Al2O3 catalysts for methanol synthesis using parallel-slurry-mixing method

JOURNAL OF FUEL CHEMISTRY AND TECHNOLOGY Volume 35, Issue 3, June 2007 Online English edition of the Chinese language journal Cite this article as: J Fuel Chem Technol, 2007, 35(3), 329−333

RESEARCH PAPER

Preparation of CuO/ZnO/Al2O3 catalysts for methanol synthesis using parallel-slurry-mixing method GUO Xian-ji1,*, LI Li-min1, LIU Shu-min1, BAO Gai-ling1, HOU Wen-hua2 1

Department of Chemistry, Zhengzhou University, Zhengzhou 450052, China

2

Key Laboratory of Mesoscopic Chemistry, Nanjing University, Nanjing 210093, China

Abstract: A series of CuO/ZnO/γ-Al2O3 catalysts with different Al2O3 contents were prepared using the parallel-slurry-mixing method, in which an aqueous solution of Cu2+ and Zn2+ mixed nitrates and an aqueous solution of Na2CO3 were poured into a precipitation vessel containing γ-Al2O3 slurry in a parallel-flow manner. The catalyst structure and its performance for methanol synthesis were investigated. The results indicated that Al2O3 as a structural promoter can inhibit CuO and ZnO crystallites from enlarging during calcination. The catalyst with a Cu/Zn/Al ratio of 4/5/1 (molar ratio) exhibits the highest catalytic activity. With an increase in Al content, the amount of (Cu0.3Zn0.7)5(CO3)2(OH)6 phase in the catalyst precursor before calcination decreases, whereas that of Cu2CO3(OH)2 phase increases; (Cu0.3Zn0.7)5(CO3)2(OH)6 in the precursor is beneficial to the catalytic activity of the final Cu/ZnO/γ-Al2O3 catalyst. Key Words: methanol synthesis; CuO/ZnO/γ-Al2O3; parallel-slurry-mixing; catalyst precursors; role of Al2O3

Methanol plays an important role in chemical and energy industries; nowadays it is mainly synthesized from syngas via a heterogeneous process using ternary copper-based Cu/ZnO/Al2O3 catalyst. Such a process is usually operated at low temperature (200−300 °C) and pressure (5−10 MPa)[1]. The copper-based catalysts are usually prepared through conventional coprecipitation method; however, recently, some novel preparation techniques have also been reported. Jensen et al.[2] developed a flame-combustion technique to prepare the copper-based catalyst by mixing acetylacetonate vapors of Cu, Zn, and Al with fuel and air, which may efficiently control the uniform dispersion of copper, zinc, and aluminum components at the molecular level, and a catalyst with large surface area, high activity, and good thermostability is obtained. Shen et al.[3] prepared a ceria-supported copper catalyst through coprecipitation; the catalyst is active for methanol synthesis even at a temperature below 200 °C. Using the chemical reduction-deposition method, Shen et al.[4] obtained a class of nickel-modified multiwalled carbon nanotube material Ni/MWCNT, and Ni/MWCNT-promoted Cu-ZnO-Al2O3 catalyst prepared through coprecipitation method exhibited high activity for methanol synthesis. Although the conventional precipitation is a mature method to prepare the copper-based catalyst, various

modifications have been made, which has attracted much attention. These modifications may be summarized as two categories: (1) addition of different metal elements such as Zr, V, Ce, Ga, and Mn[5−9]; (2) improvement of the precipitation process. Besides the typical precipitation modes such as forward, reverse, and parallel-flow and parallel-drip coprecipitations, some novel precipitation techniques like high-speed collision[10], gel-network[11], and [12] urea-hydrolysis coprecipitations have been reported recently. Li et al.[13] found that the insonation of the suspension during precipitation and aging steps could enhance the activity of copper-based catalysts appreciably. Yu et al.[14] reported that the copper-based catalyst prepared using dual-frequency ultrasonic method exhibits higher activity than that obtained by single-frequency ultrasonic method. It should be pointed out that the particles of Al-component precipitate formed in the coprecipitation processes are so fine that an effective washing of the precipitate is rather difficult. Moreover, many researches on the copper-based catalyst concerned only the effects of Cu and Zn components on its structure and property[15−22], whereas the investigation on the effects of Al component was rarely reported. The study about the effect of Al2O3 on catalyst structure and property may offer some useful information to improve the activity and thermostability of

Received: 2006-11-09; Revised: 2007-03-16 * Corresponding author. Tel: +86-371-67761876; E-mail: [email protected] Copyright©2007, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.

