Comparison of in-line NIR, Raman and UV-visible spectrometries, and at-line NMR spectrometry for the monitoring of an esterification reaction

In-line Raman, near infrared and UV-visible spectometries, and at-line low-field NMR spectrometry have been used to monitor the acid-catalysed esterification of crotonic acid and butan-2-ol. Repeat reactions were carried out in a 1 L batch reactor. Spectra taken during the reactions, along with reference ester concentrations determined by gas chromatography (GC), were used to determine the concentration of 2-butyl crotonate as the reaction proceeded. Ester concentrations were determined from 1st derivative Raman and UV-visible spectra by employing univariate calibration models, whereas the low-field NMR and NIR data required multivariate analysis by partial least squares regression. The techniques have been compared on the basis of the accuracy and between-run precision of the 2-butyl crotonate concentrations, and the ability to determine the rate constant of the reaction in the shortest possible time after the start of the reaction. The ester concentrations determined by all of the techniques were similar to those obtained by the GC reference method. In-line UV-visible spectrometry gave the poorest between-run precision. Raman and NIR spectrometries provided an estimate of the rate constant of the reaction after 90 min when the ester concentration had reached 0.09 mol dm(-3), meaning that if the rate constant at this time was not as expected then corrective action could be taken to salvage the batch.