Involvement of solid acid on Al- and Ga-doped porous silica in the Diels–Alder reaction Original Research Article

The Diels–Alder reaction of 2,3-dimethylbutadiene with p-benzoquinone using Al- and Ga-doped porous silica, i.e., MCM-41, zeolite, and amorphous silica, was studied focusing on individual effects of types, strength, and amount of acid sites and pore size of solid acid catalysts. MCM-41 and amorphous silica-based catalysts exhibited high activity for the Diels–Alder reaction, while zeolites only showed low activity. Comparison with IR spectra of adsorbed pyridine clearly indicates that solid Lewis acid sites are effective for the reaction. Turnover numbers of each catalyst were compared based on the number of total acid sites measured by NH3-temperature programmed desorption (NH3-TPD). Al-MCM-41 and Ga-MCM-41 showed higher turnover numbers than that of AlCl3, which is a typical homogeneous Lewis acid catalyst. The catalysts having stronger acid sites basically showed higher turnover numbers in the Diels–Alder reaction. The effect of pore size was examined by comparison of Ga-MCM-41 and H-type mordenite (H-MOR) evacuated at 973 K. Through dehydration of H-MOR by evacuation at 973 K, Brønsted acid sites of H-MOR were converted to Lewis acid sites of which acid strength is comparable to Ga-MCM-41. Based on the number of Lewis acid sites, Ga-MCM-41 and H-MOR after evacuation at 973 K showed the same turnover number, although they have different pore sizes. These results clearly demonstrate that the catalytic activity depends on strength and amount of Lewis acid sites, while there is not much effect of pore size on the catalytic activities of the Diels–Alder reaction of 2,3-dimethylbutadiene with p-benzoquinone.