Basic Oxides-Supported Cobalt Catalysts for One-Pot Synthesis

Article pubs.acs.org/IECR

Acidic/Basic Oxides-Supported Cobalt Catalysts for One-Pot Synthesis of Isophorone Diamine from Hydroamination of Isophorone Nitrile Yingxin Liu,† Kuo Zhou,† Meng Lu,† Lichao Wang,† Zuojun Wei,*,‡ and Xiaonian Li§ †

Research and Development Base of Catalytic Hydrogenation, College of Pharmaceutical Science, and §Institute of Industrial Catalysis, College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310014, P.R. China ‡ Key Laboratory of Biomass Chemical Engineering of Ministry of Education, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, P.R. China ABSTRACT: A series of cobalt-based catalysts supported on several acidic/basic oxides were prepared by the incipient wetness impregnation method and applied for the one-pot synthesis of isophorone diamine (IPDA) from hydroamination of isophorone nitrile (IPN). The effects of the supports and cobalt loading on the performance of the catalysts were investigated. The results showed that the Co/SiO2 catalyst with 20 wt % Co loading gave the highest conversion of IPN (90.9%) and yield of IPDA (70.4 mol %) among the supported cobalt catalysts used in this work, and it could be reused eight times without a significant decrease in the catalytic performance, which was much better than that from the commercial Raney Co. Meanwhile, the possible intermediates and reaction pathways during hydroamination of IPN were provided.

1. INTRODUCTION Isophorone diamine (IPDA) is an important alicylic amine derived from the downstream products of acetone.1 Because of the unique amino structure, IPDA is widely used as a solidifying agent for epoxy resins, showing excellent stain resistance, oil resistance, weatherability, color stability, low viscosity, and shrinkage.2 It is also a raw material to isophorone diisocyanate, which is another popular material in the fields of polyurethane and paint.3 The synthesis of IPDA from acetone can be generally divided into three steps (as shown in Scheme 1): (1) cyclo-

(IPNim) by condensation with ammonia, followed by the hydrogenation of imine; the other is the cyano group which forms aminomethyl by adding two molecules of hydrogen.1,5 According to the reaction categories, the hydroamination of IPN can be divided into two steps, that is, the iminization of the carbonyl group and the subsequent hydrogenation of imine and cyano groups.1,4 The iminization of carbonyl is usually catalyzed by Lewis acids or bases. For example, Nguyen et al.6 prepared ortho-hydroxyphenylethylimine from orthohydroxybenzophenone using chiral phosphoric acid as the catalyst in the presence of NH3-saturated methanol, with 100% of imine yield. Verardo et al.7 used NH4Cl as the catalyst to prepare diphenyl ketimine from ketone in the presence of a NH3 pressure of 5−6 MPa, with above 90% of diphenyl ketimine yield. Hahn et al.8 used TiO2 as the catalyst for the above same reaction at 120 °C and a NH3 pressure of 20 MPa, and obtained 95% of dipenyl ketimine yield. Our previous research also showed that oxides such as CaO or SiO2 were favorable to the iminization of IPN.9 The subsequent hydrogenation of imine and cyano functional groups should be catalyzed by the metal hydrogenation catalysts. Among them, cobalt, especially Raney Co, was proven to be the favorable catalyst. For example, Sauer et al.10 employed Raney Co catalyst for the hydrogenation of IPNim at 100 °C and a H2 pressure of 6 MPa, and obtained a ca. 90 mol % yield of IPDA. Ernst et al.4 used Ru−Co/γ-Al2O3 catalyst to conduct the reaction at 70−80 °C and 23 MPa H2 for 91 h, and achieved a IPDA yield of 89.7 mol %. We previously investigated the performance of Pt/C, Pd/C, Raney Ni and Raney Co catalysts for the hydrogenation of IPNim, and also

Scheme 1. Pathways of Synthesis of IPDA from Acetone

condensation of three molecules of acetone to form isophorone (IPH); (2) cyanation of IPH to form isophorone nitrile (IPN); and (3) hydroamination of IPN to form IPDA.4 In the hydroamination of IPN, two functional groups are converted into amino groups: one is the carbonyl group which first forms imine and produces intermediate product isophorone imine © XXXX American Chemical Society

