Ind. Eng. Chem. Res. 1999, 38, 1357-1363
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Ethylbenzene Oxidative Dehydrogenation on MnOx/SiO2 Catalysts Radu Craˇ ciun*,† and Nicu Dulaˇ mit¸ aˇ ‡ Department of Chemical Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, and Department of Chemical Technology, “Babes¸ -Bolyai” University, Cluj 3400, Romania
Supported MnOx on high surface area SiO2 (300 m2/g) catalysts were successfully used in ethylbenzene oxidative dehydrogenation. X-ray diffraction and X-ray photoelectron spectroscopy were employed to characterize the structure of fresh and used MnOx/SiO2 catalysts. The MnOx catalysts were prepared using the pore volume impregnation method, with MnO2 loading varying from 0.7 wt % (Mn/Si ) 0.005, atomic ratio) to 30 wt % (Mn/Si ) 0.14). The changes in the crystalline structure and dispersion of supported MnOx were related to the precursor/support interaction and the conditions used during catalyst preparation. A possible mechanism for ethylbenzene conversion to styrene on MnOx/SiO2 catalysts is proposed, where lattice oxygen from crystalline MnO2 can be used in the oxidation or oxidative dehydrogenation processes. The high selectivity in styrene (at 723 K, 24% conversion with 76% selectivity in styrene) was related to the high concentration in the MnO2 phase from the MnOx/SiO2 catalysts. The formation of the Mn3O4 phase, observed on the used catalysts, supports the proposed mechanism. These findings are of potential use for industrial applications, particularly in optimization of various oxidative dehydrogenation processes. Introduction Unsupported or supported manganese oxides are known to be active catalysts in numerous chemical processes such as CO,1-4 ethylene,5 and methanol6 oxidation; NOx7,8, H2O2,9 and O310 decomposition; oxidative coupling of methane;11-13 selective catalytic oxidation of NH3,14-16 ethylbenzene,17 or hydropyrazine to pyrazine oxidative dehydrogenation;18 selective catalytic reduction of NO with NH3,13-15,19,20 CO,11 and C2H416 hydrogenation; H2S/H2 sulfidation,21 and Hg waste removal.22 The wealth of literature data shows a variety of catalytic reactions that have been tested on MnOx catalysts. The MnOx catalytic activity has been attributed to the capability of Mn to form several oxides such as MnO2, Mn2O3, Mn3O4, or MnO and to store and provide oxygen selectively from its crystalline lattice. Because of its labile oxidation state, Mn is capable of playing the role of either a reducing agent that is oxidized (Mn2+ - ef Mn3+ - e- f Mn4+) or an oxidizing agent that is reduced (Mn4+ + e- f Mn3+ + e- f Mn2+), acting in both cases as an active component in the redox process. Studies of unsupported or Al2O3-supported MnOx catalysts identified the presence of MnO2 or mixed MnO2/ Mn2O3 phases as possible components in the catalytic process.19,21,23-25 The ratio between the MnO2 and Mn2O3 phases is a function of Mn loading and pretreatment conditions during preparation.21,23-25 The interaction of Mn with the support or other catalyst components influences its oxidation state.16 The presence of promoters such as La2O3 or CeO2 also affects the MnOx dispersion and resistance to deactivation.17,25 High MnO2 dispersion on Al2O3 support led to a significant increase in the catalytic activity, which was * To whom correspondence should be addressed. Tel: 901344-5341. E-mail:
[email protected]. † University of Pennsylvania. ‡ “Babes ¸ -Bolyai” University.
