Synergy Between Lewis Acidic Metal Sites and Silanols in Sn- and Ti-Beta for Conversion of Butenes by DRIFTS

The synergistic effect of Lewis acidic Sn/Ti sites and silanols in Sn- and Ti-Beta zeolites on the reaction pathways and mechanisms of the conversion of butenes was investigated by in situ diffuse reflectance infrared Fourier transform spectroscopy. The double bond migration reaction of n-butene to cis-2-butene followed an AB-AD mechanism, in which hydrogen abstraction (AB) from the methylene group at Lewis acidic Sn/Ti sites afforded an allyl intermediate, followed by synergistic hydrogen addition (AD) on silanols. In contrast, the following cis-trans isomerization reaction between cis-2-butene and trans-2-butene, and the activation of isobutene, followed an AD-AB mechanism via hydrogen addition to the double bond of butenes on silanols, forming an alkoxyl group rather than allyl group as the intermediate owing to the absence of a methylene group in 2-butene and isobutene.