Received: June 16, 2015 Revised: September 2, 2015 Accepted: September 6, 2015

A

DOI: 10.1021/acs.iecr.5b02184 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

flow. The hydrogen consumption was monitored by a gas chromatograph equipped with a thermal conductivity detector (TCD). TEM and STEM images were obtained using a Tecnai G2 F30 S-Twin instrument (FEI Co., USA) operated at an accelerating voltage of 300 kV. Samples were prepared by dispersing the catalyst powder in ethanol under ultrasound for 15−20 min and then dropping the suspension onto a copper grid coated with carbon film. The XPS spectrum was obtained using an Escalab Mark II Xray spectrometer (VG Co., United Kingdom) equipped with a magnesium anode (Mg Kα = 1253.6 eV), 50 eV pass energy, a 0.2 eV kinetic energy step, and 0.1 s dwelling time. Energy corrections were performed using a 1 s peak of the pollutant carbon at 284.6 eV. The sample was prepared by pressing the catalyst powder onto the surface with silver sol gel. BET specific surface area and pore structures were measured by pulsed nitrogen adsorption−desorption method at −196 °C using an ASAP 2010 instrument (Micromeritics Instrument Co.). Prior to N2 physisorption, the samples were degassed under vacuum at 250 °C overnight. 2.4. Catalytic Reaction. The iminization and one-pot hydroamination reactions were carried out in the same 100 mL stainless steel autoclave equipped with a mechanical stirrer. In the iminization of IPN to IPNim, 40 mL of methanol (solvent), 2.5 g of IPN, and 0.5 g of acidic/basic catalyst were introduced into the reactor. After the reactor was sealed and degassed to 0.001 MPa to remove air, ammonia was introduced up to 0.2 MPa with continuous stirring at 1500 rpm. Afterward, the reactor was heated to 70 °C and retained at that temperature for 4 h. The process of the one-pot hydroamination of IPN to IPDA was similar to the iminization of IPN, except that (1) the catalyst was replaced by 0.5 g of oxides-supported Co catalyst or 0.2 g of CaO and a certain amount of Raney Co; (2) 8 MPa of H2 was introduced after filling the autoclave with ammonia; and (3) the reaction was conducted at 120 °C for 8 h. 2.5. Product Analysis. The reaction products were analyzed by using an Agilent 7890 gas chromatograph equipped with a HP-5 capillary column (30.0 m × 0.32 mm × 0.25 μm) and a flame ionization detector (FID). The column temperature was raised from 100 °C to 280 °C with a heating rate of 12 °C ·min−1. Both of the injector temperature and the detector temperature were set at 280 °C. Product identification was performed with an Agilent 6890 gas chromatography system coupled to a mass spectrometer equipped with an Agilent 5973 quadrupole mass analyzer. The chromatographic analysis was conducted with an injector temperature of 280 °C and a HP-5 capillary column (30.0 m × 0.25 mm × 0.32 μm) with helium flow of 2.0 mL·min−1 and a 1:10 split ratio. The oven was heated using the following temperature program: initial temperature of 100 °C increased to 280 °C at a heating rate of 12 °C·min−1 and was maintained for 10 min. The mass spectrometer was operated in electron ionization (EI) mode at an energy of 70 eV.

observed that Raney Co showed the highest selectivity toward IPDA.9 However, the recyclability of Raney Co for the reaction was poor. Besides, a large amount of Raney Co catalyst is needed in the course of the reaction because it is easily crushed under intense agitation, which limited the industrial application of the catalyst.11,12 From the industrial point of view, there is an urgency to develop a new cobalt catalyst that meets the need in terms of activity and cost. Alternatively, supported cobalt catalysts have been successfully used in a number of reactions for years with good catalytic performance.13−23 Therefore, it is expected that this kind of catalysts could be a promising substitute in the hydrogenation of imine and cyano groups. So far, however, few researches on the hydrogenation of IPNim over supported cobalt catalysts have been reported in the literatures. Since the iminization of IPN could be catalyzed by some acidic/basic oxides and the following hydrogenation step could be catalyzed by cobalt catalyst, herein we developed a series of acidic/basic oxides supported cobalt bifunctional catalysts for the one-pot synthesis of IPDA from the hydroamination of IPN. The good dispersion of cobalt in an acidic/basic oxide support was also expected to improve the catalytic performance of cobalt. The morphology and the physicochemical properties of the as-prepared catalysts were characterized by various techniques including X-ray diffraction (XRD), hydrogentemperature-programmed reduction (H2-TPR), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and N2 adsorption/desorption isotherm. In addition, the possible intermediates and reaction pathways during the hydroamination of IPN were also investigated.

