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The development of a reaction model is often a time-consuming process, especially if unknown reactions have to be found and quantified. To alleviate the reaction modeling process, automated procedures for reaction space exploration are highly desired. We present ChemTraYzer-TAD, a new reactive molecular dynamics acceleration technique aimed at efficient reaction space exploration. The new method is based on the basin confinement strategy known from the temperature-accelerated dynamics (TAD) acceleration method. Our method features integrated ChemTraYzer bond-order processing steps for the automatic and on-the-fly determination of the positions of virtual walls in configuration space that confine the system in a potential energy basin. We use the example of 1,3-dioxolane-4-hydroperoxide-2-yl radical oxidation to show that ChemTraYzer-TAD finds more than 100 different parallel reactions for the given set of reactants in less than 2 ns of simulation time. Among the many observed reactions, ChemTraYzer-TAD finds the expected typical low-temperature reactions despite the use of extremely high simulation temperatures up to 5000 K. Our method also finds a new concerted β-scission plus O

The use of equations of state (EOS) requires the knowledge of model parameters for each molecule, but experimental data for parameter regression might not always be available. The physical soundnes...

Anharmonicity can greatly affect rate constants. One or even several orders of magnitude of deviation are found for obtaining rate constants using the standard rigid-rotor harmonic-oscillator model. In turn, reactive molecular dynamics (MD) simulations are a powerful way to explore chemical reaction networks and calculate rate constants from the fully anharmonic potential energy surface. However, the classical nature of the dynamics and the required numerical efficiency of the force field limit the accuracy of the resulting kinetics. We combine the best of both worlds by presenting an approximation that pairs anharmonic information intrinsic to classical MD with high-accuracy energies and frequencies from quantum-mechanical electronic structure calculations. The proposed scheme is applied to hydrogen abstractions in the methane system, which allows for the benchmarking of rate constants corrected by our approach against experimental rate constants. This comparison reveals a standard deviation of factor 2.6. Two archetypes of possible failure are identified in the course of a detailed investigation of the CH3• + H• → CH22• + H2 reaction. From this follows the application range of the method, within which the method shows a standard deviation of factor 2.1. The computational efficiency and beneficial scaling of the method allow for application to larger systems, as shown for hydrogen abstraction from 2-butanone by HO2•.

In part I of this paper, the reaction kinetics and thermochemistry of cyclopentanol have been studied numerically. In this part, the ignition and combustion behavior of cyclopentanol are studied experimentally in a shock tube, a rapid compression machine, a combustion vessel, and a counterflow burner. Fundamental combustion properties, such as ignition delay times, laminar flame speeds, and extinction strain rates, are reported. All measurements probe a variety of initial conditions and provide an initial evaluation of the performance of cyclopentanol under different combustion modes. The experimental results are compared with those computed using the chemical mechanism presented in Part I. Reasonable agreement is observed. The controlling kinetics of cyclopentanol oxidation is explored considering various combustion modes. Moreover, cyclopentanol is compared to a variety of C5 fuels including cyclopentane, n-pentanol, and n-pentane with respect to their combustion characteristics in order to evaluate the impact of functional groups. The auto-ignition propensity of cyclopentanol is found to be lower than that of its paraffinic and linear counterparts, while the premixed flames of all these fuels propagate with approximately identical velocities at the investigated conditions.

Biomass derived chemicals may offer sustainable alternatives to petroleum derived hydrocarbons, while also enhancing engine combustion performance with co-optimization of fuels and engines. This paper presents a numerical study on the oxidation and combustion of a novel biofuel compound, cyclopentanol. Its reaction kinetics and thermochemistry are first explored using ab initio quantum chemistry methods. Thermochemical properties are calculated for cyclopentanol and a set of its key oxidation intermediates. C-H bond dissociation energies of cyclopentanol are computed for different carbon sites. For the fuel radicals, the energy barriers of their ring-opening reactions and the potential energy surfaces of their oxidation reactions are determined. Based on the theoretical results, a chemical kinetic mechanism is proposed to describe the oxidation of cyclopentanol at low and high temperatures. The model is compared against data obtained from shock tube, rapid compression machine, combustion vessel, and counterflow burner experiments over a range of initial conditions. Furthermore, reaction pathway analysis is performed using the present mechanism to give insights into the underlying oxidation chemistry of cyclopentanol. It is found that the α-radical of cyclopentanol undergoes preferably an alcohol-specific HO2 elimination reaction to form stable cyclopentanone and this reaction can strongly retard reactivity. The major reaction pathways of β- and γ-radicals are similar to those of cyclopentyl radicals that are the sequential and formally direct reactions of fuel radicals with O2 to form cyclopentenols and HO2 radicals. The existence of the hydroxy moiety affects the bond dissociation energies and reaction barriers, slightly favoring the chain-branching channel for γ-radicals at low temperatures.

