catalysts

Article Nickel Catalyzed Conversion of into Cyclohexylamine in Water and Low Boiling Point Solvents

Yunfei Qi 1,2, Haiyun Yu 1,*, Quan Cao 2, Bo Dong 2, Xindong Mu 2 and Aiqing Mao 1

1 Key Laboratory of materials Science and Processing of Anhui Province, School of Materials Science and Engineering, Anhui University of Technology, Ma’anshan 243002, Anhui, China; [email protected] (Y.Q.); [email protected] (A.M.) 2 Key Laboratory of Bio-Based Materials, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, China; [email protected] (Q.C.); [email protected] (B.D.); [email protected] (X.M.) * Correspondence: [email protected]; Tel.: +86-532-8066-2725; Fax: +86-532-8066-2724

Academic Editor: Keith Hohn Received: 18 January 2016; Accepted: 31 March 2016; Published: 26 April 2016

Abstract: Nickel is found to demonstrate high performance in the amination of cyclohexanol into cyclohexylamine in water and two solvents with low boiling points: tetrahydrofuran and cyclohexane. Three catalysts, Raney Ni, Ni/Al2O3 and Ni/C, were investigated and it is found that the , hydrogen, the solvents and the support will affect the activity of the catalyst. In water, all the three catalysts achieved over 85% conversion and 90% cyclohexylamine selectivity in the presence of base and hydrogen at a high temperature. In tetrahydrofuran and cyclohexane, Ni/Al2O3 exhibits better activity than Ni/C under optimal conditions. Ni/C was stable during recycling in aqueous ammonia, while Ni/Al2O3 was not due to the formation of AlO(OH).

Keywords: nickel; cyclohexanol; cyclohexylamine; base; hydrogen

1. Introduction find wide application in polymers, dyes, surfactants, pharmaceuticals, and biologically active compounds [1–10]. The is usually produced through direct amination of with ammonia. As shown in Scheme1, this process includes dehydrogenation of alcohol to carbonyl compounds, amination of carbonyl compounds to imine, and hydrogenation of imine [11] to amine to produce primary, secondary and tertiary amine. Among the three, primary amine is the most useful intermediate for synthesizing value-added derivatives [12]. However, it is subject to subsequent amination due to its higher activity than ammonia and thus produces secondary and tertiary amine [5]. By now, great efforts have been made to find and develop a catalyst system to convert alcohol into primary amine. Homogeneous catalysts, such as Ru complexes, demonstrate high performance in converting secondary alcohol into primary amine [1–3]. However, these catalysts are either costly or difficult in recycling. Recently, work by Shimizu et al. showed that heterogeneous supported nickel catalysts, especially Ni/Al2O3, is a choice catalyst for amination of alcohol into a primary amine with more than 80% amine yield [13]. In their work, the toxic o-xylene ais high in boiling point (144 ˝C) and consumes more energy during the separation. Moreover, the of alcohol in o-xylene is limited due to the presence of hydroxyl group. Therefore, much work needs to be done to find and use non-toxic or low boiling solvents.

Catalysts 2016, 6, 63; doi:10.3390/catal6050063 www.mdpi.com/journal/catalysts Catalysts 2016, 6, 63 2 of 10 Catalysts 2016, 6, 63 2 of 11

SchemeScheme 1. 1.The The mechanism mechanism for amination amination of of alcohol. alcohol.

In theIn presentthe present work, work, aqueous aqueous ammonia, ammonia, tetrahydrofuran,tetrahydrofuran, and and cyclohexane cyclohexane are areemployed employed as as solvents on the amination of cyclohexanol into cyclohexylamine using Raney Ni, Ni/γ‐Al2O3 and Ni/C solvents on the amination of cyclohexanol into cyclohexylamine using Raney Ni, Ni/γ-Al2O3 and Ni/Ccatalysts. catalysts. Water Water is non is non-toxic‐toxic and andthe other the other two solvents two solvents are low are in boiling low in points. boiling points.

