Hydrotalcite Catalyst

CHINESE JOURNAL OF CATALYSIS Volume 28, Issue 4, April 2007 Online English edition of the Chinese language journal Cite this article as: Chin J Catal, 2007, 28(4): 312–316.

RESEARCH PAPER

Liquid-Phase Selective Hydrogenation of Phenol to Cyclohexanone over Pd-Ce-B/Hydrotalcite Catalyst LIU Jianliang, LI Hui, LI Hexing* Department of Chemistry, Shanghai Normal University, Shanghai 200234, China

Abstract: Supported Pd-Ce-B catalysts were prepared by chemical reduction of Pd and Ce ions that were deposited on hydrotalcite (HT) with an Al3+/(Al3++Mg2+) molar ratio of 0.2. In the reaction of liquid-phase selective hydrogenation of phenol, the as-prepared Pd-Ce-B/HT catalyst exhibited much higher activity and selectivity for cyclohexanone than Pd-Ce-B/Al2O3, Pd-Ce-B/MgO, and Pd-Ce-B/SiO2. The maximum phenol conversion and selectivity for cyclohexanone over 5.8%Pd-Ce-B/HT reached 82.0% and 80.3%, respectively. Based on the results of various characterizations, the catalyst structure-activity correlation and the promoting effects of the Ce dopant and acidic–basic properties of the support on the catalyst behavior were discussed. Key Words: palladium; cerium; boron; hydrotalcite; supported catalyst; phenol; hydrogenation; cyclohexanone

Cyclohexanone is an important chemical material and is widely used in the production of fibers, synthetic rubbers, industrial coatings, medicines, pesticides, and organic solvents [1]. Compared with the traditional method of producing cyclohexanone [1,2], the one-step process for selective hydrogenation of phenol to cyclohexanone is superior in its easy operation, small impact on the environment, and low energy consumption. However, the hydrogenation of phenol to cyclohexanol is relatively much easy. Therefore, much attention has been focused on the improvement of the selectivity for cyclohexanone, and the key issue for this is to develop suitable catalysts. In industrial plants, selective hydrogenation of phenol is carried out over Pd/Al2O3 catalysts, which have only 50%–60% selectivity for cyclo-hexanone [3]. Previous studies have shown that the addition of alkali or alkaline earth metals to Pd/Al2O3 can increase the catalyst activity and selectivity for cyclohexanone [3,4]. On the basic surface of the catalyst, phenol is adsorbed with the aromatic ring in a nonplanar mode, which favors the formation of cyclohexanone [3], whereas a proper number of acidic sites are beneficial for the isomerization of enol to cyclohexanone. Hydrotalcite (denoted as HT) is a catalyst support with controllable structure and properties [5]. By tuning the ratio of

Mg/Al, the acidic–basic properties of this material can be tailored, and the catalytic performance can be improved. In the present article, Pd-Ce-B/HT catalysts were prepared by deposition of PdCl2 and Ce(NO3)3 on the HT support with an Al3+/(Al3++Mg2+) molar ratio of 0.2 followed by chemical reduction with KBH4. The catalytic performance of these catalysts for the liquid-phase selective hydrogenation of phenol to cyclohexanone was studied.

1 Experimental 1.1 Catalyst preparation HT with an Al3+/(Al3++Mg2+) molar ratio of 0.2 was synthesized according to literature procedures [6]. The HT support (1.0 g) was impregnated with 2.0 ml of PdCl2 solution (Pd content = 0.03 g/ml) or a mixture of PdCl2 and Ce(NO3)3 solution (mass ratio of Pd/Ce = 10) for 24 h. After drying at 373 K, the precursor was calcined at 473 K for 2 h and then reduced with KBH4 aqueous solution (2.0 mol/L) containing 0.2 mol/L NaOH. The molar ratio of BH4−/Pd2+ was set at 4 to ensure complete reduction of the metallic ions. The resulting black solid was repeatedly washed with distilled water and absolute ethanol, and finally, the prepared Pd-B/HT and

Received date: 2006-10-10. * Corresponding author. Tel/Fax: +86-21-64322272; E-mail: [email protected] Foundation item: Supported by the Preliminary 973 Project (2005CCA01100), the National Natural Science Foundation of China (20377031, 20603023), the Shanghai Science and Technology Committee (05QMX1442, 0452nm070, 0552nm036), and the Shanghai Education Committee (T0402, 05DZ20). Copyright © 2007, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier BV. All rights reserved.