2. EXPERIMENTAL SECTION 2.1. Materials. IPN was synthesized in our laboratory with a purity of more than 98.5%. Raney Co was purchased from Shanghai Sun Chemical Technology Co. Ltd. Ammonia, hydrogen and nitrogen gases were provided by Hangzhou Jingong Special Gas Co. Ltd., with purity larger than 99.99%. Other chemicals were analytical reagents and purchased from Sino-pharm Chemical Reagent Co. Ltd. 2.2. Catalyst Preparation. Acidic/basic oxides such as SiO2, TiO2, CaO, NaZSM-5, and ZrO2−Al2O3 were used as the supports. All of the supported cobalt bifunctional catalysts were prepared by the incipient-wetness impregnation method.24−26 Taking 20 wt % Co/SiO2 catalyst as an example, the typical preparation steps were as follows: 3 g of SiO2 was impregnated with an aqueous solution of Co(NO3)2·6H2O (3.7 g of Co(NO3)2·6H2O dissolved in 5.3 g of H2O) at room temperature and maintained overnight. The mixture was then dried at 110 °C for 10 h, calcined at 500 °C in air for 4 h and finally reduced at 550 °C in hydrogen flow for 4 h.24,27,28 2.3. Catalyst Characterization. XRD patterns were recorded with an XRD-600 diffractometer (Shimadzu Co., Japan) using a Cu Kα radiation (λ = 0.15406 nm) in a Bragg− Brentano parafocusing optics configuration (40 kV, 40 mA). Samples were scanned from 5 to 100° with a scanning rate of 4°·min−1 and a step size of 0.02°. The crystalline phases were identified by reference to the JCPDS database. H2-TPR analysis was obtained using a FINESORB-3010 TPD instrument. The fresh catalyst was first pretreated under pure argon flow at 200 °C for 2 h and then cooled down to room temperature under argon flow. After that, a H2-TPR experiment was performed from room temperature to 900 °C at a heating rate of 10 °C·min−1 under 5% H2/Ar gas mixture

3. RESULTS AND DISCUSSION 3.1. Effect of the Supports on the Performance of Co Catalysts for IPN Hydroamination to IPDA. As reported, there are two steps to synthesize IPDA from IPN, that is, the iminization of IPN to IPNim, and the hydrogenation of the latter to IPDA. In our preliminary work,9 some solid acidic/ basic oxides, such as γ-Al2O3, SiO2, TiO2−SiO2, TiO2, and CaO B


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Industrial & Engineering Chemistry Research Table 1. Effect of the Different Oxides on the Iminization of IPN to IPNima catalyst

IPN conversion (%)

IPNim yield (mol %)

b γ-Al2O3 SiO2 TiO2−SiO2 TiO2 CaO

46.0 65.0 92.0 90.7 92.1 97.4

26.4 41.7 79.0 78.5 79.4 87.6

a

Reaction conditions: IPN, 2.5 g; methanol, 40 mL; catalyst, 0.5 g; NH3 pressure, 0.2 MPa; temperature, 70 °C; reaction time, 4 h; stirring speed, 1500 rpm. bWithout any catalyst.

Table 2. Effect of the Different Supports on the Performance of Co Catalysts for IPN Hydroaminationa

Figure 1. XRD patterns of Co/SiO2 catalysts with different Co loading.

yield (mol %) catalyst

conversion (%)

IPDA

IPAA

IPNim

others

Co/NaZSM-5 Co/ZrO2−Al2O3 Co/TiO2 Co/CaO Co/SiO2 Raney Co (2.0 g) Raney Co (1.0 g) Raney Co (0.075 g)

97.2 99.0 71.3 88.6 86.6 100 93.7 43.2

11.7 20.9 25.2 11.5 49.5 97.9 51.9 22.6

69.1 62.9 23.5 51.2 24.2 1.3 20.1 13.5

13.7 10.0 14.1 11.0 3.5 0 12.0 1.9

2.7 5.2 8.5 14.9 9.4 0.8 9.7 5.2

a

Reaction conditions: IPN, 2.5 g; methanol, 40 mL; NH3 pressure, 0.2 MPa; H2 pressure, 8 MPa; supported Co catalyst with 15 wt % loading, 0.5 g (or 0.2 g of CaO and a certain amount of Raney Co); temperature, 120 °C; reaction time, 8 h; stirring speed, 1500 rpm.

