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Liquid phase acetonitrile hydrogenation to ethylamine over a highly active and selective Ni–Co–B amorphous alloy catalyst
Applied Catalysis A: General 275 (2004) 199–206 www.elsevier.com/locate/apcata
Liquid phase acetonitrile hydrogenation to ethylamine over a highly active and selective Ni–Co–B amorphous alloy catalyst Hexing Lia,*, Yuedong Wua, Jing Zhanga, Weilin Daib, Minghua Qiaob a
Department of Chemistry, Shanghai Normal University, No. 100, Guilin Road, Shanghai 200234, PR China b Department of Chemistry, Fudan University, Shanghai 200433, PR China Received in revised form 12 July 2004; accepted 22 July 2004 Available online 1 September 2004
Abstract The ultrafine Ni–Co–B amorphous alloys with Co/(Co + Ni) molar ratio (xCo) varying from 0 to 1 was prepared by chemical reduction of mixed Ni2+ and Co2+ ions with BH4 in aqueous solution. During liquid phase acetonitrile hydrogenation to ethylamine, the specific activity (Rm) and the intrinsic activity (TON) of the Ni–Co–B catalyst first increased and then decreased with the increase of xCo from 0 to 1. The maximum activity was obtained at xCo = 0.5; the value of the activity was nearly twice as that of the Ni–B or the Co–B catalyst. Treatment of the Ni–Co–B catalyst at 873 K resulted in an abrupt decrease in the activity due both to a decrease in active surface area and, especially, to the crystallization and the decomposition of the Ni–Co–B amorphous alloy. The selectivity to ethylamine increased rapidly with xCo and then remained constant at xCo 0.5. The maximum yield of ethylamine could reach 93%, showing a good potential for industrial applications. According to kinetic studies and results of various characterization methods, such as ICP, XRD, EXAFS, XPS, SAED, TEM, DSC, TPD, and hydrogen chemisorption, the correlation of the catalytic performance to both the structural and the electronic characteristics was discussed briefly. The activation of the CBN and/or C=N bonds, the promotion on the hydrogen adsorption, and the inhibition on the ethylamine adsorption were the decisive factors responsible for the excellent activity and selectivity of the Ni–Co–B catalyst. # 2004 Elsevier B.V. All rights reserved. Keywords: Ni–Co–B amorphous catalyst; Hydrogenation; Acetonitrile; Ethylamine
1. Introduction Hydrogenation of nitriles is widely used in industry for producing diverse primary amines [1–5]. However, the utility of this method in production of ethylamine (EA) is seldom employed possibly due to the high cost and even non-availability of acetonitrile (CH3CN) in the past. Today, with the rapid development of the acrylonitrile industry, the production of the CH3CN has increased abruptly, since it is a major byproduct with the yield up to 10 wt.%. Because the applications of CH3CN are quite limited, more and more CH3CN has been accumulated. In some factories, acetonitrile is burned to reduce the storing burden, which not only wastes a resource but also causes environmental problems. * Corresponding author. Tel.: +86 2164322141; fax: +86 2164322142. E-mail address: [email protected] (H. Li). 0926-860X/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2004.07.034
In view of the industrial requirements and environmental considerations, there is a strong driving force to develop new technology to transform the CH3CN to other useful chemicals. One of the promising ways is the selective hydrogenation of CH3CN to EA, an important intermediate for producing polymers, drugs, and other fine chemicals. A problem arising from this process is the selectivity since, besides the primary amine, both the secondary and even the tertiary amines are usually formed during the nitrile hydrogenation [6,7]. Among various factors, the catalyst plays a key role in determining the selectivity to EA [8]. Raney Ni is probably the most frequently used catalyst in the nitrile hydrogenation to primary amines [1,5]. However, only less than 40% EA yield could be obtained over Raney Ni during the CH3CN hydrogenation [9]. Even in the presence of ammonia, the maximum EA yield could not exceed 80%. Up to now, only very few catalysts have been
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reported to be suitable for the title reaction. As is wellknown, amorphous alloys exhibits higher activity, better selectivity, and stronger resistance against both sulfur and amine poison in various hydrogenations [10–13]. We reported previously that the Ni–B amorphous alloy exhibited comparable activity to that of Raney Ni but much higher selectivity to EA [6]. The Co–B amorphous alloy was even more selective to EA but its activity was very poor [14]. Based on these interesting results, we report here a novel Ni– Co–B amorphous alloy catalyst for the CH3CN hydrogenation with the aim of obtaining both high activity and selectivity to EA.
2. Experimental 2.1. Catalyst preparation The Ni–Co–B sample was prepared as follows: 34 ml of 2.0 M KBH4 aqueous solution containing 0.20 M NaOH was added dropwise over a 90 min period into a 20 ml aqueous solution containing desired amounts of NiCl2 and CoCl2 at room temperature and with vigorous stirring. The molar ratio between KBH4 and the total amount of metal was 4:1 to ensure the complete reduction of all the metallic ions in the solution. The resulting black solid was washed free from Cl and K+ ions with distilled H2O until pH = 7. Then, it was further washed with absolute alcohol (EtOH) and finally kept in EtOH until the time of use. The Co/(Co + Ni) molar ratio (xCo) in the Ni–Co–B sample was determined by ICP analysis and was adjusted by changing the content of NiCl2 and CoCl2 in the solution. For comparison, both Ni–B (xCo = 0) and Co–B (xCo = 1) were also prepared in a similar way by using a solution containing only NiCl2 or only CoCl2, respectively. 2.2. Catalyst characterization The surface morphology and particle sizes of the Ni–Co– B samples were observed by using a transmission electron micrograph (TEM, JEM-2010). Their compositions were analyzed by means of inductively coupled plasma (ICP, Jarrell-As Scan 2000). The active surface area (SM) was determined by H2 chemisorption, as described previously [15]. The temperature-programmed desorption (TPD) of H2, CH3CN or EA was carried out in the same system according to the following procedures. The catalyst surface was purged by ultrapure helium stream (purity of 99.997%) for 2.0 h at 573 K; this temperature was relatively lower than that reported by Gil [16] in order to avoid the crystallization of the amorphous catalysts. After the catalyst was cooled down to 313 K in helium stream, the probe molecules (H2, CH3CN, or EA, respectively) were fed to the catalyst by the He gas flow for 12 h until the adsorption saturated. Then, the desorption of these adsorbates was performed under Ar flow for hydrogen TPD or He flow for both CH3CN and EA TPD.