The kinetics of chemical reactions in the liquid phase are often strongly determined by the reaction solvent. Consequently, the choice of the optimal solvent is an important task in chemical proces...

We measure and model the effect of CO2 addition on equilibrium composition and volume expansion for solvent system 2-methyltetrahydrofuran (2-MTHF)/water. Our experimental data cover pressures from...

In this study (Part II), the oxidation of dimethoxymethane (DMM) is investigated and a detailed chemical reaction model developed for a comprehensive description of both high- and low-temperature oxidation processes. The sub-mechanism of DMM is implemented using AramcoMech2.0 as the base mechanism. Rate coefficients are based on analogies with those for dimethyl ether, diethyl ether, and n-pentane oxidation. Furthermore, theoretical studies from recent works are also included in the present model and new calculations for the dissociation kinetics of Q ˙ OOH radicals have been carried out at the CCSD(T)/CBS(aug-cc-pVXZ; X = D, T) // B2PLYP-D3/6-311 + + G(d,p) level of theory. For validation, new ignition delay time experiments have been performed in a shock tube (ST), a rapid compression machine (RCM), and in a laminar flow reactor covering a wide range of conditions (p = 1–40 bar, T = 590–1215 K, φ = 1). In addition, the kinetic model is validated against laminar burning velocities, jet-stirred reactor, plug flow reactor and further ST and RCM experimental datasets from the literature. Pathway and sensitivity analyses were used to identify critical reaction pathways in the DMM oxidation mechanism. These show that the reactivity of DMM at intermediate temperatures is controlled by the branching between pathways initiated on the primary or secondary fuel radical. While primary fuel radicals eventually lead to chain branching, secondary fuel radical consumption is controlled by fast β-scission over a wide range of temperatures, which inhibits reactivity.

This work focuses on the ionic liquid (IL) synthesis starting from dimethyl carbonate and 1-ethylimidazole in methanol. In-situ IR-spectroscopy is used to gain time-dependent concentrations for kinetic analysis. The reaction is found to be self-accelerating, since the solvent polarity increases with the formation of the ionic product. This corresponds to a salting-out effect. Since the reaction rate is increasing in the beginning, pre-dosing of the product is proposed to enhance the overall productivity. Therefore, the space-time-yield is calculated as the arithmetic mean of the reaction rate with a varying starting point. It turns out that pre-dosing does not enhance the productivity here, since the space-time-yield of the complete reaction progress is below the initial reaction rate. Beyond that, two sharp bends can be observed in the progress of the reaction rate coefficient that point towards sudden changes in the reaction environment. The first bend can be assigned to a phase separation of the ionic liquid on a microscopical scale. The second bend takes place when the IL concentration surpasses roughly one sixth of the methanol concentration. Above that threshold, the whole amount of methanol is captured in the solvation shell of the anion monomethyl carbonate, as quantum chemical calculations suggest. Finally, the reaction rate coefficients were calculated using quantum chemical methods with respect to the solvent environment. The theoretical results show a good accordance with experimental values for the starting compositions. Thus, theoretical calculations help to estimate productive reaction mixtures.

The chemistry of urethanes plays a key role in important industrial processes. Although catalysts are often used, the study of the reactions without added catalysts provides the basis for a deeper understanding. For the non‐catalytic urethane formation and cleavage reactions, the dominating reaction mechanism has long been debated. To our knowledge, the reaction kinetics have not been predicted quantitatively so far. Therefore, we report a new computational study of urethane formation and cleavage reactions. To analyze various potential reaction mechanisms and to predict the reaction rate constants quantum chemistry and transition state theory were employed. For validation, experimental data from literature and from own experiments were used. Quantitative agreement of experiments and predictions could be demonstrated. The calculations confirm earlier assumptions that urethane formation reactions proceed via mechanisms where alcohol molecules act as auto‐catalysts. Our results show that it is essential to consider several transition states corresponding to different reaction orders to enable agreement with experimental observations. Urethane cleavage seems to be catalyzed by an isourethane, leading to an observed 2nd‐order dependence of the reaction rate on the urethane concentration. The results of our study support a deeper understanding of the reactions as well as a better description of reaction kinetics and will therefore help in catalyst development and process optimization., Reaction kinetics and mechanisms of urethane formation and cleavage reactions were investigated based on advanced quantum chemical methods. These reactions represent important model reactions in urethane chemistry. The results confirm that both urethane formation and cleavage proceed via autocatalytic mechanisms. Several mechanisms contribute to the observed reaction rate for urethane formation. A validation using own experimental data and literature data confirmed that the used quantum chemical methods yield quantitative predictions.

