Maarten Sabbe

Carbon-Centered Radical Addition and β-Scission Reactions: Modeling of Activation Energies and Pre-exponential Factors

A consistent set of group additive values DeltaGAV degrees for 46 groups is derived, allowing the calculation of rate coefficients for hydrocarbon radical additions and beta-scission reactions. A database of 51 rate coefficients based on CBS-QB3 calculations with corrections for hindered internal rotation was used as training set. The results of this computational method agree well with experimentally observed rate coefficients with a mean factor of deviation of 3, as benchmarked on a set of nine reactions. The temperature dependence on the resulting DeltaGAV degrees s in the broad range of 300-1300 K is limited to +/-4.5 kJ mol(-1) on activation energies and to +/-0.4 on logA (A: pre-exponential factor) for 90 % of the groups. Validation of the DeltaGAV degrees s was performed for a test set of 13 reactions. In the absence of severe steric hindrance and resonance effects in the transition state, the rate coefficients predicted by group additivity are within a factor of 3 of the CBS-QB3 ab initio rate coefficients for more than 90 % of the reactions in the test set. It can thus be expected that in most cases the GA method performs even better than standard DFT calculations for which a deviation factor of 10 is generally considered to be acceptable.

First-principles kinetic modeling in heterogeneous catalysis: an industrial perspective on best-practice, gaps and needs

Electronic structure calculations have emerged as a key contributor in modern heterogeneous catalysis research, though their application in chemical reaction engineering remains largely limited to academia. This perspective aims at encouraging the judicious use of first-principles kinetic models in industrial settings based on a critical discussion of present-day best practices, identifying existing gaps, and defining where further progress is needed.

Modeling the Gas‐Phase Thermochemistry of Organosulfur Compounds

Key to understanding the involvement of organosulfur compounds in a variety of radical chemistries, such as atmospheric chemistry, polymerization, pyrolysis, and so forth, is knowledge of their thermochemical properties. For organosulfur compounds and radicals, thermochemical data are, however, much less well documented than for hydrocarbons. The traditional recourse to the Benson group additivity method offers no solace since only a very limited number of group additivity values (GAVs) is available. In this work, CBS-QB3 calculations augmented with 1D hindered rotor corrections for 122 organosulfur compounds and 45 organosulfur radicals were used to derive 93 Benson group additivity values, 18 ring-strain corrections, 2 non-nearest-neighbor interactions, and 3 resonance corrections for standard enthalpies of formation, standard molar entropies, and heat capacities for organosulfur compounds and organosulfur radicals. The reported GAVs are consistent with previously reported GAVs for hydrocarbons and hydrocarbon radicals and include 77 contributions, among which 26 radical contributions, which, to the best of our knowledge, have not been reported before. The GAVs allow one to estimate the standard enthalpies of formation at 298 K, the standard entropies at 298 K, and standard heat capacities in the temperature range 300-1500 K for a large set of organosulfur compounds, that is, thiols, thioketons, polysulfides, alkylsulfides, thials, dithioates, and cyclic sulfur compounds. For a validation set of 26 organosulfur compounds, the mean absolute deviation between experimental and group additively modeled enthalpies of formation amounts to 1.9  kJ  mol(-1). For an additional set of 14 organosulfur compounds, it was shown that the mean absolute deviations between calculated and group additively modeled standard entropies and heat capacities are restricted to 4 and 2 J  mol(-1)  K(-1), respectively. As an alternative to Benson GAVs, 26 new hydrogen-bond increments are reported, which can also be useful for the prediction of radical thermochemistry.

Group Additive Values for the Gas‐Phase Standard Enthalpy of Formation, Entropy and Heat Capacity of Oxygenates

