366 J. CHEM. RESEARCH (S), 1997 J. Chem. Research (S), 1997, 366–367† Selective Oxidation of Toluene over Complex Fe–Mo Oxides in the Absence of Molecular Oxygen Wenxing Kuang,* Yining Fan, Kaidong Chen and Yi Chen Department of Chemistry, Institute of Mesoscopic Solid State Chemistry, Nanjing University, Nanjing 210093, China Fe–Mo catalysts prepared by a sol–gel method for selective oxidation of toluene are studied in the absence of molecular oxygen; a high benzaldehyde yield and high specific activity are achieved at an atomic ratio (Mo:Fe) of about 1.0.Over the past forty years, iron molybdates have been widely used in industry for the selective oxidation of hydrocarbons, and the active component has been found to be defective iron(III) molybdate.1–3 In general, the optimum specific activity is achieved at high Mo:Fe atomic ratio (ca. 1.70 to 4.05); at lower Mo:Fe ratios the excess of Fe2O3 may lead to complete oxidation and thus reduce the catalytic activity for selective oxidation.In 1981, Germain et al.4 first reported that iron molybdates can be used as catalysts for selective oxidation of toluene to benzaldehyde. Later Zhang et al.5 pointed out that the catalyst has the highest specific activity for selective oxidation of toluene to benzaldehyde at an Mo:Fe atomic ratio of 3.45. Since all the above reactions are studied in the presence of molecular oxygen, various kinds of oxygen species may co-exist on the surface of the catalyst, which eventually results in the formation of products other than benzaldehyde. 6 In contrast to the widespread applications of iron molybdates, little is known about the fundamental nature and catalytic properties of these catalysts in the absence of molecular oxygen. In this work, we report an iron molybdate of low Mo:Fe atomic ratio (1.0) which has a high benzaldehyde yield and specific activity for selective oxidation of toluene to benzaldehyde in the absence of molecular oxygen.Besides Fe2(MoO4)3, highly dispersed Fe2O3 is also found to be an active component participating in the reaction and to be responsible for the high benzaldehyde yield and high specific activity for selective oxidation of toluene. The total amount of benzaldehyde yield and the corresponding specific activity for selective oxidation of toluene to benzaldehyde (benzaldehyde yield per BET surface area) over various Fe–Mo samples in the absence of molecular oxygen are shown in Fig. 1. For pure Fe2O3 and MoO3 samples, no product is detected under our reaction conditions, while the Fe–Mo samples exhibit high yields of benzaldehyde, indicating that iron molybdate species are the main catalytic active components. It is also pertinent to note that of these Fe–Mo samples, both the highest yield of benzaldehyde and the optimum specific activity appear at an Mo:Fe atomic ratio of 1.0. This result is different from that obtained in the presence of molecular oxygen,5 suggesting that the catalytic properties of Fe–Mo samples can be influenced by the reaction conditions.Compared to the case in the presence of molecular oxygen, only lattice oxygen exists on the surface of the oxides in the absence of molecular oxgyen. This may lead to changes in the catalytic active phase and reaction mechanism. XRD and M�ossbauer spectroscopic techniques are used to study the catalytic active phase and the relationship between structure and catalytic properties of the Fe–Mo oxide with an Mo:Fe atomic ratio of 1.0.M�ossbauer parameters of the sample are listed in Table 1. For the fresh sample, only XRD lines of Fe2(MoO4)3 are observed in the XRD pattern, while a singlet corresponding to an Fe2(MoO4)3 phase and a doublet assigned to superparamagnetic Fe3+ ions are shown in the M�ossbauer spectrum, suggesting that the fresh sample