Low-Temperature Reductive Amination of Carbonyl Compounds over

Dec 19, 2018 - Ru/Nb2O5·nH2O exhibited high catalytic activity toward reductive amination of biomass-derived carbonyl compounds under mild reaction ...
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Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Low-Temperature Reductive Amination of Carbonyl Compounds over Ru Deposited on Nb2O5·nH2O Dian Deng,† Yusuke Kita,† Keigo Kamata,† and Michikazu Hara*,†,‡ †

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Laboratory for Materials and Structures, Institute of Innovative Research, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8503, Japan ‡ Advanced Low Carbon Technology Research and Development Program (ALCA), Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi 332-0012, Japan S Supporting Information *

ABSTRACT: The synthesis of amines from biomass-based feedstock is in high demand given the depletion of fossil fuels. Reductive amination is one of the most straightforward synthetic methods to obtain a variety of amines; however, it is prone to denaturation and side reactions under the harsh conditions required. By fine-tuning the surface acidity of niobic acid (Nb2O5·nH2O), we evaluated the relationship between catalytic activity on reductive amination with the amount of acid sites on Nb2O5·nH2O. Ru/Nb2O5·nH2O reduced at 300 °C was found to be an efficient catalyst for the reductive amination of biomass-derived carbonyl compounds under low temperature to afford primary amines, without the formation of secondary amines or hydrogenated products. KEYWORDS: Reductive amination, Biomass, Ruthenium, Niobic acid, Acid site effect

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INTRODUCTION Amine functionality plays a vital role in organic chemistry due to its prominence in natural products, pharmaceuticals, and agrochemicals.1 Amines serve as fundamental intermediates in organic synthesis, are used as bases in many synthetic transformations, and are imperative building blocks in a variety of common polymers. Thus, many synthetic methods have been developed because of their importance. Among them, reductive amination, where a mixture of a carbonyl compound and an amine is treated with a reductant in a one-pot fashion, is one of the most powerful and versatile tools for the synthesis of various amines.2 Metal hydride reductants are wellrepresented, although they have some drawbacks associated with workup and waste management.3,4 Catalytic methods have advantages in terms of atom efficiency and can be practical alternatives to well-studied processes using metal hydride reductants if the catalyst cost contributions are acceptable. Several enzymes as well as homogeneous and heterogeneous catalysts were previously reported to be effective for reductive amination.5 It is noteworthy that commercially available, inexpensive catalysts were reported to mediate reductive amination. 6,7 For catalytic reductive amination, molecular hydrogen (H2) is considered as the most atom-economical and environmentally friendly reducing reagent, particularly in large-scale production. Indeed, this process was applied to the industrial production of lower alkylamines (C2−C5) and cycloaliphatic amines.8 Nevertheless, carbonyl feedstock is mainly derived from the petrochemical © XXXX American Chemical Society

industry. Thus, the replacement of fossil resources with renewable materials is of particular interest in view of diminishing fossil fuel resources. Recent developments in biorefinery processes have enabled the production of various renewable raw materials. For carbonyl compounds, sugarderived furfural and 5-hydroxylmethylfurfural (HMF) are available on a large scale and are emerging as some of the most promising value-added biomass-derived platform chemicals. Some catalytic systems have been developed to date for the reductive amination of biomass-derived carbonyl compounds, including levulinic acid,9−19 2,5-diformylfuran,20 furfural,21−23 glycolaldehyde,24 and HMF.25,26 Although many efforts have been dedicated to the development of efficient synthetic methods, the use of biomass-based compounds is often burdened with denaturation of the substrates and sequential reactions such as the hydrogenation of C=O bonds as well as heteroaromatics (Table S1). Therefore, there is a compelling requirement for efficient and selective catalysts that enable the reductive amination of biomass-based carbonyl compounds under mild conditions. Our group has recently conducted a detailed investigation into the performance of a series of Ru-deposited heterogeneous catalysts for the reductive amination of furfural.27 The results clearly showed that the support has a significant effect Received: August 29, 2018 Revised: November 26, 2018 Published: December 19, 2018 A

