Aerobic oxidation of cyclohexane by gold nanoparticles immobilized upon mesoporous silica
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Catalysis Letters Vol. 100, Nos. 3–4, April 2005 (Ó 2005) DOI: 10.1007/s10562-004-3454-5
Aerobic oxidation of cyclohexane by gold nanoparticles immobilized upon mesoporous silica Kake Zhua, Juncheng Hub, and Ryan Richardsa,* b
a International University Bremen, 28759 Bremen, Germany Shanghai Research Institute of Petrochemical Technology, 201208 Shanghai, P.R. China
Received 15 November 2004; accepted 1 December 2004
A series of highly efficient hybrid gold nanoparticle/mesoporous-silica catalysts were synthesized and employed for the solventfree aerobic oxidation of cyclohexane to cyclohexanol and cyclohexanone under moderate reaction conditions. The catalysts were characterized by XRD, N2 adsorption/desorption, TEM and ICP-AES, which shows the active species are monodispersed gold nanoparticles. KEY WORDS: gold; nanoparticles; mesoporous; silica; oxidation.
1. Introduction Selective oxidation of cyclohexane to cyclohexanol and cyclohexanone using air/O2 or other oxidants is the topic of many recent studies and is of particular interest because the two major products are important intermediates in the manufacturing of nylon-6 and nylon-6,6 polymers [1,2]. Additionally, cyclohexanol and cyclohexanone also find utilities as solvents for lacquers, shellacs and varnishes; stabilizers and homogenizers for soaps and synthetic detergent emulsions; and starting reagents for synthesizing insecticides, herbicides and pharmaceuticals [3,4]. The present commercial process to produce cyclohexanol and cyclohexanone on an industrial scale starting from cyclohexane and molecular oxygen is to use cobalt salt or metal-boric acid as catalysts, which gives 4% conversion and 70–85% selectivity to cyclohexanol and cyclohexanone at 150 °C under 1–2 MPa pressure [5,6]. As a homogenerous process, which entails handling of toxic solids and waste liquids, this process has some inherent drawbacks: low conversion, cost effectiveness, higher operating costs, as well as environmental concerns. Such disadvantages make it necessary and urgent to design and cultivate new heterogenerous catalysts, furthermore, a low-cost environmentally-benign solvent-free process would be preferable to the conventional solvent based process. To heterogenize this process, metal cations, metal complexes and oxides doped upon silica, alumina, zirconia, active carbon, zeolite or aluminophosphates have been developed to catalyze the reaction [7]. Mesoporous materials such as TS-1, Ti–MCM-41, Cr–MCM-41, metal containing VPO and AlVPO were also found to be * To whom correspondence should be addressed. E-mail: [email protected]
active for the aerobic oxidation of cyclohexane [8–12], however, in most cases these new catalytic systems resulted in either a low conversion or low turnover frequency. Very recently, Au–MCM-41 and Au–zeolite were reported as new systems and were found to be highly active and selective for the reaction. Au–MCM41 was prepared by an in-situ addition of HAuCl4 to the silica gel while the mesophase was forming, followed by calcination at 550
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