(88g) Studying Tertiary Amine Alkylation Kinetics with Step-Flow in a Plug Flow Reactor Using in-Line H-NMR Spectroscopy

Studying tertiary amine alkylation
kinetics with step-flow in a plug flow reactor using in-line H-NMR spectroscopy

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 within 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 alkyl halides in a batch reaction process^[2]. However,
the process is limited by a consecutive degradation reaction and the formation
of a highly viscous phase^[4]. There is an incentive to perform the
reaction in a continuous reactor to greatly enhance the process control,
scalability, flexibility and to ensure consistent product quality. This study
focuses on determining the feasibility of a continuous process and obtaining
accurate reaction kinetics in flow. Combined with the degradation reaction
kinetics, obtained in a separate study, these insights can be used to further
intensify the process.  

Methods

107%">The
methylation reaction was carried out in a plug flow reactor (PFR) with an inner
diameter of 1.75 mm and a length of 1 m. Two Shimadzu HPLC pumps fed the
reactor separately by injecting the reactants at the required reaction
temperature at the inlet of the reactor. The reactor was kept in isothermal
conditions by a thermostat oil bath and the reaction mixture was cooled at the
reactor exit. A back pressure regulator provides a constant pressure of 30 bar
to ensure the absence of a gas-phase. The well-defined point of mixing and the
excellent temperature control allow for accurate determination of the reaction
kinetics. The reaction mixture is subsequently fed through an in-line Magriktek
Spinsolve 60 MHz benchtop NMR, which has its acquisition time set at ca. 15 s. The
overall schematic overview of the setup is shown in Figure 1.

line-height:107%">

Figure
1: schematic overview of the
PFR setup and the in-line H-NMR spectrometer

107%">By
applying a stew-flow methodology, the online NMR measurements provide transient
data in between the steady state flows^[5]. This allows for efficient
yet accurate kinetic fitting with only 2 steady states. To take the residence
time distribution (RTD) into account, a tracer step-injection was carried out
to determine the RTD profile.

Results

107%">The
tracer step-injection experiment has provided a concentration profile that
characterizes the reactor, as can be seen in Figure 2. This RTD data allows for
precise modeling of the reactor system to obtain the intrinsic kinetics from
future kinetic experiments.

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

Figure
2: residence time distribution
curve of the reactor, obtained by step-injection of a tracer

107%">Preliminary
kinetic experimental results have validated the proposed methodology. The
step-flow provides insight into the kinetic rate along the entire flow spectrum
in a single experiment. The short acquisition time of the H-NMR spectroscopy
results in high-resolution kinetic data. 

Conclusion
and outlook

A
platform and methodology for determining accurate tertiary amine alkylation
kinetics has been devised. Furthermore, the degree of plug flow in the reactor
has been established with RTD measurements. Short term future work will be
focused on performing the step-flow experiments and fitting the kinetic
parameters to the response curves of the in-line H-NMR at different
temperatures. These parameters will be used together with the degradation
kinetics and the RTD profile to model the reactor. The model can be employed to
find the optimal operating conditions to best intensify the reaction. Moreover,
studying different systems with various reactants and solvents to elucidate
their individual kinetics and assumed solvent effects respectively.

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., Investigation of the Stability of Quaternary Ammonium Methyl

Carbonates, 2011

[5] S. Mozharov et al.,
Improved Method for Kinetic Studies in Microreactors Using Flow Manipulation
and Noninvasive Raman Spectroscopy, 2011