Combustion and flame 189, 433-442 (2018). doi:10.1016/j.combustflame.2017.07.037, Published by Elsevier Science, Amsterdam [u.a.]

Sustainable solvents are crucial for chemical processes and can be tailored to applications by computer-aided molecular and process design (CAMPD). Recent CAMPD methods consider not only economics but also environmental hazards and impacts. However, holistic environmental assessment needs to address the complete life cycle of solvents. Here, we propose a CAMPD framework integrating Life Cycle Assessment (LCA) of solvents from cradle-to-grave: COSMO-susCAMPD. The framework builds on the COSMO-CAMPD method for predictive design of solvents using COSMO-RS and pinch-based process models. Cradle-to-grave LCA is enabled by combining predictive LCA from cradle-to-gate using an Artificial Neural Network with gate-to-grave Life Cycle Inventory data from the process models. The framework is applied to design solvents in a hybrid extraction-distillation process. The results highlight the need for cradle-to-grave LCA as objective function: Heuristics, economics, or cradle-to-gate LCA lead to suboptimal solvent choices. COSMO-susCAMPD thus enables the holistic environmental design of solvents using cradle-to-grave LCA., Chemical Engineering Science, 245, ISSN:0009-2509

A novel lignocellulosic biofuel 2-butyltetrahydrofuran (2-BTHF) is investigated experimentally and numerically in the present study. Laminar burning velocities were measured in a combustion vessel. Ignition delay times were experimentally determined in a rapid compression machine and in a shock tube at high pressures covering a wide temperature range. A detailed chemical kinetic mechanism is proposed to describe the oxidation of 2-BTHF at both low and high temperatures. It is demonstrated that the model gives satisfactory results for the various experimental configurations. For a better understanding of the oxidation kinetics, the chemical model is further used to elucidate the reaction pathways of 2-BTHF. A comparison of its ignition behavior with other C 8 hydrocarbons is further presented and the impact of the tetrahydrofuranic ring on the fuel ignition characteristics is highlighted and analyzed.

Property predictions are essential when dealing with molecules that have not been investigated experimentally yet. The accuracy of current predictive models like predictive perturbed-chain polar statistical associating fluid theory (PCP-SAFT) and conductor-like screening model for real solvents (COSMO-RS) is limited. We propose a combination of predictive models in order to yield a higher accuracy. Information from both predictive models are combined in PCP-SAFT parameter space using a log-likelihood function. Experimental vapor pressures, enthalpies of vaporization, and liquid densities over a wide temperature range are used to evaluate the predictions. The average error in the combined property prediction is lower than the error of the individual models. In addition, the maximum error is considerably lowered.

Chemical engineering science 159, 84-92 (2017). doi:10.1016/j.ces.2016.05.038 special issue: "iCAMD – Integrating Computer-Aided Molecular Design into Product and Process Design / Edited by Claire Adjiman and André Bardow", Published by Elsevier Science, Amsterdam [u.a.]

Utilizing CO2 to produce value-added chemicals can save environmental and economic impacts. However, these savings are reduced by the cost of CO2 supply when CO2 has to be captured from dilute sour...

Through a large interdisciplinary approach the “Tailor Made Fuels from Biomass” (TMFB) cluster of excellence has been pursuing the identification of next generation biofuels. By first using chemical synthesis to procure suitable chemical components from biomass followed by initial screening experiments a large information database is compiled and can be used to guide fuel design. As a result of this method butanone has been identified as a particularly interesting target owing to its potential usage as a spark-ignition fuel, thanks to its impressive knock resistant properties. This has motivated this study to consider the fundamental combustion chemistry controlling its reactivity under engine relevant conditions. A detailed chemical kinetic model which includes both high- and low-temperature oxidation reaction pathways for butanone is developed. This model (PCFCbutanone_v1) is validated using the available experimental data from the literature (included as supplemental material) and newly measured data collected during the course of this study. Ignition delay times are measured using both a shock tube and a rapid compression machine. They are measured for φ = 1.0 butanone/air mixtures covering a range of temperatures (850–1280 K) and pressures (20 and 40 bar) that incorporates engine relevant conditions. In addition, laminar burning velocities for butanone are measured for a range of equivalence ratios (0.7–1.3) and pressures (1 and 5 bar), an important parameter considering the potential use of butanone within spark ignition engines. The new model also incorporates new quantum chemical calculations of the thermodynamic parameters for butanone and the low-temperature species associated with its oxidation. The thermodynamic parameters used in the model are necessary to calculate the reverse rate constants in the model making them important parameters, in particular in predicting the low-temperature ignition delay times presented here. Butanone shows no evidence of negative temperature coefficient behavior. However, inclusion of the low-temperature oxidation pathways is shown to be important to accurately predict the low-temperature ignition delay times of butanone. Of particular importance are the radical β-scission and HȮ2 elimination reactions which are essential in accurately predicting the ignition delay times.

