Mesoporous Al-containing MCM-41 molecular sieves: highly active catalysts for Diels–Alder reaction of cyclopentadiene with α,β-unsaturated aldehydes Original Research Article

We investigated the liquid phase Diels–Alder reaction of cyclopentadiene with α,β-unsaturated aldehydes (at 37 °C) over mesoporous Al-containing MCM-41 molecular sieves, with different Si/Al ratios, prepared by four different methods—sol–gel, hydrothermal, grafting and template cation exchange. The catalysts have been characterized for their phase purity, surface area, average pore size and acidity. The Al-containing MCM-41 catalysts prepared by different methods behaved differently in the acid-catalyzed Diels–Alder reaction. Among the different MCM-41 catalysts (having almost the same Si/Al ratio), Al:MCM-41, prepared by grafting method showed the highest activity in the Diels–Alder reaction of cyclopentadiene with crotonaldehyde, which may be due to its higher acidity. The introduction of Al into the framework of MCM-41 increased the acidity; however, the increase in the acidity up to an optimum level increased the activity. Al:MCM-41 catalyst is more catalytically active in the Diels–Alder reaction than the heterogeneous solid acids—mesoporous SBA-15, microporous H-ZSM-5 and HY zeolites, amorphous silica–alumina, amberlist 15, active alumina, silica gel and homogeneous acid catalysts—p-toluenesulfonic acid and AlCl3.