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. 2019 Oct 15;91(20):12636-12643.
doi: 10.1021/acs.analchem.9b00895. Epub 2019 Sep 24.

Monitoring Heterogeneously Catalyzed Hydrogenation Reactions at Elevated Pressures Using In-Line Flow NMR

Koen C H Tijssen, Bram J A van Weerdenburg, Hainan Zhang  1 , J W G Janssen, Martin C Feiters, P Jan M van Bentum, Arno P M Kentgens
Affiliations

Monitoring Heterogeneously Catalyzed Hydrogenation Reactions at Elevated Pressures Using In-Line Flow NMR

Koen C H Tijssen et al. Anal Chem. .

Abstract

We present a novel setup that can be used for the in-line monitoring of solid-catalyzed gas-liquid reactions. The method combines the high sensitivity and resolution of a stripline NMR detector with a microfluidic network that can withstand elevated pressures. In our setup we dissolve hydrogen gas in the solvent, then flow it with the added substrate through a catalyst cartridge, and finally flow the reaction mixture directly through the stripline NMR detector. The method is quantitative and can be used to determine the solubility of hydrogen gas in liquids; it allows in-line monitoring of hydrogenation reactions and can be used to determine the reaction kinetics of these reactions. In this work, as proof of concept we demonstrate the optimization of the Pd-catalyzed hydrogenation reactions of styrene, phenylacetylene, cyclohexene, and hex-5-en-2-one in a microfluidic context.

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Conflict of interest statement

The authors declare no competing financial interest.

Figures

Scheme 1
Scheme 1. Hydrogenation Reaction of Styrene
Figure 1
Figure 1
Schematic drawing of the stripline NMR reaction monitoring setup for heterogeneously catalyzed hydrogenation reactions.
Figure 2
Figure 2
Solubility of hydrogen gas in chloroform as a function of hydrogen gas pressure (PH). Flow rate chloroform = 80 μL/min.
Figure 3
Figure 3
Hydrogenation of styrene: (a) concentration of hydrogen before the reaction and (b) after the reaction. (c) Calculated reaction times. (d) Styrene conversion including the fitted trends based on two models (see text).
Figure 4
Figure 4
NMR spectra of the reaction mixture of the hydrogenation of styrene (544 scans), phenylacetylene (1920 scans), cyclohexene (2560 scans), and hex-5-en-2-one (2080 scans). Asterisks mark impurities in the solvent (CHCl3, 7.26 ppm). Peak at 1.6 ppm is water. Conversion percentages are the maximum conversions and do not necessarily match with the spectra. For all spectra and conversion numbers the initial substrate concentration is 10 mM, except for the styrene spectrum, which was recorded for a concentration of 100 mM.
Scheme 2
Scheme 2. Reaction Models for the Hydrogenation Reactions
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