2. Results2. Results and and Discussion Discussion Catalysts 2016, 6, 63 3 of 11 2.1. Raney2.1. Raney Ni in Ni Aqueous in Aqueous Ammonia Ammonia TableThe 1. reactivity Conversion of ofcyclohexanol cyclohexanol wasinto cyclohexylaminefirst investigated over in Raneyaqueous Ni inammonia aqueous usingammonia Raney with Ni The reactivity of cyclohexanol was first investigated in aqueous ammonia using Raney Ni catalyst, catalyst,different which amount has of already NaOH beena. commercialized. It is theoretically difficult to convert cyclohexanol whichin has aqueous already ammonia, been commercialized. because it is reversible It is theoretically to produce imine difficult through to convert dehydration cyclohexanol of carbonyl in with aqueous Entry NaOH/g Conversion/% Yield/% ammonia,NH3 [14]. because The existence it is reversible of a large to amount produce of iminewater will through do no dehydration good to the formation of carbonyl of imine. with It NH can3 [14]. The existence of a large amount of water will do no good to the formation of imine. It can be seen from be seen from Table 1 that Raney1 Ni itself0 only achieved44 44% cyclohexanol41 conversion (Entry 1). After Table1NaOH that Raney is added, Ni itself it dramatically only achieved enhanced 44% cyclohexanol its catalytic conversion activity and (Entry obtained 1). After more NaOH than is90% added, it dramaticallyconversion, enhanced indicating itsthe catalytic base2 is helpful activity0.3 for and the reaction. obtained93 This more result than is79 consistent 90% conversion, with the indicating literature the base isthat helpful base can for promote the reaction. the amination This result of alcohol is consistent [15]. However, with the it can literature be seen that from base Table can 1 Entries promote 2 the 3 0.4 95 78 aminationto 6 that of it alcohol will not [ 15affect]. However, the conversion it can even be seen further from increasing Table1 Entriesthe NaOH 2 to amount 6 that after it will 0.3 not g NaOH affect the conversionis added. even When further the Raney increasing Ni4 catalysts the NaOH0.5 in Table amount 1, Entry95 after 6, were 0.3 recycled g NaOH84 for is the added. second When time, only the Raney 57% Ni cyclohexanol conversion was achieved (Entry 7). Raney Ni before and after the reaction (Entry 6) was catalysts in Table1, Entry 6, were recycled for the second time, only 57% cyclohexanol conversion was observed by X‐ray diffraction5 (XRD) 0.6with the results 96displayed in Figure90 1. According to Scherrer achievedequation (Entry in Equation 7). Raney (1), Ni Raney before Ni and before after and the after reaction reaction (Entry was estimated 6) was observed to be 7 nm by and X-ray 15 diffractionnm in (XRD)crystallite with the size. results The displayed reason6 why in Figure the 0.7catalyst1 . According deactivates96 to Scherrer may be equation ascribed89 into Equation the growth (1), of Raney the Ni beforecrystallite and after size. reaction was estimated to be 7 nm and 15 nm in crystallite size. The reason why the 7 b 0.7 57 52 catalyst deactivates may be ascribed to the growth of the crystallite size. D = Kλ/Bcosθ (1) a Reaction conditions: Raney Ni 6 g; aqueous ammonia 28 g; cyclohexanol 1 g; 160 °C; 17 h; b Raney D “ Kλ{Bcosθ (1) Ni (Entry 6) was recycled for the second time.

Ni Intensity (a.u.) Intensity 2

1

10 20 30 40 50 60 70 80 90 2 Theta/Degree

FigureFigure 1. X-ray1. X‐ray diffraction diffraction (XRD) (XRD) patterns patterns ofof ((11)) fresh Raney Ni; Ni; (2 (2) )Raney Raney Ni Ni used used in inTable Table 1, 1Entry, Entry 6. 6.

Table 2 shows that both the conversion and yield increase as the reaction time extends, while the selectivity decreases over time. Eight hours is enough for the reaction to achieve 94% conversion. It will not significantly improve the conversion even if the reaction time is prolonged.

Table 2. Conversion of cyclohexanol into cyclohexylamine over Raney Ni with different time.

