Research on Catalytic Properties of Palladium Catalyst

June 30, 2013

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China Petroleum Processing and Petrochemical Technology 2013, Vol. 15, No. 2, pp 24-30Catalyst Research

Recieved date: 2013-01-31; Accepted date: 2013-03-30.

Corresponding Author: Professor Fu Jiquan, Telephone: +86-

10-64288291; E-mail: [email protected].

1　Introduction

Catalyst with precision metal nanoparticles deposited on a support plays a dominant role in the field of oil refin-ing, petrochemical industry and environmental protection thanks to its superior catalytic activity and selectivity. The traditional methods for preparing precious metal nanoparticles include physical method and chemical method. The catalyst preparation process using the physi-cal method is simple, but the requirement on equipment is high and the production cost is expensive; the chemical method is flexible and diversified, but it should be real-ized with chemical reagents, which will bring about some environmental pollution problems. The biological reduc-tion method for preparing precious metal nanoparticles is becoming a research focus in the field of nanometer scale technology along with the increasingly active research on greenization technology for preparation of materials[1]. Upon using biological method to prepare precious metal nanoparticles, there is no need to add additional chemi-cal reagents, and pollution is reduced along with a full utilization of rich biological resources and biomass as the reducing agent[2-4]. Huang, et al.[5] prepared gold and silver nanoparticles through reduction using camphor leaf extract, and the addition of chemical reagent was not needed during the preparation process. Carbon nanotubes

(CNTs) having good chemical stability and high specific surface area are the excellent support for active catalytic component[6-7]. The researchers have developed CNTs as the catalyst support that has been used in hydrogenation, dehydrogenation, and oxidation reactions[8-9]. The research on catalyst prepared by biological reduction method us-ing carbon nanotubes as the support has not been reported previously.This study prepared a series of catalysts using ginkgo leaf extract as a reducing agent, with MWCNTs (multi-walled carbon nanotubes) and α-Al2O3 serving as the support. The catalysts were characterized through XRD, TEM and specific surface area measurements, and the performance of catalysts was investigated through styrene hydrogena-tion to produce ethylbenzene for activity evaluation pur-poses.

2　Experimental

2.1　Preparation of extractFive gram of ginkgo leaf powder was added to a conical flask filled with 250 mL of deionized water. The conical flask was oscillated for 12 h at 60 ℃. Insoluble biomass

Research on Catalytic Properties of Palladium Catalyst Prepared by Biological Reduction Method

Zhang Feng; Fu Jiquan(School of Materials Science and Engineering, Beijing Institute of Fashion Technology, Beijing 100029)

Abstract: This paper relates to highly dispersed supported Pd/MWCNTs and Pd/α-Al2O3 catalysts prepared by biological reduction method. The physico-chemical properties and the difference in catalytic activity of Pd catalysts prepared by bio-logical reduction method and chemical method, respectively, were investigated using XRD, TEM and specific surface char-acterization methods. The catalytic properties of catalysts were studied through activity evaluation means. The test results showed that the catalysts prepared by biological method were characteristic of small Pd nanoparticle size, good dispersion and low agglomeration, while possessing a high activity and stability in styrene hydrogenation reaction in comparison with catalysts prepared via the chemical method.Key words: Pd/MWCNTs; Pd/α- Al2O3; biological reduction; ginkgo leaf; Pd nanoparticles

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was removed by centrifugation after being cooled down. The obtained supernatant liquid was the ginkgo leaf ex-tract at a concentration of 20 g/L, which was then stored in refrigerator at 4 ℃ prior to use.

2.2　Preparation of catalystA series of palladium catalysts supported on MWCNTs (including MWCNTs-0, Pd/MWCNTs-C, and Pd/MWCNTs-B) and on α-Al2O3 (including α-Al2O3-0, Pd/α-Al2O3-C, Pd/α-Al2O3-B) were prepared respectively. Herein, 0 denotes the pure support (the pure support was prepared in order to study the influence of the said support on catalytic activity), C denotes the chemical method, and B denotes the biological method. A required concentration of palladium nitrate solution was prepared. An excess of palladium solution was used to impregnate multi-walled carbon nanotubes and α-Al2O3 was used as the support for 6 h, followed by drying for 12 h at 60 ℃. Pd/MWCNTs-C catalyst and Pd/α-Al2O3-C catalyst were prepared by the chemical method. A certain amount of palladium nitrate solution was mixed with the same amount of ginkgo leaf extract. The excessive amount of the solution was used to impregnate multi-walled carbon nanotubes and α-Al2O3

for 12 h. Thus, the Pd/MWCNTs-B catalyst and Pd/α-Al2O3 -B catalyst were prepared by the biological meth-od after being dried for 24 h at 60 ℃. A series of catalysts using α-Al2O3 as the support were calcined respectively for 2 h at 500 ℃, and were then reduced in the hydrogen flow.

