Structural properties and catalytic activity of Mn-MCM-41 mesoporous molecular sieves for single-step amination of benzene to aniline

Mesoporous MCM-41 and Mn-MCM-41 with varying Si/Mn ratios such as 100, 60, 40, 20 and 10 were synthesized at room temperature under basic conditions by direct insertion of metal ions during synthesis. The bulk and surface properties of the synthesized catalysts were examined by XRD, nitrogen adsorption/desorption, FT-IR, TG-DTA, Raman spectra, ESR technique and UV–vis diffuse reflectance spectra (DRS). Results indicated that Mn ions are partially incorporated into the SiO framework and lead to the vibration of SiOMn bonds. ESR spectra showed a well-resolved sextet centred at g = 2.0 indicating distorted octahedral symmetry of Mn(II) species in Mn-MCM-41. UV–vis DRS showed co-existence of Mn2+ and Mn3+ in Mn-MCM-41; these coordinated to Si(IV) by disordered octahedral and tetrahedral environments, respectively. Mn-MCM-41 materials showed significant catalytic activity for single-step amination of benzene in acetic acid–water medium under mild reaction conditions using hydroxylamine as aminating agent. Mn-MCM-41 (Si/Mn = 20) showed the highest benzene conversion (68.5%) and 100% selectivity for aniline.

Mn-MCM-41 is an efficient catalyst for single-step amination of benzene. The catalyst interacts with hydroxylamine in acidic medium to give NH3+ radicals which react with benzene to give protonated aminocyclohexadienyl intermediates. The unstable intermediate is oxidized by the catalyst to give aniline.Figure optionsDownload as PowerPoint slide