Scheme 1. (a) Traditional routes used in current industry. (b) Direct oxidation of toluene to benzaldehyde or homologues by O2 in this work.

Fig. 1. (a) Atomic (001) surface model of the M1 phase MoVTeNbO complex oxides and the ODH reaction ODH of ethane to ethene. (b) V=O active site on the (001) plane of M1 phase and the oxidation of propane to acrylic acid. (c) Atomic (001) surface model of the M1 phase and the oxidation of toluene to maleic anhydride. (d) TeOx/MVTN with TeOx clusters on (001) surface of the M1 phase MoVTeNbO complex oxides in suitable density and self-organized structure presented in this work, for toluene oxidation by O2 to benzaldehyde.

Fig. 2. (a) Configuration of toluene adsorption (inset) and corresponding energy with the density of TeOx on MVTN. (b-d) Atomic models about toluene reaction to benzaldehyde over MVTN (b) and TeOx/MVTN (d), with intermediates and transition states, corresponding to the energy steps diagram (c). Atomic models about the reaction of benzaldehyde to benzoic acid over MVTN (e) and TeOx/MVTN (g), with the corresponding energy steps diagram (f). The reactive area of MVTN and TeOx/MVTN involved the V=O active site containing the pentameric ensembles (S4)2-S2-(S7)2.

Fig. 3. HRTEM images of MVTN (a) and TeOx/MVTN (b). Raman (c), FT-IR (d), EPR (e), and V 2p (f) XPS spectra of MVTN and TeOx/MVTN. Vanadium k-edge XAS spectra (V foil, VO2, and V2O5 were used as references). Normalized XANES (g), Radial distribution function from EXAFS (h), and wavelet transform maps (i) of Fourier transformed k2-weighted χ(k). (j-l) Tellurium k-edge XAS spectra (Te foil, TeO2, and H6TeO6 were used as references). Normalized XANES (j), EXAFS (k), and wavelet transform maps (l) of Fourier transformed k2-weighted χ(k).

Fig. 4. (a) Catalytic performance for toluene reaction over MVTN and TeOx/MVTN (toluene: 0.6 μL·min?1, vaporized in the line; O2: 5 mL·min?1; N2: 20 mL·min?1; cat.: 0.3 g; 400 °C). (b) Effect of TeOx loading on product selectivity (toluene: 1 μL·min?1, vaporized in the line; O2: 5 mL·min?1; N2: 20 mL·min?1; cat.: 0.3 g; 400 °C). (c) Effect of molar ratio of O2 to toluene on the catalytic performance over TeOx/MVTN (toluene: 0.6 μL·min?1, vaporized in the line; O2: 2-6 mL·min?1; N2: 19-23 mL·min?1; cat.: 0.3 g; 400 °C). (d) Effect of the WHSV on the catalytic performance over TeOx/MVTN (toluene: 0.6 μL·min?1, vaporized in the line; O2: 5 mL·min?1; N2: 0-30 mL·min?1; cat.: 0.3 g; 400 °C). (e) Prolonged reaction of TeOx/MVTN (toluene: 1 μL·min?1, vaporized in the line; O2: 5 mL·min?1; N2: 20 mL·min?1; cat.: 0.3 g; 350 °C). Arrhenius plots (f), dependences of the reaction rate on the partial pressure of toluene (g) and O2 (h) over MVTN and TeOx/MVTN (toluene: 0.2-1.6 μL·min?1, vaporized in the line; O2: 1-7 mL·min?1; balance N2; total flow rate: 25 mL·min?1; cat.: 0.3 g; 250-400 °C). MD simulation (MSD) of toluene (i) and O2 (j) molecules with different atomic concentrations on the surface of MVTN and TeOx/MVTN (toluene: 0.8%-4%; O2: 9.6%-24%). Dependences of toluene and O2 diffusion coefficients obtained through MSD fitting on the concentration of toluene (k) and O2 (l).

Fig. 5. In-situ DRIFTS characterizations to capture the reaction intermediates over MVTN (a) and TeOx/MVTN (b) (toluene: 1 μL·min?1, vaporized in the line; O2: 5 mL·min?1; N2: 20 mL·min?1; cat.: 0.02 g; 350 °C for MVTN and 400 °C for TeOx/MVTN; time: 0-35 min). (c) Adsorption energy of carbon dioxide on MVTN and TeOx/MVTN, the inset shows the adsorption model of carbon dioxide on MVTN and TeOx/MVTN. (d) Te 3d XPS of TeOx/MVTN after use in toluene/O2 and toluene/Ar, respectively, at 400 °C.

Fig. 6. Proposed reaction mechanism for p-chlorotoluene oxidation to p-chlorobenzaldehyde over TeOx/MVTN at the V=O active site located at the pentameric ensembles (S4)2-S2-(S7)2. The inset shows the calculated energy profiles in eV. The atomic configurations (side view) of the reaction intermediates of p-chlorotoluene oxidation to p-chlorobenzaldehyde are shown in the reaction cycle.