Green and Sustainable Tandem Catalytic Approach for Fine-Chemicals Synthesis Using Octahedral MnO2 Molecular Sieve: Catalytic Activity versus Method of Catalyst Synthesis

Octahedral MnO2 molecular sieve (OMS) was prepared using a hydrothermal synthesis route in the absence and the presence of urea. OMS was also prepared by a solvent-free method. Catalysts were characterized by the complementary combination of X-ray diffraction, N2 adsorption, scanning electron microscopy, UV–visible spectroscopy, thermogravimetric analysis, and NH3 and CO2 temperature-programmed desorption techniques. Catalysts were investigated in the oxidation of benzyl alcohols, hydration of aromatic/heterocyclic nitrile, and condensation reactions (Knoevenagel and Aldol). Furthermore, applicability of the catalyst was extended in one-pot tandem conversion of benzyl alcohol to 2-benzylidene-malononitrile, (E)-chalcone, and 2,3-dihydro-1,5-benzothiazepine. 2-Benzylidene-malononitrile and (E)-chalcone were prepared by a two-step tandem reaction from benzyl alcohol, whereas 2,3-dihydro-1,5-benzothiazepine was prepared by a three-step tandem reaction from benzyl alcohol. Recycling study showed that no significant decrease in the catalytic activity was observed even after five recycles. To the best of our knowledge, this is the first report that shows that OMS materials obtained using different synthesis methods exhibited significant differences in their catalytic activity in the above-mentioned reactions.