A KINETIC INVESTIGATION OF COSOLVENT EFFECTS IN OXYFUNCTIONALIZATION OF N-HEXANE BY HYDROGEN-PEROXIDE ON TS-2

Systematic experiments of n-hexane conversion to n-hexanol and n-hexanone by hydrogen peroxide in the presence of TS-2, in absence as well as in presence of a co-solvent, were performed. Methanol, acetone, and acetonitrile were used. The reaction was monitored by GC analysis of the organic phase and by measurements of the volume of the oxygen evolved as a function of time. Iodometric titration of hydrogen peroxide was performed at the end of the 5-h test. Independent measurements of the n-hexane solubility equilibrium were also performed. The catalyst was characterized by AA, XRD, FTIR, and UV-visible spectroscopy. These data allowed a kinetic analysis of the initial rates of oxidation on the secondary carbons (n-hexane-2ol and -3ol) and a kinetic model is proposed for these two reactions, yielding the following rate equation: r(i0) = k(i)[H2O2]2/{1 + K[H2O2]/[Solvent]} (i = 1, 2). When applied to rates measurement free of pore diffusion limitations, this model suggests that the active site can accommodate two H2O2 molecules which is discussed in light of the Ti environment in TS-2 proposed recently by Trong On et al. (Catal. Letter 16, 85 (1992) and J. Mol. Catal. 74, 233 (1992)). The rate of the parallel reaction of H2O2 decomposition is shown to be of first order with respect to the concentration of hydrogen peroxide: r3 = k3[H2O2]. The effect of the co-solvent on initial rates of alcohol formation and secondary oxidation to ketones is discussed. (C) 1994 Academic Press, Inc.