Understanding Alkene Interaction with Metal-Modified Zeolites: Thermodynamics and Mechanism of Bonding in the π‑Complex

Zeolites modified with metal cations are perspective
catalysts
for converting light alkenes to valuable chemicals. A crucial step
of the transformation is an alkene interaction with zeolite to afford
π-complex with metal cations. The mechanism of alkene bonding
with cations is still unclear. To address this problem, propene adsorption
on H+ (Bro̷nsted acid site), Na+, Ca2+, Zn2+, Co2+, Cu2+, Cu+, and Ag+ cationic sites in ZSM-5 zeolite has been
studied by quantum chemical calculations in terms of adsorption enthalpy,
νCC frequency, and natural bond
orbital (NBO) analysis together with natural energy decomposition
analysis (NEDA). It is revealed that the conventional concept of σ-
and π-bonding is only partially applicable to alkene interaction
with metal cations in zeolites. The orbital interaction between an
alkene molecule and a metal site is more complex. Several different
bonding mechanisms have been identified depending on the nature and
electron configuration of the metal cation. This finding explains
the complex correlations observed for propene π-complex stability
and νCC frequency shift or charge transfer
from the alkene molecule. The results provide the basis for further
understanding the interactions between alkenes and inorganic solid
Bro̷nsted and Lewis acids.