Chemoselective hydrogenation of crotonaldehyde to crotyl alcohol over Pt/Ce x Sm 1-x O 2-δ catalysts

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1 Indian Journal of Chemistry Vol. 49A, January 2010, pp Chemoselective hydrogenation of crotonaldehyde to crotyl alcohol over Pt/Ce x Sm 1-x O 2-δ catalysts Ling-Yun Jin, Li-Ping Tao, Xi-Jing Liu, Guan-Qun Xie, Ji-Qing Lu & Meng-Fei Luo* Key Laboratory of the Ministry of Education for Advanced Catalysis Materials, Institute of Physical Chemistry Zhejiang Normal University, Jinhua , China mengfeiluo@zjnu.cn Received 6 November 2009; accepted 16 December 2009 Pt/Ce x Sm 1-x O 2-δ catalysts have been prepared with impregnation method using H 2 PtCl 6 as precursor. The selective vapor-phase hydrogenation of crotonaldehyde to crotyl alcohol over the catalysts has also been studied. It is found that introduction of Sm promotes the selective hydrogenation of C=O bond to crotyl alcohol due to formation of strong Lewis acidic sites. For Pt/Ce 0.8 Sm 0.2 O 2-δ catalysts, with increasing reduction temperatures, the crotonaldehyde conversion decreases while the selectivity to crotyl alcohol increases. Also, during the reaction process there is carbon deposition on the catalyst surface, which results in decrease, both in the catalytic activity as well as selectivity. Keywords: Catalyst, Supported catalysts, Hydrogenation, Chemoselective hydrogenation, Selective hydrogenation, Alcohols, Unsaturated alcohols, Aldehydes, Platinum IPC Code: Int. Cl. 9 B01J21/00 Unsaturated alcohols are important intermediates used in fragrance industries and pharmaceutical manufacturing 1. However, chemoselective hydrogennation of α, β-unsaturated aldehydes to unsaturated alcohols is a challenging task 2,3, since hydrogenation is preferred at the C=C bond rather than the C=O bond for both thermodynamic and kinetic 4,5. Pt catalysts have been widely studied in selective hydrogenation of α, β-unsaturated aldehydes such as crotonaldehyde 6 and cinnamaldehyde 7. However, monometallic platinum catalysts is mainly favorable for the hydrogenation of C=C bond 8. Therefore, it is necessary to improve the selectivity of the catalyst to C=O bond as well as to investigate the factors that affect the selectivity. Generally speaking, the selectivity to the α, β-unsaturated alcohol is affected by the structure of aldehydes, the nature of active components, the metal-support and the particle size of the active metal 9,10. It was reported that the selectivity to crotyl alcohol can be obtained by using bimetallic catalysts, active supports and by adding promoters 11. Group VIII metals supported on TiO 2 12, reducible supports such as CeO TiO 2 6,16, SnO 2 17 and ZnO 18 have been intensively investigated. Herein, we report the selective hydrogenation of crotonaldehyde in vapor phase over Pt catalysts supported on Ce x Sm 1-x O 2-δ solid solutions (x=1, 0.9, 0.8, 0.5). The effects of the reduction temperature and Lewis acidic sites on the catalytic performance of the catalysts have also been discussed. Experimental The Ce x Sm 1-x O 2-δ supports were prepared by sol-gel method. Ce(NO 3 ) 3 and Sm(NO 3 ) 3 dissolved in deionized water were mixed to form a Ce-Sm nitrate mixed solution. Citric acid with twice the molar amount of total metal cations was added to obtain a mixed nitrate and citrate solution. After being stirred for several hours, the clear solution was evaporated to obtain a gel, and then dried at 80 C, followed by a calcination at 500 C in air for 4 h at a heating rate of 5 C /min. The Pt/Ce x Sm 1-x O 2-δ catalysts were prepared by impregnating the supports with aqueous solutions of H 2 PtCl 6. The excessive solvent was evaporated and the samples were dried overnight at 110 C. The Pt content in the catalyst was 3 wt%. BET surface areas of the catalysts were determined on a Quanta Chrome Autosorb-1-MP apparatus using N 2 adsorption isotherm at -196 o C by Brunauer- Emmett-Teller (BET) method. X-ray power diffraction (XRD) patterns were obtained with a PANalytical X Pert PROMPD system, using Cu-Kα radiation. The working voltage was 40 kv and the working current was 40 ma. The intensity data were collected with a scanning rate of 1.0 /min. The mean crystallite sizes were analyzed using the Scherrer equation, where the Scherrer constant (partial shape factor) was taken as Raman measurements were performed on a Reinshaw RM1000 with a confocal microprobe Raman system using an excitation laser line of 514 nm.

