Research Article pubs.acs.org/journal/ascecg
Ru-Catalyzed Hydrogenation−Decarbonylation of Amino Acids to Bio-based Primary Amines Jasper Verduyckt,† Robin Coeck,† and Dirk E. De Vos*,† †
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
Downloaded via NEW MEXICO STATE UNIV on July 6, 2018 at 18:12:34 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
S Supporting Information *
ABSTRACT: Amino acids are considered to be a valuable renewable resource for the production of bio-based chemicals. Here, the Ru-catalyzed hydrogenation−decarbonylation toward primary amines is presented. In contrast to the direct Pdcatalyzed decarboxylation, this catalytic system operates at much lower temperatures, safeguarding the stability of the primary amines. Moreover, instead of producing CO2, the cleaved carbon is released as CH4, which can be recycled for other applications (e.g., energy purposes). After a general catalyst screening, the Ru-based system is optimized for the model reaction of L-valine in water, resulting in isobutylamine yields of up to 87%. Stronger acidity in the aqueous solution improves the stability of isobutylamine, but simultaneously promotes the parallel formation of 2-amino-3-methylbutane, which is the most important side product. This tradeoff is controlled by adding an appropriate amount of H3PO4. Besides pH, the other reaction parameters are also screened: H2 pressure, temperature, and catalyst-to-substrate ratio. Kinetic profiles are recorded to gain a thorough understanding of the reaction network and the product stability. Finally, the stability and broad applicability of the Ru/C catalyst are demonstrated. KEYWORDS: Ru, Indirect decarboxylation, Amino acids, Bio-based, Primary amines, Water
■
INTRODUCTION Primary, aliphatic amines are very important building blocks in the chemical industry; they are essential intermediates in the production of numerous agrochemicals, pharmaceuticals, surfactants, etc.1 For example, ethylamine and isopropylamine are used in the synthesis of herbicides, insecticides, and fungicides, such as simazine, bentazon, imazapyr, fenamiphos, and iprodione.1 In some cases, these amines can be produced via amination of the corresponding alcohol with ammonia using dehydration catalysts, such as zeolites, SiO2, or Al2O3, at temperatures up to 500 °C.1 Processes based on the “hydrogen borrowing” concept have become more important; in this case, supported metal catalysts based on Ni, Co, Cu, or Fe are used at temperatures up to 250 °C.1 Recently, even more active homogeneous catalysts have been developed, such as the Ru pincer and Ru triphos complexes, which allow to run this reaction at temperatures as low as 111 °C.2−4 A major drawback of these two processes is the resulting product mixture, which generally contains secondary and tertiary amines next to the desired primary amine. Therefore, a high excess of ammonia is often used to shift the equilibrium toward the primary amine.1−4 Another significant route to aliphatic amines is reductive amination of the corresponding carbonyl compound with ammonia and H2 as the reducing agent. This process adopts the same catalysts as the “hydrogen borrowing” concept and operates at atmospheric pressures and at temperatures up to 160 °C. Also, in this case, an excess of © 2017 American Chemical Society
ammonia is necessary to guarantee high selectivities to the primary amine.1 In a recent review, Froidevaux et al. describe different routes toward bio-based amines.5 Generally, these routes involve amination of bio-based compounds, such as isosorbide, 2,5bis(hydroxymethyl)furan, vanillin, etc. However, only from chitosan or amino acids can fully bio-based nitrogen-containing chemicals be obtained; in many other cases, the nitrogenous reactant (e.g. ammonia) is not recycled from a bio-based source, but rather directly provided by the Haber−Bosch process. In this work, we propose an alternative, sustainable route to primary, aliphatic amines based on the indirect decarboxylation of amino acids (i.e., via hydrogenation− decarbonylation). Amino acids are considered as a valuable renewable resource, since these nitrogen-containing compounds are readily available in a sustainable way via fermentation.6−8 Moreover, they will become more and more available via the hydrolysis of proteinrich waste streams.6 Press cakes from, e.g., soybeans and algae, poultry feathers, and dried distillers grains with solubles, are common waste fractions with a protein content of up to 90%.9 Sanders and co-workers estimate that if 10% of the fossil transportation fuel is replaced by biomass-derived fuels, over Received: December 23, 2016 Revised: February 8, 2017 Published: February 20, 2017 3290