2.3　Characterization of catalystsThe crystalline structure of catalysts was analyzed using a D8 Advance high-power X-ray diffractometer with a ro-tating target (made by the Bruker company), operating at a tube voltage of 40 kV, a tube current of 250 mA, a scan step size of 0.02°/step, and a scan range of 10°—90°. The morphology of catalysts was observed using a JOEL’s JEM2100 transmission electron microscope. The high-resolution TEM images were obtained by a high magnifi-cation transmission electron microscope (JEM2100) at an accelerating voltage of 200 kV after dipping the ultra-thin carbon film in reaction solution followed by drying. Spe-cific surface area and pore volume of each catalyst were measured by utilizing a pore distribution and specific

surface measuring instrument made by the Beijing Jing-weigaobo Science and Technology Development Center, with a P/P0 in the range of 0.05—0.95.

2.4　Evaluation of catalyst activityThe reaction of styrene hydrogenation to form ethylben-zene was used as the probe reaction for evaluating the catalyst activity. The main reaction proceeds according to Formula (1), and this reaction only generates ethylben-zene under this condition, with the catalytic selectivity reaching 100%. 0.1 g of catalyst (with a particle size of 60-100 mesh) was weighed and added to the continuous micro-reactor device. H2 flow was controlled at a rate of 80 ml/min, and the space velocity on catalyst was 20 h-1, with anhydrous ethanol used as solvent at a volume ratio of 1:1. The reaction temperature was 120 ℃, and sam-pling was performed at regular intervals. The composition of reaction products was analyzed by a GC-7890 Ⅱ gas chromatography system (made by the Beijing Tianmei Instrument Company), equipped with a HJ-1 capillary column measuring Ф 0.25 mm×25 m and a FID detector, with the temperature in column equating to 100 ℃, in the sample room—200 ℃, and in the detection room—200 ℃. The content of each component was calculated by means of the area normalization method.

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3　Results and Discussion

3.1　Characterization of catalysts

3.1.1　XRD study

Six prepared catalysts were characterized by XRD meth-od in order to investigate whether the palladium loading could affect the structure of catalyst support and the crys-talline structure of palladium on the catalyst.Figure 1 depicts XRD patterns of a series of catalysts sup-ported on MWCNTs, and Figure 2 shows XRD patterns of a series of catalysts supported on α-Al2O3. It can be seen from the spectral line of MWCNTs-0 shown in Fig-ure 1 that the diffraction peaks of crystal planes (002) and (101) in carbon nanotubes appear at 2θ=25.87° and 42.7°, respectively, while other peaks are not obvious. The dif-

Zhang Feng, et al. Research on Catalytic Properties of Palladium Catalyst Prepared by Biological Reduction Method

China Petroleum Processing and Petrochemical Technology

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fraction peak of crystal plane (002) is wide, and its dif-fraction peak is high, suggesting that the degree of long-range order of nanostructure is poor. This may be caused by superposition of diffraction peaks of impurities such as amorphous carbon and graphite particles. It is known that the carbon nanotubes-supported Pd still retains the characteristics diffraction peaks of crystal planes (002) and (101) in carbon nanotubes and their diffraction peak intensity is relatively weak upon comparing the spectral lines of MWCNTs-0, Pd/MWCNTs-C and Pd/MWCNTs-B presented in Figure 1. These results indicated that the structure of carbon nanotubes was not destroyed, and could serve as excellent support of palladium. It can be learned from Figure 1 that Pd/MWCNTs-B has charac-teristics diffraction peaks of crystal planes (111), (200), (220) and (311) of Pd at 2θ of 40.06°, 46.59°, 68.13° and 82.11°, respectively upon comparing XRD patterns of Pd/MWCNTs-C and Pd/MWCNTs-B. This has revealed that ginkgo leaf extract could reduce Pd2+ ions to Pd0 species, which were then deposited on the MWCNTs support. So ginkgo leaf extract is a good reducing agent for Pd2+ ions.

