287274 Bifurcation in Hexane Cracking On ZSM-5 and Faujasite Zeolites. A QM/MM/Qct Study

Tuesday, October 30, 2012: 1:00 PM
320 (Convention Center )
Frerich J. Keil, Department of Chemical Engineering, Hamburg University of Technology, Hamburg, Germany

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.

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