(654d) 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

[1]   R. Horn,
O. Korup, M. Geske, U. Zavyalova, I. Oprea, R. Schlögl, Rev. Sci. Inst. 2010, 81, 064102

[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

[4]   M. Reinke, J. Mantzaras,
R. Schaeren et al., Comb. Flame 2004, 136, 217-240

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