A density functional theory study of the α-olefin selectivity in Fischer–Tropsch synthesis

We studied the α-olefin selectivity in Fischer–Tropsch (FT) synthesis using density functional theory (DFT) calculations. We calculated the relevant elementary steps from C2 to C6 species. Our results showed that the barriers of hydrogenation and dehydrogenation reactions were constant with different chain lengths, and the chemisorption energies of α-olefins from DFT calculations also were very similar, except for C2 species. A simple expression of the paraffin/olefin ratio was obtained based on a kinetic model. Combining the expression of the paraffin/olefin ratio and our calculation results, experimental findings are satisfactorily explained. We found that the physical origin of the chain length dependence of paraffin/olefin ratio is the chain length dependence of both the van der Waals interaction between adsorbed α-olefins and metal surfaces and the entropy difference between adsorbed and gaseous α-olefins, and that the greater chemisorption energy of ethylene is the main reason for the abnormal ethane/ethylene ratio.