We present a hybrid modeling strategy for the prediction of octanol/water partition coefficients for alcohol ethoxylate surfactants of varying chain lengths. The strategy makes use of molecular dynamics (MD) simulations for the generation of molecular conformations in the presence of a solvent. A clustering of the conformations from the MD trajectories was carried out based on principal component analysis of their dihedral angles. Representative conformations thus selected were then used in the conductor-like screening model for realistic solvents (COSMO-RS). Each conformation has been assigned a weight using an equation derived on the basis of its probability of occurrence in the MD trajectory. Experimental partition coefficients were reproduced within conformation-independent accuracy of COSMO-RS despite the size and flexibility of the ethoxy chain otherwise posing a challenge on their solvation modeling.

k(E) can be calculated either from the Rice-Ramsperger-Kassel-Marcus theory or by inverting macroscopic rate constants k(T). Here, we elaborate the inverse Laplace transform approach for k(E) reconstruction by examining the impact of k(T) data fitting accuracy. For this approach, any inaccuracy in the reconstructed k(E) results from inaccurate/incomplete k(T) description. Therefore, we demonstrate how an improved mathematical description of k(T) data leads to accurate k(E) data. Refitting inaccurate/incomplete k(T), hence, allows for recapturing k(T) information that yields more accurate k(E) reconstructions. The present work suggests that accurate representation of experimental and theoretical k(T) data in a broad temperature range could be used to obtain k(T,p). Thus, purely temperature-dependent kinetic models could be converted into fully temperature- and pressure-dependent kinetic models.

To ensure a high efficiency of 5-hydroxymethylfurfural (HMF) synthesis, improved solvents for the extraction of HMF from a reactive aqueous solution were identified using the predictive thermodynamic model COSMO-RS. Utilizing COSMO-RS as a basis for a systematic solvent selection has the advantage of potentially saving significant time and effort by computationally screening several thousand possible solvents. Factors including temperature, concentration, and fructose addition were used for experimental validation of the predictive power of COSMO-RS. Continuous extraction experiments confirmed also kinetics and phase separation to be important for technical implementation. COSMO-RS predicted o-propylphenol and o-isopropylphenol to have partition coefficients as high as 10.02 and 9.82, which are roughly five times higher than the partition coefficient of the previously known, most effective solvent: 2-methyltetrahydrofuran (PHMF = 2). Therefore, the identification of o-propylphenol and o-isopropylphenol as...

The COSMO-SAC model for the calculation of activity coefficients has been improved by incorporating three-dimensional geometric information on molecules. This information is added to the model by means of the effective contact area of the molecules. We define a procedure to compute this contact area by using a probing sphere. The probing sphere rolls around the COSMO surfaces of two contacting atoms to define a spherical cap on each atom. The segments of the COSMO surfaces within the spherical caps are marked, and the summation of their areas defines the contact area for a pair of contacting molecules. The effective contact areas are used to compute the σ-profiles and the Onsager screening energies in the calculation of the surface activity coefficients, allowing us to remove one parameter from the COSMO-SAC model. The σ-profiles and effective areas need to be calculated only once and can be stored in a database. We show that the new model, named COSMO-3D because some three-dimensional information (the po...

