(17a) Coupling Online NMR Spectroscopy With Micro-Reaction Technology: A New Way for Quantitative Studies of Fast Reaction Kinetics in Technical Systems

INTRODUCTION

NMR
spectroscopy offers unmatched structural information of complex fluid mixtures.
Simultaneously, a quantification of all investigated species is possible
without previous calibration, which, in contrast to optical spectroscopy,
enables an easy access to reaction kinetics and even allows the identification
of unknown species, e.g. side- or intermediate
products. The method has a high resolution and allows
chemically similar compounds to be resolved. It is useful both for equilibrium
and kinetic studies. Hence, NMR spectroscopy is a powerful tool for reaction
engineering investigations. It is particularly suited for studies of
complex reaction networks with intermediates that cannot be isolated [1,2].
Nevertheless, the potential of the method has not yet been fully taken
advantage of.

In
the present contribution, methods are presented that allow studying complex reaction
networks with NMR spectroscopy under industrially relevant conditions. Recent
advances in this field were achieved by coupling micro-reaction technology with
flow NMR spectroscopy. This led to the development of a new probe head that enables
studying fast reaction kinetics with this favorable spectroscopic method.

COUPLING
REACTORS WITH NMR SPECTROSCOPY

NMR
flow probes can be directly coupled to conventional reactors. The sample is pumped
from the reactor to the NMR flow probe head where the composition of the
mixtures is monitored. Pressure resistant and thermostated lines can be used to
ensure that neither composition nor pressure nor temperature is changed upon
analysis. The method is therefore especially advantageous when the mixture
contains intermediates which cannot be isolated and have to be analyzed in
situ. Different economically important reaction networks from industrial
organic chemistry have successfully been studied in this way. Due to
limitations of the speed of the sample transfer from the reactor to the NMR
probe, the set-up with external reactors is not applicable for kinetic suites
of reactions with time constants below 5-10 minutes.

COUPLING MICRO-REACTION TECHNOLOGY WITH NMR SPECTROSCOPY

In
order to enable investigations of fast reactions with online NMR spectroscopy a
new NMR probe head was developed in co-operation with the Mainz Institute of
Microtechnology, Germany (IMM) and the University of Tübingen, Germany. A thermostated
micro-mixer was mounted directly in the NMR probe head and was coupled with a
capillary NMR flow probe with a solenoidal coil design. The combination leads
to a massive reduction of the residence time between the micro-mixer and the
probe. Thus, reactions with time constants of only seconds can be studied with
this technique.

A
pulsation free continuous flow of the reactants through the micro-mixer and the
probe is provided by two syringe pumps. The new probe head can be used in a
stationary flow- or an instationary stop-flow mode. In the flow mode, the
reaction time between the mixing and the detection is adjusted by setting the
flow rates of the reactants. In the stop-flow mode the reactants are mixed and
transferred to the probe at the maximal flow rate. Then, the flow is stopped
and the probe acts as an instationary reactor in which spectra are acquired at
predefined time intervals throughout the reaction.

A
focus of the development was put on thermal management. Isothermal conditions
even for strongly exothermic or endothermic reactions are guaranteed by liquid
tempering of the whole flow path of the reactants. The applicability of the
novel NMR probe head for monitoring fast reactions was proven in different test
studies. The new micro-mixer based probe head design is a breakthrough in
online monitoring of fast reactions with NMR spectroscopy.

Figure 1: CAD drawing of the novel probe
micro-mixer NMR head

REFERENCES

[1] M. Maiwald, H. Fischer, Y. K. Kim, K.
Albert, H. Hasse: J. Magn. Res. 166, (2004), 135-146.

[2] E. J. Kibrik, O.
Steinhof, G. Scherr, W. R. Thiel, H. Hasse: J. Appl. Polym. Sci. 128, (2013), 3957-3963.