Author:

Abstract:

Amino acids are naturally highly functionalized bio-molecules with applications in many different sectors such as food, animal feed, pharmaceuticals, cosmetics etc. Besides, their potential as an alternative renewable feedstock for petroleum-based bulk and fine chemicals cannot be overlooked. Simple selective modification and defunctionalization reactions can provide a plethora of (nitrogenous) chemicals and introduce these molecules into the bio-based chemical industry. In this dissertation, we mainly focused on the selective deamination and decarboxylation of amino acids, in particular glutamic acid, into value-added chemicals. Within the natural amino acid pool, glutamic acid is among the most interesting amino acid. It is a non-essential amino acid, consists of an additional carboxylic acid group and is efficiently produced by large-scale fermentation. Additional bulk quantities can be provided by cheap protein-rich side streams from agro-industry and biofuel production. Many of these protein streams are currently underused, leading to losses of renewable nitrogen into the environment. Consequently, the valorization of bio-derived proteins and amino acids to nitrogenous materials or chemicals can substantially increase their value and recycle organic nitrogen from natural resources. Their potential applications and currently acquired valorization strategies were illustrated and discussed in an initial literature overview. The direct deamination and decarboxylation are two attractive strategies for the conversion of amino acids into respectively a,w-bifunctional carboxylic acids, and amines and amides. However, state-of-the-art methodologies are often associated with salt waste production, expensive or hazardous co-catalysts or they are inappropriate for glutamic acid. Therefore, new drop-in strategies have been developed using existing heterogeneous catalytic systems, originating from biomass and fossil feedstock processing. Special attention has been paid on meeting the principles of green chemistry. In the first part of this thesis, the deamination of glutamic acid was investigated in order to produce bio-based glutaric acid ‒ a C5 dicarboxylic acid ‒ and derivatives. The deamination strategy is based on the metal-catalyzed hydrodenitrogenation (HDN), which is a well-known strategy in the petrochemical industry for removing nitrogen from crude oil fractions. In order to facilitate C-N bond cleavage and to recycle nitrogen into a valuable co-product, glutamic acid is modified to N,N‑dimethylglutamic acid in an initial step via a mild reductive N-alkylation with Pd/C. Subsequent C‑N hydrogenolysis in methanol eventually yields dimethyl glutarate and trimethylamine (TMA). Platinum immobilized on a titania support is found to be the most performant catalyst and is able to produce dimethyl glutarate and TMA in excellent yields. A Fourier transform infrared spectroscopy study and additional kinetic experiments revealed that the excellent C-N hydrolysis activity and selectivity originate from the interplay between the metal and the moderate acidity of the support. The catalytic system is able to convert the complete acidic amino acid fraction (viz. glutamic acid and aspartic acid) to a mixture of dimethyl glutarate and dimethyl succinate. In this way, complicated amino acid separation and extensive downstream processing can be avoided. In the second part, the aqueous direct decarboxylation of (pyro)glutamic acid has been investigated in order to recycle the nitrogen atom directly into the product, 2‑pyrrolidone. The direct decarboxylation strategy is based on the Pd-catalyzed decarboxylation of fatty acids, which is a well-investigated strategy in the context of biomass deoxygenation. Since the reaction is typically performed at temperatures above 250 °C, glutamic acid is present as its cyclic condensation product pyroglutamic acid, which can be efficiently decarboxylated to 2-pyrrolidone. Unfortunately, side products are formed due to the consecutive degradation of 2‑pyrrolidone, initiated by hydrolysis of the lactam ring. In order to minimize lactam hydrolysis, careful support selection is crucial. A Pd supported on alumina catalyst shows the highest 2‑pyrrolidone yield. In a final part, this strategy has been extended to the production of C3‑C5 N‑alkyl‑2‑pyrrolidones, by combining the reductive N-alkylation of glutamic acid and the decarboxylation in a consecutive fashion. Both reaction steps can be catalyzed by the same Pd/Al2O3 catalysts. The reductive N-alkylation typically requires mild constant H2 pressures and an excess of the carbonyl compound (viz. aldehyde or ketone), to achieve high mono N-alkylated glutamic acid yields. The additional alkyl chain promotes the lactam stability against hydrolysis during the decarboxylation, as a result of steric and electronic effects. Nevertheless, it also influences the reactivity against decarboxylation, eventually resulting in lower N‑alkyl‑2‑pyrrolidone yields compared to 2‑pyrrolidone. Neutralization of N‑alkylpyroglutamic acid with ammonia, however, decreases the hydrolysis rate and improves the selectivity; especially N‑butyl‑2‑pyrrolidone and N‑isobtyl‑2-pyrrolidone can be obtained in good yields. Both the reductive N-alkylation and decarboxylation can be combined in a consecutive fashion in a one-pot process by simply changing the gas atmosphere and temperature, preventing intermediate product purification.