LIU Jianliang et al. / Chinese Journal of Catalysis, 2007, 28(4): 312–316

Pd-Ce-B/HT catalysts were stored in ethanol for use. Inductively coupled plasma (ICP) analysis showed that the loadings of Pd in the two catalysts were 5.6% and 5.8% (mass fraction), respectively. 1.2 Catalyst characterization The structure of the samples was confirmed by X-ray diffraction (XRD) on a Rigaku Dmax-3C powder diffractometer with Cu Kα radiation. The surface electronic state of the catalysts was analyzed by X-ray photoelectron spectroscopy (XPS) on a Perkin Elmer PHI 5000 ESCA system with Al Kα radiation. All the binding energy (BE) values were calibrated by using the C 1s line (284.6 eV) as a reference. The surface morphology was observed by transmission electron microscopy (TEM) using a JEOL JEM 2011 instrument. The thermal stability was determined by differential scanning calorimetry (DSC) on a Shimadzu DSC-60 analyzer. The composition of the samples was analyzed by inductively coupled plasmaatomic emission spectroscopy (ICP-AES) with a Jarrell-Ash Scan 2000 instrument. The active surface area (Aact) was measured by CO chemisorption on a Quantachrome ChemBET 3000 apparatus.

lysts were treated in a N2 flow at 873 K for 2 h, indicating the decomposition of Pd-Ce-B and the aggregation of particles. Meanwhile, the typical HT diffraction peak disappeared, showing that the heat treatment destroyed the structure of the HT support. The cerium phase was not observed even after annealing at higher temperatures. This may be explained by considering the low content of the Ce promoter and the high dispersion of the Ce species in the alloys.

1.3 Activity test Phenol hydrogenation was carried out in a 250 ml stainlesssteel autoclave under the following reaction conditions: 1.0 g of catalyst, 5.0 g of phenol, 40 ml of ethanol, 1.0 MPa H2, 393 K, and 800 r/min. By measuring the drop of the H2 pressure in the autoclave, both the specific activity (the H2 uptake rate per gram of Pd, RHm, mmol/(h·g)) and the intrinsic activity (the H2 uptake rate per m2 of Pd, RHS, mmol/(h·m2)) were calculated using the ideal gas equation. The products were analyzed on a gas chromatograph (GC 9800, Shanghai Kechuang, China) equipped with an FID and a capillary column (AT-FFAP). The conversion of phenol and the selectivity for cyclohexanone were determined under the following conditions: injection temperature, 493 K; detector temperature, 513 K; column temperature, 393–473 K; temperature ramping rate, 10 K/min.

2 Results and discussion The XRD patterns in Fig. 1 show that the structure of the as-prepared HT support can be identified by a series of typical diffraction peaks centered at 2θ = 22.6º, 34.2º, 38.2º, 45.2º, 60.2º, and 61.5º, being indexed to (006), (102), (105), (108), (110), and (113) reflection [6]. No metallic Pd diffraction peak for 5.6%Pd-B/HT and 5.8%Pd-Ce-B/HT can be observed, which is due to the amorphous structure of Pd-B and Pd-Ce-B and their high dispersion on the HT support. Metallic Pd(111), (200), and (220) diffraction peaks could be observed at 2θ = 40.2º, 46.7º, and 68.2º [7] when the as-prepared cata-

Fig. 1 XRD patterns of the support and catalyst samples (1) Hydrotalcite (HT), (2) 5.6%Pd-B/HT, (3) 5.8%Pd-Ce-B/HT, (4) 5.8%Pd-Ce-B/HT after being treated at 873 K in N2 flow for 2 h

