(641a) Autocatalysis in Surfactant Systems: Kinetics & Model of Tertiary Amine Oxidation in Water

Autocatalysis in Surfactant
Systems: Kinetics & Model of Tertiary Amine Oxidation in Water

Arnab
Chaudhuri^a (a.chaudhuri@tue.nl),
Wyatt Winkenwerder^b (Wyatt.Winkenwerder@nouryon.com),
John van der Schaaf^a (J.Vanderschaaf@tue.nl)

Department of Chemical Engineering, TU Eindhoven, 5612 AZ,
Eindhoven, Netherlands

Nouryon-Surface Chemistry, Croton River Center 281 Fields
Lane, Brewster 10509, New York, United States of America

With increasing concerns regarding global
warming, the chemical industry has been moving towards more sustainable
processes. A main focus has been the reduction of solvents in chemical
reactions. Eliminating solvents from processes can lead to great improvements
in sustainability and process economy ^{[1, 2]}.

One such reaction which is traditionally
produced in the industry using solvents is the oxidation of fatty tertiary
amines to amine oxides with hydrogen peroxide; in a batch process ^[3].
If solvents are eliminated, this reaction begins as a biphasic mixture due to
the presence of an organic (tertiary amine) and aqueous (hydrogen peroxide)
phase. As the tertiary amine oxides are formed, the reactants are solubilized
by the zwitterionic, surface active amine oxides. This solubilization effect
greatly increases the reaction rates compared to the initial biphasic mixture.
Thus, tertiary amine oxidations show typical autocatalytic reaction behavior. This
inherent characteristic can therefore be used to develop a more sustainable
process with reaction rates comparable to solvent systems.

      (A)                                                
                  (B)

Figure 1: A) Kinetic data obtained for tertiary
amine oxidation. B) An example of the fitting obtained for the modified phase
transfer model.

The results obtained with batch experiments at
318-358K (Fig 1A) confirm that the reaction is kinetically limited and
illustrates autocatalytic behavior. There is however, an upper limit on the
temperature as degradation of the amine oxides occurs at higher values ^[4].

The reaction can be described by a phase
transfer model as suggested in literature ^[5]. In this model, initially,
there is only surface reaction at the interface of the organic and aqueous
phases which forms amine oxides. At the critical micellar concentration (cmc)
of the amine oxides, micellar structures form which solubilizes the organic
phase. This leads to reactions at the micellar interface. Here, reaction rates
increase drastically due to the increase in the surface area. While there is
good agreement between this model and the data, we have observed that the
acceleration of the reaction occurs at concentrations of amine oxide much
higher than the cmc. Therefore, we have also investigated an alternative model
for micellar autocatalysis based on interfacial tension and energy dissipation
rates.

The Arrhenius analysis (Fig 2A) illustrates
that the activation energy of the pseudophase/micellar reaction is almost half
of the surface reaction activation energy (41 kJ/mol vs 83 kJ/mol).Consequently,
a simple method which can be used to accelerate the reaction times is initially
seeding the reaction mixture with amine oxides^[6] (Fig 2B). This
method already illustrates how solvents can be eliminated from this reaction
while making reaction times/reactor volumes more viable for sustainable
processes.

(A)              
                                                        (B)                                                                    

Figure 2: A) Arrhenius analysis of the surface
reaction vs the pseudophase reaction B) Results illustrating the acceleration
obtained in batch by initially seeding with amine oxide.

The results presented in this abstract illustrate
how we are better able to design sustainable and/or continuous processes which
uses the micellar autocatalytic properties inherently existing in the reaction
to eliminate the use of solvents. The understanding of the intrinsic reaction
kinetics and mechanism therefore aids in making existing industrial processes
more sustainable. We will further discuss other methods, such as catalysis
which can further aid to bridge the gap between the reactions times observed
with solvent and the neat system.

References

[1]       K.
Boodhoo, A. Harvey, Process Intensification: An Overview of Principles and
Practice, in: K. Boodhoo, A. Harvey (Eds.), Process Intensif. Green Chem,2013:
pp. 1–31.

[2]       Stankiewicz,
A. I., J.A. Moulijn, Process Intensification : Transfroming Chemcial
Engineering, Chem. Eng. Prog. 2000 22–34.

[3]       G.L.K.
Hoh, D.O. Barlow, A.F. Chadwick, D.B. Lake, S.R. Sheeran, Hydrogen Peroxide
Oxidation of Tertiary Amines 1963,p268–271.

[4]       G.P.
Shulman, W.E. Link, Thermal decomposition of dimethyllaurylamine oxide,
J. Am. Oil Chem. Soc. 41 1964, p329–331.

[5]       T.
Buhse, R. Nagarajan, D. Lavabre, J.C. Micheau, Phase-Transfer Model for the
Dynamics of “Micellar Autocatalysis,” J. Phys. Chem. A. 1997, p3910–3917.

[6]       P.R.
Kust, J.F. Rathman, Synthesis of Surfactants by Micellar Autocatalysis:
N,N-Dimethyldodecylamine N-Oxide, Langmuir. 1995,p3007–3012.