DOI: 10.1021/acssuschemeng.6b03140 ACS Sustainable Chem. Eng. 2017, 5, 3290−3295
Research Article
ACS Sustainable Chemistry & Engineering 100 million tons of protein will be accessible.10 Separation of the resulting amino acid mixture after protein hydrolysis remains a challenge; however, new technologies, such as electrodialysis, adsorption, and fractional precipitation, are emerging.11−16 Poliakoff and colleagues have emphasized the role of amino acids as platform chemicals in the bio-based industry.6 Increasing attention is given to the catalytic transformation of these compounds to, e.g., nitriles, via oxidative decarboxylation,17−20 or amino alcohols, via hydrogenation.21,22 Direct decarboxylation, on the other hand, constitutes an interesting method to convert these amino acids to amines. There are reports on decarboxylation reactions of amino acids using enzymes, as well as using Cu-based catalysts and organocatalysts (e.g., α,β-unsaturated ketones that resemble the pyridoxal 5′-phosphate cofactor); however, these suffer from several shortcomings.11,12,23,24 The former, enzymatic technology requires an expensive cofactor and strict control of the pH, and results in a low volumetric productivity.11,12,25 The latter, homogeneous catalysts require the use of an organic solvent, and a high catalyst loading; the organocatalyst cannot be easily recycled.23,24 Heterogeneous chemocatalysis might overcome these issues. The direct Pd-catalyzed decarboxylation was first described for the synthesis of long-chain alkanes from fatty acids at 300 °C.26,27 Recently, this technology was extended to the conversion of the bio-based itaconic and (pyro)glutamic acid to methacrylic acid and 2-pyrrolidone, respectively, at 250 °C in water.28,29 Very recently, we were able to selectively produce pyrrolidine from L-proline in water at 235 °C.30 A careful modification of Pd with Pb and a low H2 pressure were necessary to suppress side reactions, such as ring opening hydrogenolysis and multiple dehydrogenation. However, when this optimized catalytic system was applied for the decarboxylation of L-leucine, only 15% yield of the corresponding primary amine was obtained (see Scheme S1 in the Supporting Information). Therefore, the direct Pd-catalyzed decarboxylation is not fit for the production of primary amines from amino acids. In this work, we present an alternative chemocatalytic approach for the decarboxylation of amino acids, inspired by the hydrogenation−decarbonylation of fatty acids.31−33 This technology operates at much lower temperatures than the Pdbased system, which is crucial for the stability of the primary amine products.
Figure 1. Catalyst screening for the hydrogenation−decarbonylation of L-valine to isobutylamine. Reaction conditions: L-valine (0.2 mmol), 5 mol % Me, 1 equiv H3PO4, water (2 mL), 6 h, 150 °C, 2 bar N2 and 40 bar H2.
dispersion of Ru (23%; see Table S1 in the Supporting Information). Next, the amount of H3PO4 added was varied from 0 to 3 equiv using Ru/C, since the pH of the aqueous solution is considered to be an important parameter for this reaction (Figure 2). Without the addition of H3PO4, the reactivity of L-
■
RESULTS AND DISCUSSION First, several commercial noble-metal catalysts were screened for the hydrogenation−decarbonylation of L-valine (1) to isobutylamine (3) using 5 mol % metal at 150 °C and 40 bar H2 for 6 h in water (Figure 1). One equiv of H3PO4 was added, since an acidic aqueous medium favors the initial carboxylic acid hydrogenation.21,22 Pt- and Pd-based catalysts show practically no conversion of L-valine (1), while Rh/C and Rh/Al2O3 exhibit conversions of 24% and 21%, respectively. In these cases, a large part is hydrogenated to valinol (2); the rest is mainly decarbonylated to yield isobutylamine (3). However, only the Ru-based catalysts form a significant amount of isobutylamine (3). Ru/Al2O3 yields 24% isobutylamine (3) at 57% conversion, while Ru/C produces 42% isobutylamine (3) at full conversion. Besides the side products found in solution, there is a great amount of carbon loss to the gas phase (cf r. inf ra). The high activity of Ru/C can be attributed to the high specific surface area of the support (640 m2/g) and to the high
Figure 2. Variation of the amount of H3PO4 added. Reaction conditions: L-valine (0.2 mmol), 5 mol % Ru/C, water (2 mL), 6 h, 150 °C, 2 bar N2 and 40 bar H2.
