Theoretical investigation of propane cracking and dehydrogenation in zeolites
Creators: name: Alaithan, Zainab Ali
Publisher: name: Imperial College London
Year: 2022

description: Cracking and dehydrogenation of alkanes are two important reactions in the petrochemical industry for the production of petrochemicals and transportation fuels. In this thesis, we used Density Functional Theory (DFT) and Molecular Dynamics (MD) to theoretically investigate the main factors that influence propane cracking and dehydrogenation inside zeolites. Zeolites are the main catalysts used in the industry to promote the cracking reaction. In the first chapter, we used DFT to investigate the influence of the zeolite pore size and geometry on the intrinsic and apparent activation energies of propane cracking. The intrinsic activation energy is the energy required to cross the energy barrier between the adsorbed reactant state and the product state, while the apparent activation energy, which is measured experimentally, inevitably includes the adsorption energy into the active site. Our results indicate that the observed differences in cracking reaction rates in different zeolites are dominated by the adsorption energies. The intrinsic activation energies are relatively insensitive to the pores’ sizes and geometries. Commonly used acidity descriptors were also investigated to determine whether they indicate zeolite activity. Our results show that for alumino-silicate frameworks, the calculated acidity descriptors such as deprotonation energies and ammonia adsorption energies don’t correlate with the calculated cracking activation energies. Since pore size and geometry also influence the diffusion of products out of the zeolite’s pore, we have also investigated the effect of pore size and channels dimensionality on the diffusion of propylene (a molecule of high industrial importance). The influence of several factors such as temperature, loading, pore diameter, and Al/H distribution was investigated. Propylene diffusion was calculated in three one-dimensional-medium pore alumino-silicate frameworks and one two-dimensional-pore zeolite for the sake of comparison. Our results demonstrate that at high loading and temperature, the diffusion of propylene in one-dimensional pore systems deviates from the Arrhenius equation and diffusivity remains constant with temperature due to the dominant effect of the propylene-propylene interactions. Also, when loadings are compared ’per accessible volume,’ the self-diffusivity in one-dimensional pore zeolites is similar to that of two-dimensional pore zeolites. Finally, the Al/H distributions can influence the diffusion of propylene when hydrogen bonds protrude perpendicular to the channels axis and interact more strongly with propylene. The catalytic potential of cobalt encapsulated in zeolites for propane and ethane dehydrogenation has sparked interest in this area. In the third chapter, we used DFT to study the geometries and energies of cobalt clusters encapsulated in a zeolite vacancy. The reactions mechanisms of propane dehydrogenation over cobalt oxide and metallic cobalt clusters were then investigated. Our results show that propane dehydrogenation is more favorable thermodynamically and kinetically over metallic cobalt than cobalt oxide. The detailed theoretical understanding of the dehydrogenation reaction mechanisms over cobalt clusters can help design catalysts with improved activity and stability.