The adsorption of 1-hexene and 3,3-dimethyl-1-butene on Ru-MCM 41

Abstract

The effect of heat-treatment under reduced pressure (> 10- 5 torr) at 623 K on a sample of MCM 41 prepared using a ruthenium surfactant as a template (Ru-MCM 41) has been studied by sorption of nitrogen, 1-hexene, and 3,3-dimethyl-1-butene. Nitrogen sorption (77 K) shows a slight decrease in the BET surface area (330 to 282 m2g- 1) and pore volume (0.19 to 0.16 cm3g- 1) as a result of the heat-treatment. However, adsorption of 1-hexene at 303 K shows a doubling of the monolayer uptake of 1-hexene on heat-treatment, which corresponds to an increase in both (1-hexene derived) BET surface area (144 to 295 m2g- 1) and pore volume (0.12 to 0.16 cm3g- 1). For the non heat-treated sample, less than ~ 50 % of the available surface was accessible to 1-hexene, suggesting the 1-hexene is sterically hindered compared with nitrogen. However heat-treatment of this sample resulted in improved accessibility of the surface and the pore system to the 1-hexene molecules. This increase in 1-hexene uptake without a corresponding change in the nitrogen BET surface area may explain the observed increase in catalytic activity for 1-hexene hydrogenation following heat-treatment of the Ru-MCM 41. The adsorption at 303 K of 3,3-dimethyl-1-butene, which is more bulky isomer of 1-hexene, yielded a (3,3-dimethyl-1-butene derived) BET surface area of 161 m2g- 1 on the heat-treated sample, i.e. about half that of the 1-hexene derived value (295 m2g- 1). The pore volume determined from this sorptive was also lower (0.13 cm3g- 1) than that for 1-hexene (0.16 cm3g- 1) on the heat-treated sample. This suggests that adsorption of 3,3-dimethyl-1-butene, which approximates to a spherical shape, is more sterically hindered compared with that of the long chain 1-hexene.