Catalytic Transformations of 1‑Butene over Palladium. A Combined Experimental and Theoretical Study

Applying a density
functional approach to slab models of planar,
(111), and rough, (110), Pd surfaces, we determined the isomerization
free energy barriers of 1-butene to be significantly lower than the
hydrogenation barriers. Microkinetic modeling allows one to mirror
the kinetic experiments on conversions of 1-butene at the corresponding
single-crystal surfaces in a qualitative fashion. Despite the inherent
limitations of such kinetic modeling, theoretical predictions are
fully supported by experimental data using Pd model catalysts: i.e.,
Pd­(111) and Pd(110) surfaces. The isomerization mechanism was calculated
to proceed via an initial dehydrogenation of 1-butene to 1-buten-3-yl
as an intermediatein contrast to the commonly proposed 2-butyl
intermediate, associated with the Horiuti–Polanyi mechanism.
Our modeling results rule out the original assumption that isomerization
has to start with a hydrogenation step to rationalize the dependence
of isomerization on hydrogen. However, this hydrogen dependence may
arise in the second step, after an initial dehydrogenation, as suggested
by the experimental data under hydrogen-deficient conditions.