Epoxidation and Partial Oxidation of Propene With Molecular Oxygen and Water Over Gold-Titanium Catalysts

1.      Introduction

The direct epoxidation of propene with a mixture of H₂ and O₂ over gold deposited on Ti-containing oxides like Ti-SiO₂ or TS-1 ¹ is an attractive alternative to the traditional processes used for the synthesis of propene oxide (PO). It is beneficial because it occurs in a single step and water is the only by-product. A disadvantage, however, is that the hydrogen efficiency is typically low, making it uneconomical. In 2009, Huang et al. ² and Ojeda et al. ³ reported the possibility of  performing the epoxidation with molecular oxygen and H₂O. This is an ideal process due to the low cost of the oxygen, which can be obtained from the air, and that it only requires a single step. Huang et al. ² indicated that three conditions should be fulfilled for this reaction to occur: small Au nanoparticles, presence of water vapor in the feed and a Ti-containing support with isolated titanium sites. In a later work ⁴ they also explained the necessity of adding an alkali salt to the catalyst which would interact with the Au cluster activating the O₂ and stabilizing it.

During the oxidation of propene with molecular oxygen, the product distribution is highly dependent on the particle size of the gold nanoparticles and on the presence of co-reactants like hydrogen or water⁵. Acrolein and CO₂ are the main side products. The production of acrolein (CH₂=CHCHO) can be considered beneficial, since it is, like PO, a highly demanded chemical intermediate, used in the synthesis of amino acid methionine or glutaraldehyde, a biocide. Therefore, understanding the influence of the reaction parameters, such as temperature, residence time and concentration of reactants in the performance of the reaction and in the ratio PO/acrolein can be of high interest.

In this work we performed a kinetic study of the epoxidation of propene with O₂ in the presence of water over gold-titanium catalysts, understanding the distribution between the production of PO and acrolein.

2.      Materials and Methods

Two different types of Ti-containing supports were prepared: Ti/SiO₂ and TS-1. The former was synthetized by grafting of Ti on silica while the latter was obtained by hydrothermal synthesis. The gold was deposited on these supports by deposition-precipitation method, using aurochloric acid and ammonia. Various catalysts with different Au loadings were prepared.

The catalytic tests were carried out in flow in a packed bed quartz reactor. Helium  was passed through an evaporating vessel with water at a controlled temperature so a constant water flow could be obtained. The outlet gases were monitored with an online compact GC.

3.      Results and Discussion

Both supports synthetized, Ti-SiO₂ and TS-1, contained tetrahedrally coordinated titanium dispersed in a matrix of silica. High gold loading efficiencies were obtained in the former support, synthetizing Au nanoparticles between 2 and 5nm. For the deposition on TS-1 the loading efficiency was not so high, so several methods were tested, including an alkali pre-treatment that creates surface defects where the Au species can be strongly fixed. In this case, the Au clusters were slightly bigger.

The kinetics of the epoxidation of propene with O₂-H₂O were studied, considering factors like temperature, concentration of reactants and residence time. In parallel, a comparative study was carried out between the epoxidation of propene with molecular O₂ and H₂O and the same reaction with H₂ and O₂. In both cases the influence of the support and the Au-loading was analyzed, addressing, as well, the deactivation of the catalyst. In this analysis, we also evaluated the influence of the presence of alkali ions, both in the gold deposition and in the epoxidation.

4.      Conclusions

The direct epoxidation of propene with molecular oxygen and water is an excellent process due to the low cost of the reactants and the reaction occurs in a single step. At the same time, it is a very clean technology, since water is the main by-product and the oxygen used for the oxidation can be obtained from the air  A further understanding of the mechanism could allow to evaluate its feasibility for industrial implementation and the possibilities for improvement. 

5.      References

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2.         Huang, J.; Akita, T.; Faye, J.; Fujitani, T.; Takei, T.; Haruta, M., Propene Epoxidation with Dioxygen Catalyzed by Gold Clusters. Angewandte Chemie International Edition 2009, 48, 7862-7866.

3.         Ojeda, M., Catalytic epoxidation of propene with H₂O-O₂ reactants on Au/TiO2. 2009.

4.         Huang, J.; Takei, T.; Ohashi, H.; Haruta, M., Propene epoxidation with oxygen over gold clusters: Role of basic salts and hydroxides of alkalis. Applied Catalysis A: General 2012, 435–436, 115-122.

5.         Lee, S.; Molina, L. M.; Lopez, M. J.; Alonso, J. A.; Hammer, B.; Lee, B.; Seifert, S.; Winans, R. E.; Elam, J. W.; Pellin, M. J.; Vajda, S., Selective Propene Epoxidation on Immobilized Au6-10 Clusters: The Effect of Hydrogen and Water on Activity and Selectivity. Angewandte Chemie-International Edition 2009, 48, 1467-1471.