GUO Xian-ji et al. / Journal of Fuel Chemistry and Technology, 2007, 35(3): 329−333

copper-based catalyst. It has been reported that optimum Cu/Zn ratio is largely dependent on the method of preparation[23], but it is not clear whether the optimum Al content is also related to the method of preparation. In this study, a series of Cu-based catalysts with various Al contents were prepared using a parallel-slurry-mixing technique. The relationships among the catalyst structure, catalytic activity, and the Al content were investigated.

1.4

Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku D/MAX-3B diffractometer with Ni-filtered Cu-Kα radiation at a scanning rate of 5º/min. Before the measurement was made, the samples were ground and sieved to less than 300 meshes. 1.5

1 1.1

Catalyst preparation

Catalytic test

Catalytic tests were carried out using a MR-GC 80 microreactor equipped with an on-line gas chromatograph. A total of 0.5 mL of catalyst sample diluted with quartz sand (both with a particle size of 0.45−0.90 mm) at a volume ratio of 1:1 was placed in a stainless-steel tubular reactor, and then reduced in situ by passing the feed gas at a space velocity of 8000 h−1. The reaction of methanol synthesis was operated under 483−553 K, 4 MPa, H2/CO/CO2 = (66−67)/(31−32)/(1−2), and an inlet space velocity of 10000 h−1. The product stream was analyzed with an on-line 103-type gas chromatograph instrument equipped with a GDX-103 column and a TCD detector. 1.3

IR spectra

Experimental

A series of copper-based catalysts, CuO/ZnO/γ-Al2O3, with a fixed Cu/Zn ratio (Cu/Zn = 4/5) and different Al contents were prepared using the parallel-slurry-mixing technique. γ-Al2O3–water slurry was first obtained in a beaker, with γ-Al2O3 ground and sieved to less than 300 mesh. An aqueous solution containing copper nitrate and zinc nitrate and an aqueous solution of sodium carbonate were simultaneously added into the precipitating vessel containing the γ-Al2O3–water slurry in parallel-flow manner with vigorous stirring. The final suspension was aged at 70 °C for 2 h, then filtered and washed, and subsequently dried to give the catalyst precursors. After grinding, the precursors were calcined in an oven to form the reference catalysts (unreduced). 1.2

XRD patterns

Thermal analysis of catalyst precursors

Thermal analysis of the catalyst precursors (DTA) was performed with a CDR-1 thermal analyzer by heating a sample (20 mg) at ramp of 20 K⋅min−1 from 298 to 773 K in nitrogen flow (25 mL⋅min−1). Before the DTA measurement was made, all samples were thoroughly dried at 353 K.

Infrared (IR) spectra were measured in the range of 400–1000 cm−1 with a Perkin-Elmer 1710 FT-IR spectrometer using a KBr pellet containing the corresponding sample.

2 2.1

Results and discussion Catalytic activity and Al content

Fig. 1 shows the change in methanol yield and CO conversion over the copper-based catalyst prepared using the parallel-slurry-mixing technique with temperature. For comparison, the one-step parallel flow coprecipitation method[24] was also employed to prepare a copper-based catalyst with a Cu/Zn/Al ratio of 4/5/1. These two catalysts with the same composition exhibit no obvious difference in the catalytic activity, although the catalyst prepared using the parallel-slurry-mixing technique exhibits a relatively broad active-temperature range and gives a higher catalytic activity at high temperature (553 K). However, the parallel-slurry-mixing technique is more advantageous, because washing of the precipitate in the parallel-slurry-mixing precipitation technique, where the Al component is added in the form of γ-Al2O3 slurry, is much easier than that in the one-step parallel flow coprecipitations technique, as mentioned previously. It was pointed out that the promoting effect of Al2O3 on the catalytic activity is dependent on its improvement of the catalyst structure[18]. Catalytic activities of copper-based catalyst with various Al contents are shown in Table 1. The catalyst with a Cu/Zn/Al ratio of 4/5/1 exhibits the highest activity, which is much higher than that of the binary Cu/ZnO catalyst with the same Cu/Zn ratio. The BET surface area of the former is 45 m2/g and that of the latter is only 27 m2/g. This indicates that Al2O3 is a typical structural promoter. The optimum Al content of the ternary copper-based catalyst is found to be approximately 10.0% (by mol), which is similar to the value of 9.5% obtained by Zhao et al.[21] using a pH-following-the-track technique. Very high Al content leads to a decline in the catalytic activity. This suggests that the optimum Al content is not dependent on the method of preparation.

