ARTICLE pubs.acs.org/IECR
Enhancement of Metal Dispersion and Selective Acetylene Hydrogenation Catalytic Properties of a Supported Pd Catalyst Jun-Ting Feng, Xiao-Yan Ma, David G. Evans, and Dian-Qing Li* State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing, The People’s Republic of China, 100029 ABSTRACT: Pd(NH3)2Cl2/MgAl-CO32--layered double hydroxide (LDH) precursor has been synthesized in situ on the surface of spherical Al2O3 using urea as a precipitant. Pd(NH3)2Cl2 particles were observed highly dispersed on the surface of MgAl-LDH/ Al2O3 by Scanning electron microscopy. After calcinations, a PdO/MgO-Al2O3 catalyst precursor was obtained. In the process of calcination, the MgAl-LDH crystallites grown on the surface of Al2O3 prevented the migration and aggregation of Pd2þ, therefore the PdO particles with uniform size still highly dispersed on the surface of MgO-Al2O3. As a comparison, a PdO/Al2O3 catalyst precursor was prepared by a conventional impregnation method. Low temperature N2 adsorption-desorption, temperature programmed desorption of hydrogen and ammonia showed that PdO/MgO-Al2O3 possessed larger surface area, higher metal dispersion, and lower surface acidity compared with PdO/Al2O3. After reduction, the selective hydrogenation of acetylene was studied over Pd/MgO-Al2O3 and Pd/Al2O3 catalysts. The Pd/MgO-Al2O3 exhibited higher activity, selectivity and better stability.
1. INTRODUCTION The ethylene stream from a naphtha cracking unit contains a small quantity of acetylene, which is a poison in the downstream process of ethylene polymerization.1 Selective hydrogenation of acetylene to ethylene in an ethylene-rich feedstock is one of the most utilized methods used to lower the acetylene level.2-4 Supported Pd-based catalysts are efficient and widely utilized for this reaction.5,6 Spherical alumina having a large surface area and high crush strength is highly effective as a catalyst carrier when used in a fixed bed. The spherical particles permit uniform packing of the material bed, whereby variations in the pressure drop through the bed are minimized and channeling of the feed stream of reaction components is essentially eliminated.7 However, the acidity of spherical alumina surface gave rise to the formation of oligomer/green oil, which would block the active sites and caused the decrease of catalyst life during acetylene hydrogenation reaction.8 Therefore, the catalyst performances could be improved by using different supports or promoters9-11 to obtain a high acetylene activity and high ethylene selectivity, as well as a long lifetime. Layered double hydroxides (LDHs), also known as hydrotalcite-like materials, 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.12 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 cations, such as Mg2þ and Al3þ; A-- denotes an organic or inorganic anion with negative charge n, such as CO32-, and Cl-; x (= [M3þ]/([M2þ] þ [M3þ])) is the value of the stoichiometric coefficient.13 The M2þ and M3þ cations are uniformly dispersed within the layers without the formation of “lakes” of like cations. LDHs are widely used as both r 2011 American Chemical Society
catalyst supports and as catalysts in their own right.14 After calcination, LDHs crystallites were transformed into complex metal oxides (LDO), which was also widely used as a basic catalyst or catalyst support.15,16 Supported Pd catalysts were usually prepared by conventional impregnation method. However, Pd catalysts prepared by this 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(NH3)2Cl2/MgAl-LDH crystallites have been synthesized on the surface of microspherical Al2O3 with appropriate surface area and surface acidity, which acts as both support and sole source of Al3þ cations. After calcination and reduction, a highly dispersed Pd/MgO-Al2O3 catalyst was obtained. As a comparison, a Pd/Al2O3 catalyst was prepared by a conventional impregnation method, followed by calcination and reduction. The structures and properties of these two 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, urea, hydrochloric acid, and hexamethylenetetramine (HMT) were all A.R. grade and were used without further purification. The aluminum powder had a purity of 99.6% and Received: July 14, 2010 Accepted: December 3, 2010 Revised: December 3, 2010 Published: January 11, 2011 1947
