Kai Leonhard

Cellulose and hemicellulose valorisation: an integrated challenge of catalysis and reaction engineering

Platform molecules have recently been in the focus of numerous investigations as intermediates for transformation of lignocellulosic biomass into fuels and chemicals. Herein we focus on challenges associated with technical implementation of the production of selected platform molecules. 5-(Hydroxymethyl)-furfural, furfural and levulinic acid were chosen to illustrate pitfalls and limitations of currently available catalytic reaction systems. Owing to the prominent reactivity, high polarity and high boiling points of most biomass-derived platform chemicals, the design of efficient, economic and environmentally benign chemical processes offers a number of difficulties. Challenges include not only a selective synthesis of such platform chemicals and their further transformation into potential products but also methods to allow an energy efficient product separation and sufficient catalyst stability under the applied reaction conditions. In this review we discuss three approaches to tackle the integration of catalytic transformations, reaction engineering and product separation. First, extraction-assisted synthesis methods are discussed. In the next step the integration of such extraction-assisted approaches into reaction cascades is considered. Finally, one-pot transformations of lignocellulose-derived carbohydrates into value-added products without isolation of the intermediate platform chemicals are outlined. The present status of predictive tools facilitating the selection of suitable solvent systems for extraction systems is discussed to outline current opportunities and constraints of a theoretical process design and optimization. In summary, this review provides an overview of recent progress with regard to strategies for process integration of chemo-catalytic biomass valorisation and highlights challenges associated with such approaches.

Modelling cellulose solubilities in ionic liquids using COSMO-RS

Recently, ionic liquids have been studied as promising solvents for cellulose. Since ionic liquids can be combined freely from different cations and anions, possible candidates are numerous. Therefore, a time efficient method to screen ionic liquids with respect to their solvent power is mandatory to find an optimal combination. By applying a model with predictive capabilities, the relative solubilities of cellulose were computed using COSMO-RS combined with different types of models describing the combinatorial contribution to the chemical potential. The results are in good qualitative agreement with those from the scarce experimental data available in the literature. On the basis of these calculations, insight into the solvation mechanism within these systems was obtained. A screening of more than 2000 ionic liquids pointed to the anion as mainly being responsible for the respective dissolving power, suggesting some new ionic liquids as potential candidates for cellulose dissolution.

Screening of new solvents for artemisinin extraction process using ab initio methodology

The solubility of artemisinin in a range of conventional and novel solvents was evaluated using the COSMO-RS approach, and verified experimentally as well as against literature data. The computational method was improved by calibrating against a limited set of experimental data, enhancing the accuracy of the calculations. The optimised method was shown to be in reasonable agreement with experimental data; however, lack of reliable experimental data is identified as an issue. Several novel solvents perceived as green alternatives to conventional solvents were targeted and shown to offer good solubility of artemisinin. Extraction from Artemisia annua by carbonate solvents was experimentally verified.

Correlation of the solubility of low-volatile organic compounds in near- and supercritical fluids. Part I: applications to adamantane and β-carotene

Abstract A new simplified form of an equation of state suitable for the correlation of pVT -data is presented. The new equation of state is capable to model pVT -data in the low- and high-pressure regions as well as in the critical region. Taking a fugacity approach as a basis, we have calculated the solubility of several low-volatile solid organic compounds, in particular of the globular molecule adamantane in CO 2 and the natural dyestuff β-carotene in three compressed gases, i.e. CO 2 , CClF 3 , and N 2 O. For all systems except β-carotene+N 2 O solubility data measured up to 100 MPa and more (180 MPa for β-carotene+CO 2 and+CClF 3 ) are available. Experimental results for the systems β-carotene+CO 2 and+CClF 3 are also reported. Here, adamantane serves as an example for a relatively high-soluble substance, while β-carotene represents low-soluble solutes. Solubility maxima, which have been found experimentally as a function of pressure or density at constant temperature, are considered in the correlations.

