Effect of steam on the modification of Br?nsted/Lewis acidity of Nb-Mn mixed oxide catalysts

The effect of water on the modification of acidic properties of Nb2O5 and Nb1.3MnOx catalysts was investigated using the cracking of cumene as model reaction, and compared to the behavior of a HZSM-5 catalyst. Nb1.3MnOx exhibited stronger Lewis acidity than Nb2O5, which translated into a higher selectivity towards alpha-methylstyrene formed on Lewis acid sites (LAS) by dehydrogenation of cumene. Steam enhanced strongly the conversion of cumene over both Nb-based catalysts. The products distribution on Nb-based catalysts was also deeply modified in the presence of steam, the selectivity towards alpha-methylstyrene decreasing strongly in favor of benzene, which is formed on Bronsted acid sites (BAS) by dealkylation of cumene. In contrast, the performances of HZSM-5 for cumene cracking and the products distribution were only marginally modified in the presence of steam. A kinetic model based on the elementary steps of the cumene reaction pathways (dealkylation and dehydrogenation) was used to estimate the ratio of LAS to BAS in absence and presence of water over Nb1.3MnOx. The activation energy of the cracking reaction was higher than that of the dehydrogenation reaction. The model described correctly the changes in the catalyst activity induced by addition of approximate to 2 V% of water, which resulted in a decrease in the [LAS]/[BAS] ratio from approximatively 3 to 1.