Table 3. Effect of Co Loading on the Performance of Co/ SiO2 Catalysts for IPN Hydroaminatioan

Figure 2. H2-TPR profile of 20 wt % Co/SiO2 catalyst.

yield (mol %) catalyst 10 15 20 25 30

wt wt wt wt wt

% % % % %

conversion (%)

IPDA

IPAA

IPNim

others

82.0 86.6 90.9 85.5 82.4

35.9 49.5 70.4 61.1 53.9

23.2 24.2 7.0 9.1 10.2

3.5 3.5 4.4 2.4 2.8

19.4 9.4 9.1 12.9 15.5

Co/SiO2 Co/SiO2 Co/SiO2 Co/SiO2 Co/SiO2

a

Reaction conditions: IPN, 2.5 g; methanol, 40 mL; NH3 pressure, 0.2 MPa; H2 pressure, 8 MPa; Co/SiO2 catalyst, 0.5 g; temperature 120 °C; reaction time, 8 h; stirring speed, 1500 rpm.

Table 4. Stability Results of 20 wt % Co/SiO2 Catalyst for IPN Hydroaminationa yield (mol %) cycle

conversion (%)

IPDA

IPAA

IPNim

others

1 2 3 4 5 6 7 8

90.9 90.5 90.0 89.8 89.1 88.9 88.6 88.5

70.4 69.2 68.8 68.8 67.1 66.4 65.7 65.0

7.0 6.9 7.4 7.7 9.6 8.9 10.5 13.0

4.4 3.5 3.0 4.6 3.4 2.1 0.8 0.2

9.1 10.9 10.8 8.7 9.0 11.5 11.6 10.3

Figure 3. TEM images of 20 wt % Co/SiO2 catalyst: (a) 200 nm, (b) 10 nm, (c) 50 nm, and (d) 1 nm.

a

Reaction conditions: IPN, 2.5 g; methanol, 40 mL; NH3 pressure, 0.2 MPa; H2 pressure, 8 MPa; catalyst, 0.5 g; temperature, 120 °C; reaction time, 8 h; stirring speed, 1500 rpm.

were used as catalysts for the iminization of IPN, and the results are shown in Table 1. It can be seen that CaO, SiO2, and TiO2 C


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Figure 4. STEM, STEM HAADF, and EDX images of 20 wt % Co/SiO2 catalyst.

Among the supported cobalt catalysts used in this work, SiO2 supported 15 wt % Co catalyst exhibited the best catalytic performance, which achieved the highest IPDA yield of 49.5 mol %. TiO2 ranked the second, with IPDA yield of 25.2 mol %; while basic oxides CaO and NaZSM-5 supported cobalt catalysts showed the worst performance, with only around 11 mol % of IPDA yield. Although both acidic and basic oxides were catalytic active for the iminization of IPN, the experimental results shown in Table 2 revealed that acidic oxides, especially weak acidic oxide such as SiO2, seemed to be more favorable to the one-pot hydroamination reaction of IPN while combined with Co. Furthermore, the largest specific surface area of SiO2 among the support (for example, the general specific surface areas of TiO2 and SiO2 are around 50 m2·g−129 and 250 m2·g−1,30 respectively) should be another important reason for the excellent catalytic performance. The catalytic performance of commercial Raney Co has also been investigated for comparison. It can be seen from Table 2 that the catalytic performance of Raney Co strongly depended on the amount of the catalyst: 2.0 g of Raney Co could convert almost 100% of IPN with 97.9 mol % of IPDA yield, while 0.075 g of Raney Co only achieved 43.2% conversion of IPN and 22.6 mol % yield of IPDA, which was far less than Co/SiO2 with the same Co content (0.5 g of Co/SiO2 with 15 wt % Co loading contained only 0.075 g of Co). To reach the similar yield of IPDA (about 50%), the amount of Raney Co catalyst should be around 1.0 g, which is about 13 times higher than that of Co/SiO2 catalyst. On the basis of cost considerations, Co/SiO2 catalyst was more suitable for the one-pot hydro-

Figure 5. Decomposed Co 2p XPS spectrum of 20 wt % Co/SiO2 catalyst.

