Now, Suljo Linic and co-workers have developed a catalyst–membrane PDH system that can surpass thermodynamic limits to reach more than 140% conversion with more than 98% propylene selectivity without noticeable deactivation for more than 90 h under relatively low temperatures (500 °C) using a hydrogen-permeable membrane to remove the H2 by-product. The catalyst–membrane hollow-fiber system consists of a Pt1Sn1/SiO2 PDH catalyst packed into an Al2O3 hollow-fiber tube containing an inner SiO2 H2-permeable membrane. The Pt1Sn1/SiO2 PDH catalyst is packed into the tube side containing the propane feed, while inert Ar weep gas is used to carry the separated H2 to the shell side.
The SiO2 layer selectively separates H2 from propane and propylene with stable membrane performance exceeding the Knudsen separation limits for more than 20 h under traditional catalytic conditions (580 °C). This high separation performance was ascribed to the strong driving force created by the sweep and the sieving effect of the SiO2 layer that excludes C3 molecules and allows H2 permeation to the shell side. The PDH catalytic performance and H2 separation efficiency have been co-designed by optimizing the sweep: feed ratios and the weight hourly space velocity to realize large Damköhler numbers and low Péclet numbers, indicating large conversion rates and high H2 permeation rates, respectively.
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