Preparation and Selective Acetylene Hydrogenation Catalytic

ARTICLE pubs.acs.org/JPCC

Preparation and Selective Acetylene Hydrogenation Catalytic Properties of Supported Pd Catalyst by the in Situ Precipitation
Reduction Method Xiao-Yan Ma, Yuan-Yuan Chai, David G. Evans, Dian-Qing Li, and Jun-Ting Feng* State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China ABSTRACT: Pd/MgAl-layered double hydroxide (LDH) was synthesized in situ on the surface of spherical Al2O3 to obtain a Pd/MgAl-LDH/Al2O3 catalyst, using hexamine as both a precipitant for LDH and a reductant for Pd2þ. After calcination and reduction, another Pd/MgO-Al2O3 catalyst was obtained. As a comparison, the Pd/Al2O3 catalyst was prepared by the conventional impregnation method. Low-temperature N2 adsorption
desorption, NH3 temperature-programmed desorption, and scanning electron microscopy results showed that Pd/MgAlLDH/Al2O3 and Pd/MgO-Al2O3 catalysts possessed larger surface area, lower surface acidity, uniform Pd particle size, and specific Pd shape compared with the Pd/Al2O3 catalyst. The catalytic performances of the catalysts were then studied in the selective hydrogenation of acetylene. In comparison to Pd/Al2O3, Pd/MgAl-LDH-Al2O3 and Pd/MgO-Al2O3 catalysts exhibited not only higher activity due to the uniform size and specific shape of Pd particles which provided more catalytic active sites but also better selectivity because of the lower surface acidity and strong metal/support interaction.

1. INTRODUCTION Ethylene is an important raw material for industrial products, particularly for polyethylene production. Typically, ethylene produced from thermal or catalytic cracking contains a small portion of acetylene, which is a poison in the downstream process of ethylene polymerization.1,2 Selective hydrogenation using supported Pdbased catalysts is one of the most effective methods to remove a trace amount of acetylene in ethylene feed stock.3,4 Alumina has been widely used as a catalyst support in fluidized beds or suspended beds because of its microporosity, high specific surface area, excellent physical strength, and resistance against acid and alkali.5 Compared with supports of irregular shape, spherical alumina has the advantage that abrasion of the particles is lower as a result of their uniform particle size and smooth surfaces. Therefore, the lifetime of the catalyst using alumina as a support can be significantly extended.6 However, the acidity of a spherical alumina surface gave rise to the formation of deposited hydrocarbon residues which could block the active sites and cause the decrease of catalyst life during an acetylene hydrogenation reaction.7 Therefore, the catalytic performances should be improved by using other supports or promoters2,8,9 to obtain a high acetylene activity and high ethylene selectivity, as well as a long lifetime. Layered double hydroxides (LDHs) are a class of synthetic two-dimensional nanostructured anionic clays with alkalescence whose structure can be described as containing brucite-like layers in which a fraction of the divalent cations have been replaced isomorphously by trivalent cations giving positively charged sheets with change-balancing anions in hydrated galleries between the layers.10 Their general formula can be expressed as [M2þ1
xM3þx(OH)2]xþ (An
)x/n 3 mH2O, where M2þ and M3þ are di- and trivalent metal r 2011 American Chemical Society

cations; An
denotes an organic or inorganic anion with negative charge n; and x (= [M3þ]/([M2þ] þ [M3þ])) is the value of the stoichiometric coefficient.11 The M2þ and M3þ cations are uniformly dispersed within the layers due to the crystal orientation effect. After calcination, LDH crystallites were transformed into complex metal oxides (LDO).12 Both LDHs and LDO can be widely used as a basic catalyst support or a promoter.13,14 Pd catalysts prepared by the conventional impregnation method have an inhomogeneity in Pd2þ distribution over the support, owing to the surface tension of the impregnating solution and other solvent effects. Moreover, the weak interactions between the support and metal ion species lead to the migration and aggregation of Pd2þ during subsequent calcination at high temperature, resulting in an increase in the inhomogeneity of the Pd2þ distribution. In this work, Pd/MgAl-LDH was synthesized in situ on the surface of spherical Al2O3 particles using hexamine as both a precipitant for LDH growth and a reductant for Pd2þ. After calcination and reduction of the resulting Pd/MgAl-LDH/Al2O3 catalyst, another catalyst Pd/MgO-Al2O3 was obtained. As a comparison, the impregnated Pd catalysts were also prepared. The structures and properties of the catalysts, including their catalytic performance in the selective hydrogenation of acetylene, have been investigated.

