posted on 2024-01-09, 08:31authored byZhaochen Miao, Yingxin Sun, Sheng Han, Qianggen Li
The
catalytic performance of gallium-modified acidic FAU (Ga-FAU)
zeolite in the methanol to propene (MTP) process was theoretically
investigated by a two-layer ONIOM (our own N-layered integrated molecular
orbital and molecular Mechanics) method. The whole MTP mechanism includes
two cycles: the polymethylbenzene (polyMB) cycle and the alkene cycle.
The polyMB cycle consists of the direct internal H-shift pathway,
spiro pathway, methyl-transfer pathway, and paring pathway. Compared
with the previous theoretical results on the MTP process on the pure
acidic Si/Al FAU, the addition of Ga atoms into the Si/Al H-FAU could
reduce the contribution of the polyMB cycle and increase the reactivity
of the alkene cycle on the MTP process. A careful analysis of activation
free energy barriers on the transition states (TSs) shows that the
internal H-shift step is the rate-determining step for the direct
internal H-shift pathway and the paring pathway. The rate-determining
step for the spiro and methyl-transfer pathways is the methylation
of the PMB molecule. The methylation of the propene molecule is the
rate-determining step for the alkene cycle. The polyMB and alkene
cycles have almost the same reactivity on the Ga-FAU zeolite. For
the polyMB cycle, the different elementary steps are in the following
order of reactivity: internal methyl transfer > deprotonation >
C–C
bond cracking > ring contraction > methylation > internal
H-shift.
The order in the alkene cycle is different: deprotonation > proton
transfer > beta-scission > methylation. The differential charge
density
(DCD), local orbital locator (LOL), and reduced density gradient (RDG)
revealed the direction of electron flow in different fragments in
the TS structures and the nature of interactions between the fragments
in the TSs.