GUO Xian-ji et al. / Journal of Fuel Chemistry and Technology, 2007, 35(3): 329−333 24 21 CO conversion x / %

Yield w / mL.(g.h)

−1

0.8

0.6

0.4

18 15 12 9 6

0.2

3 0

0.0 480

500

520

540

480

560

500

Temperature T / K

520

540

560

Temperature T / K

Fig. 1 CH3OH yield and CO conversion vs reaction temperature z parallel-flow precipitation catalyst CuO/ZnO/Al2O3 (Cu:Zn:Al = 4:5:1); ▲ parallel-slurry-mixing precipitation catalyst CuO/ZnO/γ-Al2O3 (Cu:Zn:Al = 4:5:1);  parallel-flow precipitation catalyst CuO/ZnO (Cu:Zn = 4:5)

Table 1 Catalytic activities of copper-based catalyst with various Al contents Sample No

2.2

Cu : Zn : Al

Methanol yield

CO conversion

w / mL⋅(g⋅h)−1

x/%

S1

4:5:0

0.65

18.0

S2

4 : 5 : 0.6

0.75

19.9

S3

4:5:1

0.77

20.8

S4

4 : 5 : 1.6

0.71

19.5

S5

4 : 5 : 2.4

0.59

16.0

Phase structure of the catalyst precursors

Fig. 2 shows DTA curves of the catalyst precursors with various Al contents.

The precursor of binary CuO/ZnO catalyst exhibits a relatively sharp exothermic peak around 563 K, whereas the ternary catalyst precursors give much broader exothermic peaks and higher decomposition temperatures. This suggests that Al component retards the decomposition of the precursor during calcination. With the increase in the Al content, the DTA profile of ternary catalyst evolves from single peak to double peaks, suggesting a phase change in the catalyst precursor. Thus, the phases of the ternary catalyst precursors are obviously related to the Al content. This is also supported by the XRD patterns, as shown in Fig. 3.

▼

▼ ●

▼ ●▼ ▼ ● ●●

5

●

● ●

1

4 3 2 2

1

10

20

30

40 2θ /

400

500

600

700

T/K Fig. 2 DTA profiles of catalyst precursors

50

60

70

o

Fig. 3 XRD patterns of catalyst precursors 1: CuO/ZnO/γ-Al2O3 (Cu:Zn:Al = 4:5:1); 2: CuO/ZnO/γ-Al2O3 (Cu:Zn:Al = 4:5:2.4) ● Cu2(OH)2CO3 ; ▼ (Cu0.3Zn0.7)5(CO)2(OH)6

1: CuO/ZnO (Cu:Zn = 4:5); 2: CuO/ZnO/γ-Al2O3 (Cu:Zn:Al = 4:5:0.6); 3: CuO/ZnO/γ-Al2O3 (Cu:Zn:Al = 4:5:1.0); 4: CuO/ZnO/γ-Al2O3 (Cu:Zn:Al = 4:5:1.6); 5: CuO/ZnO/γ-Al2O3 (Cu:Zn:Al = 4:5:2.4)

In the case of low Al content, the main phase is (Cu0.3Zn0.7)5(CO3)2(OH)6 and the amount of Cu2CO3(OH)2 phase is rather small; an increase in Al content can suppress the formation of the former but favor the formation of the

GUO Xian-ji et al. / Journal of Fuel Chemistry and Technology, 2007, 35(3): 329−333

latter. There has been a serious controversy on the active sites of copper-based catalyst for methanol synthesis. Despite the diversified standpoints, it is irrefutable that both pure copper and pure zinc oxide are inactive for methanol synthesis, whereas the coexistence of copper and zinc oxide components is the basis for conferring catalytic activity for methanol synthesis. Therefore, the synergic promotion of copper and zinc oxide has received much attention. For example, Jansen et al.[19] pointed out that the Cu(I)/ZnO with oxygen vacancies is the active phase for methanol synthesis, and Zhao et al.[20] further expressed the optimum active site unit in the form of

5 4 3 2

1

10

20

30

O

Cu

Zn

. The synergic promotion between copper and zinc oxide brings about the enhanced activity of the reduced copper-based catalysts (Cu/ZnO/Al2O3 or Cu/ZnO). With a fixed Cu/Zn ratio, the uniform dispersion of Cu in ZnO leads to an intimate contact between Cu and ZnO components; therefore, this type of active sites are formed to the largest extent to obtain the best synergy and the highest catalytic activity for methanol synthesis. Moreover, as shown by XRD in Fig. 3, it is reasonable to assume that the active sites are formed from the (Cu0.3Zn0.7)5(CO3)2(OH)6 phase in the precursor during the subsequent calcination and reduction process. Fang et al.[25] also pointed out that (CuZn)5(CO3)2(OH)6 is a main phase in the catalyst precursor to obtain a highly active catalyst for methanol synthesis. 2.3