2. EXPERIMENTAL SECTION 2.1. Materials. Mg(NO3)2 3 6H2O, Al(NO3)3 3 9H2O, PdCl2, NaCl, hydrochloric acid, and hexamethylenetetramine (HMT) Received: December 16, 2010 Revised: March 24, 2011 Published: April 12, 2011 8693

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The Journal of Physical Chemistry C were all A.R. grade and were used without further purification. The aluminum powder had a purity of 99.6% and an average particle size of 50 μm. The deionized water used in all experiments had a conductivity of less than 10
6 S cm
1. 2.2. Synthesis of Spherical Alumina and MgO-Modified Alumina. An alumina sol was synthesized by digesting aluminum powder in hydrochloric acid.15 Aluminum powder (30.0 g) was dispersed in deionized water (100 mL) in a four-neck flask, followed by slow addition of 15% hydrochloric acid (125 mL) using a peristaltic pump, with vigorous stirring at about reflux temperature. After one-third of the amount of hydrochloric acid had been added into the reactor, a magnetic separation procedure was conducted to remove Fe and Cu impurities through external circulation of the reaction mixture using another peristaltic pump.16 The final alumina sol contained 12.7% aluminum with an Al/Cl molar ratio of 1.8. A solution of 40% HMT (14.6 g) was added dropwise to the alumina sol (50.0 g) below 10 °C. Microspherical gel particles were prepared by dispersing the resulting mixed sol in a column filled with distillate oil at 85
95 °C by means of a drop distributor.17 The resulting gel particles were immersed in the oil and aged at 140
145 °C for 4
6 h to decompose the HMT completely. Any NH4Cl remaining in the pores of the hydrated alumina microspheres was thoroughly removed by washing using deionized water. The samples were dried at 120 °C until there was no further weight loss. After calcination at 1150 °C for 8 h in air, a mixture of spherical θ- and R-Al2O3 particles was obtained. An amount of 1.7 g of Mg(NO3)2 3 6H2O was dissolved in 35.0 mL of deionized water. An amount of 5.0 g of spherical Al2O3 was then added to this solution and impregnated for 4 h. After drying and calcination at 500 °C for 4 h, MgO-modified Al2O3 support was obtained. 2.3. Preparation of the Pd/MgAl-LDH/Al2O3 Catalyst. PdCl2 (0.0062 g) and NaCl (0.0044 g) were dissolved in 10.0 mL of deionized water to make a Na2PdCl4 solution. Mg(NO3)2 3 6H2O (1.70 g) and HMT (3.25 g) were dissolved in 35.00 mL of deionized water to make a mixed solution. The two solutions were mixed well and then added to a beaker with 5.0 g of spherical Al2O3 particles. After aging at 150 °C for 6 h, the particles were thoroughly washed with deionized water until the pH value of the washings reached 7. The resulting Pd/MgAl-LDH/Al2O3 catalyst was obtained after drying at 70 °C for 12 h. The MgAl-LDH/Al2O3 precursor was prepared by the same method. 2.4. Preparation of Pd/MgO-Al2O3 Catalyst. The Pd/MgAlLDH/Al2O3 catalyst was heated in air with a ramping rate of 10 °C min
1 to 450 °C and calcined at that temperature for 4 h, followed by slow cooling to room temperature. The resulting sample is denoted PdO/MgO-Al2O3 catalyst precursor. After reduction, a Pd/MgO-Al2O3 catalyst was obtained. 2.5. Preparation of Pd/Al2O3, MgO-Modified Pd/Al2O3, and Im-Pd/MgO-Al2O3 Catalysts. As a comparison, the impregnated catalysts were also prepared. PdCl2 (0.0060 g) and NaCl (0.0041 g) were dissolved in 5.0 mL of deionized water to make a Na2PdCl4 solution. An amount of 5.0 g of Al2O3, MgOmodified Al2O3, or MgAl-LDH/Al2O3 was added to the resulting Na2PdCl4 solution and then impregnated for 12 h. After drying, calcination, and reduction, Pd/Al2O3, MgO-modified Pd/Al2O3, and Im-Pd/MgO-Al2O3 catalysts were obtained. 2.6. Synthesis of Pristine Pd/MgAl-LDH. Pristine Pd/MgAlLDH was prepared as a reference sample by means of the precipitation
reduction method. PdCl2 (0.0628 g) and NaCl (0.0484 g) were dissolved in 10.0 mL of deionized water to make