It can also be learned from XRD patterns that there was a diffraction peak of Pd0 phase at 33.04°, indicating that a part of Pd0 species was also deposited on the surface of MWCNTs. The reason was that the grain size of Pd0 supported on the surface of MWCNTs was small, and it could be oxidized easily by the oxygen in air. So it could be reduced first in hydrogen stream before the commence-ment of catalytic reaction in the presence of Pd/MWCNTs catalyst.It can be seen from the XRD patterns of Pd/α–Al2O3-C and Pd/α–Al2O3-B presented in Figure 2 that the support-ed Pd samples show characteristics diffraction peaks of crystal planes (111), (200) and (220) of Pd species at 2θ of 39.67°, 46.0° and 67.02°, respectively. This suggests that palladium species also existed in the form of single element on the surface of α–Al2O3 support.

3.1.2　TEM study

XRD characterization confirmed that the palladium load-ing did not affect the structure of support, and palladium existed in the form of single element. TEM characteriza-tion was carried out in order to further study the surface morphology of the catalyst as well as the concentration and particle size of palladium particles.Figure 3 shows HRTEM images of Pd/MWCNTs cata-lysts. It can be seen from Figure 3 that the MWCNTs supported Pd species retain a good tubular morphology.

Figure 2　XRD patterns of α-Al2O3 and Pd/α- Al2O3 catalyst samples

Figure 1　XRD patterns of MWCNT and Pd/MWCNTs catalyst samples

Figure 3　HRTEM images of Pd/MWCNTs catalysts

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We can see from the HRTEM images by comparing Pd/MWCNTs-C and Pd/MWCNTs-B that Pd nanoparticles are supported on the inner wall of MWCNTs in the Pd/MWCNTs-C catalyst. However, the distribution of Pd nanoparticles is more uniform, and the Pd nanoparticles are supported on the outer and inner walls in the Pd/MWCNTs-B catalyst prepared by the biological reduc-tion method. It can be concluded that Pd nanoparticles had obvious lattice fringes as demonstrated by the high-resolution lattice image of the Pd/MWCNTs-B catalyst. Thus, the formation of crystalline Pd was verified, which was consistent with the XRD characterization results. The particle size distribution of Pd nanoparticles existing in the form of single element in Pd/MWCNTs-B and Pd/MWCNTs-C catalysts was calculated, with the results shown in Figure 5. We can see that when the particle size is small, the range of particle size distribution is narrow, and the average size of Pd particles is about 4 nm in the Pd/MWCNTs-B catalyst. We also can see that when the particle size is big, the range of particle size distribution is broader, and the average size of Pd particles is about 14.8 nm in the Pd/MWCNTs-C catalyst.Figure 4 shows HRTEM images of Pd/α-Al2O3 catalyst. It can be seen from Figure 4 that α-Al2O3-supported Pd retains a good morphology. By comparing TEM images of Pd/α-Al2O3-C and Pd/α-Al2O3-B we can know that pal-

ladium nanoparticles supported on the surface of α-Al2O3

can agglomerate to certain extent to form larger particles than that prepared by the biological method. However, palladium particle distribution in Pd/α-Al2O3-B catalyst prepared by the biological reduction method is more uni-form and the particle is finer. This phenomenon may be attributed to the existence of biomass which makes the distribution of active components of catalyst more uni-form during its preparation by the biological method, and Pd species exist in the form of single element in a reduced state. Thus, the agglomeration of active components was effectively avoided during the process of calcination. It should be noted that Pd supported on α-Al2O3 at first existed in an ionic form, then turned to be single elemen-tal Pd after calcination and reduction that took place in chemical immersion method. However, Pd was reduced directly on the support upon being treated by the biologi-cal reduction method. The two preparation methods were different in nature. It can be concluded that the supported Pd nanoparticles had obvious lattice fringes upon analyz-

Figure 5　Particle size distribution for supported Pd nanoparticles Frequency

Figure 4　HRTEM images of Pd/α- Al2O3 catalysts



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ing the high-resolution lattice images of Pd nanoparticles obtained by the biological reduction method. This out-come confirmed the existence of crystalline Pd, which was consistent with the XRD characterization results.

3.1.3　Specific surface area

The specific surface area and pore volume of two series of catalysts were characterized, and the results are listed in Table 1.

