Role of the support in the skeletal isomerization of 3,3-dimethyl-1-butene on oxided and sulfided CoMo/Al 2O 3 catalysts

Abstract

The skeletal isomerization of 3,3-dimethyl-1-butene (33DMB1), a typical test for the Brønsted acidity, was carried out at 200°C on a series of CoMo/Al 2O 3 catalysts with a variable atomic ratio r = % Co ( Co + Mo) . This series of catalysts was completed by an unsupported catalyst with r = 30%. On oxided catalysts, a significant decrease of the activity can be observed for low cobalt contents, which shows that the Mo/Al 2O 3 catalyst possesses the greatest number of Brønsted sites. There is a slight linear decrease of the total acidity (determined by temperature-programmed desorption of ammonia) when the cobalt content decreases. A maximum of the Lewis acidity is thus obtained for the promoted catalysts with about 30% cobalt content. On sulfided catalysts (10% H 2S in H 2, 300°C), a similar trend can be observed, the activities remaining close to those measured on oxided precursors. The most striking feature is that promoted catalysts have practically the same initial activity as the bare support. Yet, whereas the CoMo/Al 2O 3 catalysts are very stable, the activity of Al 2O 3 decreases rapidly when the sulfidation state of catalyst is maintained by dimethyldisulfide (DMDS) added to 33DMB1 On the contrary, the support is quite stable when the reaction is carried out in the presence of H 2S. Isomerization most probably occurs on the support which possesses Brønsted sites stable only under a partial pressure of H 2S. Except for Mo/Al 2O 3, there is practically no acid site active in 33DMB1 isomerization on the sulfide phases. Brønsted sites present on molybdenum are rapidly deactivated.