attributed to an efficient MnO2 exposure to the reaction feed.1,23 The MnO2 dispersion was found to depend on the manganese precursor and loading, the preparation method, and the post-preparation thermal treatment. Thus, a manganese acetate precursor leads to a higher dispersion of Mn2O3 on γ-Al2O3 support in comparison with a manganese nitrate precursor, because of a stronger interaction with the support surface.24 A multiple-step treatment with diluted manganese nitrate solutions also led to a good MnOx dispersion. The structural transformation of MnOx depends on the calcination temperature during pretreatment. Thus, low-temperature calcination and Mn loading led to formation of more MnO2 phase whereas high Mn loadings and calcination temperatures favored formation of Mn2O3 or Mn3O4 phases.23-25 The great interest in the study of transition-metal oxide-based catalysts is an indication of their importance in chemical industry, particularly in designing cheep and selective (waste-free) catalysts for oxidation and oxidative dehydrogenation processes. Despite the considerable effort devoted to the study of MnOx catalysts, problems such as identification of the active sites, MnOx crystalline phase transformations during preparation or catalysis, and surface or bulk structural changes are still a matter of debate and intensive studies. Herein, we present the influence of loading and catalysts pretreatment on the catalytic activity (conversion and selectivity) of MnOx/SiO2 catalysts, used in the ethylbenzene (EB) oxidative dehydrogenation reaction. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses were employed to obtain information about the bulk and surface structure, dispersion, and crystallinity of the SiO2-supported MnOx catalysts. Conversion and selectivity data from ethylbenzene oxidative dehydrogenation are summarized and presented in correlation with the observed MnOx/SiO2 catalyst structure. Information from the structural characterization of fresh and used catalysts as well as from the catalytic activity data was used to understand
10.1021/ie9803725 CCC: $18.00 © 1999 American Chemical Society Published on Web 02/11/1999
1358 Ind. Eng. Chem. Res., Vol. 38, No. 4, 1999
and propose a possible reaction mechanism for EB to styrene conversion. Experimental Section Catalyst Preparation. The support was prepared using silica gel (Davison Chemical Co.), finely ground ( 723 K) and on catalysts with low Mn loading and less MnO2 phase on the silica surface (high selectivity in toluene and benzene). A recent review paper summarizes the best industrial catalysts used in the EB oxidative dehydrogenation reaction.34 It was suggested that coke formation (in the first stage of the process) and a constant amount of surface metal oxides (based on Mo, Ce, Sn, or P) are the active components of the catalyst. From the present study, it is also observed that the presence and stability of MnO2, the active component of the catalyst, are crucial to maintain a good selectivity in styrene. The flexibility of the MnOx structure also plays an important role in optimization of the process. Other factors, such as promoters (Ce, La, Pr, or Nb oxides) or additives (Ba), may also influence the conversion and selectivity of the EB oxidative dehydrogenation process.17,25,35 On MnOx/ SiO2 catalysts, conditions such as low temperatures (≈723 K) and low conversions (≈20-30%) are essential to obtain high selectivity in styrene. Similar reaction conditions (conversion < 60% and low temperatures, 698-723 K) have been suggested in the literature as being the optimum for this process.34 Conclusions MnOx-supported on high surface area SiO2 catalysts were found to be active in the EB oxidative dehydrogenation process. Conversions and selectivity are strongly dependent on the MnOx structure and reaction conditions. The active component of the catalysts, which ultimately determined the selectivity in styrene, was identified as being the MnO2 phase. XRD and XPS analyses of the MnOx/SiO2 catalysts calcined at 500 °C indicated that Mn was present mostly as crystalline MnO2 and Mn2O3. The ratio between the two oxides, Mn dispersion, and MnOx particle size are strongly dependent on the Mn loading and calcination temperature used during catalyst preparation. Analysis of the used catalysts showed that Mn oxidation state degenerates from Mn4+ to Mn2+ by providing oxygen to the reactants (EB) and thus leading to formation of the less active and selective Mn3O4 species. Besides the fact that this study shows the catalytic activity of MnO2 in EB oxidative dehydrogenation, it also provided a clear relationship between the MnOx structure and its catalytic activity. This information may provide the knowledge necessary for designing more efficient and selective catalysts. Acknowledgment The authors are grateful for the assistance of Dr. John Vohs from Chemical Engineering Department, University of Pennsylvania, and of Dr. James E. Jackson and Dr. Simon Garrett from Chemistry Department, Michigan State University, for facilitating the access to XRD and XPS instrumentation and for useful discussions and suggestions regarding this material. Literature Cited (1) Mooi, J.; Selwood, P. W. J. Am. Chem. Soc. 1952, 74, 2461.
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Received for review June 9, 1998 Revised manuscript received October 30, 1998 Accepted November 18, 1998 IE9803725