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Industrial & Engineering Chemistry Research 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. An alumina sol was synthesized by digesting aluminum powder in hydrochloric acid.17 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.18 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.19 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. 2.3. Preparation of Pd(NH3)2Cl2/MgAl-LDH/Al2O3 Catalyst Precursor. PdCl2 (0.0180 g) and NaCl (0.0122 g) were dissolved in 2.0 mL deionized water to make a Na2PdCl4 solution. Mg(NO3)2 3 6H2O (3.40 g) and urea (5.50 g) were dissolved in 8.0 mL of deionized water to make a mixed solution. The two solutions were mixed well and then added to a beaker with 10.0 g of spherical Al2O3 particles. After aging at 130 °C for 6 h, the particles were washed with deionized water until the pH value of the washings reached 7. The resulting Pd(NH3)2Cl2/MgAlLDH/Al2O3 catalyst precursor with nPd/nMg molar ratio of 0.5/66 was obtained after drying at 70 °C for 12 h. MgAlLDH/Al2O3 precursor was prepared by the same method. If a LDH phase with Mg/Al molar ratio of 2 is spontaneously formed on the surface of Al2O3, 0.675 g (6.75 wt.%) of Al2O3 is involved in the LDH phase, this means that LDH microcrystallites could cover most of the entire surface. 2.4. Preparation of PdCl2/Al2O3 Catalyst Precursor. As a comparison, a PdCl2/Al2O3 catalyst precursor was prepared by a conventional impregnation method. PdCl2 (0.0121 g) and NaCl (0.0082 g) were dissolved in 10.0 mL deionized water to make a Na2PdCl4 solution. 10.0 g of spherical Al2O3 was added to the resulting Na2PdCl4 solution and then impregnated for 12 h. The PdCl2/Al2O3 catalyst precursor was obtained after drying at 70 °C for 12 h. 2.5. Preparation of PdO/Al2O3 and PdO/MgO-Al2O3 Catalyst Precursors. The PdCl2/Al2O3 and Pd(NH3)2Cl2/MgAlLDH/Al2O3 precursors were 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 samples are denoted PdO/Al2O3 and PdO/MgO-Al2O3. 2.6. Synthesis of Pristine Pd(NH3)2Cl2/MgAl-LDH. Pristine Pd(NH3)2Cl2/MgAl-LDH was prepared as a reference sample. Al(NO3)3 3 9H2O (2.25 g), Mg(NO3)2 3 6H2O (3.07 g), and urea (10.1 g) were dissolved in 50.0 mL of deionized water to make a mixed solution. PdCl2 (0.0161 g) and NaCl (0.0122 g) were dissolved in 10.0 mL deionized water to make a Na2PdCl4 solution. The two solutions were mixed well and then transferred to an autoclave and aged at 130 °C for 6 h. The precipitate was
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centrifuged and thoroughly washed with deionized water until the pH value of the filtrate reached 7. The final Pd(NH3)2Cl2/ MgAl-LDH with nPd/nMg/nAl molar ratio of 0.5/66/33 was obtained after drying at 70 °C for 12 h. Pd(NH3)2Cl2/MgAlLDH with other Pd contents were 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). The low temperature N2 adsorptiondesorption 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. The specific surface area was calculated according to the Brunauer-Emmett-Teller (BET) method based on the adsorption isotherm. Temperature programmed reduction (TPR), temperatureprogrammed desorption (H2-TPD), NH3 temperature programmed desorption (NH3-TPD), and H2-O2 titration of the catalysts were conducted on a Tianjin XQ TP-5080 chemisorption instrument. About 100 mg of catalyst precursors were loaded in a quartz reactor, followed by heating at 400 °C for 2 h in argon. TPR was carried out with a heating ramp rate of 5 °C min-1 in a stream of 10% H2 in Ar, with a total flow rate of 40 mL min-1. The outlet gas was passed through a cold trap to remove the moisture produced during reduction. After TPR process, the sample reactor was cooled to 50 °C and maintained for 1 h in a stream of 10% H2 in Ar to ensure the saturated adsorption of H2 on the surface of catalysts. Following the adsorption step, the sample reactor was flushed with Ar for 30 min to obtain a low and stable background. Subsequently, H2-TPD was performed in a stream of Ar with a flow rate of 40 mL min-1 and temperature ramp of 10 °C min-1. NH3-TPD was carried out in the same way. 