DOI: 10.1021/acssuschemeng.8b04324 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

Table 1. Specific Surface Area, CO-Pulse Chemisorption Results, and Amounts of Acid Sites in the Supported Ru Catalystsa physical property

acidity

entry

catalyst

specific surface area (m2/g)a

Ru particle size (nm)b

Ru dispersion (%)c

Lewis acid (μmol/g)d

Brønsted acid (μmol/g)d

1 2 3 4 5

Ru/Nb2O5·nH2O-200 Ru/Nb2O5·nH2O-300 Ru/Nb2O5·nH2O-400 Ru/Nb2O5·nH2O-500 Ru/Nb2O527

148 144 110 72 112

2.9 5.8 6.6 11.6 5.5

46 22 20 12 24

219 224 176 100 175e

21 33 27 18 37e

a

Specific surface area was obtained from BET measurements. bRu dispersion determined with CO-pulse titration technique. The stoichiometry of CO/Ru = 0.6 was assumed.31 cThe Ru particle size (nm) is estimated from the dispersion. dAmounts of acid sites were estimated from pyridineadsorbed FT-IR measurements.30 eAcid amount was recalculated by using identical coefficients with other samples. The same equipment was used for H2-temperature-programmed reduction (H2-TPR) experiments. Each sample (ca. 50 mg) was placed in a U-shape quartz reactor fitted with a thermocouple for continuous temperature measurement. The sample was heated to 800 °C (10 °C min−1) under a flow of 4.99% H2/Ar. The effluent gas was passed through a molecular sieve trap to remove water and then through a thermal conductivity detector (TCD). For the unreduced precursor of Ru/Nb2O5·nH2O, approximately 50 mg of Ru(NO)(NO3)3−Nb2O5·nH2O was mounted in a quartz tube, and the sample was heated with a ramp rate of 3.33 °C min−1 from 50 to 800 °C in a flow of 4.99% H2/Ar (30 mL min−1). Procedure for IR Measurements. The amounts of Lewis and Brønsted acid sites on the supported catalysts were estimated using Fourier transform infrared spectroscopy (FT-IR; FT/IR 6100 Jasco) measurements for pyridine-adsorbed samples at room temperature with a mercury cadmium telluride (MCT) detector. Samples were pressed into self-supporting disks (20 mm diameter, ca. 20 mg) and placed in an IR cell attached to a closed glass-circulation system. Prior to pyridine adsorption, the sample was dehydrated by heating at 200 °C for 1 h under vacuum. The intensities of the bands at 1450 cm−1 (pyridine coordinatively bonded to Lewis acid sites) and 1540 cm−1 (pyridinium ions formed by Brønsted acid sites) were plotted against the amounts of pyridine adsorbed on the Lewis and Brønsted acid sites of the samples, respectively. The samples were exposed to pyridine vapor at room temperature until a saturation intensity of the absorption peak was obtained. The Lewis acid density was calculated from the integrated area of the peak at 1450 cm−1 and the reported formula.30 The amounts of Brønsted and Lewis acid sites on the samples were estimated from the maximum band intensities and molecular absorption coefficients at 1450 and 1540 cm−1. For quantification, molar integral extinction coefficients of 1.65 and 2.79 cm2 μmol−1 were used for the characteristic bands of pyridinium ions formed by Brønsted acid sites and pyridine bound to Lewis acid sites, respectively. FT-IR spectra of adsorbed CO (CO-IR) were also recorded. The samples were initially pressed to make self-supporting disks (20 mm diameter, ca. 20 mg) and placed in an IR cell attached to a closed glass-circulation system. Prior to measurement, the disks were pretreated under H2 (ca. 30 kPa) at 200 °C for 1 h, followed by exposure to a vacuum for 1 h. CO as a probe molecule was then introduced into the IR system at room temperature until adsorption became saturated. Uncoordinated CO on Ru in the system was removed in vacuo for 0.5 h for the FT-IR measurement. Catalytic Reductive Amination of Furfural. The catalytic reaction was conducted in an 18 mL stainless-steel autoclave equipped with a magnetic stirrer. Catalyst (20 mg) was loaded into the reactor with 5 mL of NH3/MeOH (8 mmol NH3) and 0.5 mmol of furfural. No pretreatment of the catalyst was conducted prior to reaction. The autoclave was purged with hydrogen several times to remove air and then pressurized with H2 at 3 MPa. The autoclave was heated under stirring to initiate the reaction. After completion of the reaction, the autoclave was cooled to room temperature. Chlorobenzene was added to the reaction mixture as an external standard, and the mixture was diluted with methanol and analyzed by gas