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a Na2PdCl4 solution. Mg(NO3)2 3 6H2O (3.00 g), Al(NO3)3 3 9H2O (2.21 g), and HMT (5.75 g) were dissolved in 190.0 mL of deionized water to make a mixed solution. The two solutions were mixed well and then added to a beaker. After aging at 150 °C for 6 h, the precipitate was centrifuged and thoroughly washed with deionized water until the pH value of the filtrate reached 7. The final Pd/MgAl-LDH with nPd/nMg/nAl ratio of 2/ 66/33 was obtained after drying at 70 °C for 12 h. Pd/MgAlLDH with other Pd contents was prepared by the same method. 2.7. Analysis and Characterization. Powder XRD patterns were recorded on a Shimadzu XRD-600 X-ray powder diffractometer (Cu Ka radiation, λ = 0.15406 nm) between 3° and 70° 2θ, with a scan speed of 10° min
1. Elemental analysis was performed using a Shimadzu ICPS-75000 inductively coupled plasma emission spectrometer (ICP-ES) and an Elementar Vario EL elemental analyzer. The morphology, structure, and grain size of the samples were examined using a Hitachi S-4700 scanning electron microscope (SEM) and JEOL J-3010 high-resolution transmission electron microscopy (HRTEM). The low-temperature N2 adsorption
desorption experiments were carried out using a Quantachrome Autosorb-1 system. The Barrett
Joyner
Haldenda (BJH) method was used to calculate the pore volume and the pore size distribution. Surface elemental analysis was performed using ESCALAB250 X-ray photoelectron spectroscopy (XPS). H2 and NH3 temperature-programmed desorption (TPD) of the catalysts was conducted on a Tianjin XQ TP-5080 chemisorption instrument. H2-TPD and NH3-TPD were carried out in a stream of argon with a flow rate of 40 mL min
1 and a temperature ramp of 10 °C min
1. Hydrogen consumption and NH3 desorption were monitored by a thermal conductivity detector (TCD) linked to a computer data acquisition system. The TCD signals were calibrated using 5 μL of H2 and NH3, respectively, as standards. 2.8. Catalytic Activity Tests. All the catalytic reactions were carried out in an Xian Quan WFS-3015 fixed-bed microreactor over the temperature range 40
100 °C, with a space velocity (GHSV) of 10 056 h
1, total volume gas flow of 167.6 mL min
1, and reaction pressure of 0.4 MPa. 1 mL Pd catalysts with the size of 200
300 μm were placed in a quartz tubular reactor (7 mm i.d.) between two quartz plugs. Before starting the reaction, the catalyst was pretreated at 150 °C for 4 h in hydrogen with a flow rate of 5 mL min
1 and cooled to the required reaction temperature. The reactant stream contained 0.91% acetylene in ethylene, and the H2/acetylene ratio was 2. The reactants and products were analyzed by gas chromatography (GC) with a flame ionization detector online using a PLOT capillary column (0.53 m  50 mm). Acetylene conversion and ethylene selectivity as used herein are defined as follows9 acetylene conversion ¼

ethylene selectivity ¼

C2 H2 ðinletÞ
C2 H2 ðoutletÞ C2 H2 ðinletÞ C2 H4 ðoutletÞ
C2 H4 ðinletÞ C2 H2 ðinletÞ
C2 H2 ðoutletÞ