valine (1) was indeed lower, resulting in a conversion of only 79%. Moreover, the pH of the reaction mixture increased from 6 to 10 during the reaction, because of the formation of basic (side) products (cf r. inf ra). This led to the production of 20% isobutyl alcohol (6), which was the main product left in solution. In the other cases, the reaction solution remained acidic (pH ≤ 6). This afforded a switch in selectivity toward the formation of isobutylamine (3), as well as full conversion. However, when only 1 equiv H3PO4 was added, there was still a great amount of carbon lost from solution. Conversely, the addition of 2 equiv of H3PO4 remarkably increased the yield of isobutylamine (3) to 69%. Further increasing the amount of 3291
DOI: 10.1021/acssuschemeng.6b03140 ACS Sustainable Chem. Eng. 2017, 5, 3290−3295
Research Article
ACS Sustainable Chemistry & Engineering
reported at rather low temperatures.33,34 Furthermore, 2amino-3-methylbutane (4) is simultaneously formed with isobutylamine (3) in a parallel path. These observations result in the reaction network depicted in Scheme 1. The formation of other side products, such as diisobutylamine (5), isobutyl alcohol (6), and propane (7), is initiated by the dehydrogenation of isobutylamine (3). The resulting imine can then be hydrolyzed to the aldehyde. Both reactive intermediates can react via a condensation with isobutylamine (3) to produce the secondary amine after hydrogenation. However, isobutyraldehyde can also undergo hydrogenation or decarbonylation, yielding isobutyl alcohol (6) and propane (7), respectively. The presence of propane (7) and other volatiles was illustrated via gas-phase Fourier transform infrared (FTIR) spectroscopy (see Figure S1 in the Supporting Information). Remarkably, especially CH4 and only traces of CO were detected, meaning that CO is hydrogenated to CH4 under the reaction conditions. This is substantiated by the concomitant detection of intermediates in this hydrogenation reaction: traces of formaldehyde were observed in the gas phase and traces of methanol were detected in the liquid phase. Moreover, the lowtemperature methanation of CO adsorbed on Ru has been reported.35,36 In contrast to other decarboxylation reactions, the carbon that is cleaved off in this reaction is therefore not lost in the form of CO2; rather, it can be captured as CH4. CH4 can be separated from H2 in the gas phase via an adsorption or membrane process.37,38 To rule out any acid-catalyzed C−C cleavage, L-valinol (2) was reacted under the same conditions as L-valine (1). In the presence of Ru, the same product distribution was obtained as in the reaction with L-valine (1), while, in the absence of Ru, L-valinol (2) was not converted at all. Moreover, to corroborate the reaction mechanism, additional kinetic experiments starting from L-valinol (2) were performed (Figure S2 in the Supporting Information). When the H2 pressure was reduced from 40 bar to 20 bar, the conversion of L-valinol (2) to isobutylamine (3) was twice as fast. This confirms the route via dehydrogenation and subsequent decarbonylation. Finally, by comparing the two kinetic profiles in Figure 3, one can derive that (i) the amino acid is indeed hydrogenated faster in a more acidic environment and, more importantly, (ii) isobutylamine (3) is much more stable in a more acidic solution. The side reactions initiated by dehydrogenation occur to a far lesser extent at 2 equiv of H3PO4 and, accordingly, the yield of isobutylamine (3) only decreases slightly with time. On the other hand, the parallel formation of 2-amino-3-methylbutane (4) increases as the acidity increases, resulting in a lower maximum yield of isobutylamine (3) at 2 equiv of H3PO4, viz. 71% after 1 h, 30 min. Conversely, at 1 equiv of H3PO4, an optimal yield of 82%
H3PO4 to 3 equiv decreases the yield of isobutylamine (3) again to 65%, because of the enhanced formation of 2-amino-3methylbutane (4). To get a more precise view on the origin and stability of the products and to substantiate the indirect decarboxylation mechanism, two kinetic experiments are presented, in which the formation of the different products was followed as a function of time for the reaction with 1 and 2 equiv of H3PO4 (Figure 3). From both experiments, it is clear that L-valine (1)
Figure 3. Time course of the hydrogenation-decarbonylation of Lvaline using 1 equiv (top) or 2 equiv (bottom) of H3PO4. Reaction conditions: L-valine (0.2 mmol), 5 mol % Ru/C, H3PO4, water (2 mL), 150 °C, 2 bar N2 and 40 bar H2.
