Heterogeneous Ru-Based Catalysts for One-Pot Synthesis of Primary Amines from Aldehydes and Ammonia

The direct reductive amination of carbonyl compounds with NH3 and H2 is an alternative route to produce primary amines in practical production. The search for efficient and selective catalysts has attracted great interest. In the present work, the reductive amination of heptaldehyde with NH3 was investigated over a Ru-based catalyst. The product selectivities were found to be related with the supports of Ru. The alumina with spinel structure (γ-Al2O3, θ-Al2O3)-supported Ru catalysts exhibited selectivity favoring primary amines (94% yield) at 100% heptaldehyde conversion under optimal conditions. Purely basic (MgO, CaO) and relative acidic (Nb2O5, SnO2, MCM-41, HZSM-5) supports showed relatively poor selectivity towards primary amines (0%–53% yield). The reductive amination mechanism was also proposed. The Schiff base N-[heptylene]heptyl-1-amine was a key intermediate. Ru/γ-Al2O3 was shown to be an excellent hydrogenolysis catalyst to selectively produce primary amine by amination and hydrogenolysis of N-[heptylene]heptyl-1-amine. OPEN ACCESS Catalysts 2015, 5 2259


Introduction
Primary amines are known to be key compounds in chemical industry and biology, which are important building blocks for production of many chemical products, such as polymers, solvents, dyes, surfactants, agrochemicals, and biologically-active compounds [1][2][3][4].Due to their importance, much effort has been devoted to developing various approaches for the synthesis of primary amines, such as the alkylation of halides with NH3 [5,6], the reduction of nitriles [7,8], hydrocyanation and reduction of alkenes [9], hydroamination of olefins [10,11], amination of alcohols [12][13][14], and the reductive amination of carbonyl compounds [15][16][17].Among the methods, the reductive amination of carbonyl compounds with NH3 was the most attractive and practical way of producing primary amines both in industry and academic laboratories.The overall transformation is highly atom-efficient and economic.However, one-pot amination of carbonyl compounds with NH3 generally suffers from a main issue that the chemoselectivity is relatively poor, as the initially-generated amines are more nucleophilic than ammonia itself, thus often resulting in a mixture of products of primary, secondary, and tertiary amines [18].Thus, the highly-selective synthesis of primary amines remains full of challenges.
In attempts to achieve high yield of aliphatic primary amines, much work has been performed in the last decades.Since the first catalyzed amination of aldehydes or ketones using NH3 and H2 with Ni catalyst was reported by Mignonac in 1919 [19], both homogeneous and heterogeneous catalysts were used for the direct reductive amination of carbonyl compounds [20][21][22][23][24][25][26][27][28][29][30].For example, the reductive amination of aliphatic and aromatic ketones catalyzed by homogeneous Ir and Rh complexes was presented in the patent literature [20], but the selectivity of a primary amine was poor and the reaction conditions were strictly rigorous.Beller et al. reported the first example of a homogeneous Rh catalyst to catalyze reductive amination of aldehydes with NH3 to afford primary amines.Although high selectivity and yield towards aromatic primary amines was achieved, high pressures and temperatures were required (135 °C, 65 bar H2).Moreover, the yields of the aliphatic primary amines from aliphatic aldehydes were still poor under similar reaction condition [21].Recently, homogeneous methods were reported to prepare primary amines from ketones with reductants such as ammonium formate [22,23], silane [24,25], NaBH4 [26], the Hantzsch ester [27], NaBH3CN [28], nBu3SnH [29], and Zn(BH4)2 [30] instead of hydrogen.Compared to these expensive and environmentally-problematic reducing agents, H2 was a cleaner and more economic reducing agent in practical production.