2 50 INDIAN J CHEM, SEC A, JANUARY 2010 The acidic properties of the catalysts were studied by temperature-programmed desorption of ammonia (NH3-TPD) and Fourier transform infrared spectroscopy (FTIR). The sample (0.1 g) was first reduced at 500 C for 1 h in H 2, then it was cooled down to 50 C, and a flow of NH 3 was passed through for 30 min. The gaseous or physisorbed NH 3 was removed by N 2 at 100 C for 1 h and then the sample was heated to 700 C at a rate of 20 C min -1. The desorbed NH 3 was monitored continuously via a TCD detector. The FTIR spectra of pyridine adsorption on the catalysts reduced at 500 C were recorded on a Nicolet NEXUS 670 spectrometer in the range cm -1. The catalysts were dried at 100 C for 1 h before pyridine treatment. A self-supported catalyst pellet was brought into contact with pyridine directly. Then the catalyst was kept at 120 C for 1 h to remove physisorbed pyridine. When the catalyst was cooled to room temperature, the IR spectrum was recorded in the spectral range cm -1 with 128 scans and at a resolution of 4 cm -1 using KBr background at room temperature. The catalytic behavior of the catalysts in vaporphase hydrogenation of crotonaldehyde (2-butenal) was tested in a fixed bed reactor at atmospheric pressure. In each run, 100 mg of catalyst was used. Before the reaction, the catalyst was reduced in a flowing mixture of 6% H 2 in N 2 at different temperatures (O 2 and H 2 O were removed by Pt/Al 2 O 3 and 5A molecular sieves). The catalyst was then cooled down in hydrogen to the reaction temperature (80 C). A reaction mixture of 1% (volume ratio) crotonaldehyde in H 2 was obtained by passing H 2 flow (26 ml/min) through a thermostabilized saturator (0 C) containing the unsaturated aldehyde. The products were analyzed by gas chromatography (Shimadzu-2014) equipped with a flame ionic detector (FID), which was attached with a DB-Wax capillary column (30 m 0.25 mm 0.25 µm). Results and discussion Figure 1 shows the XRD patterns of the Ce x Sm 1-x O 2-δ mixed oxides. The diffraction patterns are consistent with those of the typical fluorite cubic structure of CeO 2 (JCPDS: ), and no diffraction peaks of Sm 2 O 3 are observed. Table 1 lists the lattice parameters, mean crystallite sizes and surface areas of the Ce x Sm 1-x O 2-δ mixed oxides. It is found that all these samples are of cubic CeO 2 phases. With increasing Sm content, the surface area of Ce x Sm 1-x O 2-δ decreases except for Ce 0.9 Sm 0.1 O 2-δ. Additionally, the lattice parameters of Ce x Sm 1-x O 2-δ mixed oxides are larger than those of pure CeO 2 due to the fact that the ionic radius of Sm 3+ cation ( nm) is larger than that of Ce 4+ cation (0.097 nm), and the formation of solid solution leads to a lattice expansion. The decrease in the mean crystallite sizes of the Ce x Sm 1-x O 2-δ mixed oxides with the increasing Sm content may be attributed to the formation of Ce x Sm 1-x O 2-δ solid solution. A similar phenomenon has also been found in the Ce x Zr 1-x O 2 solid solution 19. Table 2 lists the conversion of crotonaldehyde and selectivity to main products for chemoselective hydrogenation of crotonaldehyde over different Pt/Ce x Sm 1-x O 2-δ catalysts. Before testing, the CeO θ ( o ) Ce0.5Sm0.5O2-δ Ce0.8Sm0.2O2-δ Ce0.9Sm0.1O2-δ Fig. 1 XRD patterns of Ce x Sm 1-x O 2-δ mixed oxides. Table 1 BET surface areas, lattice parameters and crystallite sizes of the Ce x Sm 1-x O 2-δ mixed oxides Sample Phase Lattice parameter (nm) Crystallite size (nm) Surface area (m 2 g -1 ) Ce 0.5 Sm 0.5 O 2-δ Cubic Ce 0.8 Sm 0.2 O 2-δ Cubic Ce 0.9 Sm 0.1 O 2-δ Cubic CeO 2 Cubic Table 2 Pt/Ce x Sm 1-x O 2-δ catalysts for chemoselective hydrogenation of crotonaldehyde. [Pt content = 3 wt%; reaction temperature: 80 C] Catalysts Conv. Sel. (%) (%) Crotyl alcohol Butanal Butanol Pt/CeO Pt/Ce 0.9 Sm 0.1 O 2-δ Pt/Ce 0.8 Sm 0.2 O 2-δ Pt/Ce 0.5 Sm 0.5 O 2-δ

