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PdPb catalyzed decarboxylation of proline to pyrrolidine: highly selective formation of a bio-based amine in water Jasper Verduyckt, Maarten Van Hoof, Free De Schouwer, Marike Wolberg, Mert Kurttepeli, Pierre Eloy, Eric M. Gaigneaux, Sara Bals, Christine E. A. Kirschhock, and Dirk E. De Vos ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02561 • Publication Date (Web): 16 Sep 2016 Downloaded from http://pubs.acs.org on September 18, 2016
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PdPb catalyzed decarboxylation of proline to pyrrolidine: highly selective formation of a biobased amine in water Jasper Verduyckt,a Maarten Van Hoof,a Free De Schouwer,a Marike Wolberg,a Mert Kurttepeli,b Pierre Eloy,c1 Eric M. Gaigneaux,c2 Sara Bals,b Christine E. A. Kirschhock,a Dirk E. De Vos*a a
Centre for Surface Chemistry and Catalysis, Department of Microbial and Molecular Systems,
KU Leuven - University of Leuven, Leuven Chem&Tech, Celestijnenlaan 200F, Post Box 2461, 3001 Heverlee, Belgium b
Electron Microscopy for Materials Science, UA - University of Antwerp, Groenenborgerlaan
171, 2020 Antwerp, Belgium c
Institute of Condensed Matter and Nanosciences, c1 SUrface Characterization platform and
c2
MOlecules Solids & reactiviTy, UCL – Université catholique de Louvain, Place L. Pasteur 1,
Post Box L4.01.09, 1348 Louvain-la Neuve, Belgium
ABSTRACT. Amino acids have huge potential as platform chemicals in the bio-based industry. Pd catalyzed decarboxylation is a very promising route for the valorization of these natural compounds derived from protein waste or fermentation. We report that the highly abundant and non-essential amino acid L-proline is very reactive in the Pd catalyzed decarboxylation. Full conversions are obtained with Pd/C and different Pd/MeOx catalysts; this allowed to identify the
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different side reactions and hence to map the reaction network. Due to the high reactivity of pyrrolidine, the selectivity for pyrrolidine was initially low. By carefully modifying Pd/ZrO 2 with Pb in a controlled manner – i.e. via two incipient wetness impregnation steps – the selectivity increased remarkably. Finally, a thorough investigation of the reaction parameters resulted in an increased activity of this modified catalyst and an even further enhanced selectivity under a low H2 pressure of 4 bar at 235°C in water. This results in a very selective and sustainable production route for the highly interesting pyrrolidine.
KEYWORDS. PdPb, decarboxylation, L-proline, pyrrolidine, bio-based amine, water
Introduction Amines are versatile building blocks for the synthesis of organic compounds, such as dyes, agrochemicals and pharmaceuticals.1 Pyrrolidine is for example used in the synthesis of nicotine and rifamycin antibiotics.2,3 Currently, this secondary amine is produced via the high temperature dehydration of 1,4-butanediol in the presence of ammonia using e.g. a ThO2-Al2O3 catalyst at 300°C.1 The diol on its turn is synthesized via oxidation of the fossil-based butane to maleic anhydride, which is subsequently hydrogenated to 1,4-butanediol in the DAVY process.4 The different redox transformations render this route to pyrrolidine very inefficient. In this work we propose an alternative sustainable pathway for the synthesis of this important amine, starting from the amino acid L-proline as a renewable resource. Amino acids can be produced in a green way via fermentation.5,6 More importantly however, they will become increasingly available from protein rich waste streams.7 Common agricultural waste fractions such as poultry feathers, dried distillers grains with solubles and press cakes from rapeseed, soybeans, algae, etc. contain up to 90% of proteins.8 Sanders et al. predict that in this
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way more than 100 million tons of protein will be available if 10% of the fossil transportation fuel is replaced by biomass-derived fuels.9 Work-up of these proteins includes extraction, hydrolysis and subsequent separation of the amino acids. This final step still remains a big challenge, though new technologies based on electrodialysis and adsorption are emerging.10–14 Amino acids have huge potential as platform chemicals in the bio-based industry.7 They can be catalytically converted to valuable nitriles via oxidative decarboxylation,15–18 and to amino alcohols via hydrogenation.19,20 Another very interesting class of valorization reactions is the direct decarboxylation to form amines. This reaction is e.g. catalyzed by enzymes or by α,βunsaturated ketones that mimic the pyridoxal 5’-phosphate cofactor.10,11,21 The use of enzymes suffers however from the need for an expensive cofactor in combination with a strict pH control and from a low volumetric productivity.10,11,22 The use of homogeneous organocatalysts on the other hand suffers from the mandatory use of an organic solvent in combination with high catalyst loading and difficult catalyst recycling.21 A viable alternative is the direct Pd catalyzed decarboxylation, which was first applied to the decarboxylation of fatty acids for the production of diesel-like compounds at 300°C.23,24 Typically, the reaction is conducted with zerovalent Pd nanoparticles on a support. Recently, this technology was successfully adapted for the decarboxylation of itaconic acid to methacrylic acid and of (pyro)glutamic acid to 2-pyrrolidone in water at 250°C.25,26 The extension of this technology to the production of amines from other amino acids has so far not been reported, obviously due to the instability of amines under these reaction conditions. In this work the Pd catalyzed decarboxylation is successfully extended to the first highly selective formation of a bio-based amine in water, using L-proline as the substrate. An essential
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step to obtain high selectivities was the careful modification of the Pd catalyst with Pb via a new, controlled two-step incipient wetness approach.