showed good catalytic performance for the iminization of IPN to IPNim. Since cobalt was reported to be the best metal for the hydrogenation of IPNim to IPDA, which was also verified in our work, it is rational to combine cobalt with those acidic/ basic oxides for the one-pot hydroamination of IPN to IPDA. With regard to this, we prepared and tested a series of cobaltbased catalysts supported on several commercial available acidic/basic oxides with 15 wt % cobalt loading for the one-pot hydroamination of IPN to IPDA, and the results are shown in Table 2. D


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Figure 6. Total ion current spectra of the components in IPN hydroamination. (A) Isophorone (IPH); (B) IPN; (C) 3,3,5trimethylcyclohexylamine; (D) 1,3,3-trimethyl-6-azabicyclo[3,2,1]octane; (E) 3,3,5-trimethylcyclohexanol; (F) 1,3,3-trimethyl-7-imino-6-azabicyclo[3,2,1]octane; (G) 3-aminomethyl- 3,5,5- trimethylcyclohexanol; (H) 3,5,5-trimethyl-6-amino-7-azabicyclo[3,2,1]octane; (I) isophoronelamine (IPDA); (J) isophoroneimine (IPNim). (a) IPN; (b) products of iminization; (c) products of hydroamination.

which was assigned to the amorphous SiO2 (JCPDS-ICDD Card No. 05-0490).31 The three obvious diffraction peaks at 2θ = 44.2°, 51.5°, and 75.8° in each pattern corresponded to the Co (111), (200), and (220) crystal planes, showing that cobalt mainly existed in the form of Co0. The weak diffraction peaks at 2θ = 42.4° and 61.5°, respectively, corresponded to the CoO (200) and CoO (220) (JCPDS-ICDD Card No. 48-1719), showing the existence of a small amount of CoO, which could be ascribed to the insufficient reduction of the cobalt in the catalyst precursor to Co0, or most probably, part oxidation of Co0 on the surface of the catalyst during the storage time. For each pattern, the Co average crystallite sizes derived from the Co (111) diffraction peaks were around 16 nm, which seemed largely independent of the cobalt loading from 10 wt % to 30 wt %. To determine the optimal reduction temperature in the process of the cobalt catalyst preparation, the reducibility of the 20 wt % Co/SiO2 catalyst precursor was investigated by H2TPR, and the corresponding result is shown in Figure 2. It can be seen that three obvious reduction peaks were observed at 360 °C, 470 °C and 650 °C, respectively. It is generally believed that cobalt ions in Co/SiO2 mainly exist in the form of Co3O4 after calcination. The major reduction peaks located at 360 and 470 °C were attributed to the reduction of Co3O4 in two reduction steps Co (III) → Co (II) → Co (0).15,32,33 It was reported that cobalt silicates with a spinel structure could be formed due to the combination of cobalt ions with the hydroxyl on the silica surface during the impregnation process.34,35 Therefore, the small reduction peak around 650 °C was mainly attributed to the reduction of cobalt silicates. Cobalt silicate is generally regarded difficult to be reduced and cannot form dispersive metal particles with reaction activity after reduction, thus having no activity for hydrogenation.35−37 It can be seen from Figure 2 that the reduction of Co3O4 to Co0 finished below 600 °C, which suggested that almost all Co3O4 particles in Co/SiO2 were reduced to Co0 active sites at our reduction temperature of 550 °C. This result was in agreement with that of XRD. After the 20 wt % Co/SiO2 catalyst was reduced at 550 °C for 4 h, the BET surface area, total pore volume, and average pore diameter were 221 m2·g−1, 0.67 m3·g−1, and 11.2 nm, respectively. Figure 3 shows the TEM images of the 20 wt % Co/SiO2 catalyst. Quite different from the XRD pattern in which only one average size of cobalt crystallites is obtained, it is

