Fatty Acids under Candlelight: Operando Spectroscopy and Catalysis during Branched Unsaturated Fatty Acid Hydrogenation

Abstract

The results described in this PhD dissertation contribute to the more efficient and sustainable production of C18 branched saturated fatty acids from branched unsaturated fatty acids. These molecules are desirable ingredients in personal health products and are produced at a large scale annually. In Chapter 2, we describe the application of zeolite catalysts in the ring-opening of γ-stearolactone, an odorous by-product present in C18 branched unsaturated fatty acids that must be removed from the reaction mixture. No sources are available in the literature on this chemical reaction, and so we relied on papers describing the ring-opening of the much smaller γ-valerolactone, a molecule obtained via the hydrogenation of levulinic acid. As such, we expected that zeolites with strong acidity and high total acidity would perform best in the γ-stearolactone ring-opening reaction. We modified the large pore zeolite Beta and found that it led to a decrease in the total acidity and strong acidity. Surprisingly, the activity was increased, leading us to believe that other acidic properties play a role in the ring-opening reaction. We found that the Brønsted/Lewis acid site ratio of the zeolites was governing ring-opening activity. Lewis acid sites in zeolites are known to cause double bond isomerisation, which improves γ-stearolactone conversion by reducing the reverse formation of the lactone. We show in Chapter 3 that attenuated total reflectance infrared spectroscopy is an excellent tool for monitoring liquid phase reactions involving lactones. We report on operando attenuated total reflectance infrared spectroscopy in a custom hydrogenation autoclave during the hydrogenation of levulinic acid into γ-valerolactone and the ring-opening of γ-stearolactone, both reactions performed at elevated temperatures and pressures. Using principal component analysis, we reveal the sources of variance contained within the spectroscopic datasets. Despite the large amount of variance caused by temperature variations, partial least squares regression allowed us to predict the concentrations of γ-valerolactone and the onset temperature of γ-valerolactone formation during levulinic acid hydrogenation. Because of the low concentrations of γ-stearolactone in the medium during the experiments, the chemical variance caused by γ-stearolactone conversion was overshadowed by the variance caused by temperature variations during the temperature ramp. As such, determination of the onset temperature of γ-stearolactone ring-opening using partial least squares was impossible. By performing the regression in the isothermal stages of the experiments, we removed variance originating from temperature variations and constructed a model to predict the γ-stearolactone concentrations during the experiments. In Chapter 4, we develop a methodology based on operando spectroscopy to follow fatty acid hydrogenation during batch reactor operation. We show that while in theory operando Raman and attenuated total reflectance infrared spectroscopy should be able to study fatty acid hydrogenation, fluorescence spectroscopy is the more suitable technique. The fatty acid feedstock used in this work is highly fluorescent. We found that during branched unsaturated fatty acid hydrogenation in the presence of zeolite, a redshift of the fluorescence signal occurred and that this could be suppressed by hydrogenation. Various catalysts were evaluated, and we discovered that the redshift was highly correlated with the hydrogenation activity of the catalysts. Experiments with the model fluorophore fluoranthene gave insight into the nature of the fluorophores in the feedstock, which we believe are (alkyl-branched) polycyclic aromatic hydrocarbons. The developed methodology we expect can be used in industrial-scale fatty acid hydrogenation reactors to roughly predict the time required until complete hydrogenation of the fatty acids.