The XPS spectra (Fig. 2) show that the Pd species in the 5.6%Pd-B/HT and 5.8%Pd-Ce-B/HT catalysts were present in a metallic state, corresponding to a BE value of 335.0 eV (Pd 3d5/2), which agrees well with the BE of metallic Pd. However, the B species existed in two states, the elemental alloying B with a lower BE and the oxidized B (mainly B2O3) with a higher BE. The BE of the alloying B shifted positively compared to the standard BE value of amorphous boron powder (187.1 eV), implying the formation of a Pd-B amorphous alloy and the transfer of partial electrons from B to Pd, which makes Pd electron enriched and B electron deficient. The spectrum of Ce 3d shows that almost all the surface cerium was present in a +3 valence state with the BE of 885.3 eV and 903.8 eV [8,9]. As no metallic Ce was present, it can be concluded that only the Pd2+ can be reduced under the present conditions. Fig. 3 shows the typical TEM images of the as-prepared catalysts. It can be observed that the Pd particles were uniformly dispersed on the surface of the HT support. Doping of cerium led to a remarkable decrease in the particle size and a

LIU Jianliang et al. / Chinese Journal of Catalysis, 2007, 28(4): 312–316

Fig. 2

XPS spectra of 5.6%Pd-B/HT (1) and 5.8%Pd-Ce-B/HT (2) samples

more uniform dispersion. DSC curves (Fig. 4) clearly show that there was only one exothermic peak for the as-prepared Pd-B, 5.6%Pd-B/HT, and 5.8%Pd-Ce-B/HT samples, which might be ascribed to the crystallization of the Pd-B amorphous alloy. The crystallization temperature of 5.6%Pd-B/HT was 39 K higher than that of the ultrafine Pd-B, indicating the stabilization effect of the support on the amorphous structure. Meanwhile, the crystallization temperature of 5.8%Pd-Ce-B/HT was 15 K higher than that of 5.6%Pd-B/HT, which suggests that the doping of Ce

Fig. 4 DSC curves of Pd-B (1), 5.6%Pd-B/HT (2), and 5.8%Pd-Ce-B/HT (3) samples

Fig. 3 TEM photographs of 5.6%Pd-B/HT (a) and 5.8%Pd-Ce-B/HT (b) samples

was favorable for the improvement of thermal stability of the Pd-B amorphous alloy. The typical evolution of the reaction components with reaction time during the liquid-phase phenol hydrogenation is plotted in Fig. 5. Obviously, the main products obtained under the current conditions were cyclohexanone and cyclohexanol. The cyclohexanone content reached a maximum value and then decreased as a function of time, indicating that cyclohexanone was an intermediate product of the phenol hydrogenation. Table 1 shows the composition, active surface area, and catalytic hydrogenation performances of the different catalysts. Comparing the catalytic activity of 5.6%Pd-B/HT with that of 5.8%Pd-Ce-B/HT, it can be found that incorporation of Ce could effectively enhance both the hydrogenation rate (RHm and RHS) and the cyclohexanone selectivity. The maximum

LIU Jianliang et al. / Chinese Journal of Catalysis, 2007, 28(4): 312–316

Fig. 5 Reaction profiles of phenol hydrogenation over different catalysts (a) 5.6%Pd-B/HT, (b) 5.8%Pd-Ce-B/HT; (1) Phenol, (2) Cyclohexanone, (3) Cyclohexanol (Reaction conditions: 1.0 g catalyst, 5.0 g phenol, 40 ml ethanol, p(H2) = 1.0 MPa, T = 393 K, stirring rate = 800 r/min.)

HT displayed the maximum catalytic activity and cyclohexanone yield. This indicates that the support with appropriate acid–base sites favored the formation of cyclohexanone. On the one hand, on base sites phenol molecules adsorbed in the nonplanar form, which was beneficial for the formation of cyclohexanone. On the other hand, a suitable amount of acid sites favored the catalytic isomerization of enol to cyclohexanone [10]. In addition, the unique sandwich structure of HT had much influence on the catalytic activity and selectivity [11]. HT is a kind of anion clay, consisting of positively charged layers and interlayer anions. First, the strong interaction between Pd2+ and HT was favorable for the increase in the dispersion of active sites and the Pd loading (see Table 1), which could account for the improvement of the activity. Second, the base sites of the interlayer anions could adsorb phenol in a nonplanar form and enhance the cyclohexanone selectivity. Third, the reactants could diffuse into the inner space of the interlayer, which was different from the bulk conditions. This possibly decreased the activity energy and increased the cyclohexanone selectivity.