amination of IPN to IPDA. It was therefore selected for the following research. During the hydroamination of IPN to IPDA, the main byproduct was 3-aminomethyl-3,5,5-trimethyl cyclohexanol (IPAA), which might be generated by the direct hydrogenation of IPN (Table 2). Generally, IPN iminization to IPNim is a reversible reaction. Higher reaction temperature may cause the decomposition of IPNim back to IPN, and further be hydrogenated to IPAA. On the basis of our experimental results (not shown here), the optimized temperature for iminization is 70 °C, and 120 °C for hydrogenation of IPNim. As a sequence, if the hydrogenation selectivity of −CNH to −C−NH2 is lower than that of −CO to −C−OH, or larger amount of IPN is formed due to the decomposition of IPNim, the selectivity toward byproduct IPAA will be higher, leading to a lower selectivity to IPDA. Besides IPAA and IPNim, other byproducts during the hydroamination of IPN were not strictly quantified, whose possible formation pathways will be discussed in section 3.3. 3.2. Effect of Co Loading on the Performance of Co/ SiO 2 Catalyst for IPN Hydroamination to IPDA. 3.2.1. Catalytic Activity and Recyclability. The catalytic performances of Co/SiO2 catalysts with different Co loading (10, 15, 20, 25, and 30 wt %) are shown in Table 3. It can be seen that the yield of IPDA ranged from 35.9 to 70.4 mol % in the experimental reaction conditions. Generally speaking, higher Co loading means more active sites in a unit weight of catalyst for the reaction. However, due to the aggregation of Co particles, a higher Co loading may lead to a low catalytic efficiency. As shown in Table 3, the catalytic performance of the Co/SiO2 catalysts was obviously affected by Co loading. The conversion of IPN increased from 82.0% to 90.9% when Co loading varied from 10 to 20 wt %, and then decreased with the increase in Co loading. The yield of IPDA increased from 35.9% to 70.4% with increasing Co loading from 10 to 20 wt %, and then decreased when Co loading was further increased. Among the Co/SiO2 catalysts tested, the 20 wt % Co/SiO2 catalyst exhibited the highest performance, with 90.0% conversion of IPN and 70.4 mol % yield of IPDA. More importantly, the 20 wt % Co/SiO2 catalyst could be recycled more than eight times with only less than 5 mol % of decrease in the yield of IPDA as shown in Table 4. 3.2.2. Catalyst Characterization. Figure 1 shows the XRD patterns of Co/SiO2 catalysts with different Co loading. All the five catalysts showed a broad diffraction peak at 2θ = 20.8°, E


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Figure 7. Mass spectra of the main components in the products of IPN hydroamination. (A) Isophorone (IPH); (B) IPN; (C) 3,3,5trimethylcyclohexylamine; (D) 1,3,3-trimethyl-6-azabicyclo[3,2,1]octane; (E) 3,3,5-trimethylcyclohexanol; (F) 1,3,3-trimethyl-7-imino-6-azabicyclo[3,2,1]octane; (G) 3-aminomethyl- 3,5,5- trimethylcyclohexanol; (H) 3,5,5-trimethyl-6-amino-7-azabicyclo[3,2,1]octane; (I) isophoronelamine (IPDA); (J) isophoroneimine (IPNim).

size around 160 nm (Figure 3b), which are composed of hundreds of small individual Co crystallites as shown in Figure 3c, whereas the smaller Co particles only have sizes in the range of 3−5 nm (Figure 3a). Both kinds of the particles are proven

interesting to see that there are two uniform sizes of Co particles at different magnifications of the TEM images, which was also observed by Storsæter et al.38 and Barbier et al.39 The larger Co particles exist as agglomerates with the mean particle F


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Industrial & Engineering Chemistry Research Scheme 2. Possible Reaction Pathways for the Synthesis of IPDA from IPN

IPN and the possible reaction routes, we analyzed the products of iminization and hydroamination from IPN, respectively, by the GC−MS method. The typical MS total ion current (TIC) spectra are shown in Figure 6, and the MS spectra of the main components and impurities are shown in Figure 7. It can be seen from Figure 6 that the iminization of IPN had higher selectivity and less byproducts, while the subsequent hydroamination reaction generated a lot of byproducts. As reported, the byproducts are generated from the high reactive intermediates in the process of the iminization of carbonyl and the multistep hydrogenation of cyano.54 On the basis of the characteristics of all the functional groups in IPN and IPNim molecules, we concluded the main reasons of the formation of the byproducts are as follows: (1) In the iminization process, a part of the carbonyl group in IPN was directly hydrogenated to 3,3,5-trimethylcyclohexanol. (2) High activity of imine: imine is very reactive at the iminization temperature of 70−120 °C. The intramolecular heterocyclic reactions may occur to form 1,3,3trimethyl-6-azabicyclo[3,2,1] octane (D in Figure 7), 3,5,5trimethyl-6-amino-7-azabicyclo[3,2,1] octane (F in Figure 7) and 1,3,3-trimethyl-7-imino-6-azabicyclo[3,2,1] octane (H in Figure 7). (3) Cyano leaving: during the hydrogenation of IPNim, the cyano group in IPNim tended to leave by elimination reaction,51 or to be further hydrogenated to 3,3,5trimethylcyclohexylamine (compound C in Figure 7). (4) Isomerization of the products: in the hydrogenation process, isomerization of imine easily happened to form secondary amine, such as 3,3,5-trimethylcyclohexylamine (I in Figure 7).55 According to the structure of identified impurities, the possible routes of the iminization and hydroamination of IPN are given as shown in Scheme 2. It should be noted that the dimerization and trimerization of IPDA and other intermediates may probably occur because of their high reactivity. However, these compounds having high boiling points, were not detected by our GC−MS technique, and not included in the present study.

