| This thesis uses computational chemistry to investigate the catalytic cracking mechanisms of hydrocarbons on solid and liquid catalysts (alumino-silicates and chloroaluminates, respectively). Several aspects, such as initial steps, overall catalytic cycle, type of acid catalyst, and the nature of carbenium and carbonium ions as intermediates, were investigated. Transition state searching is used as the main tool to understand these aspects. Cluster models and periodic models are both applied as two approaches in the study.; The geometry and relative energies of torsional conformers of centrally protonated C4H11+ were studied with ab initio methods. Twenty-nine combined levels of theory were used to optimize the geometry to evaluate the performance of lower levels of approximation upon this challenging structure. The results show that all conformers lie within a 4 kJ mol-1 range, with the lowest-energy conformer being either trans or gauche with a staggered dihedral for the bridging proton.; The mechanisms of hydrocarbon cracking were studied using DFT computations. Firstly, the beta-scission mechanism of physisorbed and chemisorbed pentenium ions, as catalyzed by small alumino-silicate and chloroaluminate fragments, was investigated. A qualitatively different mechanism, with qualitatively different intermediates, was observed on the chloroaluminate fragment, due only to the different basicity of the two types of catalyst. Secondly, cracking of an all-trans n-hexane, via idealized Lewis-acid and Bronsted-acid chloroaluminate catalysis (a crude model of an ionic liquid), was examined. The overall catalytic cycles, including initiation steps, propagation steps and termination steps for the same reaction, were examined. Thirdly, larger models of the catalysts were also applied to several reactions studied above, to infer how to better mimic real systems and how relevant the idealized catalyst mechanism might be to the real solid or liquid catalytic systems.; A transition state searching method (NEB method) was tested on the elementary isomerization HCN &rlhar2; CNH and the beta-scission mechanism of 2-hexenium ion on Al 2Cl7- catalyst, using a periodic model. This method could be useful in future mechanism studies with larger catalyst models. The beta-scission transition state obtained with optimal parameters for the NEB method were then compared with the one from a cluster model calculation with a more exact, local surface-walking transition state searching method. The results from higher-level (GGA) and lower-level (LDA) density functional theories indicate good agreement on the reaction pathway, with GGA providing more accurate energies. However, to mimic the real ionic liquid system, an improvement should be done on the current replication model to better represent the radius shell structure of ions in an ionic liquid. |
