(278b) Bifurcation in Hexane Cracking On ZSM-5 and Faujasite Zeolites. A QM/MM/Qct Study

Bifurcation
in hexane cracking on ZSM-5 and Faujasite zeolites. A QM/MM/QCT
study.

Quantum
mechanics/Molecular mechanics (QM/MM) models are applied to
investigate the adsorption and cracking of n-hexane on ZSM-5 and
Faujasite zeolite structures. The local chemistry of bond-breaking
and forming at the acid site, and the medium to long range
interactions of the zeolite lattice with the substrates were
calculated by dispersion-corrected density functional theory (DFT,
B97-D functional) combined with molecular mechanics (MM, CHARMM).

The adsorbed
molecules investigated are characterized by their thermodynamic
properties (adsorption energy and enthalpy). From intrinsic energy
barriers, intrinsic rate coefficients were calculated by means of
transition state theory. All the thermodynamic properties are
compared with experimental data.

The influence of the
zeolite type on the thermodynamic properties is also discussed.

The results reveal
that the kinetics of cracking is insensitive to differences in acid
strengths. The thermodynamical data obtained are mainly influenced by
the adsorption energy of n-hexane on ZSM-5 and/or Y structures. The
size of the pores of the zeolite type can lead to a stronger or
weaker adsorption energy.

The reaction pathway
using the quasi-classical trajectory method (QCT) is used for
investigating bifurcation phenomena. The bifurcation process occurs
when many products are obtained starting from the same transition
state. QCT models realistic velocities by populating vibrational
modes as a function of temperature. The nuclei are propagated
classically, which results in an efficient method for exploring
reaction paths starting from the transition state.

The bifurcating
routes for n-hexane cracking leads to a full range of intermediates
and final products. The calculated product distribution is in
accordance with the experimental results.