Entry Time/h Conversion/% Yield/% Selectivity/%

1 4 71 68 95

2 6 83 76 91

3 8 94 81 86

4 12 95 82 86

5 17 96 82 85

6 24 98 83 84

7 30 98 83 84

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Table 1. Conversion of cyclohexanol into cyclohexylamine over Raney Ni in aqueous ammonia with different amount of NaOH a.

Entry NaOH/g Conversion/% Yield/% 1 0 44 41 2 0.3 93 79 3 0.4 95 78 4 0.5 95 84 5 0.6 96 90 6 0.7 96 89 7 b 0.7 57 52 a Reaction conditions: Raney Ni 6 g; aqueous ammonia 28 g; cyclohexanol 1 g; 160 ˝C; 17 h; b Raney Ni (Entry 6) was recycled for the second time.

Table2 shows that both the conversion and yield increase as the reaction time extends, while the selectivity decreases over time. Eight hours is enough for the reaction to achieve 94% conversion. It will not significantly improve the conversion even if the reaction time is prolonged.

Table 2. Conversion of cyclohexanol into cyclohexylamine over Raney Ni with different time.

Entry Time/h Conversion/% Yield/% Selectivity/% 1 4 71 68 95 2 6 83 76 91 3 8 94 81 86 4 12 95 82 86 5 17 96 82 85 6 24 98 83 84 7 30 98 83 84 Reaction conditions: Raney Ni 6 g; aqueous ammonia 28 g; cyclohexanol 1 g; 160 ˝C; 0.5 g NaOH.

The effect of cyclohexanol concentration on the reaction was displayed in Table3, indicating that higher concentration leads to lower selectivity. Because theoretically the cyclohexylamine produced can further react with cyclohexanol to produce dicyclohexyl amine and even tricyclic hexylamine [6]. However, only dicyclohexyl amine was produced while tricyclic hexylamine was not detected in this work. Further investigation demonstrated that dicyclohexyl amine could not be converted into cyclohexylamine under the reaction conditions in Table3.

Table 3. Conversion of cyclohexanol into cyclohexylamine over Raney Ni with different amount of cyclohexanol.

Entry Alcohol/g Conversion/% Yield/% Selectivity/% 1 1 96 88 93 2 1.5 93 86 92 3 2 89 76 85 4 2.5 93 72 77 Reaction conditions: 6 g Raney Ni, 28 g aqueous ammonia, 0.5 g NaOH, 160 ˝C, 17 h.

2.2. Ni/Al2O3 and Ni/C Systems Supported catalyst is generally more popular due to its less use of active metal. Moreover, the support is found to exert great influence to the activity of the catalyst [13,16,17]. Two supports Al2O3 and active carbon were investigated in aqueous ammonia with the results shown in Table4. From Entries 1 to 3, as the temperature was raised form 160 ˝C to 200 ˝C, it resulted in an increase of conversion from 51% to 71%. The conversion increases with the reaction time prolonging and then Catalysts 2016, 6, 63 4 of 10

reaches a plateau for both Ni/Al2O3 and Ni/C catalysts (Entries 4 to 6 and 10 to 12). Through the comparison between Entry 3 and 7, Entry 8 and 9, the conversion increased from 48% to 71% and 37% to 87%, respectively, indicating that NaOH can improve the catalytic activity of both Ni/Al2O3 and Ni/C. In the amination reaction, hydrogen also participated in the dehydrogenation and the hydrogenation process. The effect of hydrogen is also tested, and as shown in Table4 Entries 3, 6, 9 and 10 for Ni/Al2O3 and Ni/C, the conversion increases from 71% to 87% and 87% to 92%, respectively, suggesting that hydrogen has positive, though slight effect on the reaction for both Ni/Al2O3 and ˝ Ni/C. At the same temperature of 160 C (Entries 1 and 9), Ni/C exhibits better activity than Ni/Al2O3, indicating the support may affect the activity of nickel [13]. From Figure2, Ni in the fresh Ni/Al 2O3 and Ni/C catalysts were 14 nm and 6 nm, respectively, in crystallite size based on the Scherrer equation. 2 2 C (1211 m /g) is larger than Al2O3 (163 m /g) in surface area. Therefore, Ni on C gives higher Ni surface areas than that of Ni on Al2O3. The difference in nickel surface area could explain the better Catalystsactivity 2016 of, Ni/C 6, 63 catalyst. 5 of 11