on the activity and selectivity. The use of an acidic Nb2O5 support resulted in excellent performance with respect to both the activity and selectivity toward primary amine products compared to other supports. However, there is only a limited understanding of the role of different acid sites in the reactions. Therefore, to clarify the influence of the type and amount of acid on reductive amination, we have focused on niobic acid (Nb2O5·nH2O), which possesses both Brønsted and Lewis acid sites.28,29 The surface acidity of Nb2O5·nH2O was fine-tuned by systematically changing the reduction temperature. We investigated the catalytic activity of the supported Ru catalyst toward the reductive amination of furfural under mild reaction conditions. In addition, mechanistic studies were conducted to probe the key factors in the reductive amination of furfural.

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EXPERIMENTAL SECTION

Materials. Niobic acid (Nb 2 O 5 ·nH 2 O) was supplied by Companhia Brasileira de Metallurgia e Mineraçaó (CBMM). Ruthenium(III) nitrosyl nitrate solution (Sigma-Aldrich, 1.4 wt % Ru) was used as a ruthenium precursor. All other reagents were obtained from Sigma-Aldrich, Tokyo Chemical Industry, Wako Pure Chemical Industries, and Kanto Chemical and were used as received. Preparation of Ru/Nb2O5·nH2O-X Catalysts. In a typical procedure for the preparation of 1 wt % Ru/Nb2O5·nH2O, the Nb2O5·nH2O support was added into aqueous Ru(NO)(NO3)3 solution, and the mixture was evaporated at 60 °C until dry. The solid was then further dried under vacuum at 50 °C for 2 h. The dried sample was reduced at 200, 300, 400, or 500 °C under 5% H2/Ar flow for 2 h with a heating rate of 3.33 °C min−1. The samples are denoted as Ru/Nb2O5·nH2O-X, where X is the reduction temperature. For comparison, Ru/Nb2O5 reference catalysts were prepared.27 Characterization. Brunauer−Emmett−Teller (BET) specific surface areas of the samples were measured by nitrogen adsorption−desorption at −196 °C (Quantachrome Nova-4200e). Prior to the adsorption measurements, the samples were degassed in situ under vacuum at 150 °C for 1 h. The BET surface area was determined using the multipoint BET algorithm in the P/P0 range from 0.05 to 0.3. X-ray diffraction (XRD; Ultima IV, Rigaku) patterns of all samples were obtained using Cu Kα radiation (40 kV, 40 mA) in the 2θ range of 15−85°. X-ray photoelectron spectroscopy (XPS; ESCA-3200 Shimadzu) measurements were performed using Mg Kα radiation (1486.6 eV). Samples were pressed into pellets and fixed on doublesided carbon tape. The binding energies were calibrated using sputtered Au (4f7/2 peak at 84 eV). The metal dispersion and particle size were further characterized by CO-pulse chemisorption (BELCAT-A BEL Japan) at 50 °C using a He flow of 30 mL min−1 with intermittent pulses of 50 mL min−1 (9.53% CO in He). Before these measurements, the sample was reduced under a H2/Ar flow at 200 °C for 1 h. Adsorbed hydrogen on the reduced catalysts was removed by purging with He (30 mL min−1) at 50 °C for 15 min. The Ru dispersion was calculated by assuming a CO/Ru stoichiometry of 0.6:1. B

