(763d) Operando Attenuated Total Reflection-Infrared Spectroscopic Detection of Catalytic Solid-Liquid Interfaces: Design Aspects of Spectroscopic Cells

In heterogeneous catalysis, optimal conditions for the catalytic reaction and for a spectroscopic investigation may deviate strongly. However, coupling catalytic and spectroscopic measurements of the same sample can be advantageous (relevance of the spectroscopic data, signal correlation, etc.) and finding experimental conditions to achieve complementary measurements is an essential part of operando spectroscopic characterization. Furthermore, the application of operando spectroscopy requires the design of spectroscopic reactor cells, which mimic the conditions of a conventional reactor providing similar catalytic performance and allow surface-sensitive detection of the catalyst in its working state.^[1]

Currently, the most suitable spectroscopic technique for in situ spectroscopic study of catalytic solid-liquid interfaces is attenuated total reflection-infrared (ATR-IR) spectroscopy, which allows for the detection of adsorbates on technical powder catalysts even in the presence of strongly absorbing solvent.^[2] In the context of operando spectroscopic characterization of a catalytic solid-liquid interface, the application of supported polycrystalline catalysts and their deposition as a thin layer is crucial for achieving realistic catalytic performance.^[3] In this presentation, recent results on operandospectroscopic characterization of heterogeneous hydrogenation catalysis in liquid-phase are presented, and design aspects for the construction of ATR-IR spectroscopic cells suitable for the detection of catalytic solid-liquid interfaces under working conditions are discussed.

For operando monitoring of the catalytic solid-liquid interface during semi-batch hydrogenation a fixed-bed flow-through reactor was integrated into a recycle loop.^[4] The catalytic performance and spectroscopic detection of various spectroscopic reactor cells with different inlet and reactor geometries were assessed using a fast reaction, the asymmetric hydrogenation of the C=C bond in phenylcinnamic acid on CD-modified Pd/TiO₂. In a slurry reactor operated at atmospheric hydrogen pressure under vigorous stirring a turnover-frequency (TOF) of 1200 mol_product mol_Pd^-1 h^-1and an enantiomeric excess (ee) of 45% can be achieved in isopropanol.

In general, commercially available ATR-IR spectroscopic cells have a hole-drilled inlet and a reactor geometry with rounded-off edges. Employing such a flow-through reactor cell very sluggish mass exchange was observed at the catalyst bed leading to a severely mass transport limited hydrogenation (TOF < 20 h^-1) and a deteriorated enantioselectivity (ee on the level of 25%). A custom-made spectroscopic cell with a slit-inlet and a rectangular reactor shape has significantly better fluid flow dynamics, which is reflect in the higher catalytic performance with a TOF of 450 h^-1and an ee of 45%. The spectroscopic measurement of the deposited catalyst sample is also very sensitive to the mass transport and different, catalytically more relevant surface processes were captured using the new spectroscopic cell.

In addition to the inlet geometry, the influence of other design parameters like the height of the reactor cell, the properties of the supported catalyst, the liquid flow rate and the quantity of deposited catalyst was investigated. The mass transfer in the porous catalyst layer depending on the reactor geometry was also investigated using mathematical modelling, and computational and experimental results are compared.

References

[1] N. E. Tsakoumis, A. P. E. York, D. Chen, M. Ronning, Catal. Sci. Tech. 2015, 5, 4859-4883.

[2] F. Zaera, Chem. Soc. Rev. 2014, 43, 7624-7663.

[3] L. Rodríguez-García, K. Hungerbühler, A. Baiker, F. Meemken, Catal. Today., 2016, in press.

[4] F. Meemken, P. Müller, K. Hungerbühler, A. Baiker, Rev. Sci. Instrum. 2014, 85, 17.