to be Co through energy dispersive spectroscopic analysis (data not shown) and lattice fringe spacing measurement (Figure 3d), in which 0.2206 nm of the lattice fringe spacing corresponds to the Co (0) (111) crystal plane (JCPDSICDD Card No. 15-0806).40 The Co particles with larger size are also observed in a typical STEM image as shown in Figure 4, in which the light particles that are definitely the same as the dark ones in the TEM image Figure 3 (b) are proven again to be Co by chemical mapping recorded with the HAADF (high angle annular dark field) detector.41 The electronic states and surface characteristics of the 20 wt % Co/SiO2 catalyst were investigated by XPS, and the results are shown in Figure 5. The typical spectrum was characterized by Co 2p3/2 and Co 2p1/2 spin−orbit components separated by 15.0 eV and a shallow feature in between. The main peak at the binding energy (BE) of 778.1 eV could be assigned to a Co0 2p 3/2 configuration, while the peak at 793.1 eV could be attributed to the Co0 2p1/2 configuration,42,43 indicating that cobalt species in Co/SiO2 catalyst mainly existed in the form of zero valence. This result was in agreement with those of XRD and H2-TPR. The Co 2p spectrum also had a weak Co 2p3/2 peak at 780.2 eV and a weak Co 2P1/2 peak at 795.2 eV, which were attributed to the CoII 2p3/2 and CoII 2p1/2 configuration. The weak peaks at 783.4 and 798.2 eV represented the shakeup peaks for CoII 2p3/2 and CoII 2p1/2, respectively, which were the important fingerprints of CoO to distinguish it from the other forms of cobalt.31,37,44−46 3.3. Products Analysis. Because of the coexistence of several active functional groups, such as amino, hydroxyl, carbonyl, cyano, and CC, a lot of side reactions may occur during the hydroamination of IPN, such as the incomplete addition of carbonyl and cyano groups, the formation of alcohol from the addition of carbonyl, and the N-heterocyclization through the condensation reaction between amines and imines.47−50 Based on the summary of the previous work and a series of experiments on the hydroamination of carbonyl compounds in NH3 and H2 atmosphere, Gomez et al.51 put forward the mechanism for the formation of secondary and tertiary amines from the dehydration-condensation of primary amine, imine, and carbonyl, and the formation of cyano from the deamination of imine. Krupka et al.52,53 have explained the mechanism of these complex reactions on the basis of the classical theory proposed by von Braun and Greenfield, in which condensation of imine and primary amine gave secondary amine and tertiary amine by eliminating amino. To determine the product distribution of the hydroamination of

4. CONCLUSIONS A series of acidic/basic supported Co bifunctional catalysts, such as Co/ZrO2−Al2O3, Co/NaZSM-5, Co/TiO2, Co/CaO, and Co/SiO2 were successfully prepared via the incipientwetness impregnation method and were used for the one-pot catalytic hydroamination of IPN to IPDA. The effects of the supports and Co loading on the performance of the Co catalyst were investigated. The experimental results showed that the Co catalyst supported on SiO2 with 20 wt % Co loading exhibited G


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the best catalytic performance for IPN hydroamination. At the reaction conditions used in this work, the conversion of IPN reached 90.9%, and the yield of IPDA was up to 70.4 mol %. After being reused eight times, the catalyst also showed high catalytic performance. In addition, the structure of intermediates and main byproducts generated during the hydroamination of IPN were analyzed by GC−MS, the possible reaction routes were speculated, and the main reasons were analyzed, which may provide guidance for the synthesis of IPDA.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 13588810769. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Natural Science Foundation of China (21106134, 21276230, and 21476211), the Zhejiang Provincial Natural Science Foundation of China (LY14B060003), and the Program for Zhejiang Leading Team of S&T Innovation (2011R50002).

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Basic Oxides-Supported Cobalt Catalysts for One-Pot Synthesis

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