Mechanisms of Double-Bond Isomerization Reactions of n-Butene on Different Lewis Acids

Understanding olefin isomerization mechanisms and structures of acid sites is critical in many catalytic reactions in petroleum chemistry. Here, we report the mechanism and structural investigation on the double-bond isomerization reaction of n-butene over two different Lewis acid sites from three-membered rings (3MRs) formed by the reconstruction of silanol nests in dealuminated zeolite beta (denoted Si-beta) and AlCl3 in AlCl3@gamma-Al2O3. The results obtained using a combination of diffuse-reflectance infrared Fourier-transform spectroscopy (DRIFTS) and density functional theory (DFT) calculations showed that the double-bond migration (DBM) of n-butene to form cis-2-butene on the Si-beta catalyst follows an abstraction-addition (AB-AD) mechanism, which proceeds via a sigma-bonded allylic species as the intermediate; while the cis-trans isomerization between cis-2-butene and trans-2-butene proceeds through an addition-abstraction (AD-AB) mechanism with alkyl reversal catalyzed by silanol nests. On the AlCl3@gamma-Al2O3 catalyst, hydrogen addition to butene hardly occurs because of the high-energy barriers and a low amount of surface hydrogenation. Consequently, both double-bond migration of n-butene and cis-trans transformations of 2-butene proceed via an AB-AD mechanism. cis-2-Butene isomerization involves initial transformation to n-butene, and n-butene is then transformed to trans-2-butene by sigma-bonded allylic intermediates. The catalytic performances of the two catalysts in butene conversions further verified the results from DRIFTS and DFT studies. This work clarifies the origins of olefin isomerization mechanisms on different Lewis acid sites at the atomic level.

Automated summary

Understanding the mechanisms of olefin isomerization on Si-β and AlCl3@gamma-Al2O3 catalysts through DRIFTS and DFT studies revealed the importance of the AB-AD mechanism in the process. The results clarified the origins of olefin isomerization mechanisms at the atomic level and were further verified by the catalytic performances of the two catalysts in butene conversions.