DOI: 10.1021/acssuschemeng.8b04324 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering chromatography (GC; GC-17A, Shimadzu) with an InertCap 17MS capillary column (internal diameter = 0.25 mm, length = 30 m) and a flame ionization detector (FID). Larger-Scale Reductive Amination of Furfural. The largerscale reductive amination was conducted in a 100 mL stainless-steel autoclave equipped with a glass vessel containing a magnetic stirring bar (TVS-N2, Taiatsu Techno Corporation). Catalyst (500 mg) was loaded into the reactor with 0.5 mmol of furfural and 80 mL of MeOH. The autoclave was purged with gaseous ammonia several times to remove air and then pressurized with H2 at 3 MPa. After completion of the reaction, the autoclave was cooled to room temperature, and the reaction mixture was evaporated to dryness. The crude product was purified by flash column chromatography (EPCLC AI-580-S, Yamazen) using a Yamazen Amino Column (EtOAc/ hexane = 3:7) to give the analytically pure 1 (1.27 g, 87% yield).

may also explain the slight increase in Lewis acidity over Ru/ Nb2O5·nH2O-X, because pyridine can react with cationic metal species. The acidity of the catalysts reduced at higher temperature decreased compared with that for Ru/Nb2O5· nH2O-300, which could be a result of the decrease in Nb2O5· nH2O surface area. Reductive Amination of Furfural over Ru/Nb2O5· nH2O. All of these materials were tested in the benchmark reductive amination of furfural using ammonia and hydrogen to give furfurylamine (1) under milder conditions than the optimized conditions in our previous report (Table 2). In addition to the desired product, some byproducts were formed. N-Furfurylidenefurfurylamine (2) is the possible intermediate of reductive amination generated through the condensation of 1 with furfural. 2,4,5-Tris(2-furyl)imidazoline (3) is irreversibly formed in the absence of a catalyst. To our delight, overalkylated products were not obtained in all cases. Ru/ Nb2O5·nH2O-300 gave an excellent yield (89%) of the desired amine as the sole product (entry 2). Even at 50 °C, reductive amination proceeded smoothly with Ru/Nb2O5·nH2O-300 (entry 3). The durability of the Ru/Nb2O5·nH2O was demonstrated by recycling experiments on the reductive amination of furfural, and the recovered catalyst could be reused without significant decrease even after two reuses (Figure S2). Large-scale reductive amination of furfural was conducted to confirm the effectiveness of the Ru/Nb2O5·nH2O catalyst (Scheme 1). The catalyst reduced at 200 °C showed poor reactivity (entry 1). Further increase in the reduction temperature led to less active catalysts (73 and 62% yields). The reference Ru/Nb2O5 catalyst showed similar catalytic performance to that for Ru/Nb2O5·nH2O-400 (entries 4 vs 6). While Ru/Nb2O 5·nH2O-200 possessed a high metal dispersion, it gave a lower yield of 1, which indicates that another factor is involved in the reductive amination of carbonyl compounds catalyzed by Ru/Nb2O5·nH2O. The oxidation states and electronic states of Ru were examined by hydrogen-temperature-programmed reduction (H2-TPR), XPS, and CO-adsorbed FT-IR. The reductive behavior and degree of reduction of the Ru/Nb2O5·nH2O catalysts were investigated using H2-TPR (Figure 2). The unreduced precursor presented two hydrogen consumption peaks: a shoulder peak at 154 °C and a major peak at 170 °C (Figure S1), which were ascribed to the decomposition of ruthenium nitrosyl nitrate into RuO2 and the reduction of RuO2, respectively. The TPR profile indicated that the Ru species could be completely reduced before 200 °C during the catalyst preparation stage. The H2-TPR profiles of all reduced samples showed an identical peak at around 90 °C, and the TCD signal intensity decreased in the following order: Ru/Nb2O5·nH2O200 > Ru/Nb2O5·nH2O-300 > Ru/Nb2O5·nH2O-400 > Ru/ Nb2O5·nH2O-500. The amounts of RuO2 species decreased with increased reduction temperature and could be assigned to the reduction of RuO2. XPS was used to measure the near-surface oxidation states of Ru on Nb2O5·nH2O (Figure 3). As a result of the overlapping of the Ru 3d3/2 and C 1s peaks and that of the Ru 3p3/2 and Nb 3s peaks, only the Ru 3d5/2 peak was used for discussion. For Ru/Nb2O5·nH2O-200, the Ru 3d5/2 region was centered at 281.2 eV > 280.4 eV for Ru/Nb2O5·nH2O-300 > 280.3 eV for Ru/Nb2O5·nH2O-400 > 280.1 eV for Ru/Nb2O5·nH2O-500. RuO2 with an oxidation state of Ru4+ and metallic Ru have been reported with average binding energy (BE) values of ca. 281.4 and 279.8 eV, respectively.32 There is an obvious shift in

