Modification effect of cerium in gas hydrogenation of acrylonitrile over Pd/Al₂O₃ catalysts

Abstract Low-loaded Pd (0.05 wt.%)/Al₂O₃ catalysts doped with cerium were prepared by wet impregnation method, then calcined and reduced by H₂ at 773K. Selective gas phase hydrogenation of acrylonitrile over as-prepared catalysts was carried out in fixed bed reactor. The investigation indicated that the selectivity of hydrogenation of acrylonitrile to propionitrile over all the as-prepared catalysts was nearly 100%. And the addition of cerium improved the activity and stability greatly compared with Pd/Al₂O₃ sample studied as reference. All these may be due to the "electronic and structural effect" according to various characterizations.
Keywords Hydrogenation; Acrylonitrile; Cerium.

1. INTRODUCTION
Ceria exhibits very interesting properties as promoter of highly dispersed metal catalysts. In particular, it shows that ceria can substantially modify the chemical behavior of metal/Al₂O₃ binary systems. As to Pd/Al₂O₃ especially, palladium has the ability of promoting the reduction of ceria when the catalysts are reduced by hydrogenation. The change in valence of the Ce cation from +4 to +3 may drastically modify the catalytic activity and selectivity ^[1], which has been narrated clearly by S. Bernal and his co-workers ^[2]. Though many articles reported recently have described that cerium could modify the structure and electronic states of noble metals supported on Al₂O₃ carrier to enhance the activity of the catalyst, most of the works emphasize on automotive exhaust gas depollution where they increase the performance and the stability of the three-way catalysts (TWCS). Their results verified that the addition of cerium to Pd/Al₂O₃ enhanced the catalytic activity greatly. However, there are few works published on the promotion effect of selective hydrogenation of unsaturated functional groups. Moreover, the modification induced by cerium was found to be varied between laboratories ^[3,4]. This suggests that the preparation method play a key role in determining their properties of the rare earth oxides-promoted catalysts. In our experiments, the influence of cerium loading, the method of introducing the promoter (either before the metal or simultaneously with it) were investigated concerning the effect to selective hydrogenation of acrylonitrile to propionitrile in fixed bed glass micro-reactor. Considering of the high price of palladium that hindered the massive application in modern industry, we prepared a series of low-loaded palladium catalysts containing 0.05 wt.% Pd to investigate the catalytic behavior of low-loaded palladium catalyst and cerium doped catalysts. The selectivity of as-prepared catalysts was all nearly 100% and it seemed that the addition of cerium to Pd/Al₂O₃ system improved the activity of hydrogenation of acrylonitrile to propionitrile greatly due to its electronic effect and structural modification to the catalysts which have been verified by XRD, XPS characterizations. Recently, various catalysts have been adopted in hydrogenation of acrylonitrile, such as amorphous Ni-B/SiO₂ alloy ^[9], Pd (0.5 wt.%)/Al₂O₃ ^[10], while low-loaded Pd-RE/Al₂O₃ showed a better prospect for continuous reaction in fixed-bed reactor because of the easily gathering of amorphous particles and the deactivation of high-loaded Pd/Al₂O₃.

