A Computational Study on the Aerobic Oxidation of Benzene to Phenol in Cu/Exchanged Y Zeolite

In heterogeneous catalysis transition metal-containing zeolites, in particular copperexchanged zeolites, have been studied experimentally [1, 2] as well as theoretically. Zeolites have large surface areas and different active sites (Brønsted acid sites and transition metal centers) which play a significant role in catalysis. In order to study the catalytic oxidation of benzene to phenol using dioxygen, the adsorption of dioxygen, benzene, and their co-adsorption to Cu−exchanged Y zeolite have been investigated by means of Fourier transfrom infrared (FTIR) spectroscopy [1, 2] and model calculations at the combined quantum mechanics and molecular mechanics (QM/MM) level using the QM-Pot program. The cost-effective QM/MM approach, which treats the active part of a system by QM methods, and the rest by a classical force field giving a reliable prediction of chemical properties has become an important tool for computational modeling of catalyzed reactions. In the QM/MM framework, the QM calculations have been performed using BP86 and B3LYP exchange-correlation functionals, employing triple-zeta quality basis sets and the MM calculations are done with a polarizable force field. High-level ab initio (CCSD(T), MRCISD+Q) [2] calculations have also been performed for a simple CuO2 model system to give proofs that DFT correctly represents, and predicts the structural and electronic properties of CuO2 moieties in the present study. The model Cu−Y zeolite structures have been determined with single Cu(I) centers. Adsorption complexes at two different sites (II and III) in Y zeolite with different Si/Al patterns have been studied. On adsorption of O2 to Cu−exchanged Y zeolite, end-on coordination to the Cu atom prevails at site II whereas site III shows side-on coordination. O2 binds more strongly at site III than at site II and the triplet electronic states are the most low-lying energy states. But, small singlet-triplet splitting indicates that the impact of singlet states on the properties of adsorbed O2 is an interesting aspect. The calculated harmonic frequency of dioxygen adsorbed at site III agrees well with the experimental IR result. In several adsorption complexes, the assignment of the O2 stretching vibration was difficult due to its strong coupling with framework vibrations. An η-coordination of benzene to copper was observed for all adsorption complexes. Its interaction with Cu is stronger for site III than for site II, and the benzene−copper complexes show higher binding energies than those of dioxygen. A comparison between experimental IR results and theoretical harmonic vibrational frequencies is discussed for different benzene−copper adsorption complexes. In case of the simultaneous adsorption of O2 and C6H6, the main focus is at site III. Both O2 and C6H6 are attached to the same Cu(I) center. O2 changes its bonding pattern from side-on in single adsorption to end-on in co-adsorption processes, whereas C6H6 binds to Cu as η as in the case of single adsorption. A possible multistep reaction pathway is proposed for the aerobic oxidation of benzene to phenol in Cu−Y zeolite on the triplet, singlet and broken-symmetry singlet potential energy surfaces. The oxygen-insertion into the C−H bond of benzene occurs, and a σ-complex is formed. Simultaneous formation of the O−H bond and cleavage of the C−H bond result in a phenol-complex. Local electron correlation calculations were performed to deliver high-level estimates for the reaction barrier.