A complete and consistent set of 60 Benson group additive values (GAVs) for oxygenate molecules and 97 GAVs for oxygenate radicals is provided, which allow to describe their standard enthalpies of formation, entropies and heat capacities. Approximately half of the GAVs for oxygenate molecules and the majority of the GAVs for oxygenate radicals have not been reported before. The values are derived from an extensive and accurate database of thermochemical data obtained by ab initio calculations at the CBS-QB3 level of theory for 202 molecules and 248 radicals. These compounds include saturated and unsaturated, α- and β-branched, mono- and bifunctional oxygenates. Internal rotations were accounted for by using one-dimensional hindered rotor corrections. The accuracy of the database was further improved by adding bond additive corrections to the CBS-QB3 standard enthalpies of formation. Furthermore, 14 corrections for non-nearest-neighbor interactions (NNI) were introduced for molecules and 12 for radicals. The validity of the constructed group additive model was established by comparing the predicted values with both ab initio calculated values and experimental data for oxygenates and oxygenate radicals. The group additive method predicts standard enthalpies of formation, entropies, and heat capacities with chemical accuracy, respectively, within 4 kJ mol(-1) and 4 J mol(-1) K(-1) for both ab initio calculated and experimental values. As an alternative, the hydrogen bond increment (HBI) method developed by Lay et al. (T. H. Lay, J. W. Bozzelli, A. M. Dean, E. R. Ritter, J. Phys. Chem.- 1995, 99, 14514) was used to introduce 77 new HBI structures and to calculate their thermodynamic parameters (Δ(f)H°, S°, C(p)°). The GAVs reported in this work can be reliably used for the prediction of thermochemical data for large oxygenate compounds, combining rapid prediction with wide-ranging application.

Kinetic Modeling of α-Hydrogen Abstractions from Unsaturated and Saturated Oxygenate Compounds by Carbon-Centered Radicals

Hydrogen abstractions are important elementary reactions in a variety of reacting media at high temperatures in which oxygenates and hydrocarbon radicals are present. Accurate kinetic data are obtained from CBS-QB3 ab initio (AI) calculations by using conventional transition-state theory within the high-pressure limit, including corrections for hindered rotation and tunneling. From the obtained results, a group-additive (GA) model is developed that allows the Arrhenius parameters and rate coefficients for abstraction of the α-hydrogen from a wide range of oxygenate compounds to be predicted at temperatures ranging from 300 to 1500 K. From a training set of 60 hydrogen abstractions from oxygenates by carbon-centered radicals, 15 GA values (ΔGAV°s) are obtained for both the forward and reverse reactions. Among them, four ΔGAV°s refer to primary contributions, and the remaining 11 ΔGAV°s refer to secondary ones. The accuracy of the model is further improved by introducing seven corrections for cross-resonance stabilization of the transition state from an additional set of 43 reactions. The determined ΔGAV°s are validated upon a test set of AI data for 17 reactions. The mean absolute deviation of the pre-exponential factors (log A) and activation energies (E(a)) for the forward reaction at 300 K are 0.238 log(m(3)  mol(-1)  s(-1)) and 1.5 kJ mol(-1), respectively, whereas the mean factor of deviation between the GA-predicted and the AI-calculated rate coefficients is 1.6. In comparison with a compilation of 33 experimental rate coefficients, the between the GA-predicted values and these experimental values is only 2.2. Hence, the constructed GA model can be reliably used in the prediction of the kinetics of α-hydrogen-abstraction reactions between a broad range of oxygenates and oxygenate radicals.

Kinetic modeling of α-hydrogen abstractions from unsaturated and saturated oxygenate compounds by hydrogen atoms.

Hydrogen-abstraction reactions play a significant role in thermal biomass conversion processes, as well as regular gasification, pyrolysis, or combustion. In this work, a group additivity model is constructed that allows prediction of reaction rates and Arrhenius parameters of hydrogen abstractions by hydrogen atoms from alcohols, ethers, esters, peroxides, ketones, aldehydes, acids, and diketones in a broad temperature range (300-2000 K). A training set of 60 reactions was developed with rate coefficients and Arrhenius parameters calculated by the CBS-QB3 method in the high-pressure limit with tunneling corrections using Eckart tunneling coefficients. From this set of reactions, 15 group additive values were derived for the forward and the reverse reaction, 4 referring to primary and 11 to secondary contributions. The accuracy of the model is validated upon an ab initio and an experimental validation set of 19 and 21 reaction rates, respectively, showing that reaction rates can be predicted with a mean factor of deviation of 2 for the ab initio and 3 for the experimental values. Hence, this work illustrates that the developed group additive model can be reliably applied for the accurate prediction of kinetics of α-hydrogen abstractions by hydrogen atoms from a broad range of oxygenates.