For the practical application, the homogeneous catalytic system had the problems in the catalyst recovery and reuse.Heterogeneous catalysts could be separated easily after the reaction by simple filtration, which are more ideal for large-scale application.Up to now, most of the heterogeneous catalysts for the amination of carbonyl compounds with NH3 and H2 were reported in patent literatures, such as Ni, Co, and noble metal-based solid catalysts [31].There were only a few papers about the reductive amination of aliphatic primary aldehydes or ketone with solid catalyst and seldom clarify its process [32][33][34].For instance, Raney Ni was commonly applied to reductive amination of aldehydes with ammonia in the early literature, but it had several disadvantages including the requirement of high H2 pressure (0.2-12.5 MPa) and low activity and selectivity of the final products [32].Bodis and co-workers used carbon supported group VIII noble metal (Pt, Pd, Rh, and Ru) as catalysts for the reductive amination of butyraldehyde with NH3.It was found that Rh and Ru catalysts were active and selective in the production of primary amine under 50 bar H2 pressure, while Pt and Pd catalysts favored for the secondary amine formation [33].MgO/Al2O3 supported Ru was applied to reductive amination of 2-butyl-2-ethyl-4-cyan-butyraldehyde [34].The moderate yield of diamine was obtained, but high pressures and temperatures were also required.Thus, an environmentally-sustainable and cost effective process for the production of aliphatic primary amines by selective reductive amination of aliphatic aldehydes with NH3 is of great fundamental, as well as practical, interest.
The Ru-based catalysts are the most active catalysts for the reductive amination of carbonyl compounds to the corresponding amines.However, few reports about the comparison study on the different types of catalyst supports were found.The catalytic amination process was also seldom to be clarified.In this work, we reported that supported Ru nanoparticles catalyst, synthesized from various cheap metal oxides as the supports, acted as effective heterogeneous catalysts for the selective synthesis of aliphatic primary amines directly from aliphatic aldehydes, NH3 and H2 under relatively mild conditions.The choice of a suitable support was found to play an important role in the determining the catalytic selectivity of Ru catalyst.The effects of reaction solvent and reaction temperature were discussed in detail.The reaction mechanism of reductive amination of aldehyde with NH3 was also proposed.
XRD patterns of Ru-based catalysts were also shown in Figure S1.It can be seen that except for Ru/ZSM-5 and Ru/MCM-41, there is no observable peak corresponding to crystalline Ru species, such as Ru or RuO2.The absence of Ru species peaks in the catalysts implies that Ru might be highly dispersed on the supports (Al2O3, ZrO2, CeO2, CaO, MgO, Nb2O5, SnO2).The results were further supported by the result of elemental mapping images of Ru/γ-Al2O3 (see Figure S2).HRTEM analysis showed that Ru/γ-Al2O3, Ru/MgO, and Ru/Nb2O5 exhibited small Ru particles with average particle sizes of 3.5 nm, 4.0 nm, and 4.2 nm, respectively (see Figure S3).These indicated that the surface area of supports, dispersion of metal, and crystal size were not the key factors for the catalytic selectivity of primary amines, other factors (e.g., acid-base character) might favorably influence the catalytic performance of Ru-based catalysts.

Effect of Various Solvents on the Reaction
The effect of solvents was studied with 5% Ru/γ-Al2O3 as the catalyst.The results were listed in Table 2. Notably, the choice of solvents has an obvious influence on the activity of the catalyst.Among these solvents examined, methanol was found to be the best solvent for reductive amination of heptaldehyde, giving high 2a yield of 94% (Table 2, entry 1).The reaction in cyclohexane, THF, and toluene obtained relatively moderate yield of 2a (Table 2, entries 2-4, 72%-84%).When water was used as the solvent, the 2a yield sharply decreased to 32% (Table 2, entry 7).Similarly, when the reaction was conducted in 1,4-dioxane and methyl tert-butyl ether (MTBE), the desired 2a yields of 39% and 34% were obtained, respectively (Table 2, entries 5 and 6).Thus, Ru/γ-Al2O3 and methanol were the suitable catalyst and solvent for the reductive amination of heptaldehyde.Then, all reactions were performed in methanol with Ru/γ-Al2O3 as the catalyst.