Monte Carlo simulations of neon and argon using ab initio potentials

Gibbs ensemble Monte Carlo simulations of neon and argon have been performed with pair potentials taken from literature as well as with new ab initio potentials from just above the triple point to close to the critical point. The densities of the coexisting phases, their pair correlation functions, the vapour pressure and the enthalpy and entropy of vaporization have been calculated. The influence of the potential choice and of the addition of the Axilrod-Teller (AT) three-body potential on the above mentioned properties have been investigated. It turns out that an accurate ab initio two-body potential in connection with the AT potential yields very good results for thermodynamic properties of phase equilibria.

Automated Discovery of Reaction Pathways, Rate Constants, and Transition States Using Reactive Molecular Dynamics Simulations

We provide a methodology for deducing quantitative reaction models from reactive molecular dynamics simulations by identifying, quantifying, and evaluating elementary reactions of classical trajectories. Simulations of the inception stage of methane oxidation are used to demonstrate our methodology. The agreement of pathways and rates with available literature data reveals the potential of reactive molecular dynamics studies for developing quantitative reaction models.

Monte Carlo simulations of nitrogen using an ab initio potential

A new ab initio pair potential for nitrogen has been calculated at CCSD(T) level with aug-cc-pVDZ and -pVTZ correlation consistent basis sets. The results were extrapolated to approximate the basis set limit. This potential was used within Gibbs ensemble Monte Carlo (GEMC) simulations to obtain the densities of the coexisting phases, the vapour pressure and the enthalpy of vaporization from 70 K to close to the critical point. The influence of several 3-body interactions (an approximate anisotropic triple dipole potential derived by Stogryn, the isotropic triple dipole potential by Axilrod and Teller (AT), and a 3-body induction potential on the above mentioned properties was investigated. Satisfactory agreement with experimental data was observed. To determine whether the remaining deviations between experimental and computed data are due to inaccuracies in the 2-body or 3-body potential, the 2-body potential was rescaled to reproduce experimental 2nd virial coefficients accurately, and some of the calculations were repeated with the new potential. It turns out that an accurate 2-body potential only in connection with the AT potential yields accurate results for the thermodynamic properties phase equilibria.

An equation of state describing the critical region: extension to high pressure

Abstract In this paper, an equation of state is presented, which is able to model the pρT behaviour of fluids in the critical region, in the low-density limit and at high pressure. The equation is developed in the framework of a local mean field model describing density fluctuations in a fluid. This model is based on an earlier developed model, which was employed to describe the pρT behaviour in the critical region for several substances. It is extended here to high pressure. For the representation of the critical isotherm, argon is used as reference substance. In addition to the experimental data taken from the literature, some state points at high pressure are estimated by the Monte-Carlo simulation on the basis of ab-initio pair potentials. These data were included in the development of the equation of state in order to assure physically correct behaviour at high pressure. With this equation, the pρT behaviour of argon, sulfur hexafluoride, methane, ethylene, nitrous oxide and carbon dioxide is modeled.

What is Wrong with Quantitative Structure–Property Relations Models Based on Three-Dimensional Descriptors?

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.

Quantum Mechanically Based Estimation of Perturbed-Chain Polar Statistical Associating Fluid Theory Parameters for Analyzing Their Physical Significance and Predicting Properties

We have computed molecular descriptors for sizes, shapes, charge distributions, and dispersion interactions for 67 compounds using quantum chemical ab initio and density functional theory methods. For the same compounds, we have fitted the three perturbed-chain polar statistical associating fluid theory (PCP-SAFT) equation of state (EOS) parameters to experimental data and have performed a statistical analysis for relations between the descriptors and the EOS parameters. On this basis, an analysis of the physical significance of the parameters, the limits of the present descriptors, and the PCP-SAFT EOS has been performed. The result is a method that can be used to estimate the vapor pressure curve including the normal boiling point, the liquid volume, the enthalpy of vaporization, the critical data, mixture properties, and so on. When only two of the three parameters are predicted and one is adjusted to experimental normal boiling point data, excellent predictions of all investigated pure compound and mixture properties are obtained. We are convinced that the methodology presented in this work will lead to new EOS applications as well as improved EOS models whose predictive performance is likely to surpass that of most present quantum chemically based, quantitative structure-property relationship, and group contribution methods for a broad range of chemical substances.