Continuous selective oxidation of alcohols in supercritical carbon dioxide
Sustainable process-product development & green chemistry
SCF as Solvent Substitutes (T1-8)
Keywords: supercritical fluids, alcohol oxidation, supported noble metal catalysts, in situ spectroscopy
Supercritical fluids (SCFs) have gained considerable attention in various technical applications, particularly in extraction, separation, crystallization, and polymer technologies. More recently, their unique physical properties, which lie between those of liquids and gases, have also been exploited in the field of heterogeneous catalysis [1]. Particularly, the improved mass transport properties at the solid/fluid interface, the elimination of the gas/liquid interface compared to conventional gas/liquid reactions and the process intensification appear attractive. Along with the matchless physical properties and the environmental and technical benefits, carbon dioxide, with its low critical temperature and pressure, its non-toxic behaviour and its readily availability, is the solvent of “choice”. In oxidation reactions, it can additionally be regarded as a “safe” reaction medium.
In the present contribution we present the selective oxidation of benzyl alcohol in supercritical carbon dioxide as an important example for a mild selective oxidation reaction [2,3]. Different noble metal catalysts were tested in a continuous process and compared to conventional gas-liquid operations. A strong dependence on the phase behaviour was found using scCO2 as solvent and the reaction rates greatly exceeded those observed in conventional liquids, e.g. toluene. For the selective oxidation of benzyl alcohol over 0.5%Pd/Al2O3 a turnover frequency higher than 2000 h-1 (rate related to the number of Pd-surface atoms) was found. This is significantly higher than rates reported up to now for selective oxidation of alcohols in liquid phase and it is attributed to improved mass transport both in the bulk fluid phase and at the solid/fluid interface. The studies were supported both by kinetic investigations as well as in situ spectroscopic studies applying attenuated total reflection infrared spectroscopy (ATR-IR) and X-ray absorption spectroscopy (XAS) [3].
To widen the scope of supercritical carbon dioxide application, we have extended the approach investigating the effect of co-solvents and expanded liquids [4]. Moreover, continuous processing of a solid alcohol, cinnamyl alcohol, was studied [5]. These results show that not only supercritical fluids but also expanded solvents may exhibit advantageous properties for heterogeneous catalytic reactions.
[1] Baiker, A. Chem. Rev. 1999, 9,9 453; Subramaniam, B. Appl. Catal. A, 2001, 212, 199; Hyde, J. R.; Licence, P.; Carter, D.; Poliakoff, M. Appl. Catal. A 2001, 222, 119; Grunwaldt, J.-D.; Wandeler, R.; Baiker, A. Catal. Rev. Sci. Eng. 2003, 45, 1.
[2] Caravati, M.; Grunwaldt, J.-D.; Baiker, A. Catal. Today 2004, 91-92, 1
[3] Grunwaldt, J.-D.; Caravati, M.; Baiker, A. J. Phys. Chem. B 2006, 110, 9916; Caravati, M.; Grunwaldt, J.-D.; Baiker, A. Phys. Chem. Chem. Phys. 2005, 7, 278;
[4] Caravati, M.; Grunwaldt, J.-D.; Baiker, A. Appl. Catal. A 2006, 298, 50.
[5] Caravati, M.; Meier, D.; Grunwaldt, J.-D.; Baiker, A. J. Catal. 2006, 240, 126.
Presented Monday 17, 15:20 to 15:40, in session SCF as Solvent Substitutes (T1-8).