Reaction Pathways
The reductive amination of carbonyl compounds generally proceeds through the formation of imines via the reaction of aldehydes with NH3 and subsequent hydrogenation (Scheme 1).To get more information concerning the reaction process of reductive amination of aldehydes with NH3, the effect of reaction temperatures on the product selectivity with Ru/γ-Al2O3 catalyst was investigated.The results were shown in Figure 1.As expected, with increase in the reaction temperature from 40 °C to 80 °C, the yield of 2a improved from 7% to 94%.At low temperature (40 °C), the yield of 2a was only 7%.The yield of 2a increased to 53% when the temperature enhanced to 60 °C.A highest yield of 2a could be obtained at 80 °C.Raising the reaction temperature to 100 °C did not further increase the selectivity of the primary amine with 2a yield of 93%.On the other hand, low temperature brought negative effects on the further transformation of Schiff base 2b by hydrogenation or hydrogenolysis.The yield of 2b was about 77% at 40 °C, which declined to 0.1% when the reaction temperature increased from 40 °C to 80 °C.The similar Schiff base intermediate was also found in the previous studies [20,33,38].A closer look was taken to analyze the products after the reaction at 80 °C.Apart from major product 2a, very little 2b (<1%), 2d (<1%), 2c (<0.1%), 2e (<0.5%) were detected in the reaction mixture with GC/MS analysis.According to the Scheme 1, the expected intermediate heptyl-1-imine [CH3(CH2)5CH=NH] was not observed in our experiments probably for its poor stability.The main equilibria for heptaldehyde are shown in Scheme 2.  Some control experiments were carried out with Ru-γ-Al2O3 as the catalyst to study the mechanism of reductive amination of aldehydes with ammonia.First, when 3 MPa N2 instead of H2 was used, no primary amine was formed in the reaction, which suggested that H2 was indispensable for the amination of heptaldehyde.We also carried out the reaction at 40 °C for 2 h in 3 MPa H2 and 0.4 MPa NH3, which was then pressurized with H2 to 3 MPa again and conducted at 80 °C for another 2 h.Surprisingly, the Schiff base 2b formed at 40 °C was mostly converted to primary amine 2a at 80 °C.Few 2c was The reductive amination of heptaldehyde to 2a proceeds in several consecutive steps.In the first stage, the amination of heptaldehyde gives initial intermediate heptyl-1-imine via the reaction of heptaldehyde with NH3, which is then reduced to the 2a by hydrogenation.In the second, the initially-formed 2a would further react with heptaldehyde immediately to form the Schiff base 2b intermediate.The imine 2b may directly be reduced to the 2c by hydrogenation or added with NH3 to form corresponding geminal diamine and subsequent hydrogenolysis to produce 2a.The selectivity of 2a and 2c was governed by the relative rate of hydrogenolysis and hydrogenation.If the rate of hydrogenolysis is faster than that of hydrogenation, the hydrogenolysis towards primary amine would be the dominant pathway.The results of our experiments with Ru/γ-Al2O3 conducted at different temperatures (40-80 °C) showed that Schiff base 2b can be easily produced at the low temperature of 40 °C.However, only small amount of the Schiff base was further transformed to 2a by hydrogenolysis at low temperature, which needed a high activation energy [33].High temperatures would accelerate hydrogenolysis rates and benefit for the production of more primary amine.The yield of 2a is much higher at 80 °C (94%) than at 40 °C (7%).The result suggested that the adsorption-desorption equilibrium of the Schiff base 2b was shifted much more to benefit hydrogenolysis to form primary amine at high temperature.Thus, amination and hydrogenolysis of the Schiff base towards a primary amine was the dominant pathway at high temperature.These results were also in agreement with the literature [33].The results of our experiments also showed that only very little 2c was formed in the catalysis system when the Ru/γ-Al2O3 was used as the catalyst, which may be due to the weak chemisorption of 2b intermediate on the Ru/γ-Al2O3 surface, resulting in a low rate of hydrogenation to 2c.Therefore, Ru/γ-Al2O3 was proved to be good hydrogenolysis catalyst, which is preferential to produce a primary amine.
To elucidate the reaction mechanism proposed, a kinetic study following the formation of key products with time was also carried out.As shown in Figure 2, at the beginning of the reaction, heptaldehyde was quickly converted to 2b imines.When the reaction was reacted for 5 min, more than 95% conversion of heptaldehyde was completed; the 2b yield of 55.5% and less than 0.5% 2c were obtained.It was worth to note that no desired 2a was detected in the reaction mixture in the first stage.This may be due to the more nucleophilic of 2a than ammonia, resulting in the immediate reaction of the initially-formed 2a with the remaining heptaldehyde to formed 2b imines.When the reaction time proceeded for 15 min, no heptaldehyde remained and 2a started to accumulate.At the same time, 2b imines in the reaction mixture reached the highest concentration.As the reaction continues, the concentration of 2b imines decreased gradually, while the concentration of 2a increased rapidly.After reaction for 120 min, most of 2b intermediate was converted to 2a with 94% yield, and only trace amounts of 2b imines was remained.During the process, 2c were detected in a very low concentration.The results indicated that 2b intermediate was mainly reacted with NH3 to form geminal diamine and subsequent hydrogenolysis to produce 2a.Hydrogenation of 2b intermediate to 2c was the minor pathway.The results were in good agreement with the mechanism mentioned above, which confirmed that Ru/γ-Al2O3 was a good hydrogenolysis catalyst.