is composed of an Fe2(MoO4)3 phase and highly dispersed Fe2O3. After the reaction, XRD lines for crystalline b-FeMoO4 and Fe2(MoO4)3 are both observed in the XRD pattern.In the M�ossbauer spectrum two new doublets, which can be assigned to b-FeMoO4, are found in addition to the singlet for Fe2(MoO4)3 and the doublet for Fe3+ ions. As shown in Table 1, the amount of Fe2(MoO4)3 decreases after the reaction and, surprisingly, that of the highly dispersed superparamagnetic Fe2O3 decreases. Moreover, the amount of newly produced b-FeMoO4 species is much larger than the decrease in the amount of Fe2(MoO4)3 species.All these results suggest that the highly dispersed Fe2O3 may also parti- *To receive any correspondence (e-mail: physchem@nju.edu.cn). †This is a Short Paper as defined in the Instructions for Authors, Section 5.0 [see J. Chem. Research (S), 1997, Issue 1]; there is therefore no corresponding material in J. Chem. Research (M). Fig. 1 The catalytic properties for selective oxidation of toluene to benzaldehyde over Fe–Mo samples in the absence of molecular oxygen Table 1 M�ossbauer parameters of the Fe–Mo sample (Mo:Fe=1:1) M�ossbauer parameters Relative Sample IS (mm Sµ1) QS (mm Sµ1) H (kOe) Iron species area (%) Fresh 0.39 0.40 0.90 0 00 Fe2O3 Fe2(MoO4)3 34 66 After reaction 1.12 1.11 0.39 0.40 0.96 2.54 0.90 0 0000 b-FeMoO4 b-FeMoO4 Fe2O3 Fe2(MoO4)3 36 34 11 19J.CHEM. RESEARCH (S), 1997 367 cipate in the reaction, so that both Fe2(MoO4)3 and the highly dispersed Fe2O3 are the catalytic active species.The first stage in the reduction of Fe2(MoO4)3 with H2 was reported to be7 4Fe2(MoO4)3+5H2h8b-FeMoO4+Mo4O11+5H2O As mentioned above, both Fe2(MoO4)3 and the highly dispersed Fe2O3 phases participate in the reaction, the mechanism for which may then be suggested as 4Fe2(MoO4)3h8b-FeMoO4+Mo4O11+5[O] (1) 2Fe2O3+Mo4O11h4b-FeMoO4+[O] (2) where [O] is lattice oxygen. The total reaction equation can be written as 4Fe2(MoO4)3+2Fe2O3h12b-FeMoO4+6[O] (3) It is easy to find from eqn.(3) that the optimum atomic ratio of n(Fe2O3):n[Fe2(MoO4)3] is 1:2, i.e., Mo:Fe=1.0, which is well supported by our experimental results. Experimental The complex Fe–Mo oxides were prepared by a sol–gel method. Iron(III) nitrate, ammonium molybdate and citric acid (the molar ratio of citric acid to metallic ions was 1:3) were dissolved in water and mixed together, then nitric acid solution was added under constant stirring until the precipitates had completely dissolved.The above solutions were kept in a water batch at 333 K for gelation. The gels thus prepared were first dried at 393 K and then calcined at 673 K to afford the oxides. The oxides (250 mg) were located in a pulse microreactor and the catalytic properties were determined under the conditions of 623 K, 0.2 MPa, helium flow-rate 40 ml minµ1, and 1.45 mmol toluene/pulse. All samples were pretreated in a helium flow at 673 K for the removal of adsorbed oxygen species. The reaction products, detected by using an on-line gas chromatography, were found to be chiefly benzaldehyde and H2O. The support of the National Natural Science Foundation of China and SINOPEC is gratefully acknowledged. Received, 6th May 1997; Accepted, 17th June 1997 Paper E/7/04247G References 1 G. D. Kolovertnov, G. K. Boreskov, V. A. Dzis’Ko, B. I. Popov, D. V. Tarasova and G. G. Belugina, Kinet. Catal., 1965, 6, 950. 2 J. M. Leroy, S. Peirs and G. Tridot, C. R. Acad. Sci., Ser. C, 1971, 218. 3 I. De La Torre, G. Acosta and M. Hernandez, Rev. Inst. Mex. Pet., 1979, 11, 68. 4 J. E. Germain and R. Laugier, C. R. Acad. Sci., Ser. C, 1973, 276, 1349. 5 H. Zhang, Z. Li and X. Fu, Cuihua Xuebao, 1988, 9, 331. 6 S. L. T. Andersson, J. Catal., 1986, 98, 138. 7 H. Zhang, J. Shen and X. Ge, J. Solid State Chem., 199