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RESULTS AND DISCUSSION Catalyst Preparation and Characterization. The supported ruthenium catalysts were prepared as described previously with some modifications.12 Table 1 shows the physicochemical properties for the Ru/Nb2O5·nH2O samples. Data for the reference Ru/Nb2O5 that was reported in our previous work27 are also listed for comparison (entry 5). The amounts of acid were normalized using identical coefficients. As expected, the surface area decreased from 148 to 72 m2 g−1 as the reduction temperature was increased from 200 to 500 °C. The Ru particle size and dispersion of 1 wt % Ru/Nb2O5· nH2O catalysts were measured by CO-pulse chemisorption. The Ru dispersion for the 1 wt % Ru/Nb2O5·nH2O catalysts decreased with an increase in the reduction temperature. The metal dispersion was inversely proportional to the particle size; therefore, it is reasonable that the Ru particle size increased with the reduction temperature. The sintering of the Ru particles on Ru/Nb2O5·nH2O-500 could contribute to a decline of the ruthenium dispersion at this higher reduction temperature. The type and concentration of acid sites on Ru/Nb2O5· nH2O series catalysts were examined via FT-IR measurements of the adsorbed pyridine (Table 1 and Figure 1). When the

Figure 1. FT-IR spectra for pyridine adsorbed on Ru/Nb2O5·nH2O catalysts prepared at different reduction temperatures.

reduction temperature was increased from 200 to 500 °C, the surface acidity showed a volcano-shaped trend, with the maximum value over the catalyst reduced at 300 °C. The decrease in the number of Brønsted acid sites in Ru/Nb2O5· nH2O-X relative to the parent Nb2O5·nH2O can be accounted for by the ion exchange of bridging hydroxyl group protons for Ru3+ ions and by partial removal of the surface acidic OH groups. The presence of incompletely reduced RuOx species C

DOI: 10.1021/acssuschemeng.8b04324 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering Table 2. Reduction Amination of Furfural over Supported Ruthenium Catalystsa

yield (%) entry

catalyst

conv. (%)

1

2

3

1 2 3b 4 5 6

Ru/Nb2O5·nH2O-200 Ru/Nb2O5·nH2O-300 Ru/Nb2O5·nH2O-300 Ru/Nb2O5·nH2O-400 Ru/Nb2O5·nH2O-500 Ru/Nb2O5

>99 >99 >99 >99 >99 >99

31 89 74 73 62 76

49 trace 2 16 29 5

2 trace 7 trace 6 trace

Reaction conditions: catalyst (0.02 g), furfural (0.5 mmol), NH3 in MeOH (1.6 M, 5 mL), H2 (4 MPa), 70 °C, 4 h. Conversion and yield were determined by GC analysis. bRun at 50 °C for 12 h.