Ni/C Ni/Al O Ni 2 3

Intensity(a.u.) Al O 2 3

10 20 30 40 50 60 70 80 90 2 Theta/Degree

Figure 2. XRD patterns of fresh Ni/Al2O3 and Ni/C. Figure 2. XRD patterns of fresh Ni/Al2O3 and Ni/C.

Table 4. Catalytic activity of Ni/Al2O3 and Ni/C in aqueous ammonia. Table 4. Catalytic activity of Ni/Al2O3 and Ni/C in aqueous ammonia. Entry Catalyst T/°C t/h H2/MPa NaOH/g Conversion/% Yield/% ˝ Entry Catalyst T/ C t/h H2/MPa NaOH/g Conversion/% Yield/% 1 Ni/Al2O3 160 17 0 0.3 51 42 1 Ni/Al2O3 160 17 0 0.3 51 42 2 Ni/Al2O3 180 17 0 0.3 54 49 2 Ni/Al2O3 180 17 0 0.3 54 49 3 Ni/Al2O3 200 17 0 0.3 71 65 4 Ni/Al2O3 200 8 1 0.3 63 54 3 Ni/Al2O3 200 17 0 0.3 71 65 5 Ni/Al2O3 200 14 1 0.3 82 72 6 Ni/Al2O3 200 17 1 0.3 87 79 4 Ni/Al2O3 200 8 1 0.3 63 54 7 Ni/Al2O3 200 17 0 0 48 41 8 Ni/C 160 17 0 0 37 34 5 Ni/Al2O3 200 14 1 0.3 82 72 9 Ni/C 160 17 0 0.3 87 81 10 Ni/C 160 17 1 0.3 92 85 6 Ni/Al2O3 200 17 1 0.3 87 79 11 Ni/C 160 6 1 0.3 76 69 12 Ni/C 160 24 1 0.3 91 81 7 Ni/Al2O3 200 17 0 0 48 41 13 Ni/C 180 17 1 0.3 91 86 8 ReactionNi/C conditions: 160 cyclohexanol 17 1 g, aqueous0 ammonia0 28 g, catalyst 137 g (Ni 10 wt %). 34

9 Ni/C 160 17 0 0.3 87 81

10 Ni/C 160 17 1 0.3 92 85

11 Ni/C 160 6 1 0.3 76 69

12 Ni/C 160 24 1 0.3 91 81

13 Ni/C 180 17 1 0.3 91 86

Reaction conditions: cyclohexanol 1 g, aqueous ammonia 28 g, catalyst 1 g (Ni 10 wt %).

2.3. Recycling of the Catalysts

Ni/C exhibits better activity than Ni/Al2O3, thus its stability during the reaction is further investigated. The results, as shown in Figure 3, indicate that in the presence of NaOH and H2, Ni/C could be reused six times without losing its activity and each cycle generated more than 80% cyclohexylamine yield. However, after being recycled two times, Ni/Al2O3 (Table 4 Entry 6) became less active. It only achieved 61% conversion and 53% yield for the second run. The XRD characterization shown in Figures 4 and 5 demonstrated that Ni/C retained its structure after being recycled, while Al2O3 reacted with water and produced AlO(OH). From Figures 4 and 5, Ni in

Catalysts 2016, 6, 63 5 of 10

2.3. Recycling of the Catalysts

Ni/C exhibits better activity than Ni/Al2O3, thus its stability during the reaction is further investigated. The results, as shown in Figure3, indicate that in the presence of NaOH and H 2, Ni/C could be reused six times without losing its activity and each cycle generated more than 80% cyclohexylamine yield. However, after being recycled two times, Ni/Al2O3 (Table4 Entry 6) became less active. It only achieved 61% conversion and 53% yield for the second run. The XRD characterization Catalysts 20162016,, 66,, 6363 66 of 1111 shown in Figures4 and5 demonstrated that Ni/C retained its structure after being recycled, while