ð5Þ

ð6Þ

3. RESULTS AND DISCUSSION 3.1. Structure and Morphology of Pd/MgAl-LDH. In this work, HMT was used as both a precipitant and a reductant to 8694

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Figure 1. XRD patterns of Pd/MgAl-LDH with a Pd/Mg/Al molar ratio of 0/66/33 (a), 0.5/66/33 (b), 1/66/33 (c), 2/66/33 (d), and 5/ 66/33 (e).

Figure 2. TEM image of Pd/MgAl-LDH with a Pd/Mg/Al molar ratio of 2/66/33.

synthesize Pd/MgAl-LDH in situ on the surface of Al2O3. HMT gradually decomposed into formaldehyde and ammonia at high temperature. Ammonia acted as a precipitant for LDH, and formaldehyde acted as a reductant for Pd2þ. To investigate the structure and morphology of Pd/MgAl-LDH on the surface of Al2O3 in detail, pristine Pd/MgAl-LDH with different nPd/nMg/nAl ratios was prepared. The powder XRD patterns for Pd/MgAl-LDH are shown in Figure 1. The XRD patterns are similar to those reported in the literature for LDH phases;18 the peaks at low angle arise from the basal (003) and higher-order (006 and 009) reflections, and the peaks around 60° 2θ arise from the (110) and (113) reflections. The diffraction peaks of Pd (111) and (220) can be observed at about 2θ = 40.1° and 67.9° in Figure 1 (d) and (e). The lattice parameters a (= 2d110) and c (= d003 þ 2d006 þ 3d009) of Pd/MgAl-LDH are essentially identical to those of MgAlLDH. This suggests that the addition of PdCl42
anions has little impact on the structure of MgAl-LDH. Pd particles are mainly dispersed on the surface of MgAl-LDH crystallites. The TEM image of Pd/MgAl-LDH with a Pd/Mg/Al molar ratio of 2/66/33 (Figure 2) showed that the LDH prepared by the precipitation
reduction method has good crystallinity. In addition, Pd particles, most in shape of tetrahedral and plate-like triangular, were highly dispersed on the surface of MgAl-LDH. The catalytic activity and selectivity of metallic catalysts are strongly dependent on the shape of the active particles. Specific

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Figure 3. XRD patterns of Al2O3 (a), Pd/MgAl-LDH/Al2O3 (b), PdO/MgO-Al2O3 (c), and the standard patterns of MgAl-CO3-LDH, MgO, and Pd.

shapes can provide more corners, edges, and faces, which could increase the number of catalytic active sites, change the surfaceto-volume ratio, and eventually cause a higher catalytic activity.19 Tetrahedral nanoparticles are composed of (111) facets with a large fraction of the surface atoms being present on edges and corners.20 For plate-like triangular, Pd (111) is also the dominating facet because the top and bottom faces accounting for >70% of the surface are terminated in (111), while the side faces enclosed by (100) account for Pd/MgAl-LDH/Al2O3 > Pd/Al2O3 over the temperature range 40
100 °C in Figure 11(a). When the reaction temperature maintained 50 °C, the catalytic activities of Pd/MgO-Al2O3 and Pd/MgAl-LDH/Al2O3 are 241% and 68% higher than that of Pd/Al2O3. Preferable catalytic activity of these two catalysts can be attributed to large surface area, uniform Pd particle size, and specific shape which could provide more catalytic active sites. 8697