All the gases were of ultrahigh purity and were further purified by Chrompack clean-oxygen filter, molecular sieve and MnO subsequently. The temperature was raised from 313 to 900 K at 20 K/min. The amorphous structure of the Ni–Co–B catalysts was confirmed by both X-ray powder diffraction (XRD, Bruker AXS D8-Advance with Cu Ka radiation) and selective area electronic diffraction (SAED, HU-11B), and even studied by extended X-ray absorption fine structure (EXAFS, BL-10B) measurements carried out in the National Laboratory of High Energy Physics (KEK, Tsukuba, Japan). The crystallization process of the Ni–Co– B sample was monitored by differential scanning calorimetry (DSC, Perkin-Elmer) under N2 atmosphere at the heating rate of 10 K/min. X-ray photoelectron spectroscopy (XPS) was performed on a Perkin-Elmer PHI 5000C ESCA system using Al Ka radiation to determine the surface electronic states of the catalysts. All the binding energy (BE) values were calibrated by using C1S = 284.6 eV as a reference. The surface composition was determined by using 0.13, 2.43, and 2.50 as the PHI sensitivity factors to B1S, Ni2P3=2 , Co2P3=2 , respectively, offered by Perkin-Elmer Company [17]. During all the above characterizations, the wet Ni–Co–B sample soaked with EtOH was used; it was dried in situ in the atmosphere of ultra pure argon (purity of 99.99%, treated with a Chrompack clean-oxygen filter) to protect the sample from oxidation. 2.3. Activity test Liquid phase hydrogenation of CH3CN was carried out in a 200 ml autoclave in which the as-prepared Ni–Co–B catalyst containing 0.5 g metal (Ni or/and Co), 30 ml EtOH, and 10 ml CH3CN were mixed at PH2 = 3.0 MPa and T = 383 K. In order to ascertain the role of mass transfer, the catalyst amount was varied from 0.3 to 2.0 g and the speed of agitation was varied from 1000 to 1700 rpm. In view of the observation that the reaction rate was independent of the stirring rate and that it varied linearly with catalyst amount, it could be concluded that the stirring rate of 1200 rpm was high enough to eliminate the mass transfer [9]. Keeping the hydrogen pressure at 3.0 MPa, we sampled the reaction mixture every 15 min for GC analysis (GC 102) under the following conditions: FID detector, 2 m column filled with GDX-102/407, injector temperature 373 K, oven temperature 418 K, detector temperature 473 K, and N2 carrier gas (30 ml/min). The initial activity per gram of metal (specific activity, Rm = mmol/(h g)) was calculated according to the CH3CN conversion within the first hour. The turnover number (TON) was also calculated based on the Rm value and the surface metallic atoms determined by hydrogen chemisorption. From the product distribution in the reaction mixture, the selectivities to various products at given reaction time were obtained. The reproducibility of the results was checked by repeating the runs at least three times on the same batch of the catalyst and for another three times for a different batch of the catalyst; it was found to be within
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acceptable limits (5% for the same batch of the catalyst and 10% for the different batches of the catalyst).
3. Results and discussion As shown in Fig. 1, the TEM morphology revealed that the fresh Ni–Co–B sample with xCo of 0.50 was present in the form of ultrafine spherical particles with the average diameter of around 20 nm. After the sample had been treated at 873 K for 2 h in N2 atmosphere, severe gathering of the particles was observed. In comparison with the fresh Ni–B (xCo = 0) and Co–B (xCo = 1) samples, one can see that the increase of xCo resulted in a slight increase of the particle size. These results could account for the increase of the active surface area (SM) with the xCo and the abrupt decrease of SM after being treated at 873 K, as shown in Table 1. The SAED images attached in Fig. 1 demonstrated that all the fresh Ni–Co–B, Ni–B and Co–B displayed diffractional cycles indicative of amorphous structure [18]. After the sample had been treated at 873 K for 2 h, only individual bright dots were observed for the Ni–Co–B sample due to its crystallization. Further evidence for this conclusion was obtained from XRD patterns. As shown in Fig. 2, the fresh Ni–Co–B sample exhibited only one broad peak around 2u = 458, a typical peak indicative of amorphous alloy structure
Fig. 2. XRD patterns of (a) the fresh Ni–Co–B sample (xCo = 0.5), (b) the Ni–Co–B sample (xCo = 0.5) after being treated at 573 K for 2 h in N2 flow, and (c) the Ni–Co–B sample (xCo = 0.5) after being treated at 873 K for 2 h in N2 flow.
[19]. Treating the Ni–Co–B samples at T < 523 K in N2 flow for 2 h had no appreciable influence on the XRD pattern. However, when the treating temperature was increased further, various diffractional peaks corresponding to metallic Ni and Co, crystalline Ni–B, Co–B and Ni–Co– B alloys appeared, indicating the occurrence of crystal-
Fig. 1. TEM morphologies and SAED images of (a) the fresh Ni–Co–B sample (xCo = 0.5), (b) the Ni–Co–B sample (xCo = 0.5) after being treated at 873 K for 2 h in N2 flow, (c) the fresh Ni–B sample (xCo = 0), and (d) the fresh Ni–B sample (xCo = 1).