Scope of Reactions
Under the optimal conditions for the synthesis of 2a, a series of aldehydes (aliphatic and aromatic aldehydes) was aminated with Ru/γ-Al2O3 catalyst in the presence of NH3 in order to study the scope of the present catalytic system.The results were exhibited in Table 3. Linear aliphatic aldehydes (C5-C10, Table 3, entries 1-6) and aromatic aldehyde (4-methylbenzaldehyde, Table 3, entry 7) were selectively transformed to the corresponding primary amines with excellent yields (89%-97%), while only small amounts of primary alcohols and imines by-products were detected by the GC/MS analysis.When 2-octantone was used, the corresponding 2-octylamine yield of 97% was obtained (Table 3, entry 8).Furfurylamine yield of 75% was obtained (Table 3, entry 9), when furan-2-carbaldehyde was used.

Recycle Study of Catalyst
From the view point of green chemistry, it is important to examine the possibility of recovery and reusability of Ru/γ-Al2O3 catalyst.The capability of recycling Ru/γ-Al2O3 catalysts was investigated by the reductive amination of heptaldehyde with NH3 and H2 at 80 °C for 2 h.After completion of the benchmark reaction, the catalyst was removed from the reaction mixture by filtration, washed with ethanol for several times, dried at 50 °C for 12 h, and then reused directly without other treatment.The Ru-based catalyst could be reused four times without a significant drop in 2a yield and catalytic activity (see Figure S4).93% of 2a yield in the initial experiment with fresh Ru/γ-Al2O3 remained in the fourth run, probably caused by the weight loss of catalysts during the recovery process.The results demonstrated the sufficient stability of Ru/γ-Al2O3 catalyst in the reductive amination of heptaldehyde to primary amine 2a.

Catalyst Materials and Preparation
Commercially available inorganic compounds were used without further purification.MCM-41 and HZSM-5 zeolite with a SiO2/Al2O3 ratio of 38 were supplied by Nankai University (Tianjin, China).ZrO2 was prepared by dropping a 5 M ammonia solution into a 0.4 M aqueous solution of ZrOCl2•8H2O with vigorous stirring.Then, the precipitate was filtered, washed with distilled water several times, dried, and then sintered at 500 °C for 3 h.CaO, MgO, and Nb2O5 were synthesized by sintering of their hydroxides at 500 °C for 3 h.Ru/γ-Al2O3 (Ru loading = 5 wt.%) was synthesized by the impregnation method.In a typical synthesis, RuCl3•nH2O dissolved in deionized water was mixed with γ-Al2O3.The mixture was dried at 110 °C for 12 h and, subsequently, reduced in hydrogen gas at 500 °C for 3 h.Ru loaded on various supports (CaO, MgO, Nb2O5, ZrO2, CeO2, SnO2, ZSM-5, MCM-41, α-Al2O3, θ-Al2O3) were also prepared by the impregnation method under the same conditions (Ru loading = 5 wt.%).

