Synthesis, structural configuration and DFT molecular orbital studies of [Mn-2[benzo[b]oxazole] acetonitrile] complexes encapsulated in ZSM-5: Direct synthesis of phenol by benzene hydroxylation

Abstract

In situ synthesis of Mn-BOA [BOA = 2[benzo[b]oxazole] acetonitrile] complexes with loadings from 7.8 to 12.3 wt% in the microporous structure of zeolite ZSM-5 (Si/Al = 38) by the zeolite synthesis method was reported. The structure and characterization of the prepared samples have been identified by elemental analyses, XRD, XPS, EPR, FT-IR, UV-vis, TGA/DTGA, mass spectroscopy and pore structure analysis by nitrogen adsorption at −196 °C. A monotonic increase in crystallinity up to Mn-BOA(10.3%)/Z was shown and hence must be attributed to templating effect of manganese complex. Concurrently, the surface areas and pore volumes were decreased, suggesting that the complex remained intact within the zeolite walls. The XPS analysis of Mn-BOA(9.3%)/Z and Mn-BOA(12.3%)/Z confirm that manganese exists in the +2 oxidation state, though the latter only has a satellite feature around 646 eV which could be a result of the presence of Mn2O3. The EPR of neat complex in solution exhibited broad spectra corresponding to nearest neighbor spin-spin interactions, whereas Mn-BOA(9.1%)/Z showed well resolved metal hyperfine features as in case of the spectra in dilute frozen solutions. Theoretically bond lengths, bond angles, energies of the highest occupied (HOMO), lowest unoccupied (LUMO) molecular orbital levels, reactivity parameters and the molecular electrostatic potential (ESP) have been calculated for all samples. The catalytic activity of the synthesized samples towards benzene hydroxylation to phenol using H2O2 as a green oxidant at room temperature was evaluated. The effects of contact time, catalyst amount, and concentration of benzene and H2O2 on the conversion rate were investigated. The encapsulated complexes showed higher activities compared to that of the neat complex.