Table 1　BET surface area measurements

No. Sample S, m2/g Vp, cm3/g

1 MWCNTs-0 139.16 0.63

2 Pd/MWCNTs-C 165.47 0.55

3 Pd/MWCNTs-B 169.55 0.49

4 α–Al2O3-0 102.42 0.32

5 Pd/α–Al2O3-C 91.99 0.67

6 Pd/α–Al2O3-B 149.66 0.46

It can be seen from Table 1 that the specific surface of catalyst samples varies to different extent ranging from 139.16 m2/g for support itself to 165.47 m2/g for catalyst prepared by the chemical method and 169.55 m2/g for catalyst prepared by the biological method due to the use of different methods for supporting palladium with MWCNTs. The reason is that the carbon nanotube is char-acteristic of a tubular structure. The overall specific sur-face of catalyst increases due to large specific surface area of the palladium particles that are deposited on inner and outer walls of the nanotubes. The specific surface area of catalyst prepared by the biological method was larger than that of catalyst prepared by the chemical method (as evidenced by comparison between Pd/MWCNTs-C and Pd/MWCNTs-B). The reason may be that Pd particles prepared by the biological method were larger in dimen-sion, and were deposited on the inner and outer walls in a reduced state, leading to larger specific surface area. The pore volume of palladium catalyst with MWCNTs functioning as the support was smaller. This is because Pd particles supported on MWCNTs were embedded inside the carbon nanotubes, resulting in the blocking of a part of carbon nanotubes. Some carbon nanotubes were hol-low, leading to the reduction of effective pore volume. This finding was consistent with the results shown by

TEM measurements.For catalysts supported on α-Al2O3, the specific surface area of Pd/α-Al2O3-C prepared by the chemical method was 91.99 m2/g, which was lower than 102.42 m2/g for the α-Al2O3-0 support. However, the specific surface area of Pd/α-Al2O3-B prepared by the biological method was 149.66 m2/g, which was greater than the α-alumina support. This is probably because palladium particles prepared by the chemical method agglomerated on the surface of α-Al2O3 during the process of calcination, and the particle size was greater, which blocked a part of the carbon nanotubes. The palladium nanoparticles supported on the surface were mainly in the form of single element on the Pd/α-Al2O3-B catalyst, and its particle size was small, which would not cause the blocking of nanotubes. The protective effect of biomass could be better utilized, so agglomeration would not easily take place during calcination. Due to large specific surface area of small particles, the overall specific surface area of the catalyst increased. The increase of pore volume was probably due to the acid etching effect of palladium nitrate solution (with a pH value of 1.0) on the surface of support dur-ing the process of impregnation, leading to an increased pore volume[10]. The pore volume of Pd/α-Al2O3-B was smaller compared with Pd/α-Al2O3-C due to the buffering effect of biomass, which could weaken the acid etching effect during the process of catalyst preparation by the biological method.

3.2　Evaluation of catalyst activityThe conversion of styrene during catalytic hydrogenation reaction on Pd/α-Al2O3-C and Pd/α-Al2O3-B catalysts is shown in Figure 6.

Figure 6　Effect of reaction time on conversion of styrene■—Pd/α-Al2O3-B; ●—Pd/α-Al2O3-C

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It can be seen from Figure 6 that two catalysts have a high activity and stability in catalytic hydrogenation reac-tion of styrene. The activity of each catalyst is still high even after continuous reaction for 20 h. The catalytic per-formance of catalyst prepared by the biological method is better. The space velocity on styrene hydrogenation catalyst was increased from 20 h-1 to 200 h-1 in order to evaluate the catalytic activity of various catalysts. The activities of 6 catalyst samples were evaluated, and the re-sults are shown in Figure 7. It can be seen from Figure 7 that the catalytic activity of catalysts varies significantly. The catalytic activity of MWCNTs-0 and Pd/α-Al2O3-0 comprising pure support is close to zero, indicating that pure support materials MWCNTs and α-Al2O3 have no catalytic activity.