Characterization
XRD patterns were taken by a Bruker D8 Advanced X-ray diffractometer using CuKa radiation (λ = 1.5147Å, Karlsruhe, Germany).High resolution TEM images were collected by a field emission Tecnai G2F20 electron microscope (FEI, Hillsboro, OR, USA).The morphologies were measured by a field emission Hitachi S-4800 scanning electron microscope (Tokyo, Japan).N2 adsorption-desorption measurements were carried out at 77 K using a micromeritics ASAP 2020 sorptometer (Micromeritics, Norcross, GA, USA).The catalysts were heated at 423 K for 6 h prior to analysis.The surface area of the catalysts was calculated using the Brunauer-Emmett-Teller (BET) equation based on the adsorption data at relative pressure P/P0 values between 0.01-0.05.

General Procedure of Catalytic Reactions
The activity of the catalysts was evaluated by reductive amination of aldehydes with NH3 and H2.Unless otherwise stated, all reactions were performed at 80 °C for 2 h.In a typical reaction, heptaldehyde (6.6 mmol), Ru/γ-Al2O3 (50 mg, containing 0.025 mmol Ru), n-dodecane (0.58 mmol), and CH3OH (20 mL) were injected to a 100 mL stainless steel autoclave.The autoclave was closed, flushed with H2 several times to remove air, and charged with 0.4 MPa NH3 and 3 MPa H2 at room temperature.The autoclave was then carried out at the desired temperature under stirring for 2 h.After the reaction, the reaction mixture was cooled to room temperature and then depressurized.Conversion and yields of products were detected by GC (Shimadzu GC-2010, Kyoto, Japan) with Agilent capillary column DB-5 (Kyoto, Japan) and n-dodecane was used as an internal standard.The products were also identified by GC/MS (Shimadzu GCMS-QP2010) equipped with Agilent capillary column DB-5MS.

Conclusions
In summary, we have shown that the alumina with spinel structure (γ-Al2O3, θ-Al2O3)-supported Ru samples were active and selective catalysts for direct reductive amination of aldehydes with NH3 and H2 to corresponding primary amines.Up to 94% yield for 1-heptylamine could be obtained for the reductive amination of heptaldehyde using Ru/γ-Al2O3 catalyst.High selectivity of primary amines were also achieved in the case of the linear aliphatic aldehydes (C5-C10) and aromatic aldehydes.It was found that the supports have an important influence on the product selectivity of Ru catalyst.Compared with purely basic (MgO, CaO) and relative acidic (Nb2O5, SnO2, MCM-41, HZSM-5) supports, amphoteric supports (θ-Al2O3, γ-Al2O3, ZrO2, CeO2) were more favorable for the primary amine formation.A reductive amination mechanism was also proposed.The Schiff base 2b acted as a key intermediate for the reductive amination of heptaldehyde.Ru-based catalysts were a good hydrogenolysis catalyst.In this reaction, the addition of Schiff base 2b with NH3 to form corresponding geminal diamine and subsequent hydrogenolysis to produce 2a was the dominate pathway.

Scheme 1 .
Scheme 1. Main reactions of the reductive amination of carbonyl compounds with NH3 and H2.

Table 1 .
Amination of heptaldehyde over various catalysts a .

Table 2 .
Amination of heptaldehyde in various solvents a .
b Yields were determined by GC based on aldehyde.