(13c) 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

1.        
Hayashi, T.; Tanaka, K.; Haruta, M., Selective Vapor-Phase Epoxidation of
Propylene over Au/TiO₂Catalysts in the Presence of Oxygen and Hydrogen.
J. Catal. 1998, 178, 566-575.

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.;
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Propene Epoxidation on Immobilized Au6-10 Clusters: The Effect of Hydrogen and
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