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Table 1 Structural characteristics and catalytic properties of the as-prepared catalystsa Catalyst
Bulk compound (at.%)
xCo
xSurf Co
SM (m2/g)
Rm (mmol/(h g))
TON (h 1)
ttotal (min)
Selectivity (%) EA
DEA
Ni–B Ni–Co–B-3 Ni–Co–B-5 Ni–Co–B-8 Co–B Crystallized Ni–Co–B-5
Ni75.2B24.8 Ni50.6Co21.9B27.5 Ni35.6Co34.7B29.7 Ni12.5Co57.5B30.0 Co75.4B25.6 Ni35.6Co34.7B29.7
0 0.30 0.50 0.80 1.00 0.50
0 0.20 0.30 0.71 1.00 0.50
19.6 18.1 17.0 16.4 15.8 2.8
90.8 113.0 146.3 129.6 66.4 7.9
18.2 24.5 33.7 31.0 16.4 11.1
150 120 90 105 210 –
67.6 83.1 93.0 93.0 92.8 –
32.4 16.9 6.7 6.7 6.7 –
a
Reaction conditions: a catalyst containing 0.5 g Ni and/or Co, 30 ml EtOH, and 10 ml CH3CN at PH2 = 3.0 MPa and T = 383 K.
lization, together with partial decomposition. The amorphous structure was further confirmed by the extended EXAFS, from which the RDF curves could be obtained by the fast Fourier transformation [20]. Only one broad peak ˚ was observed for all the fresh Ni–Co– around R = 1.7–2.4 A B samples regardless of the xCo, showing that they had no long-range ordering but only short-range ordering structure confined within the first-near-neighbor atom layer [21]. From the DSC analysis (Fig. 3), one can see that the Ni–Co– B sample with xCo of 0.5 exhibited only one exothermic peak around 729.9 K. Taking into account that the Ni–B displayed an exothermic peak around 614 K [9] while the Co–B showed an exothermic peak around 765 K [14], one could conclude that the Ni and Co in the Ni–Co–B sample were present in the whole amorphous alloy rather than in the individual Ni–B and Co–B amorphous alloys. The alloying between Ni and Co resulted in the shift of the crystallization temperature toward to that of the Co–B amorphous alloy. The XPS spectra in Fig. 4 demonstrated that both the nickel and cobalt species in the Ni–Co–B sample (xCo = 0.5) were present only in their metallic states, corresponding to binding energy (BE) of 852.6 eV in Ni2P3=2 level and 778.2 eV in Co2P3=2 level, respectively. The asymmetrical feature of the XPS peaks for Ni2P and Co2P demonstrated that both the metallic Ni and the metallic Co were present in different chemical environments, possibly owing to the co-
Fig. 3. DSC pattern of the Ni–Co–B sample (xCo = 0.5).
existence of the Ni–Co–B alloy and trace of both the Co–B and the Ni–B alloys. However, the boron species were present in both the alloying B and oxidizing B (B2O3), corresponding to BE of 188.1 eV and 192.5 eV, respectively. Comparing to the standard BE of pure B (187.1 eV) [22], one immediately found that the alloying B was positively charged, indicating that partial electrons transferred from alloying B to the metal (Ni and/or Co) in the Ni–Co–B amorphous alloy [23–25]. This could be explained by the assumption of Imanaka et al. that the bonding electrons of the B occupied the vacant d-orbitals of metallic Ni or Co [26]. The BE of the metallic Ni in the Ni–Co–B was 0.4 eV lower than that in the Ni–B. However, the BE value of the metallic Co in the Ni–Co–B was only 0.1 eV lower than that in the Co–B, which was within the error range of our XPS measurements. These results suggested that the metallic Ni accepted more electrons from the alloying B than the metallic Co in the Ni–Co–B amorphous alloy. As the BE values of the alloying B in the Ni–Co–N, Ni–B and Co–B amorphous alloys were almost the same, the higher electron density on the Ni atom in the Ni–Co–B sample could be mainly attributed to the B-enrichment, as shown in Table 1. According to our previous paper, the theoretical calculations using ab initio DFT method [27] demonstrated that the increase of the B content would contribute more electrons to the metallic Ni or Co in their corresponding amorphous alloys. When one compared the xCo determined by ICP and that determined by XPS (xSurf Co ), it was obvious that the Ni– Co–B surface was enriched with Ni. The Ni-enrichment increased with the increase of xCo from 0 to 0.5 and then slightly decreased when the xCo further increased. Fig. 5 shows the TPD curves of H2, CH3CN and EA molecules over the Ni–B, Co–B and Ni–Co–B (xCo = 0.5) amorphous catalysts, respectively. One can see that the Co– B exhibited much stronger adsorption for both the CH3CN and EA but weaker adsorption for hydrogen than the Ni–B. Although the Ni and Co contents were almost equivalent in the Ni–Co–B amorphous catalyst at xCo = 0.5, the position of the main desorption peak on the Ni–Co–B was close to that on the Ni–B catalyst, possibly due to the Ni-enrichment. From the TPD curves, it was also found that the Ni–Co–B catalyst exhibited weaker adsorption for CH3CN and EA than either the Ni–B or the Co–B, possibly owing to the higher electron density on the Ni active sites in the Ni–Co–
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Fig. 4. XPS spectra of the Ni–Co–B sample (xCo = 0.5).
B, which could repel the lone electron pair on the nitrogen atom in either the CH3CN or the EA molecule. As shown in Fig. 6, the CH3CN conversion increased almost linearly with the reaction time, indicating that the CH3CN hydrogenation was zero-order with respect to CH3CN concentration. A slight deviation from the straight
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Fig. 5. (a) Hydrogen, (b) acetonitrile and (c) ethylamine TPD curves over Ni–B, Co–B and Ni–Co–B (xCo = 0.5) amorphous alloys.
line was observed at the end of CH3CN hydrogenation, possibly due to the extremely low CH3CN concentration or/ and the accumulation of reaction products, which might poison the catalyst. At low CH3CN conversion (<50%), only EA was identified in the reaction mixture. With the increase of CH3CN conversion, diethylamine (DEA)
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Fig. 6. Change of the CH3CN conversion and the selectivities to EA and DEA with the reaction time over the Ni–Co–B (xCo = 0.5) amorphous alloy catalyst. Reaction conditions: 0.5 g catalyst, 10 ml CH3CN, 30 ml EtOH, T = 383 K, PH2 = 3.0 MPa, stirring rate = 1200 rpm.