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The catalytic activity of MgO-modified Pd/Al2O3 and Im-Pd/ MgO-Al2O3 is also shown in Figure 11(a). In comparison to the Pd/Al2O3 catalyst, the MgO-modified Pd/Al2O3 catalyst exhibited similar acetylene conversion over the temperature range 40
100 °C indicating that the modification of MgO is unable to improve the catalytic activity. The acetylene conversion of the Im-Pd/MgO-Al2O3 catalyst is significantly higher than that of the Pd/Al2O3 catalyst but still lower than that of Pd/MgAlLDH/Al2O3 and Pd/MgO-Al2O3 catalysts with specific Pd shape prepared by the in situ precipitation
reduction method. In a series reaction of the type AfBfC (where A represents acetylene, B ethylene, and C ethane), selectivity for the formation of B is dependent on the conversion of reactant A.29 As shown in Figure 11(b), the ethylene selectivity decreased with the increase of acetylene conversion due to the fact that the ethylene is produced as an intermediate in the acetylene hydrogenation reaction. In the presence of acetylene, the hydrogenation of ethylene is very slow.30 However, the ethylene selectivity decreased dramatically when the conversion of acetylene approached 100%. It is therefore necessary to compare catalysts

at similar conversions. Pd/MgO-Al2O3 and Pd/MgAl-LDH/ Al2O3 catalysts exhibited distinctly higher ethylene selectivity than Pd/Al2O3 which can be ascribed to the lower surface acidity.

Figure 8. Pd3d XPS pattern of Pd/MgAl-LDH-/Al2O3.

Figure 9. NH3-TPD profiles of Al2O3 (a), Pd/MgAl-LDH/Al2O3 (b), PdO/MgO-Al2O3 (c), MgO-modified Pd/Al2O3 (d), and Im-PdO/ MgO-Al2O3 (e).

Figure 6. Size distributions of Pd nanoparticles in Pd/Al2O3 (a), Pd/ MgAl-LDH/Al2O3 (b), and Pd/MgO-Al2O3 (c) catalysts.

Figure 7. HRTEM images of Pd/MgAl-LDH/Al2O3 (a) and Pd/Al2O3 (b). 8698

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Table 2. NH3-TPD Result of Support and Catalysts Qt

D

S

(mmolNH3 3 g
1)

(μmolNH3 3 m
2)

(m2 g
1)

Al2O3

0.140

2.98

47

Pd/MgAl-LDH/

0.150

2.67

56

0.155

2.50

62

MgO-modified

0.143

2.42

59

PdO/Al2O3 Im-PdO/MgO-

0.154

2.52

61

sample

Al2O3 PdO/MgOAl2O3

Al2O3

Figure 10. H2-TPD profiles of Pd/Al2O3 (a), Pd/MgAl-LDH/Al2O3 (b), PdO/MgO-Al2O3 (c), MgO-modified Pd/Al2O3 (d), and Im-PdO/ MgO-Al2O3 (e).

On the basis of the mechanisms in the literature,31 acetylene hydrogenation was suggested to take place on the active sites on the Pd surface, and ethylene hydrogenation was believed to take place on the support or on the deposited hydrocarbon residues. The carbon deposits accumulated on the support acted as a hydrogen bridge for the hydrogen spillover from Pd to the support facilitating ethylene hydrogenation to ethane. The lower concentration of acidity sites on the surface of Pd/MgO-Al2O3 and Pd/MgAl-LDH/Al2O3 catalysts considerably reduced deposited hydrocarbon residues, thus the ethylene selectivity was improved.32 Moreover, the modification of metal/support interactions, in the case of a less acidic support, also enhanced the ethylene desorption and decreased the consecutive hydrogenation to ethane. The ethylene selectivity of MgO-modified Pd/Al2O3 and ImPd/MgO-Al2O3 is also shown in Figure 11(b). Although the introduction of MgO and LDO could efficiently decrease the surface acidity of the Al2O3 support, the ethylene selectivity of MgO-modified Pd/Al2O3 and Im-Pd/MgO-Al2O3 catalysts was not as good as Pd/MgO-Al2O3 and Pd/MgAl-LDH/Al2O3 catalysts. The ethylene selectivity of the catalyst was affected not only by the nature of the support but also by the interaction between metal and support. Compared with the impregnation process, the interaction between metal and support was strengthened

