The direct conversion of benzene to phenol over Fe-ZSM-5 zeolite is investigated using quantum mechanical/molecular mechanical (QM/MM) calculations on a large model consisting of 683 SiO2 units (2084 atoms). The active-site model for benzene hydroxylation involves a mononuclear iron-oxo species (FeIII=O) located at the ion-exchangeable Al site of ZSM-5 zeolite. The proposed catalytic cycle is partitioned into four steps: (i) H atom abstraction, (ii) O atom insertion, (iii) phenyl migration, and (iv) phenol release. The decomposition of nitrous oxide plays an important role in the formation of the iron-oxo species and in the avoidance of the unstable Fe II state at the active site. The catalytic reaction occurs on the sextet and quartet potential energy surfaces. The quartet state plays a major role in the course of the reaction, whereas the sextet state lies lower in energy in the entrance channel and in the final stages of the reaction. The QM/MM calculations show that the nanopores of the zeolite framework would decrease the activation barriers in the transition states involved in the reaction pathway and accelerate the exchange of phenol and benzene in the final stages of the reaction. The environmental effect from the zeolite framework promotes the conversion of benzene to phenol.
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