Studies on the Hydrogenation of Acetonitrile over Fresh Mo2C/γ-Al2O3Catalyst by In-situ IR Spectroscopy

Zhang Jing; Wu Weicheng; Liu Chuang; Ding Xiaoguang; Chu Gang; Zhang Jianguo

(1. Liaoning Shihua University, College of Chemistry, Chemical Engineering and Environmental Engineering, Fushun 113001; 2. Liaoning Shihua University, School of Computer and Communication Engineering, Fushun 113001; 3. School of Medical Instrument and Food Engineering, University of Shanghai for Science and Technology, Shanghai 200093)

Studies on the Hydrogenation of Acetonitrile over Fresh Mo2C/γ-Al2O3Catalyst by In-situ IR Spectroscopy

Zhang Jing1; Wu Weicheng1; Liu Chuang1; Ding Xiaoguang2; Chu Gang1; Zhang Jianguo3

(1. Liaoning Shihua University, College of Chemistry, Chemical Engineering and Environmental Engineering, Fushun 113001; 2. Liaoning Shihua University, School of Computer and Communication Engineering, Fushun 113001; 3. School of Medical Instrument and Food Engineering, University of Shanghai for Science and Technology, Shanghai 200093)

The adsorption of acetonitrile, the co-adsorption of acetonitrile with CO, and hydrogenation of acetonitrile on fresh Mo2C/γ-Al2O3catalyst were studied by in situ IR spectroscopy. It was found out that CH3CN exhibited strong interaction with the fresh Mo2C/γ-Al2O3catalyst and was adsorbed mainly on Moδ+sites of fresh Mo2C/γ-Al2O3catalyst. Moreover, CH3CN could affect the shifting of IR spectra for CO adsorption towards a lower wave number. The IR spectroscopic study on acetonitrile hydrogenation showed that CH3CN could be easily hydrogenated in the presence of H2on the Mo2C/γ-Al2O3catalyst. Furthermore, it was observed that CH3CN could be selectively hydrogenated to imines on fresh Mo2C/γ-Al2O3catalyst. Additionally, the active sites of fresh Mo2C/γ-Al2O3catalyst might be covered with coke during the hydrogenation reaction of acetonitrile. The treatment of catalyst with hydrogen at 673 K could not completely remove coke deposits on the surface of the Mo2C/γ-Al2O3catalyst.

fresh Mo2C/γ-Al2O3catalyst; hydrogenation; acetonitrile; in situ IR spectroscopy

1 Introduction

Amines are widely used in industry as intermediates for manufacture of agrochemicals, pesticides, polymers, pharmaceuticals, and other chemicals[1]. They can be prepared by different ways, for example, via alkylation of ammonia or reductive amination of oxo-compounds[2]. One important route for the production of amines is hydrogenation of the corresponding nitriles[3]. The nitriles can be hydrogenated in liquid-phase[4]or in gas-phase[5-8]. In general, products formed during hydrogenation of nitriles cover the primary, secondary, and tertiary amines.

The catalytic hydrogenation of acetonitrile (CH3CN) is an important route for the production of ethylamine, which has wide industrial applications. Many studies have been performed on Raney nickel catalysts[9-11]. It has been reported that this reaction can be catalyzed by a variety of transition metals, especially by those from the group VIII of the periodic table[3]. The catalyst configuration, including the nature of the metal and support, is important in determining the selectivity of nitrile hydrogenation[12]. Huang and Sachtler[13]propose that, among the transition metal catalysts, Ru displays a highest selectivity to primary amine, while over Pd and Pt catalysts, secondary and tertiary amines are preferentially formed; the selectivity of Ni and Rh is between these extremes.