Al2Ni/AlO3 reacted22O33 and with Ni/C water catalysts and increasedincreased produced fromfrom AlO(OH). 14 nm to Fromto 23 nm Figures and 64 nmand toto5 Ni7 nm in Ni/Alafter being2O3 recycled,recycled,and Ni/C catalystsrespectivelyrespectively increased inin crystallite from 14 nm size to 23according nm and toto 6 nm thethe to Scherrer 7 nm after equation. being recycled, High Resolution respectively Transmission in crystallite sizeElectron according Microscopy to the Scherrer (HRTEM)(HRTEM) equation. analysis High of Ni/Al Resolution22O33 catalysts Transmission before and Electron after thethe Microscopy reactionreaction as (HRTEM) shown analysisinin Figures of Ni/Al 6 and 27O furtherfurther3 catalysts proved before thatthat some and after of Ni the were reaction lostlost fromfrom as thethe shown surface in of Figures Al22O336 because and7 further of thethe provedformationformation that someof AlO(OH). of Ni were The InductivelyInductively lost from the Coupled surface Plasma of Al2 Ooptical3 because emission of the spectrometry formation of(ICP(ICP AlO(OH).‐‐OES) Theanalysis Inductively demonstrated Coupled Plasma thatthat 11% optical Ni emission on Ni/Al spectrometry22O33 was lost.lost. (ICP-OES) As toto analysisNi/C catalyst, demonstrated HRTEM that analysis (Figures 8 and 9) indicated that less Ni on C was lost than that on Al2O3, which is consistent 11%analysis Ni on Ni/Al (Figures2O 38 wasand 9) lost. indicated As to Ni/C that less catalyst, Ni on C HRTEM was lost analysis than that (Figures on Al2O8 3and, which9) indicated is consistent that with the fact that only 0.5% Ni on C was lost by ICP‐OES analysis. The substantial loss of metal and lesswith Ni onthe C fact was that lost only than 0.5% that Ni on on Al C2 Owas3, which lost by is ICP consistent‐OES analysis. with the The fact substantial that only loss 0.5% of Nimetal on Cand was the growth of the crystallite size may be the reason for the deactivation of the Ni/Al2O3 catalyst; while lostthe by growth ICP-OES of the analysis. crystallite The size substantial may be the loss reason of metal for the and deactivation the growth of of the the Ni/Al crystallite2O3 catalyst; size while may be thethe small decrease of cyclohexylamine yield of Ni/C catalyst after being recycledrecycled inin Figure 3 may the reason for the deactivation of the Ni/Al2O3 catalyst; while the small decrease of cyclohexylamine be ascribed to slight loss of Nickel and growth of crystallite size. It can thus be concluded from yieldbe ofascribed Ni/C catalystto slight afterloss of being Nickel recycled and growth in Figure of crystallite3 may be ascribedsize. It can to thus slight be loss concluded of Nickel from and the above analysis that Ni/C is a better choice than Ni/Al2O3 during the amination of cyclohexanol growththe above of crystallite analysis size. that ItNi/C can is thus a better be concluded choice than from Ni/Al the2O above3 during analysis the amination that Ni/C of is cyclohexanol a better choice inin aqueous ammonia. than Ni/Al2O3 during the amination of cyclohexanol in aqueous ammonia.

ConversionConversion Yield 100100 Yield

8080

6060 % %

4040

2020

00 123456123456 Recycling times

˝ FigureFigure 3. 3.Recycling Recycling of of Ni/C Ni/C catalyst. catalyst.catalyst. Conditions:Conditions: 160 160 °C,°C,C, 17 17 h, h, 1 1 MPa MPa H H22,, 2 1, g 1 gcyclohexanol,cyclohexanol, cyclohexanol, 1 g 1 10 g 10wt wt % % Ni/CNi/C catalyst, catalyst,catalyst, 28 28 g g (25 (25(25 wt wt %) %) aqueous aqueous ammonia.ammonia.

