Surface-Initiated Gas-Phase Epoxidation of Propylene with Molecular Oxygen by Silica-Supported Molybdenum Oxide: Effects
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Surface-Initiated Gas-Phase Epoxidation of Propylene with Molecular Oxygen by Silica-Supported Molybdenum Oxide: Effects of Addition of C3H8 or NO and Reactor Design Zhaoxia Song Æ Naoki Mimura Æ Susumu Tsubota Æ Tadahiro Fujitani Æ S. Ted Oyama
Received: 5 October 2007 / Accepted: 19 October 2007 / Published online: 8 November 2007 Springer Science+Business Media, LLC 2007
Abstract The gas-phase epoxidation of propylene was studied over MoOx/SiO2 catalysts in a reaction system with a post-catalytic bed volume. In the reaction of a mixture of propylene and propane with oxygen below 578 K, propylene oxide (PO) was mainly formed from the oxidation of propylene. It was found that the oxidation reaction was very sensitive to the temperature of the post-catalytic space more than the temperature of the catalyst bed, strongly indicating that radical reactions occurring in the post-catalytic bed free space were responsible for the PO formation. The addition of NO increased propylene conversions and PO selectivity at low conversions, confirming that radical reactions were involved in the propylene reactions. Keywords Epoxidation Silica-supported molybdenum oxides Post-catalytic reaction Radical reaction
Z. Song N. Mimura (&) S. Tsubota T. Fujitani S. T. Oyama Research Institute for Innovation in Sustainable Chemistry, National Institute of Advanced Industrial Science and Technology (AIST), 16-1, Onogawa, Tsukuba Ibaraki 305-8569, Japan e-mail: [email protected] S. T. Oyama Environmental Catalysis and Nanomaterials Laboratory, Department of Chemical Engineering (0211), Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA
1 Introduction Propylene oxide (PO) is an important chemical feedstock in the chemical industry, and is currently manufactured by multi-step methods, such as the chlorohydrin process, which uses Cl2 and H2O, or variants of the Halcon process which use organic hydroperoxides [1]. The above industrial processes are not only complex and costly, but also produce large amounts of side products or co-products. Thus, the development of a process for the one-step production of PO from propylene, especially using molecular oxygen as oxidant, is highly desirable. Recently, we reported that a reactor having a postcatalytic bed volume, was effective in forming PO over silica-supported titanium oxide [2, 3] and molybdenum oxide [4, 5] catalysts using molecular oxygen as an oxidant. It was suggested that the metal oxide clusters present in these catalysts are effective for the generation of allyl radicals from propylene at relatively low temperature, and PO is produced through gas-phase chain reactions. It has been reported that the homogeneous gas-phase oxidation of a mixture of propane and propylene with molecular oxygen can produce PO with much higher efficiency than that with propane or propylene alone [6]. Co-feeding NO with C3H6 and O2 enhanced both conversion and PO selectivity on supported silver catalysts. However, a silver catalyst containing molybdenum as a promoter
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