(243c) 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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