AlOOH

NiNi Intensity (a.u.) Intensity Intensity (a.u.) Intensity 22 Al O Al2 O3 2 3

11

1010 20 20 30 30 40 40 50 50 60 60 70 70 80 80 90 90 22 ThetaTheta (Degree)(Degree)

FigureFigure 4. XRD 4. XRD patterns patterns of of (1 ()(1 fresh)) freshfresh Ni/Al Ni/Al222OO33;;;( ((22)) Ni/Al Ni/Al22O2O33 after3 after thethe the reaction.reaction. reaction.

Catalysts 2016, 6, 63 6 of 10 Catalysts 20162016,, 66,, 6363 77 of 1111

NiNi

2 Intensity (a.u.) Intensity (a.u.) Intensity (a.u.)

1

2020 30 30 40 40 50 50 60 60 70 70 80 80 90 90 2 Theta/Degree

FigureFigure 5. 5. XRD XRD patterns patterns of of ( ((11))) fresh freshfresh Ni/C; Ni/C; ( (2)) Ni/C after after sixsix six timestimes times reaction.reaction. reaction.

Figure 6.6. HighHigh Resolution Transmission Electron Microscopy (HRTEM) (HRTEM) micrographs of the the freshfresh Ni/Al222OO33 3catalyst.catalyst.catalyst.

FigureFigure 7. 7. HRTEM HRTEM micrograph micrograph of of Ni/Al Ni/Al22OO33 catalystcatalystcatalyst after after thethe the reaction.reaction. reaction.

Catalysts 2016,, 66,, 63 63 78 of 1011 Catalysts 2016, 6, 63 8 of 11

Figure 8. HRTEM micrographs of the fresh Ni/C catalyst. FigureFigure 8.8. HRTEM micrographsmicrographs ofof thethe freshfresh Ni/CNi/C catalyst.

Figure 9.9. HRTEM micrographsmicrographs ofof Ni/CNi/C catalyst after six times reaction. Figure 9. HRTEM micrographs of Ni/C catalyst after six times reaction.

2.4. Conversion in Low Boiling Point Solvents Although cyclohexanolcyclohexanol could could be be efficiently efficiently converted converted in aqueous in aqueous ammonia, ammonia, the high the boilinghigh boiling point ofpoint water of water may lead may to lead high to energy high energy consumption consumption during during the separation the separation process. process. Two solventsTwo solvents with lowwith boiling low boiling points tetrahydrofuranpoints tetrahydrofuran (THF) and (THF) cyclohexane and cyclohexane were therefore were investigated. therefore investigated. Amination ofAmination cyclohexanol of cyclohexanol in non-aqueous in non solvents‐aqueous goes solvents on well goes with on the well absence with the of water.absence The of water. polarity The of thepolarity two solventsof the two follows solvents the follows order THF the order > cyclohexane, THF > cyclohexane, which influences which influences the solubility the solubility of both the of substrateboth the substrate and NH andin them. NH3 in It them. can be It seen can frombe seen Table from5 (Entries Table 5 1(Entries to 4) that, 1 to in 4) THF, that, NaOHin THF, plays NaOH a both the substrate 3and NH3 in them. It can be seen from Table 5 (Entries 1 to 4) that, in THF, NaOH moreplays importanta more important role than role H . than The baseH2. The can base improve can theimprove conversion the conversion from 13% from to 50% 13% while to 50% hydrogen while plays a more important role2 than H2. The base can improve the conversion from 13% to 50% while ˝ ˝ hashydrogen no such has dramatic no such effect. dramatic Raising effect. the Raising temperature the temperature from 160 fromC to 160 180 °CC to can 180 lead °C tocan an lead increase to an ofincrease conversion of conversion from 50% from to 85% 50% (Entry to 85% 4 (Entry and 5). 4 In and the 5). case In the of cyclohexane, case of cyclohexane, the base the is not base soluble is not insoluble it. The in effectit. The of effect the base of the in cyclohexanebase in cyclohexane was thus was not thus investigated. not investigated. As can beAs seencan be from seen Table from5 EntriesTable 5 6 Entries and 7, hydrogen 6 and 7, canhydrogen improve can the improve conversion the from conversion 31% to 96%. from Comparing 31% to 96%. Entries Comparing 5 and 7, moreEntries cyclohexylamine 5 and 7, more yieldcyclohexylamine is obtained in yield cyclohexane, is obtained indicating in cyclohexane, the solvent indicating can affect the the solvent activity can of Ni/Alaffect theO ,activity as the polarity of Ni/Al of2 cyclohexaneO3, as the polarity is smaller of thancyclohexane that of THF. is smaller In cyclohexane, than that it isof easierTHF. forIn affect 2the3 activity of Ni/Al2O3, as the polarity of cyclohexane is smaller than that of THF. In cyclohexanolcyclohexane, toit is adsorb easier on for the cyclohexanol surface of Al toO adsorband facilitate on the surface the catalytic of Al2O reaction.3 and facilitate However, the the catalytic major cyclohexane, it is easier for cyclohexanol to2 adsorb3 on the surface of Al2O3 and facilitate the catalytic disadvantagereaction. However, of cyclohexane the major lies disadvantage in its low ability of cyclohexane to solubilize lies some in its polar low alcohol.ability to In solubilize that case, some THF maypolar be alcohol. a better In choice. that case, THF may be a better choice.