a

character, even though all catalysts were exposed to air before the XPS measurements. This result and the H2-TPR profile together demonstrate that the Ru/Nb2O5·nH2O-200 catalyst is incompletely reduced, perhaps as a result from the strong interaction between the support and metal. The electronic states of Ru nanoparticles have a significant effect on the selectivity toward reductive amination or hydrogenation of the furan ring.27 To further probe the electronic state of ruthenium particles on Nb2O5·nH2O, COadsorbed FT-IR measurements were conducted (Figure 5). Three bands were observed for the Ru/Nb2O5·nH2O-300 catalyst. The bands at around 2026 cm−1 are attributed to linearly adsorbed CO on metallic Ru. The two bands appeared at 2082 and 2142 cm−1 are assigned to CO species linearly adsorbed on partially oxidized Ru particles. Samples reduced at higher temperature also showed similar surface electronic properties, which is indicated by the same shapes and wavenumbers of the three bands for the catalysts reduced at 300 °C and higher temperatures. These materials exhibit different surface acidity, although the supported Ru nanoparticles showed the same electronic state. The CO absorption bands were shifted toward lower wavenumber with decreased reduction temperature, and the extent of the red-shift of the Ru-CO absorption band was 8 cm−1 from Ru/Nb2O5·nH2O300 (2028 cm−1) to Ru/Nb2O5·nH2O-200 (2020 cm−1). A similar shift was observed on the bands assigned to Run+-CO and Run+-(CO)3. A red-shift in IR spectra generally points out a comparatively low transition energy of CO species adsorbed on the ruthenium surface.33 The expectation of a blue-shift for the CO absorption band is contradictory to the XPS results; the blue-shift is observed on more oxidized Ru particles of Ru/ Nb2O5·nH2O-200. It was reported that the absorption of CO on metal species can be differentiated not only by the electron transfer but also by other factors such as the geometric structure changes of the metal species and the dipole−dipole effect.34,35 A strong dipole−dipole effect was evident for CO molecules adsorbed on small metal particles (99% at 1 h. Further reaction increased the yield of 1 and decreased the yield of 2. This time course is almost the same as our previous system so that reductive amination proceeded through the same reaction pathway (Scheme 2). Aldehyde reacts with ammonia to give the imine 4, which is hydrogenated with Ru hydride species produced in situ by the dissociation of molecular hydrogen to afford the desired primary amine 1. The generated 1 is in equilibrium with the dimer imine 2

entry

catalyst

conv. (%)

7

8

1 2 3 4 5 6

Ru/Nb2O5·nH2O-200 Ru/Nb2O5·nH2O-300 Ru/Nb2O5·nH2O-400 Ru/Nb2O5·nH2O-500 Nb2O5·nH2O none

29 42 44 26 34 19

27 42 42 27 32 19

29 43 43 31 34 16

a Reaction conditions: catalyst (0.02 g), 5 (0.5 mmol), H2O (10 mmol), MeOH (5 mL), 70 °C, 2 h.

E

DOI: 10.1021/acssuschemeng.8b04324 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering nH2O-300 and Ru/Nb2O5·nH2O-400 accelerated the hydrolysis of 5 to give benzaldehyde (7) and aniline (8), both with yields around 40% (entries 2 and 3). Hydrolysis using Ru/ Nb2O5·nH2O-300 and Ru/Nb2O5·nH2O-400 was faster than the reaction without the catalyst, which indicates that acid sites or Ru particles can accelerate the hydrolysis. However, there was no difference in catalytic performance between these catalysts. These results suggest that the high catalytic activity of Ru/Nb2O5·nH2O-300 for hydrogenation leads to excellent performance for the low-temperature production of primary amines and is due to a larger amount of acid sites. This may be attributed to the activation of imines by acid sites, which facilitates attack by Ru hydride species generated by metallic Ru and H2 (step 2).38,39 In order to examine the correlation of the acid strength with the catalytic activity, the FT-IR spectra for acetone-adsorbed Ru/Nb2O5·nH2O-300 and -400 were measured (Figure S3). There was no significant difference in band positions of the C=O stretching vibration between both catalysts, indicating that the catalytic activity is not likely a result of the acid strength. Applications to Other Catalytic Reactions. With an active catalyst in hand, we studied its general applicability for the reductive amination of carbonyl compounds (Table 5). For

