The skeletal isomerization in ferrierite: A theoretical assessment of the bi-molecular conversion of cis-butene to iso-butene

It is still not totally clear as to whether the skeletal isomerization of linear butenes to iso-butene in ferrierite occurs via a mono-molecular or bi-molecular process. To try and shed more light on this process, quantum chemical calculations were undertaken on both mechanisms. A large cluster model (H53O52Si35Al) has been employed here to study the bi-molecular process and these results are contrasted to the mono-molecular results previously reported by the author using the same model. The results suggest that a bi-molecular process can indeed result in the formation of iso-butene, as well as longer chained by-products as exemplified by 2,4,4-trimethylpent-2-ene. A rate determining step of 18.6 kcal/mol is found for the bi-molecular process, involving CC bond formation between the two monomers. The barrier is also predicted to be considerably lower than that of the mono-molecular reaction (24.5 kcal/mol). Nevertheless, given that 2,4,4-trimethylpent-2-ene has a considerably lower barrier to reaction, and is more energetically favourable, iso-butene product to might not be expected to form in large quantities via this route.

Despite considerable experimental effort, it is still unclear as to whether the skeletal isomerization of linear butenes to iso-butene in ferrierite occurs via a mono-molecular or bi-molecular process with time on stream.Insight into both processes is provided by quantum chemical calculations on both mechanisms using a large cluster model (H53O52Si35Al).Figure optionsDownload high-quality image (237 K)Download as PowerPoint slideHighlights
► QM assessment of the mono-molecular and bi-molecular conversion of linear butenes to iso-butene in FER.
► A bi-molecular mediated process displays lower activation barriers than the monomolecular process.
► A bi-molecular process favours longer chained side-products compared to the monomolecular process.