2. EXPERIMENTAL
2.1 Catalyst Preparation
Pd(0.05 wt.%)/Al₂O₃ catalysts were prepared by impregnation at incipient wetness of the support with aqueous solutions of PdCl₂. The support utilized is g-Al₂O₃(40-50mesh).
    Pd-CeO₂/Al₂O₃ catalysts containing 0.05 wt.% palladium were prepared by conventional wet impregnation technique (consecutive impregnation or co-impregnation method). And the content of cerium is 1 wt.%,3 wt.%,5 wt.%,8 wt.%,10 wt.% respectively (calculated by cerium atom). Precursors utilized were PdCl₂, Ce(NO₃)₃.
    First, Ce(NO₃)₃ and PdCl₂ as precursors were dissolved in de-ion water. For the catalysts prepared by consecutive impregnation method, the g-Al₂O₃ support was added to an aqueous solution of Ce(NO₃)₃, the mixture was kept at room-temperature for 24h with agitation occasionally to make active species distribute symmetrically on the support, then dried in an oven at 393K for 3h followed by calcination at 773K for 3h with heating rates of 10K/min. Then the mixed support was added to an aqueous solution of PdCl₂, kept at atmosphere temperature for 24h, then dried at 393K for 3h, calcined at 773K for 3h and reduced by H₂ at 773K for 2h with the heating rate also at 10K/min.
    For the catalysts prepared by co-impregnation method, aqueous solutions of Ce(NO₃)₃ and PdCl₂ were added to g-Al₂O₃ support at the same time. The following steps were the same as above.
2.2 Characterizations
The information of structural position of the Pd-CeO₂/Al₂O₃ catalysts prepared by successive impregnation method were obtained from the Powder X-ray diffraction(XRD) patterns of the reduced samples measured in a Rigaku D/max 2550VB/PC apparatus using a filtered Cu Ka radiation（l＝0.154056nm）. The scanning range is 10-80º. The surface electronic states of the selected sample Pd-CeO₂ (3 wt.%)/Al₂O₃ was determined by the binding energy gained from X-ray photoelectron spectroscopy (XPS). The X-ray photoelectron spectra was obtained by using Al Ka radiation (1486.6eV) through a Perkin-Elmer PHI 5000C ESCA system. All binding energy (BE) values were calibrated by using the value of C 1s of contaminant carbons (284.6eV) as a reference.
2.3 Catalytic Tests
Gas phase acrylonitrile hydrogenation was carried out in a fixed bed glass micro-reactor at atmospheric pressure in the temperature range of 100-200ºC. The weight of catalyst was 20mg mixed with 380mg g-Al₂O₃ (40-50 mesh) to adjust the residence time of raw materials in the fixed bed. Before rising up to the first reaction temperature, the catalyst was contacted with the reaction mixture prepared by bubbling hydrogenation gas through a thermo-stabilized saturator (39ºC) in which raw material acrylonitrile was contained and the flux of hydrogenation was 100ml/min. The effluents were analyzed by on line gas chromatography in a GC-102 chromatographic instrument produced by Shanghai Analytical Apparatus Factory provided with a FID detector and SE30-PEG20M packed column.

3. RESULTS AND DISCUSSION
The following reactions may occur during the gas phase hydrogenation of acrylonitrile over various as-prepared catalysts:
CH₂=CHC≡N + H₂ → CH₃CH₂C≡N ①
CH₂=CHC≡N + H₂ → CH₂＝CHCH₂NH₂ ②
CH₂=CHC≡N + H₂ → CH₃CH₂CH₂NH₂ ③
    Under the present reaction conditions, however, no other production but propionitrile was identified, showing that the hydrogenation of acrylonitrile followed reaction ①nearly quantitatively. The behavior could be attributed to the high selectivity of palladium to double bond.

Figure 1. XRD patterns of Pd-CeO₂/Al₂O₃ prepared by consecutive impregnation method