Results & Discussion Pd catalyst screening and unraveling the complex reaction network In an initial screening different Pd catalysts were tested for the decarboxylation of L-proline (1) to pyrrolidine (2) with water as the solvent, using 4 mol% Pd at 6 bar N2 and 225°C for 6 h (Table 1, entries 1-6). These reaction conditions were inspired by those recently reported by our group for the decarboxylation of pyroglutamic acid.26 In general, L-proline (1) is very reactive for the decarboxylation, since almost all catalysts attain full conversion already at 225°C. The high conversions obtained with different catalysts allowed to identify the side products and hence to map the reaction network (Scheme 1). Due to its high reactivity, the selectivity to pyrrolidine (2) was initially low (2-33%). The first side route is the dehydrogenation of pyrrolidine (2) to form pyrrole (3 - pathway A), which was observed especially for Pd/Al2O3 and Pd/ZrO2. This reaction is known to be Pd catalyzed; group-IV metal oxide supports, like ZrO2 and TiO2, are known to favor the pyrrolidine dehydrogenation.27 Pathway B leads to a plethora of side products, starting with a single dehydrogenation to form 1-pyrroline (4), which was identified using 1H-NMR and GC-MS. This cyclic imine is hydrated to pyrrolidin-2-ol and its linear counterpart γ-aminobutyraldehyde. The cyclic amino alcohol is then again dehydrogenated to form 2-pyrrolidone (5). The latter is rather stable, but can be hydrolyzed to a minor extent, which consecutively opens the route to propionic acid (9).26 The linear amino aldehyde is less stable and is most probably the origin of more side products, such as butyric acid (10), γ-hydroxybutyric acid (11) and γ-butyrolactone (12). The most important pathway C is initiated
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by the ring opening hydrogenolysis of pyrrolidine (2).28 H2 can be generated in situ in the watergas shift reaction of CO formed in decarbonylation reactions (cfr. infra).26 In this way n-butylamine (6) is formed, which is subsequently fully defunctionalized to propane (7) and other volatile compounds. The presence of propane (7) was proven by performing gas phase Fourier transform infrared spectroscopy (FTIR) on the headspace of the reaction in entry 1 (Table 1). n-Butylamine (6) was merely observed in trace amounts, since it is highly reactive and readily defunctionalized to propane (7) and other volatiles. This was demonstrated with a reaction under the same conditions, starting from the linear amine: n-butylamine (6) was converted for 92% and only traces of products could be found in solution, while CO, CO 2, NH3 and propane (7) were detected in the gas phase. The equilibrium between CO and CO2 was clearly in favor of CO2 (CO/CO2 = 0.04); this confirms the occurrence of the water-gas shift reaction under these conditions. Finally, if L-proline (1) is not converted quickly enough, it gets the opportunity to react through a double intermolecular condensation; this gives rise to the diketopiperazine cyclodi-L-prolyl (8). This reaction represents most of the conversion in the blank reaction (Table 1, entry 13). The detailed identification of the observed compounds is given in the supporting information.
Table 1. Pd catalyst screening for the decarboxylation of L-proline to pyrrolidine.a
1 2 3 4 5 6
Catalyst Pd/C Pd/Al2O3 Pd/BaSO4 Pd/CaCO3 Pd/MgAl2O4 Pd/ZrO2
Conversion [%] Pyrrolidine DKP b >99 2 99 6 99 10
