Selective conversion of cyclohexane to cyclohexanol and cyclohexanone using a gold catalyst under mild conditions Yi-Jun Xu, Philip Landon, Dan Enache, Albert F. Carley, M. W. Roberts, and Graham J. Hutchings , * School of Chemistry, Cardiff University, Cardiff, CF 10 3TB, UK Received 3 January 2005; accepted 11 January 2005 The oxidation of cyclohexane to cyclohexanol and cyclohexanone are investigated using supported gold catalysts using mild conditions of temperature and pressure. These catalysts are found to show some limited activity at 70 °C. However, the gold catalysts do not exhibit significantly different behaviour from supported Pt or Pd catalysts, and the selectivity observed is solely a function of conversion which in turn is a function of reaction time. It is clear that at very low conversions very high selectivities can be observed, but high selectivity under these mild reaction conditions cannot be maintained at higher conversions. KEY WORDS: oxidation; cyclohexane; gold; cyclohexanol; cyclohexanone. 1. Introduction One of the most important recent discoveries con- cerning catalysis has been the discovery that gold, long considered to be an inert noble metal, is a highly reactive heterogeneous catalyst. This has led, in the last 2–3 years, to a dramatic increase in interest in catalysis by gold [1], although the first indications were made nearly two decades ago with the prediction that gold would be the most effective catalyst for acetylene hy- drochlorination [2] and the seminal studies by Haruta et al. [3] concerning the low temperature oxidation of carbon monoxide. These early studies have now gener- ated this new interest, in particular in the design of selective oxidation catalysts based on supported gold nanocrystals. For example, gold catalysts have been shown to be very selective for the oxidation of alcohols to acids [4–6] and we have shown that complete speci- ficity for glycerol oxidation to glyceric acid can be observed [7,8]. In addition, gold catalysts have been shown to be highly selective for the synthesis of hydro- gen peroxide from the direct oxidation of hydrogen [9,10]. Gold is therefore rapidly becoming a versatile oxidation catalyst that is capable of catalysing processes that other metals cannot achieve with such specificity. Activation of C–H bonds in alkanes is known to be very difficult to achieve selectively at temperatures below 100 °C [11,12]. To date, the lowest temperature for selective oxidation of the alkane C–H bond has been achieved by Thomas et al. using transitional metal ions substituted molecular sieve catalysts at 100 °C [11–14]. We therefore wished to investigate the activation of C–H bonds at temperatures below this, and in particular we have examined the use of supported gold catalysts and we have found that they can be effective at tem- perature as low as 70 °C. One of the most important alkane activation pro- cesses currently operated industrially is the oxidation of cyclohexane to cyclohexanol and cyclohexanone, which in a recent review has been acknowledged to be a reac- tion that continues to be a significant challenge [15]. The aerobic oxidation of cyclohexane is central to the pro- duction of nylon-6 and nylon-6,6 and the worldwide production exceeds 10 6 tonnes per annum. Commer- cially the process is operated at 150–160 °C with cobalt naphthenate as an initiator for the radical oxidation process that gives 70–85% selectivity at 4% conversion. Operation at high conversions leads to total oxidation and consequently this large-scale commercial process has been designed to operate at low conversion. There is clearly scope to produce a more efficient process but given the current focus on green chemistry the main scope is to design a system that gives 100% specificity. One approach has been to use hydrogen peroxide as oxidant, and promising results have been obtained with vanadium phosphate catalysts [16] and Cr-MCM-41 [17]. However, it is recognized that the desired ideal oxidant is air (or its reactive component dioxygen, O 2 ) and the observation of high selectivity with air has been more elusive which can be attributed to the fact that air oxidations are intrinsically non-selective and difficult to control [12,18]. Ratnasamy and co-workers [19] have shown that l 3 -oxo-bridged Co/Mn cluster complexes were very selective as homogeneous catalysts, but it is generally recognised that a heterogeneous catalyst is preferred. Recently, Nowotny et al. [20] have investi- gated immobilization of cobalt complexes but leaching occurs under reaction conditions. One of the most * To whom correspondence should be addressed. E-mail: hutch@cf.ac.uk Catalysis Letters Vol. 101, Nos. 3–4, June 2005 (Ó 2005) 175 DOI: 10.1007/s10562-005-4886-2 1011-372X/05/0600–0175/0 Ó 2005 Springer Science+Business Media, Inc.