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 H2 and NH3 respectively as standards. H2-O2 titration was carried out at 120 °C in a stream of N2. Before titration, the sample was saturated with hydrogen and then purged by flushing argon for 30 min. The same flushing under argon was performed between each titration cycle. After pretreatment, pulses of oxygen were introduced until the full saturation of the catalyst was achieved. The chemisorbed oxygen was then titrated by hydrogen. Afterward a second oxygen titration was carried to confirm the volume of H2. The dispersion of Pd on the catalysts was calculated from the volume of H2 used for the titration of O2 by the following simplified equation:20 Dð%Þ ¼
2 VTH M 10 - 3 100 3 22:4 W P
ð1Þ
where D (%) = the dispersion of Pd, VH T = the volume of H2 used for the titration of O2 (mL), M = the relative molecular mass of Pd, W = mass of catalyst (g), and P = Pd mass fraction of the catalyst (%). 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 1948
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(GHSV) of 10 056 h-1, total volume gas flow of 167.6 mL min-1, and reaction pressure of 0.4 MPa. One milliliters 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 follows:11 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
ð2Þ ð3Þ
3. RESULTS AND DISCUSSION 3.1. Structure and Morphology of Pristine Pd(NH3)2Cl2/ MgAl-LDH. The powder XRD patterns for Pd(NH3)2Cl2/
MgAl-LDH with different nPd/nMg/nAl molar ratios are shown in Figure 1a-d. The XRD patterns are similar to those reported in the literature for LDH phases;21 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 lattice parameters a (= 2d110) and c (= d003 þ 2d006 þ 3d009) of Pd(NH3)2Cl2/MgAl-LDH are essentially identical to those of MgAl-LDH. This suggests that the addition of PdCl42- anions has little impact on the structure of MgAlLDH. Pd(NH3)2Cl2 particles are mainly dispersed on the surface of MgAl-LDH crystallites. Subsequent work focused on the Pd(NH3)2Cl2/MgAl-LDH catalyst precursor with an nPd/nMg/ nAl ratio of 0.5/66/33. After calcinations, Pd(NH3)2Cl2/MgAlLDH was transformed into PdO/MgAl-LDO. The characteristic reflections of MgAl-LDO can be observed in Figure 1e, consistent with the positions and relative intensities reported for MgAlLDO.22 This indicates that the layer hydroxyl groups and interlayer carbonate anions have decomposed with loss of water and carbon dioxide. Some Al3þ ions took the replace of Mg2þ and existed in the crystal lattice of MgO and the others react with oxygen to form amorphous alumina during the calcinations process. However, no characteristic reflections of PdO was observed because of the low Pd content and small particle size. SEM images of MgAl-LDH (Figure 2a) and Pd(NH3)2Cl2/ MgAl-LDH (Figure 2b) showed that the LDHs prepared by the hydrothermal method have good crystallinity and uniform size. Because of the interaction between Pd(NH3)2Cl2 and MgAlLDHs, the Pd(NH3)2Cl2 particles with a size of 10-30 nm can be seen highly dispersed on the surface of MgAl-LDH. Some representative examples are marked with arrows in Figure 2b. 3.2. Structure and Porosity of Pd(NH3)2Cl2/MgAl-LDH/ Al2O3 and PdO/MgO-Al2O3. The powder XRD patterns for Al2O3 support (a), Pd(NH3)2Cl2/MgAl-LDH/Al2O3 (b), and the calcined product PdO/MgO-Al2O3 (c) are shown in Figure 3. The pattern of the Al2O3 shown in Figure 3a is a superposition of the characteristic reflections of θ-Al2O3 and R-Al2O3. The pattern of Pd(NH3)2Cl2/MgAl-LDH/Al2O3 is a superposition of the characteristic reflections of Al2O3 and those of the LDH, as
Figure 1. XRD patterns of Pd(NH3)2Cl2/MgAl-LDH with nPd/nMg/ nAl ratios of 0/66/33 (a), 0.5/66/33 (b), 1/66/33 (c), 2/66/33 (d), 5/66/33 (e), and the calcined product PdO/MgAl-LDO (f).
