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Steam Reforming of Ethanol Over ZnO and ZnO/Al2O3 Supported Catalysts

Mengnan Chen, Chang Hwan Kim, and Levi Thompson. Chemical Engineering, University of Michigan, 2300 Hayward St., Ann Arbor, MI 48109

Hydrogen is an important feedstock for the production of fertilizers, clean-burning transportation fuels, and chemicals.  Hydrogen also holds great promise as a fuel for high efficiency fuel cells and its production from renewable feedstocks could significantly reduce our nation's dependence on foreign energy sources and fossil fuels.  A promising, near-term renewable source for hydrogen is ethanol derived from biomass including energy plants, waste materials from agro-industries, forestry residue materials, or the organic fractions of municipal solid waste. Presently available catalysts do not appear to be sufficiently effective for commercial use [1].

Research described in this paper investigated the use of ZnO and ZnO/Al2O3 supported catalysts for the Steam Reforming of Ethanol (SRE). Metals including Pt, Rh, Pd and Ag were added to the supports using a dry impregnation method, and selected properties were listed in Table 1. The SRE experiments were carried out at 723K in a fixed bed quartz reactor with a molar ratio of ethanol to water 1:4. Reaction rates listed in Table 1 were those measured after 30 minutes on stream. Hydrogen and CH3CHO formation rates were similar for the ZnO supported catalysts, except for Pt/ZnO. A significant amount of CH4 produced by the Pt/ZnO catalyst suggests that CH3CHO decomposed to CH4 over this material. Surface areas for the ZnO supported catalysts were 20 m2/g.  In an effort to increase the surface area and perhaps improve the metal dispersion, we prepared a series of catalysts using supports where ZnO was dispersed onto g-Al2O3. The ZnO loading (20 wt%) corresponded to monolayer surface coverage and the surface areas were typically 130 m2/g. While rates and selectivities for the ZnO and ZnO/Al2O3 supports were similar, H2 formation selectivities for the ZnO/Al2O3 supported catalysts were better than those for the ZnO supported catalysts suggesting differences in their surface chemistries. Surface chemical and structural properties for the catalysts were characterized using infrared spectroscopy and transmission electron microscopy.  Differences in the densities of surface acid and base sites appeared to account for differences in selectivity.  These and other results will be presented in this paper.

Table 1. Catalytic conversions and reaction rates at 723K after 30 minutes on stream.

Sample

Metal

Ethanol 

Formation Rate Ámol/(s∙g)

 

Loading /%

Conversion /%

H2

CH3CHO

C2H4

CH4

Acetone

ZnO

-

52

56

36

9

0

5

Pt/ZnO

1

94

135

61

1

12

6

Rh/ZnO

2

51

81

60

14

1

3

Pd/ZnO

2

41

69

68

16

0

4

Ag/Al2O3

2

100

0

2

169

0

0

ZnO/Al2O3

-

54

60

45

5

0

2

Pd/ZnO/Al2O3

2

44

82

48

3

0

3

Ag/ZnO/Al2O3

2

40

79

52

4

0

3

Corresponding author: Levi T. Thompson , ltt@umich.edu

References

[1] Agus H., Sandun F., Naveen M., Sushil A., Energy & Fuels 2098, 19 (2005).