Stat Phys, 1986, 42, 185-200. [3] R. Kopelman, Science, 1988, 241, 1620-1626.

Unravelling the impact of obstacles in diffusion and kinetics of an enzyme catalysed reaction

Lattice gas automata simulations of diffusion-limited reactions in heterogeneous media exhibit fractal-like kinetics, which is a generalised mass action kinetics with time-dependent rate constants. We develop a two dimensional lattice gas automata simulation of the Michaelis-Menten mechanism in diffusion-limited conditions to investigate the effect of density and size of obstacles on reactant diffusion and rate coefficients. In order to simulate more physicochemical realistic conditions, reactants rotate and interact according to their specific orientation. We also model weak interaction forces between reactants and obstacles. Our results show that obstacle density and size affect diffusion, first- and second-order rates. We also find that particle rotations and weak force interactions among particles lead to a significant decay in the fractal-like kinetic exponent h. These results suggest that the effects of fractal-like kinetics disappear under less restricted conditions than previously believed in lattice based simulations.

A 2D → 3D Polycatenated Metal–Organic Framework: Synthesis, Structure, Magnetic and Catalytic Study

AbstractA mixed‐ligand 3D metal–organic framework, [Ni(1,2‐cpd)(bpe)(H2O)]n (1) [1,2‐cpd = cis‐cyclopentane‐1,2‐dicarboxylate, bpe = 1,2‐di(4‐pyridyl)ethylene], has been constructed. Topological analysis revealed that the structure is a 3‐c uninodal 2D + 2D → 3D polycatenated net with the Schläfli symbol 63. The compound is catalytically active towards the epoxidation reaction in heterogeneous media. It catalyses almost all types of olefinic substrates with equal efficiency. After reaction it can be recovered quite easily and can be used for further reaction without any loss of activity for several cycles. Variable‐temperature magnetic susceptibility measurement reveals the presence of a weak antiferromagnetic interaction in compound 1.

Calcined Eggshell Meal: New solid support for the Knoevenagel reaction in heterogeneous media

The Knoevenagel condensation is easily carried out without a solvent at room temperature using CalcinedEggshell Meal (CEM). Taking into account environmental and economical considerations, the handling ofCEM used here presents many advantages such as easier separation and recovery from the reaction mixtureand thus, enhanced recycling possibilities, which are now well established in fine organic synthesis.

Анализ химического состава продуктов разложения российского вещества VX в условиях каталитического метанолиза

Analytical platform for the testing new decontaminating formulations, involving identification of toxic destruction products and determination of the depth of decontamination under conditions mimicking real decontamination scenarios, was developed. The possibility of application of catalytic methanolysis for destruction of a highly toxic compound О -isobutyl S -[2-( N,N -diethylamino)ethyl] methylphosphonothioate referred to Russian VX (hereinafter, RVX) in homogeneous and heterogeneous reaction media are studied. Conditions for extraction of residual RVX from the reaction mixture, as well as GC−MS analysis of RVX were found and optimized. Destruction products of RVX, formed by its La-catalyzed methanolysis were identified. Efficiency of the catalytic formulation for decontamination of RVX-contaminated plastic surfaces was studied. The obtained results provide evidence for a high efficiency of the catalytic methanolysis for chemical neutralization of RVX and allow this process to be recommended for decontamination of engineering equipment. Key words: toxic organophosphorus compounds, VX-group compounds, catalytic methanolysis, gas chromatography−mass spectrometry, decontamination (Russian) L.K. Gustyleva, E.I. Savel’eva, N.S. Khlebnikova, A.S. Radilov Research Institute of Hygiene, Occupational Pathology and Human Ecology Federal State Unitary Enterprise Federal Medical Biological Agency, p/o Kus’molovsky, Leningrad Region, Russian Federation

Alkaline Hydrolysis of an Aromatic Ester: Kinetic Studies Using Thermal Power Profiles

AbstractThe reaction kinetics of ethyl benzoate saponification with sodium hydroxide in a heterogeneous reaction medium was investigated. The rate of reaction in a heterogeneous liquid‐liquid system incorporates a chemical kinetics term as well as the mass transfer between both phases. The reaction was carried out in a reaction calorimeter and the overall reaction rate was determined from the obtained thermal profiles. The mass transfer parameters were calculated numerically, taking into account those reaction rate constants determined in our previous study, where the above‐mentioned reaction was conducted in a homogeneous reaction medium. The dependence of the mass transfer parameters and reaction rates on different heterogeneity stages and temperatures is described.

Synthesis and Characterization of Boron Trifluoride Doped High Performance Polyaniline

We report simple synthesis of boron trifluoride (BF3) doped defect free high performance polyaniline (HPPANI) in two step method. Firstly, HPPANI was prepared via self‐stabilization dispersion polymerization method in a heterogeneous reaction medium. Second step involves doping of emeraldine base form of HPPANI with boron trifluoride under reduced vacuum. The resultants BF3 doped HPPANI have been well characterized by using UV‐Visible spectroscopy, Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and thermogravimetry. The spectroscopic data indicated that the interaction between HPPANI and BF3.Thermogravimetry studies revealed that the BF3 doping improved the thermal stability of defects free PANI.