Figure 7　Effect of different catalysts on conversion of styrene■—MWCNTs-0; ●—Pd/MWCNTs-C; ▲—Pd/MWCNTs-B;

▼—Pd/α-Al2O3-0; ◆—Pd/α-Al2O3-C; —Pd/ α-Al2O3-B

Upon comparing Pd/MWCNTs-C, Pd/MWCNTs-B and Pd/α-Al2O3-C and Pd/α-Al2O3-B catalysts, it can be learned that the catalytic activity of catalyst using MWCNTs as the support was higher than that of catalyst using α-Al2O3 as the support, which might be probably attributed to the nanometer effect of carbon nanotubes. The structure of carbon nanotube itself provides a larger specific surface area, which significantly increases the support area and contributes to the uniform distribution of active compo-nent of catalyst. This view has already been demonstrated from previous characterizations. Therefore, the catalysts using MWCNTs as the support have a higher activity based on the same amount of catalyst support.The results showed that Pd/MWCNTs-C and Pd/MWCNTs-B catalysts all had a higher activity with car-bon nanotubes serving as the support. The high activity of

these two catalysts identified at the beginning of reaction was resulted from the hydrogen reduction of catalysts. Pd/MWCNTs-B catalyst began to show a much higher activ-ity than that of Pd/MWCNTs-C catalyst in 2.5 h after the start of run. The reason is that the dispersion of palladium particles prepared by the biological reduction method was better, resulting in a uniform distribution of palladium on the inner and outer walls of carbon nanotubes with high stability. However, the stability of Pd/MWCNTs-C cata-lyst prepared by the chemical method was lower, which was consistent with the results obtained from TEM char-acterization. The activity of two catalysts all decreased due to the loss of active component with the extension of reaction time. Although the activity of both of them decreased, the activity of Pd/MWCNTs-B was slightly higher than that of Pd/MWCNTs-C after 2.5 h of reaction. In general, the activity of catalysts prepared by the bio-logical method was higher than that of catalysts prepared by the chemical method.The activity of Pd/α-Al2O3-B catalyst prepared by the bio-logical method was even higher than that of Pd/α- Al2O3-C catalyst prepared by the chemical method as indicated by the two curves of catalysts using α-Al2O3 as the sup-port.

4　Conclusions

1) MWCNTs containing no functional groups can be used as a good support for palladium catalyst. The MWCNT-supported catalyst had a higher activity compared with traditional Al2O3-supported catalyst, and had revealed considerable development potential.2) The biological reduction method using ginkgo leaf extract can directly reduce Pd2+ ions to Pd0 species. The biological method has the advantages of good dispersion, small particle size of active component and low agglom-eration of Pd particles.3) Palladium catalysts prepared by the biological reduc-tion method had higher activity and better stability than that of catalysts prepared by the chemical method under the reaction system adopted by this study.

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Massive Production of Zeolite Catalyst for Phenol/Acetone Production

On March 20, 2013 the Petrochemical Research Institute of Jilin Petrochemical Company (JPC) had for the first time successfully manufactured one m3 of the MCM-49 zeolite, symbolizing its capability to produce in commer-cial scale the MCM-49 zeolite. This catalyst is expected to be applied on JPC’s 125 kt/a phenol/acetone unit, which can bring about an incremental economic benefit amounting to 15 million RMB a year. Currently JPC adopts the zeolite Y to manufacture iso-propanol on the phenol/acetone unit. Since high benzene/olefin ratio and high temperature are required for conduct-ing the isopropanol alkylation reaction, there are a lot of disadvantages such as large volume of circulating materi-als in the system, high energy consumption for product separation, low production efficiency, and excessive by-products. The catalyst needs to be flushed periodically with hot benzene for regeneration, which would result in high safety hazards and extended non-productive dura-tion.Application of the MCM-49 zeolite can reduce the ben-zene/olefin ratio and reaction temperature during the alkylation reaction along with decreased raw materials

and energy consumption. Furthermore, this catalyst does not need benzene flushing and can extend its service life to 3—6 years, thus avoiding safety hazards arising from flushing of zeolite by hot benzene, reducing catalyst re-placement frequency, and extending production cycle to raise the techno-economic level of cumene production to a higher rung.It is told that this research institute has performed optimi-zation tests in commercial scale at the side cut of produc-tion unit after implementing two 1500-hour stability tests running at a temperature of 130 ℃, a propylene space ve-locity of 1.0 h-1, a benzene/olefin ratio of 6, and a recycle ratio of 8 and 6, respectively. This unit has been operating for 5000 h to achieve a propylene conversion of 100%. Upon using a composite catalyst consisting of mainly the HMCM-49 zeolite for transalkylation of poly-isopropylbenzenes with benzene at a temperature of 190 ℃ the conversion of di-isopropylbenzene and tri-iso-propylabenzene reached 58.3% and 50.3%, respectively, which would command a domestic leading position with all indicators reaching or exceeding those of imported catalysts.

2013,15(2):24-30

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Research on Catalytic Properties of Palladium Catalyst