appeared gradually, causing a rapid decrease in the selectivity to EA. However, no triethylamine (TEA) was detected throughout the reaction process. Since the selectivity was strongly dependent on the reaction progress, the comparison of the selectivities between different catalysts was conducted at 100% CH3CN conversion, which was just corresponding to the maximum EA yield. As shown in Table 1, the initial activity (Rm) of the Ni– Co–B amorphous alloy first increased and then decreased with the increase of xCo, which was in accordance with the reaction time needed for the total conversion of CH3CN. The maximum Rm (146.3 mmol/(h g)) was obtained at xCo = 0.5, which was nearly twice that of either the Ni–B (90.8 mmol/ (h g)) or the Co–B (66.4 mmol/(h g)). This was mainly attributed to its higher intrinsic activity (see the TON values in Table 1). Treatment of the Ni–Co–B amorphous alloy at 873 K for 2 h resulted in an abrupt decrease in the activity due to the remarkable decrease in both the SM and the intrinsic activity (TON). The Ni–Co–B amorphous alloy catalyst could be used repetitively for more than five times without significant decrease in the activity. After reactions, the ICP analysis revealed that only less than 2.3 ppm metallic Ni and Co were detected in the product mixture, indicating that the leaching of active metal species from the catalyst surface during the above hydrogenation could be neglected. After CH3CN hydrogenation over the fresh Ni– Co–B catalyst had proceeded for 30 min, the reaction mixture was filtered and then allowed the mother liquor (filtrate) to react for another 120 min at the same reaction conditions. No significant activity was observed, demonstrating that the hydrogenation occurs on the surface of the Ni–Co–B alloy. By plotting log Rm against 1/T, the apparent activation energies of the CH3CN hydrogenation over the Ni–Co–B (xCo = 0.50), Ni–B and Co–B catalysts were determined as 26, 37 and 46 kJ/mol, respectively, which
again confirmed that the intrinsic activity changed in the order of Ni–Co–B > Ni–B > Co–B. Kinetic studies demonstrated that, over the Ni–Co–B amorphous catalyst (xCo = 0.5), the effect of initial CH3CN concentration on Rm was not significant, indicating that the reaction was zero-order with respect to CH3CN. However, as shown in Fig. 7, the Rm increased almost linearly with the PH2 from 1.0 to 5.0 MPa, implying that the CH3CN hydrogenation was first-order with respect to PH2 . Such phenomena were also observed on other Ni–Co–B catalysts with different xCo, even including the Ni–B or the Co–B [9,14]. From the TPD curves in Fig. 5, one can conclude that the catalyst surface was almost saturated with CH3CN even at very low CH3CN concentration in bulk solution, while only little portions were covered with hydrogen during its competitive adsorption against CH3CN, since the adsorption
Fig. 7. Dependence of the specific activity (Rm) on the hydrogen pressure (PH2 ).
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for CH3CN was much stronger than that for hydrogen. The hydrogenation rate increased with PH2 owing to the increase of the surface hydrogen concentration. However, as the surface CH3CN has already reached saturation, the fraction of the increased surface CH3CN caused by increasing the CH3CN concentration in bulk solution, or the fraction of the decreased CH3CN caused by increasing the hydrogen adsorption, was not significant enough to affect the hydrogenation rate. Besides the PH2 , the difference between the adsorption strengths for hydrogen and for CH3CN played a decisive role in determining the amount of the hydrogen adsorbed on the catalyst surface and in turn, the hydrogenation rate. No poisoning effect of CH3CN on the hydrogenation rate was observed, even when its concentration was extremely high, suggesting that the CH3CN concentration in the solution had little influence on the competitive adsorption between hydrogen and CH3CN, since the catalyst surface was already saturated with CH3CN. The Co–B amorphous alloy exhibited lower activity than the Ni–B, owing to its stronger adsorption for CH3CN and weaker adsorption for hydrogen. The Ni– Co–B amorphous alloy was more active than the Ni–B and the Co–B, owing to the weaker adsorption for CH3CN but stronger adsorption for hydrogen, which was favorable for the hydrogen adsorption. At very high xCo (>0.5), the activity of the Ni–Co–B catalyst decreased slightly, since too many Ni active sits were replaced by less active Co sites, taking into account that the total amount of Ni and Co was fixed at 0.5 g in each run of the activity test. Perhaps the decreases in both the Ni-enrichment and the B-enrichment at high xCo may also account for the decrease in the activity. Table 1 also reveals that the selectivity to EA changed in the order of Co–B Ni–Co–B Ni–B. Over the Ni–Co–B amorphous alloys, the selectivity to EA increased rapidly with the increase of xCo from 0 to 0.5. The maximum selectivity (93%) was obtained at xCo = 0.5, which was the same as that of the Co–B. No significant change in selectivity was observed when xCo further increased. According to Braun’s mechanism [28–30], the CH3CN hydrogenation might proceed in the following scheme:
EA and thus, effectively inhibit the condensation between imine and EA, taking into account that such condensation should occur on the catalyst surface [4]. Only at very low concentration of CH3CN could the EA be adsorbed by the catalyst, resulting in the rapid formation of DEA, as shown in Fig. 6. The promoting effect of the Co-dopant on the selectivity to EA could be mainly attributed to the electronenrichment of the Ni active sites. On one hand, as shown in Fig. 5, although the adsorption for CH3CN was slightly decreased, the high electron density on the Ni active sites could greatly weaken the adsorption for EA and thus inhibit the condensation between imine and EA, which favored the selectivity to EA since the formation of DEA was inhibited. On the other hand, according to the adsorption mechanism [31], the electronic interaction between the C=N group in imine and the metallic active sites might be a forward donation of the electrons from the HOMO of the C=N bond, i.e. from the pC=N to the dz2 orbital of the metal atom, and the back donation from the dx2 y2 orbital of the metal atom to the LUMO, i.e. p*C=N. Since the p*C=N was an antibonding orbital, the increased back electron donation to the p*C=N that resulted from the high electron density on the Ni active sites could also make the C=N bond more activated toward hydrogenation, which was also favorable for selectivity to EA [32].
Acknowledgment This work was supported by the Nation Natural Science Foundation of China (20377031), the Natural Science Foundation of Shanghai Science and Technology Committee, and the Shanghai Education Committee.
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Liquid phase acetonitrile hydrogenation to ethylamine over a highly active and selective Ni–Co–B amorphous alloy catalyst Hexing Lia,*, Yuedong Wua, Jing Zhanga, Weilin Daib, Minghua Qiaob a
Department of Chemistry, Shanghai Normal University, No. 100, Guilin Road, Shanghai 200234, PR China b Department of Chemistry, Fudan University, Shanghai 200433, PR China Received in revised form 12 July 2004; accepted 22 July 2004 Available online 1 September 2004
Abstract The ultrafine Ni–Co–B amorphous alloys with Co/(Co + Ni) molar ratio (xCo) varying from 0 to 1 was prepared by chemical reduction of mixed Ni2+ and Co2+ ions with BH4 in aqueous solution. During liquid phase acetonitrile hydrogenation to ethylamine, the specific activity (Rm) and the intrinsic activity (TON) of the Ni–Co–B catalyst first increased and then decreased with the increase of xCo from 0 to 1. The maximum activity was obtained at xCo = 0.5; the value of the activity was nearly twice as that of the Ni–B or the Co–B catalyst. Treatment of the Ni–Co–B catalyst at 873 K resulted in an abrupt decrease in the activity due both to a decrease in active surface area and, especially, to the crystallization and the decomposition of the Ni–Co–B amorphous alloy. The selectivity to ethylamine increased rapidly with xCo and then remained constant at xCo 0.5. The maximum yield of ethylamine could reach 93%, showing a good potential for industrial applications. According to kinetic studies and results of various characterization methods, such as ICP, XRD, EXAFS, XPS, SAED, TEM, DSC, TPD, and hydrogen chemisorption, the correlation of the catalytic performance to both the structural and the electronic characteristics was discussed briefly. The activation of the CBN and/or C=N bonds, the promotion on the hydrogen adsorption, and the inhibition on the ethylamine adsorption were the decisive factors responsible for the excellent activity and selectivity of the Ni–Co–B catalyst. # 2004 Elsevier B.V. All rights reserved. Keywords: Ni–Co–B amorphous catalyst; Hydrogenation; Acetonitrile; Ethylamine
1. Introduction Hydrogenation of nitriles is widely used in industry for producing diverse primary amines [1–5]. However, the utility of this method in production of ethylamine (EA) is seldom employed possibly due to the high cost and even non-availability of acetonitrile (CH3CN) in the past. Today, with the rapid development of the acrylonitrile industry, the production of the CH3CN has increased abruptly, since it is a major byproduct with the yield up to 10 wt.%. Because the applications of CH3CN are quite limited, more and more CH3CN has been accumulated. In some factories, acetonitrile is burned to reduce the storing burden, which not only wastes a resource but also causes environmental problems. * Corresponding author. Tel.: +86 2164322141; fax: +86 2164322142. E-mail address: [email protected] (H. Li). 0926-860X/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2004.07.034
In view of the industrial requirements and environmental considerations, there is a strong driving force to develop new technology to transform the CH3CN to other useful chemicals. One of the promising ways is the selective hydrogenation of CH3CN to EA, an important intermediate for producing polymers, drugs, and other fine chemicals. A problem arising from this process is the selectivity since, besides the primary amine, both the secondary and even the tertiary amines are usually formed during the nitrile hydrogenation [6,7]. Among various factors, the catalyst plays a key role in determining the selectivity to EA [8]. Raney Ni is probably the most frequently used catalyst in the nitrile hydrogenation to primary amines [1,5]. However, only less than 40% EA yield could be obtained over Raney Ni during the CH3CN hydrogenation [9]. Even in the presence of ammonia, the maximum EA yield could not exceed 80%. Up to now, only very few catalysts have been
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reported to be suitable for the title reaction. As is wellknown, amorphous alloys exhibits higher activity, better selectivity, and stronger resistance against both sulfur and amine poison in various hydrogenations [10–13]. We reported previously that the Ni–B amorphous alloy exhibited comparable activity to that of Raney Ni but much higher selectivity to EA [6]. The Co–B amorphous alloy was even more selective to EA but its activity was very poor [14]. Based on these interesting results, we report here a novel Ni– Co–B amorphous alloy catalyst for the CH3CN hydrogenation with the aim of obtaining both high activity and selectivity to EA.
2. Experimental 2.1. Catalyst preparation The Ni–Co–B sample was prepared as follows: 34 ml of 2.0 M KBH4 aqueous solution containing 0.20 M NaOH was added dropwise over a 90 min period into a 20 ml aqueous solution containing desired amounts of NiCl2 and CoCl2 at room temperature and with vigorous stirring. The molar ratio between KBH4 and the total amount of metal was 4:1 to ensure the complete reduction of all the metallic ions in the solution. The resulting black solid was washed free from Cl and K+ ions with distilled H2O until pH = 7. Then, it was further washed with absolute alcohol (EtOH) and finally kept in EtOH until the time of use. The Co/(Co + Ni) molar ratio (xCo) in the Ni–Co–B sample was determined by ICP analysis and was adjusted by changing the content of NiCl2 and CoCl2 in the solution. For comparison, both Ni–B (xCo = 0) and Co–B (xCo = 1) were also prepared in a similar way by using a solution containing only NiCl2 or only CoCl2, respectively. 2.2. Catalyst characterization The surface morphology and particle sizes of the Ni–Co– B samples were observed by using a transmission electron micrograph (TEM, JEM-2010). Their compositions were analyzed by means of inductively coupled plasma (ICP, Jarrell-As Scan 2000). The active surface area (SM) was determined by H2 chemisorption, as described previously [15]. The temperature-programmed desorption (TPD) of H2, CH3CN or EA was carried out in the same system according to the following procedures. The catalyst surface was purged by ultrapure helium stream (purity of 99.997%) for 2.0 h at 573 K; this temperature was relatively lower than that reported by Gil [16] in order to avoid the crystallization of the amorphous catalysts. After the catalyst was cooled down to 313 K in helium stream, the probe molecules (H2, CH3CN, or EA, respectively) were fed to the catalyst by the He gas flow for 12 h until the adsorption saturated. Then, the desorption of these adsorbates was performed under Ar flow for hydrogen TPD or He flow for both CH3CN and EA TPD.
All the gases were of ultrahigh purity and were further purified by Chrompack clean-oxygen filter, molecular sieve and MnO subsequently. The temperature was raised from 313 to 900 K at 20 K/min. The amorphous structure of the Ni–Co–B catalysts was confirmed by both X-ray powder diffraction (XRD, Bruker AXS D8-Advance with Cu Ka radiation) and selective area electronic diffraction (SAED, HU-11B), and even studied by extended X-ray absorption fine structure (EXAFS, BL-10B) measurements carried out in the National Laboratory of High Energy Physics (KEK, Tsukuba, Japan). The crystallization process of the Ni–Co– B sample was monitored by differential scanning calorimetry (DSC, Perkin-Elmer) under N2 atmosphere at the heating rate of 10 K/min. X-ray photoelectron spectroscopy (XPS) was performed on a Perkin-Elmer PHI 5000C ESCA system using Al Ka radiation to determine the surface electronic states of the catalysts. All the binding energy (BE) values were calibrated by using C1S = 284.6 eV as a reference. The surface composition was determined by using 0.13, 2.43, and 2.50 as the PHI sensitivity factors to B1S, Ni2P3=2 , Co2P3=2 , respectively, offered by Perkin-Elmer Company [17]. During all the above characterizations, the wet Ni–Co–B sample soaked with EtOH was used; it was dried in situ in the atmosphere of ultra pure argon (purity of 99.99%, treated with a Chrompack clean-oxygen filter) to protect the sample from oxidation. 2.3. Activity test Liquid phase hydrogenation of CH3CN was carried out in a 200 ml autoclave in which the as-prepared Ni–Co–B catalyst containing 0.5 g metal (Ni or/and Co), 30 ml EtOH, and 10 ml CH3CN were mixed at PH2 = 3.0 MPa and T = 383 K. In order to ascertain the role of mass transfer, the catalyst amount was varied from 0.3 to 2.0 g and the speed of agitation was varied from 1000 to 1700 rpm. In view of the observation that the reaction rate was independent of the stirring rate and that it varied linearly with catalyst amount, it could be concluded that the stirring rate of 1200 rpm was high enough to eliminate the mass transfer [9]. Keeping the hydrogen pressure at 3.0 MPa, we sampled the reaction mixture every 15 min for GC analysis (GC 102) under the following conditions: FID detector, 2 m column filled with GDX-102/407, injector temperature 373 K, oven temperature 418 K, detector temperature 473 K, and N2 carrier gas (30 ml/min). The initial activity per gram of metal (specific activity, Rm = mmol/(h g)) was calculated according to the CH3CN conversion within the first hour. The turnover number (TON) was also calculated based on the Rm value and the surface metallic atoms determined by hydrogen chemisorption. From the product distribution in the reaction mixture, the selectivities to various products at given reaction time were obtained. The reproducibility of the results was checked by repeating the runs at least three times on the same batch of the catalyst and for another three times for a different batch of the catalyst; it was found to be within
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acceptable limits (5% for the same batch of the catalyst and 10% for the different batches of the catalyst).
3. Results and discussion As shown in Fig. 1, the TEM morphology revealed that the fresh Ni–Co–B sample with xCo of 0.50 was present in the form of ultrafine spherical particles with the average diameter of around 20 nm. After the sample had been treated at 873 K for 2 h in N2 atmosphere, severe gathering of the particles was observed. In comparison with the fresh Ni–B (xCo = 0) and Co–B (xCo = 1) samples, one can see that the increase of xCo resulted in a slight increase of the particle size. These results could account for the increase of the active surface area (SM) with the xCo and the abrupt decrease of SM after being treated at 873 K, as shown in Table 1. The SAED images attached in Fig. 1 demonstrated that all the fresh Ni–Co–B, Ni–B and Co–B displayed diffractional cycles indicative of amorphous structure [18]. After the sample had been treated at 873 K for 2 h, only individual bright dots were observed for the Ni–Co–B sample due to its crystallization. Further evidence for this conclusion was obtained from XRD patterns. As shown in Fig. 2, the fresh Ni–Co–B sample exhibited only one broad peak around 2u = 458, a typical peak indicative of amorphous alloy structure
Fig. 2. XRD patterns of (a) the fresh Ni–Co–B sample (xCo = 0.5), (b) the Ni–Co–B sample (xCo = 0.5) after being treated at 573 K for 2 h in N2 flow, and (c) the Ni–Co–B sample (xCo = 0.5) after being treated at 873 K for 2 h in N2 flow.
[19]. Treating the Ni–Co–B samples at T < 523 K in N2 flow for 2 h had no appreciable influence on the XRD pattern. However, when the treating temperature was increased further, various diffractional peaks corresponding to metallic Ni and Co, crystalline Ni–B, Co–B and Ni–Co– B alloys appeared, indicating the occurrence of crystal-
Fig. 1. TEM morphologies and SAED images of (a) the fresh Ni–Co–B sample (xCo = 0.5), (b) the Ni–Co–B sample (xCo = 0.5) after being treated at 873 K for 2 h in N2 flow, (c) the fresh Ni–B sample (xCo = 0), and (d) the fresh Ni–B sample (xCo = 1).