In recent years, early transition metal carbides are of interest because they have many superior properties such as high melting points, good thermal and catalytic behavior, and excellent electronic characteristics[14-16]. Importantly, early transition metal carbides, such as molybdenum carbide (Mo2C), have attracted much attention because they show catalytic properties resembling the Group VIII metals in a number of hydrogen-involved reactions[17]. Therefore, it is meaningful to study the hydrogenation of CH3CN on Mo2C catalyst because this idea may open up a new way for the hydrogenation of acetonitrile.

In this paper, hydrogenation of CH3CN on fresh Mo2C/ γ-Al2O3catalyst was studied using the in situ IR spectros-copy. In situ IR spectroscopy is a predominant technique in the surface characterization of supported catalysts[18-19]. Based on these studies, we attempted to study the structure–performance relationship for CH3CN hydrogenation over fresh Mo2C/γ-Al2O3catalyst.

2 Experimental

2.1 Synthesis of fresh Mo2C/γ-Al2O3catalyst

A fresh Mo2C/γ-Al2O3catalyst was prepared by the temperature programmed reaction (TPR) of MoO3/γ-Al2O3. A MoO3/γ-Al2O3catalyst sample consisting of 10% (mass fraction) of Mo was prepared by the incipient impregnation method. The support γ-Al2O3(SBET=108 m2/g, Degussa) was impregnated with an aqueous solution of ammonium heptamolybdate (analytical grade), followed by drying at 397 K overnight and then calcination at 773 K for 240 min. The MoO3/γ-Al2O3sample was pressed into a self-supporting wafer of approximately 15 mg/cm2. The wafer was placed in a quartz IR cell equipped with CaF2windows, in which the in situ carburization could be performed. The Mo2C/Al2O3catalyst was prepared via carburization in a flowing H2stream containing 20% of CH4using the temperature-programmed reaction (TPR) method. The temperature was increased from room temperature (RT) to 573 K in 30 min and from 573 to 1033 K in 460 min, and then the temperature was maintained at 1033 K for 60 min. The carburized sample was cooled down to RT in a flowing H2stream containing 20% of methane.

2.2 IR spectroscopic characterization

The IR spectroscopic experiments were carried out under the following conditions: (1) The cell was evacuated to 1.3×10-3Pa, prior to being exposed to a mixture of CO (with a partial pressure of 1.3×103Pa) and CH3CN (with a partial pressure of 1.3×103Pa) for co-adsorption. (2) The cell was exposed to a mixture of CH3CN (with a partial pressure of 1.3×103Pa) and H2(with a partial pressure of 1.3×104Pa) at different temperatures. (3) The fresh Mo2C/ γ-Al2O3catalyst was treated by a CH3CN/H2(1.3×103Pa/1.3×104Pa) mixture at 773 K for 30 min prior to being subject to adsorption of CO; the inactivated Mo2C/Al2O3catalyst was regenerated by H2treatment at 673 K for 120 min, and then was subject to adsorption of CO.

All IR spectra were collected at room temperature on a Nicolet Impect 410 Fourier transform infrared spectrometer with a resolution of 4 cm-1and 64 scans in the region of 4 000—1 000 cm-1. All the spectra were subtracted by the spectrum of the sample recorded before adsorption. Unless otherwise indicated, the spectra were obtained after the system was evacuated to 10-3torr in order to obtain the spectrum of chemisorbed CO.

3 Results and Discussion

3.1 Adsorption of CH3CN on fresh Mo2C/γ-Al2O3catalyst

Figure 1 compares the IR spectrum of CH3CN adsorbed on fresh Mo2C/γ-Al2O3catalyst and IR spectrum recorded after the outgassing of the gas phase CH3CN at RT. It is found out that the adsorbed CH3CN on fresh Mo2C/γ-Al2O3gave IR bands at 2 312, 2 285, 2 249, 2 177, 1 447, and 1 372 cm-1. Four IR bands at 2 312, 2 285, 2 249, and 2 177 cm-1could be ascribed to the stretching vibration mode of CN species[20], while the bands at 1 447 and 1 372 cm-1were attributed to the deformation vibration of unsaturated C-H3groups[20]. The two bands appearing at 2 312 and 2 285 cm-1might be assigned to CH3CN bonded to the surface Lewis acid sites and to the Fermi resonance band, respectively[21-22]. A tentative assignment for the IR bands is given in Table 1.

