On-line measurement of chemical reactions by a compact NMR spectrometer

ERNESTO DANIELI; MARIO H. M. KILLNER; YAMILA GARRO LINCK; JUAN PERLO; FEDERICO CASANOVA; BERNHARD BLÜMICH

Hersonissos

Congreso; Euromar 2013. Magnetic Resonance Conference; 2013

On-line nuclear magnetic resonance (NMR) spectroscopy analysis of chemical reactions provides important information for process development and optimization. However, the use of conventional high-field NMR spectrometers for this purpose is limited because of the large size of this equipment and the required maintenance which makes it difficult to install them in the chemistry labs, next to the fume hoods where the reactions are carried out. For this application desktop spectrometers based on permanent magnet technology appear as a powerful alternative. Examples of the use of low-field medium-resolution NMR spectrometers for the on-line monitoring of chemical reactions have been presented in the past. However, the reported performance was always far from the one required for industrial applications or in routine chemical process analysis to study relevant chemical reactions. Technically relevant chemical reactions performed in industry using microreactor technology like Knoevenagel condensations, Suzuki reactions, or transfer hydrogenations are run in the low concentration limit and involve a number of chemical components with complex spectra. Small sample concentrations are typically available, either due to economical reasons, or in order to have control over the reaction kinetics, which can be altered by highly exothermic effects. To allow high quality understanding of such reactions high spectral resolution and sensitivity are required. In this work we present real time 1H NMR spectra measured under the fume hood with a new compact 1 T system offering improved resolution and sensitivity. In combination with a flow-through setup, different chemical reactions, such as the transfer hydrogenation of acetophenone to 1-phenylethanol or the base-catalyzed transesterification of vegetable oils with a short chain alcohol to produce biodiesel were studied in batch mode. Reaction yields and kinetic constants are calculated for different reaction conditions. Additionally, we have studied the effects of flow rate, cell characteristics, and tubing lengths on the accuracy to measure concentrations of a flowing mixture with the aim of optimizing the flow settings.