present catalytic system was applied to the reductive amination of biomass-derived carbonyl compounds. The reductive amination of levulinic acid with ammonia was initially performed to give 5-methylpyrrolidone in 90% yield (entry 5). There are only two reports of the reductive amination of levulinic acid with ammonia,9,14 both of which required harsh reaction conditions to obtain a high yield of the pyrrolidone. Ru/Nb2O5·nH2O catalyst could give satisfactory yields for biomass-derived carbonyl compounds such as 5-hydroxymethylfurfural and isophorone (entries 6 and 7).

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SUMMARY The temperature used for reduction of the catalyst had a significant influence on the low-temperature reductive amination over Ru-deposited Nb 2O 5·nH 2O. From the characterization results, it can be concluded that the catalyst prepared at a reduction temperature of 300 °C had the maximum catalytic activity for reductive amination. The high performance of the catalyst can be attributed to a high density of acid sites. In addition, Ru/Nb2O5·nH2O exhibited high catalytic performance for the reductive amination of biomassderived carbonyl compounds.

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Table 5. Reductive Amination of Carbonyl Compoundsa

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b04324.

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Experimental details, one table (comparison of previously reported reductive amination systems), 19 figures (TPR profile, reuse experiments, FT-IR spectra, and NMR spectra) (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yusuke Kita: 0000-0003-2455-7076 Keigo Kamata: 0000-0002-0624-8483 Michikazu Hara: 0000-0003-3450-5704 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Advanced Low Carbon Technology Research and Development Program (ALCA) of the Japan Science and Technology Agency (JST) (JPMJAL1205). D.D. would like to thank the LOTTE Foundation for scholarship.

Reaction conditions: Ru/Nb2O5·nH2O-300 (0.02 g), carbonyl compound (0.5 mmol), NH3 in MeOH (1.6 M, 5 mL), H2 (3 MPa) at 70 °C, 4 h. bH2 (2 MPa), 15 h. cIsolated as hydrochloride salt. d120 °C, 24 h. e90 °C. f100 °C, 12 h. a

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REFERENCES

(1) Roose, P.; Eller, K.; Henkes, E.; Rossbacher, R.; Höke, H. Amines, Aliphatic. In Ullmann’s Encyclopedia of Industrial Chemistry; Wiley-VCH: Weinheim, Germany, 2015. (2) Tripathi, R. P.; Verma, S. S.; Pandey, J.; Tiwari, V. K. Recent Development on Catalytic Reductive Amination and Applications. Curr. Org. Chem. 2008, 12, 1093−1115. (3) Abdel-Magid, A. F.; Mehrman, S. J. A Review on the Use of Sodium Triacetoxyborohydride in the Reductive Amination of Ketones and Aldehydes. Org. Process Res. Dev. 2006, 10, 971−1031.

the aromatic aldehydes, the corresponding primary amines were obtained in about 90% yield (entries 1 and 2) It is noteworthy that the present catalytic system is compatible with reduction-sensitive functional groups such as nitro and iodide groups. We next explore the application of our Ru-based catalyst for the reductive amination of ketones, resulting in the high yields of the primary amines (entries 3 and 4). The F

DOI: 10.1021/acssuschemeng.8b04324 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

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DOI: 10.1021/acssuschemeng.8b04324 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX