Investigating tertiary amine alkylation/benzylation kinetics with ramp-flow in a plug-flow reactor using in-line 1H NMR spectroscopy

Quaternary ammonium salts are readily utilised within the chemical industry as, e.g., phasetransfer catalysts and anti-microbials, and their annual production capacity exceeds 1 million tonnes. These surfactants are conventionally produced in batch Menshutkin reactions from tertiary amines and an alkyl halide. With an incentive to move towards a more efficient continuous process, a better understanding of its kinetics is paramount. This paper explores a novel method to determine kinetic parameters, by introducing a linear flow ramp, combined with in-line Proton Nuclear Magnetic Resonance (H-1 NMR) analytics. Batch reactions between N,N-dimethyldecylamine (DMDA) and benzyl chloride (BnCl) have been performed in various solvents to examine solvent effects, and to validate the ramp-flow method. H-1 NMR spectrometry showed that acetonitrile had the highest reaction rate, while methanol (MeOH) was deemed to have the best overall potential with regards to process intensification and product separation. Kinetic data was also obtained in a Plug-Flow Reactor (PFR) subjected to ramp-flow, under continuous measurement of the outlet stream composition. Between 20 and 70 degrees C, DMDA quaternisation rates were determined in MeOH, using both BnCl and methyl iodide reactants. This resulted in the successful elucidation of the respective Arrhenius parameters. Furthermore, the data from the ramp-flow method has been compared with the data obtained in batch, and in the PFR under conventional steady-state conditions. In general, all three methods were in good agreement, and it was concluded that the proposed method is valid. This method can thus provide a more efficient way to determine highly accurate kinetic parameters. (C) 2021 The Authors. Published by Elsevier B.V. on behalf of Institution of Chemical Engineers.

Automated summary

This study introduces a novel method to determine kinetic parameters of quaternary ammonium salts reactions through linear flow ramp and in-line Proton Nuclear Magnetic Resonance (H-1 NMR) analytics. The results show that methanol is the most suitable solvent for the reaction, and the proposed method has potential for efficient production and product separation.