Heterogeneous strong base catalysis in supercritical carbon dioxide by mesoporous alumina and sulfated mesoporous alumina

The development of solid strong base catalysts utilizable in green but acidic medium of scCO(2) is reviewed. The strong base sites on mesoporous alumina and sulfated mesoporous alumina that had been generated by severe treatment at 773 K under vacuum (10(-4) Torr) were not neutralized by the compressed Lewis acidic molecules of CO2, promoting a representative strong base-catalyzed reaction of the Tishchenko reaction as well as a typical base-catalyzed reaction of the Knoevenagel reaction in scCO(2). Infrared spectroscopy of the adsorbed pyrrole, temperature-programmed desorption of CO2, and the poisoning by a very weak Bronsted acid of methanol have revealed that the average strengths of the base sites on mesoporous alumina and sulfated mesoporous alumina are weaker than that on conventional gamma-alumina like JRC-ALO-4, but that they have a small number of strong base sites which function even in scCO(2) medium. It was found that the addition of a slight amount of THF cosolvent into scCO(2) remarkably accelerates the Tishchenko reaction over sulfated mesoporous alumina; the reaction rate in the scCO(2)-THF medium was 1.5-fold and 2-fold faster than those in ordinary organic solvents such as benzene and THF and that in pure scCO(2), respectively. The unique structures of mesoporous alumina and sulfated mesoporous alumina have been fully characterized by N-2 adsorption-desorption measurements and XRD analyses.