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Rh-catalyzed hydrogenation of amino acids, protein hydrolyzates and peptides

Book - Dissertation

The use of fossil feedstocks for the production of fuels and chemicals has led and still leads to growing environmental damage and concerns. Therefore, technologies are now developed for using bio-based feedstocks to replace the fossil counterparts. Especially the valorization of lignocellulosic biomass into chemicals has received much attention, whereas the use of proteins as feedstock is much less investigated. Nevertheless, proteins are highly functionalized and contain fixated N in their backbone and side chains, making it an ideal feedstock for the production of value-added chemicals. Moreover, in industry many protein-rich waste streams are produced (for instance dried distillers grains with solubles and seed cakes from biofuel production, poultry feather meal…) which are now mainly burned or used as animal feed which are low-value applications. Therefore, this dissertation focuses on the valorization of these protein-rich waste streams for the production of value-added chemicals. In a first part, the hydrogenation of amino acids to amino alcohols is studied, using a bimetallic Rh-MoOx/SiO2 catalyst. The literature on this subject is scarce and the substrate scope is limited to amino acids with non-interfering functional groups in the side chain. The hydrogenation of all natural amino acids is presented in this dissertation, including those with interfering functional groups in the side chain. Although C-O hydrogenolysis leading to the formation of amines occurs as a side reaction, the amino acid hydrogenation generally gives high conversions and selectivities to the amino alcohols. Unfortunately, the S-containing amino acids, cysteine and methionine containing a thiol and thioether functional group, act as catalyst poisons. This is problematic when the application to protein hydrolysates is targeted, since these amino acids are always present in proteins. Nevertheless, the issue could be addressed by oxidizing the thiol and thioether moieties to a sulfonic acid and sulfone group respectively, via an elegant performic acid oxidation procedure. Subsequent hydrogenation of cysteic acid and methionine sulfone results in high conversions and selectivities to the amino alcohols. Finally, a protein hydrolysate from bovine serum albumin, a model protein for animal-derived protein-rich waste streams, is successfully hydrogenated with 90% overall conversion and 88% overall selectivity to amino alcohols after 48 h. This result is highly promising and could provide opportunities for the valorization of protein-rich waste streams to value-added chemicals. For example, the mixture of amino alcohols could be used as ethanolamine-type crosslinking agents for the production of polyurethanes. The hydrogenation of peptides to peptide-derived polyols is studied in the second part of this dissertation. The use of peptides could be more efficient compared to individual amino acids since proteins need only be partially hydrolyzed and re-assembled. The resulting bio-based polyols could be used as polymer precursor in the polyurethane synthesis. Enzymatic hydrolysis of proteins with trypsin, a specific endopeptidase that cleaves peptide bonds after a lysine or arginine residue, is employed in this work; so the length and amino acid sequence of the resulting peptides are completely known. Subsequently, an oxidation of cysteine and methionine to cysteic acid and methionine sulfone respectively, is performed to avoid catalyst poisoning during hydrogenation. Several strategies (changing the solvent system, increasing the amount of catalyst, evaluating the effect of H3PO4 addition, decreasing to volume of oxidation mixture to avoid overoxidation, adjusting the pH) have been investigated, however, up to now, no hydrogenation of carboxylic acid groups in the peptides was observed. It is believed that peptide solvation issues hamper the hydrogenation and therefore, experiments using NaBH4 as a reductant are now being conducted. Preliminary results of the reduction of an esterified di- or tripeptide of glycine, were promising, as hydrogenation is observed; so the next step will be to test the reduction with esterified peptides as substrate. In the final part, hydrogenation reactions using a mesoporous Ru-AAO as catalyst are investigated. The anodized aluminum oxide supports are synthesized according to a four-step procedure. First, the platelets are electropolished resulting in a clean Al surface. Then, a first anodization step is performed and irregular pores are formed. These pores are chemically etched in the third step and finally, regular pores are formed in the second anodization. The AAOs are thoroughly characterized and highly ordered, tubular mesopores are identified. Subsequently, the supports are impregnated with Ru-precursor solutions. Depositing Ru inside the pores of the AAO was not straightforward and the contact angle of the Ru-precursor solution with the Al2O3 phase seemed critical: an aqueous solution with a high contact angle resulted in the deposition of a thick Ru-layer on the outer surface of the AAO. In contrast, the use of an ethanolic Ru-precursor solution with a lower contact angle, enabled the deposition of Ru inside the pores which was clearly visualized using HAADF-STEM-EDX. The Ru-AAO catalysts were then used in the hydrogenation of levulinic acid with high conversion and selectivity to γ-valerolactone, however, due to the acidic conditions, Al2O3 dissolved and recrystallized on the AAO outer surface. Reductions of toluene and butanal to methylcyclohexane and butanol respectively, did not lead to dissolution of the AAO and good conversions and selectivities are obtained, proving that Ru-AAOs have high catalytic activity.
Publication year:2019
Accessibility:Closed