3 NOTES 51 catalyst was reduced in situ at 500 C and then reacted at 80 C. Data were obtained after 30 min of time on stream. It may be noted that the catalytic performance of the Pt/Ce x Sm 1-x O 2-δ catalysts is greatly affected by the component of the support. With the increase of Sm content, the catalytic activity of the Pt/Ce x Sm 1-x O 2-δ catalysts (in form of conversion of crotonaldehyde) decreases gradually from 32.0% to 12.3%. This indicates that the introduction of Sm into the CeO 2 support leads to an inhibition of catalytic activity. As seen from Table 2, with increasing Sm content, the selectivity to crotyl alcohol increases initially and then decreases. When small amount of Sm was added (x=0.8), the selectivity is enhanced remarkably from 38.5% (over Pt/CeO 2 ) to 73.1% (over Pt/Ce 0.8 Sm 0.2 O 2-δ ). However, with further increase of Sm content (such as Ce 0.5 Sm 0.5 O 2-δ ) in the catalysts, the selectivity to crotyl alcohol decreases. NH 3 -TPD profiles of Pt/Ce x Sm 1-x O 2-δ catalysts reduced in situ at 500 C are shown in Fig. 2. Desorption peaks of α (below 250 C) and β (about 375 C) are seen for all catalysts. Moreover, it is noticed that no desorption peak is observed at higher than 500 C for Pt/CeO 2 and Pt/Ce 0.9 Sm 0.1 O 2-δ catalysts. However, with increasing content of Sm, strong peak of γ at about 620 C appears for Pt/Ce 0.8 Sm 0.2 O 2-δ catalyst, while it gets remarkbly weak with further increasing content of Sm (Pt/Ce 0.5 Sm 0.5 O 2-δ catalyst). This indicates that the strongest acidic site is found in the Pt/Ce 0.8 Sm 0.2 O 2-δ catalyst. In order to determine the nature of acidic sites, pyridine adsorption experiment was conducted. Figure 3 shows the IR spectra of adsorbed pyridine on Pt/Ce 0.8 Sm 0.2 O 2-δ and Pt/Ce 0.9 Sm 0.1 O 2-δ catalysts. Bands at 1453 attributed to Lewis acidic sites 20 and 1615 cm -1 attributed to Lewis acidic sites and/or vibrations of water molecules 21 are observed, while the band at 1450 cm -1 attributed to Brönsted acidic sites 22 is very weak. This indicates that the surface acidity of these catalysts mainly consist of Lewis acidic sites. It has been reported that Lewis acidic site is favorable for the adsorption of C=O bond and promotes the formation of crotyl alcohol 16,23, therefore, the formation of strong Lewis acidic site may be the main reason that the selectivity of Pt catalysts supported on Ce x Sm 1-x O 2-δ solid solution increases. In addition to possessing the strongest acidic site, the Pt/Ce 0.8 Sm 0.2 O 2-δ catalyst also shows the highest selectivity to crotyl alcohol (Table 2). Considering the highest selectivity to crotyl alcohol, the effect of reduction temperatures on Intensity (a. u.) α β Pt/Ce0.5Sm0.5O2-δ Pt/Ce0.8Sm0.2O2-δ Temperature ( o C) Pt/Ce 0.8 Sm 0.2 O 2-δ catalyst was further studied at different temperatures ( C). With increasing reduction temperature, the conversion of crotonaldehyde decreased while the selectivity to crotyl alcohol increased. By the Pt dispersion obtained from CO chemical absorption (conducted on Quantachrome CHEMBET-3000), the crystallite size of Pt reduced at 300 and 500 C are 4.9 and 6.4 nm, respectively. It indicates that with a rise in the reduction temperature, the particle size of Pt increases, and the catalytic activity declines while the selectivity to crotyl alcohol increases. The decrease in the activity is probably due to the decline in the active sites of the catalyst resulting from the increase in particle size of Pt 11,18. However, in the case of large γ Pt/Ce0.9Sm0.1O2-δ Pt/CeO2 Fig. 2 NH 3 -TPD profiles of Pt/Ce x Sm 1-x O 2-δ catalysts. Absorbance (a. u.) Pt/Ce0.8Sm0.2O2-δ Pt/Ce0.9Sm0.1O2-δ Wave number (cm -1 ) Fig. 3 IR spectra of pyridine adsorption on Pt/Ce 0.8 Sm 0.2 O 2-δ and Pt/Ce 0.9 Sm 0.1 O 2-δ catalysts.

