Spatially Resolved Raman Spectroscopy In Catalytic Packed Bed Reactors

Spatially Resolved Raman Spectroscopy in Catalytic Packed Bed Reactors

Measuring spatially resolved species and temperature profiles in catalytic reactors has recently gained attention in the literature and several reactor designs with or without sampling probes have been developed [1,2,3,4]. As reviewed by Urakawa and Baiker [5], also the measurement of spatially resolved spectroscopic data in catalytic reactors becomes more and more important as catalysts are dynamic systems that continuously adapt to the local chemical potential in the reactor. However, in situ spectroscopy cells are usually adapted to the spectroscopic technique and in most cases do not reflect the flow and temperature field in a technical reactor. Measuring spectroscopic profiles along the centerline of a catalytic fixed bed reactor, e.g. by fiber optics, would be an approach closer to industrial reality because fixed bed reactors are frequently used for chemical processes. Furthermore no thermal gradients are induced by radiation losses which can be the case in commercial or self-designed spectroscopic cells equipped with windows for optical access.

In the present paper we demonstrate how spatially resolved Raman spectroscopy using fiber optic probes can be applied to monitor the state of the catalyst in a fixed bed along the flow direction in a catalytic partial oxidation reaction. The oxidative dehydrogenation of ethane to ethylene on MoO_x supported on g-Al₂O₃ spheres is used as demonstration example. After determining spatial resolution and offset of the used fiber optics by means of mapping a sharp transition between an excellent Raman scatterer (sulfur) and a poor Raman scatterer (graphite) arranged as cylinders in the reactor tube, the reactor is filled with the catalyst and a simultaneous spatially resolved measurement of gas species and Raman spectra is conducted. In situ measurements are complemented by ex situ XRD and Raman measurements on the used catalyst extracted layer by layer from the reactor tube. The latter Raman measurements were conducted under a confocal microscope. Two reaction zones were observed in the catalyst bed. At the entrance, ethane reacts with gas phase oxygen to ethylene, carbon monoxide, carbon dioxide and water. Depending on the loading, MoO_x is present in this oxidation zone either as crystalline MoO₃ or as polymolybdate. The temperature maximum is observed at the end of the oxidation zone where about 500°C are reached. Upon complete O₂ consumption the color of the catalyst changes from white or gray respectively to violet, which was identified by ex situ XRD and Raman spectroscopy to be MoO₂. The lattice oxygen removed by this reduction preferentially oxidizes C₂H₄ to CO₂. Interestingly C₂H₆ is much less prone to deep oxidation than C₂H₄ and no CO but only CO₂ is formed by reaction of C₂H₄ with lattice oxygen. Towards the end of the catalyst bed a mixture of polymolybdate and MoO₂ is observed indicating that the catalyst bed had not reached steady state yet and was still in the process of being reduced to MoO₂.

 

References

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[2]   J. Sxa, D. L. A. Fernandez, F. Aiouache et al., Analyst 2010, 135, 2260

[3]   M. Bosco, F. Vogel, Catal.Today 2006, 116, 348

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[5]   A. Urakawa, A. Baiker, Top. Catal. 2009, 52, 1312-1322