Figure 1 IR spectra of CH3CN adsorbed on Mo2C/γ-Al2O3catalyst at RT (a) and catalyst sample after evacuation (b)

3.2 Co-adsorption of CH3CN with CO on fresh Mo2C/γ-Al2O3catalyst

Figure 2 shows the IR spectra of co-adsorbed CO with CH3CN on the fresh Mo2C/γ-Al2O3catalyst. Figure 2a shows the spectrum of CO adsorbed on fresh Mo2C/γ-Al2O3catalyst. It is shown that CO species adsorbed on a fresh Mo2C/γ-Al2O3sample gave two characteristic IR bands at 2 056 and 2 196 cm-1[23]. The band at 2 056 cm-1could be assigned to the linearly adsorbed CO on Moδ+(0 <δ< 2) sites of fresh Mo2C/γ-Al2O3, while the band at 2 196 cm-1might be assigned to the CCO species that were formed from the reaction of CO with the surface-active carbon atoms of Mo2C/γ-Al2O3catalyst[23]. The spectrum of CH3CN absorbed on fresh Mo2C/γ-Al2O3catalyst is shown in Figure 2c. Four IR bands at 2 312, 2 285, 2 249, and 2 177 cm-1were identified.

When CO was pre-adsorbed on the catalyst sample and CH3CN was then introduced (Figure 2b), four bands ascribed to the stretching vibration mode of CN species at 2 312, 2285, 2 249, and 2 177 cm-1were also observed, while the band at 2 196 cm-1, which was assigned to the CCO species, disappeared. In addition, the band at 2 056 cm-1shifted to 1 971 cm-1, and the intensity decreased greatly. Figure 2d shows the IR spectrum of CH3CN which was pre-adsorbed on the Mo2C/γ-Al2O3catalyst prior to the introduction of CO. Compared with the result from Figure 2a, it is found that the band at 2 056 cm-1shifted to 2 010 cm-1, which was accompanied by a decrease in intensity, while the band at 2 177 cm-1was shifted to a higher wave number.

As shown in Figure 2, it is suggested that the surface sites of fresh Mo2C/γ-Al2O3catalyst were susceptible of CO and CH3CN adsorption, and the presence of CH3CN had a significant influence on the frequency of CO vibration. Moreover, CH3CN could affect the shifting of IR spectra for CO adsorption towards a lower wave number. These results might indicate that CH3CN was adsorbed mainly on Moδ+sites of the fresh Mo2C/γ-Al2O3catalyst. Furthermore, the red shift of n(CO) led by co-adsorption of CH3CN might be interpreted in terms of the electronic effects of p-donation of CH3CN. Stanislaus, et al. proposed a scheme for the chemisorption of CH3CN[24].

Figure 2 IR spectra of CO adsorbed at RT on Mo2C/γ-Al2O3catalyst: (a) CO alone; (b) CO adsorbed at first, followed by adsorption of CH3CN; (c) CH3CN alone; (d) CH3CN adsorbed at first, followed by adsorption of CO

Figure 3 IR spectra of CH3CN/H2(1.3×103Pa/1.3×104Pa) adsorbed on Mo2C/γ-Al2O3catalyst at different temperatures

