Rh-Catalyzed Hydrogenation of Amino Acids to Biobased Amino Alcohols: Tackling Challenging Substrates and Application to Protein Hydrolysates

Abstract

While protein-rich biomass waste is nowadays mainly used for animal feed, conversion of its amino acid constituents to nitrogenous chemicals is a potential higher value route. To that end, the hydrogenation of amino acids to amino alcohols was studied in this work. Using a bimetallic Rh–MoOx/SiO2 catalyst, glutamic acid was for the first time hydrogenated to the aminodiol in high yield. By minimizing partial reduction and consecutive hydrogenolysis and by suppressing the competitive cyclization to pyroglutamic acid (and derivatives thereof), glutamidiol was obtained in 77% yield at 70 bar H2 and 80 °C. High yields (typically >80%) and selectivities were also achieved for most other natural amino acids, except for the S-containing amino acids cysteine and methionine, which act as catalyst poisons. This limitation was overcome by applying a simple oxidation step with performic acid prior to the hydrogenation. The system was applied successfully to a mixture of amino acids obtained by hydrolysis of preoxidized bovine serum albumin. Amino alcohols were produced with high overall conversion (>90%) and selectivity (88%) without the need for an intermediate, expensive, and difficult separation step. The reaction proceeds with very high atom economies for both carbon and nitrogen and generates only water as a byproduct.