Kinetic studies of catalytic dehydration of tert-butanol on zeolite NaHZSM-5

Using a combination of FTIR spectroscopy and GC kinetic methods, we have studied in detail the adsorption and dehydration reaction of tert-butanol ( t-BuOH) on H-ZSM-5. From FTIR studies of the kinetics of t-BuOH adsorption, we estimate the diffusion coefficient of t-BuOH in the zeolite channels as 5 × 10 −11 cm 2s −1 at 23°C. The course of the dehydration reaction with time (in the temperature interval 23–60°C) is followed by simultaneous growth of a peak for adsorbed water, changes in the stretching and deformation vibrations for the adsorbed organic, and changes in the nature of the H-bonded species. These changes show that dehydration is accompanied by rapid isomerization and oligomerization of the butene product (the rates of isomerization and oligomerization substantially exceed the rate of dehydration), making it impossible to identify the intermediate for the dehydration reaction (be it carbonium ion or alkoxy species). Under reaction conditions the oligomers (most likely linear C8 species) remain adsorbed on the active sites inside the zeolite channels, resulting in rapid deactivation of the sites. When the sites within the zeolite pores are completely deactivated, the dehydration reaction proceeds on sites located at the external surface of the zeolite crystallites. The latter sites are deactivated much more slowly than the former ones under our reaction conditions.