Computational Study and Kinetic Analysis of the Aminolysis of Thiolactones

The aminolysis of three differently α-substituted γ-thiolactones (C4H5OSX, X = H, NH2, and NH(CO)CH3) is modeled based on CBS-QB3 calculated free energies corrected for solvation using COSMO-RS. For the first time, quantitative kinetic and thermodynamic data are provided for the concerted path and the stepwise path over a neutral tetrahedral intermediate. These paths can take place via an unassisted, an amine-assisted, or a thiol-assisted mechanism. Amine assistance lowers the free energy barriers along both paths, while thiol assistance only lowers the formation of the neutral tetrahedral intermediate. Based on the ab initio calculated rate coefficients, a kinetic model is constructed that is able to reliably describe experimental observations for the aminolysis of N-acetyl-dl-homocysteine thiolactone with n-butylamine in THF and CHCl3. Reaction path analysis shows that for all conditions relevant for applications in polymer synthesis and postpolymer modification, an assisted stepwise mechanism is operative in which the formation of the neutral tetrahedral intermediate is rate-determining and which is mainly amine-assisted at low conversions and thiol-assisted at high conversions.

Group Additive Kinetics for Hydrogen Transfer Between Oxygenates.

Hydrogen abstraction reactions involving oxygenates in gaseous phase play an important role in many biomass-related conversion processes. In this work, group additivity is used to provide Arrhenius parameters in a temperature range of 300-2500 K for hydrogen abstractions between oxygenate compounds such as alcohols, ethers, esters, acids, ketones, diketones, aldehydes, hydroxyperoxides, alkyl peroxides, and unsaturated ethers and ketones. The group additive values for Arrhenius parameters of hydrogen transfer reactions of the type O--H--C and O--H--O are derived from CBS-QB3 calculations in the high-pressure limit. From a total set of 118 reactions, 43 group additivity values are determined. Inclusion of an additional 37 corrections accounting for cross-resonance effects in the transition state further improves the accuracy of the model. For a set of 25 ab initio calculated and 60 experimental rate coefficients, group additive modeling reproduces rate coefficients within a mean factor of deviation of ∼3. Hence, the developed group additive models can be reliably used for an accurate and fast prediction of the kinetics of hydrogen abstractions involving oxygenates.

Thiol-Michael addition in polar aprotic solvents: nucleophilic initiation or base catalysis?

The thiol-Michael addition of ethanethiol to ethyl acrylate, methyl vinylsulfone and maleimide initiated by ethyl-, diethyl-, triethylamine and triethylphosphine in tetrahydrofuran (THF) is investigated at room temperature for concentrations ranging from 0.5 to 2 mol L−1 for the reactants and 0.03 to 0.3 mol L−1 for the initiators. Rate coefficients for all elementary steps in a reaction scheme consisting of both the base catalyzed and the nucleophile initiated mechanism are calculated using CBS-QB3 corrected for solvation with COSMO-RS. Diffusional limitations are taken into account using the coupled encounter pair model. The ab initio apparent kinetic parameters are used in a microkinetic model and simulated conversions agree well with experimental data. Competition with the aza-Michael addition is shown to be insignificant. Regardless of the choice of ene or catalyst, conversion is governed by an anionic cycle in which first an addition from the thiolate to the ene occurs, followed by a rate-controlling proton transfer to the obtained Michael adduct anion from another thiol. For acrylates and vinylsulfones, the addition of the thiolate to the ene is quasi-equilibrated, while for maleimides this elementary reaction has a positive affinity, explaining their large reactivity. The choice of catalyst or ene strongly affects the initiation mechanism. Using tertiary phosphines only nucleophilic initiation takes place while with tertiary amines, only base catalysis occurs. For primary and secondary amines both initiation mechanisms contribute. The presented kinetic parameters and the insights on diffusional limitations are critical for the further optimization of thiol-Michael additions for polymer conjugation.

Ab initio coverage-dependent microkinetic modeling of benzene hydrogenation on Pd(111)

The effect of hydrogen coverage on the kinetics of benzene hydrogenation on Pd(111) has been investigated with optPBE-vdW density functional theory calculations and a coverage-dependent microkinetic model. The dominant reaction path consists of the consecutive hydrogenation of carbon atoms located in ortho positions relative to the previously hydrogenated carbon atom, independent of the hydrogen coverage. Increasing the hydrogen coverage destabilizes all surface species, which leads to weaker adsorption and increased rate coefficients for the hydrogenation steps due to stronger destabilization of reactants than transition states. The catalytic activities simulated using the constructed coverage-dependent microkinetic model exceed those obtained using a low-coverage microkinetic model by several orders of magnitude and are comparable to experimentally observed activities. The rate coefficients to which the global rate is most sensitive depend on the reaction conditions and differ from those calculated using low coverage kinetics. Therefore, properly accounting for coverage dependence on the kinetics and thermodynamics of catalytic hydrogenation reactions is not only required for an accurate DFT-based prediction of the catalytic activity but also for a correct understanding of the reaction mechanism.