Propane dehydrogenation on an extra-framework and framework-embedded metal site within ZSM-5 zeolite from first-principles microkinetic simulations

Isolated single metal site within zeolite exhibits promising catalytic performances towards propane dehydrogenation (PDH); however, the underlying relationship between the local structure of the intrinsic site and the catalytic behavior remains elusive. Herein, the possible structures, relative stabilities and catalytic performances of three isolated single metal cations (M2+, M is Co, Ni, or Cu) within ZSM-5 zeolite were theoretically investigated by combining density functional theory calculations and microkinetic simulations. Both the extra-framework site and the framework-embedded site were taken into account for the sitting of a single metal cation. The isolated divalent M2+ sites are more stable than the corresponding univalent MOH+ sites and both kinds of structures display distinct dependence of stability on temperature. The stepwise pathway rather than the concerted pathway is followed for direct PDH under the reaction conditions of 853 K. Microkinetic simulations reveal that PDH activity increases in the sequence of Co < Ni < Cu within the same local environment of the active site. At the framework-embedded site, the divalent M2+ cations exhibit higher catalytic activity compared to univalent MOH+ cations; on the contrary, the MOH+ motifs are more active than the divalent M2+ ones at the extra-framework site. It seems that the dissociative adsorption enthalpy of propane on isolated metal sites can be utilized to describe PDH activity variation with different metal cations. Simulation results demonstrate that the framework-embedded single Cu site derived from the silanol nest within ZSM-5 zeolite exhibits outstanding PDH activity, while it may suffer from the weakness of stability. This work thus highlights the importance of the local environment of single metal active sites within zeolites and may open up novel avenues for the screening of high-performance catalysts for PDH.