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 scCO₂ 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 scCO₂. Infrared spectroscopy of the adsorbed pyrrole, temperature-programmed desorption of CO₂, and the poisoning by a very weak Brönsted 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 γ-alumina like JRC-ALO-4, but that they have a small number of strong base sites which function even in scCO₂ medium. It was found that the addition of a slight amount of THF cosolvent into scCO₂ remarkably accelerates the Tishchenko reaction over sulfated mesoporous alumina; the reaction rate in the scCO₂–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 scCO₂, respectively. The unique structures of mesoporous alumina and sulfated mesoporous alumina have been fully characterized by N₂ adsorption–desorption measurements and XRD analyses.