shown in Figure 3b, which confirms that Pd(NH3)2Cl2/MgAlLDH has been formed on the Al2O3 support. After calcinations, Pd(NH3)2Cl2/MgAl-LDH was transformed into PdO/MgOAl2O3, as shown in Figure 3c. The N2 adsorption-desorption isotherms (not shown) of the Al2O3, Pd(NH3)2Cl2/MgAl-LDH/Al2O3, and PdO/MgOAl2O3 were all of Type IV with an obvious hysteresis loop.23 The shape of the hysteresis loop in each case was a superposition of Types H1 and H3. This is generally taken to indicate that samples have both tubular and parallel slit-shaped capillary pores, which are caused by the gas escaping during calcination and the stacking of alumina microcrystallites.24 The specific surface area of Pd(NH3)2Cl2/MgAl-LDH/Al2O3 (54 m2 g-1) was slightly higher than that of Al2O3 itself (47 m2 g-1); this may be because that the obtained Pd(NH3)2Cl2/MgAl-LDH microcrystallites increased the surface roughness of Al2O3 support. The total pore volume (0.42 cm3 g-1), and the most probable pore size (32.1 nm) were lower than the corresponding values for Al2O3 (0.48 cm3 g-1 and 40.9 nm), the reason being that a fraction of the Al2O3 pore volume was taken up by LDH microcrystallites. After calcinations, the total pore volume and the most probable pore size of PdO/MgO-Al2O3 (0.46 cm3 g-1 and 37.2 nm) were slightly higher than the corresponding values for the Pd(NH3)2Cl2/MgAl-LDH/Al2O3 precursor, while the specific surface area (51 m2 g-1) was somewhat lower. This may be because that the decomposition of layer hydroxyl groups and interlayer anions in LDH, with associated loss of water and carbon dioxide, led to a decrease in the size of the particles, and the resulting MgAl mixed oxides therefore took up less of the pore volume of Al2O3 than the MgAl-LDH precursor. The specific surface area (45 m2 g-1), most probable pore size (0.47 cm3 g-1) and total pore volume (37.7 nm) of PdO/ Al2O3 were all slightly smaller than the corresponding values for Al2O3. This may be because that a number of PdO microcrystallites aggregated together during the drying process and the aggregations blocked some of the pores in the Al2O3 support. 3.3. Morphology of Pd(NH3)2Cl2/MgAl-LDH/Al2O3 and PdO/MgO-Al2O3. As a comparison, a PdCl2/Al2O3 catalyst precursor and the calcined product PdO/Al2O3 were prepared by a conventional impregnation method. SEM images of Al2O3 support (a), PdCl2/Al2O3 (b), MgAl-LDH/Al2O3 (c), 1949
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Figure 2. SEM micrographs of MgAl-LDH (a) and Pd(NH3)2Cl2/MgAl-LDH (b).
Figure 3. XRD patterns of Al2O3 support (a), Pd(NH3)2Cl2/MgAlLDH/Al2O3 (b), and the calcined product PdO/MgO-Al2O3 (c).
Pd(NH3)2Cl2/MgAl-LDH/Al2O3 (d), PdO/Al2O3 (e), and PdO/ MgO-Al2O3 (f) are shown in Figure 4. Comparison with pristine Al2O3, shows that congeries of Pd containing particles with a size of 50-300 nm blocked most of the pores on the surface of Al2O3, and that the palladium-containing phase exhibited low dispersion, as shown in Figure 4b. In MgAl-LDH/Al2O3 (Figure 4c), MgAl-LDH microcrystallites having good crystallinity and uniform size grew homogeneously on the surface of Al2O3. Comparison with the micrographs of MgAl-LDH/Al2O3 and Pd(NH3)2Cl2/MgAl-LDH/Al2O3 shows that some palladium-containing particles with a size of 10-30 nm are dispersed homogeneously on the surface of the latter, as marked with arrows in Figure 4d. The high dispersion of palladium-containing particles on the surface of MgAl-LDH/Al2O3 can be ascribed to the interaction between Pd(NH3)2Cl2 and LDH in the synthesis process. In the case of the calcined product PdO/Al2O3, a large quantity of PdO was congregated on the surface of Al2O3, as shown in Figure 4e, which can be attributed to the migration of Pd2þ during drying and calcination processes. As for PdO/MgOAl2O3, PdO particles with much smaller size dispersed homogeneously on the surface of MgO-Al2O3. Examples are marked with arrows in Figure 4f. This is because that the presence of LDH crystallites prevented the congregation of Pd2þ during the calcination process. These micrographs clearly confirm that PdO/MgO-Al2O3 possessed higher surface dispersion than
