Ru-Catalyzed HydrogenationâDecarbonylation of Amino Acids to Bio

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Ru-catalyzed hydrogenation-decarbonylation of amino acids to biobased primary amines Jasper Verduyckt, Robin Coeck, and Dirk E. De Vos ACS Sustainable Chem. Eng., Just Accepted Manuscript • Publication Date (Web): 20 Feb 2017 Downloaded from http://pubs.acs.org on February 21, 2017

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ACS Sustainable Chemistry & Engineering

Ru-catalyzed hydrogenation-decarbonylation of amino acids to biobased primary amines Jasper Verduyckt,a Robin Coeck,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. E-mail: [email protected]

ABSTRACT. Amino acids are considered as a valuable renewable resource for the production of biobased chemicals. Here, the Ru-catalyzed hydrogenation-decarbonylation towards primary amines is presented. In contrast to the direct Pd-catalyzed 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 CH 4, which can be recycled for e.g. energy purposes. After a general catalyst screening, the Ru-based system was 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 parallel formation of 2-amino-3-methylbutane, which is the most important side product. This trade-off was controlled by adding an appropriate amount of H3PO4. Next to pH, the other reaction parameters were screened as well: H2 pressure, temperature and catalyst-to-substrate ratio. Kinetic profiles were recorded to gain a thorough understanding of the

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reaction network and of the product stability. Finally, the stability and broad applicability of the Ru/C catalyst were demonstrated.

KEYWORDS. Ru, indirect decarboxylation, amino acids, biobased, 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 towards the primary amine.1–4 Another significant route to aliphatic amines is reductive amination of the corresponding carbonyl compound with ammonia and H2 as 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 ammonia is necessary to guarantee high selectivities to the primary amine.1


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In a recent review, Froidevaux et al. describe different routes towards biobased amines. 5 In general, these routes involve amination of biobased compounds, such as isosorbide, 2,5-bis(hydroxymethyl)furan, vanillin, etc. However, only from chitosan or amino acids fully biobased nitrogen containing chemicals can be obtained; in many other cases, the nitrogenous reactant, e.g. ammonia, is not recycled from a biobased 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 hydrogenationdecarbonylation. 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 protein rich 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 coworkers estimate that if 10% of the fossil transportation fuel is replaced by biomass-derived fuels, over 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, like electrodialysis, adsorption and fractional precipitation, are emerging.11–16 Poliakoff and colleagues emphasize the role of amino acids as platform chemicals in the biobased 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-


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unsaturated ketones that resemble the pyridoxal 5’-phosphate cofactor; however, these suffer from several shortcomings.11,12,23,24 The former, enzymatic technology needs an expensive cofactor, a 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, a high catalyst loading and 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 biobased itaconic and (pyro)glutamic acid to methacrylic acid and 2pyrrolidone, 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, like 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 (Supporting Information, Scheme S1). The direct Pd-catalyzed decarboxylation is thus 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 Pd-based system, which is crucial for the stability of the primary amine products.

Results and discussion First, several commercial noble metal catalysts were screened for the hydrogenationdecarbonylation of L-valine (1) to isobutylamine (3) using 5 mol% metal at 150°C and 40 bar H2


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for 6 h in water (Figure 1). 1 eq. 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 a conversion 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 lot of carbon loss to the gas phase (cfr. infra). 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 dispersion of Ru (23%, Table S1).

Figure 1. Catalyst screening for the hydrogenation-decarbonylation of L-valine to isobutylamine. Reaction conditions: L-valine (0.2 mmol), 5 mol% Me, 1 eq. H3PO4, water (2 mL), 6 h, 150°C, 2 bar N2 and 40 bar H2. Next, the amount of H3PO4 added was varied from 0 to 3 eq. using Ru/C, since the pH of the aqueous solution is considered to be an important parameter for this reaction (Figure 2). Without


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the addition of H3PO4 the reactivity of L-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, due to the formation of basic (side) products (cfr. infra). 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 towards the formation of isobutylamine (3), as well as full conversion. However, when only 1 eq. H3PO4 was added, there was still a lot of carbon lost from solution. Conversely, the addition of 2 eq. remarkably increased the yield of isobutylamine (3) to 69%. Further increasing the amount of H3PO4 to 3 eq. decreases the yield of isobutylamine (3) again to 65%, owing to the enhanced formation of 2-amino-3-methylbutane (4).

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. 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


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2 eq. H3PO4 (Figure 3). From both experiments it is clear that

L-valine

(1) is quickly

hydrogenated to valinol (2), after which the -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 reported at rather low temperatures. 33,34 Furthermore, 2-amino-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, like 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 spectroscopy (Figure S1). 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 in the liquid phase. Moreover, low temperature 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 thus 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


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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). When the H2 pressure was lowered 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, comparing the two kinetic profiles in Figure 3, one can derive that the amino acid is indeed hydrogenated faster in a more acidic environment and more importantly, that 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 eq. H3PO4 and accordingly, the yield of isobutylamine (3) only decreases slightly with time. On the other hand, parallel formation of 2-amino-3-methylbutane (4) increases with increasing acidity, resulting in a lower maximum yield of isobutylamine (3) at 2 eq. H3PO4, viz. 71% after 1 h 30 min. Conversely, at 1 eq. H3PO4 an optimal yield of 82% isobutylamine (3) was realized after 1 h 20 min. The Supporting Information lists a detailed identification of all observed compounds.


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Figure 3. Time course of the hydrogenation-decarbonylation of L-valine using 1 eq. (top) or 2 eq. (bottom) 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. 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. Clearly, there is a trade-off between the stability of isobutylamine (3) and the parallel formation of 2-amino-3-methylbutane (4), determined by the acidity of the aqueous solution. Therefore the amount of H3PO4 added was fine-tuned for the reaction at 1 h 20 min, which was the optimal time at both 1 and 2 eq. H3PO4 (Figure 4). At 1.1 eq. H3PO4 the yield of isobutylamine (3) slightly increased to 84%; there was however still some isobutyl alcohol (6) remaining. Further increasing the amount of H3PO4 to 1.2 eq. further increased the yield to 87%


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isobutylamine (3), with 6% 2-amino-3-methylbutane (4) as the only side product in solution. Adding more than 1.2 eq. H3PO4 gives rise to more additional 2-amino-3-methylbutane (4) than can be compensated by the stabilization of isobutylamine (3), and so the yield decreases again.

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. Further screening of the reaction conditions was then performed with 1.2 eq. H3PO4 at 1 h 20 min. Figure S3 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-to-substrate 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 (Figure S4). Similarly, the reaction temperature can be decreased to 135°C, still yielding 84% isobutylamine (3) after 6 h (Figure S5). 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 (99

22 c

21

2

99

61

19

3

>99

76

6d

4

>99

84

7d

5

>99

40

27

6

>99

36

23 c

7

>99

50

27 d

8

>99

33 d

26

9

>99

51 d

17

10

12

>99 d

-

-

a

Conversion. b Selectivity. c Methylamine is very volatile, yields are therefore underestimated by analyzing the aqueous solution. d The enantiopurity is lost in the process.


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Xa [%]

Major product

Sb [%]

Minor product

Sb [%]

11

>99

e

e

e

e

12

>99

36 d

20

13

>99

31 d

26

14

>99

19

15

15

>99

14 d

11

16

>99

42 d

32

17

80

36 d

16

18

90

32 d

7

19

Ru-Catalyzed HydrogenationâDecarbonylation of Amino Acids to Bio

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