Consideration of some sampling problems in the on-line analysis of batch processes by low-field NMR spectrometry

Abstract

A low-field medium-resolution NMR spectrometer, with an operating frequency of 29 MHz for 1H, has been assessed for on-line process analysis. A flow cell that incorporates a pre-magnetisation region has been developed to minimise the decrease in the signal owing to incomplete polarisation effects. The homogeneous esterification reaction of crotonic acid and 2-butanol was monitored using a simple sampling loop; it was possible to monitor the progression of the reaction through changes in CH signal areas of butanol and butyl crotonate. On-line analysis of heterogeneous water-toluene mixtures proved more challenging and a fast sampling loop system was devised for use with a 5 L reactor. The fast sampling loop operated at a flow rate of 8 L min(-1) and a secondary sampling loop was used to pass a sub-sample through the NMR analyser at a slower (mL min(-1)) rate. It was shown that even with super-isokinetic sampling conditions, unrepresentative sampling could occur owing to inadequate mixing in the reactor. However, it was still possible to relate the 1H NMR signal obtained at a flow rate of 60 mL min(-1) to the composition of the reactor contents.

Simultaneous 19F–1H medium resolution NMR spectroscopy for online reaction monitoring

“…Since only recently promising MR-NMR benchtop instruments with acceptable performance came to market, a process integrated instrument could accordingly be developed on the basis of presently emerging laboratory instruments. Within this study, a different approach from [16][17][18][19][20][21][22] was realised: A simple 1/16 00 FEP tube with an ID of 0.04 00 (1.02 mm) was used as a flow probe. The thermal isolation between the flow tube and the NMR probe was realised with a simple 5 mm glass Dewar tube inserted into a benchtop MR-NMR spectrometer.…”

On‐Line Monitoring of Chemical Reactions by using Bench‐Top Nuclear Magnetic Resonance Spectroscopy

“…Although the first magnet prototypes were already considered a breakthrough in the field and proved useful for chemical analysis, [19][20][21][22][23][24][25][26][27][28][29] their initial performance in terms of sensitivity and resolution was not good enough to study standard reactions at typical low concentrations. Most often, educts are dissolved in protonated solvents at concentrations of just a few percent, so high sensitivity is needed to detect the small signals of the reacting molecules and high resolution is required to minimize overlap of the weak reactant signals with the large solvent lines.…”

Medium Resolution NMR at 20 MHz: Possibilities and Challenges

“…An alternative could be MR-NMR with its inherent high chemical selectivity. First examples are described in literature 17,[20][21][22] . In this work the modified minispec mq20 19 was used to monitor the reaction progress in an emulsion polymerization 26 .…”

“…In contrast to relaxometry methods for determination of water and oil content [12][13][14][15][16][17] , prerequisite for frequency resolved NMR proton spectra is a sufficient spectral resolution of the NMR spectrometer in the range of 0.2 ppm full width half maximum (FWHM) 18 and frequency or B 0 field stability. The general feasibility of NMR spectroscopy with adequate resolution at low magnetic fields have already been shown by different groups [19][20][21][22] . The technique is named medium resolution NMR (MR-NMR) in the following.…”