PdO/Al2O3. In addition, MgAl-LDO crystallines also dispersed uniformly on the surface of Al2O3 because of the stronger interaction force between LDH and alumina support. 3.4. TPR and Metal Dispersion Studies of PdO/MgO-Al2O3 Catalyst Precursor. TPR profiles obtained for the PdO/Al2O3 and PdO/MgO-Al2O3 catalyst precursors are shown in Figure 5. The reduction peak at around 60 °C in each curve can be attributed to the reduction of PdO.25 The reduction temperature of PdO/MgO-Al2O3 (52 °C) is lower than the corresponding value for PdO/Al2O3 (61 °C), while the reduction peak area is somewhat higher. This could be due to the smaller crystallite size in the former leading to complete reduction of PdO particles. H2TPD profiles from 20 to 450 °C of Pd/Al2O3 and Pd/MgOAl2O3 are given in Figure 6. The profiles both showed at least two domains of H2 desorption peaks. Peaks below 100 °C are generally attributed to H2 desorbed from metal particles, while high temperature peaks are attributed to H2 located in subsurface layers and/or to spillover H2.26 Hydrogen uptake values for H2-O2 titration and metal dispersion are shown in Table 2 along with the palladium loadings in the material as determined by ICP. The degree of metal dispersion in Pd/MgO-Al2O3 is 8.9% higher than that in Pd/Al2O3. 3.4. NH3-TPD of Pd(NH3)2Cl2/MgAl-LDH/Al2O3 and PdO/ MgO-Al2O3. The acidic properties of Al2O3, PdO/Al2O3, Pd(NH3)2Cl2/MgAl-LDH/Al2O3 and PdO/MgO-Al2O3 were investigated using the NH3-TPD technique. The NH3-TPD profiles from 120 to 600 °C of Al2O3 (a), Pd(NH3)2Cl2/MgAlLDH/Al2O3 precursor (b), and PdO/MgO-Al2O3 (c) are shown in Figure 7 (any peaks below 120 °C can be attributed to physical desorption) and the corresponding parameters are listed in Table 3. The acidity of PdO/Al2O3 is quite similar to that of Al2O3 support because of the low PdO loading. Thus, the NH3TPD profile of PdO/Al2O3 was not shown. Figure 7a shows two broad peaks centered around 190 and 330 °C; their separation leads to the assumption that at least two types of acid sites exist on the surface of samples.27 The low temperature peak could be ascribed to weak acid sites, whereas the high temperature peak is typical of medium acidity sites which could give rise to the deposition of green-oils (oligomers) in selective hydrogenation of acetylene.28 In comparison of Al2O3 support, the high temperature peak of Pd(NH3)2Cl2/MgAl-LDH/Al2O3 and PdO/MgO-Al2O3 related to medium acidity sites disappeared. Although the total acid amount of Pd(NH3)2Cl2/MgAl-LDH/ Al2O3 and PdO/MgO-Al2O3 are higher than that of Al2O3 support, the acidity density are lower, as shown in Table 3. This implies that the uniformly growth of LDH or the corresponding 1950
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Figure 4. SEM micrographs of Al2O3 support (a), PdCl2/Al2O3 (b), MgAl-LDH/Al2O3 (c), Pd(NH3)2Cl2/ MgAl-LDH/Al2O3 precursor (d), PdO/ Al2O3 (e), and PdO/MgO-Al2O3 (f).
Figure 5. TPR curves of PdO/Al2O3 (a) and PdO/MgO-Al2O3 (b) catalyst precursors.
calcined product LDO could reduce the surface acidity of alumina support. 3.5. Catalytic Activity. The catalytic performances of Pd/ Al2O3 and Pd/MgO-Al2O3 during the selective hydrogenation of acetylene for 2 h at different temperatures, are shown in Figure 8. It was observed that for both Pd/Al2O3 and Pd/MgO-Al2O3, the conversion of acetylene increased with increasing temperature, while the ethylene selectivity decreased due to the fact that the ethylene is produced as an intermediate in acetylene hydrogenation reaction. High temperature favors the hydrogenation of ethylene because the activation energy of ethylene hydrogenation is higher than that of acetylene hydrogenation in the presence of catalysts. When the conversion of acetylene approached 100%, the ethylene selectivity was very low. This is because that in the presence of acetylene, the hydrogenation of ethylene is very slow.29 Pd/MgO-Al2O3 exhibited distinctly higher catalytic activity and selectivity than Pd/Al2O3 over the temperature range 60-100 °C. Preferable catalytic activity of Pd/MgO-Al2O3 catalyst might be attributed to higher specific surface area and 1951
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Table 3. NH3-TPD Result of Al2O3, Pd(NH3)2Cl2/MgAlLDH/Al2O3, and PdO/MgO-Al2O3 total acid amount
acid density
specific surface
(mmol NH3 g-1) (μmol NH3 m-2) area (m2 g-1)
sample Al2O3 Pd(NH3)2Cl2/
0.140 0.143
2.9 2.6
47 54
0.145
2.7
51
MgAl-LDHs/Al2O3 PdO/MgO-Al2O3
Figure 6. H2-TPD curves of Pd/Al2O3 (a) and Pd/MgO-Al2O3 (b) catalysts.