3.3 Hydrogenation of CH3CN on fresh Mo2C/γ-Al2O3catalyst

Figure 3 shows the IR spectra of CH3CN/hydrogen (1.3×103Pa/1.3×104Pa) adsorbed on fresh Mo2C/γ-Al2O3catalyst at different temperatures. It is found out that the band at 1 579 cm-1appeared at 298 K in additionto characteristic bands of CH3CN (2 312, 2 285, and 2 249 cm-1). The band at 1 579 cm-1increased in intensity with an increasing temperature, while the characteristic bands of CH3CN(2 312, 2 285, and 2 249 cm-1) decreased in intensity. When the reaction temperature reached 573 K, the typical bands associated with CH3CN disappeared almost completely, and a strong band at 1 579 cm-1was observed. The band at 1 579 cm-1might be assigned to the bending vibrations of N—H bond in CH3CHxNHythat was derived from hydrogenation of CH3CN[25]. The band at 1 579 cm-1showed a decrease in intensity after the temperature reached 723 K and disappeared at 773 K. The desorption of CH3CHxNHyspecies at high temperature might be the reason relating to the disappearance of the band at 1 579 cm-1. The hydrogenation of CH3CHxNHyoccurring at high temperature might be another possible reason for disappearance of the band at 1 579 cm-1. Additionally, a new band at 1 664 cm-1was observed at 423 K. The band at 1 664 cm-1was suggested to be related to C=N species. These results clearly suggest that CH3CN could be selectively hydrogenated to imines on fresh Mo2C/γ-Al2O3catalyst.

We investigated the effect of γ-Al2O3support on the hydrogenation of CH3CN. The IR spectra of CH3CN/hydrogen (1.3×103Pa/1.3×104Pa) adsorbed on the γ-Al2O3support at different temperatures are shown in Figure 4. The adsorbed CH3CN on γ-Al2O3support gave IR bands similar to that of CH3CN in the gas phase, indicating that CH3CN was weakly or physically absorbed on the γ-Al2O3support. Moreover, the spectra in Figure 4 did not show obvious change with an increasing temperature. These results suggest that the fresh Mo2C/γ-Al2O3catalyst exhibited high activity for the hydrogenation of CH3CN.

3.4 Adsorption of CH3CN on fresh Mo2C/γ-Al2O3catalyst at different temperatures

We also studied the influence of reaction temperature on the absorption of CH3CN on fresh Mo2C/γ-Al2O3catalyst, with the results presented in Figure 5. It is observed that the band at 1 579 cm-1appeared at room temperature. As it has been discussed above, the band at 1 579 cm-1should be assigned to the vibrations of CH3CHxNHythat was derived from hydrogenation of CH3CN. This indicates that CH3CN could be easily hydrogenated in the presence of H2on the Mo2C/γ-Al2O3catalyst. The band at 1 579 cm-1increased in intensity with an increasing temperature. This phenomenon could be explained as follows: The decomposition reaction of CH3CN occurred on the surface of fresh Mo2C/γ-Al2O3catalyst, while hydrogen resulted from the decomposition reaction of CH3CN could react with CH3CN. Thus, the intensity of band at 1 579 cm-1would increase with an increasing temperature.

Figure 4 IR spectra of CH3CN/H2(1.3×103Pa/1.3×104Pa) adsorbed on γ-Al2O3at different temperatures

Figure 5 IR spectra of CH3CN (1.3×103Pa) adsorbed on Mo2C/γ-Al2O3catalyst at different temperatures

Additionally, a band at 2 177 cm-1was also observed in Figure 5, while we did not find the band at 2 177 cm-1when CH3CN was absorbed on the γ-Al2O3support (Figure 4). These results indicate that the band at 2 177 cm-1was formed from the adsorption of CH3CN on Moδ+sites of the fresh Mo2C/γ-Al2O3catalyst, and the band at 2 177 cm-1could be ascribed to the end-on mode of CH3CN. It is generally believed that the adsorption of CH3CN on the surface of catalyst has an influence on the selectivity of CH3CN hydrogenation reaction. Sachtler, et al.[26]studied the reaction mechanism of CH3CN molecules on the transition metal.It is found that CH3CN is adsorbed on the surface of Ru by multiple bonds (Ru=N), and the mobility of bonded species is minimal on the Ru surface. However, CH3CN is adsorbed on the surface of Pt or Pd by single bonds, and the adsorbed groups are sufficiently mobile so that they can react with each other also at low temperature. The formation of secondary and tertiary amines requires the mobility of adsorbed groups, which can react with each other. Thus, Pt is highly selective for tertiary amine.