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Table 1 Structural characteristics and catalytic properties of the as-prepared catalystsa Catalyst
Bulk compound (at.%)
xCo
xSurf Co
SM (m2/g)
Rm (mmol/(h g))
TON (h 1)
ttotal (min)
Selectivity (%) EA
DEA
Ni–B Ni–Co–B-3 Ni–Co–B-5 Ni–Co–B-8 Co–B Crystallized Ni–Co–B-5
Ni75.2B24.8 Ni50.6Co21.9B27.5 Ni35.6Co34.7B29.7 Ni12.5Co57.5B30.0 Co75.4B25.6 Ni35.6Co34.7B29.7
0 0.30 0.50 0.80 1.00 0.50
0 0.20 0.30 0.71 1.00 0.50
19.6 18.1 17.0 16.4 15.8 2.8
90.8 113.0 146.3 129.6 66.4 7.9
18.2 24.5 33.7 31.0 16.4 11.1
150 120 90 105 210 –
67.6 83.1 93.0 93.0 92.8 –
32.4 16.9 6.7 6.7 6.7 –
a
Reaction conditions: a catalyst containing 0.5 g Ni and/or Co, 30 ml EtOH, and 10 ml CH3CN at PH2 = 3.0 MPa and T = 383 K.
lization, together with partial decomposition. The amorphous structure was further confirmed by the extended EXAFS, from which the RDF curves could be obtained by the fast Fourier transformation [20]. Only one broad peak ˚ was observed for all the fresh Ni–Co– around R = 1.7–2.4 A B samples regardless of the xCo, showing that they had no long-range ordering but only short-range ordering structure confined within the first-near-neighbor atom layer [21]. From the DSC analysis (Fig. 3), one can see that the Ni–Co– B sample with xCo of 0.5 exhibited only one exothermic peak around 729.9 K. Taking into account that the Ni–B displayed an exothermic peak around 614 K [9] while the Co–B showed an exothermic peak around 765 K [14], one could conclude that the Ni and Co in the Ni–Co–B sample were present in the whole amorphous alloy rather than in the individual Ni–B and Co–B amorphous alloys. The alloying between Ni and Co resulted in the shift of the crystallization temperature toward to that of the Co–B amorphous alloy. The XPS spectra in Fig. 4 demonstrated that both the nickel and cobalt species in the Ni–Co–B sample (xCo = 0.5) were present only in their metallic states, corresponding to binding energy (BE) of 852.6 eV in Ni2P3=2 level and 778.2 eV in Co2P3=2 level, respectively. The asymmetrical feature of the XPS peaks for Ni2P and Co2P demonstrated that both the metallic Ni and the metallic Co were present in different chemical environments, possibly owing to the co-
Fig. 3. DSC pattern of the Ni–Co–B sample (xCo = 0.5).
existence of the Ni–Co–B alloy and trace of both the Co–B and the Ni–B alloys. However, the boron species were present in both the alloying B and oxidizing B (B2O3), corresponding to BE of 188.1 eV and 192.5 eV, respectively. Comparing to the standard BE of pure B (187.1 eV) [22], one immediately found that the alloying B was positively charged, indicating that partial electrons transferred from alloying B to the metal (Ni and/or Co) in the Ni–Co–B amorphous alloy [23–25]. This could be explained by the assumption of Imanaka et al. that the bonding electrons of the B occupied the vacant d-orbitals of metallic Ni or Co [26]. The BE of the metallic Ni in the Ni–Co–B was 0.4 eV lower than that in the Ni–B. However, the BE value of the metallic Co in the Ni–Co–B was only 0.1 eV lower than that in the Co–B, which was within the error range of our XPS measurements. These results suggested that the metallic Ni accepted more electrons from the alloying B than the metallic Co in the Ni–Co–B amorphous alloy. As the BE values of the alloying B in the Ni–Co–N, Ni–B and Co–B amorphous alloys were almost the same, the higher electron density on the Ni atom in the Ni–Co–B sample could be mainly attributed to the B-enrichment, as shown in Table 1. According to our previous paper, the theoretical calculations using ab initio DFT method [27] demonstrated that the increase of the B content would contribute more electrons to the metallic Ni or Co in their corresponding amorphous alloys. When one compared the xCo determined by ICP and that determined by XPS (xSurf Co ), it was obvious that the Ni– Co–B surface was enriched with Ni. The Ni-enrichment increased with the increase of xCo from 0 to 0.5 and then slightly decreased when the xCo further increased. Fig. 5 shows the TPD curves of H2, CH3CN and EA molecules over the Ni–B, Co–B and Ni–Co–B (xCo = 0.5) amorphous catalysts, respectively. One can see that the Co– B exhibited much stronger adsorption for both the CH3CN and EA but weaker adsorption for hydrogen than the Ni–B. Although the Ni and Co contents were almost equivalent in the Ni–Co–B amorphous catalyst at xCo = 0.5, the position of the main desorption peak on the Ni–Co–B was close to that on the Ni–B catalyst, possibly due to the Ni-enrichment. From the TPD curves, it was also found that the Ni–Co–B catalyst exhibited weaker adsorption for CH3CN and EA than either the Ni–B or the Co–B, possibly owing to the higher electron density on the Ni active sites in the Ni–Co–
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Fig. 4. XPS spectra of the Ni–Co–B sample (xCo = 0.5).
B, which could repel the lone electron pair on the nitrogen atom in either the CH3CN or the EA molecule. As shown in Fig. 6, the CH3CN conversion increased almost linearly with the reaction time, indicating that the CH3CN hydrogenation was zero-order with respect to CH3CN concentration. A slight deviation from the straight
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Fig. 5. (a) Hydrogen, (b) acetonitrile and (c) ethylamine TPD curves over Ni–B, Co–B and Ni–Co–B (xCo = 0.5) amorphous alloys.
line was observed at the end of CH3CN hydrogenation, possibly due to the extremely low CH3CN concentration or/ and the accumulation of reaction products, which might poison the catalyst. At low CH3CN conversion (<50%), only EA was identified in the reaction mixture. With the increase of CH3CN conversion, diethylamine (DEA)
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Fig. 6. Change of the CH3CN conversion and the selectivities to EA and DEA with the reaction time over the Ni–Co–B (xCo = 0.5) amorphous alloy catalyst. Reaction conditions: 0.5 g catalyst, 10 ml CH3CN, 30 ml EtOH, T = 383 K, PH2 = 3.0 MPa, stirring rate = 1200 rpm.
