The Direct Epoxidation of Propene in the Explosive Regime in a Microreactor - A Study Into the Reaction Kinetics

Introduction

            Gold-titania catalysts are an attractive option for a future process to produce propene oxide directly¹. At mild conditions, they are capable of directly epoxidizing propene at a selectivity of typically > 90% using a mixture of hydrogen and oxygen. Before this system can be used on a large scale, however, the catalyst system needs some further improvements. The hydrogen efficiency (the ratio of propene oxide produced in relation to the co/side product water) needs to be improved, as at this moment it is typically only 10-30%. The catalyst stability is also an important factor, as now in the first hours of operation, the catalyst activity typically drops by a factor of 2-4. A kinetic study to investigate the kinetics of the epoxidation, water formation, catalyst deactivation, and catalyst reactivation can provide valuable information for improvements to the catalyst system and the way the catalyst is operated in a process.

            In this study, we are utilizing a microreactor system to perform a kinetic investigation into the propene epoxidation over a gold-titania catalyst. The usage of a microreactor system for the this reaction using a mixture of hydrogen, oxygen, and propene allows for the safe operation in the explosive regime. In literature, the concentrations of the reactants are typically limited to 10 vol% for each of the reactants in a large inert gas stream, to circumvent the possibility of working with explosive gas mixtures. Microreactors have a great potential for carrying out a reaction safely inside of the explosive region of the gas mixture used for a number of reasons. Firstly, microreactors have only a small volume. For our system, the energy liberated of an explosion in the microreactor would be less than 1 J, which would not be able to affect the its integrity. Secondly, the excellent temperature control that is possible in a microreactor, due to the large external area compared to the reactor volume, will diminish or even remove the risk of a runaway. Most importantly, the characteristic diameter of the reactor is smaller than the mean free path of the molecules, preventing flame propagation inside the channels since the molecules transfer their energy to the wall instead of each other.

            In this study, we are presenting the advantages of using a microreactor system for the epoxidation of propene using a gold-titania catalyst system, and use the microreactor system for the determination of kinetic expressions for the epoxidation, catalyst deactivation, and catalyst reactivation.

Results

            A typical propene epoxidation reaction experiment over a gold titania catalyst does not show a constant catalytic activity. Aside from the desired epoxidation reaction, a consecutive reaction of propene oxide (intermediates) causes the catalyst to deactivate by producing strongly adsorbing species (primarily) carboxylates, which only desorb very slowly (reactivation)². To describe these relatively complex changing activity, we developed a relatively simple algebraic expression based on published kinetics. This expression provided three reaction rate constants with a physical meaning: the rate constants for the epoxidation, the deactivation, and reactivation. Since we could use the microreactor to perform the reaction at a wide range of experimental condition, we obtained these parameters as a function of the feed gas concentrations.

            It is shown that the propene concentration does not influence the propene oxide formation rate, however, higher propene concentrations significantly reduce the catalyst deactivation rate. This indicates that propene oxide is not involved in the rate determining step in the epoxidation reaction and it also indicates that propene decreases the amount of one of the species involved in the catalyst deactivation. Hydrogen increases the rate of the epoxidation reaction, while it only has a minor influence on the rate of deactivation and reactivation. This indicates that hydrogen is involved in the rate determining step for the epoxidation. Oxygen has a beneficial effect on the epoxidation reaction, at low concentrations the rate increases significantly, at higher concentrations, the rate increases more slowly. This indicates that oxygen is involved in the rate determining step in the epoxidation reaction. These observations for hydrogen, oxygen and propene is in agreement with the most common view in literature, that the formation of a peroxide species out of hydrogen and oxygen is the rate determining step in the epoxidation ^{3, 4}.

Oxygen slightly decreases the deactivation rate and is beneficial for the catalyst reactivation. This is in support with suggestions in literature that strongly adsorbed consecutive oxidation products of propene oxide are deactivating the catalyst. A consecutive oxidation of these species to produce carbon dioxide, which desorbs more easily, us in line with our observations⁵.

After determining the reaction rate dependency of all reactants, the benefits of using a microreactor were demonstrated. For the gold on titania dispersed on silica catalyst used in this study, it was possible to select reaction conditions comprising of 20 vol% of propene, and 40 vol% hydrogen and oxygen. At these conditions a propene oxide yield over 3 times higher than is obtainable at the conventional 10-10-10 vol% conditions could be reached (5.2e-7 mol/g_catalyst/s compared to 1.5e-7 mol/g_catalyst/s).

 

Conclusions

            A microreactor was used as a valuable tool for the determination of the reaction kinetics for the direct epoxidation of propene over a gold-titania catalyst. A simple kinetic model was developed, expressing the catalyst behavior as a function of a epoxidation reaction rate constant, a deactivation rate constant, and a re-activation rate constant. The dependency of these rate constant on the feed gas composition has been determined. The rate constant for the epoxidation was only significantly dependant on the hydrogen and oxygen concentrations, supporting the view most common in literature that the formation of a peroxide on the gold nanoparticles is the rate determining step in the epoxidation. The benefits of using a microreactor were demonstrated for this reaction system. The option of operating this reaction in the explosive region allowed us to increase the productivity by over a factor of 3.

 

References

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