Table 2. Analytical Data and Metal Dispersion of Pd/Al2O3 and Pd/MgO-Al2O3 Catalysts Pd loading wt.(%)a
V0 (μL)
D (%)
Pd/Al2O3
0.051
13.0
85.2
Pd/MgO-Al2O3
0.052
14.0
92.8
sample
a
As determined by ICP.
Figure 7. NH3-TPD curves of Al2O3 (a), Pd(NH3)2Cl2/MgAl-LDH/ Al2O3 (b), and PdO/MgO-Al2O3 (c).
Figure 8. Effect of varying the reaction temperature on the activity (a) and selectivity (b).
Pd dispersion resulting in the formation of more active centers. The higher ethylene selectivity of Pd/MgO-Al2O3 catalyst can be ascribed to the lower surface acidity. On the basis of the mechanisms in the literature,30 acetylene hydrogenation was suggested to take place on the active sites on Pd surface while most of the carbonaceous deposits were found to be accumulated on the support. The carbon deposits acted as a hydrogen bridge for the hydrogenation spillover from Pd to the support facilitating ethylene hydrogenation to ethane. The lower concentration of acidity sites on the surface of Pd/MgO-Al2O3 catalyst considerably reduced coke deposition, thus the ethylene selectivity was improved.31 In addition, the modification of metal/support
interactions, which in the case of a less acidic support and smaller Pd particles could enhance the ethylene desorption and also decrease the consecutive hydrogenation to ethane. Pd/Al2O3 and Pd/MgO-Al2O3 catalysts were investigated for a longer reaction time at 80 °C and the results of time on stream analysis are shown in Figure 9. The activity over the Pd/Al2O3 catalyst decreased significantly with increasing time on stream. This deactivation has been ascribed to the deposition of oligomers consisting of different alkenes, dienes, and, to a lesser extent, alkanes and carboxylicacids on the catalyst surface, which partly block the active sites of the Pd/Al2O3 catalyst.32,33 The activity over the Pd/MgO-Al2O3 catalyst decreased to a lesser 1952
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surface of the MgAl-LDH/Al2O3 because of the interaction between Pd(NH3)2Cl2 and LDH. After calcination and reduction, a Pd/MgO-Al2O3 catalyst was obtained. In comparison of the Pd/Al2O3 catalyst prepared by a conventional impregnation method, Pd/MgO-Al2O3 catalyst possessed higher Pd dispersion, larger surface area and lower surface acidity. The selective hydrogenation of acetylene was studied using Pd/MgO-Al2O3 and Pd/Al2O3 as catalysts. Because of higher specific surface area and Pd dispersion, the Pd/MgO-Al2O3 catalyst exhibited higher catalytic activity over the temperature range 60-100 °C. In addition, the lower concentration of surface acidity sites and the modification of metal/support interactions in Pd/MgO-Al2O3 catalyst considerably reduced coke deposition, thus the ethylene selectivity of Pd/MgO-Al2O3 catalyst was also higher than that of Pd/Al2O3.
’ AUTHOR INFORMATION Corresponding Author
*Tel.: þ86 10 64436992. Fax: þ86 10 64425385. E-mail:
[email protected].
’ ACKNOWLEDGMENT This work was supported by the National Key Technology R&D Program (2009BAE89B01), 973 Program (2011CBA00506), the National 863 Project and the National Natural Science Foundation of China. ’ REFERENCES
Figure 9. Time-on-stream analysis on the activity (a) and selectivity (b).
extent which can be attributed to the catalyst possessing higher specific surface area and more active centers. Ethylene selectivity over the Pd/MgO-Al2O3 catalyst exhibited this better stability after attaining a steady state within a period of 5 h on stream, whereas the Pd/Al2O3 catalyst only reached a steady state after 10 h on stream. This is mainly because that the former catalyst exhibits lower surface acidity. In case of selective hydrogenation of acetylene, surface acidity of the support favor the deposition of carbonaceous which acted as a hydrogen bridge facilitate ethylene hydrogenation to ethane. Thus, the ethylene selectivity of Pd/Al2O3 catalyst with higher concentration of surface acidity sites needed a longer period to reach steady state. As for Pd/ MgO-Al2O3, lower surface acidity and smaller Pd particles suppressed the deposition of carbonaceous and strengthened the metal/support interactions which could enhance the desorption of ethylene. Therefore, ethylene selectivity over the Pd/ MgO-Al2O3 catalyst had a lower period to reach steady state.
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