appeared gradually, causing a rapid decrease in the selectivity to EA. However, no triethylamine (TEA) was detected throughout the reaction process. Since the selectivity was strongly dependent on the reaction progress, the comparison of the selectivities between different catalysts was conducted at 100% CH3CN conversion, which was just corresponding to the maximum EA yield. As shown in Table 1, the initial activity (Rm) of the Ni– Co–B amorphous alloy first increased and then decreased with the increase of xCo, which was in accordance with the reaction time needed for the total conversion of CH3CN. The maximum Rm (146.3 mmol/(h g)) was obtained at xCo = 0.5, which was nearly twice that of either the Ni–B (90.8 mmol/ (h g)) or the Co–B (66.4 mmol/(h g)). This was mainly attributed to its higher intrinsic activity (see the TON values in Table 1). Treatment of the Ni–Co–B amorphous alloy at 873 K for 2 h resulted in an abrupt decrease in the activity due to the remarkable decrease in both the SM and the intrinsic activity (TON). The Ni–Co–B amorphous alloy catalyst could be used repetitively for more than five times without significant decrease in the activity. After reactions, the ICP analysis revealed that only less than 2.3 ppm metallic Ni and Co were detected in the product mixture, indicating that the leaching of active metal species from the catalyst surface during the above hydrogenation could be neglected. After CH3CN hydrogenation over the fresh Ni– Co–B catalyst had proceeded for 30 min, the reaction mixture was filtered and then allowed the mother liquor (filtrate) to react for another 120 min at the same reaction conditions. No significant activity was observed, demonstrating that the hydrogenation occurs on the surface of the Ni–Co–B alloy. By plotting log Rm against 1/T, the apparent activation energies of the CH3CN hydrogenation over the Ni–Co–B (xCo = 0.50), Ni–B and Co–B catalysts were determined as 26, 37 and 46 kJ/mol, respectively, which
again confirmed that the intrinsic activity changed in the order of Ni–Co–B > Ni–B > Co–B. Kinetic studies demonstrated that, over the Ni–Co–B amorphous catalyst (xCo = 0.5), the effect of initial CH3CN concentration on Rm was not significant, indicating that the reaction was zero-order with respect to CH3CN. However, as shown in Fig. 7, the Rm increased almost linearly with the PH2 from 1.0 to 5.0 MPa, implying that the CH3CN hydrogenation was first-order with respect to PH2 . Such phenomena were also observed on other Ni–Co–B catalysts with different xCo, even including the Ni–B or the Co–B [9,14]. From the TPD curves in Fig. 5, one can conclude that the catalyst surface was almost saturated with CH3CN even at very low CH3CN concentration in bulk solution, while only little portions were covered with hydrogen during its competitive adsorption against CH3CN, since the adsorption
Fig. 7. Dependence of the specific activity (Rm) on the hydrogen pressure (PH2 ).
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for CH3CN was much stronger than that for hydrogen. The hydrogenation rate increased with PH2 owing to the increase of the surface hydrogen concentration. However, as the surface CH3CN has already reached saturation, the fraction of the increased surface CH3CN caused by increasing the CH3CN concentration in bulk solution, or the fraction of the decreased CH3CN caused by increasing the hydrogen adsorption, was not significant enough to affect the hydrogenation rate. Besides the PH2 , the difference between the adsorption strengths for hydrogen and for CH3CN played a decisive role in determining the amount of the hydrogen adsorbed on the catalyst surface and in turn, the hydrogenation rate. No poisoning effect of CH3CN on the hydrogenation rate was observed, even when its concentration was extremely high, suggesting that the CH3CN concentration in the solution had little influence on the competitive adsorption between hydrogen and CH3CN, since the catalyst surface was already saturated with CH3CN. The Co–B amorphous alloy exhibited lower activity than the Ni–B, owing to its stronger adsorption for CH3CN and weaker adsorption for hydrogen. The Ni– Co–B amorphous alloy was more active than the Ni–B and the Co–B, owing to the weaker adsorption for CH3CN but stronger adsorption for hydrogen, which was favorable for the hydrogen adsorption. At very high xCo (>0.5), the activity of the Ni–Co–B catalyst decreased slightly, since too many Ni active sits were replaced by less active Co sites, taking into account that the total amount of Ni and Co was fixed at 0.5 g in each run of the activity test. Perhaps the decreases in both the Ni-enrichment and the B-enrichment at high xCo may also account for the decrease in the activity. Table 1 also reveals that the selectivity to EA changed in the order of Co–B Ni–Co–B Ni–B. Over the Ni–Co–B amorphous alloys, the selectivity to EA increased rapidly with the increase of xCo from 0 to 0.5. The maximum selectivity (93%) was obtained at xCo = 0.5, which was the same as that of the Co–B. No significant change in selectivity was observed when xCo further increased. According to Braun’s mechanism [28–30], the CH3CN hydrogenation might proceed in the following scheme:
EA and thus, effectively inhibit the condensation between imine and EA, taking into account that such condensation should occur on the catalyst surface [4]. Only at very low concentration of CH3CN could the EA be adsorbed by the catalyst, resulting in the rapid formation of DEA, as shown in Fig. 6. The promoting effect of the Co-dopant on the selectivity to EA could be mainly attributed to the electronenrichment of the Ni active sites. On one hand, as shown in Fig. 5, although the adsorption for CH3CN was slightly decreased, the high electron density on the Ni active sites could greatly weaken the adsorption for EA and thus inhibit the condensation between imine and EA, which favored the selectivity to EA since the formation of DEA was inhibited. On the other hand, according to the adsorption mechanism [31], the electronic interaction between the C=N group in imine and the metallic active sites might be a forward donation of the electrons from the HOMO of the C=N bond, i.e. from the pC=N to the dz2 orbital of the metal atom, and the back donation from the dx2 y2 orbital of the metal atom to the LUMO, i.e. p*C=N. Since the p*C=N was an antibonding orbital, the increased back electron donation to the p*C=N that resulted from the high electron density on the Ni active sites could also make the C=N bond more activated toward hydrogenation, which was also favorable for selectivity to EA [32].
Acknowledgment This work was supported by the Nation Natural Science Foundation of China (20377031), the Natural Science Foundation of Shanghai Science and Technology Committee, and the Shanghai Education Committee.
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