3.1 XRD            
Figure 1 shows the diffraction patterns of Pd/Al₂O₃ and palladium catalysts supported on CeO₂/Al₂O₃ supports that contain cerium from 1 wt.% to 10 wt.% prepared by consecutive impregnation method. Depending on the temperature, palladium can exist in a form of a metallic Pd or PdO. But from the XRD patterns following, it seems that all the Pd species were reduced to Pdº because no PdO characteristic peak emerged on the patterns. And there was a weak peak of Pd⁰ (2q=40º) for the Pd/Al₂O₃ without the addition of cerium owning to the low concentration of palladium on Al₂O₃ support. But with the concentration of cerium increasing, the peak Pdº became much smaller. And only a very small peak could be observed with the content of cerium more than 3 wt.%, which indicated that the dispersing degree got improved ^[5]. On the other side the peaks of ceria could not be seen on the pattern of Pd-CeO₂ (1wt.%)/Al₂O₃ accounting for the good dispersing degree of ceria on Al₂O₃ support until the concentration of cerium reached 5 wt.%. With the further addition of cerium, the intensity of the characteristic peaks of ceria (2q=45.66º, 33.18º, 47.30º, 56.52º,76.42º) were even greater than the g-Al₂O₃ support (2q=37.44º, 45.66º, 66.96º) which represented the formation of large crystals of CeO₂. At the site of 2q=61.08º, Pd/Al₂O₃ exhibited a peak which seemed to be the characteristic peak of d- Al₂O₃, but no existence of d- Al₂O₃ could be observed after adding cerium to the catalysts, representing that the strong interaction of cerium and g-Al₂O₃ prevented g-Al₂O₃ from transforming to other kinds of crystalloid. Afterwards, a sharp peak referring to a- Al₂O₃ (2q=26.60º) also emerged on Pd/Al₂O₃, decreasing quickly with the addition of cerium, further verifying the strong interaction between cerium and g-Al₂O₃, i.e., cerium has the ability to stabilize the g-Al₂O₃ support. Then we can conclude that cerium interacted with the support g-Al₂O₃ to weaken the interaction between Pd and Al₂O₃, resulting to free Pd from the support. Marvin has got the same result that addition of foreign cations could stabilize the surface of g-Al₂O₃^[6]. Though the existence of Pd could facilitate the reduction of CeO₂ that would be discussed later, no peaks of low valent Ce cation were observed on the XRD patterns.


Figure 2. Binding energy of various species on Pd-CeO₂/Al₂O₃

3.2 XPS  
To estimate the electronic states of the species on Pd-CeO₂/Al₂O₃, we measured the binding energies of the species on Pd-CeO₂ (3 wt.%)/Al₂O₃ prepared by consecutive impregnation method. The result in Figure 2 showed that binding energy of Pd_5/2 was 335.60eV, a little higher than that of metallic palladium (335.2eV). This value revealed that Pd species were well reduced to metal palladium in agreement with XRD result. But the peak was shifted to higher energy by 0.4eV, manifesting that no alloy containing Pd was formed because the alloy formation usually showed the shift by more than 1eV. It proved that Pdº was free on the support, further verifying the result of XRD patterns. On the pattern of Ce 3d, Ce⁴⁺ exhibits the binding energy of 909.25eV, 6eV lower than the value reported by Junko ^[7], indicating that CeO₂ was rich in electrons. Considering of the low content of Pd, the ceria's richness in electrons must be due to the interaction between ceria and alumina. And there emerged two characteristic peaks of Ce³⁺ at 890.38eV and 888.48eV site (a shoulder peak) on the pattern, which illuminated that CeO₂ had been reduced partly, and the peak of 878.68eV should represent the existence of Ceº. Though CeO₂ is very difficult to be reduced at conventional atmosphere, it would not be so stable with the existence of Pd. That is, Pd facilitated the reduction of CeO₂ to low valent Ce species ^[7]. The reduction of CeO₂ to Ce₂O₃ might produce new active sites to improve catalytic activity ^[8].

Figure 3. Conversion as a function of temperature over as-prepared catalysts
Reaction conditions: P=1 atm, Space velocity=1600h^-1, acrylonitile/H₂ (molar ratio)=9:1, Time on stream = 30min

