Model Study of the Impregnation of Mo on Al2O3 Examined by Raman Spectroscopy
Advancing the chemical engineering fundamentals
Interfacial & Colloidal Phenomena - II (T2-6b)
Keywords: Impregnation, Hydrotreating, Raman Spectroscopy, Phosphomolybdates
Model Study of the Impregnation of Mo on Al2O3 Examined by Raman Spectroscopy
Pablo Beato§, Kim Johannsen§*, Xenia Faber§
§) Haldor Topsøe A/S, Nymøllevej 55, 2800 Lyngby, Denmark
*) Corresponding author, kjo@topsoe.dk
Materials containing Ni or Co together with Mo on an alumina carrier are used mainly as hydrotreating catalysts for removing sulphur and nitrogen from different oil fractions in oil refineries. These catalysts are the main key for producing green fuels such as ultra low sulphur diesels (ULSD), which play an important part in reducing SO2 emissions from vehicles[1]. The Ni/Mo catalysts can be prepared in several different ways, but the usual technique involves dissolving Mo and Ni-precursors in a suitable solution and impregnating this solution onto a pre-shaped alumina carrier. The impregnated carrier is calcined and sulfidized to generate the active hydrotreating catalyst.
During the impregnation step complex chemical and physical interactions between the species in the solution and the solid surface occur, which in part are responsible for the active structure of the final catalyst[2]. To understand the ongoing physico-chemical processes during the impregnation and calcination step is thus crucial for further improvement of these highly active catalysts.
Raman spectroscopy is a powerful tool for the investigation of the vibrational modes of polymolybdate species and contrary to IR spectroscopy relatively insensitive to O-H-stretching vibrations. Metal-oxygen vibrations of complexes in aqueous solutions can therefore be measured without interference from the O-H stretching frequencies down to energies of ca. 50 cm-1.
In this work, we have used Raman spectroscopy to study the structure of phospomolybdates in solution and their interaction with the carrier during impregnation as a model system for molybdenum precursors in hydrotreating catalysts. Furthermore the structural development of the phospomolybdates was studied during drying and calcination using in-situ Raman spectroscopy.
For the conditions used in our study preliminary results indicate the formation of the Strandberg type H2P2Mo5O234- and the Keggin type P2Mo18O626- and no other phosphomolybdates. The Strandberg and Keggin ions retain their structure upon impregnation into the pore system of the alumina carrier and also upon drying of the impregnated carrier. Only upon heating to T>180°C does a decomposition of the phosphomolybdates take place.
References:
[1] H. Topsøe, B.S. Clausen and F.E. Massoth: “Hydrotreating Catalysis – Science and Technology”, Springer Berlin 1996.
[2] J. A. Bergwerff, L. G. A. Van de Water, T. Visser, P. De Peinder, B. R. G. Leliveld, K. P. De Jong, B. M. Weckhuysen, Chem. Eur. J. 11 (2005) 4591.
Presented Thursday 20, 10:05 to 10:25, in session Interfacial & Colloidal Phenomena - II (T2-6b).
