Active sites for ethanol oxidation over SnO2-supported molybdenum oxides

doi:10.1016/S0926-860X(99)00430-5

Applied Catalysis A: General

Volume 193, Issues 1–2, 28 February 2000, Pages 195-202

https://doi.org/10.1016/S0926-860X(99)00430-5 Get rights and content

Abstract

SnO₂-supported molybdenum oxides with varying coverage were synthesized and used for the catalytic oxidation of ethanol. The catalysts were obtained from precipitation of SnCl₄ by ammonia in the presence of (NH₄)₂Mo₇O₂₄ (A). Some catalysts were also prepared by impregnation of (NH₄)₂Mo₇O₂₄ on SnO₂ (B) for comparison. It was verified that molybdenum oxides inhibited the sintering of SnO₂ crystals during calcination for preparation A, resulting in homogeneous systems with high specific areas. The solids were characterized by FTIR, temperature programmed reduction (TPR), DRS-UV, XPS and X-ray diffraction (XRD). The molybdenum coverage was determined by oxygen pulses after reduction at 400°C under hydrogen. The results indicated two structurally different superficial sites. Four-coordinated molybdates were preferentially formed on the surface of co-precipitated catalysts at low molybdenum loading, while six-coordinated polymolybdates were obtained in other cases. Bulk MoO₃ oxide was also observed at very high loading. The turnover numbers (TONs) measured for ethanol oxidative dehydrogenation suggested that dispersed, four-coordinated molybdates were the active phase. These species also gave higher selectivity to acetic acid.

Introduction

Acetic acid is currently produced from ethanol by dehydrogenation to acetaldehyde in the gas phase and subsequent oxidation in the liquid phase. Using two reactors consequently increases costs. Moreover, the dehydrogenation catalysts deactivate while environmental problems are associated with the homogeneous systems. Allakhverdova et al. [1], [2] demonstrated the technical feasibility of the production of acetic acid from ethanol in a single step using catalysts based on Sn–Mo oxides in the presence of water steam. In fact, catalysts based on Sn–Mo have been pointed out as efficient for alcohol oxidation into aldehydes [3], [4], unsaturated hydrocarbons [5], esters [6] and acids [1], [2], [7].

Sn–Mo oxides can be obtained from the impregnation of a molybdenum salt on pre-formed tin oxide SnO₂ [3], [4] or on hydrated SnO₂ (stannic acids) [5]. Some authors mention also the possibility of synthesizing the catalysts by co-precipitation [1], [2] of molybdenum and tin salts.

The catalytic performance of these materials has not only been correlated to the presence of molybdenum cations dissolved in the crystalline structure of SnO₂ [1], [2], [5], but also more recently, to the formation of superficial molybdate species, which are easily reducible and more reactive [3], [4]. Ai [6] showed that the behavior of this system is strongly related to its acid–base characteristics. New studies are thus necessary to better characterize the active species.

In this paper, we discuss the nature of superficial sites on SnO₂-supported molybdenum oxides prepared at different molybdenum loadings and compare the impregnation and co-precipitation techniques.

Section snippets

Methodology

Two sets of catalysts were prepared. The first one (preparation A) was obtained from the precipitation of SnO₂·xH₂O, by addition of ammonium hydroxide to an aqueous solution of SnCl₄ and ammonium heptamolybate (NH₄)₂Mo₇O₂₄ (HMA), followed by evaporation of the solution. Four catalysts, A1, A2, A3 and A4, were synthesized with molybdenum loading of 6.4, 9.2, 14.6 and 24.6 wt.%, respectively. The second preparation (B) was performed by wet impregnation of a solution of HMA on pre-formed SnO₂. The

Results

The XRD patterns of SnO₂-supported molybdenum oxides are shown in Fig. 1. The samples prepared by co-precipitation (A) are poorly crystallized and present only the peaks referring to SnO₂ (cassiterite phase). The samples prepared by impregnation (B) present the SnO₂ pattern with a higher degree of crystallinity. Some authors studied SnO₂ for its capacity to dissolve cations such as Sb and Mo. Montgolfier et al. showed [9] by EPR the presence of Mo⁺⁵ dissolved into crystalline SnO₂. Okamoto et

Discussion

XRD patterns and BET analyses have shown that the preparation method has a strong influence upon the texture of the final SnO₂-supported molybdenum oxides. As expected, the XRD pattern and specific area are almost undisturbed after impregnation of molybdenum on pre-formed SnO₂. Molybdenum trioxide, which is observed at high molybdenum loading in these catalysts, is then concluded to grow from the surface of the support as the molybdenum loading increases. Superficial Mo enrichment observed by

Conclusion

SnO₂-supported molybdenum oxides were obtained by co-precipitation and impregnation methods. Two series of catalysts were prepared with various molybdenum loadings. It was shown that the presence of molybdenum oxides inhibits crystal growth of the SnO₂ support, leading to material with a high specific area. In this case, four-coordinated species are dispersed on the external surface of the solid after calcination. A model was proposed, according to which six-coordinated species are occluded

Acknowledgements

We thank Ricardo Aderne, Carlos Andre A.C. Perez and Prof. Martin Schmal (NUCAT/COPPE/UFRJ, Rio de Janeiro) for DRS-UV and XPS analyses. We acknowledge INT/MCT (Instituto Nacional de Tecnologia), CNPq (Conselho Nacional de Desenvolvimento Cientı́fico e Tecnológico) and FUJB (Fundação Universitária José Bonifácio) for financial support.

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Active sites for ethanol oxidation over SnO2-supported molybdenum oxides

Abstract

Introduction

Section snippets

Methodology

Results

Discussion

Conclusion

Acknowledgements

J. Catal.

J. Catal.

J. Catal.

J. Catal.

Polyhedron

J. Catal.

Appl. Catal. A

Appl. Catal.

Active sites for ethanol oxidation over SnO₂-supported molybdenum oxides