Quantitative NMR Spectroscopy Study of Chemical Synthesis of
Bio-based Materials: Formation of Formyl-oxy-stearic Acid from Oleic Acid
Agnes Fröscher1, Erik von Harbou1, Werner Thiel2, Hans Hasse1,
1Laboratory of Engineering Thermodynamics, University of Kaiserslautern, 2Department of Chemistry, University of Kaiserslautern
Fatty acids are an interesting class of renewable raw materials for the chemical industry. They might play an important role in the future production of biopolymers. One of the most important fatty acids is oleic acid, which can be obtained from vegetable oils. To enable polymerization, oleic acid has to be modified. An interesting modification is converting the oleic acid to hydroxy-stearic acid. The central step in this process is the acidic catalyzed formation of formyl-oxy-stearic acid (FSA) from oleic acid (OA) and formic acid (FA). The studied system shows a large miscibility gap between a formic acid rich phase and an oleic acid rich phase.
In this work, we investigate the liquid-liquid equilibrium (LLE) and chemical equilibrium as well as the homogeneously catalyzed reaction kinetics in the system OA-FA-FSA. Despite the large miscibility gap, the reaction can be carried out in the homogeneous phase. The suitable region is identified by a comprehensive study of the entire system at different temperatures. Since the analysis of the complex reacting mixture is difficult with conventional analytical techniques, we apply quantitative 1H- and 13C-NMR spectroscopy. NMR spectroscopy facilitates the analysis of complex fluid mixtures in situ without sample preparation. The method has a high resolution and thus even chemically similar compounds such as OA and FSA can be differentiated and analyzed. Furthermore, unknown compounds, e.g. side- or intermediate products, can be identified by NMR spectroscopy. Contrarily to optical spectroscopy, no previous calibration is needed for quantitative analysis.
Furthermore, a spatially resolving NMR technique is applied that enables the investigation of the (reactive) LLE in situ in the NMR tube. By using a magnetic field gradient in combination with specially shaped excitation pulse the composition of the mixture is analyzed not by averaging over a large volume but in small slices at different positions in the sample tube. Thus, the composition in both liquid phases can be directly measured and no separation of the two phases prior to the analysis is necessary. By heating up the sample directly within the NMR spectrometer, the LLE can be investigated at different temperatures using only a single sample.
Based on the new data models of the LLE, the chemical equilibrium, and the reaction kinetics are parameterized. They are used for a preliminary conceptual process design for the production of formyl-oxy-stearic acid.
More generally, we demonstrate that quantitative NMR spectroscopy is a powerful and versatile tool for studying systems related to chemical synthesis of bio-based materials as it provides the data of the physicochemical properties that are needed for the design, optimization, and model-based scale-up of the industrial process.