Catalysts 2016, 6, 63 8 of 10

Table 5. Conversion of cyclohexanol into cyclohexylamine over Ni/Al2O3 in tetrahydrofuran (THF) and cyclohexane.

˝ Entry Solvents T/ CH2/MPa NaOH/g Conversion/% Yield/% 1 THF 160 0 0 13 11 2 THF 160 1 0 11 9 3 THF 160 0 0.3 50 46 4 THF 160 1 0.3 50 41 5 THF 180 1 0.3 85 76 6 cyclohexane 180 0 0 31 28 7 cyclohexane 180 1 0 96 87

Reaction conditions: cyclohexanol 1 g, Ni/Al2O3 (Ni 10 wt %) 1 g, solvent 25 mL, NH3 0.4 MPa, 17 h.

Table6 shows the activity of Ni/C in cyclohexane and THF. Similar with Ni/Al 2O3, NaOH still ˝ plays a more important role than H2 (Entries 1, 2 and 3) in THF. At 180 C in the presence of H2 and NaOH, only 32% conversion was achieved for Ni/C catalyst (Entry 4). While at 180 ˝C in cyclohexane, 42% conversion was obtained (Entry 5). Compared with Table5, Ni/Al 2O3 is superior to Ni/C both in THF and cyclohexane.

Table 6. Conversion of cyclohexanol into cyclohexylamine over Ni/C in THF and cyclohexane.

˝ Entry Solvent T/ CH2/MPa NaOH/g Conversion/% Yield/% 1 THF 160 0 0 11 9 2 THF 160 0 0.3 22 20 3 THF 160 1 0 12 10 4 THF 180 1 0.3 32 28 5 cyclohexane 180 1 0.3 42 35

Reaction conditions: cyclohexanol 1 g, Ni/C (Ni 10 wt %) 1 g, solvent 25 mL, NH3 0.4 MPa, 17 h.

3. Experimental Section

3.1. Materials Cyclohexanol (ě99%), cyclohexylamine (ě99%), aqueous ammonia (25%–28%), NaOH (ě99%), cyclohexane (ě99%) and tetrahydrofuran (ě99%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Nickel nitrate hexahydrate (ě98%) was purchased from Tianjin Science and Technology Co., Ltd. (Tianjin, China). Active carbon was supplied by Cabot Co., Ltd. (Boston, MA, USA). γ-Al2O3 was purchased from ShanDong Aluminium Industry Co., Ltd. (Shandong, China). Raney Ni was obtained from Dalian General Chemical Industry Co., Ltd. (Dalian, China). And 1, 6-hexanediol (>99%) was bought from Aladdin (Shanghai, China).