4 52 INDIAN J CHEM, SEC A, JANUARY Conversion/ Selectivity (%) Conv.& sel. (%) Crotonaldehyde Crotyl alcohol Butanal Butanol Others After reaction Fresh Time on stream (min) Time on stream (min) Fig. 4 Catalytic performance for Pt/Ce 0.8 Sm 0.2 O 2-δ catalyst reaction at 80 C after reduction at 500 C. particles, the exposed Pt(111) surface planes rises, which improves the hydrogenation of C=O bond 16,24. Therefore, the selectivity to crotyl alcohol increases with increasing reduction temperature. Another reason for the enhancement of the selectivity to crotyl alcohol with the reduction temperature is related to the reducible properties of the support Ce 0.8 Sm 0.2 O 2-δ. When the catalyst is reduced at high temperature, the reduction degree of Ce 0.8 Sm 0.2 O 2-δ is improved and the amount of Ce 3+ increases, which donate more electrons to platinum, leading to a rise in the electron density of platinum. It depresses the adsorption of C=C bond and increases the selectivity to crotyl alcohol. Figure 4 displays the catalytic performance of Pt/Ce 0.8 Sm 0.2 O 2-δ catalyst reduced at 500 C for hydrogenation of crotonaldehyde at 80 C. It may be noted that main reaction products are crotyl alcohol, butanal and butanol, and the hydrogenation of C=O bond is comparatively dominant during the reaction. As seen in the figure, the crotonaldehyde conversion decreases sharply in the first 30 min and then reaches a relatively steady level (about 10%) after about 90 min. The selectivity to crotyl alcohol decreases but selectivity to butanal increases with time on stream, after 210 min, while that to crotyl alcohol and butanal reach 53% and 35%, respectively. This indicates that the advantage of hydrogenation of C=O bond is gradually replaced by Raman shift (cm -1 ) Fig. 5 Raman spectra of Pt/Ce 0.8 Sm 0.2 O 2-δ catalysts before and after reaction. that of C=C bond and the catalytic activity decreases with time on stream. To further study the deactivation of the catalytic activity, Raman spectra of Pt/Ce 0.8 Sm 0.2 O 2-δ catalysts before and after reaction were recorded (Fig. 5). The fresh catalyst shows two Raman bands at 460 and 600 cm -1, which can be attributed to the F 2g vibration mode of the fluorite structure of CeO 25 2 and the oxygen vacancies 26, respectively. Compared with the fresh catalyst, the catalyst after reaction shows a new band centered at 1580 cm -1, which is assigned to the carbon deposit 27. The Pt/C interface is formed due to the carbon deposit on the Pt catalysts surface, which is main favorable for the formation of butanal 28, and therefore, the selectivity to crotyl alcohol decreases. Moreover, the active sites Pt(111) are covered by the carbon deposit and thus the catalytic activity decreases, which leads to the decrease in both the crotonaldehyde conversion and the selectivity to crotyl alcohol. The present study shows that for Pt/Ce x Sm 1-x O 2-δ catalysts, the introduction of Sm into the support is favorable for the selectivity to crotyl alcohol due to the strong Lewis acidic sites. With reduction temperatures the crystallite size of Pt increases, so that the crotonaldehyde conversion decreases while the selectivity to crotyl alcohol remarkably increases. Furthermore, due to the carbon deposition on the catalyst surface, both the catalytic activity and the selectivity decrease with time on stream.

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