Hydrogen is a clean fuel which does not generate carbon dioxide and has attracted a great deal of attention for use as a fuel gas in polymer electrolyte fuel cells. Hydrogen can be produced via the steam reforming of methane (SRM), where the conversion of methane is limited because of the thermodynamic equilibrium. Membrane reactors are expected to reduce the operating temperature by shifting the SRM via the use of hydrogen permselective membranes. A limited number of papers have reported on membrane reactors for methane reforming that use porous membranes: steam reforming of methane using alumina membranes [1, 2] and silica membranes [3-6], although a number of studies of membrane reactors for SRM using dense membranes such as palladium have been reported.

Silica is one of the most attractive materials for hydrogen-selective membranes because of its amorphous structure, where silica network allows the permeation of small molecules such as hydrogen. However, the stability of silica membranes in steam is not sufficient for the application to SRM membrane reactor. In this study, will be described the improvement of the hydrothermal stability by adding metal ions such as Ni into silica. Metal doped silica sol was prepared by hydrolysis and condensation of tetraethoxysilane (TEOS) mixed with Ni(NO₃)₂ in ethanol solutions. Ni-doped silica membranes prepared by the sol-gel method showed high stability in hydrothermal conditions (500 degree C, H₂O=90-400 kPa). Hydrothermal treatment of the membranes before exposed to H₂ was found to be quite effective to prevent the further densification of Ni-doped amorphous silica networks due to reduction in H₂ and sintering in steam [7-10].

Another problem to be solved is the enhancement of catalytic activity of catalytic membranes where a limited amount of catalyst can be impregnates. In this study, a bimodal structure of catalytic supports is proposed [6] for a catalytic membrane reactor, consisting of a microporous silica top layer, for the selective permeation of hydrogen, and an α-alumina support layer, for catalytic reaction of the steam reforming of methane. An α-alumina support layer with a bimodal structure was prepared by impregnating γ-Al₂O₃ inside α-Al₂O₃ microfiltration membranes (1 µm in pore diameter), and then immersing the membranes in a nickel nitrate solution, resulting in a bimodal catalytic support. The bimodal catalytic support showed a large conversion of methane at a high space velocity compared with a conventional catalytic membrane with a monomodal structure. The enhanced activity of Ni-catalysts in bimodal catalytic supports was confirmed by hydrogen adsorption measurements. A bimodal catalytic membrane, i. e., a Ni-doped silica membrane coated on a bimodal catalytic support, showing an approximate selectivity of hydrogen over nitrogen of 100 with a hydrogen permeance of 0.5-1x10^-5 m³ m^-2 s^-1 kPa^-1 was examined for the steam reforming of methane. The reaction was carried out at 500 °C, and the feed and permeate pressures were maintained at 100 and 20 kPa, respectively. Methane conversion could be increased up to approximately 0.7 beyond the equilibrium conversion of 0.44 by extracting hydrogen from the reaction stream to the permeate stream.

Keywords: Membrane reactor; Hydrogen; Bimodal catalytic membrane; Steam reforming of methane; Microporous silica membrane

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Acknowledgement

This work was partly supported by the R&D Project for High Efficiency Hydrogen Production/Separation System using Ceramic Membranes funded by the New Energy and Industrial Technology Development Organization (NEDO), Japan.