3.2. Catalyst Preparation

Ni/Al2O3 and Ni/C (Ni 10 wt %) catalysts were prepared by impregnation. A mixture of γ-Al2O3 ˝ or C and an aqueous solution of Ni(NO3)2¨ 6H2O was evaporated at 40 C under reduced pressure, ˝ then dried at 110 C for 12 h. The as-synthesized Ni/Al2O3 and Ni/C were reduced in a tubular ˝ furnace under a flow of H2 at 500 C for 6 h.

3.3. Catalyst Characterization X-ray diffraction (XRD) measurements were carried out by a Bruker D8 Advanced X-ray diffractometer using Cu Kα radiation (λ = 1.5147 Å, Karlsruhe, Germany). High Resolution Transmission Electron Microscopy (HRTEM) measurements were carried out using an emission Tecnai G2F20 electron microscope (FEI, Hillsboro, OR, USA). Catalysts 2016, 6, 63 9 of 10

The Brunauer-Emmett-Teller (BET) surface area of Al2O3 and active carbon was measured using ˝ a micromeritics (ASAP-2020 M + C). Samples were pretreated at 180 C for 3 h under vacuum. N2 adsorption/desorption isotherms were measured at 77 K. The surface areas were determined from adsorption values using the Brunauer-Emmett-Teller (BET) surface method. The loss of Ni was determined using a Thermo IRIS IntrepidIIXSP Inductively Coupled Plasma Emission Spectrometer (Labcompare, South San Francisco, CA, USA).

3.4. Typical Procedures for the Reactions Aqueous ammonia, cyclohexanol, NaOH and Raney Ni were charged into an autoclave, which was sealed and then heated under magnetic stirring at a preset temperature. Tetrahydrofuran (or cyclohexane) and cyclohexanol were poured into the autoclave. The catalyst Ni/Al2O3 or Ni/C was then charged into the autoclave immediately after reduction under the protection of N2. The autoclave purged three times with H2 was pressured to 0.4 MPa NH3 under stirring for 30 min. After being pressured to 1 MPa H2, the autoclave was heated under stirring at a preset temperature.

3.5. Analytical Methods Gas chromatography (GC) analysis was conducted using a Varian-450 gas chromatograph (Varian, Salt Lake City, UT, USA) with a flame ionization detector. The temperature of the column (DB-5, 30 m ˆ 0.32 mm ˆ 0.25 µm) was maintained at 60 ˝C for 1 min and then raised to 280 ˝C with a ramp rate of 16 ˝C/min for 1 min. The flowing rate of nitrogen was 1 mL/min with a split ratio of 30:1. The yield of cyclohexylamine and the conversion of cyclohexanol were calculated using 1, 6-hexanediol as an internal standard.

4. Conclusions It was found in the amination of cyclohexanol into cyclohexylamine over Ni catalysts in aqueous ammonia, THF and cyclohexane that the base had positive effect on the activity of Raney Ni, Ni/Al2O3 and Ni/C in aqueous ammonia and THF. Hydrogen could also improve the conversion of cyclohexanol. In aqueous ammonia, Ni/C showed higher activity, thus was a better choice than Ni/Al2O3. While in THF and cyclohexane, Ni/Al2O3 was superior to Ni/C. The reasons why different supports influence the activity of Ni may be ascribed to their varying Ni surface areas. Stability study shows that Ni/C is better than Ni/Al2O3 in aqueous ammonia. This work is helpful for us to further understand the factors that could affect the amination of alcohol.

Acknowledgments: This research was supported by grants from the National Natural Science Foundation of China (No. 11204003, No. 21273260, No. 21433001, and 21303238), the Natural Science Foundation of Anhui Province (No. 1508085SMB209), the Anhui Provincial Key Science Foundation for Outstanding Young Talent (No. 2013SQRL022ZD) and Shandong Provincial Natural Science Foundation for Distinguished Young Scholar, China (No. JQ201305). Author Contributions: Y.Q. and H.Y. conceived and designed the experiments; Q. Y. performed the experiments; Y.Q., H.Y., Q.C and X.M. analyzed the data; B.D. and A. M. contributed reagents/materials/analysis tools; Y.Q. wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.

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