cyclohexanone yield was increased from 48.2% to 65.6%. The promoting effect of Ce could be attributed to the improvement of Pd dispersion (Aact). On the other hand, Ce3+ species can act as Lewis basic sites and increase the adsorption and activation of the reactants, resulting in the increase in the intrinsic activity (RHS). Moreover, this adsorption was in a nonplanar form, favoring the production of cyclohexanone [3] and thus increasing the cyclohexanone selectivity. When the 5.8%Pd-Ce-B/HT catalyst was treated at higher temperature, the catalytic activity and the selectivity for cyclohexanone greatly decreased. On the one hand, this was due to the aggregation of Pd-Ce-B particles and the decrease in active surface area, which caused the decrease in catalytic activity. On the other hand, the heat treatment resulted in the crystallization of Pd-B and the loss of the electronic effect because of the decomposition of these alloys and even caused the decomposition of HT [6], which decreased both the intrinsic activity (RHS) and the cyclohexanone selectivity. Concerning the catalytic properties of Pd-Ce-B supported on different supports, it can be observed that 5.8%Pd-Ce-B/

Table 1 Structural characteristics and catalytic properties of the as-prepared catalysts Catalyst

Composition

Aact/(m2/g)

RHm a/(mmol/(h·g))

RHS b/(mmol/(h·m2))

Selectivityc (%)

Yieldd (%)

5.6%Pd-B/HT

Pd67.9B32.1

86.1

162.9

1.89

69.2

48.2

5.8%Pd-Ce-B/HT

Pd65.4Ce3.8B30.8

99.2

259.8

2.62

80.3

65.6

3.9%Pd-Ce-B/Al2O3

Pd63.5Ce4.5B32.0

149.9

164.4

1.10

57.4

20.8

4.0%Pd-Ce-B/MgO

Pd67.5Ce5.9B26.6

84.6

98.7

1.17

86.0

36.4

3.5%Pd-Ce-B/SiO2

Pd71.7Ce6.5B21.8

139.9

43.8

0.31

72.3

17.4

Crystallized 5.8%Pd-Ce-B/HTe

Pd65.4Ce3.8B30.8

68.6

40.6

0.59

58.3

13.4

a

Hydrogen consumption per hour per gram of Pd.

b

Hydrogen consumption per hour per m2 of Pd.

c

The cyclohexanone selectivity obtained at the maximum cyclohexanone yield.

d

The maximum cyclohexanone yield obtained under the reaction conditions shown in Fig. 5.

e

The crystallized catalyst obtained by treating the fresh sample at 873 K in N2 flow for 2.0 h.

LIU Jianliang et al. / Chinese Journal of Catalysis, 2007, 28(4): 312–316

3 Conclusions

Mol Catal (China), 2005, 19(2): 115 [3] Neri G, Visco A M, Donato A, Milone C, Malentacchi M,

The Pd-Ce-B/HT catalyst could be prepared by deposition of PdCl2 and Ce(NO3)3 on the HT support and chemical reduction with KBH4. This catalyst was effective for the hydrogenation of phenol to cyclohexanone. The support with different acidic and basic properties could affect the adsorption form of phenol, leading to a significant difference in the yield of objective product cyclohexanone. The maximum cyclohexanone yield was obtained on the 5.8% Pd-Ce-B/HT catalyst with appropriate acid–base sites.

Gubitosa G. Appl Catal A, 1994, 110(1): 49 [4] Mahata N, Raghavan K V, Vishwanathan V, Park C, Keane M A. Phys Chem Chem Phys, 2001, 3(13): 2712 [5] Costantino U, Marmottini F, Nocchetti M, Vivani R. Eur J Inorg Chem, 1998, (10): 1439 [6] Narayanan S, Krishna K. Appl Catal A, 1998, 174(1–2): 221 [7] Harpeness R, Gedanken A. Langmuir, 2004, 20(8): 3431 [8] Teterin Y A, Teterin A Y, Lebedev A M, Utkin I O. J Electron Spectrosc Relat Phenom, 1998, 88–91: 275 [9] Kasten L S, Grant J T, Grebasch N, Voevodin N, Arnold F E,

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