3.3Hydrogenation of Acrylonitrile
Figure 3 shows the conversion of hydrogenation reaction as a function of reaction temperature over Pd/Al₂O₃ and Pd-CeO₂/Al₂O₃ catalysts prepared by consecutive impregnation and co-impregnation method respectively. And there were no other products but propionitrile that can be identified. For Pd/Al₂O₃, acrylonitrile conversion shifted with temperature sharply, first increased with temperature and then decreased greatly, getting a maximum at about 140ºC. While to Pd-CeO₂/Al₂O₃ catalysts, it exhibited trivial influence of acrylonitrile conversion in the range of 100-200ºC. This could be explained by that the addition of cerium decreased the activation energy of the reaction. And the catalytic activity of the catalysts with the addition of cerium was much higher than Pd only catalyst as can be seen from Figure 3. But the content of cerium played a key role to the activity of the as-prepared catalysts. The conversion always got a maximum at the concentration of 3wt.% cerium no matter what impregnation method was adopted, and decreased quickly with the further addition of cerium. To Pd-CeO₂ (10 wt.%)/Al₂O₃ prepared by co-impregnation method, the activity was even lower than Pd/Al₂O₃ and the activity of the catalysts prepared by consecutive impregnation method was much better than those prepared by co-impregnation method. All these may be due to the interaction of Pd, cerium and Al₂O₃ by both electronic and structural modification as had been characterized by XRD and XPS. And large crystals of CeO₂ would appear on the Al₂O₃ support if the content of cerium reached beyond 5 wt.%, which may wall up the holes of the support to prevent the sufficient contact of active sites and raw materials, resulting in the decline of catalytic activity. Due to the same reason, catalysts prepared by co-impregnation method were not so active because CeO₂ was much easier to form on the surface of Pd to inhibit the contact of Pd and acrylonitrile. Combined with XPS result, the peak of Pdº shifted to higher energy by 0.4eV, indicated the interaction of cerium decreased the electron density of palladium which may drop the activity of the catalyst and conversion of the hydrogenation reaction. But the binding energy of ceria declined greatly, which may prove the formation of cerium aluminate to make a new active site come into being. And Pd facilitated the reduction of Ce⁴⁺ to Ce³⁺, the existence of Ce³⁺ may lead to the formation of new active sites and improvement of the activity too. What's more, we found in our experiments that cerium improved the catalyst's stability greatly. In order to investigate the difference of their stability in short time, we increased the runoff of acrylonitrile (85g acrylonitrile/gcat.h, Reaction Temperature=140ºC, P=1atm). And the result proved that doped cerium catalyst exhibited a better ability to retard deactivation. Pd/Al₂O₃ deactivated after continuous reaction for 2.5h while life span of Pd-CeO_x/Al₂O₃ was more than 5h. The difference of their stability illuminated that cerium doped catalyst kept its catalytic activity for a longer time at severe reaction conditions.

4. CONCLUSIONS
We have examined CeO₂/Al₂O₃ as promoter of the Pd catalyst for acrylonitrile hydrogenation and confirmed that cerium had positive effects on the catalyst catalytic and stable performance after calcination and reduction at 773K. This is due to the interaction of the Pd and metal oxides. First, the sintering of the dispersed palladium particles was retarded with the addition of cerium and correspondingly the catalyst activity was preserved even after calcination and reduction at 773K. And the existence of Pd facilitated the reduction of CeO₂ to low valent species to form new active sites to improve the catalytic activity.

REFERENCES
[1] L Fan, K Fujimoto, J. Catal., 1997, 172 : 238.
[2] S Bernal, J J Calvino, M.A. Cauqui et al., Catal. Today, 1999, 50 : 175
[3] R P Underwood, A.T.Bell., J. Catal.,1988, 109 : 61.
[4] E Schultz, A. L. Borer, R. Prins., Catal. Lett., 1992, 14: 279.
[5] J Tu, Y L Fu, P.Y.Ling., Chin J.Phy.Chem.(Huaxue wuli xuebao), 2001, 14 : 1
[6] M F L. Johnson., J. Catal., 1990, 123: 245
[7] J  Mitome, E Aceves, U S Ozkan., Catal. Today., 1999, 53 : 597
[8] F B Noronha, M A S Baldanza, R S Monteiro et al. Appl. Catal. A: General., 2001, 210 : 275
[9] Hui Li, Hexing Li, Wei-Lin Dai et al., Appl. Catal. A: General., 2001, 207: 151
[10] Faqiang Wang, Zuhui Xu, Guoguang Zhou. Shanghai Normal University. Acta. 2002, 31:42

[ Back ] [ Home ] [ Up ] [ Next ]