Cobalt-Containing Molecular Sieves as Catalysts for the Low Conversion Autoxidation of Pure Cyclohexane

摘要

CoAPO molecular sieves have already been studied as solid acid catalysts, but the incorporation of cobalt into alu-minophosphate and aluminosilicate structures could also open a route toward heterogenization of cobalt autoxidation catalysis. The autoxidation of cyclohexane is an important bulk process, which is currently carried out in the presence of small amounts of dissolved cobalt salts. Reactions proceed by a free radical mechanism, yielding primarily cyclohexyl hydroperox-ide (cHHP). Cobalt ions accelerate the overall reaction as new radicals are generated by cHHP decomposition in a Haber-Weiss cycle. As the primary oxidation products are more susceptible to oxidation than the substrate, conversion must be kept low to obtain reasonable selectivities for the products cy-clohexanol and cyclohexanone. Precipitation deactivates the homogeneous catalysts. Besides the technical advantages of catalyst heterogeneity, the catalyst activity, stability, and selectivity are thus important factors to be considered in developing new heterogeneous catalysts for this process. We studied the autoxidation of cyclohexane in the presence of CoAPO-5 and CoAPO-11 molecular sieves, monitoring activity and selectivity by product formation and oxygen consumption. Overall cobalt distribution and coordination were studied by elemental analysis, X-ray photoelectron spectroscopy (XPS), and UV-Vis reflectance spectroscopy (DRS) to evaluate heterogeneity and stability. Isolation of the cobalt sites by incorporation intothe molecular sieve framework seems to be critical for obtaining active and truly heterogeneous autoxidation catalysts. These solid catalysts show an improved stability and selectivity over the classical cobalt naphthenate catalyst.