How well do mean breakthrough curves predict mixing‐controlled reactive transport?

Macroscopic transport models calibrated by flux‐averaged breakthrough curves of conservative compounds do not necessarily characterize mixing well because such breakthrough curves do not provide information on fluctuations of concentration within the solute flux, which may influence mean reaction rates. We numerically examine the validity of macroscopic transport models, which are capable of describing all details of flux‐averaged breakthrough curves, for predicting a mixing‐controlled bimolecular precipitation reaction in heterogeneous media. We consider a homogeneous, isotropic medium with an elliptical, low‐permeability inclusion and random heterogeneous fields. For the single‐inclusion case, slow advection through the inclusion results in a multimodal breakthrough curve with enhanced tailing. We vary the hydraulic conductivity contrast and Peclet number to investigate the performance of a “perfect” macroscopic transport model for predicting the total precipitated mass within the domain and the peak concentration difference between the conservative and reactive cases at the outflow boundary. The results indicate that such a model may perform well in media with either very small or very high permeability contrast or at low Peclet number. In the high‐contrast case, most flow takes place in preferential flow paths, resulting in a small variance of the flux‐weighted concentration, even though the offset in the breakthrough between the slow and fast travel paths is substantial. Maximum relative errors in terms of total precipitated mass and the peak concentration difference between the conservative and reactive cases occur at intermediate permeability contrasts and large Peclet numbers. Numerical simulations on random heterogeneous fields confirm the finding of the single‐inclusion case. Thus, in cases with intermediate hydraulic conductivity contrast, making macroscopic models fit flux‐averaged concentration breakthrough curves better may not improve the prediction of mixing‐controlled reactive transport, and it becomes necessary to quantify and account for the variability of conservative concentrations in the flux in order to formulate an appropriate macroscopic transport model that predicts mixing‐controlled reactive transport.

The role of molecular modeling in confined systems: impact and prospects

Molecular modeling at the electronic and atomistic levels plays an important and complementary role to experimental studies of confinement effects. Theory and atomistic simulation can provide fundamental understanding, determine the limits of well known macroscopic laws such as Kelvin's equation, provide predictions for systems that are difficult to study via experiment (e.g. adsorption of highly toxic gases), and can be used to gain detailed molecular level information that may not be accessible in the laboratory (e.g. the local structure and composition of confined phases). We describe the most important and useful methods that are based firmly on quantum mechanics and statistical mechanics, including ab intio and classical density functional theories, and Monte Carlo and molecular dynamics simulation. We discuss their strengths and limitations. We then describe examples of applications of these methods to adsorption and equilibrium properties, including testing the Kelvin equation, determination of pore size distributions and capillary phenomena. Applications to self and transport diffusion, including single-file and anomalous diffusion, and viscous flow in nanoporous materials are described. The use of these methods to understand confinement effects on chemical reactions in heterogeneous media is treated, including effects on reaction equilibria, rates and mechanism. Finally we discuss the current status of molecular modeling in this area, and the outlook and future research needs for the next few years. The treatment is suitable for the general technical reader.

A Tripodal Peptidic Titanium Phosphonate as a Homochiral Porous Solid Medium for the Heterogeneous Enantioselective Hydration of Epoxides

AbstractA porous, homochiral titanium‐phosphonate material based on a tripodal peptide scaffold was used as a heterogeneous reaction medium for the enantioselective hydration (>99%) of styrene oxide. This titanium‐phosphonate material, which was shown to contain confined chiral spaces, was prepared by polymerization of L‐leucine onto a tris(2‐aminoethyl)amine initiator, followed by capping with phosphonate groups and completed by non‐aqueous condensation with titanium isopropoxide. Circular dichroism confirmed that the peptide tethers yielded a secondary structure. X‐ray powder diffraction and transmission electron microscopy supported by a semi‐empirical model showed the likely formation of a porous, lamellar material that was quantified by nitrogen adsorption.

Coupling of mass transfer and reactive transport for nonlinear reactions in heterogeneous media

Fast chemical reactions are driven by mixing‐induced chemical disequilibrium. Mixing is poorly represented by the advection‐dispersion equation. Instead, effective dynamics models, such as multirate mass transfer (MRMT), have been successful in reproducing observed field‐scale transport, notably, breakthrough curves (BTCs) of conservative solutes. The objective of this work is to test whether such effective models, derived from conservative transport observations, can be used to describe effective multicomponent reactive transport in heterogeneous media. We use a localized formulation of the MRMT model that allows us to solve general reactive transport problems. We test this formulation on a simple three‐species mineral precipitation problem at equilibrium. We first simulate the spatial and temporal distribution of mineral precipitation rates in synthetic hydraulically heterogeneous aquifers. We then compare these reaction rates to those corresponding to an equivalent (i.e., same conservative BTC) homogenized medium with transport characterized by a nonlocal in time equation involving a memory function. We find an excellent agreement between the two models in terms of cumulative precipitated mass for a broad range of generally stationary heterogeneity structures. These results indicate that mass transfer models can be considered to represent quite accurately the large‐scale effective dynamics of mixing controlled reactive transport at least for the cases tested here, where individual transport paths sample the full range of heterogeneities represented by the BTC.