COMPUTATIONAL MODELLING OF FISCHER-TROPSCH REACTIONS MECHANISM: ALKYL AND ALKENYL PATHWAYS

• Main Author: PAMUNGKAS (NIM : 20514013), GAWANG
• Format: Theses
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/22241
• Institution: Institut Teknologi Bandung

Gas-to-liquid (GTL) technology has been widely developed to convert natural gas into synthetic liquid fuel. GTL technology consists of three steps i.e., syngas generation, syngas convertion, and hydrocracking. Syngas (CO and H2) is produced from coal or natural gas via steam reforming. Syngas is converted into mixture of hydrocarbons by Fischer-Tropsch reactions, the hydrocarbons mixture is then undergone hydrocracking to give desired products. In Fischer-Tropsch reactions, hydrocarbons are produced by CO and H2 reactions on the surface of transition metal. There are many reviews on the reaction mechanisms of the Fischer-Tropsch reactions, but still remains debate in the formation of product, mainly α-olefin. Two major mechanisms were proposed to explain propagation and termination steps on the Fischer-Tropsch reactions, i.e. alkyl and alkenyl mechanisms. Computational modelling of Fischer-Tropsch reactions mechanism on Fe4-cluster have been conducted by density functional theory (DFT) approach using UB3LYP functional and LANL2DZ basis set. The purpose of this study is to describe the steps of α-olefin formation on Fischer-Tropsch reactions by alkyl and alkenyl mechanisms. The computational modelling shows that the formation of α-olefin on the alkenyl mechanism more preferable than alkyl mechanism. In the alkenyl propagation steps, the activation energy (Ea) and ΔEreaction of vinyl formation are 190.14 kJ/mol (m=14) and -70.64 kJ/mol, respectively. In other hand, the the activation energy (Ea) and ΔEreaction of alkyl formation are 256.77 kJ/mol (m=14) and -27.30 kJ/mol. Both kinetic and thermodynamic aspects implies that the formation of vinyl species more favorable than alkyl species. Moreover, the alkyl termination steps need the activation energy (Ea) and ΔEreaction to form α-olefin aproximate 615.84 kJ/mol and 10.07 kJ/mol, respectively. In other hand, the alkenyl termination steps show that the activation energy (Ea) and ΔEreaction of α-olefin formation are 454.26 kJ/mol and 21.74 kJ/mol, respectively. The alkenyl termination steps were kinetically favorable than alkyl termination steps. Although it seems different if we considered thermodynamic aspects, the alkyl termination steps relatively stable than alkenyl termination steps.