M. A. Liauw

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.

Electrically Heated Microreactors for Methanol Steam Reforming

Hydrogen generation for automotive fuel cell application faces the demands of fast load alternations. These may be met in an excellent manner by microstructured reactors for methanol steam reforming. Calculations reveal fast heating when using heat transfer oil and negligible temperature gradients in the catalyst layers. By solving the differential equation for heat transport with boundary conditions adjusted for the microchannel system, the temperature gradients in microstructured foil stacks may be estimated. The results were verified experimentally by gradient measurements in electrically heated lab-scale reformers (demonstrators, i.e. pre-prototype stage). Effects on temperature gradients and conversion by changing the material of both reformer body and foils were examined, as well as effects due to constrained temperature gradients. These studies were performed with PdZn-catalyst systems based on nanoparticle washcoats both in the steady state and during dynamic operation. The hydrogen output of the demonstrators corresponded to 200Wel taking into account a system efficiency of the propulsion system of about 40%. After load changes, the hydrogen output reached its new value within 10 s to a degree of 90%. After temperature changes of 80K in the range 230–310°C, the hydrogen output reached its new value after 240 s to a degree of 90%. The secondary reaction to dimethyl ether was extremely dependent on the reactor material during load changes.

Characterization of a Gas/Liquid Microreactor, the Micro Bubble Column: Determination of Specific Interfacial Area

The micro bubble column [1] was one of the first microstructured devices being specially designed for gas/liquid processing. Within a short time, the experimental performance of this microreactor was demonstrated for the fluorination of toluene using elemental fluorine, as an example of use with industrial relevance. However, the understanding of fundamental reactor characteristics had to be postponed to succeeding, more detailed investigations. In this context, the objective of the work described in this article is to determine specific interfacial areas as a main parameter of reactor performance and to develop on this basis a first reactor model for gas/liquid reactions in the micro bubble column. This reactor model enables then proper prediction of suitable candidate reactions for processing in this microreactor.

Periodic Operation in Microchannel Reactors

Microchannel reactors promise to display unusually fast response behavior to sharp changes of feed composition in heterogeneous catalytic systems. In an attempt to use this feature for the elucidation of the transient kinetics and possibly for performance enhancement, three model reaction systems are selected. CFD assisted reactor design and catalyst preparation studies are presented.

Microstructured Catalysts for Methanol-Steam Reforming

For the application of endothermic methanol-steam reforming e.g. in automobile systems based on hydrogen-driven PEM (proton exchange membrane) fuel cells, active and microstructure-compatible catalyst layers were examined.

Highly Active N,O Zinc Guanidine Catalysts for the Ring-Opening Polymerization of Lactide

New zinc guanidine complexes with N,O donor functionalities were prepared, characterized by X-Ray crystallography, and examined for their catalytic activity in the solvent-free ring-opening polymerization (ROP) of technical-grade rac-lactide at 150 °C. All complexes showed a high activity. The fastest complex [ZnCl2 (DMEGasme)] (C1) produced colorless poly(lactide) (PLA) after 90 min with a conversion of 52 % and high molar masses (Mw =69 100, polydispersity=1.4). The complexes were tested with different monomer-to-initiator ratios to determine the rate constant kp . Furthermore, a polymerization with the most active complex C1 was monitored by in situ Raman spectroscopy. Overall, conversion of up to 90 % can be obtained. End-group analysis was performed to clarify the mechanism. All four complexes combine robustness against impurities in the lactide with high polymerization rates, and they represent the fastest robust lactide ROP catalysts to date, opening new avenues to a sustainable ROP catalyst family for industrial use.

A New Microstructure Device for Fast Temperature Cycling for Chemical Reactions

A microstructure reactor for fast periodic temperature changes will be presented. The developed reactor allows to reach temperature differences of several 10 K in the time range of several seconds. The device is made of stainless steel and electrically heated with high power resistor cartridges. It is cooled with deionized water as cooling fluid. The test facility is based on a microcomputer control system, which allows the half cycle time, i.e. the time between the lowest and the highest temperature of one cycle, to be adjusted from minutes to seconds. Depending on the half cycle time temperature changes of up to 200 K are achievable.

Concerning the Role of Supercritical Carbon Dioxide in SN1 Reactions

A series of SN 1-type reactions has been studied under various conditions to clarify the role of supercritical carbon dioxide (scCO2 ) as reaction medium for this kind of transformations. The application of scCO2 did not result in higher yields in any of the experiments in comparison to those under neat conditions or in the presence of other inert compressed gases. High-pressure UV/Vis spectroscopic measurements were carried out to quantify the degree of carbocation formation of a highly SN 1-active alkyl halide as a function of the applied solvent. No measureable concentration of carbocations could be detected in scCO2 , just like in other low polarity solvents. Taken together, these results do not support the previously claimed activating effect via enhanced SN 1 ionization due to the quadrupolar moment of the supercritical fluid.

Process analytical technology (PAT) applied to biomass valorisation: a kinetic study on the multiphase dehydration of xylose to furfural

We present a methodology for evaluating the kinetic parameters of biomass transformations employing process analytical technology (PAT). The salient feature of this strategy involves the determination of reaction rates and activation energies for biomass conversions under process conditions without influence from possible side reactions or fouling of optical probes. PAT was applied to the dehydration of the C5-sugar xylose to the platform chemical furfural as a model reaction system. The reaction was monitored in situ in a biphasic 2-methyltetrahydrofuran/water (2-MTHF/H2O) system by ATR mid-IR and Raman spectroscopy at different reaction temperatures. We evaluated the reaction kinetics for the dehydration of xylose to furfural and the side reaction of humin formation by chemometric and kinetic modelling of the inline data. The activation energy for xylose dehydration (151 ± 3 kJ mol−1) was slightly higher than that for humin formation (139 ± 7 kJ mol−1). This insight allowed for the proposal of an optimised reactor concept to maximise the yields of furfural and minimise the production of humins.

Selective oxidation of isoprene to citraconic anhydride

The isoprene oxidation to citraconic anhydride (CA, methylmaleic anhydride) over a V-Ti-O catalyst was studied with steady-state experiments, concentration cycling, fiber-optical in situ diffuse reflectance (DR) VIS spectroscopy and TGA. The results suggest both a reversible deactivation by deposits and a Mars–van Krevelen mechanism. Upon exposure to isoprene, the oxidized catalyst quickly changes its state, presumably by carbon deposition and reduction. This process is significantly faster than the regeneration under oxygen. In addition, the reaction seems to follow a ‘rake’ mechanism where CA is formed via a sequence of intermediates. The consecutive oxidation of CA leads to a selectivity maximum.