Unreduced catalyst

40

50

60

70

2θ / ° Fig. 4 XRD patterns of unreduced catalysts 1: CuO/ZnO (Cu:Zn = 4:5); 2: CuO/ZnO/γ-Al2O3 (Cu:Zn:Al = 4:5:0.6); 3: CuO/ZnO/γ-Al2O3 (Cu:Zn:Al = 4:5:1); 4: CuO/ZnO/γ-Al2O3 (Cu:Zn:Al = 4:5:1.6); 5: CuO/ZnO/γ-Al2O3 (Cu:Zn:Al = 4:5:2.4)

IR spectra of pure CuO, pure ZnO, and binary CuO/ZnO samples obtained by parallel flow precipitation[24] proved that pure CuO and pure ZnO exhibit two sharper absorption peaks at 583, 531 cm−1 and 515, 430 cm−1, respectively, whereas the binary CuO/ZnO shows only a broad absorption band around 482 cm−1 due to the interaction between CuO and ZnO. The IR spectra of ternary CuO/ZnO/γ-Al2O3 with various Al contents exhibit a broader absorption band in the range of 400−600 cm−1, as shown in Fig. 5, which is similar to that of binary CuO/ZnO. 1 2 3

The unreduced catalysts were obtained through calcination of the precursors. As shown in Fig. 4, XRD peaks at 2θ = 35.6, 38.8, and 49.0º are characteristic of CuO and those at 2θ = 36.3, 31.8, 34.5, 56.5, 63.0, and 68.1º are ascribed to ZnO. The broadening of XRD peaks of ternary catalysts suggests that the crystallinity of CuO and ZnO in ternary catalysts is lower than that in the binary catalyst, owing to the fact that Al component restrains copper and zinc component crystallites from enlarging during calcination. Al2O3 acts as a structure promoter; high Al content favors a uniform dispersion of different phases. Whereas, as mentioned previously, an increase in Al content results in a decline in (Cu0.3Zn0.7)5(CO3)2(OH)6 phase and an increase in Cu2CO3(OH)2 phase; this is disadvantageous to the formation of active sites. Thus, it could be reasonably concluded that addition of proper amounts of Al2O3 in the binary Cu/ZnO is necessary to obtain a ternary catalyst with high activity for methanol synthesis.

4

1000

800

600

ν /cm

400

-1

Fig. 5 IR spectra of catalysts (unreduced) 1: CuO/ZnO/γ-Al2O3 (Cu:Zn:Al = 4:5:0.6); 2: CuO/ZnO/γ-Al2O3 (Cu:Zn:Al = 4:5:1); 3: CuO/ZnO/γ-Al2O3 (Cu:Zn:Al = 4:5:1.6); 4: CuO/ZnO/γ-Al2O3 (Cu:Zn:Al = 4:5:2.4)

The broadening of IR absorption band with increasing Al content also reflects an enhancement of the interaction

GUO Xian-ji et al. / Journal of Fuel Chemistry and Technology, 2007, 35(3): 329−333

between CuO and ZnO. As mentioned previously; however, an increase in Al2O3 content is unfavorable for the uniform dispersion of CuO and ZnO phase, which results in a weakened interaction between the two components.

3

promoter. Appl Catal B, 2001, 29(2): 207−215. [8] Li J T, Zhang W, Gao L, Gu P, Sha K, Wan H. Methanol synthesis on Cu-Zn-Al and Cu-Zn-Al-Mn catalysts. Appl Catal A, 1997, 165(1−2): 411−417. [9] Zhang X T, Chang J, Wang T J, Fu Y, Tan T W. Methanol

Conclusions

synthesis catalyst prepared by two-step precipitation combined with addition of surfactant. Journal of Fuel Chemistry and

(1) A series of CuO/ZnO/γ-Al2O3 catalysts with high activity for methanol synthesis were prepared using parallel-slurry-mixing method. Such a preparation process, especially the step of precipitate washing, is much easer than the coprecipitation process. (2) The optimum Al content in the ternary copper-based catalyst is approximately 10.0% (by mol), and it is not dependent on the method of preparation. (3) Al component acts as a structural promoter and restrains the active component grains from enlarging during the calcination of catalyst precursors. (4) The formation of catalyst precursor phase depends on Al content. With a low Al content, (Cu0.3Zn0.7)5(CO3)2(OH)6 is the main phase in the precursors; with an increase in Al content, (Cu0.3Zn0.7)5(CO3)2(OH)6 phase decreases and Cu2CO3(OH)2 phase increases. The phase (Cu0.3Zn0.7)5(CO3)2(OH)6 is beneficial to the catalytic activity of the final Cu/ZnO/γ-Al2O3 catalyst.

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