Selective Oxidation of CO in H2 by Permeation through Catalytically Active Zeolite Membranes.

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  • Hasegawa Yasuhisa
    Department of Applied Chemistry, Graduate School of Engineering, Kyushu University
  • Sotowa Ken-Ichiro
    Department of Applied Chemistry, Graduate School of Engineering, Kyushu University
  • Kusakabe Katsuki
    Department of Applied Chemistry, Graduate School of Engineering, Kyushu University
  • Morooka Shigeharu
    Department of Applied Chemistry, Graduate School of Engineering, Kyushu University

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When CO is removed from H2 by selective oxidation using a catalytically active membrane, the CO is oxidized during its permeation through the catalytic layer to the permeate side. Since the oxidation rate of CO is proportional to the inverse of its concentration, the rejection of CO using an H2-selective microporous layer which is overlaid on the zeolite layer is also effective. In this study, Y-type zeolite membranes with different thicknesses were prepared on the outer surface of porous α-Al2O3 tubes by hydrothermal synthesis. The membranes were modified to different extents by ion-exchange with Pt ions. Gas permeation properties of the membranes were then determined using single-component H2 and CO, as well as mixtures of H2, CO, O2 and Ar. Thus, the effects of the membrane thickness, the amount of Pt loading and the size of micropores on CO oxidation could be evaluated. For the Pt/NaY membrane prepared by hydrothermal synthesis for 12 h and ion-exchanged using a 2.0-mmol L–1 solution of [Pt(NH3)4]Cl2, the H2 permeance and the H2/CO separation factor were 1.1 × 10–6 mol m–2 Pa–1 s–1 and 1.9 at 473 K. The thickness of the zeolite layer had no detectable effect on CO oxidation. When the membrane was ion-exchanged with a 10.0-mmol L–1 solution of [Pt(NH3)4]Cl2, the CO/H2 mole fraction ratio on the permeate side was minimized to 0.007 at 493 K for a feed of H2:CO:O2:Ar = 4:2:1:93 on a molar basis. The membranes were further modified with SiO2 using dip-coating and chemical vapor deposition techniques. The latter technique led to a membrane, the outer surface of which was coated with an H2-selective microporous SiO2 layer. This resulted in a decrease in CO concentration in the zeolite layer. Thus the CO oxidation proceeded effectively in the catalytically active zeolite layer, and the CO/H2 mole fraction ratio on the permeate side was 0.01 at 523 K for a feed of H2:CO:O2:Ar = 4:2:1:93 on a molar basis.

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