On the radical cracking of n-propylbenzene to ethylbenzene or toluene over Sn/Al2O3-Cl catalysts under reforming conditions
Résumé
Two series of chlorided-alumina-supported Sn catalysts were synthesized with different precursors, SnCl2 or SnBu4, and with various contents of Sn. The acidity of the catalysts including 0.2 wt% Sn was characterized by FTIR adsorption–desorption of 2,6-dimethylpyridine (Brønsted acidity) and pyridine (Lewis acidity) and compared with that of Al2O3–Cl. The catalytic activity of the synthesized materials was investigated for the transformation of n-propylbenzene under reforming conditions. The results show that the incorporation of Sn into Al2O3–Cl is not harmless. Neither the nature of the Sn precursor nor the content of Sn evenly affects the acidity and the distribution of the isomerized and cracked products. The correlation between the product distribution obtained for the transformation of n-propylbenzene and the acidity of the Al2O3–Cl and the 0.2SnSnCl2/Al2O3–Cl catalysts supports the formation of isopropylbenzene and benzene catalyzed by Brønsted acid sites via carbenium ion chemistry. In contrast, the production of toluene and ethylbenzene occurs via radical chemistry. The formation of these products is assumed to be catalyzed by Lewis acid sites with different strengths. The stability of the proposed radical intermediate species is consistent with the involvement of stronger Lewis acid sites in the production of toluene compared with those involved in that of ethylbenzene. The catalytic cycles responsible for the formation of toluene and ethylbenzene via radical pathways over an Alsingle bondO pair are reported. Finally, it is worth noting that benzene is always the major product of the cracked compounds.