is quickly hydrogenated to valinol (2), after which the α-amino alcohol is converted to isobutylamine (3). This primary amine is most probably formed via decarbonylation of the amino aldehyde, which is in equilibrium with valinol (2). Ru-catalyzed decarbonylation of octadecanal and furfural has already been
Scheme 1. Reaction Network for the Hydrogenation−Decarbonylation of L-Valine
a
The detailed identification of the observed (numbered) compounds is given in the Supporting Information. 3292
DOI: 10.1021/acssuschemeng.6b03140 ACS Sustainable Chem. Eng. 2017, 5, 3290−3295
Research Article
ACS Sustainable Chemistry & Engineering isobutylamine (3) was realized after 1 h, 20 min. The Supporting Information lists a detailed identification of all observed compounds. Clearly, there is a tradeoff between the stability of isobutylamine (3) and the parallel formation of 2-amino-3methylbutane (4), determined by the acidity of the aqueous solution. Therefore, the amount of H3PO4 added was finetuned for the reaction at 1 h, 20 min, which was the optimal time at both 1 and 2 equiv of H3PO4 (see Figure 4). At 1.1
Table 1. Hydrogenation−Decarbonylation of Natural Amino Acids
Figure 4. Fine-tuning of the amount of H3PO4 added. Reaction conditions: L-valine (0.2 mmol), 5 mol % Ru/C, water (2 mL), 1 h 20 min, 150 °C, 2 bar N2 and 40 bar H2.
equiv of H3PO4, the yield of isobutylamine (3) slightly increased to 84%; however, there was still some isobutyl alcohol (6) remaining. Further increasing the amount of H3PO4 to 1.2 equiv further increased the yield to 87% isobutylamine (3), with 6% 2-amino-3-methylbutane (4) as the only side product in solution. Adding more than 1.2 equiv of H3PO4 gives rise to more additional 2-amino-3-methylbutane (4) than can be compensated by the stabilization of isobutylamine (3), and, therefore, the yield decreases again. Further screening of the reaction conditions was then performed with 1.2 equiv of H3PO4 at 1 h, 20 min. Figure S3 in the Supporting Information shows that 20 bar H2 suffices to reach full conversion and more than 80% yield of isobutylamine (3), using 5 mol % Ru/C at 150 °C. Moreover, the catalyst-tosubstrate ratio can be decreased to only 1 mol % Ru without significantly affecting the yield of isobutylamine (3), although a longer reaction time is necessary in this case (see Figure S4 in the Supporting Information). Similarly, the reaction temperature can be decreased to 135 °C, still yielding 84% isobutylamine (3) after 6 h (see Figure S5 in the Supporting Information). Furthermore, the Ru/C catalyst could be used for at least three runs without an appreciable loss in isobutylamine (3) yield, and no Ru was leached from the catalyst (
