NaCl-Promoted CuO–RuO 2 /SiO 2 Catalysts for Propylene Epoxidation with O 2 at Atmospheric Pressures: A Combinatorial Micro-reactor Study S ¸ ule Kalyoncu • Derya Du ¨ zenli • Isik Onal • Anusorn Seubsai • Daniel Noon • Selim Senkan • Zafer Say • Evgeny I. Vovk • Emrah Ozensoy Received: 16 September 2014 / Accepted: 24 November 2014 / Published online: 9 December 2014 Ó Springer Science+Business Media New York 2014 Abstract A combinatorial approach is used to investigate several alkali metals promoted Cu–Ru binary oxide cata- lysts with improved catalytic performance in the propylene partial oxidation. 2 %Cu/5 %Ru/c–SiO 2 catalyst had the best yield with high propylene conversion and propylene oxide (PO) selectivity. Among the promoters screening in the study, NaCl promotion significantly increased the PO selectivity accompanied by some attenuation in the total propylene conversion. It was proposed that binary oxide catalysts revealed a greater number of exposed catalytically active adsorption sites as compared to monometallic oxide counterparts according to XPS and FTIR results. Besides NaCl addition alters the structure, yielding a significantly improved PO selectivity without any change in the particle size of Cu and Ru oxide according to XRD analysis. Keywords Propylene Á Epoxidation Á Ru Á Cu Á SiO 2 1 Introduction Propylene oxide (PO) is a very important chemical feed- stock for the production of a wide variety of commodity chemicals, such as polyether polyols and propylene glycol [1]. Currently, chlorohydrin and organic hydroperoxide processes are two of the commonly used industrial pro- cesses for PO synthesis in the chemical industry. These processes lead to the generation of a large amount of waste water and organic byproducts. Thus, they are not preferable due to their environmental and economical drawbacks [2]. Because of the deficiencies of the aforementioned PO production processes, novel methods of producing PO have been explored which included direct oxidation of propyl- ene using various catalytic systems and proper oxidants, such as H 2 O 2 [3–5]O 2 –H 2 gas mixture [6–9] and N 2 O[10, 11]. However, high costs of these oxidants restrict the commercialization of these processes. Therefore, the direct gas-phase epoxidation of propylene to PO by molecular oxygen has been a focus of interest as an attractive alter- native from both economical and environmental standpoints. The recent discovery of the highly active Au/TiO 2 cat- alysts in various catalytic reactions led to the use of Au in conjunction with other reducible metal oxide support materials [7–9, 12–15]. However, since highly selective Au catalysts typically exhibit low propylene conversion and require hydrogen co-feeding, such catalysts are industrially less promising for PO production [8]. Successful results obtained from modified Ag catalysts in the gas phase epoxidation of ethylene by molecular oxygen led to many studies on propylene epoxidation over different support materials and modifiers [16–28]. However the conversion and the selectivity of Ag-based catalysts in PO production were lower than those for ethylene oxide production as a S ¸ . Kalyoncu Á D. Du ¨zenli Á I. Onal (&) Department of Chemical Engineering, Middle East Technical University, Ankara 06800, Turkey e-mail: ional@metu.edu.tr A. Seubsai Department of Chemical Engineering, Kasetsart University, Bangkok 10900, Thailand D. Noon Á S. Senkan Department of Chemical and Biomolecular Engineering, UCLA, Los Angeles, CA 90095-1592, USA Z. Say Á E. I. Vovk Á E. Ozensoy Department of Chemistry, Bilkent University, Ankara 06800, Turkey E. I. Vovk Boreskov Institute of Catalysis, Novosibirsk 630090, Russian Federation 123 Catal Lett (2015) 145:596–605 DOI 10.1007/s10562-014-1454-7