(224e) Kinetic Studies of Quaternary Ammonium Salt Degradation for Process Intensification Purposes

Kinetic studies of
quaternary ammonium salt degradation for process intensification purposes

Roel
Kleijwegt ( 11.0pt;font-family:" arial>r.j.t.kleijwegt@tue.nl),
Wyatt Winkenwerder, John van der Schaaf

text-autospace:none"> " arial>Department of Chemical Engineering and Chemistry, Eindhoven
University of Technology, font-family:" arial> Het
Kranenveld 14, 5612 AZ Eindhoven, The Netherlands

Introduction

107%">Quaternary
ammonium salts (QASs) are applied in the chemical industry as e.g., detergents phase-transfer
catalysts and fuel additives^[1-3]. Conventionally, these compounds are
synthesized by methylation of (fatty) tertiary amines with methyl halides. In
pursuit of safer and more renewable synthetic pathways, an incentive for the
use of a dimethyl carbonate (DMC) reagent has arisen^[2]. Previous
studies have shown that its methylation rate is poor and typically an order of
magnitude lower than the halide counterparts^[4]. Therefore, process
intensification seems paramount to make DMC a viable alternative to methyl
halides. Increasing the reaction temperature will increase the reaction rate,
however, the maximum temperature is limited by a consecutive degradation
reaction of the QAS to volatile species. Quantification of these decomposition
reaction kinetics is essential for further optimization of the overall process.

Methods

107%">The
kinetic data can be measured using a TA Instruments Q500 instrument for thermogravimetric
analysis (TGA), because of the low volatility of the QAS and the high
volatility of the degradation products. The TGA measurements were carried out
in a TA Instruments Q500. The kinetic data is subsequently fitted according to
Arrhenius behavior to determine the kinetic parameters. Although the methyl
carbonate QASs are of particular interest, other anions will also be measured
for comparison. The number of long-chain substitutions on the nitrogen will
also be varied. These variations provide insight into the effect of these
parameters on the QAS’s stability.

Results

The standard
isoconversional analysis has been used to determine the activation energy as a
function of the conversion (α).

line-height:107%">

line-height:107%"> color:#44546A">Figure 1: activation
energy vs. conversion of QASs without dependency (a) and with dependency (b)

107%">In
Figure 1a, the activation energy can be described as relatively independent of
the conversion, indicating the absence of any physical effects or complicated
kinetics. In Figure 1b, however, there a clear trend is apparent for higher
conversions. The increase of the activation energy results from the formation
of an involatile degradation product, of which the evaporation rate is limiting
the weight loss. This leads to an increasing resistance of the weight loss and
thus an increase in the determined activation energy.

line-height:107%;page-break-after:avoid"> line-height:107%;font-family:" arial>

Figure
2: fitted TGA data of an
example QAS without a low volatility compound (a) and with a low volatility
compound (b)

107%">With
the knowledge regarding the chemical and physical mechanisms of the QAS
compounds, they have been accordingly fitted to first order kinetics (Figure
2a) and first order kinetics with consecutive evaporation (Figure 2b). The
hypothesis that the tridodecyl-QASs transition into an evaporation limited
regime throughout their degradation is confirmed by Figure 2b.

Conclusion
and outlook

The
thermal stability and the degradation kinetic rates of a range of different
QASs have been determined. Even the evaporation limited compounds could be
analyzed using TGA. From the acquired parameters a clear trend has become
apparent. The compounds with a methyl carbonate anion are the least stable and
decompose at the highest rates. Furthermore, chloride compounds have proven to
be more stable than the bromide counterparts. Lastly, with an increasing number
of long-chain substituents on the nitrogen, the thermal stability decreases. In
future work, the range of different QASs will be extended and used to predict
the thermal and kinetic parameters of other compounds. Also, the formation
kinetics will be determined and used to design a continuous reactor type and optimize
its operating conditions.

107%">References

[1] C. M. Starks, Phase-Transfer
Catalysis. I. Heterogeneous Reactions Involving Anion Transfer by Quaternary
Ammonium and Phosphonium Salts, 1971

[2] Y. Ono, Catalysis
in the production and reactions of dimethyl carbonate, an environmentally
benign building block, 1997

[3] S. H. Pyo et al., Dimethyl
carbonate as a green chemical, 2017

[4] D.E. Weisshaar et
al., Kinetic Study of the Reaction of Dimethyl Carbonate with Trialkylamines,
2009