Journal of chemical information and modeling 58(7), 1343-1355 (2018). doi:10.1021/acs.jcim.8b00078, Published by American Chemical Society, Washington, DC

The oxidation of n-butyl formate, a potential biofuel candidate, is studied using three different experimental approaches. Ignition delay times have been measured for stoichiometric mixtures of fuel and air for pressures of about 20 and 90 bar at temperatures from 846 up to 1205 K in a high-pressure shock tube. A rapid compression machine has been used to determine the low-temperature ignition delay times for stoichiometric mixtures at pressures close to 20 bar over the temperature range from 646 K up to 861 K. Laminar burning velocities have been determined for stoichiometric ratios ranging from 0.8 to 1.2 using the high-pressure chamber method combined with an optical Schlieren cinematography setup in order to acquire experimental data at elevated pressures of about 10 bar and a temperature of 373 K. A detailed kinetic model has been constructed including high-temperature and low-temperature reaction pathways. The enthalpies of formation, entropies, and specific heats at constant pressure for the fuel, its primary radicals, and several combustion intermediates have been computed with the CBS-QB3 methods and included in the mechanism. This model was validated successfully against the presented data and used to elucidate the combustion of this interesting ester. The importance of accurate inclusion of the low-temperature peroxy chemistry has been highlighted through sensitivity and reaction path analysis. This study presents the first combustion study of n-butyl formate and leads to an improved understanding of the chemical kinetics of alkyl ester oxidation.

This work describes the application of a hybrid two-step enantioselective separation process. As a first step, pertraction using supported liquid membranes provides an initial enrichment, while the following preferential crystallization delivers the enantiopure crystals as final product. Mandelic acid in water was studied as a model system. Using a suitable chiral selector, pertraction provides enrichments exceeding 10% and reaching up to 20% in the permeate phase, which was sufficient to allow for subsequent selective preferential crystallization. Based on the individual performances of pertraction and crystallization, overall yields and productivities are estimated. The calculated productivities are compared with values achievable in alternatively applicable chromatographic separation processes using chiral stationary phases. The realized hybrid pertraction-crystallization process is in its present state for the example considered still inferior to preparative chromatography. Strategies for further improvement are suggested.

For predictive applications, equation of state (EOS) models have to describe all relevant physical interactions accurately. In this contribution, the vapor–liquid equilibria of various dipolar two-center Lennard-Jones model molecules are determined by molecular simulation, as function of molecular elongation and deflection angle of the dipole. It is shown that present PC-SAFT-based EOS models require additional adjustable parameters in order to describe the orientational effects of the dipole-moment. We present extensions of the model to avoid the additional parameters and apply the extended equations to model systems and real molecules.

Combustion and flame 158(8), 1444-1455 (2011). doi:10.1016/j.combustflame.2010.12.028, Published by Elsevier Science [u.a.], Amsterdam [u.a.]

10th European Congress of Chemical Engineering, ECCE 2015, Nice, France, 28 Aug 2015 - 29 Sep 2015; Chemical engineering research and design : CERD 115(B, SI) 433-442 (2016). doi:10.1016/j.cherd.2016.09.029 special issue: "10th European Congress of Chemical Engineering / Edited by Martine Poux, Jerzy Bałdyga, Xavier Joulia and Michel Meyer", Published by Elsevier, Amsterdam

Quantitative structure-property relations (QSPR) employing descriptors derived from the three-dimensional (3D) molecular structure are frequently applied for property prediction in various fields of research. However, there is no common understanding of the necessary degree of detail to which molecular structure has to be known for reliable descriptor evaluation, but computational methods used vary from simplified molecular mechanics up to rigorous ab initio programs. In order to quantify the yet unknown error due to this heterogeneity, widely used 3D molecular descriptors from diverse fields of application are evaluated for molecular structures computed by different computational methods. The results clearly indicate that the widespread, exclusive use of the most stable molecular conformation as well as too simplistic computational methods yield systematically erroneous descriptor values with misleading information for the inferred structure-property relations. Thus, generating an awareness and understanding of this fundamental problem is considered an important first step to make 3D QSPR a generally accepted property prediction method.

Integrierter Prozess zur Trennung chiraler Systeme mit Verbindungsbildung Dipl.-Ing. (FH) L. Gou (E-Mail: gou@mpi-magdeburg.mpg.de), Dr. H. Lorenz, Dipl.-Ing. S. Robl, Dr. K. Leonhard, Prof. K. Schaber, Prof. A. Seidel-Morgenstern Max-Planck-Institut fur Dynamik komplexer technischer Systeme, Sandtorstrase 1, D-39106 Magdeburg, Germany Universitat Karlsruhe, Institut fur Technische Thermodynamik und Kaltetechnik, Engler-Bunte-Ring 21, D-76131 Karlsruhe, Germany RWTH Aachen, Lehrstuhl fur Technische Thermodynamik, Schinkelstrase 8, D-52062 Aachen, Germany Otto-von-Guericke-Universitat Magdeburg, Institut fur Verfahrenstechnik, Universitatsplatz 2, D-39106 Magdeburg, Germany DOI: 10.1002/cite.201050380