Role of Spatial Constraints of Bronsted Acid Sites for Adsorption and Surface Reactions of Linear Pentenes

The Bronsted acid sites of H-ZSM-5 and ferrierite reversibly adsborb linear pentenes via hydrogen bonding, rapidly isomerizing the double bond. On H-ZSM-5, dimerization of adsorbed pentenes occurs at a slower rate and leads to pentyl ester covalently bound to the surface. Pentene adsorbed on zeolites with narrower pores, such as ferrierite, remained stable in a hydrogen-bonded state even up to 423 K. Comparing the differential heat of adsorption of 2-pentene on silicalite and ferrierite allowed for the determination of the enthalpy difference between physically adsorbed pentene in ZSM-5 and the localized hydrogen-bonded pi-complex at Bronsted acid sites, -36 kJ/mol. The activation energy (35 kJ/mol) for dimerization is almost identical to this enthalpy difference, suggesting that the rate-determining step is associated either with the mobilization of pi-bonded 2-pentene or with the equally large activation barrier to form an alkoxy group via a carbenium-ion transition state. In a closed system, the dimerization rate is first order in the concentration of the pi-complex that is both in equilibrium with the mobile pentene phase and in production of the carbenium ion that reacts with the mobile pentene. Overall, the alkoxy group is -41 +/- 7 kJ/mol more stable than physisorbed pentene, establishing a series of energetically well-separated groups of reactive surface species.