We investigated the liquid phase Diels-Alder reaction of cyclopentadiene with alpha, beta-unsaturated aldehydes (at 37 degreesC) 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 SUM 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. (C) 2002 Elsevier Science B.V. All rights reserved.