Figure 11. Conversion of acetylene versus reaction temperature (a) and ethylene selectivity versus conversion (b).

during the in situ precipitation
reduction process in which the reduction of Pd2þ was accompanied by the in situ growth of MgAlLDH on the surface of alumina. Thus, Pd/MgAl-LDH/Al2O3 and Pd/MgO-Al2O3 catalysts prepared by the in situ precipitation
reduction method with strong metal/support interaction exhibited higher ethylene selectivity. In summary, preferable catalytic performances of Pd/MgAl-LDH/Al2O3 and Pd/MgO-Al2O3 can be attributed to the supporting method of Pd particles and the unique role of the LDH precursor. On the basis of the above experimental results, Pd/Al2O3, Pd/ MgAl-LDH/Al2O3, and Pd/MgO-Al2O3 catalysts were investigated for a longer reaction time at 50 °C, and the results are shown in Figure 12. As shown in Figure 12(a), the activity over the Pd/ Al2O3 catalyst decreased significantly with increasing time on stream. The deactivation can be ascribed to the deposition of coke and oligomers on the catalyst surface, which partly block the active sites of the Pd/Al2O3 catalyst.33 The activity over the Pd/MgAlLDH/Al2O3 and Pd/MgO-Al2O3 catalysts decreased to a lesser extent because of larger surface area and more active centers. Moreover, the ethylene selectivity over Pd/MgAl-LDH/Al2O3 and Pd/MgO-Al2O3 catalysts also exhibited better stability than the Pd/Al2O3 catalyst, as shown in Figure 12(b). It is because that surface acidity of the support could favor the deposition of 8699

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The catalytic performances were studied in the selective hydrogenation of acetylene. Pd/MgAl-LDH/Al2O3 and Pd/ MgO-Al2O3 catalysts exhibited much higher activity and ethylene selectivity than other catalysts prepared by the impregnation method. Preferable catalytic activity can be attributed to the uniform size and specific shape of Pd particles which increased the number of catalytic active sites. Higher ethylene selectivity can be ascribed to the lower surface acidity and strong metal/ support interaction which considerably reduced the deposited hydrocarbon residues and enhanced the desorption of ethylene.

’ AUTHOR INFORMATION Corresponding Author

*Address: Box 98, 15 Bei San Huan East Road, Beijing 100029, China. Fax: þ86 10 64425385. Tel.: þ86 10 64451007. E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by the 973 Project (2011CBA00506) and the National Natural Science Foundation of China. ’ REFERENCES

Figure 12. Time-on-stream analysis on the activity (a) and selectivity (b).

hydrocarbon which acted as a hydrogen bridge facilitated ethylene hydrogenation to ethane. Thus, the ethylene selectivity of the Pd/ Al2O3 catalyst with higher concentration of surface acidity sites needed a longer period to reach steady state. However, lower surface acidity and uniform Pd particles of Pd/MgAl-LDH/Al2O3 and Pd/ MgO-Al2O3 catalysts suppressed the deposition of carbonaceous and strengthened the metal/support interactions which could enhance the desorption of ethylene.

4. CONCLUSIONS Pd/MgAl-LDH was synthesized on the surface of spherical Al2O3 by the in situ precipitation
reduction method to obtain a Pd/MgAl-LDH/Al2O3 catalyst using hexamine as both a precipitant for LDH and a reductant for Pd2þ. After calcination and reduction, another catalyst Pd/MgO-Al2O3 was obtained. For comparison, Pd/Al2O3, MgO-modified Pd/Al2O3, and Im-Pd/ MgO-Al2O3 catalysts were prepared by the conventional impregnation method. NH3-TPD results indicated that the introduction of MgO and MgAl-LDO could efficiently decrease the surface acidity of the Al2O3 support. SEM and H2-TPD results showed that Pd/MgAl-LDH/Al2O3 and Pd/MgO-Al2O3 catalysts prepared by the in situ precipitation
reduction method possessed higher dispersion and specific Pd shape compared with the impregnated catalysts.

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