(582ag) Catalytic Aqueous-Phase Reforming of Methanol to Produce Hydrogen

I. Coronado^a,*, M. Stekrova^b, L. García Moreno^c, , M. Reinikainen^a, P. Simell^a, R. Karinen^b, J. Lehtonen^a

^aVTT Technical Research Centre of Finland Ltd., Espoo, FI-02044 VTT, Finland

^b Department of Chemical and Metallurgical Engineering, Aalto University, Espoo, 02150, Finland

^c Department of Chemical and Energy Technology, Rey Juan Carlos University, c/Tulipan s/n, Móstoles, 28933, Spain

*Corresponding author: irene.coronado@vtt.fi

Aqueous-phase reforming (APR) is an energetically efficient reaction to process water fractions derived from biofuel production in processes including pyrolysis and Fischer-Tropsch (FT) reaction. The organic compounds commonly found in those fractions can be upgraded to different added value molecules such as hydrogen. H₂ can be for instance further utilized in the hydrotreatment of the pyrolytic bio-oil and to adjust the CO:H₂ ratio in the FT reaction. Moreover, applying APR reduces the environmental impact caused by organic-water disposal ¹.

The operation conditions of APR, low temperature and moderate pressure, introduce kinetic and mass transfer limitations that negatively affect the activity and selectivity. Furthermore, the feedstock composition and hydrothermal conditions compromise the stability of the catalysts. Platinum-based catalysts are widely applied in APR because of their high H₂ selectivity ². The selectivity to hydrogen decreases over less expensive Ni-based catalysts ³. Nevertheless, they exhibit high activity in APR and the hydrogen production can be increased by metal promotion and selection of suitable supports ⁴. Accordingly, this study presents unpublished, highlighted results of the methanol aqueous-phase reforming over γ-alumina supported nickel doped with copper and cerium.

1. Scope

Aiming at high activity and hydrogen selectivity in APR, the catalysts screened for this work were nickel and nickel promoted with copper and cerium. The metals were supported on β-SiC, α- and γ-Al₂O3, and ZrO₂. Additionally, platinum on alumina was considered as a bench mark. Except the α-alumina supported, which was a commercial catalyst, the rest of catalysts tested for this work were prepared by impregnation of metal precursors on the indicated supports. The catalysts were characterized by atomic absorption spectroscopy (AAS) or X-ray fluorescence (XRF), nitrogen physisorption, and X-ray diffraction (XRD) to estimate the amount of active metals, surface properties, and surface species and particle size respectively. These features were regarded to assess the performance of the catalysts.

Methanol was selected as a model compound for catalytic APR because it is a major compound, representative of pyrolysis and FT reaction waters. The experiments were conducted in a continuously operated tubular reactor at 230°C and 32 bar. The gaseous and liquid products were analysed by online and offline gas chromatograph respectively. The results were evaluated considering the carbon yield to carbon containing gases and the product yield.
 

2. Results and discussion

Nickel on β-silicon carbide and α-alumina exhibited inferior results compared to the rest of catalyst. The carbon yield to carbon containing gases (CtG) was under 1% for both catalysts. The low values can be attributed to the lower surface area of these catalysts that suggest poor metal dispersion. Although the CtG was similar for Ni/ β-SiC and Ni/ α-Al₂O₃, the later catalyst yielded more methane. In contrast, the inert character of β-SiC might have allowed the more selective and abundant production of hydrogen by lower participation of this in secondary reactions.

The CtG of Ni/ γ-Al₂O₃ was 5 times higher than the value obtained with the platinum-based catalyst. In contrast, the hydrogen selectivity was higher over Pt / γ-Al₂O₃ than over the supported nickel. The experiments revealed higher methane selectivity also over zirconia supported nickel catalyst. Higher alkane yields and lower CtG was attributed to the lower surface area and larger nickel crystallites that characterized the zirconia catalyst.

To improve the activity and selectivity of the Ni/ γ-Al₂O₃, this catalyst was promoted with copper and cerium. Hydrogen yield and selectivity over the Cu-doped catalyst were significantly improved. Moreover, the addition of cerium increased the CtG and resulted in higher hydrogen yield and lower alkane production. The enhancements were attributed to a synergic effect of Cu and Ce with nickel.

3. Conclusions

The goal of this study was to find a nickel-based catalyst that exhibited high activity and hydrogen selectivity in the APR of methanol. Nickel on γ-alumina showed considerable activity to convert methanol. However, the hydrogen selectivity was lower compared to the values commonly obtained with platinum-based catalysts. Promotion of Ni/ γ-Al₂O₃ by copper and cerium increased the hydrogen production and selectivity indicating a synergy of metals for the studied reaction. The catalysts proved their suitability to convert methanol into hydrogen by APR. Therefore, the authors encourage further research to optimize the metal loadings, and prove the suitability of the catalysts for APR of real water fractions of refinery.

 

References

1. Coronado I, Stekrova M, Reinikainen M, Simell P, Lefferts L, Lehtonen J, Int J Hydrogen Energy 2016, 41, 11003-11032.

2. Shabaker JW, Davda RR, Huber GW, Cortright RD, Dumesic J, J Catal 2003, 215, 344–52.

3. Huber GW, Dumesic J, Catal Today 2006, 111, 119–32.

4. Davda RR, Shabaker JW, Huber GW, Cortright RD, Dumesic J, Appl Catal B Environ 2005, 56, 171–86.

Acknowledgement

This work was funded by the Academy of Finland (Project no. 285398).