Figure 6 IR spectra of CO adsorbed at RT

3.5 Changes in reaction active sites of Mo2C/γ-Al2O3before and after CH3CN hydrogenation

Figure 6 compares the IR spectra of CO adsorbed on fresh Mo2C/γ-Al2O3catalyst before and after CH3CN hydrogenation reaction. CO species adsorbed on fresh Mo2C/γ-Al2O3sample gave a main characteristic IR band at 2 056 cm-1(Figure 6a). After Mo2C/γ-Al2O3catalyst was treated with CH3CN at 773 K for 30 min, the band at 2 056 cm-1shifted to lower frequencies at 2 094 cm-1. It is obvious that the adsorption band at 2 056 cm-1decreased in intensity after CH3CN was subject to hydrogenation reaction (Figure 6c). The fact that the intensity of the band at 2 056 cm-1decreased obviously which might be attributed to the elimination of some surface carbon atoms of carbide species. The amount of active sites of Mo2C decreased remarkably. Therefore, these results suggested that the coke deposited on the Mo2C/γ-Al2O3catalyst might be formed during the hydrogenation reaction of CH3CN[23]. Hydrogen treatment at high temperature can remove deposited carbon species, and this is a conventional method to remove the deposited coke from the surface of Mo2C. Figure 6d displays the IR spectrum of CO adsorption on H2-treated fresh Mo2C/γ-Al2O3at 673K. After the catalyst was reduced by H2at 673 K, the band at 2 056 cm-1shifted to 2 045 cm-1. According to our previous study results[23], the H2-treatment could eliminate the surface polymeric carbon species of carbide catalysts, and the carbon species in the bulk carbide material might be also removed. This could be the main reason why the band at 2 056 cm-1shifted to 2 045 cm-1. Furthermore, it is obvious that the intensity of band at 2 045 cm-1in Figure 6d was slightly stronger than that of 2 056 cm-1in Figure 6c, indicating to the elimination of some carbon atoms on the surface of Mo2C/γ-Al2O3by H2treatment. However, it is found that the intensity of the band at 2 045 cm-1in Figure 6d dramatically decreased as compared to the intensity of the band at 2 056 cm-1in fresh Mo2C/γ-Al2O3catalyst (Figure 6c). This result suggests that the treatment with hydrogen at 673 K could not completely remove the coke deposits from the surface of Mo2C/γ-Al2O3catalyst.

4 Conclusions

Hydrogenation of CH3CN on fresh Mo2C/γ-Al2O3catalyst was studied by in situ IR spectroscopy. It is found out that CH3CHxNHywas formed after CH3CN adsorption on the Mo2C/γ-Al2O3catalyst. The IR results of hydrogenation of CH3CN indicated that the fresh Mo2C/γ-Al2O3catalyst exhibited high activity for the hydrogenation of CH3CN. Moreover, CH3CN could be selectively hydrogenated to imines on fresh Mo2C/γ-Al2O3catalyst. However, coke deposits on the Mo2C/γ-Al2O3catalyst might be formed during the hydrogenation reaction of CH3CN. The treatment of catalyst with hydrogen at 673 K could not completely remove the coke deposits on the surface of the Mo2C/γ-Al2O3catalyst.

Acknowledgements: This work is financially supported by the National Natural Science Foundation of China (No. 21573101), the Liaoning Provincial Natural Science Foundation (No. 2014020107), the Program for Liaoning Excellent Talents in Universities (No. LJQ2014041), and is also sponsored by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry (SRF for ROCS, SEM).

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date: 2015-02-15; Accepted date: 2015-05-13.

Zhang Jing, E-mail: jingzhang_dicp@ live.cn.