Gas-phase epoxidation of propylene over small gold ensembles on TS-1 B. Taylor a , J. Lauterbach b , W.N. Delgass a, * a Forney Hall of Chemical Engineering, 480 Stadium Mall Drive, School of Chemical Engineering, Purdue University, West Lafayette, IN 47907, USA b Department of Chemical Engineering, University of Delaware, Newark, DE 19716, USA Received 30 September 2004; received in revised form 18 February 2005; accepted 18 February 2005 Available online 10 May 2005 Abstract A series of Au/TS-1 catalysts with varying gold and titanium contents was prepared by deposition–precipitation (DP) and examined at 140–200 8C in a 10/10/10/70 vol.% mixture of hydrogen, oxygen, propylene and helium at a space velocity of 7000 mL/h/g cat . Gold loading was found to be closely related to titanium loading, implying that low gold loadings using deposition–precipitation results in an inherently small number of very active sites. Forcing the gold loading to higher values resulted in poor activity and stability. A catalyst prepared with a Si/ Ti = 36 and a gold loading of 0.05 wt% produced 116 g PO /h/kg cat at 200 8C, which is the highest rate thus reported for a TS-1-based catalyst, with no evidence of deactivation during the 40 h temperature program. Catalysts prepared with lower titanium and gold contents resulted in very active catalysts when rates were normalized to the total gold content, 350 g PO /h/g Au at 200 8C for 0.01 wt% Au/TS-1 (Si/Ti = 500), indicative of the more efficient use of gold and titanium for the epoxidation reaction. The low gold loadings coupled with the absence of gold particles in TEM micrographs make it likely that, in these materials, significant activity is attributable to gold entities much smaller than 2 nm. # 2005 Elsevier B.V. All rights reserved. Keywords: Epoxidation; Gold; TS-1; Propylene oxide; Propylene 1. Introduction Propylene oxide (PO) is a valuable chemical intermediate with end-products ranging from toothpaste additives to the structural units of block co-polymers. While Ag/a-Al 2 O 3 catalysts have been used for decades to produce ethylene oxide from ethylene and molecular oxygen, the silver surface attacks the relatively weak allylic bond of propylene resulting in complete oxidation. As a result, commercial production of PO has been limited to liquid-phase methods (chlorohydrin and hydroperoxidation processes), which suffer from the production of chlorinated by-products, stoichiometric co-products and/or expensive oxidants. The direct, heterogeneous gas-phase epoxidation of propylene using an inexpensive and abundant oxidant such as molecular oxygen would present an environmentally benign and potentially economically viable method for the production of PO. In 1998, Haruta et al. found that Au/TiO 2 prepared by deposition–precipitation (DP) selectivity epoxidized propy- lene using a mixture of oxygen and hydrogen as the oxidant [1]. Activity was assigned to the presence of hemispherical 2–5 nm particles, as observed using high-angle annular dark-field-scanning transmission electron microscopy (HAADF-STEM), in intimate contact with the support material [2]. Although these early catalysts suffered from low PO yields and deactivation, improvements were made through the use of supports with a higher degree of titanium isolation. Mixed oxide materials such as supported titanias and the titanosilicates TS-1, TS-2 Ti-b, Ti-MCM-41, and Ti- MCM-48 [3–20] improved the catalyst stability and PO yield, however, the PO rate was not sufficiently high to be considered commercially viable. Further focus on the engineering of an appropriate support material has recently led to the development of the first Au-Ti-based catalyst with www.elsevier.com/locate/apcata Applied Catalysis A: General 291 (2005) 188–198 * Corresponding author. Tel.: +1 765 494 4059; fax: +1 765 494 0805. E-mail address: delgass@ecn.purdue.edu (W.N. Delgass). 0926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2005.02.039