Heterogeneous catalytic application of octahedral molecular sieves of manganese oxides - Oxidation, nitration, and condensation reactions

This research demonstrates the heterogeneous catalytic application of manganese oxide octahedral molecular sieves (OMS) for three different kinds of reactions. In particular K-OMS-2 which has a cryptomelane type of structure and its acid-exchanged form H-K-OMS-2 were used as catalysts for oxidation of cyclohexane, regioselective nitration of phenolic compounds, and synthesis of 2-aminopdiphenylamine. The products of these reactions are of great industrial value for synthesis of fine chemicals or their use as intermediates for synthesizing other important compounds. ^ The first part of this thesis deals with oxidation of cyclohexane using t-butyl hydroperoxide as oxidation and H-K-OMS-2 as a catalyst. High conversions were obtained due to presence of both Lewis and Brønsted acid sites in the catalyst. A turn over number (TON) of 73 was obtained in just 24 hours at 80°C and if we consider only contributions of active manganese sites then the TON is 287. A mechanism has been proposed with a dual active site participation theory. Both Lewis acid sites and Brønsted acid sites participate in the reaction resulting in higher conversion and higher selectivity for the desired products. ^ In the second part of this thesis K-OMS-2 was successfully used for catalyzing nitration of phenolic compounds. A clean nitration process has been demonstrated, where the reaction is done at room temperature with a short reaction time. The reaction times vary from 30-120 minutes depending on the substrate. In all the cases 100% conversions were obtained with high selectivity for the ortho-nitro products. A mechanism has been proposed for this mild nitration process explaining the reasons for high regioselectivity. ^ The last part of this thesis concerns the solid acid catalyzed condensation reaction between phenylhydroxylamine and aniline to synthesize 2-aminodiphenylamine. The use of H-K-OMS-2 in this synthesis produces the ortho isomer of aminodiphenylamine with ∼96% selectivity as compared to the para isomer. This is very unique as all other solid acids give predominantly the para isomer. The stabilization of the nitrenium ion intermediate where the aniline attacks at the 2-position was suspected to be the reason behind high ortho selectivity. ^